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Quantitative phase transformation behavior in TiNi shape memory alloy thin films

Published online by Cambridge University Press:  01 October 2004

Bo-Kuai Lai
Affiliation:
Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106
H. Kahn
Affiliation:
Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106
S.M. Phillips
Affiliation:
Department of Electrical Engineering, Arizona State University, Tempe, Arizona 85287
Z. Akase
Affiliation:
Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106
A.H. Heuer*
Affiliation:
Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106
*
a)Address all correspondence to this author. e-mail: heuer@case.edu
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Abstract

Phase transformations in near-equiatomic TiNi shape memory alloy thin films were studied, and the phase fraction evolutions were quantitatively correlated to the stress and resistivity of the films. TiNi thin films with compositions of 50.1, 51.1, and 51.7 at.% Ti all exhibited transformation temperatures between 65 and 100 °C, low residual stresses at room temperature (RT), and high recoverable stresses, thus making them suitable for microactuators in microelectromechanical systems. Low residual stresses at RT, less than 50 MPa, can be obtained even when only a small quantity of martensite, less than 30%, is present. Phase fraction evolution during complete thermal cycles (heating and cooling) was studied using elevated temperature x-ray diffraction, combined with quantitative Rietveld analysis. R-phase always appeared in these near-equiatomic TiNi thin films during cooling but did not have a noticeable effect on the stress–temperature hysteresis curves, which mainly depend on the phase fraction evolution of martensite. On the other hand, the occurrence of R-phase determined the variation of film resistivity. Martensite, austenite, and R-phase coexisting within a single grain were observed using transmission electron microscopy.

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Articles
Copyright
Copyright © Materials Research Society 2004

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References

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