Phase transformations and phase stability of the AlCuFe alloys with low-Fe content

Phase transformation and microstructure of the AlCuFe alloys of the approximate compositional range of 20-50 at.% Cu and 1-10 Fe at.% have been investigated for annealed samples by means of differential thermal analysis, magneto-thermal analysis, scanning electron microscopy, electron probe analysis and powder X-ray diffraction. Representative phase transformations categorized as polymorphic, discontinuous precipitation, quasi-binary eutectoid, and ternary transition reaction (U-type) phase transformation are presented. These phase transformations are found to have a common feature, which is characterized by a dissolution of a primitive CsCl-type cubic phase and a formation of a face-centered orthorhombic phase. The crystal structures of the cubic and orthorhombic phases are determined from the powder diffraction data using routines for indexing DICVOL, for intensity extraction and ab initio structure determination EXPO and for Rietveld refinement FULPROF. A face-centered orthorhombic cell (a=8.1530(3), b=14.1370(4), c=10.0736(4), Vol=1161.0(7), Z=4, at room temperature) with a starting structural model of ζ1-Al3Cu4 (space group Fmm2, Z=4) approximates the observed pattern of powder sample quenched from 620 °C fairly well. Refined orthorhombic cell dimensions are obtained by in situ high-temperature X-ray diffraction at 463, 553 and 636 °C. These phase transformations are sensitive to the heat treatment and the alloy composition. They can be suppressed at moderate cooling rates. The metastable structures for both slowly cooled and as-cast alloys are found to contain the cubic phase replacing the equilibrium orthorhombic phase.