Experimental and molecular dynamics studies of phase transformations during cryogenic thermal cycling in complex TiNi-based crystalline/amorphous alloys

In TiNi-based crystalline/amorphous alloys, superelasticity in crystalline phase coordinating the dislocation sinking in amorphous phase lead to a high ductility and outstanding anti-fatigue properties. We performed cryogenic thermal cycling, between 77 K and 303 K, on the complex TiNi-based alloys consisting of a major B2 austenite phase, an interdendritic amorphous phase, and a minor B19′ martensite phase in the as-cast state. The critical martensitic phase transformation stress (σ_m) increased with the number of thermal cycles, reaching a maximum at 10 cycles. The initial B19’ martensite which is confined in the amorphous phase transformed to B2 austenite due to thermal induced stable transformation. A lamellar structure of alternating amorphous and crystalline layers dominantly grew into the amorphous matrix as a consequence of the thermal fatigue during the cryogenic thermal cycling. Initial cell for the molecular dynamic simulations was carefully prepared to contain three different phases. Cyclic compressive loading and cryogenic thermal cycling simulations were consistent with the experimental results.