Orientation Dependence of the Yield Strength of the B2 Phase in Stress-Free and Stress-Assisted Aged Ti-51.5at.%Ni Single Crystals

The article reports for the first time the effects of Ti₃Ni₄ particle variants on the orientation dependence of the yield strength of the B2 phase on Ti-51.5at.%Ni single crystals under compression after stress-free and stress-assisted aging at 823 K for 1 h. Understanding the yield strength of the B2 phase is crucial because it determines the superelasticity interval, stress hysteresis, and cyclic stability. A decrease in the number of particle variants from four to one as a result of stress-assisted aging weakened the orientation dependence of the yield strength of the B2 phase. For four particle variants, the σ_cr(M_d) differed by 60% in the <001> and <111> orientations (2100 and 1300 MPa). By contrast, for one particle variant, the σ_cr(M_d) differed by only 20% (1630 and 1350 MPa). Transmission electron microscopy showed that the number of particle variants did not affect the yield strength of the B2 phase in the case of the {001} <001> and {011} <001> dislocation slip observed in the <111> orientation. However, the number of particle variants strongly influenced the yield strength of the B2 phase in the case of {114}_B2 mechanical twinning, observed in the <001> orientation because of the different arrangement of the twinning planes relative to one or four particle variants and the long-range internal compressive stress formed one particle variant.