Individual phase deformation and flow correlation to macroscopic constitutive properties of DP1180 steel

Dual phase (DP) steels are broadly deployed by the automotive industry to enhance strength and ductility for light weighting. As DPs micro-constituents’ grain size is frequently on the same order of sizes probed with nanoindentation, an inverse technique was combined with nanoindentation load-depth curves to extract the stress-strain behavior of ferrite and martensite in DP1180. Determining the appropriate depth to evaluate the properties of the micro-constituent phases enabled computation of the flow behavior of each individual phase. Post indentation microscopy correlated the microstructure to the spatially resolved indentation arrays. The post-indentation impression was characterized to eliminate indentation size effects and probe bluntness influences. The yield strength of ferrite, martensite and DP1180 were determined to be 370, 948 and 836 MPa respectively while the corresponding strain hardening exponents (n) were computed via an inverse method as 0.175, 0.025 and 0.11. Mechanical properties obtained from the indentation method exhibited good agreement with the experimentally measured macroscopic yield stress (922 MPa) and hardening exponent (0.11) of DP1180.