Lattice defects generated by cyclic thermomechanical loading of nanocrystalline NiTi wires

In the talk, i will summarize recent results of the systematic analysis of unrecovered strains and permanent lattice defects observed by TEM in nanocrystalline NiTi wires subjected to wide range of cyclic thermomechanical loads in tension including shape memory and recovery stress cycling. It will be shown that the nanocrystalline wires tend to transform into compound twinned B19’ martensite when the transformation proceeds under low stress without generating any permanent lattice defects and unrecovered strain. When the martensitic transformation proceeds under external stress, however, it generates unrecovered strains and leaves behind various types of permanent lattice defects in austenite, mainly slip dislocations and deformation bands containing B19’ residual martensite and/or {114} austenite twins that will be presented and discussed.