Hydrogen-associated decohesion and localized plasticity in a high-Mn -two phase- lightweight steel
Hydrogen embrittlement (HE) is one of the most dangerous embrittlement problems in metallic materials and advanced high-strength steels (AHSS) are particularly prone to HE with the presence of only a few parts-per-million of H. However, the HE mechanisms in these materials remain elusive, especially for the lightweight steels where the composition and microstructure significantly differ from the traditional plain-carbon steels. Here we focus on a high-Mn and high-Al lightweight steel and unravel the effects of H-associated decohesion and localized plasticity on its H-induced catastrophic failure.
Driven by the increasing demand for passenger safety and weight reduction in the automotive industry, advanced Fe-Mn-Al-C alloys with low density and specific high strength have gained great attention in recent decades. Unfortunately, the concomitant issue is that these high strength materials are prone to hydrogen embrittlement (HE) as evidenced by the serious degradation of their load-bearing capacity with the presence of typically only a few parts-per-million of H, which impedes their further application due to an incomplete understanding of the HE mechanisms in multi-length scales particularly the synergistic interplay fundamentals between H and defects in the complex microstructure. In our project, we focus on a high-Mn and high-Al lightweight steel with an austenite and ferrite two-phase microstructure and unravel the effects of H-associated decohesion and localized plasticity on its H-induced catastrophic failure. The HE in this alloy is driven by both, H-induced intergranular cracking along austenite-ferrite phase boundaries and H-induced transgranular cracking inside the ferrite. The mechanisms of such H-induced damage behavior were studied regarding of thermal desorption analysis, fractography, H distribution behavior at boundaries, H effect on dislocation, H-assisted damage statistical analysis, and crystallographic facet orientations. The H-induced intergranular cracking is attributed to the H-enhanced decohesion mechanism (HEDE), while H-induced transgranular cracking inside the ferrite involves a competition between HEDE and hydrogen enhanced localized plasticity mechanism. Those findings provide some new insights into the boundary conditions of different HE mechanisms in high-strength alloys, their interplay and synergistic effects on damage evolution.