Experimental investigation of plasticity mechanisms of martensite by micron-sized uni-axial tensile tests

Lath martensite is the key constituent in advanced steels that provides the overall strength. Martensite is known as hard and brittle, but recent evidence shows significant plasticity before  fracture, which should be related to the underlying lath microstructure. Therefore, the influence of lath and block boundaries on martensite plasticity through uni-axial tensile testing of individual microconstituents is studied.

MSc student: ing. René Vaes
Daily supervisors: dr.ir. J.P.M. Hoefnagels and C. Du MSc
Project supervisor: prof.dr.ir.  M.G.D. Geers

Low carbon martensite, also known as "lath martensite" is a polycrystalline phase which is present in martensitic and multi-phase high strength steels, e.g. Dual Phase steels. Martensite has a well-defined internal (crystalline) hierarchical microstructure (Figure 1 [S. Morito,  2003]). While a lot is known about the effect of martensite in advanced steels, little is known about martensite on its own. The morphology and crystallography of martensite in different alloys is well defined by previous research, while the mechanical properties and deformation behavior are still unknown.

The goal of this research is to experimentally investigate which plasticity mechanisms occur in martensite during micron-sized uni-axial tensile tests. Miniature sized specimens are fabricated by FIB milling from a single prior austenite grain. Specific mechanisms may lead to conformation on the predicted presence of retained austenite in the microstructure of martensite. By using different orientations of the martensite laths, the local strain can be related to the microstructural features of the different specimens.

This research requires miniature sized specimens, which means that the design of experiments is of attentive considerations. Tensile specimens are fabricated within martensite packets with lath or block boundaries orientated 45° (see Figure 2a) and parallel to the loading direction. EBSD orientation maps (Figure 2b) and pole figures (Figure 2c)  of the top and bottom surfaces confirm single packet specimens with vertical through-thickness boundaries. A uni-axial tensile test procedure was developed, based on a home-built Nano-Tensile Stage (Figure 3) with highly accurate alignment and force measurements.