Effect of Grain Size and Testing Temperature on Low-Cycle Fatigue Behavior and Plastic Deformation Mode of Ti-2Al-2.5Zr
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THE Ti-2Al-2.5Zr alloy is usually applied in the severe conditions of low temperature and dynamic loading, such as in the pipeline system of liquid hydrogen, oxygen in a missile engine, or experimental equipment for polar investigations. So far, most investigations[1–3] have focused on the effect of the strain rate and loading mode on the deformation behavior of the hcp Ti-2Al-2.5Zr alloy at room temperature (RT). To the authors’ best knowledge, little information is available on the cyclic deformation behavior of the hcp Ti-2Al-2.5Zr alloy at low temperature. Slip and twinning are two primary plastic deformation modes in hcp Ti alloys. It is well known that plastic deformation is mainly controlled by the nucleation and motion of dislocations in fcc crystalline materials with medium or high stacking-fault energy (SFE), e.g., Cu, Ni, and Al at RT. Dislocation sources and pileups are H. WANG, Doctoral Student, Y.L. XU, Graduate Student, Q.Y. SUN, Associate Professor, L. XIAO, Professor, and J. SUN, Professor, are with the State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an, 710049, People’s Republic of China. P. GE, Senior Engineer, is with the Northwest Institute for Nonferrous Metal Research, Xi’an 710016, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted January 7, 2009. Article published online August 20, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
expected to exist within individual grains, with deformation twins being difficult to activate.[4,5] In recent years, however, twins and stacking faults were observed in deformed Al, Cu, and Ni when the grain size was refined to nanometer scale.[6–8] The factors that resulted in a transition of the plastic deformation mode from slip to twinning, and their effect on low-cycle fatigue (LCF) life of metals, remain unclear. It is known, however, that twinning plays an important role in maintaining a homogeneous plastic deformation in fcc and bcc materials at low temperature. Twinning usually has a detrimental effect on the mechanical properties of these materials. In contrast, plastic anisotropy was often observed in hcp titanium alloys, due to their limited individual slip modes at RT. Twinning can provide more individual plastic deformation than slip. Little information is available about the dependence of cyclic deformation modes and LCF behavior of the hcp Ti-2Al-2.5Zr alloy on grain size and testing temperature. High-manganese austenitic steel, which is also named twinning-induced plasticity (TWIP) steel, has an fcc crystalline structure with a low SFE of ~40 mJ/m2. Deformation twinning frequently occurs in these alloys, and large strain hardening and good ductility are displayed in this material. Ueji suggested[9] that deformation twinning was severely restricted in fine-grained TWIP steel when the grains were refined from 49.6 to 1.8 lm. Conversely, in hcp metals such as Ti, Zr, and Mg with a high SFE, deformation twinning is a common VOLUME 40
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