Abstract
The effect of heat treatments on the microstructure and mechanical properties of Ni base superalloys was investigated. The tested alloys were manufactured by investment casting under various conditions of superheat. These alloys were solution treated at 1120 and 1180 degrees C for 2 h followed by air cooling before aging process at 845 degrees C for 24 h. The volume fraction, V-f, of TCP (Tetragonal Close Packed) phases decreases as the solution treatment temperature increases. The grain size of aged specimens treated at 1180 degrees C was found to be coarser than that treated at 1120 degrees C. The V-f of primary gamma' particles in aged alloys treated at 1120 degrees C was higher than that of alloys treated at 1180 degrees C. However, V-f of secondary gamma' precipitates in aged alloys treated at 1180 degrees C was found to be larger than that in alloys treated at 1120 degrees C. Hardness values of aged alloys treated at 1120 degrees C were lower than those measured for 1180 degrees C treated alloys. The effect of solution heat treatments on the corrosion behavior of the tested Ni base superalloys was also studied via monitoring corrosion rates of these alloys in 1.0 M H2SO4 solution at 25 degrees C. Rates of corrosion were monitored based on ICP-AES (inductively coupled plasma atomic emission spectrometry) method of chemical analysis via determination of Ni2+ in solution after each corrosion test. The amount of nickel released into the corrosive medium was taken as a measure of the corrosion rate. The corroded surfaces were examined by the light microscope. Results obtained revealed that the corrosion rate of the aged alloys treated at 1120 degrees C was much higher than that of aged alloys treated at 1180 degrees C. At the same temperature, alloys with fine microstructure were found to resist corrosion compared to those with coarse microstructure. The electrochemical behavior of such alloys was also studied in 1.0 M H2SO4 solution based on polarization measurements. The four tested Ni alloys exhibited a passive behavior in 1.0 M H2SO4 solution, and their anodic behavior in the active dissolution region was discussed.