High-temperature deformation processing maps for a NiTiCu shape memory alloy

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Bikas Maji and Madangopal Krishnan Materials Science Division, Bhabha Atomic Research Centre, Mumbai 400085, India

Upadrasta Ramamurtya) Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India (Received 12 May 2011; accepted 27 July 2011)

The properties of widely used Ni–Ti-based shape memory alloys (SMAs) are highly sensitive to the underlying microstructure. Hence, controlling the evolution of microstructure during high-temperature deformation becomes important. In this article, the “processing maps” approach is utilized to identify the combination of temperature and strain rate for thermomechanical processing of a Ni42Ti50Cu8 SMA. Uniaxial compression experiments were conducted in the temperature range of 800–1050 °C and at strain rate range of 103 and 102 s1. Two-dimensional power dissipation efficiency and instability maps have been generated and various deformation mechanisms, which operate in different temperature and strain rate regimes, were identified with the aid of the maps and complementary microstructural analysis of the deformed specimens. Results show that the safe window for industrial processing of this alloy is in the range of 800–850 °C and at 0.1 s1, which leads to grain refinement and strain-free grains. Regions of the instability were identified, which result in strained microstructure, which in turn can affect the performance of the SMA.

I. INTRODUCTION

Shape memory alloys (SMAs) possess the ability to recover their original shape either after deformation (superelastic behavior) or through a postdeformation heating (shape memory effect) due to a reversible thermoelastic austenite–martensite phase transformation.1 Among a wide variety of SMAs, the Ni–Ti (NiTi)-based ones are the most popular due to their attractive properties, such as low elastic anisotropy, high ductility, biocompatibility, high corrosion, and abrasion resistance.1–6 Therefore, they are either used or being considered for a wide variety of applications such as couplings, valves, actuators, and biomedical implants. In most of these applications, the SMA components are in the form of either thin wires or sheets or tubes. This smaller final size of the components means a fairly large number of thermomechanical processing steps become essential for breaking down of the large cast ingots to the required sizes. Also, hot deformation is an essential process for production of high-quality semifinished products of NiTi alloys.1 Since the final microstructure controls the structural as well as functional properties and performance of SMAs, which are known to be sensitive to parameters a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.257 2484

J. Mater. Res., Vol. 26, No. 19, Oct 14, 2011

http://journals.cambridge.org

Downloaded: 14 Mar 2015

such as grain size and texture, it becomes imperative to exercise careful control over the process parameters during high-temperature deformation. Extensive studies have been carried out about various ther