Origin of twinning mode hierarchy in NiTi – A critical understanding

Experimental evidence has universally demonstrated the coexistence of { 11 1 ¯ } Type-I twins and 〈 011 〉 Type-II twins—defined as lattice-invariant deformation—within the same habit plane variant of the B19’ martensite in Ni-Ti alloys. This coexistence is unexpected because, although { 11 1 ¯ } Type-I twins represent a valid solution derived from the phenomenological theory of martensite crystallography, they are not conjugate to the experimentally dominant 〈 011 〉 Type-II twins. A clear explanation for the widespread occurrence of { 11 1 ¯ } Type-I twinning mode has been elusive. This study shows calculations of twin boundary (TB) energies for both twinning modes in Type-I and found that { 11 1 ¯ } twinning is favored over { 011 } since the TB energy is much lower by a factor of three. The accurate determination of TB energy requires ab-initio calculations because the classical interatomic potentials miss the correct positioning of the lattice-motif points. High-resolution experimental images at the atomic level confirm the positioning of Ni and Ti atoms and agree precisely with our calculations. Furthermore, experimental electron diffraction images and simulated images agree only when the correct interface is constructed as in this study. With the correct offsets, further shear and shuffles establish twin nucleation and migration barriers that elucidate a preference for { 11 1 ¯ } Type-I twinning mode in NiTi alloys. In summary, the current study employs Density Functional Theory (DFT) and three recently proposed Molecular Dynamics (MD) potentials to gain further insights into twinning in NiTi, while also serving as a critical evaluation of the proposed MD models.