Phase diagram of Ti_50-x Ni_50+x: Crossover from martensite to strain glass

We systematically investigated the variation in transition behavior and physical properties over a wide excess Ni (acting as defect) concentration range (x=0-2.5) in Ti_50-x Ni_50+x alloys. This enables the establishment of an updated quantitative phase diagram for this important system. The phase diagram shows not only the well-known parent phase and martensite phase but also a premartensitic state and a strain glass state. Our experiments were able to determine quantitatively the borders of these states, the latter two having been unclear so far. The new phase diagram shows that a crossover from martensite to strain glass occurs at x=1.3, and the appearance of a "premartensitic phase" below a critical temperature Tnd for defect-containing compositions (x>0). We propose that point defects (excess Ni here) play two roles in a ferroelastic/martensitic system: (i) changing the thermodynamic driving force for the formation of long-range strain order (martensite) and (ii) creating random local stress that favors a premartensitic nanostructure and strain glass. Our work enables a simple explanation for several long-standing puzzles, such as the appearance of premartensitic nanostructure, the vanishing of transition latent heat with increasing Ni content and the anomalous negative temperature coefficient of electrical resistivity in Ni-rich Ti-Ni alloys.