ON THE INFLUENCE OF TIN ON THE FORMATION OF THE SIGMA-PHASE IN THE FE-CR SYSTEM

The influence of tin on the formation of the sigma-phase, caused by an isothermal annealing at 700-degrees-C, in Fe-Cr alloys with near equiatomic compositions was studied by means of Fe-57 and Sn-119 Mossbauer spectroscopy. It was shown that the addition of up to 5.5 at% Sn to the Fe Cr alloys retards the formation of the sigma-phase significantly. The kinetics of the transformation follows the Johnson-Mehl equation. The evidence was found that tin precipitates on the grain boundaries if its nominal concentration exceeds almost-equal-to 2 at%. For a given nominal concentration of added tin the size of grains was found to be the same in the alpha- and sigma-phases, but it was smaller for samples with higher Sn content. The composition of the sigma-phase remains constant at a value of 46.7 at% Cr and 1.4 at% Sn, regardless of the nominal tin content.