*4.2. Effect of Microstructures on Mechanical Properties*

The previous section has shown that cold deformation could promote strain-induced martensite formation. Compared with the austenite with a face centered cubic (fcc) crystal structure, the ά-martensite with a body-centered cubic (bcc) crystal structure has fewer slip directions and a larger lattice resistance, which means that ά-martensite is more difficult to slip in the process of plastic deformation [30]. The strain-induced martensite formed during cold deformation could effectively improve the strength of the samples. Furthermore, although there are large blocks of untransformed austenite existing in CR samples, the results of TEM and EBSD have indicated that a high density grain boundary existed in these austenite structures. The grain boundary can hinder the movement of dislocations, which could reduce the stress concentration and improve the strength of the samples. Meanwhile, the increasing volume fraction of martensite and dislocation density as the cold rolling reduction increased resulted in the increasing Vickers hardness [31].

However, the high density dislocation in martensite and its uneasy sliding contribute to its poor plasticity. The high density of the grain boundary in the untransformed austenite structure will significantly hinder the movement of dislocations. Furthermore, austenite structure refinement could enhance the stability of austenite, which inhibits the martensite transformation during the plastic deformation, resulting in a decrease in the ductility [32].
