Thermal stability of quasicrystals in an icosahedral-phase-strengthened aluminum alloy

The thermal stability and decomposition products of the quasicrystalline I-phase in a powder-processed Al-2.8Cr-1.51Mn-0.28Co-0.31Zr alloy (in at. %) have been studied using thermal analysis, electron microscopy studies on heat-treated samples, and in situ TEM heating studies. The alloy was consolidated through blind-die compaction and exhibited a nano-composite FCC Al matrix with ≈25% by volume of I-phase dispersoids. In samples heat treated ex situ at 300–450 °C, two transformations were observed. Initially, the I-phase decomposed by transforming to a ternary Al45(Cr,Mn)7 phase. This decomposition was accompanied by ejection of Co, which formed Al9Co2. After longer exposures, the Al45(Cr,Mn)7 transformed to an Al12(Cr,Mn) phase with a coarse irregular morphology. In situ TEM observations on sections through powder particles were used to identify the onset times for the initial decomposition process, and these were used together with activation energies determined by Kissinger analysis of isothermal DSC data to construct transformation curves. TEM measurements of the Co content from dispersoids in the samples heat treated ex situ were used to determine the extent of I-phase decomposition, and these data matched well with the trends expected from the curves. These observations provide a useful insight into the processing and operating conditions that might be used for such QC-reinforced Al alloys.