Rapid Solidification of Austenitic Stainless Steels by Splat Quenching

This thesis explores the phase transformations and microstructural evolution that are observed in rapid solidification of austenitic stainless steels by splat quenching. Splat quenching is an experimental method that produces rapidly solidified structures under cooling rates that are comparable to that of additive manufacturing. In additive manufacturing, specifically selective laser melting (SLM), metal is subjected to a very rapid heating and cooling that produces cooling rates in the range from 105-106 C/s. This rapid solidification produces microstructures that deviate from equilibrium. Five compositions of austenitic stainless steel with varying Cr/Nieq were studied to assess the effect of composition and cooling rate on the solidification microstructures that take place during splat quenching. The five compositions studied in this thesis were produced via arc melting to provide feedstock for splat quenching experiments. Splat quenched samples were analyzed for the presence of microsegregation, phase content, and solidification mode and morphology. Analysis techniques included electron dispersive spectroscopy, backscatter imaging, secondary imaging, and electron backscatter diffraction. Cooling rates achieved during splat quenching were evaluated utilizing electrolytic etching to estimate cell size of the solidification microstructures. Based on these analyses, the cooling rates were estimated to be in excess of 106 C/sec with a solidification velocity range of 0.1-1 m/s. The phase content of the splat quench microstructures as a function of alloy composition agreed well with the current rapid solidification literature.