EFFECTS OF VANADIUM AND CHROMIUM ADDITION ON S'-PHASE PRECIPITATION IN A 2091-ALUMINUM-ALLOY

Age-hardening characteristics and precipitate structures have been exa mined in 2091 alloys (Al-2.15%Li-2.04%Cu-1.5%Mg-0.14%Zr) with and with out 0.15%V or 0.19%Cr addition, with special attention to the relation ship between S'-phase precipitation and the dispersoid particles which are formed during ingot homogenization. It was found that S'-phase pr ecipitates mainly on dislocation loops around incoherent Zr-bearing an d V-bearing dispersoid particles. In contrast, coherent Zr-bearing dis persoids and incoherent coarse Cr-bearing ones provided no nucleation site for S'-precipitate. The numerical density of S'-precipitates was not sufficient to contribute to hardening, corresponding to the low de nsity of incoherent Zr-bearing and V-bearing dispersoids. The low dens ity of V-bearing dispersoids was attributed to the low concentration o f vanadium in solution before ingot homogenization, which was caused b y the formation of a number of coarse V-bearing constituent particles during solidification. To avoid this, small 2091+V alloy ingots were p repared by chill-casting. The density of V-bearing dispersoids was sig nificantly increased, leading to a finer distribution of S'-precipitat es as well as a higher hardness level, which was comparable to that ob tained in the 2098 alloy T8-treated.