Microstructures across the alpha-epsilon transformation in iron under dynamical loading

Under increasing pressure iron undergoes a displacive transition from a BCC to an HCP phase, producing multiple orientations, and possibly creating a complex microstructure [1]. Back to ambient pressure, the reversion process fragments even further the microstructure, leaving only traces of the transformation history. We aim to understand this history from the post-mortem microstructure.

Single crystal iron samples are shock loaded at different intensities using a laser pulse. A momentum trap prevents multiple waves to transit within the material and permits its recovery. The resulting microstructure is mapped in terms of crystal orientations using the EBSD technique. The orientation of each grain can only be explained by certain transformation sequences or scenarios, taking into account the possibility of phase change and twinning. Improbable scenarios are sorted out using Hadamard jump conditions on the grain boundaries and a Schmid factor.

Evidence of the α-ε-α transformation cycle is found with some grains either explained by the Burgers path, with an additional twinning in ε-Fe before the release, or having formed between two ε-Fe grains.

References

[1] E. Boulard et al. “Following the phase transitions of iron in 3D with X-ray and diffraction under extreme conditions”. In: Acta Materialia 192 (2020), pp. 30–39. 1359-6454. doi: 10.1016/j.actamat.2020.04.030.