EFFECTS OF GROWTH-RATE ON CARBIDES AND MICROPOROSITY IN DS200+HF SUPERALLOY

The effects of growth rate on the carbide morphology and microporosity were investigated using DS200 + Hf superalloy, between 16.7 x 10(-6) and 266.7 x 10(-6) m s-1. The fact that the shape factor remained almost unchanged with the growth rate indicates that the shape of the carbide particles does not directly depend on the cooling rate in this alloy. The stability of carbide particles was considered in terms of the interfacial energy between the carbide and matrix interface and the fluctuation of carbide composition. It was observed that the carbide/gamma-matrix interfacial area per unit volume as a function of growth rate remained almost unchanged (especially above 66.7 x 10(-6) m s-1), indicating that the rate of coarsening of carbides during solidification is not affected by the carbide/matrix specific interface energy. One of the factors which determines the rate controlling step for the coarsening of carbide particles is suggested to be Ti in the interdendritic and grain-boundary regions, and Hf in the vicinity of the incipient melting region.