Causes of Partial or Complete Reversibility of Martensitic Transformation in Alloys of Iron

Martensitic transformation implies a change in crystal structure by lattice variant shear as well as the formation of lattice defects by various amounts of lattice invariant shear. Crystallographic reversibility defines conditions under which the reverse reaction leads to the restoration of a defect-free high temperature phase (γ). This requires a lattice variant shear in opposite direction to the previous martensitic transformation. As alloys of iron are usually non-reversible a systematic study has been conducted to define the parameters which favour reversibility. Different Fe-Ni-based alloys have been studied to explore the effects of heating rates and alloying elements. The course of the retransformation is followed by DSC investigations. Optical and TEM investigations have been done to describe the change of macro- and microstructure after retransformation

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

Martensitic transformation in Fe-Ni-based alloys is accompanied with an increase of volume (ΔVγ-α≈3%). Lattice invariant shear leads to additional dilatometric changes for non-random transformations. As the result of predominantly ΔVγ-α a large thermal hysteresis (ΔTH) 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, ΔVγ-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, ΔVγ-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 ΔTH is observed during an ausaging sequence for conditions close to maximum volume fraction and minimum size of γ'

NiTi WIRES FOR ORTHODONTIC APPLICATION

In order to characterize NiTi orthodontic wires microstructure, transformation temperatures, and deformation behaviour have been investigated. Various states of dislocation density were established by additional heat treatment of a work hardened material. The as recieved wire and the change in microstructure due to the chosen heat treatment were described by optical light microscopy. The stress induced transformation behaviour is related with the experimental results in transformation characteristic measurements and microstructural observations