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I.

INTRODUCTION

SHAPE-RECOVERY behavior of the nearly equiatomic NiTi alloys is sensitive to the thermal and/or mechanical process schedules.[1–3] In case of the Ti–rich alloys, the shape recovery is manifested by the martensite–austenite transformation during heating of the samples.[4–8] In the process of shape recovery, the amount of recoverable strain is the convoluted effect of deformation accommodation due to martensite reorientation (MR), variants coalescence, and detwinning process.[9] The mechanisms related to MR and variants coalescence are determined by the overall microstructural constitution including the contribution of defects, precipitates, and interfaces. On the other hand, the detwinning process is the manifestation of intrinsic mechanical response of the individual martensite variant. In this context, the onset of slip-controlled plastic deformation limits the detwinning process within the martensite variants and adversely affects the shaperecovery behavior in both one-way and two-way modes. Therefore, mechanical behavior of the individual martensite variant is an important component of the observed shape-memory effect (SME). The nanoindentation or instrumented indentation technique has already been established as the reliable tool for measurement of mechanical properties in the microstructural length scale in wide range of materials.[10,11] The available loading modes allow to determine various mechanical parameters viz., hardness, elastic modulus, creep parameters, deformation work, fracture toughness, work hardening behavior,[12–27] etc. The technique has been further employed A. SINHA, Assistant Professor, and S. DATTA, Associate Professor, are with the School of Materials Science and Engineering, Bengal Engineering and Science University, Howrah 711103, India. Contact e-mail: [email protected] P.C. CHAKRABORTI, Professor, is with the Department of Metallurgical and Material Engineering, Jadavpur University, Kolkata 700032, India. P.P. CHATTOPADHYAY, Professor, is with the Department of Metallurgy and Materials Engineering, Bengal Engineering and Science University. Manuscript submitted January 29, 2012. Article published online November 9, 2012 1722—VOLUME 44A, APRIL 2013

extensively to study the pseudoelasticity and stress–strain behavior of NiTi alloy.[5,6,28–33] For example, the onset of predominantly elastic deformation of the twinned martensite was identified and estimated from the nanoindentation response. Rajagopalan et al.[34] has also reported the elastic modulus value of the B19’ martensite prior to twinning using spherical indentation, which has been found to be in agreement with the values obtained from neutron diffraction and macroscopic extensometry. It is well known that the surface deformation profile of an indent can be either sink-in or pile-up depending on the strain-hardening behavior of the materials.[35] In low-strain-hardening materials, pile-up tends to occur due to incompressible plastic deformation, whereas sink-in occurs in the case of high-strain-hardening materi