Martensitic Transformation in Submicron Cu-Al-Ni Pillar
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Martensitic Transformation in Submicron Cu-Al-Ni Pillar Lifeng Liu1,2, Yumei Zhou1 and Lan Lv1,2 1
State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, PR China 2 Center for Advancing Materials Performance from the Nanoscale (CAMP-Nano) & Hysitron Applied Research Center in China (HARCC), State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, PR China ABSTRACT The transformation plateau on the strain-stress curve is the characteristic of superelasticity of bulk shape memory alloys upon tension/compression loading. However, recent studies show that such transformation plateau is hard to see when the sample size of shape memory alloys decreases to submicrons. In order to see what happened in such small scale samples during loading, in-situ compression test has been done with single crystal Cu-14.2Al4.0Ni (wt %) submicron pillars. Our in-situ observation during compression demonstrates that the stress-induced martensitic transformation indeed occurs in submicron pillars, but is not suppressed. Furthermore, the transformation proceeds in a sequential nucleation-growthnucleation dominated mode, but not the transient way like that in bulk materials. As a result, the stress keeps increasing throughout the transformation and no obvious transformation plateau can be detected. However, the underlying reason for such contrast transformation behaviors between our submicron pillars and bulk materials still needs further investigation. INTRODUCTION Shape memory alloys are widely used in many fields due to its unique properties of superelasticity (SE) and shape memory effect (SME). SE behavior, on account of stress-induced martensitic transformation (SIMT) with a large nonlinear recoverable strain, always appears a flag shaped hysteresis loop with a strain plateau on the strain-stress curve upon loading and unloading [1-3]. The plateau strain (or transformation strain) depends on the transformation shape strain and crystal orientation, so it often characterizes a certain martensitic transformation and can be conversely used to estimate the transformation, per se [2,4-8]. Recent studies show that for small scale pillars, the superelasticity plateau can be rarely seen during loading and unloading [7,14-16]. For example, the 380 nm diameter NiTi pillar, no transformation plateau can be observed from the stress-strain curve; the transformation plateau becomes ill-defined for the CuAlNi microwires when the diameter decreases from 74 μm to 23 μm. Accordingly, it was deduced the transformation was suppressed due to the high critical stress for nucleation [9-13]. Such contrast between bulk materials and small scale pillars has attracted keen attention and many works have been done to understand such interesting phenomenon. However, the underlying mechanism and direct evidence for the disappearance of transformation plateau until now still remains unclear. In order to see what happened during loading and unloading in small scale pillars, we focus o
