abstract

This presentation explores a new strategy of utilizing local heterogeneity within the lattice to design and create NiTi shape memory alloys of extraordinary superelasticity. The lattice heterogeneity may involve chemical, crystallographic as well as microstructural features, often on subgrain, lattice and unit cell scales. The extraordinary superelasticity may include high superelastic stresses in excess of 1 GPa, low temperature sensitivity of the critical stress for transformation, and wide superelastic temperature windows in excess of 100 °C.

Such a heterogeneous landscape of the lattice exerts severe mechanical resistance to the lattice distortion of a martensitic phase transformation, which is a process of dual nature of a mechanical lattice shape change and of a thermal transformation. This, in the case of near- equiatomic NiTi, forces the usual B2->B19′ martensitic transformation into the B2->R transformation of a smaller lattice distortion and then further into a state of “strain glass” distortion with such a miniscule lattice distortion that is barely detectable experimentally. In addition, a martensitic transformation often also involves atomic shuffling, which produces no crystal shape change and thus is not directly affected by the mechanical resistance of the heterogeneous matrix to shape change. Such suppression of the lattice distortion and divorce between the lattice distortion and the atomic shuffling for a martensitic transformation create a highly peculiar situation, by putting the martensite into a much less clearly defined crystallographic and thermodynamic state and the transformation through a much less certain path. Such “extraordinary” conditions offer us a new opportunity to design and create shape memory alloys of extraordinary superelastic properties.

ESOMAT 2024 CONFERENCE SECRETARIAT

Via Filippo Turati, 8
20121 Milano Mi – Italy
Tel. +39 02 76021132
Email: esomat2024@aimnet.it