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

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.