Comparison between mechanisms and microstructures of α − γ, γ − ϵ and α − ϵ − α phase transitions in iron

The α↔ϵ, α→γ and γ→ϵ transformations have been characterised in diamond anvil cells under hydrostatic compression conditions. In situ x-ray diffraction of single or oligo-crystals and ex situ SEM-EBSD measurements have been analyzed with multi-grain techniques. The mechanisms of α ↔ ϵ transitions are martensitic, following Burgers paths whiwh requires a high plastic activity. A memory effect of the reversion exists in the vast majority of the sample: the starting orientation of α-Fe single crystal is recovered. Small grains of α-Fe exhibit a new orientation compatible with Burgers path, possibly associated to twinning in ϵ-Fe. Close to the α − γ − ϵ-Fe triple point (8.7
GPa, 750 K), the α → γ transformation undergoes via diffusion and recrystallization of γ-Fe, and γ → ϵ transformation is martensitic but involves no plasticity. As a result, the microstuctures in ϵ-Fe produced by a direct α → ϵ transformation and by α → γ → ϵ transitions path are very different.