Features of Multistage Thermoelastic B2-R-B19' Martensitic Transformations in Heterophase Single Crystals of Ti-Ni Alloys

摘要

In heterophase Ti50.0-xNi50.0+x (N... = 0.8-1.0 at.%) single crystals after aging at 823 K for 1.5 h at the heating/cooling rate (15 +/- 1) K/min and stepwise aging at 823 K for 1.5 h plus aging at 573-623 K for 1 h, multi-stage martensitic transformations are observed: there are three peaks of heat release in the DSC (differential scanning calorimetry) curves. The first peak is associated with a B2-R-transformation and increased electrical resistance rho and the other two correspond to the development of an R-B19'-martensitic transformation occurring in two stages, and are followed by a two-stage drop of electrical resistance in the rho(T) plot. The high-temperature stage of the R-B19'-martensitic transition is associated with transformations under the action of local stress fields in the vicinity of disperse Ti3Ni4 particles measuring similar to 400 nm, while the low-temperature stage is associated with transformations in the spaces between large particles containing fine disperse beta'-phase particles isostructural with respect to the matrix, measuring 3-5 nm, which are formed during slow cooling from 823 K or in the course of low-temperature aging at 573-623 K.