CREEP OF METALS CONTAINING HIGH-VOLUME FRACTIONS OF UNSHEARABLE DISPERSOIDS .2. EXPERIMENTS IN THE AL-AL2O3 SYSTEM AND COMPARISON TO MODELS

The tensile and compressive creep properties of coarse-and fine-graine d dispersion-strengthened aluminum with 25 vol.% submicron alumina dis persoids are presented for temperatures between 335 degrees C and 500 degrees C and stresses between 30 MPa and 110 MPa. For all stresses in vestigated, the minimum creep rate is higher in tension than in compre ssion, because cavitation is the main deformation mechanism in tension . In compression, however, dislocation creep is the dominant deformati on mechanism al all stresses for the large-grained material and at hig h stresses for the fine-grained material, while diffusional creep domi nates in the fine-grained material at low stresses. The apparent stres s exponents for both diffusional creep and dislocation creep are much higher than for unreinforced aluminum, indicating that the dispersoids strongly inhibit both mechanisms. The threshold stresses determined e xperimentally for dislocation creep are significantly higher than thos e predicted by existing climb or detachment models, which consider the interaction of a single dislocation with dispersoids. Since transmiss ion electron microscopy reveals that several dislocations typically in teract with a single dispersoid, the modified threshold stress model p resented in the theoretical companion article [1] is applicable, where by the stress of dislocation pile-ups upon the threshold-controlling d islocation is taken into account. Good agreement is found between the experimentally determined threshold stresses and theoretical predictio ns from that model. The same model can also satisfactorily explain the very high measured values of the apparent activation energy. (C) 1997 Acta Metallurgica Inc.