Microstructure structure and mechanical properties of coherent precipitation strengthened ultrahigh strength maraging stainless steel

Ultra-high strength maraging stainless steels possess many important applications such as in aircraft landing gears owing to their excellent strength and good process ability. However, traditional ultra-high strength maraging stainless steels are facing the challenge of balancing strength and ductility while pursuing ultra-high strength. This is mainly due to the semi-coherent or non-coherent relationship between the precipitated nanoparticles and the body-centered cubic (BCC) martensitic matrix. In this work, a novel ultra-high strength maraging stainless steel (Fe-7.95Cr-13.47Ni-3.10Al-1.83Mo-0.03C-0.23Nb, weight percent, %) is designed using a cluster formula approach. Alloy ingots are prepared by vacuum induction melting under an argon atmosphere, followed by hot rolling at 950 ℃ and multiple passes of cold rolling. Finally, the alloy is aged at 500 ℃ for 288 h. Microstructural characterizations of the alloy in different aging states are performed using electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). As a result, the martensitic structure of the alloy is fragmented and elongated, with high-density dislocations (~1.8×10<sup>–3</sup> nm<sup>–2</sup>) and a large number of coherent B2-NiAl nanoparticles (<5 nm) observed in the BCC martensitic matrix after cold rolling and aging. In terms of mechanical properties, the alloy exhibits significant age-hardening, with a peak-aged hardness of 651 HV after ageing treatment. It also demonstrates an extraordinarily high yield strength (<i>σ</i><sub>YS</sub> = 2.3 GPa) and a decent elongation (El = 3.6%), indicating a well-balanced strength-ductility property. Finally, the origins of the ultra-high strength in the novel alloy are discussed in depth, showing that the ultra-high strength of this stainless steel comes from the strengthening effect of different microstructures. This study provides valuable guidance for designing high-performance ultra-high strength maraging stainless steels.