Local thermal expansions and lattice strains in Elinvar and stainless steel alloys

Local thermal expansions and lattice strains in the Elinvar alloy ${\mathrm{Fe}}_{49.66}{\mathrm{Ni}}_{42.38}{\mathrm{Cr}}_{5.49}{\mathrm{Ti}}_{2.47}$ (Ni Span C) and the stainless steel $\text{SUS}304{\mathrm{Fe}}_{71.98}{\mathrm{Ni}}_{9.07}{\mathrm{Cr}}_{18.09}{\mathrm{Mn}}_{0.86}$ (AISI304) were investigated by the temperature-dependent Cr, Fe, and Ni $K$-edge extended x-ray absorption fine-structure (EXAFS) measurements, combined with the path-integral effective classical potential Monte Carlo (PIECP MC) theoretical simulations. From the EXAFS analysis of the Elinvar alloy, the local thermal expansion around Fe is found to be considerably smaller than the ones around Ni and Cr. This observation can be understood simply because Fe in the Elinvar alloy exhibit an incomplete Invar-like effect. Moreover, in both the Elinvar and SUS304 alloys, the local thermal expansions and the lattice strains around Cr are found to be larger than those around Fe and Ni. From the PIECP MC simulations of both the alloys, the first-nearest neighbor Cr-Fe pair shows extraordinarily large thermal expansion, while the Cr-Cr pair exhibits quite small or even negative thermal expansion. These findings consequently indicate that the lattice strains in both the Elinvar and SUS304 alloys are concentrated predominantly on the Cr atoms. Although the role of Cr in stainless steel has been known to inhibit corrosion by the formation of surface chromium oxide, the present investigation may interestingly suggest that the Cr atoms in the bulk play a hidden new role of absorbing inevitable lattice strains in the alloys.