MICROSTRUCTURE AND MECHANICAL-BEHAVIOR OF CR-CR2HF IN-SITU INTERMETALLIC COMPOSITES

A detailed investigation of the effects of microstructural changes on the mechanical behavior of two in situ intermetallic composites with C r and Cr2Hf phases in the Cr-Hf system was performed. The nominal comp ositions (at. pct) of the alloys were Cr-5.6Hf (hypoeutectic) and Cr-1 3Hf (eutectic). The study included evaluations of strength, ductility, and fracture toughness as a function of temperature and creep behavio r. Two microstructures in each alloy were obtained by heat treatments at 1250 degrees C (fine microstructure) and 1500 degrees C (coarse mic rostructure).;A decrease in elastic strength (stress at the onset of i nelastic response in the load-deflection curve) with the coarsening of the microstructures was noted for both alloys below 1000 degrees C. T he Cr-13Hf ahoy retained strength to a higher test temperature, relati ve to Cr-5.6Hf alloy, under both microstructural conditions. The alloy s showed no evidence of ductility at room temperature. However, in the coarse microstructure of the Cr-5.6Hf alloy, the primary Cr exhibited ductility at and above 200 degrees C; ductility in primary Cr could b e seen only at and above 1000 degrees C for the fine microstructure. I n other words, the temperature at which ductility was first observed d ecreased from about 1000 degrees C to about 200 degrees C due to high- temperature heat treatment in this alloy. Both microstructures of Cr-5 .6Hf alloy showed a significant increase in fracture toughness with in creasing test temperature. However, the increases in fracture toughnes s with temperature for the Cr-13Hf alloy microstructures were relative ly small. Both alloys showed about four orders of magnitude reduction in steady-state creep rates relative to pure Cr at 1200 degrees C. The results are analyzed in the light of deformation characteristics and fracture micromechanisms. The effects of microstructural factors, such as the size and continuity of phases, solubility levels of Hf as well as interstitial elements in Cr, on the observed mechanical behavior a re discussed.