Strengthening Aluminum By Zirconium and Chromium

Yan, Shi

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Strengthening Aluminum By Zirconium and Chromium

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The Al-Zr system is used to form a thermally stable strengthening phase in high temperature aluminum-base casting alloys. These alloys have good strength at elevated temperature due to the precipitation of coherent metastable Al3Zr particles upon decomposition of the supersaturated Al-Zr solid solution by a carefully designed heat treatment. Formation of the Al3Zr particles occurs by a peritectic reaction, which decrees that once formed, the particles cannot be dissolved by a solid-state homogenization process. Accordingly, melting the alloy must serve as the homogenization step of the precipitation hardening process; and solidification during casting must serve as the quenching step. Unfortunately, a prohibitively fast solidification rate is necessary to obtain a solid solution with as little as 0.4% Zr in Al. It is found that adding Cr to Al-0.4wt%Zr binary alloy makes it easier to form the supersaturated solid solution, and the ternary Al-0.4wt%Zr- 0.8wt%Cr alloy has better room and elevated temperature tensile properties than the binary Al- 0.4wt%Zr alloy. Various one-step and two-step isothermal aging cycles were investigated in order to arrive at the optimum aging schedule for the Al-0.4wt%Zr-0.8wt%Cr. It is found that soaking the alloy at 400C for 24 hours is optimum; and employing a two-step aging schedule reduces the aging time without sacrificing strength. The two- step aging schedule includes soaking the alloy at 375C for 3 hours and then at 425C for an additional 12 hours. Examination of the precipitates that form in the Al-0.4wt%Zr-0.8wt%Cr with High Resolution Transmission Electron Microscopy (HRTEM) shows that they have the L12 crystal structure. Energy Dispersive Spectrometry (EDS) shows that the particles contain only aluminum and zirconium whereas the matrix is a solid solution of chromium in aluminum. Hence, it is suggested that zirconium strengthens the Al- 0.4wt%Zr-0.8wt%Cr alloy by a precipitation hardening mechanism and chromium further enhances the strength by solid solution strengthening.

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