Multiaxial stress–strain response and displacive transformations in NiTi alloy from first principles

Abstract

Present ab initio study was focussed on a response of NiTi martensite to a superposition of shear and tensile or compressive stresses acting normally to the shear planes. The theoretically predicted base-centered orthorhombic (BCO) ground-state structure was found unstable under uniaxial compression and two transformations, one from orthorhombic to a monoclinic symmetry and the other back from monoclinic to orthorhombic symmetry, were observed in the computational model. The former transformation shows that the uniaxial compressive stress of about 4 GPa destabilizes the BCO structure by reducing its symmetry to the experimentally observed monoclinic one. However, superposition of small shear stresses remarkably lowers the compressive stress necessary for this destabilization. The latter transformation then draws the crystal lattice to the B19 structure. The theoretical shear strength of NiTi martensite was subsequently computed as a function of the normal stress. The results obtained show that the effect of the normal stress is surprisingly opposite to that calculated for NiTi austenite and other cubic metals, i.e., that the shear strength is lowered by the compressive normal stress and vice versa.