Axial ratio of an Fe–30Ni–0.23C bct martensite determined by neutron diffraction

When an Fe–30Ni–0.23C alloy is cooled below Ms temperature lower than RT, in situ neutron diffraction has revealed that newly transformed bct martensite shows a high axial ratio, c/a-value. When the sample is heated from 100 K to RT, the c/a apparently decreases. Such a change in c/a indicates that, even below RT, there is carbon diffusion to induce tempering effect. When a quenched martensite is tempered at 473 K, the c/a is drastically decreased.