Elinvar Effect by Nanoscale Displacive Phase Transformation in Martensites

Ferroelastic alloys suffer modulus softening prior to the martensitic transformation, but still exhibit normal cooling-hardening effect below the martensitic transformation finish temperature, M_f. Here we report an unprecedented phenomenon that a martensitic state can show nearly temperature-independent modulus (i.e., Elinvar effect) below M_f, as exhibited in Ni-Fe-Mn-Ti alloys. The Elinvar martensites have a hierarchical domain structure, where two types of orthorhombic nanomartensites with distinct atomic modulation periods and magnetic states coexist within an individual ferroelastic domain. Their small free-energy difference gives rise to a nanoscale displacive phase transformation from one to another, and the moderate modulus softening compensates for the ever-present elastic hardening, leading to the wide temperature range Elinvar effect. This finding is a new insight of martensitic phase transformation, which might help to discover technically desired properties, such as the temperature-independent modulus and good ductility at cryogenic temperatures.