Evidence for short-time limit of martensite deaging in shape-memory alloys: Experiment and atomistic simulation

Abstract

It is well known that martensite aging effects in shape memory alloys can be simply removed when the aged martensite experiences a reverse transformation to the parent phase followed by cooling back to the martensite state. This “deaging” process has been known to be very fast but it remains a question as to whether there exists a short-time limit for such a fast deaging process. In this letter, we report that there indeed exists a short-time limit for the deaging. We have studied the aging and deaging of a Au-49.5Cd shape memory alloy, and found that complete removal of the previous aging, as manifested by the recovery of martensite transition start temperature (Ms), occurs only after aging in the parent phase for more than 500 s. Shorter time holding/aging in the parent phase results in a higher Ms as compared with the fully deaged case. Therefore, there is a fast relaxation process during the deaging or parent phase aging process. Atomistic simulations suggest that the origin of the observed time-dependent deaging arises from the change in short-range configurations of point defects, being the same as that of the martensite aging. As a result, it is possible to unify the microscopic mechanism of aging in both martensite and parent phase; both are due to a symmetry-conforming short-range ordering tendency of point defects.