Lath martensite is a key microstructural component of a broad class of advanced high strength steels, playing a crucial role in strengthening, toughness and failure mechanisms, including hydrogen embrittlement [1,2]. The mechanical response of martensite critically depends on its hierarchical microstructure, composed of subgrains (packets, blocks, laths) often combined with secondary phases such as retained austenite to form tough austenite/martensite nanolaminates. The austenite-martensite (fcc-bcc) transformation controls the formation of the martensite microstructure. Despite its relevance for applications, there is no established theory for this transformation.

To clarify the mechanism of transformation, we have performed atomistic simulations of the interface reproducing the major experimental TEM and HRTEM observations in Fe alloys. The atomistic model reveals for the first time the structure and motion of the athermal and glissile fcc austenite/bcc martensite interface in steels [3]. The interface structure consists of [-101](111)fcc screws, as envisioned by previous theories, and [1-11](-101)bcc screws with kinks, which was not envisioned before. The atomistic findings have guided the formulation of a new, parameter-free double-shear predictive theory of martensite crystallography. Theory predictions show that the fcc/bcc lattice parameter ratio is the key factor controlling the shape deformation (i.e. the in-situ transformation strain), which can achieve more than 90%, namely three times the existing experimental estimates [4].

The theory has been further validated on high-resolution digital image correlation measurements on a model FeNiMn steel [5], revealing $\sim$40% shape deformation in this alloy. The theory predictions show a correlation between lattice parameter ratio/shape deformation and uniform elongation per austenite volume fraction in a number of low alloyed steels, i.e. increased ductility with increased shape deformation [5]. Furthermore, the crystallographic theory has been implemented in a multi-scale crystal plasticity framework. We have shown that the large shape deformation predicted in low-C steels can explain the observed lath martensite plasticity, that is dominated by apparent lath boundary sliding mediated by thin retained austenite films [6].

The experimental validation on a range of lath martensites, including Fe-Ni-Mn and low-C steels, demonstrates that the theory can be used for guiding design of novel and tougher advanced high-strength steels.

References

1. E. De Moor, S. Lacroix, A. Clarke, J. Penning, J.G. Speer, Metall. Mater. Trans. A, 39 (2008), 2586.

2. D. Raabe, S. Sandlöbes, J. Millán, D. Ponge, H. Assadi, M. Herbig, P.-P. Choi, Acta Mater., 61 (2013), 6132.

3. F. Maresca, W.A. Curtin, Acta Mater. 134, (2017), 302.

4. H.K.D.H. Bhadeshia, ISIJ Int., 42 (2002), 1059.

5. F. Maresca, E. Polatidis, M. Smid, H. Van Swygenhoven, W.A. Curtin, Acta Mater., 200 (2020), 246.

6. F. Maresca, V.G. Kouznetsova, M.G.D. Geers, W.A. Curtin, Acta Mater, 156 (2018), 463.