Influence of solidification rate on precipitation and microstructure of directional solidification IN792+Hfsuperalloy

The effect of solidification rate on the precipitation and microstructure of directional solidification IN792 + I-If alloy was studied. The solidification sequence and the initial precipitation temperature of different phases were determined by the observation of the quenched microstructure combined with the differential thermal analysis measurement. The script carbide was turned into faceted carbide with the drop of solidification rate. It was concluded by microstructure analysis that the faceted carbide was pushed by the gamma solid front before it was captured. The incorporation of gamma phase into the faceted carbide was due to the dendrite growth of the carbide toward one point and the mergence of the dendrites. Some long carbide bars were formed along the grain boundaries by continual reaction of eutectic (gamma + MC carbide) at a solidification rate of 0.5 mu m/s. Two zones, the gamma' forming elements enriched zone and depleted zone, were found in the residual liquid area. Eutectic gamma/gamma' nucleated in the gamma' forming elements enriched zone. The eta-phase precipitation was controlled by the ratio of (Ti + Hf + Ta + W)/Al in the residual liquid. The growth of eutectic gamma/gamma' increased the ratio and induced the eta-phase precipitation. A lower solidification rate decreased the ratio by sufficient diffusion and hence efficiently suppressed the eta-phase precipitation.