Precipitation phenomena in a powder-processed quasicrystal-reinforced Al-Cr-Mn-Co-Zr alloy

The thermal behavior of a powder-processed Al-2.5Cr-1.35Mn-0.25Co-0.31Zr alloy (in at.%) has been studied by comparing hardness and SEM data from as-consolidated and heat-treated samples of the bulk alloy with data from in situ TEM heating experiments on sections through individual particles of the as-atomized powder. The starting microstructure consisted of an Al matrix with I-phase quasicrystalline dispersoids, and the distribution of the I-phase depended on the powder particle size. Most of the powder (≈ 70% by volume) exhibited a cellular dendritic Al microstructure with minority phases at the cell boundaries and I-phase at the powder particle surfaces. Upon heat treatment, the consolidated alloy retained the initial hardness and microstructure up to about 400 °C. At this point the alloy hardened by about 6%, and then softened significantly at higher temperatures. The main microstructural change was the precipitation of a plate-like ternary Al11(Cr,Mn)2 phase within the cellular Al matrix. In situ TEM observations on powder particles with the cellular dendritic microstructure at temperatures of 400–450 °C revealed the kinetics of the precipitation for this ternary phase. The kinetic data gave Avrami exponents of approximately 3 in the nucleation and growth regime. The temperature dependence of the rate constants gave an activation energy of 277 kJ/mol, which is consistent with Cr diffusion in the Al matrix being the rate-limiting process for the precipitation of Al11(Cr,Mn)2. The onset of the precipitation at 350–400 °C should not restrict the application of such alloys, but it may limit the processing if the decomposition of the supersaturated solid solution is to be avoided.