Among the Dual-Phase (DP) steel microstructure constituents, lath martensite is generally considered to be the hard and brittle phase that determines strength, while restricting ductility. However, recent studies have shown that lath martensite islands can deform in a considerable ductile manner in DP steels, and that fully lath martensitic steels show strong plasticity preferentially occurred over the habit plane, as explained by various competing micromechanical deformation mechanisms, including so-called ‘substructure boundary sliding’. However, it is unclear if full martensite insights can be extended to lath martensite islands in DP steel, for which the substructure boundaries often cross the full martensite islands [1].

Therefore, to unravel the anisotropy of lath martensite plasticity in DP steels, the martensite habit plane orientation and the surrounding DP morphology was correlated to the degree of martensite plasticity, in 3 classes of in-situ SEM-DIC experiments: ’1D’ isolated, well-defined, micrometer sized tensile tests on a single ferrite-martensite phase boundary [2], novel ’2D’ tests that introduce more deformation complexity on ~100μm-wide, ~1μm-thin, through-thickness microstructures [3], and statistical analysis on more traditional ’3D’ bulk bending tests that exhibit the full multi-phase complexity. For the ‘1D’ and ‘2D’ tests, two-sided microstructure maps were aligned to yield two-sided, nanoscale microstructure-resolved strain fields using our recent nanomechanical testing framework,[4] while slip system activity fields were obtained with our novel Slip Systems based Local Identification of Plasticity (SSLIP) method.[5]

Combining all results reveals that the orientation of the habit plane determines how plasticity percolates through the lath morphology in cases of a favorably oriented habit plane. Plasticity bands occur predominantly between lath boundaries at lower global deformations, while substructure boundary sliding appears to activate upon larger deformations. Interestingly, strong plasticity occurs almost always over the habit plane, even when it has relatively unfavorable orientations, revealing a considerable slip resistance contrast between in-habit-plane and out-of-habit-plane slip.

References

[1] C. Du, J.P.M. Hoefnagels, S. Kölling, M.G.D. Geers, J. Sietsma, R. Petrov, V. Bliznuk, P.M. Koenraad, D. Schryvers & B. Amin-Ahmadi, Martensite crystallography and chemistry in dual phase and fully martensitic steels. Materials Characterization, 139, 411 (2018).

[2] C. Du, J.P.M. Hoefnagels, L.I.J.C. Bergers, and M.G.D. Geers, A Uni-Axial Nano-Displacement Micro-Tensile Test of Individual Constituents from Bulk Material, Experimental Mechanics, 57, 1249 (2017).

[3] T. Vermeij, J. Wijnen, R.H.J. Peerlings, M.G.D. Geers & J.P.M. Hoefnagels, A quasi-2D integrated experimental-numerical approach to high-fidelity mechanical analysis of metallic microstructures, In preparation (2023).

[4] T. Vermeij, J. A. C. Verstijnen, T. J. J. Ramirez y Cantador, B. Blaysat, J. Neggers & J. P. M. Hoefnagels, A nanomechanical testing framework yielding front&rear-sided, high-resolution, microstructure-correlated SEM-DIC strain fields, Experimental Mechanics 62, 1625 (2022).

[5] T. Vermeij, R.H.J. Peerlings, M.G.D. Geers & J.P.M. Hoefnagels, Automated identification of slip system activity fields from digital image correlation data. Acta Materialia, 243, 118502 (2023).