Microstructure and mechanical properties of TiNi alloy associated with B₄C addition and rolling deformation

Ti₅₀Ni₅₀ alloys were prepared by vacuum arc melting technology associated with the addition of B₄C particles. The B₄C and TiNi matrix undergo a complete in-situ reaction, resulting in the formation of two micro/nano-sized reinforcing phase particles TiB₂ and TiC. The present study investigates the effects of different mass fractions of B₄C, reinforcing phase particles, and cold-rolling deformation/annealing on the microstructure and mechanical properties of the alloy. Microstructural observations indicate that the generated reinforcing phase particles and the matrix precipitate phase Ti₃Ni₄ have good coherency at the interface with the matrix. After multiple cold-rolling and annealing cycles, the reinforcing phase particles are significantly fractured and refined. Nano-scale recrystallized grains and subgrain structures appear in the alloy, and high-density dislocations are observed around the reinforcing phase particles. Tensile mechanical property tests show that the reinforcing phase particles can significantly improve the mechanical properties of TiNi alloys. When the B₄C addition is 0.26%, the tensile fracture strength and elongation after fracture reach 621.8 MPa and 6.2%, which are 29% and 21.5% higher, respectively, compared to the original alloy. After 4 cold-rolling passes and annealing (60% deformation amount), corresponding values further increase to 547.1 MPa and 15.48%. The improvement of mechanical properties associated with reinforcing phase particles can be understood through particle dispersion strengthening, load-bearing strengthening, and thermal mismatch dislocation strengthening. The contribution of cold-rolling and annealing to the mechanical properties mainly originates from grain refinement strengthening, dislocation strengthening, and precipitation strengthening of micro-nano particles. The fracture mode of the alloy changes from a ductile-brittle mixed fracture mode to a plastic fracture mode after deformation annealing.