Prediction of Indentation Behavior of Superelastic TiNi
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INTRODUCTION
A considerable amount of work has been done to study superelastic and shape memory effects in TiNi alloys. TiNi exhibits superelastic effect through stressinduced martensitic transformation, i.e., phase transformation between parent austenite phase and martensite phase, leading to high strain during deformation. During loading, the parent austenite phase transforms to twinned martensite phase followed by detwinning of martensite, i.e., reorientation of martensitic variants, accompanied by large strain. The reverse transformation from martensite to austenite occurs upon unloading, accompanied by large recoverable strain. Due to superelastic effect, TiNi alloys show enhanced wear and dent resistance compared to other conventional materials.[1] Conventional bearing materials such as AISI 52100, AISI M-50, AISI 440C steels, and carbon steels are susceptible to denting and wear during high loading. It is expected that bearing made from superelastic TiNi alloys provides superior wear and dent resistance during high impact loading conditions. In order to assess the applicability of superelastic TiNi in such applications, a fundamental understanding of its behavior under indentation loading is essential. Instrumented indentation methods have been extensively used in recent years to assess mechanical properties of materials. Hardness and elastic modulus are frequently calculated from load-depth data obtained RABIN NEUPANE, MASc Materials Engineering, and ZOHEIR FARHAT, Associate Professor, are with the Department of Process Engineering and Applied Science, Materials Engineering Program, Dalhousie University, Halifax, NS B3J 2X4, Canada. Contact e-mail: [email protected] Manuscript submitted February 17, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
from micro- and nano-indentation tests. Much work has been undertaken to exploit indentation techniques to study materials and coatings at small indentation scale.[2–10] The study of indentation behavior of conventional materials is complex and involves elastic, elasto-plastic, and plastic deformations underneath an indenter. For superelastic materials, such as TiNi, the above deformations are coupled with a reversible stressinduced martensitic transformation, which adds to the complexity of the problem. Current indentation theories and models[6,11–13] deal mainly with the deformation of single-phase materials with no phase transition and fail to explain behavior when extended to materials that undergo phase transformation during indentation. More recently, researchers attempted to extend current theories to study the mechanical properties of multi-phase functional materials.[14–16] The most commonly employed method in determining hardness and elastic modulus from load–displacement curves, generated from indentation experiments, was developed by Oliver and Pharr.[7] This method is developed for single-phase materials and produces excellent results. However, when applied to multi-phase materials that undergo phase change during indentation, such as superelastic
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