Atomistic study of the effect of crystallographic orientation on the twinning and detwinning behavior of NiTi shape memory alloys

Understanding the effect of crystallographic orientation on the twinnin/detwinning mechanisms in NiTi shape memory alloys at an atomistic scale can help to control and tune the mechanical properties and failure behavior of such materials. In this work, we employed classical molecular dynamics (MD) and density functional theory (DFT) computational methods to better understand how twinning and detwinning occurs through a combination of slip, twin, and shuffle on 〈0 1 0〉, 〈1 1 0〉, and 〈1 1 1〉 crystallographic orientations under uniaxial tensile test. Elastic constants including Young's Modulus (E), Bulk modulus (B), Poisson's ratio (ν), and Shear Modulus (G) are obtained and computed for resultant stress-induced martensite variants as a function of crystallographic orientation using DFT calculations. In addition, computational nanoindentation tests are carried out using MD simulations to evaluate the effect of crystallographic orientation on the twinning and detwinning characteristics in martensite in NiTi alloys under sphere indenter, both qualitatively and quantitatively. Based on a careful polyhedral template matching (PTM) and dislocation analysis (DXA) by taking into account the textures, it is determined that the microscopic stress-strain and load-displacement responses strongly depend on the crystallographic orientation. Our findings reveal that the size of twinned and detwinned zones in martensite increases in the order of 〈1 1 1〉 < 〈0 1 0〉 < 〈1 1 0〉. Based on DFT results, against 〈1 1 1〉 direction, abrupt changes in the free energy-strain curves occurs at 4% strain in 〈0 0 1〉, and 8% strain in 〈1 1 0〉 directions. The twinning and detwinning mechanisms are controlled by monoclinic martensite (B19′) → orthorhombic martensite (B19) phase transformation in 〈1 1 0〉 orientation and by body-centered orthorhombic martensite (BCO) → an intermediate structure (B19″) → monoclinic martensite (B19′) phase transformation in 〈0 0 1〉 orientation. Finally, the predicted orientation-dependent critical energy release rate is analyzed to examine the effect of the twinning and detwinning process on the fracture toughness of the material. Our results show that reducing the density of twins results in increasing the critical energy release rate. Therefore, the fracture stress intensity increases in the order of 〈0 0 1〉 < 〈1 1 0〉 < 〈1 1 1〉.