Solidification of undercooled Fe-Cr-Ni alloys: Part II. Microstructural evolution

Results are reported on microstructures of Fe-Cr-Ni alloys, solidified over a range of undercoolings and quenched during or after recalescence. Alloys studied contained 70 wt pct Fe and with Cr varying from approximately 15 to 20 wt pct. The three lower Cr alloys were hypoeutectic (with fee as primary phase in equilibrium solidification); the two higher Cr alloys were hypereutectic (with bcc as primary phase in equilibrium solidification). Results obtained are in agreement with predictions based on thermal analyses previously presented; they confirm and extend the understanding gained in that work. The primary phase to solidify in the hypoeutectic alloys is bec when undercooling is greater than an amount which decreases with increasing Cr content. At the lower Cr contents, the stable fcc phase then forms by solid-state transformation of the metastable phase and its subsequent engulfment by additional fcc. At the higher Cr content, transformation is by a peritectic-like reaction in the semisolid state, except near the surface at higher undercoolings where the transformation is massive. In the hypereutectic alloys, primary solidification at all undercoolings is the stable bcc phase. Partial transformation to fcc occurs in the semisolid or solid state, depending on composition and undercooling. Formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology