Martensitic Transformation Terminated in "Gas-liquid"-type Critical Point

We found that the line of martensitic transformations (MTs) of Ni-Fe(Co)Ga is terminated in a "gas-liquid"-type critical point, CP(318K, 109MPa) of the stress-temperature phase diagram. Below CP a tensile superelastic deformation of 14% and pronounced hysteresis were obtained whereas above it an anhysteretic nonlinear deformation of similar value was observed. In-situ neutron diffraction of aforementioned single crystals during compression tests at different temperatures indicates a stress-induced MT with two-phase coexistence below CP, and continuous change of the lattice parameter from austenite to a post-critical state above CP. The Landau-type theory describes the existence of an end-point on the stress-temperature phase diagram of the first-order MT. It also predicts that CP can be experimentally observed for the alloys with low shear modulus ( a few GPa). The Fe-Pd alloy was found recently to be another example of this behavior, showing a CP at 280K and 40 MPa.