STRENGTHENING MECHANISMS IN ALUMINUM CERAMIC PARTICLE COMPOSITE ALLOYS PRODUCED BY MECHANICAL ALLOYING

Room-temperature yield strength of powder-metallurgy Al-ceramic particle composite alloys produced by mechanical alloying was analyzed from a viewpoint of microstructure which was characterized by several features: high dislocation density (in the order of 10(14) m-2), uniform dispersion of fine Al4C3 and Al2O3 particles (approximately 35 nm in size) and coarse ceramic particles (0.4-1.0-mu-m), and small grain size (approximately 0.5-mu-m). A large portion (more than 80%) of yield strength was concluded to be contributed through dispersion hardening by the line particles and particle reinforcing by the coarse particles; the contribution was greater through the former mechanism than through the latter one for a given volume fraction of particles. The former mechanism was based on detaching pinned-down dislocations at the fine particles from them, and the latter one on restricting matrix deformation by mechanical constraint around the coarse particles. Impurities in the Al matrix made a modest contribution to strength, probably through solution hardening. These mechanisms raised the strength additively. Work hardening due to a high density of dislocations introduced during processing and grain boundary strengthening due to small grain size were considered not to be principal mechanisms for determining the yield strength.