The Effect of Size on the Deformation Twinning Behavior in Hexagonal Close-Packed Ti and Mg

Transcription

1 JOM, Vol. 64, No. 10, 2012 DOI: /s Ó 2012 TMS (outside the U.S.) The Effect of Size on the Deformation Twinning Behavior in Hexagonal Close-Packed Ti and Mg QIAN YU, 1,2 RAJA K. MISHRA, 3 and ANDREW M. MINOR 1,2,4 1. Department of Materials Science and Engineering, University of California, Berkeley, CA, USA. 2. National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, CA, USA. 3. General Motors Research and Development Center, Warren, MI, USA aminor@berkeley.edu In hexagonal close-packed (HCP) structural materials, the limited activation of different slip mechanisms results in alternative deformation mechanisms, such as twinning, which become relevant to plasticity. As external/internal dimension refinement affects operative mechanisms and is commonly used to tune the mechanical properties of materials, understanding the effect of size on deformation twinning in HCP materials is a critical issue for improving their strength and ductility. Recent in situ and ex situ small-scale testing experiments have generated insights into size effects on twinning by deforming single-crystal systems with different sizes. In this article, we review some of the recent results in this field, including studies of the size-related deformation twinning behavior in Ti, Mg, and their alloys. The effect of size on deformation twinning in these systems is remarkable, resulting in a significant change in the mechanical properties of the materials. Deformation twinning can be restricted by the size effect in certain size regimes and materials but also can be promoted by the presence of surfaces at extremely small scales. The correlation of these two effects in two different HCP materials is discussed. Hexagonal close-packed (HCP) structured magnesium (Mg) and titanium (Ti) metals exhibit very attractive physical properties, both being lightweight structural materials, and Ti, in particular, has high corrosion resistance. In designing the next generation of structural materials, understanding the origin of strength and ductility at the most fundamental level in these materials provides a degree of flexibility to the materials engineer. Both the strength and ductility in metals are determined by the evolution of defects under an applied stress. The high symmetry of cubic crystals gives rise to more than five independent dislocation slip systems to accommodate macroscopic shape changes (formability) at relatively low stresses. However, the HCP structure generally has limited activation of slip systems at low temperatures, resulting in relatively poor ductility. Often, slip is accompanied by twinning to accommodate general plastic deformation in HCP metals. 1 5 Ti and Mg are examples of materials that readily twin during room-temperature deformation. It has been generally established that deformation twinning has significant influence on both the strength and the ductility of materials In advanced alloys such as twinning-induced plasticity (TWIP) steels, deformation twinning has been engineered to enhance ductility and toughness dramatically. 7,8 The coherent twin boundaries (TB) can be especially beneficial to a material s ductility compared to general grain boundaries (GB). In HCP materials, especially in Mg, certain DT modes (i.e., f10 12gh10 1 1i) have been found to be beneficial to the ductility of Mg, whereas others (i.e., f10 11gh10 1 2i) are conjectured to be detrimental. 1,11 Because of the need to enable materials strengthening in industrial applications, processing strategies have been designed to improve the mechanical performance of materials by modifying the microstructure and texture that alter the overall (Published online September 7, 2012) 1235

2 1236 Yu, Mishra, and Minor defect structure 12,13 and defect storage. For HCP materials, irrespective of the chemical composition, dimensional refinement is one common method that is used to tune the mechanical properties. 12,14 17 The grain size refinement, for example, alters slip and twinning behavior as well as grain boundary sliding. A number of studies has explored the influence of grain refinement on grain boundary sliding and slip and a number of studies have focused on the size effect on slip behavior alone. Understanding the effect of size on the deformation twinning behavior in HCP materials has been relatively unexplored, even though it is a critical issue for the development of lightweight structural materials. A number of previous studies has been performed in polycrystalline Ti and Mg metals to explore the size effect on the mechanical behavior and on deformation twinning However, the complications arising from texture characteristics in polycrystalline materials, especially fine grain materials processed by severe plastic deformation, makes direct analysis of the plasticity data difficult and ambiguous. Recently, small-scale testing has enabled the study of single-crystal specimens where in situ mechanical testing techniques provide direct analysis of the deformation mechanisms. 12,19 24 Yu et al. 21 studied the crystal size effect on deformation twinning in a Ti alloy in In this study, they used microcompression and in situ transmission electron microscopy (TEM) nanocompression experiments to study the size-related deformation twinning behavior in Ti alloy singlecrystal samples with external dimensions ranging from several micrometers to approximately 200 nm. One typical sample is shown in the scanning electron microscope (SEM) image in Fig. 1a. In Ti, the stress required for activating twinning was found to increase drastically with decreasing sample size, following a Hall Petch type relationship where the exponent was close to 1. Deformation twinning dominated the plastic deformation at the largest sizes. The twinning shear resulted in a sudden and huge strain change that can be read from the engineering stress strain curves, as shown in Fig. 1b. From the morphology of the deformed sample, the shear steps could be clearly seen (Fig. 2a). The cross-sectional TEM images of these micropillars showed the formation of a large number of deformation twins. Figure 2b shows an example of the deformation twins in a deformed micrometer-scale pillar. Similar to what is observed in bulk materials, the twin bands observed in the micropillar usually have a large aspect ratio, but the average thickness of the twins is decreased compared with that in the bulk samples. This behavior was observed until the pillar diameter reached a value of 1 lm, below which twinning deformation disappeared completely and was replaced by only dislocation plasticity. No evidence of any twinning was found from either load displacement data or microstructure data. Instead, dislocation interaction and multiplication were observed in submicrometer diameter specimens as shown in Fig. 3a. As a result, plastic deformation proceeded in a more continuous fashion as shown in both the in situ movie and the related mechanical curves (Fig. 3b). The stress level corresponding to this transition in deformation mode was over 4 GPa, a value that approaches the ideal strength of Ti alloy. Meanwhile, the Hall Petch-like relation disappeared and the stress at which the pillars deformed saturated at the same level for submicrometer pillars. Micropillar compression results for single-crystal Mg oriented for twinning showed no twinning in samples from approximately 2 lm to 10lm in diameter. Both Lilleodden 25 and Byer et al. 26 tested pure Mg single-crystal pillars with the same [0001] crystal orientation and reported pyramidal and hci type dislocations in the deformed pillars, but no twins. The size regime where the deformation twinning vanished in Mg is larger than that ob- Fig. 1. (a) SEM image of a Ti micropillar before the mechanical test. (b) Stress strain curve from the compression test of the Ti micropillar. 21

3 The Effect of Size on the Deformation Twinning Behavior in Hexagonal Close-Packed Ti and Mg 1237 Fig. 2. (a) A deformed Ti alloy micropillar; the shear step can be clearly seen. (b) Cross-sectional TEM image of the deformed micron Ti alloy pillar in (a), showing deformation twins. 21 Fig. 3. (a) TEM dark-field image captured from an in situ movie showing the dislocation activity during deformation in a submicron Ti alloy pillar, no twinning deformation occurred at this size scale. (b) A stress strain curve of a nanocompression test on a submicron Ti alloy pillar. 21 served in Ti alloy, but the trend of transition in the deformation mode from twinning dominated to dislocation dominated is similar. In Mg, it was also observed that deformation twinning reoccurred during compression of Mg pillars in extremely small samples with diameters approximately 200 nm. 20,27 Ye et al. 20 reported the formation of deformation twinning by performing in situ TEM nanocompression tests on submicron single-crystal pure Mg and Mg-0.2.wt.% Ce alloy grains oriented for deformation twinning. The sample sizes were approximately 200 nm in diameter. Twinning deformation was observed and the twin boundary migrated until almost the entire pillar had been converted to the twinned orientation. The critical stress for twinning nucleation in this case was approximately 1.26 GPa, demonstrating the significant size effect associated with the twining mechanism, which is consistent with what had been observed previously in the Ti-Al alloy. 21 Yu et al. 27 further investigated the deformation twinning behavior in pure Mg single-crystal samples through nanocompression, nanotension, and nanobending tests. The formation of deformation twins was observed under all of these different strain paths. The average nucleation stress for the contraction twin under nanocompression was approximately 750 MPa and that for extension twin in tension was approximately 800 MPa. Because of the stress gradient generated by the taper of the sample and the higher concentration of defects at

4 1238 Yu, Mishra, and Minor the contact surface where a certain amount of roughness existed, the plastic deformation in the small-scale compression testing always initialized from the contact surface, similar to that observed by Ye et al. 20 The result was the heterogeneous nucleation of a single twin. The further growth of the twin resulted in strain softening due to the easy escape of dislocations from the reoriented twinned regime to the sample surface. Interestingly, in nanotension and nanobending tests, where the contact surface issue was avoided, a different structure of deformation twinning was observed. Instead of the heterogeneous nucleation and growth of a twin in compression, a high-density of nanosized twins formed in tension. The average thickness of the nanotwins was approximately 5 nm to 10 nm, and they had a large aspect ratio, spanning the length of the gage section. The nanotwin formation was also observed in nanobending, where they nucleated right at the crack tip. Figure 4a shows a typical nanotension sample before deformation with an inset image of the nanotwinned structure after the in situ deformation. More importantly, the nanotwinned structure had a remarkable influence on both the strength and ductility of samples. Figure 4b shows the comparison of a nanotension and nanocompression curve. There is significant strain hardening in the tensile data, most likely resulting from the high density of nanotwins. The large number of twin boundaries in the nanotwinned structure can serve as barriers to dislocation motion in the sample, similar to the effect demonstrated in TWIP steel. 7,8 The above results show that a strong size effect exists in the deformation behavior of HCP metals and alloys oriented for deformation by twinning. This size effect results in a size regime where twinning does not occur (small Ti alloy samples or micrometer-scale Mg samples). For larger samples, one observes bulk-like behavior while at smaller dimensions, twinning reappears. The Mg data provide a hint of the correlation between the size effect and the surface effect on deformation twinning at the smallest dimensions. In micrometer-scale Mg samples, preexisting bulk dislocations dominate the deformation response while in the nanometer scale Mg samples, the surface nucleation of defects seems to be more important. Correlated surface nucleation of twinning dislocations is believed to be responsible for the formation of many nanotwins in tensile and bending samples as opposed to one large twin observed in the compression sample. The strong size effect on deformation twinning in HCP metals can be understood based on the sizerelated dislocation behavior and the pole mechanism in face-centered cubic 28 metals. It is correlated with the availability of dislocation sources. In the stimulated slip model proposed by Yu et al., 21 the formation of a maturate deformation twinning is accomplished by the stimulated slip plane by plane, and the stimulated slip from plane n to plane n +1 is catalyzed by promoter defects, such as screw dislocation poles. Since the surface in single-crystal materials and the grain boundary in polycrystalline materials will determine the confined volume to restrict the number of dislocations and dislocation activities, it has been commonly accepted that the number of available promoters is less in smaller volumes. Therefore, the smaller the dimension of the sample, the more weakly two adjacent slip planes are coupled by threading screw pole dislocations, and the more difficult it is for deformation twinning. The disappearance of twins in micrometer-scale samples of Mg and Ti alloy confirm to such mechanisms. As the external dimension becomes extremely small, the effect of nucleation at the surface becomes significant. The surface effect in nanoscience and engineering has been broadly studied in thin films, nanowires, and other nanomaterials Fig. 4. (a) Bright-field TEM image of a submicron Mg tensile sample before deformation; the insert TEM image shows the formation of the nanotwinned structure after the in situ nanotensile test. (b) Comparison of the stress strain curves from a nanocompression and a nanotensile test in submicron Mg samples. Strain softening was observed in the nanocompression test, while strain hardening was observed in the nanotensile test. 27

5 The Effect of Size on the Deformation Twinning Behavior in Hexagonal Close-Packed Ti and Mg 1239 Surfaces naturally introduce layers of atoms where the coordination number and the electronic structure of atoms will be different from that of internal atoms. 32 As a consequence, the physical and chemical properties of materials can be significantly different near the surface compared to the bulk. The physical change manifests in a change in the surface energy, surface residual stress, and vacancy distribution as to facilitate the surface nucleation of defects. 32,33 The influence of the surface effect on any property F of the nanoscale sample is directly associated with the surface to volume ratio, which can be simply expressed in the form of F 1/d, where d is the physical dimension of the material. It is obvious that the proportion of surface in the total volume of the material will become higher when the physical dimension of the materials becomes smaller, resulting in increased significance of the surface nucleation of defects. The total volume and surface-to-volume ratio are both affected by the gage section of a mechanical test sample, and both influence the deformation twinning behavior. The correlation between the size effect and the surface effect changes with the sample dimension. As shown in the simple schematic in Fig. 5, at relatively larger scales, as the size of the sample decreases and the number of preexisting sources (or promoters ) for twinning formation decreases, the formation of deformation twinning becomes harder and harder. The change of the stress required for nucleation of deformation twinning with only size reduction is more dramatic than the change of stress for dislocation nucleation and motion as the surface to volume ratio diminishes. 21 As shown on Fig. 5, it is conceivable that there is a size regime where the stress required for deformation twinning is higher than that for nonbasal hc+ai dislocations (both twinning and hc+ai dislocations provide nonbasal deformation components), which would explain the lack of twins seen in the micrometer-scale Mg pillars. 25,26 Simultaneously, the effect of the increased surface area leads to easier nucleation of defects. At the smallest sizes, the effect of easy nucleation of twinning dislocations at the surface can again lead to the appearance of deformation twins, but the relative stress levels for the various mechanisms would differ. With such a view of the formation of twins and nanotwins, one would expect to find nanotwins in the Ti-alloy at smaller dimensions, although this remains to be confirmed. The results discussed above from Mg and Ti alloy demonstrate that the relative size effects differ from material to material when twinning mode is activated. The nucleation and movement of defects are known to be sensitive to composition as well as kinetic factors such as temperature and strain rate. In a broad sense, however, the effect of size can be used to tune the strength of materials by regulating the physical dimension to factor in surface nucleation as well as the stress level operating within the Fig. 5. A schematic showing the correlation of the size effect and the surface effect on deformation twinning. a is a positive exponent, 0< a 1. material. Presumably, the effect of dimensional refinement observed in these small single-crystal materials applies to bulk materials with a confined internal dimension. Understanding this connection between the external size effect and the internal size effect could be a critical step in altering the strength and plasticity in HCP materials. ACKNOWLEDGEMENTS This research was supported by the General Motors Research and Development Center and performed at the National Center for Electron Microscopy and the Advanced Light Source at Lawrence Berkeley National Laboratory, which is supported by the U.S. Department of Energy under Contract # DE-AC02-05CH REFERENCES 1. M.R. Barnett, Mater. Sci. Eng. A Struct. Mater. Prop. Microstruct. Process. 464, 1 (2007). 2. J.W. Christian and S. Mahajan, Prog. Mater. Sci. 39, 1 (1995). 3. M.H. Yoo, Metall. Trans. A 12, 409 (1981). 4. G. Partridge, Met. Rev. 12, 169 (1967). 5. D.G. Westlake, Acta Metall. 14, 442 (1961). 6. K. Lu, L. Lu, and S. Suresh, Science 324, 349 (2009). 7. G. Frommeyer, U. Brux, and P. Neumann, ISIJ Int. 43, 438 (2003). 8. O. Bouaziz, S. Allain, and C. Scott, Scr. Mater. 58, 484 (2008). 9. X.Y. Lou, M. Li, R.K. Boger, S.R. Agnew, and R.H. Wagoner, Int. J. Plast. 23, 44 (2007). 10. T. Zhu, J. Li, A. Samanta, H.G. Kim, and S. Suresh, Proc. Natl. Acad. Sci. USA 104, 3031 (2007). 11. J. Koike, Metall. Mater. Trans. A Phys. Metall. Mater. Sci. 36A, 1689 (2005).

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