When steel is quenched to room temperature from an austenitic state, martensite is formed if the cooling rate is suitably high and the martensite start temperature is above room temperature.

Low- and medium-carbon martensite forms as laths, which tend to be able to have 24 different variant orientations, given a single austenitic orientation. These variants form into packets. In a packet, the variants are all formed on approximately the same habit plane (in the ~{111} family in the case of ~0.2 wt-% steel). The reason for packet formation is easy to imagine - laths probably are likely to grow into directions with the fewest volumetric restrictions, which are defined by the neighboring laths previously formed. So this is fine.

Some authors also discuss plate groups. A plate group is referred to by Stormvinter et al. [1] as a collection of variants sharing certain types of boundary misorientations – to be precise, V1-V6, V1-V16 and V1-V17 according to the variant classification used by (for example) Morito et al. [2] and Miyamoto et al. [3]

Could someone explain to me what is the special nature of the relationship between laths belonging to the same plate group? On a larger scale, what would it imply if neighboring laths belonging to the same plate group are frequently encountered in a martensitic microstructure – for example at packet boundaries? It is suggested by Stormvinter et al. [1] that variants belonging to the same plate group are formed by autocatalysis.

I would appreciate any insights and discussion on the topic.

BR, Tuomo Nyyssönen

[1] A. Stormvinter, G. Miyamoto, T. Furuhara, P. Hedström, A. Borgenstam, Effect of carbon content on variant pairing of martensite in Fe–C alloys, In Acta Materialia, Volume 60, Issue 20, 2012, Pages 7265-7274, ISSN 1359-6454

[2] S. Morito, H. Tanaka, R. Konishi, T. Furuhara, T. Maki, The morphology and crystallography of lath martensite in Fe-C alloys, In Acta Materialia, Volume 51, Issue 6, 2003, Pages 1789-1799, ISSN 1359-6454

[3] G. Miyamoto, N. Iwata, N. Takayama, T. Furuhara, Quantitative analysis of variant selection in ausformed lath martensite, In Acta Materialia, Volume 60, Issue 3, 2012, Pages 1139-1148, ISSN 1359-6454