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Grain growth and precipitation in an annealed cold-rolled Ni50.2Ti49.8 alloy
Intermetallics 15 (2007) 1538e1547 www.elsevier.com/locate/intermet
Grain growth and precipitation in an annealed cold-rolled Ni50.2Ti49.8 alloy Arvind K. Srivastava a,1, Dominique Schryvers a,*, Jan Van Humbeeck b a
EMAT, University of Antwerp, CGB, Groenenborgerlaan 171, B-2020 Antwerp, Belgium b MTM, K.U. Leuven, Kasteelpark Arenberg 44, B-3001 Heverlee (Leuven), Belgium
Received 13 February 2007; received in revised form 6 June 2007; accepted 7 June 2007 Available online 15 August 2007
Abstract In this paper we present a transmission electron microscopic study on the effect of annealing on the microstructure of a cold-rolled Ni50.2Ti49.8 ribbon. Transmission electron microscopy of the as-received sample shows the presence of alternating amorphous and crystalline bands. The crystalline bands have widths of the order of a few microns and contain amorphous nanopockets and B2 nanograins, the latter at around 20 nm diameter and preferentially oriented with their normal along the h111i direction and perpendicular to the strip surface. As-received samples were annealed for 30 min at different temperatures up to 800 C. Crystallization starts in the amorphous bands at around 350 C and finally ends up with the coarsening of the grains in the entire sample. Annealing of the samples at 450 C entirely transforms the amorphous bands into crystalline bands. At 800 C the grain size increases to 30e50 mm with a formation of a tweed kind of morphology inside the grains when observed at room temperature. Diffraction patterns from such grains reveal the presence of diffuse intensity around 1/3h110i* indicating the formation of the R-phase. NiTi2 precipitates form at 450 C while annealing at 600 C and higher yields Ni3Ti2 precipitates. For samples annealed at 500 C for a longer time, Ni4Ti3 precipitates have been observed along with the austenite to martensite transformation in the grains. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: A. Nanostructured intermetallics; B. Martensitic transformations; B. Precipitates; C. Rolling; F. Electron microscopy, transmission
1. Introduction NieTi shape memory alloys (SMAs) have been considered to be a very attractive material for practical applications due to their extraordinary shape memory and superelastic characteristics. Further improvements of the characteristics of these alloys can be achieved in many ways like changing the alloy composition or by specific thermomechanical treatments. However, control of the characteristics of the NieTi alloys by a mere changing of the composition cannot resolve all remaining problems in view of particular applications. The second method, i.e., the use of some particular thermomechanical treatments, can * Corresponding author. Tel.: þ32 (0)3 265 3247; fax: þ32 (0)3 265 3257. E-mail address: [email protected] (D. Schryvers). 1 On leave from Synchrotron Utilization and Material Research Division, Raja Ramanna Centre of Advanced Technology, Indore 452013, India. 0966-9795/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2007.06.003
effectively improve the functional properties of these shape memory alloys. These thermomechanical treatments could be annealing following cold work [1e3], aging after solution treatment [4,5] and thermal and/or stress cycling [6,7]. Amongst these, low and moderate temperature annealing following appropriate cold deformation is one of the effective methods for controlling SMA properties [8,9]. Since these NieTi alloys are very susceptible towards solid-state amorphization by cold rolling [10e13], the amount of amorphization can be controlled by the percentage of cold work performed on these alloys. The thermal treatment following the cold work leads to the nucleation and/or growth of nanograins [10,12]. Also, annealing at appropriate temperatures facilitates the formation of certain kinds of precipitates, which are further responsible for controlling the characteristics of these alloys [14]. The purpose of this paper is to present microstructural investigations on systematically annealed cold-rolled NieTi
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strips, with the emphasis on the grain growth and nucleation and growth of precipitates. 2. Experimental procedure A 40% cold-rolled Ni50.2Ti49.8 ribbon (3 mm wide and 100 mm thickness) was obtained from @MT, Herk-de-Stad, Belgium, which reached the high cold deformation through a sequence of several cold deformation steps. Samples from this asrolled ribbon were annealed for 30 min at different temperatures (350 C, 400 C, 450 C, 500 C, 550 C, 600 C, 800 C) in an argon atmosphere and subsequently furnace cooled to room temperature. In order to study the effect of a long annealing treatment, some samples were annealed for 5 h at 500 C. Differential Scanning Calorimetric (DSC) measurements were performed on an MDSC 2920 from TA Instruments on asreceived sample. First, the sample was cooled to 50 C and isothermally treated for 2 min at this temperature. It was then heated to 400 C at a rate of 10 C/min and again isothermally treated for 10 min before cooling back to 50 C. Two such cycles were performed subsequently on the same sample. Structural characterization was carried out using a Seifert 3003 T/T powder X-ray diffractometer equipped with a high temperature attachment. X-ray Diffraction (XRD) patterns were recorded at different temperatures viz. at room temperature, 100 C, 150 C, 200 C, 300 C, 400 C and finally back at room temperature. For transmission electron microscopy (TEM) samples were spark cut into 3 mm diameter discs which were mechanically polished with emery paper to remove the oxide layer. Mechanically polished discs were subjected to thinning by electropolishing with a twin jet polisher and an electrolyte of 90% methanol and 10% sulphuric acid. During electropolishing the electrolyte temperature was maintained at 9 C and the potential applied was 20 V. Two different microscopes were used for TEM characterization: a Philips CM20 microscope was used for conventional microscopy and Energy Dispersive X-ray (EDX) measurements, a LaB6 JEOL 4000EX microscope was used for high resolution transmission electron microscopy (HRTEM). All TEM investigations were performed at room temperature, except for one attempt to induce the martensitic transformation by cooling the as-received sample in a liquid nitrogen holder. Grain sizes were measured from dark field TEM images using the ImageJÒ software after automatic background subtraction setting the lowest pixel value to zero and manual threshold setting so that all bright particles receive the same pixel value above a certain value. After this the area of each particle is calculated by summing all pixels per particle followed by the calculation of the corresponding diameter assuming circular particles. 3. Results 3.1. DSC measurements Fig. 1 shows the DSC measurement of the as-received 40% cold-rolled NieTi alloy. During the first heating run, the DSC
Fig. 1. Two DSC runs of the as-received NieTi alloy: the vertical dashed line indicates room temperature.
reveals an apparent change of baseline between 100 C and 150 C for which no microstructural explanation could be found, followed by a broad exothermic peak with an onset at around 250 C and a maximum at 352 C. This peak represents the crystallization of amorphous NieTi areas, as will be concluded from the structural characterization. After cooling from 400 C, an exothermic peak appears just above room temperature while in a second heating run, after cooling to 50 C, an endothermic peak appears with a small hysteresis of about þ10 , which correlates with the R-phase transformation [15,16]. Further heating to 400 C reveals a flat baseline (without any shift as in the first run) and no exothermic peak at around 350 C, indicating that at the present rate of heating all amorphous materials have crystallized during the first run. As expected, the A-to-R transformation again appears at the same temperature upon cooling during the second run. The total latent heat during the ReA transformation is of the order of 5 J/g. The DSC curve confirms that no martensite is present after the first heating (recovery annealing to 400 C, also leading to crystallization of the amorphous bands) and consecutive cooling to room temperature. 3.2. Structural characterization Fig. 2 shows part of the XRDs of the sample taken at different temperatures. As can be seen from the figure, at room temperature the pattern shows a single broad peak corresponding to the 110 reflection of the B2 phase, indicating strong texturing. The broad peak is indicative of the presence of nanosize grains in the sample possibly including some lattice straining. On heating the full-width-at-half-maximum (FWHM) remains more or less constant up to 200 C. On further heating to 300 C a decrease in FWHM of the 110B2 peak is observed which represents the onset of the crystallization and grain growth, corresponding with the broad exothermic peak in the first cycle of the DSC curve as shown in Fig. 1. At 400 C the 110B2 peak has become quite sharp and some
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Fig. 2. Room and high temperature XRD patterns of as-received and annealed samples: (C) represents B2 phase and (B) NiTi2 precipitates.
new peaks have appeared. One of these new peaks can be indexed as the 100B2 peak indicating a loss of texture while the other reveals the NiTi2 cubic phase with lattice parameter a ¼ 1.1319 nm. Furthermore, there is a weak but systematic shift in the 110B2 peak position towards the smaller angle when moving from room temperature to higher temperatures indicating a lattice expansion at higher temperatures. The 110B2 peak shifts back to its original position when the measurement is repeated at room temperature, as is clear from the top line in Fig. 2. 3.3. Microstructural characterization Fig. 3a shows a transmission electron micrograph of the asreceived sample. The micrograph reveals the presence of alternating nanocrystalline and amorphous bands. The nanocrystalline bands (marked as ‘NC’) have widths of the order of a few microns while the amorphous regions (marked as ‘A’) have widths of a few tens of nanometers. In the nanocrystalline bands the grains are preferentially oriented in a h111iB2 direction, as can be concluded from the selected area electron diffraction (SAED) inset in Fig. 3b. This preferential orientation is perpendicular to the rolling direction (and normal to the major surface of the resulting strip), but the arc shape of the 110 type of reflections indicates that slight in-plain rotations of the nanograins around this central h111i zone exist. The average nanograin size is 20 nm in diameter, measured by choosing an area with minimal overlap of grains in the dark field micrograph obtained by selecting a 110 reflection and assuming spherical grains. Throughout the nanograin area nanosized pockets of amorphous material can be recognized in HRTEM images as seen in the image of Fig. 3c. FFT of small crystalline parts in Fig. 3c reveals that on both sides of the amorphous nanopocket the lattice is slightly deformed (no perfect 60 between 110 type reflections) and rotated with respect to one another over approximately 4 , confirming the observation of the arcs in the inset of Fig. 3b. While wide beam analysis by EDX and EELS measurements confirms the Ni50.2Ti49.8 composition, nanoprobe EELS measurements clearly show that the extended amorphous bands indeed also consist of amorphous metallic NieTi material. In
some cases, small individual crystalline nanoparticles can also be recognized inside the amorphous bands as shown in the encircled area in Fig. 3a and the HRTEM image of Fig. 3d. No martensite was observed at room temperature nor did in situ cooling in a liquid nitrogen holder induce any martensitic transformation in the electropolished samples. A sample annealed at 350 C for 30 min shows essentially the same kind of morphology as the as-received sample with alternating amorphous and nanocrystalline bands with the width of the former only slightly and locally being reduced. The average size of the nanograins has slightly increased and is now at around 20e30 nm while SAED reveals that the preferential orientation of the grains in the nanocrystalline band is still retained. Nanopockets of amorphous NieTi material are still observed inside the NC areas. In samples annealed at 400 C, an increase in grain size is observed and the average grain size as measured through bright and dark field images (Fig. 4a and b) is at around 75 nm. Moreover, although the amorphous bands are now completely crystallized they are still visible as delineated shapes in the bright and dark field images, as indicated by arrows in Fig. 4. The appearance of more polycrystalline rings in the SAED pattern (Fig. 4c) indicates that the sample has lost most of its texturing but the nanograins still preserve the parent B2 structure. As from this annealing temperature, HRTEM images do not reveal any amorphous material any more. Fig. 5a and b shows a bright and dark field micrograph of the sample annealed at 450 C. The average grain size calculated from these figures is at around 100 nm. Also, the status of crystallization is again clearly visible as the former amorphous bands can still be recognized (arrows). The SAED pattern (Fig. 5c) shows continuous polycrystalline rings. Most of the rings can be indexed by the B2 structure, however, there are a few extra rings (dashed lines) present which correspond with the formation of a new phase and can be indexed by the NiTi2 cubic phase with lattice parameter a ¼ 1.131 nm, confirming the XRD data. The interplanar lattice spacings for this NiTi2 phase are listed in the adjacent table and compared with the B2 ones, indicating that rings 1, 2, 4 and 6 indeed belong to this new phase. On further heating to 500 C the amorphous regions have completely crystallized and the band morphology in the sample has disappeared as seen from Fig. 6a. The diffraction pattern (Fig. 6b) shows a close to single zone SAED pattern, as the grains have now grown to sizes just below the micron. Annealing at 550 C yields grain sizes between 1 mm and 1.5 mm. Samples annealed at 600 C (Fig. 7a) show further coarsening of the grains and also the formation of elongated precipitates at the B2 grain boundaries. These precipitates were identified as the high temperature form of Ni3Ti2 with an I4/mmm tetragonal crystal structure and lattice parameters a ¼ 0.309 nm and c ¼ 1.354 nm by nano-diffraction and EDX. An example of the former showing the characteristic row of four super reflections for this structure is shown in Fig. 7b [17]. At an annealing temperature of 800 C (Fig. 8a) the B2 grain size increases to 30e50 mm with the formation of a tweed kind of morphology inside the grains. Diffraction
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Fig. 3. (a) Bright field TEM of as-received sample. NC and A represent nanocrystalline and amorphous regions, respectively. (b) Dark field micrograph using a 110 reflection of the same region as shown in (a) with SAED pattern showing h111i texturing as inset. (c) HRTEM revealing pockets of amorphous material inside the nanograins’ (NC) area. (d) HRTEM of a nanoparticle inside the amorphous (A) band.
patterns (Fig. 8b) from such grains reveal the presence of a diffuse intensity around 1/3h110i* indicating the onset of formation of the R-phase when observing the samples at room temperature [16,18]. Also, the number of Ni3Ti2 precipitates formed at grain boundaries increases. All these samples do not show any austenite to martensitic transformation, only once in a 500 C annealed sample a martensite region was observed. With this in mind, another set of samples was heated at 500 C for 5 h. Microstructural investigation shows quite distinct changes in these samples as compared to the samples annealed at 500 C for 30 min. In the longer annealed samples two kinds of regions have been observed, one with mostly austenite and others with martensite and austenite. Fig. 9a shows a bright field image of one of such regions where only austenite grains are present whereas Fig. 9b shows the other region where both austenite and martensitic grains are present, the latter clearly recognized by the internal twinning. Moreover, small Ni4Ti3 precipitates with
sizes between 50 nm and 100 nm can clearly be recognized (Fig. 9c) from nanoprobe diffraction patterns revealing the characteristic row of six super reflections for this structure (Fig. 9d) [19]. These precipitates are found to be present at the grain boundaries rather than within the grains and do not reveal the conventional lens shape known for these precipitates in regular superelastic NieTi material. 4. Discussion TEM shows that a 40% cold-rolled Ni50.2Ti49.8 strip contains large areas with nanocrystalline grains and amorphous pockets, the grains preferentially oriented along the h111i zone axis revealing a h111i(110) texture. The latter texture, though not exclusively, was also observed by Paula et al. [20] by synchrotron radiation in cold-rolled Ni49Ti51 and by Chang and Wu [21] by X-ray Diffraction in cold-rolled Ni50Ti50. Breuer and Klimanek explained the occurrence of this texture in b-brass material, also
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Fig. 4. (a) Bright field and (b) dark field TEM of a 400 C annealed sample. Arrows show the crystallization of the amorphous bands. (c) Corresponding SAED pattern showing diminishing of texture.
having an ordered B2 structure, by the motion of [111] superdislocations on (110) planes during rolling at room temperature [22]. Along with this process the original micron sized grains are then further broken-up into smaller nanograins but retaining their central h111i zone perpendicular to the rolling direction, leading to the arcs in the diffraction pattern shown in the inset of Fig. 3b. In-between the nanocrystalline areas fine long amorphous bands are observed and inside the nanocrystalline areas nanopockets of amorphous material are found. These observations confirm those by Kim et al. in Ni50Ti50, although in the latter work the 40% cold-rolled material only revealed nanograin material with amorphous pockets while the amorphous bands were only observed in 70% material [13]. Also, we did not observe any B190 martensite in our as-received material, which could be related with the fact that our material contains slightly more Ni which implies a lower martensitic transformation temperature (Ms). However, at a composition of 50.2 at.% Ni Ms should be above room temperature but the nanoscale of the grains and the internal strain will suppress the transformation
[23]. Still, following the conclusion by Ewert et al. [11] that material with a higher Ms yields a higher volume fraction of amorphous material when deformed might indicate that Ms in the undeformed material is above room temperature. From the DSC runs it is indeed confirmed that the martensitic transformation is suppressed since with a Ni concentration only slightly above 50% one normally expects a martensite start temperature at around 15 C [24]. After annealing the sample at 350 C for 30 min the nanograins slightly increase in average dimensions but amorphous material still remains as pockets as well as bands, confirming earlier results [10,11]. As from 400 C the amorphous bands get completely converted into nanocrystalline bands, as is evident from Fig. 4a and b, and no amorphous pockets are recognized any more. The latter indicates that these pockets, when observed, are not due to the sample preparation but are a genuine part of the nanostructure. Also, the texture diminishes at this temperature which indicates recrystallization in the NC bands while the crystallization in the amorphous bands also has no
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Fig. 5. (a) Bright field TEM of a 450 C annealed sample. The arrows show the location of the former amorphous bands. (b) Corresponding dark field micrograph also showing the crystallization of the amorphous bands. (c) SAED pattern showing the presence of B2 and NiTi2 phases: dashed and solid circles represent NiTi2 and B2 phases, respectively.
Fig. 6. (a) Bright field TEM of a 500 C annealed sample showing complete disappearance of the traces of the amorphous bands. (b) Corresponding SAED pattern showing the nearly single crystalline pattern.
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Fig. 7. (a) Bright field TEM micrograph of the sample annealed at 600 C. Arrows show Ni3Ti2 precipitates in different orientations at the B2 grain boundaries. (b) Nanoprobe diffraction pattern from a precipitate revealing the typical row of four super reflections.
tendency for texture. Upon further increase of the annealing temperature the growth rate also increases and finally at moderately high temperature of 500 C grain coarsening starts and the nanograins transform into large grains of the size of a few
microns. These grains still preserve the B2 structure with a complete loss of the earlier texture. In Fig. 10 a summary of the measured grain size distributions for the lower temperatures is presented, indicating an evolution from a sharp
Fig. 8. (a) Transmission electron micrograph of an 800 C annealed sample. Arrows indicate larger Ni3Ti2 precipitates. (b) Two arrow heads in the diffraction pattern represent the direction along which the intensity line scan in (c) has been taken. (c) Intensity line scan showing two weak peaks (marked by arrows) at 1/3 and 2/3 positions between the two major peaks.
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Fig. 9. (a) Bright field TEM micrograph of the sample annealed for 5 h at 500 C from the regions mostly containing austenite grains. (b) Bright field micrograph showing regions partially transformed to martensite, (c) micrograph showing Ni4Ti3 precipitate (darkest area is the origin of (d)) and (d) corresponding nanoprobe diffraction pattern from the precipitates.
peaked distribution of nanograins in the as-received sample to a much wider spread already at 450 C. For samples annealed at 800 C the grain size is sufficiently large to allow the observation of a tweed morphology and diffuse intensity in reciprocal space originating from the R-phase transformation. For samples annealed at 450 C reflections belonging to NiTi2 precipitates have been observed both in XRD and TEM. This confirms the observation by Paula et al. [20] who found such precipitates upon annealing of Ti51Ni49 cold-rolled samples above 400 C. However, at present it is unclear how to explain the formation of Ti-rich precipitates in a Ni-rich sample, although minor inhomogeneities at, e.g., grain boundaries could possibly account for this assisted by the relative low free energy of this phase [25]. Upon annealing at higher temperature these precipitates dissolve back into the matrix after which Ni3Ti2 precipitates are produced at the grain boundaries with numbers increasing with a further increase in temperature. Comparing this with the TTT diagram published by Nishida et al. for bulk Ni52Ti48 [26], it is seen
that in the present cold-rolled material these precipitates are formed at lower temperatures (600 C instead of 700 C at 30 min) and shorter annealing times (30 min instead of 10 h or more at 600 C), possibly due to an increased number of vacancies or other defects assisting the diffusion of atoms, since the starting composition is less favorable. The latter also confirms conclusions from earlier annealing work on less deformed NieTi alloys [27]. For longer annealed samples, i.e., 5 h at 500 C, the formation of Ni4Ti3 precipitates has been observed. This confirms that the matrix initially is rich in Ni and in this case better fits with the TTT diagram from Nishida et al. for Ni52Ti48, although they did not observe any precipitation (except an oxide phase) for equiatomic Ni50Ti50 material which is closer to the present composition of Ni50.2Ti49.8 [26]. The fact that these precipitates are found to nucleate at grain boundaries confirms observations in conventional material with low Ni supersaturation, although possible effects of lattice defects due to the severe deformations should not be neglected [28]. The latter
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memory alloys: a fundamental approach’’ and of a EU RTN program 505226 ‘‘Multi-scale modelling and characterisation for phase transformations in advanced materials’’.
References
Fig. 10. Grain size distribution for as-received, 350 C, 400 C and 450 C annealed samples.
can moreover be expected to induce local strain fields in the matrix due to which the precipitates lose their expected lens shape normally induced by the lattice mismatch between the atomic structures of the precipitate and surrounding B2 matrix. These Ni-rich precipitates leave the matrix deficient in Ni, which raises the martensitic start temperature above room temperature [24], explaining the observation at room temperature of several transformed regions in these samples, although the increase in grain size might also have affected the martensitic start temperature in the same way. In previous experiments including annealing of cold-rolled NieTi no precipitates were described although they are essential in the applications of this material [12,13]. To what extend the observed stabilization of austenite and following precipitation upon annealing have an effect on the shape memory and superelastic behaviour of these ribbons is the subject of ongoing work. 5. Conclusions The effect of cold rolling and subsequent annealing on the microstructure of a Ni50.2Ti49.8 SMA is investigated. Cold rolling of 40% introduces amorphization as well as stabilization of the B2 structure in textured nanograins. Amorphous nanosized pockets as well as amorphous bands are observed in as-received material. Post-deformation annealing for 30 min at or above 350 C leads to the crystallization of the amorphous regions with a gradual increase of the grain size up to 30e50 mm at 800 C. Formation of NiTi2 precipitates occurs at 450 C and Ni3Ti2 precipitates grow above 600 C. Longer time annealing of 5 h at 500 C yields Ni4Ti3 precipitates and grains containing twinned martensite. Acknowledgment This research work was performed with the support of an FWO project G.0465.05 ‘‘The functional properties of shape
[1] Miyazaki S, Ohmi Y, Otsuka K, Suzuki Y. Characteristics of deformation and transformation pseudo-elasticity in TieNi alloys. J Phys 1982; 43(Suppl. 12):C4-255e60. [2] Lin HC, Wu SK. The tensile behaviour of a cold-rolled and reverse transformed equiatomic TiNi alloy. Acta Metall Mater 1994;42: 1623e30. [3] Miyazaki S, Otsuka K. Deformation and transition behavior associated with the R-phase in TieNi alloys. Metall Trans A 1986;17:53e63. [4] Miyazaki S, Otsuka K. Development of shape memory alloys. ISIJ Int 1989;29:353e77. [5] Stroz D, Bojarski Z, Ilczuk J, Lekston Z, Morawiec H. Effect of thermal cycling on as quenched and aged nickel rich NieTi alloy. J Mater Sci 1991;26:1741e8. [6] Takasi T, Nakata Y, Shimizu K. Thermal cycling effects in an aged Nirich TieNi shape memory alloy. Mater Trans JIM 1987;28:883e90. [7] Wurzel D. Marforming and tempering of binary NieTi alloys including precipitation effects. Mater Sci Eng A 1999;273e275:634e8. [8] Brailovski V, Khmelevskaya IYu, Prokoshkin SD, Pushin VG, Ryklina EP, Aliev RZ. Foundations of heat and thermomechanical treatments and their effects on the structure and properties. Phys Met Metallogr 2004;97(Suppl. 1):3e5. [9] Prokoshkin SD, Khmelevskaya IYu, Brailovski V, Trochu F, Turenne S, Turilina VYu. Thermomechanical treatments and their influence on the microstructure and stress/strain diagrams of NiTi shape memory alloys. Can Metall Quart 2004;43:95e9. [10] Koike J, Parkin DM, Nastasi M. Crystal-to-amorphous transformation of NiTi induced by cold rolling. J Mater Res 1990;5:1414e8. [11] Ewert JC, Bo¨hm I, Peter R, Haider F. The role of the martensite transformation for the mechanical amorphization of NiTi. Acta Mater 1997;45: 2197e206. [12] Nakayama H, Tsuchiya K, Umemoto M. Crystal refinement and amorphization by cold rolling in TiNi shape memory alloys. Scr Mater 2001;44: 1781e5. [13] Kim Y-H, Cho G-B, Hur S-G, Jeong S-S, Nam T-H. Nanocrystallization of a Tie50.0Ni(at.%) alloy by cold working and stress/strain behavior. Mater Sci Eng A 2006;438e440:531e5. [14] Otsuka K, Ren X. Physical metallurgy of TieNi-based shape memory alloys. Prog Mater Sci 2005;50:511e678. [15] Sandrock GD, Perkins AJ, Hehemann RF. The premartensitic instability in near-equiatomic TiNi. Metall Trans 1971;2:2769e81. [16] Murakami Y, Shindo D. Changes in microstructure near the R-phase transformation in Ti50Ni48Fe2 studied by in-situ electron microscopy. Philos Mag Lett 2001;81:631e8. [17] Hara T, Ohba T, Otsuka K, Nishida M. Phase transformation and crystal structures of Ti2Ni3 alloys. Mater Trans JIM 1997;38:277e84. [18] Otsuka K, Sawamura T, Shimizu K. Crystal structure and internal defects of equiatomic TiNi martensites. Phys Status Solidi A 1971;5: 457e70. [19] Tadaki T, Nakata Y, Shimizu K, Otsuka K. Crystal structure, composition and morphology of a precipitate in an aged Ti51 at.% Ni shape memory alloy. Trans JIM 1986;27:731e40. [20] Paula AS, Canejo JH, Mahesh KK, Silva RJ, Braz Fernandes FM, Martins RM, et al. Study of the textural evolution in Ti-rich NiTi using synchrotron radiation. Nucl Instrum Methods Phys Res Sect B 2006;246: 206e10. [21] Chang SH, Wu SK. Textures in cold-rolled and annealed Ti50Ni50 shape memory alloy. Scr Mater 2004;50:937e41. [22] Breuer D, Klimanek P. Investigation of texture and dislocation density in rolled b-brass by X-ray and neutron diffraction. Mater Sci Eng A 1997;234e236:818e21.
A.K. Srivastava et al. / Intermetallics 15 (2007) 1538e1547 [23] Waitz T, Kazykhanov V, Karnthaler H-P. Martensitic phase transformations in nanocrystalline NiTi studied by TEM. Acta Mater 2004;52: 137e47. [24] Tang W. Thermodynamic study of the low-temperature phase B190 and the martensitic transformation in near-equiatomic TieNi shape memory alloys. Metall Trans A 1997;28:537e44. [25] Makifuchi Y, Terunuma Y, Nagumo M. Structural relaxation in amorphous NieTi alloys prepared by mechanical alloying. Mater Sci Eng A 1997;226e228:312e6.
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[26] Nishida M, Wayman CM, Honma T. Precipitation processes in near equiatomic TiNi shape memory alloys. Metall Trans A 1986;17: 1505e15. [27] Treppmann D, Hornbogen E, Wurzel D. The effect of combined recrystallization and precipitation processes on the functional and structural properties in NiTi alloys. J Phys IV 1995;5:C8:569e74. [28] Fan G, Chen W, Yang S, Zhu J, Ren X, Otsuka K. Origin of abnormal multi-stage martensitic transformation behavior in aged Ni-rich TieNi shape memory alloys. Acta Mater 2004;52:4351e62.
Grain growth and precipitation in an annealed cold-rolled Ni50.2Ti49.8 alloy Arvind K. Srivastava a,1, Dominique Schryvers a,*, Jan Van Humbeeck b a
EMAT, University of Antwerp, CGB, Groenenborgerlaan 171, B-2020 Antwerp, Belgium b MTM, K.U. Leuven, Kasteelpark Arenberg 44, B-3001 Heverlee (Leuven), Belgium
Received 13 February 2007; received in revised form 6 June 2007; accepted 7 June 2007 Available online 15 August 2007
Abstract In this paper we present a transmission electron microscopic study on the effect of annealing on the microstructure of a cold-rolled Ni50.2Ti49.8 ribbon. Transmission electron microscopy of the as-received sample shows the presence of alternating amorphous and crystalline bands. The crystalline bands have widths of the order of a few microns and contain amorphous nanopockets and B2 nanograins, the latter at around 20 nm diameter and preferentially oriented with their normal along the h111i direction and perpendicular to the strip surface. As-received samples were annealed for 30 min at different temperatures up to 800 C. Crystallization starts in the amorphous bands at around 350 C and finally ends up with the coarsening of the grains in the entire sample. Annealing of the samples at 450 C entirely transforms the amorphous bands into crystalline bands. At 800 C the grain size increases to 30e50 mm with a formation of a tweed kind of morphology inside the grains when observed at room temperature. Diffraction patterns from such grains reveal the presence of diffuse intensity around 1/3h110i* indicating the formation of the R-phase. NiTi2 precipitates form at 450 C while annealing at 600 C and higher yields Ni3Ti2 precipitates. For samples annealed at 500 C for a longer time, Ni4Ti3 precipitates have been observed along with the austenite to martensite transformation in the grains. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: A. Nanostructured intermetallics; B. Martensitic transformations; B. Precipitates; C. Rolling; F. Electron microscopy, transmission
1. Introduction NieTi shape memory alloys (SMAs) have been considered to be a very attractive material for practical applications due to their extraordinary shape memory and superelastic characteristics. Further improvements of the characteristics of these alloys can be achieved in many ways like changing the alloy composition or by specific thermomechanical treatments. However, control of the characteristics of the NieTi alloys by a mere changing of the composition cannot resolve all remaining problems in view of particular applications. The second method, i.e., the use of some particular thermomechanical treatments, can * Corresponding author. Tel.: þ32 (0)3 265 3247; fax: þ32 (0)3 265 3257. E-mail address: [email protected] (D. Schryvers). 1 On leave from Synchrotron Utilization and Material Research Division, Raja Ramanna Centre of Advanced Technology, Indore 452013, India. 0966-9795/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2007.06.003
effectively improve the functional properties of these shape memory alloys. These thermomechanical treatments could be annealing following cold work [1e3], aging after solution treatment [4,5] and thermal and/or stress cycling [6,7]. Amongst these, low and moderate temperature annealing following appropriate cold deformation is one of the effective methods for controlling SMA properties [8,9]. Since these NieTi alloys are very susceptible towards solid-state amorphization by cold rolling [10e13], the amount of amorphization can be controlled by the percentage of cold work performed on these alloys. The thermal treatment following the cold work leads to the nucleation and/or growth of nanograins [10,12]. Also, annealing at appropriate temperatures facilitates the formation of certain kinds of precipitates, which are further responsible for controlling the characteristics of these alloys [14]. The purpose of this paper is to present microstructural investigations on systematically annealed cold-rolled NieTi
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strips, with the emphasis on the grain growth and nucleation and growth of precipitates. 2. Experimental procedure A 40% cold-rolled Ni50.2Ti49.8 ribbon (3 mm wide and 100 mm thickness) was obtained from @MT, Herk-de-Stad, Belgium, which reached the high cold deformation through a sequence of several cold deformation steps. Samples from this asrolled ribbon were annealed for 30 min at different temperatures (350 C, 400 C, 450 C, 500 C, 550 C, 600 C, 800 C) in an argon atmosphere and subsequently furnace cooled to room temperature. In order to study the effect of a long annealing treatment, some samples were annealed for 5 h at 500 C. Differential Scanning Calorimetric (DSC) measurements were performed on an MDSC 2920 from TA Instruments on asreceived sample. First, the sample was cooled to 50 C and isothermally treated for 2 min at this temperature. It was then heated to 400 C at a rate of 10 C/min and again isothermally treated for 10 min before cooling back to 50 C. Two such cycles were performed subsequently on the same sample. Structural characterization was carried out using a Seifert 3003 T/T powder X-ray diffractometer equipped with a high temperature attachment. X-ray Diffraction (XRD) patterns were recorded at different temperatures viz. at room temperature, 100 C, 150 C, 200 C, 300 C, 400 C and finally back at room temperature. For transmission electron microscopy (TEM) samples were spark cut into 3 mm diameter discs which were mechanically polished with emery paper to remove the oxide layer. Mechanically polished discs were subjected to thinning by electropolishing with a twin jet polisher and an electrolyte of 90% methanol and 10% sulphuric acid. During electropolishing the electrolyte temperature was maintained at 9 C and the potential applied was 20 V. Two different microscopes were used for TEM characterization: a Philips CM20 microscope was used for conventional microscopy and Energy Dispersive X-ray (EDX) measurements, a LaB6 JEOL 4000EX microscope was used for high resolution transmission electron microscopy (HRTEM). All TEM investigations were performed at room temperature, except for one attempt to induce the martensitic transformation by cooling the as-received sample in a liquid nitrogen holder. Grain sizes were measured from dark field TEM images using the ImageJÒ software after automatic background subtraction setting the lowest pixel value to zero and manual threshold setting so that all bright particles receive the same pixel value above a certain value. After this the area of each particle is calculated by summing all pixels per particle followed by the calculation of the corresponding diameter assuming circular particles. 3. Results 3.1. DSC measurements Fig. 1 shows the DSC measurement of the as-received 40% cold-rolled NieTi alloy. During the first heating run, the DSC
Fig. 1. Two DSC runs of the as-received NieTi alloy: the vertical dashed line indicates room temperature.
reveals an apparent change of baseline between 100 C and 150 C for which no microstructural explanation could be found, followed by a broad exothermic peak with an onset at around 250 C and a maximum at 352 C. This peak represents the crystallization of amorphous NieTi areas, as will be concluded from the structural characterization. After cooling from 400 C, an exothermic peak appears just above room temperature while in a second heating run, after cooling to 50 C, an endothermic peak appears with a small hysteresis of about þ10 , which correlates with the R-phase transformation [15,16]. Further heating to 400 C reveals a flat baseline (without any shift as in the first run) and no exothermic peak at around 350 C, indicating that at the present rate of heating all amorphous materials have crystallized during the first run. As expected, the A-to-R transformation again appears at the same temperature upon cooling during the second run. The total latent heat during the ReA transformation is of the order of 5 J/g. The DSC curve confirms that no martensite is present after the first heating (recovery annealing to 400 C, also leading to crystallization of the amorphous bands) and consecutive cooling to room temperature. 3.2. Structural characterization Fig. 2 shows part of the XRDs of the sample taken at different temperatures. As can be seen from the figure, at room temperature the pattern shows a single broad peak corresponding to the 110 reflection of the B2 phase, indicating strong texturing. The broad peak is indicative of the presence of nanosize grains in the sample possibly including some lattice straining. On heating the full-width-at-half-maximum (FWHM) remains more or less constant up to 200 C. On further heating to 300 C a decrease in FWHM of the 110B2 peak is observed which represents the onset of the crystallization and grain growth, corresponding with the broad exothermic peak in the first cycle of the DSC curve as shown in Fig. 1. At 400 C the 110B2 peak has become quite sharp and some
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Fig. 2. Room and high temperature XRD patterns of as-received and annealed samples: (C) represents B2 phase and (B) NiTi2 precipitates.
new peaks have appeared. One of these new peaks can be indexed as the 100B2 peak indicating a loss of texture while the other reveals the NiTi2 cubic phase with lattice parameter a ¼ 1.1319 nm. Furthermore, there is a weak but systematic shift in the 110B2 peak position towards the smaller angle when moving from room temperature to higher temperatures indicating a lattice expansion at higher temperatures. The 110B2 peak shifts back to its original position when the measurement is repeated at room temperature, as is clear from the top line in Fig. 2. 3.3. Microstructural characterization Fig. 3a shows a transmission electron micrograph of the asreceived sample. The micrograph reveals the presence of alternating nanocrystalline and amorphous bands. The nanocrystalline bands (marked as ‘NC’) have widths of the order of a few microns while the amorphous regions (marked as ‘A’) have widths of a few tens of nanometers. In the nanocrystalline bands the grains are preferentially oriented in a h111iB2 direction, as can be concluded from the selected area electron diffraction (SAED) inset in Fig. 3b. This preferential orientation is perpendicular to the rolling direction (and normal to the major surface of the resulting strip), but the arc shape of the 110 type of reflections indicates that slight in-plain rotations of the nanograins around this central h111i zone exist. The average nanograin size is 20 nm in diameter, measured by choosing an area with minimal overlap of grains in the dark field micrograph obtained by selecting a 110 reflection and assuming spherical grains. Throughout the nanograin area nanosized pockets of amorphous material can be recognized in HRTEM images as seen in the image of Fig. 3c. FFT of small crystalline parts in Fig. 3c reveals that on both sides of the amorphous nanopocket the lattice is slightly deformed (no perfect 60 between 110 type reflections) and rotated with respect to one another over approximately 4 , confirming the observation of the arcs in the inset of Fig. 3b. While wide beam analysis by EDX and EELS measurements confirms the Ni50.2Ti49.8 composition, nanoprobe EELS measurements clearly show that the extended amorphous bands indeed also consist of amorphous metallic NieTi material. In
some cases, small individual crystalline nanoparticles can also be recognized inside the amorphous bands as shown in the encircled area in Fig. 3a and the HRTEM image of Fig. 3d. No martensite was observed at room temperature nor did in situ cooling in a liquid nitrogen holder induce any martensitic transformation in the electropolished samples. A sample annealed at 350 C for 30 min shows essentially the same kind of morphology as the as-received sample with alternating amorphous and nanocrystalline bands with the width of the former only slightly and locally being reduced. The average size of the nanograins has slightly increased and is now at around 20e30 nm while SAED reveals that the preferential orientation of the grains in the nanocrystalline band is still retained. Nanopockets of amorphous NieTi material are still observed inside the NC areas. In samples annealed at 400 C, an increase in grain size is observed and the average grain size as measured through bright and dark field images (Fig. 4a and b) is at around 75 nm. Moreover, although the amorphous bands are now completely crystallized they are still visible as delineated shapes in the bright and dark field images, as indicated by arrows in Fig. 4. The appearance of more polycrystalline rings in the SAED pattern (Fig. 4c) indicates that the sample has lost most of its texturing but the nanograins still preserve the parent B2 structure. As from this annealing temperature, HRTEM images do not reveal any amorphous material any more. Fig. 5a and b shows a bright and dark field micrograph of the sample annealed at 450 C. The average grain size calculated from these figures is at around 100 nm. Also, the status of crystallization is again clearly visible as the former amorphous bands can still be recognized (arrows). The SAED pattern (Fig. 5c) shows continuous polycrystalline rings. Most of the rings can be indexed by the B2 structure, however, there are a few extra rings (dashed lines) present which correspond with the formation of a new phase and can be indexed by the NiTi2 cubic phase with lattice parameter a ¼ 1.131 nm, confirming the XRD data. The interplanar lattice spacings for this NiTi2 phase are listed in the adjacent table and compared with the B2 ones, indicating that rings 1, 2, 4 and 6 indeed belong to this new phase. On further heating to 500 C the amorphous regions have completely crystallized and the band morphology in the sample has disappeared as seen from Fig. 6a. The diffraction pattern (Fig. 6b) shows a close to single zone SAED pattern, as the grains have now grown to sizes just below the micron. Annealing at 550 C yields grain sizes between 1 mm and 1.5 mm. Samples annealed at 600 C (Fig. 7a) show further coarsening of the grains and also the formation of elongated precipitates at the B2 grain boundaries. These precipitates were identified as the high temperature form of Ni3Ti2 with an I4/mmm tetragonal crystal structure and lattice parameters a ¼ 0.309 nm and c ¼ 1.354 nm by nano-diffraction and EDX. An example of the former showing the characteristic row of four super reflections for this structure is shown in Fig. 7b [17]. At an annealing temperature of 800 C (Fig. 8a) the B2 grain size increases to 30e50 mm with the formation of a tweed kind of morphology inside the grains. Diffraction
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Fig. 3. (a) Bright field TEM of as-received sample. NC and A represent nanocrystalline and amorphous regions, respectively. (b) Dark field micrograph using a 110 reflection of the same region as shown in (a) with SAED pattern showing h111i texturing as inset. (c) HRTEM revealing pockets of amorphous material inside the nanograins’ (NC) area. (d) HRTEM of a nanoparticle inside the amorphous (A) band.
patterns (Fig. 8b) from such grains reveal the presence of a diffuse intensity around 1/3h110i* indicating the onset of formation of the R-phase when observing the samples at room temperature [16,18]. Also, the number of Ni3Ti2 precipitates formed at grain boundaries increases. All these samples do not show any austenite to martensitic transformation, only once in a 500 C annealed sample a martensite region was observed. With this in mind, another set of samples was heated at 500 C for 5 h. Microstructural investigation shows quite distinct changes in these samples as compared to the samples annealed at 500 C for 30 min. In the longer annealed samples two kinds of regions have been observed, one with mostly austenite and others with martensite and austenite. Fig. 9a shows a bright field image of one of such regions where only austenite grains are present whereas Fig. 9b shows the other region where both austenite and martensitic grains are present, the latter clearly recognized by the internal twinning. Moreover, small Ni4Ti3 precipitates with
sizes between 50 nm and 100 nm can clearly be recognized (Fig. 9c) from nanoprobe diffraction patterns revealing the characteristic row of six super reflections for this structure (Fig. 9d) [19]. These precipitates are found to be present at the grain boundaries rather than within the grains and do not reveal the conventional lens shape known for these precipitates in regular superelastic NieTi material. 4. Discussion TEM shows that a 40% cold-rolled Ni50.2Ti49.8 strip contains large areas with nanocrystalline grains and amorphous pockets, the grains preferentially oriented along the h111i zone axis revealing a h111i(110) texture. The latter texture, though not exclusively, was also observed by Paula et al. [20] by synchrotron radiation in cold-rolled Ni49Ti51 and by Chang and Wu [21] by X-ray Diffraction in cold-rolled Ni50Ti50. Breuer and Klimanek explained the occurrence of this texture in b-brass material, also
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Fig. 4. (a) Bright field and (b) dark field TEM of a 400 C annealed sample. Arrows show the crystallization of the amorphous bands. (c) Corresponding SAED pattern showing diminishing of texture.
having an ordered B2 structure, by the motion of [111] superdislocations on (110) planes during rolling at room temperature [22]. Along with this process the original micron sized grains are then further broken-up into smaller nanograins but retaining their central h111i zone perpendicular to the rolling direction, leading to the arcs in the diffraction pattern shown in the inset of Fig. 3b. In-between the nanocrystalline areas fine long amorphous bands are observed and inside the nanocrystalline areas nanopockets of amorphous material are found. These observations confirm those by Kim et al. in Ni50Ti50, although in the latter work the 40% cold-rolled material only revealed nanograin material with amorphous pockets while the amorphous bands were only observed in 70% material [13]. Also, we did not observe any B190 martensite in our as-received material, which could be related with the fact that our material contains slightly more Ni which implies a lower martensitic transformation temperature (Ms). However, at a composition of 50.2 at.% Ni Ms should be above room temperature but the nanoscale of the grains and the internal strain will suppress the transformation
[23]. Still, following the conclusion by Ewert et al. [11] that material with a higher Ms yields a higher volume fraction of amorphous material when deformed might indicate that Ms in the undeformed material is above room temperature. From the DSC runs it is indeed confirmed that the martensitic transformation is suppressed since with a Ni concentration only slightly above 50% one normally expects a martensite start temperature at around 15 C [24]. After annealing the sample at 350 C for 30 min the nanograins slightly increase in average dimensions but amorphous material still remains as pockets as well as bands, confirming earlier results [10,11]. As from 400 C the amorphous bands get completely converted into nanocrystalline bands, as is evident from Fig. 4a and b, and no amorphous pockets are recognized any more. The latter indicates that these pockets, when observed, are not due to the sample preparation but are a genuine part of the nanostructure. Also, the texture diminishes at this temperature which indicates recrystallization in the NC bands while the crystallization in the amorphous bands also has no
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Fig. 5. (a) Bright field TEM of a 450 C annealed sample. The arrows show the location of the former amorphous bands. (b) Corresponding dark field micrograph also showing the crystallization of the amorphous bands. (c) SAED pattern showing the presence of B2 and NiTi2 phases: dashed and solid circles represent NiTi2 and B2 phases, respectively.
Fig. 6. (a) Bright field TEM of a 500 C annealed sample showing complete disappearance of the traces of the amorphous bands. (b) Corresponding SAED pattern showing the nearly single crystalline pattern.
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Fig. 7. (a) Bright field TEM micrograph of the sample annealed at 600 C. Arrows show Ni3Ti2 precipitates in different orientations at the B2 grain boundaries. (b) Nanoprobe diffraction pattern from a precipitate revealing the typical row of four super reflections.
tendency for texture. Upon further increase of the annealing temperature the growth rate also increases and finally at moderately high temperature of 500 C grain coarsening starts and the nanograins transform into large grains of the size of a few
microns. These grains still preserve the B2 structure with a complete loss of the earlier texture. In Fig. 10 a summary of the measured grain size distributions for the lower temperatures is presented, indicating an evolution from a sharp
Fig. 8. (a) Transmission electron micrograph of an 800 C annealed sample. Arrows indicate larger Ni3Ti2 precipitates. (b) Two arrow heads in the diffraction pattern represent the direction along which the intensity line scan in (c) has been taken. (c) Intensity line scan showing two weak peaks (marked by arrows) at 1/3 and 2/3 positions between the two major peaks.
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Fig. 9. (a) Bright field TEM micrograph of the sample annealed for 5 h at 500 C from the regions mostly containing austenite grains. (b) Bright field micrograph showing regions partially transformed to martensite, (c) micrograph showing Ni4Ti3 precipitate (darkest area is the origin of (d)) and (d) corresponding nanoprobe diffraction pattern from the precipitates.
peaked distribution of nanograins in the as-received sample to a much wider spread already at 450 C. For samples annealed at 800 C the grain size is sufficiently large to allow the observation of a tweed morphology and diffuse intensity in reciprocal space originating from the R-phase transformation. For samples annealed at 450 C reflections belonging to NiTi2 precipitates have been observed both in XRD and TEM. This confirms the observation by Paula et al. [20] who found such precipitates upon annealing of Ti51Ni49 cold-rolled samples above 400 C. However, at present it is unclear how to explain the formation of Ti-rich precipitates in a Ni-rich sample, although minor inhomogeneities at, e.g., grain boundaries could possibly account for this assisted by the relative low free energy of this phase [25]. Upon annealing at higher temperature these precipitates dissolve back into the matrix after which Ni3Ti2 precipitates are produced at the grain boundaries with numbers increasing with a further increase in temperature. Comparing this with the TTT diagram published by Nishida et al. for bulk Ni52Ti48 [26], it is seen
that in the present cold-rolled material these precipitates are formed at lower temperatures (600 C instead of 700 C at 30 min) and shorter annealing times (30 min instead of 10 h or more at 600 C), possibly due to an increased number of vacancies or other defects assisting the diffusion of atoms, since the starting composition is less favorable. The latter also confirms conclusions from earlier annealing work on less deformed NieTi alloys [27]. For longer annealed samples, i.e., 5 h at 500 C, the formation of Ni4Ti3 precipitates has been observed. This confirms that the matrix initially is rich in Ni and in this case better fits with the TTT diagram from Nishida et al. for Ni52Ti48, although they did not observe any precipitation (except an oxide phase) for equiatomic Ni50Ti50 material which is closer to the present composition of Ni50.2Ti49.8 [26]. The fact that these precipitates are found to nucleate at grain boundaries confirms observations in conventional material with low Ni supersaturation, although possible effects of lattice defects due to the severe deformations should not be neglected [28]. The latter
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memory alloys: a fundamental approach’’ and of a EU RTN program 505226 ‘‘Multi-scale modelling and characterisation for phase transformations in advanced materials’’.
References
Fig. 10. Grain size distribution for as-received, 350 C, 400 C and 450 C annealed samples.
can moreover be expected to induce local strain fields in the matrix due to which the precipitates lose their expected lens shape normally induced by the lattice mismatch between the atomic structures of the precipitate and surrounding B2 matrix. These Ni-rich precipitates leave the matrix deficient in Ni, which raises the martensitic start temperature above room temperature [24], explaining the observation at room temperature of several transformed regions in these samples, although the increase in grain size might also have affected the martensitic start temperature in the same way. In previous experiments including annealing of cold-rolled NieTi no precipitates were described although they are essential in the applications of this material [12,13]. To what extend the observed stabilization of austenite and following precipitation upon annealing have an effect on the shape memory and superelastic behaviour of these ribbons is the subject of ongoing work. 5. Conclusions The effect of cold rolling and subsequent annealing on the microstructure of a Ni50.2Ti49.8 SMA is investigated. Cold rolling of 40% introduces amorphization as well as stabilization of the B2 structure in textured nanograins. Amorphous nanosized pockets as well as amorphous bands are observed in as-received material. Post-deformation annealing for 30 min at or above 350 C leads to the crystallization of the amorphous regions with a gradual increase of the grain size up to 30e50 mm at 800 C. Formation of NiTi2 precipitates occurs at 450 C and Ni3Ti2 precipitates grow above 600 C. Longer time annealing of 5 h at 500 C yields Ni4Ti3 precipitates and grains containing twinned martensite. Acknowledgment This research work was performed with the support of an FWO project G.0465.05 ‘‘The functional properties of shape
[1] Miyazaki S, Ohmi Y, Otsuka K, Suzuki Y. Characteristics of deformation and transformation pseudo-elasticity in TieNi alloys. J Phys 1982; 43(Suppl. 12):C4-255e60. [2] Lin HC, Wu SK. The tensile behaviour of a cold-rolled and reverse transformed equiatomic TiNi alloy. Acta Metall Mater 1994;42: 1623e30. [3] Miyazaki S, Otsuka K. Deformation and transition behavior associated with the R-phase in TieNi alloys. Metall Trans A 1986;17:53e63. [4] Miyazaki S, Otsuka K. Development of shape memory alloys. ISIJ Int 1989;29:353e77. [5] Stroz D, Bojarski Z, Ilczuk J, Lekston Z, Morawiec H. Effect of thermal cycling on as quenched and aged nickel rich NieTi alloy. J Mater Sci 1991;26:1741e8. [6] Takasi T, Nakata Y, Shimizu K. Thermal cycling effects in an aged Nirich TieNi shape memory alloy. Mater Trans JIM 1987;28:883e90. [7] Wurzel D. Marforming and tempering of binary NieTi alloys including precipitation effects. Mater Sci Eng A 1999;273e275:634e8. [8] Brailovski V, Khmelevskaya IYu, Prokoshkin SD, Pushin VG, Ryklina EP, Aliev RZ. Foundations of heat and thermomechanical treatments and their effects on the structure and properties. Phys Met Metallogr 2004;97(Suppl. 1):3e5. [9] Prokoshkin SD, Khmelevskaya IYu, Brailovski V, Trochu F, Turenne S, Turilina VYu. Thermomechanical treatments and their influence on the microstructure and stress/strain diagrams of NiTi shape memory alloys. Can Metall Quart 2004;43:95e9. [10] Koike J, Parkin DM, Nastasi M. Crystal-to-amorphous transformation of NiTi induced by cold rolling. J Mater Res 1990;5:1414e8. [11] Ewert JC, Bo¨hm I, Peter R, Haider F. The role of the martensite transformation for the mechanical amorphization of NiTi. Acta Mater 1997;45: 2197e206. [12] Nakayama H, Tsuchiya K, Umemoto M. Crystal refinement and amorphization by cold rolling in TiNi shape memory alloys. Scr Mater 2001;44: 1781e5. [13] Kim Y-H, Cho G-B, Hur S-G, Jeong S-S, Nam T-H. Nanocrystallization of a Tie50.0Ni(at.%) alloy by cold working and stress/strain behavior. Mater Sci Eng A 2006;438e440:531e5. [14] Otsuka K, Ren X. Physical metallurgy of TieNi-based shape memory alloys. Prog Mater Sci 2005;50:511e678. [15] Sandrock GD, Perkins AJ, Hehemann RF. The premartensitic instability in near-equiatomic TiNi. Metall Trans 1971;2:2769e81. [16] Murakami Y, Shindo D. Changes in microstructure near the R-phase transformation in Ti50Ni48Fe2 studied by in-situ electron microscopy. Philos Mag Lett 2001;81:631e8. [17] Hara T, Ohba T, Otsuka K, Nishida M. Phase transformation and crystal structures of Ti2Ni3 alloys. Mater Trans JIM 1997;38:277e84. [18] Otsuka K, Sawamura T, Shimizu K. Crystal structure and internal defects of equiatomic TiNi martensites. Phys Status Solidi A 1971;5: 457e70. [19] Tadaki T, Nakata Y, Shimizu K, Otsuka K. Crystal structure, composition and morphology of a precipitate in an aged Ti51 at.% Ni shape memory alloy. Trans JIM 1986;27:731e40. [20] Paula AS, Canejo JH, Mahesh KK, Silva RJ, Braz Fernandes FM, Martins RM, et al. Study of the textural evolution in Ti-rich NiTi using synchrotron radiation. Nucl Instrum Methods Phys Res Sect B 2006;246: 206e10. [21] Chang SH, Wu SK. Textures in cold-rolled and annealed Ti50Ni50 shape memory alloy. Scr Mater 2004;50:937e41. [22] Breuer D, Klimanek P. Investigation of texture and dislocation density in rolled b-brass by X-ray and neutron diffraction. Mater Sci Eng A 1997;234e236:818e21.
A.K. Srivastava et al. / Intermetallics 15 (2007) 1538e1547 [23] Waitz T, Kazykhanov V, Karnthaler H-P. Martensitic phase transformations in nanocrystalline NiTi studied by TEM. Acta Mater 2004;52: 137e47. [24] Tang W. Thermodynamic study of the low-temperature phase B190 and the martensitic transformation in near-equiatomic TieNi shape memory alloys. Metall Trans A 1997;28:537e44. [25] Makifuchi Y, Terunuma Y, Nagumo M. Structural relaxation in amorphous NieTi alloys prepared by mechanical alloying. Mater Sci Eng A 1997;226e228:312e6.
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[26] Nishida M, Wayman CM, Honma T. Precipitation processes in near equiatomic TiNi shape memory alloys. Metall Trans A 1986;17: 1505e15. [27] Treppmann D, Hornbogen E, Wurzel D. The effect of combined recrystallization and precipitation processes on the functional and structural properties in NiTi alloys. J Phys IV 1995;5:C8:569e74. [28] Fan G, Chen W, Yang S, Zhu J, Ren X, Otsuka K. Origin of abnormal multi-stage martensitic transformation behavior in aged Ni-rich TieNi shape memory alloys. Acta Mater 2004;52:4351e62.