EFFECTS OF CHEMISTRY AND PROCESSING ON THE FRACTURE RELATED PROPERTIES OF P/M ALUMINUM ALLOY CT91 (FATIGUE, MICROSTRUCTURE, POWDER METALLURGY)

With the single exception of fatigue crack propagation resistance, the development of high strength powder metallurgy (P/M) aluminum alloys has been very successful. CT91, an example of this class of alloy, has been studied to determine the effect of chemistry (Co content), deformation processing (38% and 64% uniaxial upset forging), and stress relief on engineering and fracture related properties. Stress relief by compression after solution heat treatment was found to have little effect on the properties studied and made no apparent change in the microstructure. A slight reduction in strength and slight improvement in fatigue crack propagation (FCP) resistance was noted in material that had been stress relieved. Co content had the largest effect of all program variables on strength--the 0.4% Co alloy exhibits a 28 MPa (4 ksi) increase in both ultimate and yield strength compared to the 0% Co alloy. No effect of Co content on fracture toughness or FCP resistance was observed. Changing the uniaxial forging deformation from 38% to 64% produced no measureable effect on strength, toughness, or FCP resistance. There was a marked increase in scatter in the fracture related properties at the lower forging strain. This is attributed to the reduced degree of breakup and distribution of oxides found at the prior powder particle boundaries which provides sites for easy crack propagation. None of the major program variables had a significant effect on the LCF behavior of the alloys. Only single slope Coffin-Manson behavior was observed with (beta) ranging from 1.017 to 1.344. These (beta)'s are considerably higher than values reported for similar alloys in different product forms. This also is attributed to the inability of the forging process to adequately break up and distribute the oxide associated with the prior particle surfaces. TEM analysis of LCF indicated homogeneous deformation for all cases. In addition, for (DELTA)(epsilon)(,p) < 0.3%, fine slip band formation was occasionally observed. Results of LCF testing indicate that CT91 forgings do not conform to the LCF-based models of FCP behavior in the range of plastic strains studied.