Effect of Cr and Ni/Fe Ratio on the Microstructure and Mechanical Properties of γ′-Strengthened Ni–Fe-Based Alloys

To continue to meet the future materials’ requirements for advanced power generation systems, enhancing the mechanical properties and long-term phase stability of Ni–Fe-based alloys is needed. In this study, alloying modifications were made to improve the phase stability and 0.2 pct yield strength of modified cast austenitic stainless steels used for high-temperature heat exchanger applications. The focus was to investigate the influence of Cr concentration and the ratio of Ni/Fe in the alloy system on the elevated temperature yield strength, ductility, and its matrix phase stability. Over the range of experimental alloy compositions investigated, the best balance of mechanical properties in tension at 750 °C was achieved through a modest reduction in Cr as this alloy possessed a yield strength of 630 MPa and 28 pct elongation. The σ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sigma $$\end{document} phase volume fraction was also limited compared with other alloys. The modifications in Cr concentration and Ni/Fe ratio to the alloy system are independent with the γ′ precipitate solvus temperature and volume fraction formed during aging. However, decreasing Cr concentration from 21 at to 18 at.pct was observed to increase the yield strength and decrease the fraction of σ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sigma $$\end{document} phase precipitation. Reductions in the Ni/Fe ratio led to a reduction of the tensile ductility as this tended to promote the formation of discontinuous precipitation along the grain boundary.