A model for vorticity-generated segregation during electron beam surface melting

Abstract An unusual cellular pattern, two orders of magnitude greater than the scale of the solidification structure, arises from centrifugal motion of carbide particles within the melt. The vortices involved are generated from circulatory flow instabilities. The necessary driving force for the circulatory flow is the temperature differential between the plasma cavity and the rear solid-liquid interface. A consequence of the centrifugal motion is enrichment of carbide-forming elements at the peripheral region of the vortices. Chemical analysis of the carbides formed on solidification using the Philips EM420 analytical electron microscope shows them to be unusually rich in molybdenum (45–46 wt.%) and low in iron and chromium (6–12 wt.% Cr, 1.5−1.3 wt.% Fe). This is in striking contrast to an analysis of carbide phases in conventionally treated material.