Simultaneous measurement of mechanical and electrical contact resistances during nanoindentation of NiTi shape memory alloys

Shastry, VV and Ramamurty, U (2013) Simultaneous measurement of mechanical and electrical contact resistances during nanoindentation of NiTi shape memory alloys. In: ACTA MATERIALIA, 61 (14). pp. 5119-5129.

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Abstract

The nanoindentation technique can be employed in shape memory alloys (SMAs) to discern the transformation temperatures as well as to characterize their mechanical behavior. In this paper, we use it with simultaneous measurements of the mechanical and the electrical contact resistances (ECR) at room temperature to probe two SMAs: austenite (RTA) and martensite (RTM). Two different types of indenter tips - Berkovich and spherical - are employed to examine the SMAs' indentation responses as a function of the representative strain, epsilon(R). In Berkovich indentation, because of the sharp nature of the tip, and in consequence the high levels of strain imposed, discerning the two SMAs on the basis of the indentation response alone is difficult. In the case of the spherical tip, epsilon(R) is systematically varied and its effect on the depth recovery ratio, eta(d), is examined. Results indicate that RTA has higher eta(d) than RTM, but the difference decreases with increasing epsilon(R) such that eta(d) values for both the alloys would be similar in the fully plastic regime. The experimental trends in eta(d) vs. epsilon(R) for both the alloys could be described well with a eta(d) proportional to (epsilon(R))(-1) type equation, which is developed on the basis of a phenomenological model. This fit, in turn, directs us to the maximum epsilon(R), below which plasticity underneath the indenter would not mask the differences in the two SMAs. It was demonstrated that the ECR measurements complement the mechanical measurements in demarcating the reverse transformation from martensite to austenite during unloading of RTA, wherein a marked increase in the voltage was noted. A correlation between recovery due to reverse transformation during unloading and increase in voltage (and hence the electrical resistance) was found. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Item Type:	Journal Article
Publication:	ACTA MATERIALIA
Publisher:	PERGAMON-ELSEVIER SCIENCE LTD
Additional Information:	Copyright of this article is belongs PERGAMON-ELSEVIER SCIENCE LTD
Keywords:	Shape memory alloys; Nanoindentation; Austenite to martensite phase transformation; Electrical resistivity; Depth recovery ratio
Department/Centre:	Division of Mechanical Sciences > Materials Engineering (formerly Metallurgy)
Date Deposited:	23 Sep 2013 12:10
Last Modified:	23 Sep 2013 12:10
URI:	http://eprints.iisc.ac.in/id/eprint/47286

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