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Microstructural features and orientation correlations of non-modulated martensite in Ni–Mn–Ga epitaxial thin films
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ScienceDirect Acta Materialia 61 (2013) 6809–6820 www.elsevier.com/locate/actamat
Microstructural features and orientation correlations of non-modulated martensite in Ni–Mn–Ga epitaxial thin films B. Yang a,b,c, Z.B. Li a,b, Y.D. Zhang b,c,⇑, G.W. Qin a, C. Esling b,c, O. Perroud b, X. Zhao a, L. Zuo a,⇑ b
a Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China Laboratoire d’E´tude des Microstructures et de Me´canique des Mate´riaux (LEM3), CNRS UMR 7239, Universite´ de Lorraine, 57045 Metz, France c Laboratory of Excellence on Design of Alloy Metals for Low-mass Structures (DAMAS), Universite´ de Lorraine, 57045 Metz, France
Received 29 May 2013; received in revised form 25 July 2013; accepted 27 July 2013 Available online 29 August 2013
Abstract Epitaxially grown thin films with nominal composition Ni50Mn30Ga20 and thickness 1.5 lm were prepared on MgO(1 0 0) substrate with a Cr buffer layer by DC magnetron sputtering. The surface layer microstructures of the as-deposited thin films consist of non-modulated (NM) martensite plates with tetragonal structure at ambient temperature, which can be classified into the low and high relative contrast zones of clustered plates (i.e. plate colonies) with parallel or near-parallel inter-plate interface traces in secondary electron images. Orientation analyses by electron backscatter diffraction revealed that individual NM plates are composed of alternately distributed thicker and thinner lamellar variants with (1 1 2)Tetr compound twin relationship and coherent interlamellar interfaces. In each plate colony, there are four distinct plates in terms of the crystallographic orientation of the thicker lamellar variants and therefore, in total, eight orientation variants. For the low relative contrast zones, both thicker and thinner lamellar variants in adjacent plates are distributed symmetrically across their inter-plate interfaces (along the substrate edges). At the atomic level, there are no unbalanced interfacial misfits and height misfits, resulting in long and straight inter-plate interfaces with homogeneous contrast. However, in the high relative contrast zones, the thicker and thinner lamellar variants in adjacent plates are oriented asymmetrically across their inter-plate interfaces (at 45° to the substrate edges). Significant atomic misfits appear in the vicinity of the inter-plate interfaces and in the film normal direction. The former result in bending of the inter-plate interfaces, and the latter give rise to the high relative contrast between adjacent plates. Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ferromagnetic shape memory alloys; Ni–Mn–Ga thin films; Crystallographic features; Electron backscatter diffraction
1. Introduction Ferromagnetic Ni–Mn–Ga alloys are conceived as promising sensor or actuator materials for micro-electromechanical ⇑ Corresponding authors. Addresses: Laboratoire d’E ´ tude des Microstructures et de Me´canique des Mate´riaux (LEM3), CNRS UMR 7239, Universite´ de Lorraine, 57045 Metz, France. Tel.: +33 3 87315399; fax: +33 3 87315377 (Y.D. Zhang), Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China. Tel.: +86 24 23906503; fax: +86 24 23892454 (L. Zuo). E-mail addresses: [email protected] (Y.D. Zhang), [email protected] (L. Zuo).
systems or devices because of their large and highly reversible magnetic-field-induced strains with fast dynamic response [1–5]. However, such excellent performance is only attainable within a narrow microstructural window, as the magneticfield-induced shape change comes from the reorientation of ferromagnetic martensite with strong magnetocrystalline anisotropy. Typically, the martensitic transformations in these alloys are accompanied by the formation of self-accommodated groups [6] of martensitic variants, oriented differently and clustered into colonies. The microstructural texturing to eliminate undesired variants has become a prerequisite for optimizing the shape memory effect, even for single crystal alloys.
1359-6454/$36.00 Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actamat.2013.07.055
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In recent years, the magnetron sputtering technique has attracted considerable attention as a significant texturing tool for ferromagnetic Ni–Mn–Ga thin films grown epitaxially on single crystal substrates such as MgO [7–10], Al2O3 [11,12], GaAs [13,14] and other substrates [15]. Although the procedure to fabricate continuous thin films with homogeneous chemical composition and controllable film thickness can be parameterized [8,10,16–20], the microstructural and crystallographic characterization of the thin films produced remains challenging, owing to the local constraints from substrates, the specific geometry of thin films and the ultrafine microstructures of the constituent phases. So far, pioneering efforts [7–10,18,19,21–28] have been devoted to the microstructural characterization of epitaxial Ni–Mn–Ga thin films to determine their physical and mechanical behavior. The X-ray diffraction (XRD) technique was first used to analyze the phases and textures of Ni–Mn–Ga thin films [10,18,20,28]. This technique proved to be applicable, in most circumstances, to the identification of highly textured multi-phases such as austenite, modulated martensite and non-modulated (NM) martensite. In the case of cubic austenite or tetragonal NM martensite, the lattice constants can be unambiguously determined from the XRD reflection peak locations [9,10,20,24]. As for monoclinic seven-layered modulated martensite (7M or 14M), the lattice constant determination may suffer from an insufficient number of identified reflection peaks [10,18–20,29]. Without knowing precisely the monoclinic angle, the monoclinic modulated martensite needs to be simplified with a pseudoorthorhombic structure, as practiced previously for bulk alloys [10,18–20,29]. Furthermore, the structure modulation of the 7M martensite is frequently interpreted in terms of the nanotwin combination of the tetragonal NM martensite (the so-called adaptive phase), as the lattice constants of the austenite, 7M martensite and NM martensite fulfill the relation proposed by the adaptive phase theory (a7M = cNM + aNM aA, aA = b7M, c7M = aNM [29]). Scanning electron microscopy (SEM) has further revealed that individual martensitic variants in Ni–Mn– Ga thin films have a plate-like morphology and are organized in colonies [18,19,21], similar to the case of bulk alloys, but with a much finer microstructure. Here, each plate colony represents a distinct zone of martensite plates that are clustered with parallel or near-parallel plate interfaces [6]. As suggested by the secondary electron (SE) imaging, two types of martensite plate colonies may exist in epitaxial Ni–Mn–Ga thin films grown on a single crystal MgO substrate. One refers to long plates with relatively homogeneous contrast, running parallel to one edge direction ([1 0 0]MgO or [0 1 0]MgO) of the substrate. The other refers to shorter plates with relatively high contrast, oriented at 45° to the substrate edges. Close examination using atomic force microscopy [19,21,23] demonstrated that the different SE contrasts correspond to different surface corrugations of the films, i.e. the
relatively low contrast zones to low surface corrugation, and the relative high contrast zones to high surface corrugation. Previous studies on Ni–Mn–Ga thin films [9,10,19– 23,26,27,29] suggested that the high surface corrugation zones are associated with the 7M martensitic structure. Adjacent martensite plates in these zones may have a socalled a–c twin relationship [9,19,21,23,25,29], i.e. the pseudo-orthorhombic unit cells of two twinned martensite plates share a common b axis (the basis of the pseudoorthorhombic cell is set in accordance with that of the cubic basis of the austenite). The inter-plate interfaces correspond to the (1 0 1)orth plane, and they are inclined at roughly 45° to the substrate surface [9,19,21,23,27]. Highresolution scanning tunneling microscopy (STM) examinations [23] further indicated that there are still fine periodic corrugations in the interiors of 7M plates. Such fine corrugations may be related to the structure modulation of the 7M martensite [23]. In contrast, the nature of long martensite plates in the low surface corrugation zones has been studied less. Some reported that they are NM martensite [19], but others concluded that they are 7M martensite [18,21]. Using the epitaxial relationship between MgO substrate and austenite, along with the Bain orientation relationship between austenite and NM martensite, the adjacent martensite plates were deduced to be a–c twin related to the plate interfaces ((1 0 1)orth) perpendicular to the substrate surface [18,21]. As no significant corrugations can be detected in the interiors of individual plates, their microstructural and crystallographic features remain unknown. Clearly, owing to the lack of direct correlation between the ultrafine martensitic microstructures and their crystallographic orientations, the details of the configuration of lamellar variants inside martensite plate and the characteristics of intra- and inter-plate interfaces in Ni–Mn–Ga thin films are still not available. Here, an attempt is made to explore the microstructural and crystallographic features of martensite in epitaxial Ni50Mn30Ga20 thin films. By means of XRD and electron backscatter diffraction (EBSD) analyses, the crystal structures of constituent phases and the configurations of martensitic variants and their orientation correlations are addressed. 2. Experimental methods Ni–Mn–Ga thin films with a nominal composition of Ni50Mn30Ga20 and nominal thickness 1.5 lm were deposited from a cathode target of Ni46Mn32Ga22 by DC magnetron sputtering (JZCK-400DJ) at a sputtering rate of 0.2 nm s1. A 100 nm thick Cr buffer layer was precoated on the MgO(1 0 0) monocrystal substrate. The base pressure before deposition was <9.0 105 Pa. In order to obtain continuous thin films, the deposition was conducted under a fixed Ar2 working pressure of 0.15 Pa, with the substrate being held at 500 °C. The entire deposition process lasted for 2 h.
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The thickness and chemical composition of the thin films produced were measured using a stylus profiler (DEKTAK 150) and an energy dispersive spectrometer (Bruker XFlash 4010) attached to the field emission gun SEM microscope (JEOL JSM-6500F), respectively. It was verified that the as-deposited thin films have a thickness of 1.47 lm on average, and the chemical composition is Ni51.5Mn29Ga19.5, both being very close to the designed nominal values. The crystal structures of the as-deposited thin films were determined by XRD, using Co Ka radiation (k = 0.178897 nm). Considering that the thin films may have in-plane texture, two four-circle X-ray diffractometers, one with a conventional h–2h coupled scan and the other with a rotating anode generator (RIGAKU RU300) and a large-angle position sensitive detector (INEL CPS120), were used to collect a sufficient number of diffraction peaks. The geometrical configurations of the two X-ray diffractometers are illustrated schematically in Fig. 1. In the former case (Fig. 1a), the h–2h coupled scans were performed in the 2h range of 45–90° with tilt angles w varying from 0° to 10° and a step size of 1°. In the latter case (Fig. 1b), the 2h scans were conducted with tilt angle w from 0.75° to 78.75° and a step size of 1.25°. At
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each tilt angle, the sample was rotated from 0° to 360° with a step size of 5°. Two incident angles x (27.9° and 40°) were selected in order to obtain possible diffraction peaks at the low 2h (48–58°) and high 2h (82.5°) regions. The final diffraction patterns were obtained by integrating all the diffraction patterns acquired at different sample positions. The microstructures and crystallographic orientations of the as-deposited thin films were examined using the same SEM equipped with an EBSD acquisition camera and Channel 5 analysis system (Oxford HKL). In order to obtain clear EBSD patterns with detectable Kikuchi lines for reliable orientation determination, the film surfaces were subjected to slight electrolytic polish with a solution of 20% HNO3 in CH3OH at 12 V for 3 s at room temperature. Here, the Kikuchi line patterns from individual martensitic variants were acquired manually and analyzed in interactive mode. For specifying the orientations of martensitic variants, the macroscopic sample coordinate system refers to the cubic lattice basis of the MgO(1 0 0) monocrystal substrate ([1 0 0]MgO, [0 1 0]MgO and [0 0 1]MgO). The conventional monoclinic and tetragonal Bravais lattice cells are chosen to describe the crystal structure of the 7M martensite and that of the NM martensite, respectively. Traditionally, the martensite structures were referenced to the coordinate system derived from cubic coordinate system of austenite [30,31]. 3. Results 3.1. Crystal structure
Fig. 1. Schematic drawings of four-circle X-ray diffractometer using (a) a point detector (h = x) and (b) a large-angle position sensitive detector (x is different from h).
Fig. 2a shows the w-dependent XRD patterns of the asdeposited Ni–Mn–Ga thin films obtained by h–2h coupled scanning at ambient temperature. At each tilt angle w, only a limited number of diffraction peaks appear. No diffraction peaks are visible over the 2h range 48–55°. This may be a result of the specific texture of the as-deposited thin films, as several diffraction peaks in this angle range were observed in previous powder XRD measurements [32,33]. Fig. 2b presents the XRD patterns measured using a large-angle position-sensitive detector under two different incident beam conditions. Compared with Fig. 2a, extra diffraction peaks are clearly seen in the 2h range 48–55° and at the higher 2h positions (82°). These additional peaks in the 2h range 48–55° were not detected in previous studies on the Ni–Mn–Ga thin films [9,10,18,20,27,29]. By combining all the characteristic diffraction peaks in Fig. 2a and b, it is possible to conduct more reliable phase identification and lattice constant determination on the constituent phases of the as-deposited thin films, especially for the modulated martensite. Apart from the well-identified diffraction peak positions for the MgO substrate and the Cr buffer layer (the latter could also be indexed as Ni–Mn–Ga austenite), all other diffraction positions were compared with those calculated using the published crystal structure data on the monoclinic incommensurate 7M martensite [32–35] and the
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B. Yang et al. / Acta Materialia 61 (2013) 6809–6820 Table 1 Calculated lattice constants of austenite, 7M martensite and NM martensite for as-deposited Ni–Mn–Ga thin films. Phase
7M
Space group
P2/m (No. 10) NM I4/mmm (No. 139) Austenite Fm3m (No. 225)
Crystal system
Lattice constants a (nm) b (nm) c (nm) a, b, c
Monoclinic 0.4262 0.5442 4.199 a = c = 90°, b = 93.7° Tetragonal 0.3835 0.3835 0.6680 a = b = c = 90° Cubic L21 0.5773 0.5773 0.5773 a = b = c = 90°
Fig. 2. XRD patterns of as-deposited Ni–Mn–Ga thin films with MgO/Cr substrate: (a) measured by conventional h–2h coupled scanning at different tilt angles w; (b) measured by 2h scanning at two incidence angles x and integrated over the rotation angle u.
tetragonal NM martensite [36,37]. Fairly good matches were achieved between the measured and recalculated peak positions for the two types of martensite, which confirms that the 7M martensite and the NM martensite coexist in the as-deposited thin films at room temperature. It should be noted that there is always a deviation of 0.2–2° in the measured peak positions from those obtained by powder XRD diffraction [32–34]. This indicates that the lattice constants of the 7M and NM martensite in the as-deposited thin films are not exactly the same as those derived from their powder counterparts, as the substrate imposes a constraint on the as-deposited thin films. Using the measured XRD data, the lattice constants of the 7M martensite and the NM martensite were resolved. Table 1 summarizes the complete crystal structure information of the three different phases (austenite, 7M martensite and NM martensite) involved in the as-deposited thin films. This information is required for EBSD orientation analysis.
Fig. 3. (a) SE image of electrolytically polished Ni–Mn–Ga thin films, showing martensite plates that are clustered in colonies with low and high relative contrasts. The sample coordinate system is set in accordance with the basis vectors of the MgO(1 0 0) substrate ([1 0 0]MgO, [0 1 0]MgO and [0 0 1]MgO). (b) High-magnification BSE image of the squared area Z1 in Fig. 3a, showing fine lamellae distributed alternately inside each plate. The inter-plate interfaces are marked with white dotted lines, and the intraplate interfaces with blue and green solid lines. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.2. Microstructure Fig. 3a shows a typical SE image of the as-deposited thin films after slight electrolytic polishing. It is seen that the microstructure of the film surface layer is characteristic
of clustered plates with two distinct relative contrasts. The low relative contrast zones (e.g. Z1 in Fig. 3a) contain long and straight plates with their plate interfaces running parallel to one edge of the substrate (i.e. [1 0 0]MgO or
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[0 1 0]MgO), whereas the high relative contrast zones (e.g. Z2 in Fig. 3a) contain shorter and wavy plates with their plate interfaces oriented roughly at 45° with respect to the substrate edges. In each relative contrast zone, one or several plate colonies can be distinguished locally by the alignment of parallel or near-parallel inter-plate interface traces. Further examination at a higher magnification in backscattered electron imaging (BSE) mode reveals that the fine lamellae with two different brightness levels are distributed alternately inside each plate, as highlighted with green and blue lines in Fig. 3b. Of the two neighboring lamellae, one is thicker, and the other is thinner. As the BSE image brightness for a monophase microstructure with homogeneous chemical composition depends on the crystallographic orientation of the microstructural components, the thicker and thinner lamellae in each plate should have two distinct orientations, and they are referred to as two orientation variants. Using the determined lattice constants of the NM and 7M martensite and the published atomic position data [32] as initial input, the constituent phases in the surface layers of the as-deposited thin films were verified by EBSD analysis. The Kikuchi line indexation has evidenced that both low and high relative contrast zones displayed in Fig. 3a can be identified as the tetragonal NM martensite, but not the 7M martensite. Fig. 4 presents an example of the Kikuchi line pattern acquired from one martensite plate with high brightness in the high relative contrast zone and the calculated patterns using the tetragonal NM martensite structure. As the in-plate lamellar variants are too fine and beyond the resolution of the present EBSD analysis system, a single set of Kikuchi lines could not be obtained from one variant. For each acquisition, there are always mixed patterns from two neighboring variants appearing in one image, as displayed in Fig. 4a. However, the high intensity reflection lines belonging to two different variants can be well separated in the image, as outlined by the green and red triangles in Fig. 4b. By comparing Fig. 4c and d with Fig. 4a, one can find perfect matches between the acquired Kikuchi lines for the two variants and the calculated ones using the tetragonal NM martensite structure. Apparently, the martensite appearing in the surface layers of the present thin films is different from that identified previously by other groups [9,10,18,19,21,23,26,27,29]. This may be due to the different thickness of the thin films produced. In the present study, the NM martensite was found at the surface of the 1.5 lm thick thin films, while the others reported the existence of the 7M martensite at the surface of the thin films with maximum thickness 0.5 lm. Indeed, the formation of different types of martensite is very sensitive to local constraints. It is obvious that, at the film surface, the constraint from the substrate decreases with the increasing film thickness. Thus, the surface layers of thick films without much constraint from the substrate would easily transform to the stable NM martensite, as demonstrated in the present case.
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Moreover, from the distinguishable peaks of the 7M and NM martensite in the XRD patterns displayed in Fig. 2, one can see that the quantity of the 7M martensite is comparable with that of the NM martensite, which is inconsistent with the EBSD microstructural characterization. In fact, the EBSD information is obtained from the surface layer (100 nm thick) of the as-deposited thin films after slight electrolytic polishing, whereas the information from the XRD measurement is through-the-thickness to the substrate. Thus, one may consider that the 7M martensite is located underneath the EBSD-detected surface layer, and the surface layer microstructure is composed mainly of the NM martensite. This further confirms the above deduction that the NM martensite appears at the film surface for present thick films, in contrast to the formation of the 7M modulated martensite at the film surface for other thinner films [9,10,19,21–23,26,29]. Hereafter, the present paper focuses only on the orientation correlations of the NM martensite plates. 3.3. Orientation correlation Detailed EBSD orientation analyses were conducted on the NM martensite plates in the low and high relative contrast zones (Z1 and Z2 in Fig. 3a). Individual plates were identified to be composed of two alternately distributed orientation variants (lamellae) with different thicknesses, as illustrated schematically in Fig. 5a and b. According to the orientations of the thicker variants in plates, two sets of orientation plates can be distinguished, i.e. the set of Plates A, B, C and D in the low relative contrast zones and the set of Plates 1, 2, 3 and 4 in the high relative contrast zones. Thus, there are in total eight orientation variants of the NM martensite in one plate colony. For easy visualization, they are denoted as V1, V2, . . ., V8 in Fig. 5a and SV1, SV2, . . ., SV8 in Fig. 5b, where the symbols with odd subscripts correspond to the thicker (major) variants, and those with even subscripts the thinner (minor) variants. Taking the basis vectors of the MgO(1 0 0) monocrystal substrate as the sample reference frame, the measured orientations of the NM variants in the two relative contrast zones are presented in the form of {0 0 1}Tetr, {1 1 0}Tetr, {0 1 0}Tetr and {1 1 2}Tetr pole figures, as displayed in Fig. 5c and d. It is noted that the orientations of the major and minor variants in the low relative contrast zones are different from those in the high relative contrast zones. For the low relative contrast zones (Z1), the major and minor variants in Plates A, B, C and D are oriented respectively with their (1 1 0)Tetr planes and (0 0 1)Tetr planes nearly parallel to the substrate surface (Fig. 5c). In the high relative contrast zones (Z2), such plane parallelisms hold for Plate 2 and Plate 4, but with an exchange of the planes between the major and minor variants, whereas both major and minor variants in Plate 1 and Plate 3 are oriented with their (1 1 0)Tetr planes nearly parallel to the substrate surface
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Fig. 4. (a) Kikuchi line pattern acquired from one martensite plate in the high relative contrast zone (Z2) in Fig. 3a. (b) Indication of high intensity reflection lines from two adjacent variants 1 (the green triangle) and 2 (the red triangle). (c and d) Kikuchi line patterns calculated using the tetragonal structure for variants 1 and 2, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(Fig. 5d). In correlation with the microstructural observations, Plate 2 and Plate 4 are featured with higher brightness and Plate 1 and Plate 3 with lower brightness. Based on misorientation calculations, the orientation relationships between adjacent lamellar variants in the same plate were further determined from their orientation data acquired manually by EBSD. For both low and high relative contrast zones, the in-plate lamellar variants are found to have a compound twin relationship with the twinning elements K1 = (1 1 2)Tetr, K2 ¼ ð1 1 2ÞTetr , g1 ¼ ½1 1 1Tetr , g2 = [1 1 1]Tetr, P ¼ ð1 1 0ÞTetr and s = 0.412. Here, all the crystallographic elements are expressed in the crystal basis of the conventional tetragonal Bravais lattice. The present twin relationship is the same as that reported for the bulk alloys [38,39], and it is also consistent with the so-called a–c twin relationship found in the thin films [9,19,21, 23,25,29]. By means of the indirect two-trace method [40], the inplate interlamellar interface planes in the two relative contrast zones were further determined. They coincide well with the (1 1 2)Tetr compound twin planes (roughly corresponding to the (1 0 1) planes of the austenite [9,10,19,29]) and are perfectly coherent, as indicated by the light green solid squares in Fig. 5c and d. According to the {1 1 2}Tetr pole figure of the low relative contrast zones shown in Fig. 5c, the interlamellar interfaces in each plate are inclined at 47.5° towards the substrate surface, and two interlamellar interfaces from adjacent plates are positioned symmetrically either to the (0 1 0)MgO plane (for Plates A
and B) or to the (1 0 0)MgO plane (for Plates B and C). As for the high relative contrast zones (Fig. 5d), the interlamellar interfaces in Plate 1 or 3 are roughly perpendicular (88.6°) to the substrate surface, whereas the interlamellar interfaces in Plate 2 or 4 are inclined 44.4° towards the substrate surface. Moreover, the orientation relationships between two lamellae connected by an inter-plate interface in the low and high relative contrast zones were calculated. The minimum rotation angles and the corresponding rotation axes are displayed in Tables 2 and 3. The respective major and minor variants that meet at the inter-plate interface are related by a rotation of 83° around the h1 1 0iTetr axes for the major variants and a rotation of 11–14° around the h3 0 1iTetr axes for the minor variants, with certain degrees of deviation. For the inter-plate interfaces in the low relative contrast zones (Z1), the closest atomic planes from two major variants in adjacent plates are the ð1 1 2ÞTetr planes, whereas those from two minor variants in adjacent plates are the (0 1 0)Tetr planes, as outlined with black dotted rectangles in the {0 1 0}Tetr and {1 1 2}Tetr pole figures in Fig. 5c. These atomic planes closest to the interplate interfaces are all nearly perpendicular to the substrate surface and positioned symmetrically to the (0 1 0)MgO plane. As for the inter-plate interfaces in the high relative contrast zones (Z2), the closest atomic planes from two major and two minor variants in adjacent plates are the respective ð1 1 2ÞTetr and (0 1 0)Tetr planes (Fig. 5d), similar to the case in the low relative contrast zones. However,
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Fig. 5. (a and b) Schematic illustration of the geometrical configuration of NM plates. V1, V2, . . ., V8 and SV1, SV2, . . ., SV8 denote eight orientation variants in low relative contrast zones (Z1) and in high relative contrast zones (Z2), respectively. (c and d) Representation of measured orientations of inplate lamellar variants in the form of {0 0 1}Tetr, {1 1 0}Tetr, {0 1 0}Tetr and {1 1 2}Tetr pole figures. The orientations of intra- and inter-plate interface planes are indicated by light green solid squares and black dotted rectangles, respectively, in the {0 1 0}Tetr and {1 1 2}Tetr pole figures. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
they are no longer perpendicular to the substrate surface. Detailed calculations on Plate 1 and Plate 2 show that the ð 1 1 2ÞTetr planes of the major variants are inclined 42.57° and 47.24° to the substrate surface, and the (0 1 0)Tetr planes of the minor variants are inclined 48.29° and 41.68°, respectively. It should be noted that the present results on the orientation relationships between lamellar variants in two adjacent NM plates (e.g. Plate A and Plate B in the low relative contrast zones and Plate 1 and Plate 2 in the high relative contrast zones), and the orientations of in-plate interlamellar interfaces and inter-plate interfaces with respect to the substrate are similar to those reported in the literature [18,19,21,23,29]. However, the direct EBSD orientation measurements have clarified that Plate C and Plate D in the low relative contrast zones (or Plate 3 and Plate 4 in the high relative contrast zones) are not a repetition of
Plate A and Plate B (or Plate 1 and Plate 2), in terms of the orientations of the in-plate lamellar variants, the intra-plate interfaces and the inter-plate interfaces. 4. Discussion As demonstrated above, the morphology and the surface topology of the NM martensite plates in the low and high relative contrast zones are clearly different, although they appear to be composed of the same (1 1 2)Tetr compound twins that act as the primary microstructural elements. For the two distinct relative contrast zones, the crystallographic orientations of the in-plate martensitic variants with respect to the substrate surface are not the same. This may be the origin of the morphological and topological differences observed for the two relative contrast zones, as discussed below.
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Table 2 Minimum rotation angle and rotation axis between two lamellar variants connected by an inter-plate interface in low relative contrast zones. Adjacent plates
Variant pairs
Misorientation angle Rotation axis x (°)
A/B
V1–V3
82.6518
V2–V4
13.6865
V5–V7
83.0965
V6–V8
13.1146
V3–V5
83.1537
V4–V6
14.4210
V7–V1
83.0093
V8–V2
14.4189
C/D
B/C
D/A
3.4° from the h1 1 0iTetr direction 5.4° from the h0 3 1iTetr direction 3.7° from the h1 1 0iTetr direction 4.6° from the h3 0 1iTetr direction 3.6° from the h1 1 0iTetr direction 4.0° from the h3 0 1iTetr direction 3.7° from the h1 1 0iTetr direction 3.1° from the h3 0 1iTetr direction
Table 3 Minimum rotation angle and rotation axis between two lamellar variants connected by an inter-plate interface in high relative contrast zones. Adjacent plates
Variant pairs
Misorientation Rotation axis angle x (°)
1/2
SV1–SV3
82.9699
SV2–SV4
14.1760
SV5–SV7
82.7963
SV6–SV8
14.8473
SV3–SV5
82.5782
SV4–SV6
12.2859
SV7–SV1
82.8110
SV8–SV2
11.8090
3/4
2/3
4/1
3.9° from the h1 1 0iTetr direction 2.1° from the h3 0 1iTetr direction 3.4° from the h1 1 0iTetr direction 3.6° from the h0 3 1iTetr direction 4.7° from the h1 1 0iTetr direction 3.5° from the h0 3 1iTetr direction 5.0° from the h1 1 0iTetr direction 9.9° from the h0 3 1iTetr direction
4.1. Low relative contrast zone Fig. 6a illustrates the atomic correspondences of eight lamellar variants organized in four NM plates (representing one plate colony) for the low relative contrast zones (Z1), viewed from the top of the as-deposited thin films. The atomic correspondences were constructed using individually measured orientations of lamellar variants and the calculated intra- and inter-plate interface planes. The width ratio (expressed in the number ratio of atomic layers) between the minor and major variants is 0.492, being determined according to the phenomenological theory of martensitic transformation (known as WLR theory
[41,42]) under the assumption that the invariant plane is parallel to the MgO substrate surface. This width ratio is very close to 2:4 (0.5), i.e. that of the ideal ð5 2Þ stacking sequence. It should be noted that, for the sake of saving space in this paper, only two atomic layers for the minor variants and five atomic layers for the major variants were taken in Fig. 6a, to illustrate the thickness ratio between the minor variants and the major variants, the structures of intra- and inter-plate interfaces and the orientations of lamellar variants with respect to the substrate. In reality, the lamellar variants are much thicker in the nanometer range. One should not confuse this picture with the structure model from the “adaptive phase theory” [19,23,24,29,43], where the unit cell of the monoclinic 7M martensite is built based on a fixed number of atomic layers for nanotwin-combined tetragonal NM variants [19]. Fig. 6b presents a three-dimensional (3-D) display of the atomic correspondences between two alternately distributed lamellar variants with the (1 1 2)Tetr compound twin relationship in one NM plate. Fig. 6a and b shows that the in-plate interlamellar interfaces are coherent, with perfect atomic match. To further reveal the inter-plate interface features, a 3-D configuration of two adjacent NM plates was constructed using the twinned major and minor variants as blocks, as illustrated in Fig. 6c. It is shown in Fig. 6a and c that the inter-plate interfaces are incoherent, with a certain amount of atomic mismatch. These inter-plate interfaces correspond to the so-called a–c twin interfaces, as depicted in the literature [19,21]. Interestingly, the atoms from two adjacent NM plates are not totally disordered at the inter-plate interfaces, but show some periodicity. For instance, each pair of major and minor lamellae constitutes one period, and two end atoms at one period possess a perfect match at the inter-plate interface (Fig. 6a). If those coherent atoms are chosen as reference, the atoms within one period experience an increased mismatch symmetrically to the plate interface when approaching the interlamellar interface enclosed in the period. Both the periodic coherence and the symmetrical mismatch would define a straight inter-plate interface, which acts as another invariant plane for the NM martensite in the low relative contrast zones. Indeed, the width ratio required by this invariant plane is the same as that required by the invariant plane parallel to the MgO substrate surface. Because of this atomic construction, inter-plate interfaces are always straight, without the need to bend to accommodate the unbalanced interfacial atomic misfits. This boundary character was evidenced in the microstructural observation (Fig. 3a). As the combination of the lamellar variants in adjacent plates are the same, the atomic structures of all inter-plate interfaces are identical. Therefore, all inter-plate interfaces in the low relative contrast zones are parallel to one another, as demonstrated in Fig. 3b. Moreover, as the in-plate major and minor variants (i.e. (1 1 2)Tetr compound twins) have the same orientation combination for all NM plates, and they are distributed
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Fig. 6. (a) Atomic correspondences of eight lamellar variants in four NM plates for low relative contrast zones (viewed from the top of as-deposited thin films). Only Mn and Ga atoms are displayed. (b) 3-D-atomic correspondences of two alternately distributed lamellae (major and minor variants) with (1 1 2)Tetr compound twin relationship in one NM plate. The coherent twinning planes are outlined in green. c1 is the dihedral angle between the (1 1 0)Tetr plane of the major variants and the MgO substrate, and c2 is the dihedral angle between the (0 0 1)Tetr plane of the minor variants and the MgO substrate. (c) 3-D configuration of two adjacent NM plates, constructed using in-plate lamellar variants as blocks to illustrate the plate interface misfits. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
symmetrically to the inter-plate interfaces (Fig. 6c), no microscopic height misfits across the inter-plate interfaces appear in the film normal direction. The NM plates in these
zones are relatively flat, without pronounced surface relief or corrugation. Therefore, no significant relative contrast is visible between adjacent NM plates in the SE images.
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Fig. 7. (a) Atomic correspondences of eight lamellar variants in four NM plates for high relative contrast zones (viewed from the top of as-deposited thin films). Only Mn and Ga atoms are displayed. (b) 3-D-atomic illustration of the combination of major and minor variants ((1 1 2)Tetr compound twins) in Plate 1 and Plate 2. The twinning planes between lamellar variants are colored in green. c1 is the dihedral angle between the (0 0 1)Tetr plane of the major variants and the MgO substrate, and c2 is the dihedral angle between the (1 1 0)Tetr plane of the minor variants and the MgO substrate. (c) 3-D construction of two adjacent plates showing the plate interface misfits. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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4.2. High relative contrast zone Similarly, the atomic correspondences of eight lamellar variants contained in four NM plates, the 3-D-atomic correspondences of twinned lamellar variants in single NM plates and the 3-D configuration of two adjacent NM plates were constructed for the high relative contrast zones (Z2), as shown in Fig. 7. It is shown in Fig. 7a that the inter-plate interface between Plate 1 and Plate 2 has almost the same crystallographic character as that between Plate 3 and Plate 4. Therefore, only the case of Plate 1 and Plate 2 is considered. The width ratios between the minor and major variants (i.e. (1 1 2)Tetr compound twins) were calculated to be 0.47 for Plate 1 and 0.48 for Plate 2. Here, the ideal width ratio 0.5 was used to construct Fig. 7. For a real material, the deviations from the ideal width ratio may be accommodated by stacking faults. Fig. 7a shows that both major and minor variants in Plate 1 have their (1 1 0)Tetr planes near-parallel to the substrate surface, whereas the major and minor variants in Plate 2 have their (0 0 1)Tetr and (1 1 0)Tetr planes, respectively, near-parallel to the substrate surface. The intra-plate (1 1 2)Tetr compound twin interfaces in Plate 1 and Plate 2 are related by a rotation of 90° around the [1 1 0]MgO (Fig. 7a and b). It is noted that, depending on the orientation of the compound twin boundary, the (1 1 2)Tetr compound twinning may bring about the corrugations of NM plates in the high relative contrast zones, as illustrated in Fig. 7b. When the compound twinning directions do not lie in the substrate surface (e.g. Plate 2), the corrugations appear on the film surface. If the corrugation planes happen to be perpendicular to the film surface (e.g. Plate 1), no significant surface relief is created on the free surface of thin films. STM images evidenced that the free surface of the NM plates (equivalent to Plate 1 in the present work) stays smooth [23]. Fig. 7a and c shows that the microscopic plate interface outlined by the ð 1 1 2ÞTetr and (0 1 0)Tetr planes from the adjacent plates are not in mirror relation with respect to the macroscopic inter-plate interface. As the lengths of one pair of major and minor variants (considered as one period) in two plates are not the same along the macroscopic inter-plate interface (Fig. 7c), unbalanced atomic misfit can be expected. This misfit is cumulative and increases with the increased length (parallel to the film surface) and the height (normal to the film surface) of the plates. Therefore, the inter-plate interface orientation would be dominated by the orientation of the ð1 1 2ÞTetr of the major variant in either Plate 1 or Plate 2, depending on local constraints. This may be the reason why the interplate interfaces in the high relative contrast zones are bent after running for a certain length. As displayed in Fig. 7b and c, the atomic misfit also arises in the film normal direction. For example, the planar spacing of the major and minor variants in the film normal direction is 0.272 nm in Plate 1, but 0.334 (major) and 0.272 nm (minor) in Plate 2 (Fig. 7b). If it is assumed that
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the major variants in Plate 2 make a dominant contribution to the atomic misfit between the two plates at the plate interface, some regions of Plate 2 are elevated by 23% in height with respect to those of Plate 1. In the present work, the as-deposited thin films were subject to slight electrolytic polishing before the microstructural observations. The constraints induced by the atomic misfits in the height direction may be fully released at the free surface. Thus, a significant height difference exists between Plate 1 and Plate 2. In the high relative contrast zones of Fig. 3a, the NM plates with higher brightness are those with larger planar spacing in the film normal direction (e.g. Plate 2), whereas the NM plates with lower brightness have smaller planar spacing in the film normal direction. This could well account for the difference in brightness between adjacent NM plates observed in the high relative contrast zones. 5. Summary 1.5 lm thick epitaxial Ni-Mn-Ga thin films were prepared on an MgO(1 0 0) substrate with a Cr buffer layer by DC magnetron sputtering. Based upon the XRD and EBSD characterizations, the surface layer microstructures of the as-deposited thin films were identified to be composed mainly of plate-like NM martensite with a tetragonal structure at ambient temperature. The clustered NM plates with aligned inter-plate interfaces (i.e. plate colonies) exhibit different relative contrast in the SE images. The low relative contrast zones consist of long straight plates with their inter-plate interface traces parallel to the substrate edges, whereas the high relative contrast zones consist of shorter and wavy plates with their inter-plate interface traces at 45° to the substrate edges. In each plate colony, four distinct NM plates can be distinguished in terms of their crystallographic orientations with respect to the MgO substrate, within which two orientation variants (i.e. major and minor lamellae) are alternately distributed with a (1 1 2)Tetr compound twin relationship and coherent interlamellar interface. Indeed, in the low relative contrast zones, the long and straight inter-plate interfaces result from the symmetrical distribution of the respective major and minor variants in adjacent NM plates. As no microscopic height misfits across inter-plate interfaces appear in the film normal direction, the relative contrast between adjacent NM plates is not pronounced in the SE images. However, in the high relative contrast zone, the asymmetrically distributed lamellar variants in adjacent NM plates lead to the change of inter-plate interface orientation. The pronounced height misfits across inter-plate interfaces in the film normal direction give rise to surface reliefs, hence the high relative contrast between adjacent NM plates. Acknowledgements This work is supported by the 111 Project of China (Grant No. B07015), the Program for Liaoning Innovative Research Team in University (Grant No. LT2013007), the
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joint Chinese–French project OPTIMAG (Grant No. ANR-09-BLAN-0382), the Sino-French Cai Yuanpei Program (Grant No. 24013QG), and the Northeastern University Research Foundation for Excellent Doctor of Philosophy Candidates (Grant No. 200904). The authors are grateful to Professor Martin Crimp, Michigan State University, for reading the manuscript and improving the English style. References [1] Sozinov A, Lanska N, Soroka A, Zou W. Appl Phys Lett 2013;102:021902. [2] Backen A, Yeduru SR, Diestel A, Schultz L, Kohl M, Fa¨hler S. Adv Eng Mater 2012;14:696. [3] Khelfaoui F, Kohl M, Buschbeck J, Heczko O, Fa¨hler S, Schultz L. Eur Phys J Spec Topics 2008;158:167. [4] Sozinov A, Likhachev AA, Lanska N, Ullakko K. Appl Phys Lett 2002;80:1746. [5] Murray SJ, Marioni M, Allen SM, O’Handley RC, Lograsso TA. Appl Phys Lett 2000;77:886. [6] Adachl K, Perkins J, Wayman CM. Acta Metall 1988;36:1343. [7] Ranzieri P, Fabbrici S, Nasi L, Righi L, Casoli F, Chernenko VA, et al. Acta Mater 2013;61:263. [8] Yeduru SR, Backen A, Ku¨bel C, Wang D, Scherer T, Fa¨hler S, et al. Scripta Mater 2012;66:566. [9] Buschbeck J, Niemann R, Heczko O, Thomas M, Schultz L, Fa¨hler S. Acta Mater 2009;57:2516. [10] Thomas M, Heczko O, Buschbeck J, Ro¨ßler UK, McCord J, Scheerbaum N, et al. New J Phys 2008;10:023040. [11] Eichhorn T, Jakob G. Mater Sci Forum 2010;635:155. [12] Kallmayer M, Porsch P, Eichhorn T, Schneider H, Jenkins CA, Jakob G, et al. J Phys D – Appl Phys 2009;42:084008. [13] Hakola A, Heczko O, Jaakkola A, Kajava T, Ullakko K. Appl Surf Sci 2004;238:155. [14] Dong JW, Chen LC, Xie JQ, Mu¨ller TAR, Carr DM, Palmstrøm CJ, et al. J Appl Phys 2000;88:7357. [15] Castano FJ, Nelson-Cheeseman B, O’Handley RC, Ross CA, Redondo C, Castano F. J Appl Phys 2003;93:8492. [16] Yeduru SR, Backen A, Fa¨hler S, Schultz L, Kohl M. J Alloys Compd 2012. http://dx.doi.org/10.1016/j.jallcom.2012.02.151. [17] Jetta N, Ozdemir N, Rios S, Bufford D, Karaman I, Zhang X. Thin Solid Films 2012;520:3433.
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ScienceDirect Acta Materialia 61 (2013) 6809–6820 www.elsevier.com/locate/actamat
Microstructural features and orientation correlations of non-modulated martensite in Ni–Mn–Ga epitaxial thin films B. Yang a,b,c, Z.B. Li a,b, Y.D. Zhang b,c,⇑, G.W. Qin a, C. Esling b,c, O. Perroud b, X. Zhao a, L. Zuo a,⇑ b
a Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China Laboratoire d’E´tude des Microstructures et de Me´canique des Mate´riaux (LEM3), CNRS UMR 7239, Universite´ de Lorraine, 57045 Metz, France c Laboratory of Excellence on Design of Alloy Metals for Low-mass Structures (DAMAS), Universite´ de Lorraine, 57045 Metz, France
Received 29 May 2013; received in revised form 25 July 2013; accepted 27 July 2013 Available online 29 August 2013
Abstract Epitaxially grown thin films with nominal composition Ni50Mn30Ga20 and thickness 1.5 lm were prepared on MgO(1 0 0) substrate with a Cr buffer layer by DC magnetron sputtering. The surface layer microstructures of the as-deposited thin films consist of non-modulated (NM) martensite plates with tetragonal structure at ambient temperature, which can be classified into the low and high relative contrast zones of clustered plates (i.e. plate colonies) with parallel or near-parallel inter-plate interface traces in secondary electron images. Orientation analyses by electron backscatter diffraction revealed that individual NM plates are composed of alternately distributed thicker and thinner lamellar variants with (1 1 2)Tetr compound twin relationship and coherent interlamellar interfaces. In each plate colony, there are four distinct plates in terms of the crystallographic orientation of the thicker lamellar variants and therefore, in total, eight orientation variants. For the low relative contrast zones, both thicker and thinner lamellar variants in adjacent plates are distributed symmetrically across their inter-plate interfaces (along the substrate edges). At the atomic level, there are no unbalanced interfacial misfits and height misfits, resulting in long and straight inter-plate interfaces with homogeneous contrast. However, in the high relative contrast zones, the thicker and thinner lamellar variants in adjacent plates are oriented asymmetrically across their inter-plate interfaces (at 45° to the substrate edges). Significant atomic misfits appear in the vicinity of the inter-plate interfaces and in the film normal direction. The former result in bending of the inter-plate interfaces, and the latter give rise to the high relative contrast between adjacent plates. Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ferromagnetic shape memory alloys; Ni–Mn–Ga thin films; Crystallographic features; Electron backscatter diffraction
1. Introduction Ferromagnetic Ni–Mn–Ga alloys are conceived as promising sensor or actuator materials for micro-electromechanical ⇑ Corresponding authors. Addresses: Laboratoire d’E ´ tude des Microstructures et de Me´canique des Mate´riaux (LEM3), CNRS UMR 7239, Universite´ de Lorraine, 57045 Metz, France. Tel.: +33 3 87315399; fax: +33 3 87315377 (Y.D. Zhang), Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China. Tel.: +86 24 23906503; fax: +86 24 23892454 (L. Zuo). E-mail addresses: [email protected] (Y.D. Zhang), [email protected] (L. Zuo).
systems or devices because of their large and highly reversible magnetic-field-induced strains with fast dynamic response [1–5]. However, such excellent performance is only attainable within a narrow microstructural window, as the magneticfield-induced shape change comes from the reorientation of ferromagnetic martensite with strong magnetocrystalline anisotropy. Typically, the martensitic transformations in these alloys are accompanied by the formation of self-accommodated groups [6] of martensitic variants, oriented differently and clustered into colonies. The microstructural texturing to eliminate undesired variants has become a prerequisite for optimizing the shape memory effect, even for single crystal alloys.
1359-6454/$36.00 Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actamat.2013.07.055
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In recent years, the magnetron sputtering technique has attracted considerable attention as a significant texturing tool for ferromagnetic Ni–Mn–Ga thin films grown epitaxially on single crystal substrates such as MgO [7–10], Al2O3 [11,12], GaAs [13,14] and other substrates [15]. Although the procedure to fabricate continuous thin films with homogeneous chemical composition and controllable film thickness can be parameterized [8,10,16–20], the microstructural and crystallographic characterization of the thin films produced remains challenging, owing to the local constraints from substrates, the specific geometry of thin films and the ultrafine microstructures of the constituent phases. So far, pioneering efforts [7–10,18,19,21–28] have been devoted to the microstructural characterization of epitaxial Ni–Mn–Ga thin films to determine their physical and mechanical behavior. The X-ray diffraction (XRD) technique was first used to analyze the phases and textures of Ni–Mn–Ga thin films [10,18,20,28]. This technique proved to be applicable, in most circumstances, to the identification of highly textured multi-phases such as austenite, modulated martensite and non-modulated (NM) martensite. In the case of cubic austenite or tetragonal NM martensite, the lattice constants can be unambiguously determined from the XRD reflection peak locations [9,10,20,24]. As for monoclinic seven-layered modulated martensite (7M or 14M), the lattice constant determination may suffer from an insufficient number of identified reflection peaks [10,18–20,29]. Without knowing precisely the monoclinic angle, the monoclinic modulated martensite needs to be simplified with a pseudoorthorhombic structure, as practiced previously for bulk alloys [10,18–20,29]. Furthermore, the structure modulation of the 7M martensite is frequently interpreted in terms of the nanotwin combination of the tetragonal NM martensite (the so-called adaptive phase), as the lattice constants of the austenite, 7M martensite and NM martensite fulfill the relation proposed by the adaptive phase theory (a7M = cNM + aNM aA, aA = b7M, c7M = aNM [29]). Scanning electron microscopy (SEM) has further revealed that individual martensitic variants in Ni–Mn– Ga thin films have a plate-like morphology and are organized in colonies [18,19,21], similar to the case of bulk alloys, but with a much finer microstructure. Here, each plate colony represents a distinct zone of martensite plates that are clustered with parallel or near-parallel plate interfaces [6]. As suggested by the secondary electron (SE) imaging, two types of martensite plate colonies may exist in epitaxial Ni–Mn–Ga thin films grown on a single crystal MgO substrate. One refers to long plates with relatively homogeneous contrast, running parallel to one edge direction ([1 0 0]MgO or [0 1 0]MgO) of the substrate. The other refers to shorter plates with relatively high contrast, oriented at 45° to the substrate edges. Close examination using atomic force microscopy [19,21,23] demonstrated that the different SE contrasts correspond to different surface corrugations of the films, i.e. the
relatively low contrast zones to low surface corrugation, and the relative high contrast zones to high surface corrugation. Previous studies on Ni–Mn–Ga thin films [9,10,19– 23,26,27,29] suggested that the high surface corrugation zones are associated with the 7M martensitic structure. Adjacent martensite plates in these zones may have a socalled a–c twin relationship [9,19,21,23,25,29], i.e. the pseudo-orthorhombic unit cells of two twinned martensite plates share a common b axis (the basis of the pseudoorthorhombic cell is set in accordance with that of the cubic basis of the austenite). The inter-plate interfaces correspond to the (1 0 1)orth plane, and they are inclined at roughly 45° to the substrate surface [9,19,21,23,27]. Highresolution scanning tunneling microscopy (STM) examinations [23] further indicated that there are still fine periodic corrugations in the interiors of 7M plates. Such fine corrugations may be related to the structure modulation of the 7M martensite [23]. In contrast, the nature of long martensite plates in the low surface corrugation zones has been studied less. Some reported that they are NM martensite [19], but others concluded that they are 7M martensite [18,21]. Using the epitaxial relationship between MgO substrate and austenite, along with the Bain orientation relationship between austenite and NM martensite, the adjacent martensite plates were deduced to be a–c twin related to the plate interfaces ((1 0 1)orth) perpendicular to the substrate surface [18,21]. As no significant corrugations can be detected in the interiors of individual plates, their microstructural and crystallographic features remain unknown. Clearly, owing to the lack of direct correlation between the ultrafine martensitic microstructures and their crystallographic orientations, the details of the configuration of lamellar variants inside martensite plate and the characteristics of intra- and inter-plate interfaces in Ni–Mn–Ga thin films are still not available. Here, an attempt is made to explore the microstructural and crystallographic features of martensite in epitaxial Ni50Mn30Ga20 thin films. By means of XRD and electron backscatter diffraction (EBSD) analyses, the crystal structures of constituent phases and the configurations of martensitic variants and their orientation correlations are addressed. 2. Experimental methods Ni–Mn–Ga thin films with a nominal composition of Ni50Mn30Ga20 and nominal thickness 1.5 lm were deposited from a cathode target of Ni46Mn32Ga22 by DC magnetron sputtering (JZCK-400DJ) at a sputtering rate of 0.2 nm s1. A 100 nm thick Cr buffer layer was precoated on the MgO(1 0 0) monocrystal substrate. The base pressure before deposition was <9.0 105 Pa. In order to obtain continuous thin films, the deposition was conducted under a fixed Ar2 working pressure of 0.15 Pa, with the substrate being held at 500 °C. The entire deposition process lasted for 2 h.
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The thickness and chemical composition of the thin films produced were measured using a stylus profiler (DEKTAK 150) and an energy dispersive spectrometer (Bruker XFlash 4010) attached to the field emission gun SEM microscope (JEOL JSM-6500F), respectively. It was verified that the as-deposited thin films have a thickness of 1.47 lm on average, and the chemical composition is Ni51.5Mn29Ga19.5, both being very close to the designed nominal values. The crystal structures of the as-deposited thin films were determined by XRD, using Co Ka radiation (k = 0.178897 nm). Considering that the thin films may have in-plane texture, two four-circle X-ray diffractometers, one with a conventional h–2h coupled scan and the other with a rotating anode generator (RIGAKU RU300) and a large-angle position sensitive detector (INEL CPS120), were used to collect a sufficient number of diffraction peaks. The geometrical configurations of the two X-ray diffractometers are illustrated schematically in Fig. 1. In the former case (Fig. 1a), the h–2h coupled scans were performed in the 2h range of 45–90° with tilt angles w varying from 0° to 10° and a step size of 1°. In the latter case (Fig. 1b), the 2h scans were conducted with tilt angle w from 0.75° to 78.75° and a step size of 1.25°. At
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each tilt angle, the sample was rotated from 0° to 360° with a step size of 5°. Two incident angles x (27.9° and 40°) were selected in order to obtain possible diffraction peaks at the low 2h (48–58°) and high 2h (82.5°) regions. The final diffraction patterns were obtained by integrating all the diffraction patterns acquired at different sample positions. The microstructures and crystallographic orientations of the as-deposited thin films were examined using the same SEM equipped with an EBSD acquisition camera and Channel 5 analysis system (Oxford HKL). In order to obtain clear EBSD patterns with detectable Kikuchi lines for reliable orientation determination, the film surfaces were subjected to slight electrolytic polish with a solution of 20% HNO3 in CH3OH at 12 V for 3 s at room temperature. Here, the Kikuchi line patterns from individual martensitic variants were acquired manually and analyzed in interactive mode. For specifying the orientations of martensitic variants, the macroscopic sample coordinate system refers to the cubic lattice basis of the MgO(1 0 0) monocrystal substrate ([1 0 0]MgO, [0 1 0]MgO and [0 0 1]MgO). The conventional monoclinic and tetragonal Bravais lattice cells are chosen to describe the crystal structure of the 7M martensite and that of the NM martensite, respectively. Traditionally, the martensite structures were referenced to the coordinate system derived from cubic coordinate system of austenite [30,31]. 3. Results 3.1. Crystal structure
Fig. 1. Schematic drawings of four-circle X-ray diffractometer using (a) a point detector (h = x) and (b) a large-angle position sensitive detector (x is different from h).
Fig. 2a shows the w-dependent XRD patterns of the asdeposited Ni–Mn–Ga thin films obtained by h–2h coupled scanning at ambient temperature. At each tilt angle w, only a limited number of diffraction peaks appear. No diffraction peaks are visible over the 2h range 48–55°. This may be a result of the specific texture of the as-deposited thin films, as several diffraction peaks in this angle range were observed in previous powder XRD measurements [32,33]. Fig. 2b presents the XRD patterns measured using a large-angle position-sensitive detector under two different incident beam conditions. Compared with Fig. 2a, extra diffraction peaks are clearly seen in the 2h range 48–55° and at the higher 2h positions (82°). These additional peaks in the 2h range 48–55° were not detected in previous studies on the Ni–Mn–Ga thin films [9,10,18,20,27,29]. By combining all the characteristic diffraction peaks in Fig. 2a and b, it is possible to conduct more reliable phase identification and lattice constant determination on the constituent phases of the as-deposited thin films, especially for the modulated martensite. Apart from the well-identified diffraction peak positions for the MgO substrate and the Cr buffer layer (the latter could also be indexed as Ni–Mn–Ga austenite), all other diffraction positions were compared with those calculated using the published crystal structure data on the monoclinic incommensurate 7M martensite [32–35] and the
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B. Yang et al. / Acta Materialia 61 (2013) 6809–6820 Table 1 Calculated lattice constants of austenite, 7M martensite and NM martensite for as-deposited Ni–Mn–Ga thin films. Phase
7M
Space group
P2/m (No. 10) NM I4/mmm (No. 139) Austenite Fm3m (No. 225)
Crystal system
Lattice constants a (nm) b (nm) c (nm) a, b, c
Monoclinic 0.4262 0.5442 4.199 a = c = 90°, b = 93.7° Tetragonal 0.3835 0.3835 0.6680 a = b = c = 90° Cubic L21 0.5773 0.5773 0.5773 a = b = c = 90°
Fig. 2. XRD patterns of as-deposited Ni–Mn–Ga thin films with MgO/Cr substrate: (a) measured by conventional h–2h coupled scanning at different tilt angles w; (b) measured by 2h scanning at two incidence angles x and integrated over the rotation angle u.
tetragonal NM martensite [36,37]. Fairly good matches were achieved between the measured and recalculated peak positions for the two types of martensite, which confirms that the 7M martensite and the NM martensite coexist in the as-deposited thin films at room temperature. It should be noted that there is always a deviation of 0.2–2° in the measured peak positions from those obtained by powder XRD diffraction [32–34]. This indicates that the lattice constants of the 7M and NM martensite in the as-deposited thin films are not exactly the same as those derived from their powder counterparts, as the substrate imposes a constraint on the as-deposited thin films. Using the measured XRD data, the lattice constants of the 7M martensite and the NM martensite were resolved. Table 1 summarizes the complete crystal structure information of the three different phases (austenite, 7M martensite and NM martensite) involved in the as-deposited thin films. This information is required for EBSD orientation analysis.
Fig. 3. (a) SE image of electrolytically polished Ni–Mn–Ga thin films, showing martensite plates that are clustered in colonies with low and high relative contrasts. The sample coordinate system is set in accordance with the basis vectors of the MgO(1 0 0) substrate ([1 0 0]MgO, [0 1 0]MgO and [0 0 1]MgO). (b) High-magnification BSE image of the squared area Z1 in Fig. 3a, showing fine lamellae distributed alternately inside each plate. The inter-plate interfaces are marked with white dotted lines, and the intraplate interfaces with blue and green solid lines. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.2. Microstructure Fig. 3a shows a typical SE image of the as-deposited thin films after slight electrolytic polishing. It is seen that the microstructure of the film surface layer is characteristic
of clustered plates with two distinct relative contrasts. The low relative contrast zones (e.g. Z1 in Fig. 3a) contain long and straight plates with their plate interfaces running parallel to one edge of the substrate (i.e. [1 0 0]MgO or
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[0 1 0]MgO), whereas the high relative contrast zones (e.g. Z2 in Fig. 3a) contain shorter and wavy plates with their plate interfaces oriented roughly at 45° with respect to the substrate edges. In each relative contrast zone, one or several plate colonies can be distinguished locally by the alignment of parallel or near-parallel inter-plate interface traces. Further examination at a higher magnification in backscattered electron imaging (BSE) mode reveals that the fine lamellae with two different brightness levels are distributed alternately inside each plate, as highlighted with green and blue lines in Fig. 3b. Of the two neighboring lamellae, one is thicker, and the other is thinner. As the BSE image brightness for a monophase microstructure with homogeneous chemical composition depends on the crystallographic orientation of the microstructural components, the thicker and thinner lamellae in each plate should have two distinct orientations, and they are referred to as two orientation variants. Using the determined lattice constants of the NM and 7M martensite and the published atomic position data [32] as initial input, the constituent phases in the surface layers of the as-deposited thin films were verified by EBSD analysis. The Kikuchi line indexation has evidenced that both low and high relative contrast zones displayed in Fig. 3a can be identified as the tetragonal NM martensite, but not the 7M martensite. Fig. 4 presents an example of the Kikuchi line pattern acquired from one martensite plate with high brightness in the high relative contrast zone and the calculated patterns using the tetragonal NM martensite structure. As the in-plate lamellar variants are too fine and beyond the resolution of the present EBSD analysis system, a single set of Kikuchi lines could not be obtained from one variant. For each acquisition, there are always mixed patterns from two neighboring variants appearing in one image, as displayed in Fig. 4a. However, the high intensity reflection lines belonging to two different variants can be well separated in the image, as outlined by the green and red triangles in Fig. 4b. By comparing Fig. 4c and d with Fig. 4a, one can find perfect matches between the acquired Kikuchi lines for the two variants and the calculated ones using the tetragonal NM martensite structure. Apparently, the martensite appearing in the surface layers of the present thin films is different from that identified previously by other groups [9,10,18,19,21,23,26,27,29]. This may be due to the different thickness of the thin films produced. In the present study, the NM martensite was found at the surface of the 1.5 lm thick thin films, while the others reported the existence of the 7M martensite at the surface of the thin films with maximum thickness 0.5 lm. Indeed, the formation of different types of martensite is very sensitive to local constraints. It is obvious that, at the film surface, the constraint from the substrate decreases with the increasing film thickness. Thus, the surface layers of thick films without much constraint from the substrate would easily transform to the stable NM martensite, as demonstrated in the present case.
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Moreover, from the distinguishable peaks of the 7M and NM martensite in the XRD patterns displayed in Fig. 2, one can see that the quantity of the 7M martensite is comparable with that of the NM martensite, which is inconsistent with the EBSD microstructural characterization. In fact, the EBSD information is obtained from the surface layer (100 nm thick) of the as-deposited thin films after slight electrolytic polishing, whereas the information from the XRD measurement is through-the-thickness to the substrate. Thus, one may consider that the 7M martensite is located underneath the EBSD-detected surface layer, and the surface layer microstructure is composed mainly of the NM martensite. This further confirms the above deduction that the NM martensite appears at the film surface for present thick films, in contrast to the formation of the 7M modulated martensite at the film surface for other thinner films [9,10,19,21–23,26,29]. Hereafter, the present paper focuses only on the orientation correlations of the NM martensite plates. 3.3. Orientation correlation Detailed EBSD orientation analyses were conducted on the NM martensite plates in the low and high relative contrast zones (Z1 and Z2 in Fig. 3a). Individual plates were identified to be composed of two alternately distributed orientation variants (lamellae) with different thicknesses, as illustrated schematically in Fig. 5a and b. According to the orientations of the thicker variants in plates, two sets of orientation plates can be distinguished, i.e. the set of Plates A, B, C and D in the low relative contrast zones and the set of Plates 1, 2, 3 and 4 in the high relative contrast zones. Thus, there are in total eight orientation variants of the NM martensite in one plate colony. For easy visualization, they are denoted as V1, V2, . . ., V8 in Fig. 5a and SV1, SV2, . . ., SV8 in Fig. 5b, where the symbols with odd subscripts correspond to the thicker (major) variants, and those with even subscripts the thinner (minor) variants. Taking the basis vectors of the MgO(1 0 0) monocrystal substrate as the sample reference frame, the measured orientations of the NM variants in the two relative contrast zones are presented in the form of {0 0 1}Tetr, {1 1 0}Tetr, {0 1 0}Tetr and {1 1 2}Tetr pole figures, as displayed in Fig. 5c and d. It is noted that the orientations of the major and minor variants in the low relative contrast zones are different from those in the high relative contrast zones. For the low relative contrast zones (Z1), the major and minor variants in Plates A, B, C and D are oriented respectively with their (1 1 0)Tetr planes and (0 0 1)Tetr planes nearly parallel to the substrate surface (Fig. 5c). In the high relative contrast zones (Z2), such plane parallelisms hold for Plate 2 and Plate 4, but with an exchange of the planes between the major and minor variants, whereas both major and minor variants in Plate 1 and Plate 3 are oriented with their (1 1 0)Tetr planes nearly parallel to the substrate surface
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Fig. 4. (a) Kikuchi line pattern acquired from one martensite plate in the high relative contrast zone (Z2) in Fig. 3a. (b) Indication of high intensity reflection lines from two adjacent variants 1 (the green triangle) and 2 (the red triangle). (c and d) Kikuchi line patterns calculated using the tetragonal structure for variants 1 and 2, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(Fig. 5d). In correlation with the microstructural observations, Plate 2 and Plate 4 are featured with higher brightness and Plate 1 and Plate 3 with lower brightness. Based on misorientation calculations, the orientation relationships between adjacent lamellar variants in the same plate were further determined from their orientation data acquired manually by EBSD. For both low and high relative contrast zones, the in-plate lamellar variants are found to have a compound twin relationship with the twinning elements K1 = (1 1 2)Tetr, K2 ¼ ð1 1 2ÞTetr , g1 ¼ ½1 1 1Tetr , g2 = [1 1 1]Tetr, P ¼ ð1 1 0ÞTetr and s = 0.412. Here, all the crystallographic elements are expressed in the crystal basis of the conventional tetragonal Bravais lattice. The present twin relationship is the same as that reported for the bulk alloys [38,39], and it is also consistent with the so-called a–c twin relationship found in the thin films [9,19,21, 23,25,29]. By means of the indirect two-trace method [40], the inplate interlamellar interface planes in the two relative contrast zones were further determined. They coincide well with the (1 1 2)Tetr compound twin planes (roughly corresponding to the (1 0 1) planes of the austenite [9,10,19,29]) and are perfectly coherent, as indicated by the light green solid squares in Fig. 5c and d. According to the {1 1 2}Tetr pole figure of the low relative contrast zones shown in Fig. 5c, the interlamellar interfaces in each plate are inclined at 47.5° towards the substrate surface, and two interlamellar interfaces from adjacent plates are positioned symmetrically either to the (0 1 0)MgO plane (for Plates A
and B) or to the (1 0 0)MgO plane (for Plates B and C). As for the high relative contrast zones (Fig. 5d), the interlamellar interfaces in Plate 1 or 3 are roughly perpendicular (88.6°) to the substrate surface, whereas the interlamellar interfaces in Plate 2 or 4 are inclined 44.4° towards the substrate surface. Moreover, the orientation relationships between two lamellae connected by an inter-plate interface in the low and high relative contrast zones were calculated. The minimum rotation angles and the corresponding rotation axes are displayed in Tables 2 and 3. The respective major and minor variants that meet at the inter-plate interface are related by a rotation of 83° around the h1 1 0iTetr axes for the major variants and a rotation of 11–14° around the h3 0 1iTetr axes for the minor variants, with certain degrees of deviation. For the inter-plate interfaces in the low relative contrast zones (Z1), the closest atomic planes from two major variants in adjacent plates are the ð1 1 2ÞTetr planes, whereas those from two minor variants in adjacent plates are the (0 1 0)Tetr planes, as outlined with black dotted rectangles in the {0 1 0}Tetr and {1 1 2}Tetr pole figures in Fig. 5c. These atomic planes closest to the interplate interfaces are all nearly perpendicular to the substrate surface and positioned symmetrically to the (0 1 0)MgO plane. As for the inter-plate interfaces in the high relative contrast zones (Z2), the closest atomic planes from two major and two minor variants in adjacent plates are the respective ð1 1 2ÞTetr and (0 1 0)Tetr planes (Fig. 5d), similar to the case in the low relative contrast zones. However,
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Fig. 5. (a and b) Schematic illustration of the geometrical configuration of NM plates. V1, V2, . . ., V8 and SV1, SV2, . . ., SV8 denote eight orientation variants in low relative contrast zones (Z1) and in high relative contrast zones (Z2), respectively. (c and d) Representation of measured orientations of inplate lamellar variants in the form of {0 0 1}Tetr, {1 1 0}Tetr, {0 1 0}Tetr and {1 1 2}Tetr pole figures. The orientations of intra- and inter-plate interface planes are indicated by light green solid squares and black dotted rectangles, respectively, in the {0 1 0}Tetr and {1 1 2}Tetr pole figures. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
they are no longer perpendicular to the substrate surface. Detailed calculations on Plate 1 and Plate 2 show that the ð 1 1 2ÞTetr planes of the major variants are inclined 42.57° and 47.24° to the substrate surface, and the (0 1 0)Tetr planes of the minor variants are inclined 48.29° and 41.68°, respectively. It should be noted that the present results on the orientation relationships between lamellar variants in two adjacent NM plates (e.g. Plate A and Plate B in the low relative contrast zones and Plate 1 and Plate 2 in the high relative contrast zones), and the orientations of in-plate interlamellar interfaces and inter-plate interfaces with respect to the substrate are similar to those reported in the literature [18,19,21,23,29]. However, the direct EBSD orientation measurements have clarified that Plate C and Plate D in the low relative contrast zones (or Plate 3 and Plate 4 in the high relative contrast zones) are not a repetition of
Plate A and Plate B (or Plate 1 and Plate 2), in terms of the orientations of the in-plate lamellar variants, the intra-plate interfaces and the inter-plate interfaces. 4. Discussion As demonstrated above, the morphology and the surface topology of the NM martensite plates in the low and high relative contrast zones are clearly different, although they appear to be composed of the same (1 1 2)Tetr compound twins that act as the primary microstructural elements. For the two distinct relative contrast zones, the crystallographic orientations of the in-plate martensitic variants with respect to the substrate surface are not the same. This may be the origin of the morphological and topological differences observed for the two relative contrast zones, as discussed below.
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Table 2 Minimum rotation angle and rotation axis between two lamellar variants connected by an inter-plate interface in low relative contrast zones. Adjacent plates
Variant pairs
Misorientation angle Rotation axis x (°)
A/B
V1–V3
82.6518
V2–V4
13.6865
V5–V7
83.0965
V6–V8
13.1146
V3–V5
83.1537
V4–V6
14.4210
V7–V1
83.0093
V8–V2
14.4189
C/D
B/C
D/A
3.4° from the h1 1 0iTetr direction 5.4° from the h0 3 1iTetr direction 3.7° from the h1 1 0iTetr direction 4.6° from the h3 0 1iTetr direction 3.6° from the h1 1 0iTetr direction 4.0° from the h3 0 1iTetr direction 3.7° from the h1 1 0iTetr direction 3.1° from the h3 0 1iTetr direction
Table 3 Minimum rotation angle and rotation axis between two lamellar variants connected by an inter-plate interface in high relative contrast zones. Adjacent plates
Variant pairs
Misorientation Rotation axis angle x (°)
1/2
SV1–SV3
82.9699
SV2–SV4
14.1760
SV5–SV7
82.7963
SV6–SV8
14.8473
SV3–SV5
82.5782
SV4–SV6
12.2859
SV7–SV1
82.8110
SV8–SV2
11.8090
3/4
2/3
4/1
3.9° from the h1 1 0iTetr direction 2.1° from the h3 0 1iTetr direction 3.4° from the h1 1 0iTetr direction 3.6° from the h0 3 1iTetr direction 4.7° from the h1 1 0iTetr direction 3.5° from the h0 3 1iTetr direction 5.0° from the h1 1 0iTetr direction 9.9° from the h0 3 1iTetr direction
4.1. Low relative contrast zone Fig. 6a illustrates the atomic correspondences of eight lamellar variants organized in four NM plates (representing one plate colony) for the low relative contrast zones (Z1), viewed from the top of the as-deposited thin films. The atomic correspondences were constructed using individually measured orientations of lamellar variants and the calculated intra- and inter-plate interface planes. The width ratio (expressed in the number ratio of atomic layers) between the minor and major variants is 0.492, being determined according to the phenomenological theory of martensitic transformation (known as WLR theory
[41,42]) under the assumption that the invariant plane is parallel to the MgO substrate surface. This width ratio is very close to 2:4 (0.5), i.e. that of the ideal ð5 2Þ stacking sequence. It should be noted that, for the sake of saving space in this paper, only two atomic layers for the minor variants and five atomic layers for the major variants were taken in Fig. 6a, to illustrate the thickness ratio between the minor variants and the major variants, the structures of intra- and inter-plate interfaces and the orientations of lamellar variants with respect to the substrate. In reality, the lamellar variants are much thicker in the nanometer range. One should not confuse this picture with the structure model from the “adaptive phase theory” [19,23,24,29,43], where the unit cell of the monoclinic 7M martensite is built based on a fixed number of atomic layers for nanotwin-combined tetragonal NM variants [19]. Fig. 6b presents a three-dimensional (3-D) display of the atomic correspondences between two alternately distributed lamellar variants with the (1 1 2)Tetr compound twin relationship in one NM plate. Fig. 6a and b shows that the in-plate interlamellar interfaces are coherent, with perfect atomic match. To further reveal the inter-plate interface features, a 3-D configuration of two adjacent NM plates was constructed using the twinned major and minor variants as blocks, as illustrated in Fig. 6c. It is shown in Fig. 6a and c that the inter-plate interfaces are incoherent, with a certain amount of atomic mismatch. These inter-plate interfaces correspond to the so-called a–c twin interfaces, as depicted in the literature [19,21]. Interestingly, the atoms from two adjacent NM plates are not totally disordered at the inter-plate interfaces, but show some periodicity. For instance, each pair of major and minor lamellae constitutes one period, and two end atoms at one period possess a perfect match at the inter-plate interface (Fig. 6a). If those coherent atoms are chosen as reference, the atoms within one period experience an increased mismatch symmetrically to the plate interface when approaching the interlamellar interface enclosed in the period. Both the periodic coherence and the symmetrical mismatch would define a straight inter-plate interface, which acts as another invariant plane for the NM martensite in the low relative contrast zones. Indeed, the width ratio required by this invariant plane is the same as that required by the invariant plane parallel to the MgO substrate surface. Because of this atomic construction, inter-plate interfaces are always straight, without the need to bend to accommodate the unbalanced interfacial atomic misfits. This boundary character was evidenced in the microstructural observation (Fig. 3a). As the combination of the lamellar variants in adjacent plates are the same, the atomic structures of all inter-plate interfaces are identical. Therefore, all inter-plate interfaces in the low relative contrast zones are parallel to one another, as demonstrated in Fig. 3b. Moreover, as the in-plate major and minor variants (i.e. (1 1 2)Tetr compound twins) have the same orientation combination for all NM plates, and they are distributed
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Fig. 6. (a) Atomic correspondences of eight lamellar variants in four NM plates for low relative contrast zones (viewed from the top of as-deposited thin films). Only Mn and Ga atoms are displayed. (b) 3-D-atomic correspondences of two alternately distributed lamellae (major and minor variants) with (1 1 2)Tetr compound twin relationship in one NM plate. The coherent twinning planes are outlined in green. c1 is the dihedral angle between the (1 1 0)Tetr plane of the major variants and the MgO substrate, and c2 is the dihedral angle between the (0 0 1)Tetr plane of the minor variants and the MgO substrate. (c) 3-D configuration of two adjacent NM plates, constructed using in-plate lamellar variants as blocks to illustrate the plate interface misfits. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
symmetrically to the inter-plate interfaces (Fig. 6c), no microscopic height misfits across the inter-plate interfaces appear in the film normal direction. The NM plates in these
zones are relatively flat, without pronounced surface relief or corrugation. Therefore, no significant relative contrast is visible between adjacent NM plates in the SE images.
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Fig. 7. (a) Atomic correspondences of eight lamellar variants in four NM plates for high relative contrast zones (viewed from the top of as-deposited thin films). Only Mn and Ga atoms are displayed. (b) 3-D-atomic illustration of the combination of major and minor variants ((1 1 2)Tetr compound twins) in Plate 1 and Plate 2. The twinning planes between lamellar variants are colored in green. c1 is the dihedral angle between the (0 0 1)Tetr plane of the major variants and the MgO substrate, and c2 is the dihedral angle between the (1 1 0)Tetr plane of the minor variants and the MgO substrate. (c) 3-D construction of two adjacent plates showing the plate interface misfits. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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4.2. High relative contrast zone Similarly, the atomic correspondences of eight lamellar variants contained in four NM plates, the 3-D-atomic correspondences of twinned lamellar variants in single NM plates and the 3-D configuration of two adjacent NM plates were constructed for the high relative contrast zones (Z2), as shown in Fig. 7. It is shown in Fig. 7a that the inter-plate interface between Plate 1 and Plate 2 has almost the same crystallographic character as that between Plate 3 and Plate 4. Therefore, only the case of Plate 1 and Plate 2 is considered. The width ratios between the minor and major variants (i.e. (1 1 2)Tetr compound twins) were calculated to be 0.47 for Plate 1 and 0.48 for Plate 2. Here, the ideal width ratio 0.5 was used to construct Fig. 7. For a real material, the deviations from the ideal width ratio may be accommodated by stacking faults. Fig. 7a shows that both major and minor variants in Plate 1 have their (1 1 0)Tetr planes near-parallel to the substrate surface, whereas the major and minor variants in Plate 2 have their (0 0 1)Tetr and (1 1 0)Tetr planes, respectively, near-parallel to the substrate surface. The intra-plate (1 1 2)Tetr compound twin interfaces in Plate 1 and Plate 2 are related by a rotation of 90° around the [1 1 0]MgO (Fig. 7a and b). It is noted that, depending on the orientation of the compound twin boundary, the (1 1 2)Tetr compound twinning may bring about the corrugations of NM plates in the high relative contrast zones, as illustrated in Fig. 7b. When the compound twinning directions do not lie in the substrate surface (e.g. Plate 2), the corrugations appear on the film surface. If the corrugation planes happen to be perpendicular to the film surface (e.g. Plate 1), no significant surface relief is created on the free surface of thin films. STM images evidenced that the free surface of the NM plates (equivalent to Plate 1 in the present work) stays smooth [23]. Fig. 7a and c shows that the microscopic plate interface outlined by the ð 1 1 2ÞTetr and (0 1 0)Tetr planes from the adjacent plates are not in mirror relation with respect to the macroscopic inter-plate interface. As the lengths of one pair of major and minor variants (considered as one period) in two plates are not the same along the macroscopic inter-plate interface (Fig. 7c), unbalanced atomic misfit can be expected. This misfit is cumulative and increases with the increased length (parallel to the film surface) and the height (normal to the film surface) of the plates. Therefore, the inter-plate interface orientation would be dominated by the orientation of the ð1 1 2ÞTetr of the major variant in either Plate 1 or Plate 2, depending on local constraints. This may be the reason why the interplate interfaces in the high relative contrast zones are bent after running for a certain length. As displayed in Fig. 7b and c, the atomic misfit also arises in the film normal direction. For example, the planar spacing of the major and minor variants in the film normal direction is 0.272 nm in Plate 1, but 0.334 (major) and 0.272 nm (minor) in Plate 2 (Fig. 7b). If it is assumed that
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the major variants in Plate 2 make a dominant contribution to the atomic misfit between the two plates at the plate interface, some regions of Plate 2 are elevated by 23% in height with respect to those of Plate 1. In the present work, the as-deposited thin films were subject to slight electrolytic polishing before the microstructural observations. The constraints induced by the atomic misfits in the height direction may be fully released at the free surface. Thus, a significant height difference exists between Plate 1 and Plate 2. In the high relative contrast zones of Fig. 3a, the NM plates with higher brightness are those with larger planar spacing in the film normal direction (e.g. Plate 2), whereas the NM plates with lower brightness have smaller planar spacing in the film normal direction. This could well account for the difference in brightness between adjacent NM plates observed in the high relative contrast zones. 5. Summary 1.5 lm thick epitaxial Ni-Mn-Ga thin films were prepared on an MgO(1 0 0) substrate with a Cr buffer layer by DC magnetron sputtering. Based upon the XRD and EBSD characterizations, the surface layer microstructures of the as-deposited thin films were identified to be composed mainly of plate-like NM martensite with a tetragonal structure at ambient temperature. The clustered NM plates with aligned inter-plate interfaces (i.e. plate colonies) exhibit different relative contrast in the SE images. The low relative contrast zones consist of long straight plates with their inter-plate interface traces parallel to the substrate edges, whereas the high relative contrast zones consist of shorter and wavy plates with their inter-plate interface traces at 45° to the substrate edges. In each plate colony, four distinct NM plates can be distinguished in terms of their crystallographic orientations with respect to the MgO substrate, within which two orientation variants (i.e. major and minor lamellae) are alternately distributed with a (1 1 2)Tetr compound twin relationship and coherent interlamellar interface. Indeed, in the low relative contrast zones, the long and straight inter-plate interfaces result from the symmetrical distribution of the respective major and minor variants in adjacent NM plates. As no microscopic height misfits across inter-plate interfaces appear in the film normal direction, the relative contrast between adjacent NM plates is not pronounced in the SE images. However, in the high relative contrast zone, the asymmetrically distributed lamellar variants in adjacent NM plates lead to the change of inter-plate interface orientation. The pronounced height misfits across inter-plate interfaces in the film normal direction give rise to surface reliefs, hence the high relative contrast between adjacent NM plates. Acknowledgements This work is supported by the 111 Project of China (Grant No. B07015), the Program for Liaoning Innovative Research Team in University (Grant No. LT2013007), the
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joint Chinese–French project OPTIMAG (Grant No. ANR-09-BLAN-0382), the Sino-French Cai Yuanpei Program (Grant No. 24013QG), and the Northeastern University Research Foundation for Excellent Doctor of Philosophy Candidates (Grant No. 200904). The authors are grateful to Professor Martin Crimp, Michigan State University, for reading the manuscript and improving the English style. References [1] Sozinov A, Lanska N, Soroka A, Zou W. Appl Phys Lett 2013;102:021902. [2] Backen A, Yeduru SR, Diestel A, Schultz L, Kohl M, Fa¨hler S. Adv Eng Mater 2012;14:696. [3] Khelfaoui F, Kohl M, Buschbeck J, Heczko O, Fa¨hler S, Schultz L. Eur Phys J Spec Topics 2008;158:167. [4] Sozinov A, Likhachev AA, Lanska N, Ullakko K. Appl Phys Lett 2002;80:1746. [5] Murray SJ, Marioni M, Allen SM, O’Handley RC, Lograsso TA. Appl Phys Lett 2000;77:886. [6] Adachl K, Perkins J, Wayman CM. Acta Metall 1988;36:1343. [7] Ranzieri P, Fabbrici S, Nasi L, Righi L, Casoli F, Chernenko VA, et al. Acta Mater 2013;61:263. [8] Yeduru SR, Backen A, Ku¨bel C, Wang D, Scherer T, Fa¨hler S, et al. Scripta Mater 2012;66:566. [9] Buschbeck J, Niemann R, Heczko O, Thomas M, Schultz L, Fa¨hler S. Acta Mater 2009;57:2516. [10] Thomas M, Heczko O, Buschbeck J, Ro¨ßler UK, McCord J, Scheerbaum N, et al. New J Phys 2008;10:023040. [11] Eichhorn T, Jakob G. Mater Sci Forum 2010;635:155. [12] Kallmayer M, Porsch P, Eichhorn T, Schneider H, Jenkins CA, Jakob G, et al. J Phys D – Appl Phys 2009;42:084008. [13] Hakola A, Heczko O, Jaakkola A, Kajava T, Ullakko K. Appl Surf Sci 2004;238:155. [14] Dong JW, Chen LC, Xie JQ, Mu¨ller TAR, Carr DM, Palmstrøm CJ, et al. J Appl Phys 2000;88:7357. [15] Castano FJ, Nelson-Cheeseman B, O’Handley RC, Ross CA, Redondo C, Castano F. J Appl Phys 2003;93:8492. [16] Yeduru SR, Backen A, Fa¨hler S, Schultz L, Kohl M. J Alloys Compd 2012. http://dx.doi.org/10.1016/j.jallcom.2012.02.151. [17] Jetta N, Ozdemir N, Rios S, Bufford D, Karaman I, Zhang X. Thin Solid Films 2012;520:3433.
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