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Gradual martensitic transformation of B2 phase on TiCu-based bulk metallic glass composite during deformation
Intermetallics 75 (2016) 1e7
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Gradual martensitic transformation of B2 phase on TiCu-based bulk metallic glass composite during deformation S.H. Hong a, J.T. Kim a, H.J. Park a, Y.S. Kim a, J.Y. Suh b, Y.S. Na c, K.R. Lim c, J.M. Park d, *, K.B. Kim a, ** a Hybrid Materials Center (HMC), Faculty of Nanotechnology and Advanced Materials Engineering, Sejong University, 209 Neugdong-ro, Gwangjin-gu, Seoul 143-747, Republic of Korea b High Temperature Energy Materials Research Center, Korea Institute of Science & Technology (KIST), Hwarangno 14-gil 5, Seoungbuk-gu, Seoul 136-791, Republic of Korea c Light Metal Division, Korea Institute of Materials Science (KIMS), 797 Changwondaero, Seongsan-gu, Changwon-si, Gyeongnam 642-831, Republic of Korea d Global Technology Center (GTC), Samsung Electronics Co., Ltd., 129 Samsung-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do 443-742, Republic of Korea
a r t i c l e i n f o
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Article history: Received 11 November 2015 Received in revised form 10 April 2016 Accepted 4 May 2016
In this study, we investigate the progress of stress-induced martensitic transformation depending on the stress state on TiCu-based bulk metallic glass composite via systematical transmission electron microscopy analysis. On the stage of elastic deformation (before yielding), the martensitic transformation from austenite B2 phase to martensite B190 phase occurred from interface between B2 particle and amorphous matrix with ~50 nm region. Just after yielding, the martensitic transformed region was extended toward B2 particle with distance of ~130 nm from the interface. Moreover, the deformation twinning of martensite B190 phase in martensitic transformed region was also observed. When 1% plastic deformation occurred, the martensitic transformed region reached up to ~600 nm. Based on these results, it is believed that the high level of stress can be concentrated around the interface due to the elastic mismatch of B2 and amorphous phases. As a result, structural gradient morphology originated from phase transition was formed into the B2 particle during early stage of deformation. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Metallic glasses Martensitic transformation Twinning Work-hardening
1. Introduction Bulk metallic glasses (BMGs) have shown unique combinatorial mechanical properties such as high strength and large elastic limit [1e4]. However, unfavorable brittleness of the BMGs originated from shear localization at room temperature leads to catastrophic failure with no plastic manner [5,6]. Moreover, the inhomogeneous shear deformation behavior causes strain softening characteristic of BMGs [7]. These serious drawbacks limit their access to use an engineering material [8,9]. To overcome these disadvantages, several attempts have been found the solution by developing metallic glass matrix composite containing crystalline phases [10e15]. For example, the Ti- and Zr- based bulk metallic glass
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J.M. Park), [email protected] (K.B. Kim). http://dx.doi.org/10.1016/j.intermet.2016.05.002 0966-9795/© 2016 Elsevier Ltd. All rights reserved.
composites (BMGCs) with ductile dendrite phases, such as b-Ti (Zr) phase, have exhibited the improved plasticity through a severe interaction of plastic deformation mechanisms including the dislocations in the crystalline phase and the shear bands in the metallic glass matrix [11,16,17]. However, the BMGCs containing ductile dendritic phases still have a serious problem for engineering application. They show a work-softening with an occurrence of early necking right after yielding during tensile deformation. Therefore, it is noteworthy to develop work-hardenable BMG or BMG composite [18]. Recently, CuZr-based BMGCs containing intermetallic B2eCuZr phases rather than soft dendrites have been reported, which represent obvious plastic deformation and pronounced workhardening behavior after yielding during compression as well as tension [19e23]. They asserted that improved mechanical properties are mainly attributed to a stress-induced martensitic transformation behavior of austenite B2 phase to martensite B190 phase, as B2eNiTi shape memory phases. More recently, Ti-based BMGCs containing B2-phases are developed, which are also shown the
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large plasticity and work-hardening [24e28]. The large plasticity of these BMGCs depends on the volume fraction and length scale of the B2 phases embedded on metallic glass matrix [21,25]. The optimization of volume fraction (20e40 vol%) and length scale (>100 mm) of B2 phase is effective to generate the shear bands and impede the propagation of main shear band due to their higher strain accommodation capacity than that of small B2 phase with size of 10e20 mm [26,28]. In addition, they proposed that the workhardening behavior is strongly related with the hardening of the B2 phases which connected with martensitic transformation and deformation twining [28,29]. Generally, the martensitic transformation and deformation twining phenomenon of B2 phases during deformation have been widely studied in crystalline shape memory alloys (SMAs) [30e33]. Most of studies about these phenomena on SMAs are related with deformation mechanism of B2 phase itself. On the other hand, in this study, spherical B2 particles are surrounded by amorphous matrix with different mechanical characteristics such as higher modulus, large elastic limit and higher hardness [25,29]. This implies that deformation state and behavior of B2 phase in BMGC maybe different with those of SMA. Therefore, it is noteworthy to study the detailed martensitic transformation behavior of B2 particles in BMGCs at the initial stage of deformation to understand the work hardening phenomenon of B2 particle-reinforced BMGCs. In the present study, we explore the relationship between phase transformation and deformation mechanism of B2 phase containing BMGC as well as investigate martensitic transformation behavior of B2 particles on BMGCs at the early stage of deformation through systematical TEM observation [25,28]. In order to understand the progress of martensitic transformation of B2 particle on Ti-based BMGC during deformation, we performed interrupted compression test, i.e., before yielding stage, right after yielding stage and 1% plastic deformation stage and simultaneously examined the corresponding microstructural evolution according to the deformation state. 2. Experimental procedure Ti45.3Cu39.5Ni7.8Zr4.9Sn2.5 ingots were prepared by arc melting with high purity elements of 99.99 at.% in an argon atmosphere. Ingots were remelted at least 5 times to achieve chemical homogeneity. As-cast samples were produced by suction casting into a cylindrical rod-shape Cu mold with diameters of 2 mm and 50 mm in length. In order to analyze the phase and structure of casted rod samples, X-ray diffraction (XRD, Rigaku-D/MAX-2500/PC) with CuKa radiation and transmission election microscope (TEM: Technai F20) were used. Thin film specimens were prepared by ion milling with liquid nitrogen cooling. The cross sectional morphologies of the rod samples were examined by scanning electron microscopy (SEM: JEOL JSM-6390). The cylindrical samples with 2:1 aspect ratio for compression tests were prepared. The deformed samples with different degree of deformation were prepared under compressive mode at a strain rate of 1 103 s1. The lateral surface morphologies of deformed samples were investigated using SEM. Microstructural evolution of deformed samples was investigated using TEM. High resolution transmission electron microscope (HRTEM) was also performed in order to investigate the atomic structure of the samples. 3. Results and discussion Fig. 1 shows the XRD pattern, SEM backscattered electron (BSE) image and TEM bright field (BF) image of the as-cast alloy, together with selected area electron diffraction (SAED) patterns of amorphous matrix and B2 particles, respectively. For Fig. 1(a), the XRD
trace of as-cast sample shows sharp crystalline peaks corresponding B2 phase superimposed on the broad diffraction of amorphous phase. The SEM BSE image in Fig. 1(b) obtained from cross sectional area of the sample reveals typical microstructure of BMGC containing spherical particles (dark contrast) embedded in brighten matrix. The spherical particles (indicated by the arrows) can be identified as B2 phase based on XRD analysis, where the volume fraction of the B2 particles are measured as 31 ± 5 vol% through the SEM BSE images. TEM BF image in Fig. 1(c) presents the enlarged view of crystalline particle embedded amorphous matrix. The inset SAED pattern obtained from crystalline particle represent [001] zone axis of the CsCl-type austenite B2 phase and the superlattice of the {100} plane of the B2 phase is distinctly confirmed. The SAED patterns in Fig. 1(d) obtained from the matrix presents a typical diffuse hallow ring, indicating amorphous phase. The SAED patterns in Fig. 1(e) obtained from the interface area between the amorphous and B2 particle exhibits only spotty diffraction corresponding [001] zone axis of B2 phase with diffuse hallow ring. Based on these results, it is confirmed that the as-cast alloy consists of only micron-scale spherical B2 particles with amorphous matrix, and there is no other crystalline phase in interface area between amorphous matrix and B2 particle. As shown in previous study [28], Ti45.3Cu39.5Ni7.8Zr4.9Sn2.5 BMGC exhibits large plasticity of 13 ± 2%, yield strength of 1645 ± 25 MPa and ultimate strength of 1900 ± 25 MPa, together with pronounced work-hardening. The work-hardening exponent, h, can be estimated based on the Hollomon equation [34]:
S¼ Κ hε where S, K and ε are the true stress, strength coefficient and true strain, respectively. The work-hardening exponent h of the current BMGC was determined to be 0.126, which is much higher than that of alloy steels widely used in structural applications (i.e., 30CrMnSiA: 0.063, 30CrMnSiNi2A: 0.091, 40CrNiMo: 0.066) [35] as well as other Ti-based BMGC containing B2 phase with 28 ± 5 vol% (0.109) [26]. For TieCueNieZr BMGCs including B2 particles with different volume fraction, the work-hardening exponent was calculated from in the previous study [26], which reveals that h can be altered by the volume fraction of B2 phase (i.e., 28 vol%: 0.109, 38 vol%: 0.198, 55 vol%: 0.334, 100 vol%: 0.500). The noticeable work-hardening behavior of current BMGCs is a result of martensitic transformation and deformation twinning of B2 phase embedded in amorphous matrix [28]. In order to investigate progress of martensitic transformation of B2 particle during initial stage of deformation, the three types of deformed samples with different degree of deformation i.e., before yielding stage, just after yielding stage and after 1% plastic deformation stage, were prepared through interrupted compression, as shown in compressive stress-strain curves in Fig. 2(a). The SEM images in Fig. 2(b) and (c) show lateral surface morphologies of elastically deformed sample (before yield stage) and 1% plastically deformed sample, respectively. The lateral surface of elastically deformed sample shows no any evidence of plastic deformation such as shear bands, which indicates that the sample did not suffered a plastic deformation. On the other hand, the shear bands (marked by white arrows) are observed in 1% plastically deformed sample shown in Fig. 2(c). Furthermore, copious slip bands (marked by yellow arrows) discovered in B2 particle as shown in inset SEM image in Fig. 2(c). These results indicate that severe plastic deformation occurred in B2 particle during early stage of plastic deformation. Based on this result, it can be inferred that the highly stress is concentrated at interface area between amorphous matrix and B2 particle during deformation. In order to obtain deep insight for the structural evolutions on
Fig. 1. XRD patterns (a), SEM BSE image (b) and TEM BF image (c) of as-cast alloy, together with SAED patterns obtained from amorphous matrix (d) and interface area between amorphous matrix and B2 particle (e). Inset shows the SAED patterns obtained from B2 particle.
Fig. 2. True compressive stress-strain curves (a) of as-cast alloys and corresponding SEM BSE images obtained from lateral surface of elastically deformed alloy (b) and 1% plastically deformed alloy (c).
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B2 particle, we performed the TEM analysis for the three kinds of deformed samples. Fig. 3 illustrates TEM BF image (a), HRTEM images (b)-(d) and fast Fourier transform (FFT) obtained from elastically deformed sample (before yield stage). Fig. 3(b)-(d) exhibit the HRTEM images corresponding to i), ii) and iii) areas in Fig. 3(a), together with inset FFT patterns obtained from each HRTEM images, respectively. The inset FFT pattern in Fig. 3(b) and (c) were indexed as [001] zone axis of the B2 phase, respectively. This indicates that there is no phase transformation in i) and ii) areas. On the other hand, the HRTEM image (Fig. 3(d)) obtained from iii) area in Fig. 3(a), which is much closed area to the interface, reveals distinct two regions. The upper inset FFT pattern corresponding to region 1 (left side of dotted line) shows diffraction spots corresponding [001] zone axis of B2 phase, whereas lower inset FFT pattern corresponding region 2 which is identified as [210] zone axis of monoclinic B190 phase, demonstrating the stressinduced martensitic transformation from austenite B2 phase to martensite B190 phase. This is clear evidence of the stress-induced martensitic transformation occurred in vicinity of interface between amorphous matrix and B2 phase, and the distance of martensitic transformed region was less than 50 nm. Similar to NiTi phase, the occurrence of stress-induced martensitic transformation zone on B2 phases requires critical applied stress for martensitic transformation [36]. Therefore, it is believed that higher stress beyond the critical stress for martensitic transformation applied at edge region of B2 particle, leading to the formation of martensitic
transformed area with distance of 50 nm due to their elastic modulus mismatch [24]. Fig. 4 shows TEM BF, HRTEM images and corresponding FFT patterns obtained from right after yielded sample. TEM BF image in Fig. 4(a) shows interface region of the micron-scale B2 particle embedded in amorphous matrix. Fig. 4(b) displays HRTEM image and FFT pattern obtained from i) area in Fig. 4(a). The inset FFT pattern exhibits the diffraction spot corresponding [110] zone axis of B2 phase with superlattice of the {100} plane of the B2 phase, indicating well-ordered structure of B2 phase. Fig. 4(c) shows the HRTEM image obtained from ii) area with the distance of 130 nm from the interface between amorphous matrix and B2 particle. Similar to HRTEM image in Fig. 3(d), the FFT patterns obtained from region 1 and 2 in Fig. 4(c) reveals two kinds of diffraction spots. The diffraction spots corresponding region 1 (upper inset FFT) were indexed [110] zone axis of B2 phase, whereas the lower FFT pattern corresponding region 2 reveals weak sub diffraction spots (marked by yellow arrows) superimposed on diffraction spots corresponding [110] zone axis of B2 phase. These sub diffraction spots are identified as [100] zone axis corresponding monoclinic B190 phase, representing occurrence of the stress-induced martensitic transformation. HRTEM image in Fig. 4(d) obtained from iii) area exhibits closed interface region of amorphous matrix and B2 particle. The inset FFT patterns obtained B2 particle region indexed as [001] zone axis of twinned monoclinic B190 phase (marked by black and yellow arrows in center) superimposed on the [110] zone axis of B2 phase.
Fig. 3. TEM BF image (a), HRTEM images (b)e(d) corresponding i), ii) and iii) areas in Fig. 3(a) with corresponding inset FFT patterns obtained from elastically deformed alloy.
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Fig. 4. TEM BF image (a), HRTEM images (b)e(d) corresponding i), ii) and iii) areas in Fig. 4(a) with corresponding inset FFT patterns obtained from right after yielded alloy.
These results reveal that the stress-induced martensitic transformed region is extended to ~130 nm. Moreover, nano-scale deformation-induced twining of the martensite B190 phase is observed in the vicinity of interface on right after yield stage, which can be considered as an evidence of local plastic deformation of the particle [46]. From these results, it is feasible that the increase of stress concentration at interface can lead to acceleration of martensitic transformation as well as deformation twinning. Fig. 5 displays TEM BF image, HRTEM images and corresponding FFT obtained from 1% plastically deformed sample. TEM BF image in Fig. 5(a) shows the micron-scale B2 particle embedded in amorphous matrix. The i) area in Fig. 5(a) remains the B2 structure even the edge region of B2 particle undergoes severe plastic deformation [as shown in Fig. 2(c)], as confirms in HRTEM image and corresponding inset FFT pattern in Fig. 5(b). In contrast, the FFT pattern obtained from HRTEM image in Fig. 5(c) corresponding the ii) area with distance of ~600 nm from the interface reveals two kind of diffraction spots corresponding [110] zone axis of B2 phase (as marked by white arrows) and [100] zone axis of B190 phase (as marked by yellow arrows), indicating occurrence of stress-induced martensitic transformation. Moreover, as observed in right after yielded sample (in Fig. 4), the deformation-induced twining of martensite B190 phase is also observed through the inset FFT pattern in Fig. 5(d) corresponding iii) area in Fig. 5(a), exhibiting diffraction spots of [001] zone axis of twinned B190 phase (marked by black and yellow arrows in center) superimposed on the [110]
zone axis of B2 phase. And the twin boundary of twinned B190 phase is also observed in Fig. 5(d) (marked by yellow dotted line). These results indicate that the much higher stress than critical stress for martensitic transformation of B2 phase is applied to further area (~600 nm) from the interface between amorphous matrix and B2 particle. Based on TEM investigation for the microstructural evolution depending on the degree of stress-strain state, it was confirmed that the stress-induced martensitic transformation from austenite B2 phase to martensite B190 phase gradually occurred from the vicinity of interface between amorphous matrix and B2 particle to inside region of B2 particle with increasing strain. At the early stage of deformation, the origin of stress concentration at the interface is related to elastic modulus mismatch. The elastic match of reinforcing crystalline phase and amorphous matrix is the key to decide mechanical properties of BMGCs [24,29]. In current BMGC, the B2 phase has relatively lower elastic modulus as compared with that of amorphous matrix (87.8 ± 3.9 and 121.7 ± 2.5 GPa). Large difference of elastic modulus between amorphous matrix and B2 phase induces highly stress concentration at the interface between those phases upon loading. With increasing strain on elastic deformation stage, the stress concentration on the interface can be released by stress-induced martensitic transformation of B2 phase at the vicinity of interface between amorphous matrix and B2 particle (see Fig. 4). With the further increase of strain to early plastic deformation stage, the stress was continually concentrated on the
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Fig. 5. TEM BF image (a), HRTEM images (b)e(d) corresponding i), ii) and iii) areas in Fig. 5(a) with corresponding inset FFT patterns obtained from 1% plastically deformed alloy.
interface, which leads occurrence of gradual stress-induced martensitic transformation zone from vicinity of interface to further inside regions of B2 particles (shown in Figs. 4 and 5). In the TRIP steels, it has been confirmed that the critical applied stress for martensitic transformation decreases due to the plastic deformation of austenite phase, and the plastic deformation of austenite phase leads locally stress concentration at obstacles, such as twin boundaries, deformation bands, interfaces, etc., and thus the concentrated stress becomes equivalent for the austenite phase [36,38]. Alternatively, some studies on NiTi SMAs reported the reversible martensitic transformation after unloading in imposed stress [39e41]. The reverse transformation must occur upon unloading, since the formed martensite is completely unstable at temperatures above austenite finish temperature (Af) in the absence of stress [42]. However, when the NiTi alloy has deformed significantly, the dislocations were activated in the stress-induced martensite phase, which hinders the reverse phase transformation to the B2 phase, even the deformed alloy heat treated above austenite finish temperature [43e45]. In this study, the gradual martensitic transformation process of B2 particle on early deformation stage was investigated by TEM observation and interrupted compression test. The observed martensitic transformed zone on before yielding, right after yielding and 1% plastic deformation stage is about ~50, ~130 and ~600 nm. The distance of these martensitic transformed zones can be influenced by the degree of applied stress concentration and reversible martensitic
transformation after unloading the imposed stress. The systematical microstructural investigation obtained from each deformed sample clearly shows abnormal microstructural evolution with gradual martensitic transformation of B2 particle on the early stage of plastic deformation. This gradual stress-induced martensitic transformation process of B2 particle embedded on amorphous matrix on early deformation stage is definitely disentangled compared with martensitic transformation of B2 particle on large plastically deformed BMGCs [23,29,37]. Concerning the microstructural evolution of BMG matrix, a few number of tiny primary shear bands are formed around B2 particle, which have slightly interaction with B2 particle at the early stage of plastic deformation [27,29]. For late plastic deformation stage, a large number of primary and secondary shear bands are formed around B2 particle in the BMG matrix. At the same time, hierarchical lath martensite plates with deformation twinning were observed inside the large strained B2 particles. From these results, it can be speculated that the stress-induced martensites with parallel slats also can form in inside of largely plastically deformed B2 particle on the late plastic deformation stage of the current BMGC. 4. Conclusions The martensitic transformation behavior of B2 particles on TiCubased BMGC containing micron-scale spherical B2 particles was systemically investigated by TEM observation at the different
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deformation stages. Upon loading, the stress-induced martensitic transformation from austenite B2 phase to martensite B190 phase occurs firstly at the vicinity of the interface between amorphous matrix and B2 particle before yielding. With the increase of strain to 1% plastic deformation stage, martensitic transformed region of B2 phase was gradually extended from interface to inside of B2 particles with deformation twinning of martensite B190 phase. From these observations, we realized that stress-induced martensitic transformation at the interface was attributed to the elastic mismatch of amorphous phase and B2 phase. The stress-induced martensitic transformed zone is gradually extended from interface region to inside region of B2 phase depending on the deformation state.
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Acknowledgments
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This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (Ministry of Science, ICT & Future Planning) (No. 2013R1A2A2A05006550), Industrial Infrastructure Program for fundamental technologies (Project number: N0000846) and the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning(KETEP) granted financial resource from the Ministry of Trade, Industry & Energy (MOTIE, Korea). (No. 20142020103910). And the author specially thanks C.H. Shim at Advanced Analysis Center, KIST, for his technical support on TEM operation.
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Gradual martensitic transformation of B2 phase on TiCu-based bulk metallic glass composite during deformation S.H. Hong a, J.T. Kim a, H.J. Park a, Y.S. Kim a, J.Y. Suh b, Y.S. Na c, K.R. Lim c, J.M. Park d, *, K.B. Kim a, ** a Hybrid Materials Center (HMC), Faculty of Nanotechnology and Advanced Materials Engineering, Sejong University, 209 Neugdong-ro, Gwangjin-gu, Seoul 143-747, Republic of Korea b High Temperature Energy Materials Research Center, Korea Institute of Science & Technology (KIST), Hwarangno 14-gil 5, Seoungbuk-gu, Seoul 136-791, Republic of Korea c Light Metal Division, Korea Institute of Materials Science (KIMS), 797 Changwondaero, Seongsan-gu, Changwon-si, Gyeongnam 642-831, Republic of Korea d Global Technology Center (GTC), Samsung Electronics Co., Ltd., 129 Samsung-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do 443-742, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 November 2015 Received in revised form 10 April 2016 Accepted 4 May 2016
In this study, we investigate the progress of stress-induced martensitic transformation depending on the stress state on TiCu-based bulk metallic glass composite via systematical transmission electron microscopy analysis. On the stage of elastic deformation (before yielding), the martensitic transformation from austenite B2 phase to martensite B190 phase occurred from interface between B2 particle and amorphous matrix with ~50 nm region. Just after yielding, the martensitic transformed region was extended toward B2 particle with distance of ~130 nm from the interface. Moreover, the deformation twinning of martensite B190 phase in martensitic transformed region was also observed. When 1% plastic deformation occurred, the martensitic transformed region reached up to ~600 nm. Based on these results, it is believed that the high level of stress can be concentrated around the interface due to the elastic mismatch of B2 and amorphous phases. As a result, structural gradient morphology originated from phase transition was formed into the B2 particle during early stage of deformation. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Metallic glasses Martensitic transformation Twinning Work-hardening
1. Introduction Bulk metallic glasses (BMGs) have shown unique combinatorial mechanical properties such as high strength and large elastic limit [1e4]. However, unfavorable brittleness of the BMGs originated from shear localization at room temperature leads to catastrophic failure with no plastic manner [5,6]. Moreover, the inhomogeneous shear deformation behavior causes strain softening characteristic of BMGs [7]. These serious drawbacks limit their access to use an engineering material [8,9]. To overcome these disadvantages, several attempts have been found the solution by developing metallic glass matrix composite containing crystalline phases [10e15]. For example, the Ti- and Zr- based bulk metallic glass
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J.M. Park), [email protected] (K.B. Kim). http://dx.doi.org/10.1016/j.intermet.2016.05.002 0966-9795/© 2016 Elsevier Ltd. All rights reserved.
composites (BMGCs) with ductile dendrite phases, such as b-Ti (Zr) phase, have exhibited the improved plasticity through a severe interaction of plastic deformation mechanisms including the dislocations in the crystalline phase and the shear bands in the metallic glass matrix [11,16,17]. However, the BMGCs containing ductile dendritic phases still have a serious problem for engineering application. They show a work-softening with an occurrence of early necking right after yielding during tensile deformation. Therefore, it is noteworthy to develop work-hardenable BMG or BMG composite [18]. Recently, CuZr-based BMGCs containing intermetallic B2eCuZr phases rather than soft dendrites have been reported, which represent obvious plastic deformation and pronounced workhardening behavior after yielding during compression as well as tension [19e23]. They asserted that improved mechanical properties are mainly attributed to a stress-induced martensitic transformation behavior of austenite B2 phase to martensite B190 phase, as B2eNiTi shape memory phases. More recently, Ti-based BMGCs containing B2-phases are developed, which are also shown the
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large plasticity and work-hardening [24e28]. The large plasticity of these BMGCs depends on the volume fraction and length scale of the B2 phases embedded on metallic glass matrix [21,25]. The optimization of volume fraction (20e40 vol%) and length scale (>100 mm) of B2 phase is effective to generate the shear bands and impede the propagation of main shear band due to their higher strain accommodation capacity than that of small B2 phase with size of 10e20 mm [26,28]. In addition, they proposed that the workhardening behavior is strongly related with the hardening of the B2 phases which connected with martensitic transformation and deformation twining [28,29]. Generally, the martensitic transformation and deformation twining phenomenon of B2 phases during deformation have been widely studied in crystalline shape memory alloys (SMAs) [30e33]. Most of studies about these phenomena on SMAs are related with deformation mechanism of B2 phase itself. On the other hand, in this study, spherical B2 particles are surrounded by amorphous matrix with different mechanical characteristics such as higher modulus, large elastic limit and higher hardness [25,29]. This implies that deformation state and behavior of B2 phase in BMGC maybe different with those of SMA. Therefore, it is noteworthy to study the detailed martensitic transformation behavior of B2 particles in BMGCs at the initial stage of deformation to understand the work hardening phenomenon of B2 particle-reinforced BMGCs. In the present study, we explore the relationship between phase transformation and deformation mechanism of B2 phase containing BMGC as well as investigate martensitic transformation behavior of B2 particles on BMGCs at the early stage of deformation through systematical TEM observation [25,28]. In order to understand the progress of martensitic transformation of B2 particle on Ti-based BMGC during deformation, we performed interrupted compression test, i.e., before yielding stage, right after yielding stage and 1% plastic deformation stage and simultaneously examined the corresponding microstructural evolution according to the deformation state. 2. Experimental procedure Ti45.3Cu39.5Ni7.8Zr4.9Sn2.5 ingots were prepared by arc melting with high purity elements of 99.99 at.% in an argon atmosphere. Ingots were remelted at least 5 times to achieve chemical homogeneity. As-cast samples were produced by suction casting into a cylindrical rod-shape Cu mold with diameters of 2 mm and 50 mm in length. In order to analyze the phase and structure of casted rod samples, X-ray diffraction (XRD, Rigaku-D/MAX-2500/PC) with CuKa radiation and transmission election microscope (TEM: Technai F20) were used. Thin film specimens were prepared by ion milling with liquid nitrogen cooling. The cross sectional morphologies of the rod samples were examined by scanning electron microscopy (SEM: JEOL JSM-6390). The cylindrical samples with 2:1 aspect ratio for compression tests were prepared. The deformed samples with different degree of deformation were prepared under compressive mode at a strain rate of 1 103 s1. The lateral surface morphologies of deformed samples were investigated using SEM. Microstructural evolution of deformed samples was investigated using TEM. High resolution transmission electron microscope (HRTEM) was also performed in order to investigate the atomic structure of the samples. 3. Results and discussion Fig. 1 shows the XRD pattern, SEM backscattered electron (BSE) image and TEM bright field (BF) image of the as-cast alloy, together with selected area electron diffraction (SAED) patterns of amorphous matrix and B2 particles, respectively. For Fig. 1(a), the XRD
trace of as-cast sample shows sharp crystalline peaks corresponding B2 phase superimposed on the broad diffraction of amorphous phase. The SEM BSE image in Fig. 1(b) obtained from cross sectional area of the sample reveals typical microstructure of BMGC containing spherical particles (dark contrast) embedded in brighten matrix. The spherical particles (indicated by the arrows) can be identified as B2 phase based on XRD analysis, where the volume fraction of the B2 particles are measured as 31 ± 5 vol% through the SEM BSE images. TEM BF image in Fig. 1(c) presents the enlarged view of crystalline particle embedded amorphous matrix. The inset SAED pattern obtained from crystalline particle represent [001] zone axis of the CsCl-type austenite B2 phase and the superlattice of the {100} plane of the B2 phase is distinctly confirmed. The SAED patterns in Fig. 1(d) obtained from the matrix presents a typical diffuse hallow ring, indicating amorphous phase. The SAED patterns in Fig. 1(e) obtained from the interface area between the amorphous and B2 particle exhibits only spotty diffraction corresponding [001] zone axis of B2 phase with diffuse hallow ring. Based on these results, it is confirmed that the as-cast alloy consists of only micron-scale spherical B2 particles with amorphous matrix, and there is no other crystalline phase in interface area between amorphous matrix and B2 particle. As shown in previous study [28], Ti45.3Cu39.5Ni7.8Zr4.9Sn2.5 BMGC exhibits large plasticity of 13 ± 2%, yield strength of 1645 ± 25 MPa and ultimate strength of 1900 ± 25 MPa, together with pronounced work-hardening. The work-hardening exponent, h, can be estimated based on the Hollomon equation [34]:
S¼ Κ hε where S, K and ε are the true stress, strength coefficient and true strain, respectively. The work-hardening exponent h of the current BMGC was determined to be 0.126, which is much higher than that of alloy steels widely used in structural applications (i.e., 30CrMnSiA: 0.063, 30CrMnSiNi2A: 0.091, 40CrNiMo: 0.066) [35] as well as other Ti-based BMGC containing B2 phase with 28 ± 5 vol% (0.109) [26]. For TieCueNieZr BMGCs including B2 particles with different volume fraction, the work-hardening exponent was calculated from in the previous study [26], which reveals that h can be altered by the volume fraction of B2 phase (i.e., 28 vol%: 0.109, 38 vol%: 0.198, 55 vol%: 0.334, 100 vol%: 0.500). The noticeable work-hardening behavior of current BMGCs is a result of martensitic transformation and deformation twinning of B2 phase embedded in amorphous matrix [28]. In order to investigate progress of martensitic transformation of B2 particle during initial stage of deformation, the three types of deformed samples with different degree of deformation i.e., before yielding stage, just after yielding stage and after 1% plastic deformation stage, were prepared through interrupted compression, as shown in compressive stress-strain curves in Fig. 2(a). The SEM images in Fig. 2(b) and (c) show lateral surface morphologies of elastically deformed sample (before yield stage) and 1% plastically deformed sample, respectively. The lateral surface of elastically deformed sample shows no any evidence of plastic deformation such as shear bands, which indicates that the sample did not suffered a plastic deformation. On the other hand, the shear bands (marked by white arrows) are observed in 1% plastically deformed sample shown in Fig. 2(c). Furthermore, copious slip bands (marked by yellow arrows) discovered in B2 particle as shown in inset SEM image in Fig. 2(c). These results indicate that severe plastic deformation occurred in B2 particle during early stage of plastic deformation. Based on this result, it can be inferred that the highly stress is concentrated at interface area between amorphous matrix and B2 particle during deformation. In order to obtain deep insight for the structural evolutions on
Fig. 1. XRD patterns (a), SEM BSE image (b) and TEM BF image (c) of as-cast alloy, together with SAED patterns obtained from amorphous matrix (d) and interface area between amorphous matrix and B2 particle (e). Inset shows the SAED patterns obtained from B2 particle.
Fig. 2. True compressive stress-strain curves (a) of as-cast alloys and corresponding SEM BSE images obtained from lateral surface of elastically deformed alloy (b) and 1% plastically deformed alloy (c).
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B2 particle, we performed the TEM analysis for the three kinds of deformed samples. Fig. 3 illustrates TEM BF image (a), HRTEM images (b)-(d) and fast Fourier transform (FFT) obtained from elastically deformed sample (before yield stage). Fig. 3(b)-(d) exhibit the HRTEM images corresponding to i), ii) and iii) areas in Fig. 3(a), together with inset FFT patterns obtained from each HRTEM images, respectively. The inset FFT pattern in Fig. 3(b) and (c) were indexed as [001] zone axis of the B2 phase, respectively. This indicates that there is no phase transformation in i) and ii) areas. On the other hand, the HRTEM image (Fig. 3(d)) obtained from iii) area in Fig. 3(a), which is much closed area to the interface, reveals distinct two regions. The upper inset FFT pattern corresponding to region 1 (left side of dotted line) shows diffraction spots corresponding [001] zone axis of B2 phase, whereas lower inset FFT pattern corresponding region 2 which is identified as [210] zone axis of monoclinic B190 phase, demonstrating the stressinduced martensitic transformation from austenite B2 phase to martensite B190 phase. This is clear evidence of the stress-induced martensitic transformation occurred in vicinity of interface between amorphous matrix and B2 phase, and the distance of martensitic transformed region was less than 50 nm. Similar to NiTi phase, the occurrence of stress-induced martensitic transformation zone on B2 phases requires critical applied stress for martensitic transformation [36]. Therefore, it is believed that higher stress beyond the critical stress for martensitic transformation applied at edge region of B2 particle, leading to the formation of martensitic
transformed area with distance of 50 nm due to their elastic modulus mismatch [24]. Fig. 4 shows TEM BF, HRTEM images and corresponding FFT patterns obtained from right after yielded sample. TEM BF image in Fig. 4(a) shows interface region of the micron-scale B2 particle embedded in amorphous matrix. Fig. 4(b) displays HRTEM image and FFT pattern obtained from i) area in Fig. 4(a). The inset FFT pattern exhibits the diffraction spot corresponding [110] zone axis of B2 phase with superlattice of the {100} plane of the B2 phase, indicating well-ordered structure of B2 phase. Fig. 4(c) shows the HRTEM image obtained from ii) area with the distance of 130 nm from the interface between amorphous matrix and B2 particle. Similar to HRTEM image in Fig. 3(d), the FFT patterns obtained from region 1 and 2 in Fig. 4(c) reveals two kinds of diffraction spots. The diffraction spots corresponding region 1 (upper inset FFT) were indexed [110] zone axis of B2 phase, whereas the lower FFT pattern corresponding region 2 reveals weak sub diffraction spots (marked by yellow arrows) superimposed on diffraction spots corresponding [110] zone axis of B2 phase. These sub diffraction spots are identified as [100] zone axis corresponding monoclinic B190 phase, representing occurrence of the stress-induced martensitic transformation. HRTEM image in Fig. 4(d) obtained from iii) area exhibits closed interface region of amorphous matrix and B2 particle. The inset FFT patterns obtained B2 particle region indexed as [001] zone axis of twinned monoclinic B190 phase (marked by black and yellow arrows in center) superimposed on the [110] zone axis of B2 phase.
Fig. 3. TEM BF image (a), HRTEM images (b)e(d) corresponding i), ii) and iii) areas in Fig. 3(a) with corresponding inset FFT patterns obtained from elastically deformed alloy.
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Fig. 4. TEM BF image (a), HRTEM images (b)e(d) corresponding i), ii) and iii) areas in Fig. 4(a) with corresponding inset FFT patterns obtained from right after yielded alloy.
These results reveal that the stress-induced martensitic transformed region is extended to ~130 nm. Moreover, nano-scale deformation-induced twining of the martensite B190 phase is observed in the vicinity of interface on right after yield stage, which can be considered as an evidence of local plastic deformation of the particle [46]. From these results, it is feasible that the increase of stress concentration at interface can lead to acceleration of martensitic transformation as well as deformation twinning. Fig. 5 displays TEM BF image, HRTEM images and corresponding FFT obtained from 1% plastically deformed sample. TEM BF image in Fig. 5(a) shows the micron-scale B2 particle embedded in amorphous matrix. The i) area in Fig. 5(a) remains the B2 structure even the edge region of B2 particle undergoes severe plastic deformation [as shown in Fig. 2(c)], as confirms in HRTEM image and corresponding inset FFT pattern in Fig. 5(b). In contrast, the FFT pattern obtained from HRTEM image in Fig. 5(c) corresponding the ii) area with distance of ~600 nm from the interface reveals two kind of diffraction spots corresponding [110] zone axis of B2 phase (as marked by white arrows) and [100] zone axis of B190 phase (as marked by yellow arrows), indicating occurrence of stress-induced martensitic transformation. Moreover, as observed in right after yielded sample (in Fig. 4), the deformation-induced twining of martensite B190 phase is also observed through the inset FFT pattern in Fig. 5(d) corresponding iii) area in Fig. 5(a), exhibiting diffraction spots of [001] zone axis of twinned B190 phase (marked by black and yellow arrows in center) superimposed on the [110]
zone axis of B2 phase. And the twin boundary of twinned B190 phase is also observed in Fig. 5(d) (marked by yellow dotted line). These results indicate that the much higher stress than critical stress for martensitic transformation of B2 phase is applied to further area (~600 nm) from the interface between amorphous matrix and B2 particle. Based on TEM investigation for the microstructural evolution depending on the degree of stress-strain state, it was confirmed that the stress-induced martensitic transformation from austenite B2 phase to martensite B190 phase gradually occurred from the vicinity of interface between amorphous matrix and B2 particle to inside region of B2 particle with increasing strain. At the early stage of deformation, the origin of stress concentration at the interface is related to elastic modulus mismatch. The elastic match of reinforcing crystalline phase and amorphous matrix is the key to decide mechanical properties of BMGCs [24,29]. In current BMGC, the B2 phase has relatively lower elastic modulus as compared with that of amorphous matrix (87.8 ± 3.9 and 121.7 ± 2.5 GPa). Large difference of elastic modulus between amorphous matrix and B2 phase induces highly stress concentration at the interface between those phases upon loading. With increasing strain on elastic deformation stage, the stress concentration on the interface can be released by stress-induced martensitic transformation of B2 phase at the vicinity of interface between amorphous matrix and B2 particle (see Fig. 4). With the further increase of strain to early plastic deformation stage, the stress was continually concentrated on the
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Fig. 5. TEM BF image (a), HRTEM images (b)e(d) corresponding i), ii) and iii) areas in Fig. 5(a) with corresponding inset FFT patterns obtained from 1% plastically deformed alloy.
interface, which leads occurrence of gradual stress-induced martensitic transformation zone from vicinity of interface to further inside regions of B2 particles (shown in Figs. 4 and 5). In the TRIP steels, it has been confirmed that the critical applied stress for martensitic transformation decreases due to the plastic deformation of austenite phase, and the plastic deformation of austenite phase leads locally stress concentration at obstacles, such as twin boundaries, deformation bands, interfaces, etc., and thus the concentrated stress becomes equivalent for the austenite phase [36,38]. Alternatively, some studies on NiTi SMAs reported the reversible martensitic transformation after unloading in imposed stress [39e41]. The reverse transformation must occur upon unloading, since the formed martensite is completely unstable at temperatures above austenite finish temperature (Af) in the absence of stress [42]. However, when the NiTi alloy has deformed significantly, the dislocations were activated in the stress-induced martensite phase, which hinders the reverse phase transformation to the B2 phase, even the deformed alloy heat treated above austenite finish temperature [43e45]. In this study, the gradual martensitic transformation process of B2 particle on early deformation stage was investigated by TEM observation and interrupted compression test. The observed martensitic transformed zone on before yielding, right after yielding and 1% plastic deformation stage is about ~50, ~130 and ~600 nm. The distance of these martensitic transformed zones can be influenced by the degree of applied stress concentration and reversible martensitic
transformation after unloading the imposed stress. The systematical microstructural investigation obtained from each deformed sample clearly shows abnormal microstructural evolution with gradual martensitic transformation of B2 particle on the early stage of plastic deformation. This gradual stress-induced martensitic transformation process of B2 particle embedded on amorphous matrix on early deformation stage is definitely disentangled compared with martensitic transformation of B2 particle on large plastically deformed BMGCs [23,29,37]. Concerning the microstructural evolution of BMG matrix, a few number of tiny primary shear bands are formed around B2 particle, which have slightly interaction with B2 particle at the early stage of plastic deformation [27,29]. For late plastic deformation stage, a large number of primary and secondary shear bands are formed around B2 particle in the BMG matrix. At the same time, hierarchical lath martensite plates with deformation twinning were observed inside the large strained B2 particles. From these results, it can be speculated that the stress-induced martensites with parallel slats also can form in inside of largely plastically deformed B2 particle on the late plastic deformation stage of the current BMGC. 4. Conclusions The martensitic transformation behavior of B2 particles on TiCubased BMGC containing micron-scale spherical B2 particles was systemically investigated by TEM observation at the different
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deformation stages. Upon loading, the stress-induced martensitic transformation from austenite B2 phase to martensite B190 phase occurs firstly at the vicinity of the interface between amorphous matrix and B2 particle before yielding. With the increase of strain to 1% plastic deformation stage, martensitic transformed region of B2 phase was gradually extended from interface to inside of B2 particles with deformation twinning of martensite B190 phase. From these observations, we realized that stress-induced martensitic transformation at the interface was attributed to the elastic mismatch of amorphous phase and B2 phase. The stress-induced martensitic transformed zone is gradually extended from interface region to inside region of B2 phase depending on the deformation state.
[18]
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Acknowledgments
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This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (Ministry of Science, ICT & Future Planning) (No. 2013R1A2A2A05006550), Industrial Infrastructure Program for fundamental technologies (Project number: N0000846) and the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning(KETEP) granted financial resource from the Ministry of Trade, Industry & Energy (MOTIE, Korea). (No. 20142020103910). And the author specially thanks C.H. Shim at Advanced Analysis Center, KIST, for his technical support on TEM operation.
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