The effect of quenched-in vacancies upon the martensitic transformation was investigated, by means of resistometry, in monocrystals which did not suffer from interfering complications (due to other reactions of excess vacancies) because the alloy did not decompose and did not undergo order-disorder or order-order transformations during cooling. It was found that, in the furnace-cooled condition, the transformation hysteresis was only 4K but, when the specimen was rapidly quenched from a high temperature, the transformation hysteresis was greatly increased; with both Ms and Mf being highly depressed. Both Ms and Mf increased during subsequent aging. On the other hand, As and Af decreased during aging after quenching. A mechanism was proposed in which excess vacancies were assumed to migrate to the parent/martensite interface sinks; thus creating so-called jogs at the interface and making the interface less mobile. All of the observed features were accounted for qualitatively by this mechanism; including the temperature intervals between Ms and Mf, and between As and Af. After long-term aging, the transformation hysteresis curves became very similar to those for furnace-cooled samples. This behavior was associated with a tendency of the resistivity value to become almost equal to that of furnace-cooled samples; which reflected the annealing-out of excess vacancies. However, the value of (Mf + As)/2 was here much lower than that for furnace-cooled samples. This strange behavior could not be explained.

Y.Murakami, Y.Nakajima, K.Otsuka: Scripta Materialia, 1996, 34[6], 955-62