Superelasticity of TiNi-based shape memory alloys at micro/nanoscale
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Seiichiro Ii Structural Materials Unit, Research Center of Strategic Materials, National Institute for Materials Science, Tsukuba, Japan
Shyi-Kaan Wu and Chun-Hway Hsueha) Department of Materials Science and Engineering, National Taiwan University, Taipei 10617, Taiwan (Received 30 June 2014; accepted 14 October 2014)
Superelasticity of shape memory alloy (SMA) results from the reversible thermoelastic martensitic transformation. Although this property has been studied extensively at the macroscale, the study of this superelastic behavior at the micro/nanoscale is relatively new. In this work, we processed TiNi-based SMAs with different compositions and different phase transformation temperatures. Nanoindentations were performed with different peak loads and at various temperatures to systematically characterize the degree of localized stress-induced martensitic transformation at the nanoscale for each SMA. Micropillar compression tests were also performed to study the global superelastic behavior at the microscale. The physics of stress-induced martensitic transformation versus the phase transformation temperature, the testing temperature, and the peak load relations was explored and the difference between the localized and the global superelastic behaviors was discussed. Our results demonstrate the potential of integrating TiNi-based SMAs into functional micro- and nanodevices.
I. INTRODUCTION
Shape memory alloys (SMAs) have been extensively studied from the fundamental and technological viewpoints. The most commonly studied SMAs are the Ti–Ni systems with nearly equiatomic composition because of their superior shape memory effect (SME) and superelasticity (SE) resulting from its austenitic and martensitic structures.1–4 In addition to its corrosion resistance, wear resistance, and biocompatibility, Ti–Ni SMAs exhibit high damping capacity that is of great importance to devices, where minimization of vibration during operation is critical to the device performance. Below the martensite finish temperature, Mf, transformation of the parent phase to the martensitic phase is complete. When Ti–Ni SMA is deformed below the Mf temperature, a residual strain remains after unloading. However, heating the specimen up to above the austenite finish temperature, Af, the residual strain can be completely recovered and the specimen retains its original shape. This phenomenon is known as SME.5 Above the Af temperature, SMAs can demonstrate SE, in which relatively large mechanically imposed strains (up to ;8%) can be accommodated by the stress-induced martensitic a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.322 J. Mater. Res., Vol. 29, No. 22, Nov 28, 2014
http://journals.cambridge.org
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transformation. This martensitic phase reverts to the parent phase upon removal of the applied load to result in SE.5 Most existing studies on SMAs dealing with SME and SE are at the macroscale. Recent advances in semiconductor processing techniques and microdevices have
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