Influence of solidification rate on precipitation and microstructure of directional solidification IN792 + Hf superalloy

The effect of solidification rate on the precipitation and microstructure of directional solidification IN792 + Hf 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 γ solid front before it was captured. The incorporation of γ 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 (γ + MC carbide) at a solidification rate of 0.5 μm/s. Two zones, the γ′ forming elements enriched zone and depleted zone, were found in the residual liquid area. Eutectic γ/γ′ nucleated in the γ′ forming elements enriched zone. The η-phase precipitation was controlled by the ratio of (Ti + Hf + Ta + W)/Al in the residual liquid. The growth of eutectic γ/γ′ increased the ratio and induced the η-phase precipitation. A lower solidification rate decreased the ratio by sufficient diffusion and hence efficiently suppressed the η-phase precipitation.

On the mechanism of the thermal transformations in solid ammonium nitrate

The thermal transformations which take place in solid ammonium nitrate have been studied with the following techniques: differential thermal analysis, measurement of d.c. electrical conductance, optical microscopy, X-ray diffraction and nuclear magnetic resonance. It has been found that in dry ammonium nitrate, transformations between forms V, IV, II and I only take place and that the transformations appear to be of the order-disorder type. The transformations IV ⇌ III only take place in the presence of moisture and the mechanism appears to be one of dissolution and recrystallization.