Effect of disperse Ti3N4 particles on the martensitic transformations in titanium nickelide single crystals

Abstract

The effect of the size and volume fraction of Ti 3 N 4 particles in Ti-(50.3-51.5) at % Ni single crys- tals on their martensitic transformation temperatures and temperature hysteresis is studied. Aging at T = 673- 823 K leads to a nonmonotonic change in the martensitic transformation temperatures and temperature hyster- esis, which is related to a change in the Ni concentration in the matrix, the hardening of the high-temperature phase, a change in the elastic and surface energies generated upon the martensitic transformations, and the inter- nal stresses that appear because of the difference in the lattice parameters of the particles and the matrix. As a result of the high strength of the B 2 phase and the high elastic and surface energies that are generated upon the martensitic transformations due to the precipitation of particles of size d < 40 nm at an interparticle distance λ < 50 nm, the martensitic transformation temperatures decrease down to the suppression of the R - B 19' transi- tions upon cooling to 77 K. A thermodynamic description for the martensitic transformations in heterophase crystals is proposed, and an analogy between the martensitic transformations in heterophase Ti-Ni single crys- tals with nanoparticles of size d = 20-100 nm and those in single-phase Ti-Ni polycrystals with a grain size of 50-200 nm is found.