Developing NiAl-strengthened HSLA steels by controlling nanoscale precipitation and high-angle boundaries

Abstract

In this work, an attempt was made to develop NiAl precipitation-strengthened HSLA steels by introducing Al to the alloy composition of conventional HSLA steels. A novel HSLA steel with a superior combination of yield strength (1234 MPa) and total elongation (13.3%) was achieved. The precipitation behaviors of nanoparticles (including NiAl and carbides) and the collateral effects of Al on the microstructural evolution in this steel were studied. The results show high densities of Ni(Al, Mn) and (Mo, V)C precipitate in this alloy, but low precipitation kinetics. The high Al/Ni atom ratio in the alloy composition and high dislocation density promote the precipitation of Ni(Al, Mn). Additionally, the Ni(Al, Mn) nanoparticles serve as heterogeneous nucleation sites for the (Mo, V)C carbide, significantly increasing the nucleation rates of the carbide. Except for precipitation, increasing Al also increases the proportion of high-angle grain boundaries by altering martensitic transformation kinetics, resulting in a refinement of the equivalent grain size. This microstructure evolution also optimizes the matrix's dislocation distribution, allowing the steel to retain good ductility after precipitation strengthening. Finally, theoretical models are used to quantitively evaluate the contributions of the above changes induced by Al to yield strength.