Impact of ferrite/martensite interface on nanohardness in a Dual-Phase steel: A combined experimental and theoretical study

This combined experimental and theoretical study examines the impact of ferrite/martensite (F/M) interface on the hardness/indentation depth curve during nanoindentation tests on a Dual-Phase (DP) steel. Depth-dependent hardness measurements are performed and compared with the classical Indentation Size Effect (ISE), which typically predicts a monotonic decrease in hardness with increasing indentation depth. However, in ferritic regions near martensite, the present experimental investigation reveals a non-monotonic nanohardness behavior with an initial decrease due to the classic ISE, followed by an increase from a critical indentation depth and a final decrease from a second critical depth. The intermediate stage with an increase in nanohardness between these two critical depths indicates a localized hard- ening mechanism, which is not captured by the classic ISE model equation. Using a large enough dataset of representative indents performed in ferritic grains close to F/M interfaces, the experimental study demonstrates that this localized hardening is attributed to these specific interfaces acting as strong barriers to plasticity. It is also shown that the increase in nanohardness follows a clear power-law dependence on the microstructural surface geometric distance between the indent position and the nearby F/M interface location. A model, based on Strain Gradient Plasticity (SGP) theory, and explicitly accounting for this distance, is used to explain this nanohardness increase and estimate the evolutions of geometrically necessary dislocation (GND) densities between both critical indentation depths at the origin of the pronounced size-dependent strengthening. These results shed new light on the plasticity mechanisms that control hardness variations at the nanoscale in DP steels due to ferrite/martensite interface, and provide a framework for better understanding the effects of phase boundaries on localized plasticity.