MODELING FRECKLE FORMATION IN 3 DIMENSIONS DURING SOLIDIFICATION OF MULTICOMPONENT ALLOYS

The formation of macrosegregation defects known as ''freckles'' was si mulated using a three-dimensional finite element model that calculates the thermosolutal convection and macrosegregation during the dendriti c solidification of multicomponent alloys. A recently introduced algor ithm was used to calculate the complicated solidification path of allo ys of many components, which can accommodate liquidus temperatures tha t are general functions of liquid concentrations. The calculations are started from an all-liquid state, and the growth of the mushy zone is followed in time. Simulations of a Ni-Al-Ta-W alloy were performed on a rectangular cylinder until complete solidification. The results rev eal details of the formation of freckles not previously observed in tw o-dimensional simulations. Liquid plumes in the form of chimney convec tion emanate from channels within the mushy zone, with similar qualita tive features previously observed in transparent systems. Associated w ith the formation of channels, there is a complex three-dimensional fl ow produced by the interaction of the different solutal buoyancies of the alloy solutes. Regions of enhanced solid growth develop around the channel mouths, which are visualized as volcanoes on top of the mushy zone. The prediction of volcanoes differs from our previous calculati ons with multicomponent alloys in two dimensions, in which the volcano es were not nearly as apparent. These and other features of freckle fo rmation phenomena are illustrated.