  NiTi is the most used shape-memory alloy, nonetheless, a lack of
understanding remains regarding the associated structures and transitions,
including their barriers. Using a generalized solid-state nudge elastic band
(GSSNEB) method implemented via density-functional theory, we detail the
structural transformations in NiTi relevant to shape memory: those between
body-centered orthorhombic (BCO) groundstate and a newly identified stable
austenite ("glassy" B2-like) structure, including energy barriers (hysteresis)
and intermediate structures (observed as a kinetically limited R-phase), and
between martensite variants (BCO orientations). All results are in good
agreement with available experiment. We contrast the austenite results to those
from the often-assumed, but unstable B2. These high- and low-temperature
structures and structural transformations provide much needed atomic-scale
detail for transitions responsible for NiTi shape-memory effects.
