On the Effect of Volume Change and γ'-Precipitation Hardening on the Reversibility of Martensitic Transformation in Fe-Ni-Co-Ti-Alloy

Abstract
Martensitic transformation in Fe-Ni-based alloys is accompanied with an increase of volume (ΔV_γ-α≈3%). Lattice invariant shear leads to additional dilatometric changes for non-random transformations. As the result of predominantly ΔV_γ-α a large thermal hysteresis (ΔT_H) originates. This leads to a diffusion-controlled α→γ-transformation, i.e. the reverse reaction becomes crystallographically irreversible (α≠γ).
In the case of an aged Fe-Ni-Co-Ti alloy, ΔV_γ-α depends on the magnetic state of the austenite and on the volume fraction of the precipitated γ'-particles. With increasing the volume fraction of γ, ΔV_γ-α is reduced. Dislocations at the former martensite/austenite interfaces indicate incomplete reversibility of the transformation cycles, even in the ausaged condition. However, drastic reduction of ΔT_H is observed during an ausaging sequence for conditions close to maximum volume fraction and minimum size of γ'.