Martensitic transformations are acquiring an increasing technological relevance in a broad scope of high-performance materials. However, there is a lack of in-situ experimental information on the martensitic transformation behaviour of individual grains, which is essential to optimize the mechanical properties of the material. In this sense, in-situ three-dimensional synchrotron X-ray diffraction has become a powerful tool to monitor individual grains within the bulk of polycrystalline materials. In this study, we have focused on TRIP steels, which are attracting a great interest due to their high strength and good formability. The Transformation Induced Plasticity (TRIP) effect, which is considered to yield a significant contribution to the large elongation of these steels, stems from the transformation of the metastable (face-centred cubic) austenite phase into the (body-centred tetragonal) martensite phase. The martensitic transformation of the metastable austenite can be induced mechanically by applied stress or thermally by cooling. In recent experiments we have monitored in-situ the martensitic transformation of a significant number of individual grains during cooling, using an intense microbeam of high-energy X‑rays (80 keV). For each of the grains, the martensitic transformation temperature has been correlated to local microstructural parameters like the interstitial carbon concentration and the grain size. For the smallest grains we demonstrated that the grain stability of the metastable phase depends strongly on the grain size, indicating a critical size below which the martensitic transformation is completely suppressed. Future developments at synchrotron and free electron laser sources will strongly improve the spatial and time resolution and thereby lead to new opportunities to probe martensitic transformations of single grains in situ.

[1] N.H. van Dijk et al., Acta Mater. 53 (2005) 5439.
[2] E. Jiménez-Melero et al., Scripta Mater. 56 (2007) 421.

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