Role of elastic and shear stabilities in the martensitic transformation path of NiTi

Due to the importance of NiTi as a shape-memory material and the uncertainty regarding its atomisitic martensitic transformation path, a thorough investigation to understand the structural stability governing this displacive phase transformation is warranted. We investigate elastic and shear stabilities of NiTi using first-principles calculations with the highly precise full-potential linearized augmented plane-wave method. Ambiguities of the B2, R, B19, B19', and proposed B33 structures are resolved, and we establish that the P3 symmetry is preferred for the R phase of equiatomic NiTi, and the phase stability of each structure is established by examining calculated formation energies, which show agreement with direct reaction calorimetry experiments. Additionally, all single-crystal elastic constants, Young's, bulk and shear moduli, Poisson's ratio, and the Zener anisotropy of the B2, R, B19, B19', and B33 phases are calculated and presented yielding agreement with experiment that exceeds that of previous calculations. To investigate the susceptibility to shearing, generalized stacking-fault energetics are calculated for the {001}, {011}, and {111} slip planes of the B2 phase. Burgers vectors and shear resistance are established while examining atomic shuffling throughout the imposed shear; the {001} and {111} stacking faults possess high-energy barriers. By investigating various deformation mechanisms related to these stacking faults, we find an instability to < 100 >{011} slip in the B2 phase. Using this and reviewing previously proposed atomistic transformation paths, the mechanisms governing the direct martensitic transformation of NiTi between the austenite and the martensite are identified. Barrierless transformation paths from the B2 phase to the B19' phase and from the B2 phase to the B33 phase are proposed.