Spatially resolved in-situ characterisation of competing martensitic transformation pathways during nanoscratch in 316H Stainless Steel
Localised surface deformation beneath frictional contacts generates a tribolayer whose microstructure and properties differ from the bulk. In austenitic stainless steels, this tribolayer forms through two competing martensitic transformation pathways. Here, these pathways are isolated in 316H stainless steel using in-situ synchrotron X-ray nanodiffractometry combined with nanoscratch testing, which together yield spatial maps of the evolving strain field beneath a single sliding asperity. Finite element modelling interprets the resulting distribution of martensitic phases, revealing a pressure driven pathway selection where hydrostatic compression ahead of the contact suppresses $\alpha'$ formation and favours the $\gamma \rightarrow \varepsilon$ transformation, while lateral sliding relieves this constraint and introduces a shear strain driving $\varepsilon \rightarrow \alpha'$ , producing an overall sequential $\gamma \rightarrow \varepsilon \rightarrow \alpha'$ pathway in the tribolayer. Where hydrostatic constraint persists, $\varepsilon$-martensite is retained; where material piles up and is unconstrained above the surface, the transformation proceeds directly to $\alpha'$. The $\gamma$-austenite adjacent to $\alpha'$- martensite shows elevated dislocation density, indicating that $\alpha'$ formation is accommodated by plastic deformation in the surrounding matrix. This distinction could explain differences in galling performance among iron-based and cobalt-based hardfacing alloys, where the $\varepsilon$-martensite forming cobalt alloys offer superior galling resistance. The methodology presented resolves transient microstructural states inaccessible to static measurements of macroscale, multiple asperity contacts, establishing a route to mechanistic insight across tribological phenomena more broadly.