Direct Measurement of the Nanoscale Mechanical Properties of NiTi Shape Memory Alloy
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Direct Measurement of the Nanoscale Mechanical Properties of NiTi Shape Memory Alloy Gordon A. Shaw1, Wendy C. Crone2 Department of Chemistry, University of Wisconsin-Madison Madison, WI 53706, U.S.A. 2 Department of Engineering Physics, University of Wisconsin-Madison Madison, WI 53706, U.S.A. 1
ABSTRACT The mechanical properties of sputter-deposited NiTi shape memory alloy thin films ranging in thickness from 35 nm to 10 µm were examined using nanoindentation and atomic force microscopy (AFM). Indents made in films as thin as 150 nm showed partial shape recovery upon heating, although film thickness was found to have a marked effect on the results. A modified spherical cavity model is used to describe the findings, which suggest that the substrate tends block the shape memory effect as film thickness decreases below a threshold level which is specific to applied load. This has the net effect of decreasing the indent recovery below the critical film thickness. The fact that the spherical cavity model predicts the critical film thickness at which the shape memory effect is blocked indicates that the increased recovery of nanoscale indentations is due to a suppression of plastic processes rather than an enhancement of shape memory processes. INTRODUCTION NiTi shape memory alloy (SMA) is a unique material that combines resilience, good biocompatibility, and suitability for thin film deposition with a fascinating wealth of chemical and physical phenomena. Chief among these is a solid state phase change known as a martensitic transformation which allows for the shape memory effect (SME). As a result, it has been the subject of a great deal of study [1]. Nevertheless, significant questions about the complex behavior of this alloy remain. Among the unanswered questions is how its mechanical properties change as the size scale considered changes from micrometers to nanometers. Various methods have been used to probe the mechanical properties of NiTi, including uniaxial tensile and compressive loading, and microindentation [2,3]. Nanoindentation is well suited for the testing of very small volumes of material. In this technique, a load is applied to a diamond probe, which penetrates the surface of the material being tested. Analysis of the load-displacement curves then yields information about the material, particularly the Young’s modulus, and hardness [4]. Indentation shape recovery has also been demonstrated on the microscale [5]. Our nanoscale work shows good agreement with the microscale data, and also extends the measurements to thin film materials [6] as well as different kinds of deformation (nanoscratch and nanowear) [7]. It also indicates that shape recovery increases markedly at indentation depths less than 100nm. Factors influencing pseudoelastic phenomena at this size scale
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have also been investigated [8]. The present study was performed to examine the micro and nanoscale properties of NiTi SMA by examining the effect of film thickness on the shape memory properties of NiTi. EXPERIMENTAL Ni
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