Mechanism of heterogeneous distribution of Cr-containing dispersoids in DC casting 7475 aluminum alloy

The actual effective partition coefficients of Mg and Cr in a cross-section of a dendrite arm in a direct-chill(DC)-casting ingot of 7475 aluminum alloy are obtained.Meanwhile,by analyzing the microstructure,the mechanism of the heterogeneous distribution of E(Al_(18)Mg_(3)Cr_(2))dispersoids in this DC ingot is revealed.The results show that the actual effective partition coefficients of Mg and Cr are 0.650 and 1.392,respectively,and they describe the heterogeneous distributions of Mg and Cr along the direction of radius of the cross-section of the dendrite arm of the alloy.After homogenization treatment at 470℃ for 24 h,Mg diffuses uniformly,but Cr hardly diffuses.Both the concentrations of Mg and Cr and the sites of heterogeneous nucleation in the alloy are the determinants of the formation of E dispersoids simultaneously.The heat treatment at 250℃ for 72 h provides a large number of the sites of heterogeneous nucleation of the formation of fine E dispersoids that will be formed in the center of the cross-section during the subsequent heat treatment at higher temperature.

Effects of Prior Corrosion with and Without Stress on the Mechanical Properties of 7475-T761 Aluminum Alloy

The effect of prior corrosion on the mechanical properties of 7475-T761 aluminum alloy was investigated by immersion test, stress corrosion test, cathode charge method and electrochemical polarization test. Results show that prior corrosion in the solution with 3 wt% NaC1 and 0.5 wt% H202 leads to mechanical properties deterioration of 7475-T761 aluminum alloy. Moreover, the elongation decreases significantly. This is mainly attributed to electrochemical corrosion and hydrogen embrittlement, in which corrosion plays a major role. Tensile stress promotes the degradation of the mechanical properties by accelerating the pitting corrosion and hydrogen embrittlement.