Martensite-to-austenite reverse transformation in Fe-Ni-Co alloys

The reverse martensitic transformation was investigated in three Fe-Ni-Co alloys containing acicular martensite by means of dilatometry and coercive force (H_c) measurements. Two maxima were observed in the H_c -T relationships: the first H_c peak occurred at the Curie temperature of the austenite and is attributable to magnetic effects in a material containing two ferromagnetic phases; the second H_c peak, with values near 400 oe, is caused by reverse transformation wherein the remaining ferromagnetic martensite has increased coercivity due to decreased particle size and increased shape anisotropy. Dilatometric measurements display a gradual decrease in expansivity in the early stages of reversal. This observation is explained by a two-step process for shear-type transformation in which the first austenite formed has high magnetostriction and low expansivity. As the magnetostriction is lost by further heating, a large expansion occurs which counteracts contraction due to the crystallographic phase change.