Aging effects on transformation temperatures of cold worked Ni-52 at.-%Ti shape memory alloy

Abstract

In the present work, a Ni-52Ti(at.-%) shape memory alloy was cold worked under compression at various plastic strain levels of 0-20%. The cold worked alloys were then aged at temperatures in the range 723-873 K for 1 hour. It was found that aging promoted the precipitation of secondary phases in the cold worked specimens. The extent of precipitation was apparently related to the aging temperature and the amount of cold work. This, in turn, influenced the martensitic Ms, Mp, and Mf and austenitic As, Ap, and Af transformation temperatures. At temperatures between 823 and 873 K, there was no consistent trend in the transformation temperatures, owing to the inhomogeneous nature of the precipitation reaction. However, in cold worked specimens (>5%) aged at temperatures below 823 K, a depression in the peak austenitic temperatures Ap was found. In addition, the martensite peak transformation temperatures Mp initially increased (0-5% cold working), but then decreased in all cases, with the effect being more pronounced at 723-773 K. Aging at 873 K where secondary phase precipitation was copious did not seem to have a significant influence on the exhibited transformation temperatures. Apparently, at this aging temperature, alloy recovery and defect annihilation counteracted most of the effects of the accumulated elastic and friction energy components of the thermoelastic transformation.