Effects of severe ausforming on hierarchical microstructural development and mechanical performance in a martensitic high-strength steel

Martensitic high-strength steels have recently increased in significance as structural engineering materials for lightweighting in aerospace and defense applications due to their low cost, high strength and specific strength, and excellent toughness. In this study, high-strength martensitic AF9628 steel was ausformed at high temperatures (≥ 950°C) via the severe plastic deformation technique known as equal channel angular pressing (ECAP), and the hierarchical microstructures were comprehensively characterized, from the prior austenite grains to the martensite packets, blocks, and laths, and finally, to the precipitation of various carbide families. In fact, the present study is the first comprehensive work systematically revealing different carbides forming in this steel. Different combinations of ECAP processing parameters produced a wide variety of prior austenite grain sizes and morphologies that, in turn, produced different levels of martensitic refinement upon quenching. Transmission electron microscopy also confirmed the presence of various carbide families (e.g., the ε-, η-, and θ-carbides) that precipitated during tempering at 200°C and contributed to AF9628's high toughness. Overall, this study investigates how ausforming-ECAP can produce upper-performance limits of the enhanced ultimate tensile strength (beyond 2 GPa) and Charpy toughness (beyond 55 J at −40°C) in AF9628 via hierarchical microstructural refinement and morphology control. Here, process, structure, and property relationships are established for ausforming-ECAP of martensitic high-strength steels, demonstrating the wide variety of effects that unique prior austenite grain microstructures can have on mechanical performance. Overall, these findings indicate that precise control of the prior austenite grain structure can unlock novel mechanical properties in martensitic steels.