Lüders strain ductilizes a 1.2 GPa high-strength metastable steel via delaying martensite transformation

The formation of Lüders bands tends to result in premature necking and reduced ductility in metallic materials. Studies have shown that the martensite (α′) transformation can help stabilize Lüders bands and decrease Lüders strain in metastable materials. However, the impact of Lüders strain on the α′ transformation and deformation mechanisms of materials with high ductility is not well understood. This study compares the relationship between uniform strain, Lüders strain, and microstructural evolution in an austenitic stainless steel. By combining the high stability of the γ phase and Lüders strain, we believe that Lüders strain can ductilize a 1.2 GPa high-strength metastable steel via delaying martensite transformation. The main deformation mechanism is dislocation slip during the first initial Lüders strain (band 1), rather than stress-assisted martensite transformation (SAMT). Subsequently, α′ transformation is activated due to increasing true stress, leading to further plastic strain. The true stress remains basically stable during the Lüders strain 1 and cannot efficiently induce α′ transformation. In contrast, the rapid increase in stress during uniform strain further accelerates the α′ transformation in sample A10, resulting in high strength but moderate ductility in the sample with continuous yielding, like the conventional transformation-induced plasticity effect. The delayed α′ transformation behavior during Lüders strain presents a new opportunity for developing high-performance metastable steels. Tailoring plastic instability in metastable engineering presents a novel strategy for harnessing synergistic enhancements in strength and ductility. Delaying nano-micro martensite transformation induced Lüders strain ductilize a 1.2 GPa yield stress.