Effect of heat-treatment on phase stability and grain coarsening in a powder-processed Al-Ni-Co-Zr-Y alloy

Processing of Al alloys via metastable amorphous intermediates can give much higher volume fractions of dispersed strengthening phases than in conventional precipitation- or dispersion-hardened systems. Here, we report a study on an Al-Ni-Co-Zr-Y alloy processed by gas atomization and consolidated/devitrified by warm extrusion. X-ray diffraction and electron microscopy are used to reveal the effects of heat-treatments at 300-500 A degrees C for up to 96 h on the phase stability and coarsening behavior of the alloy. In all samples, the microstructure contains 22 % by volume of Al-19(Ni,Co)(5)Y-3 plates surrounded by grains of FCC Al. Samples heat-treated at 350 A degrees C and above also contain fine Al3Y and Al3Zr particles as minority phases. The softening of the alloy is limited for heat-treatment temperatures of up to 400 A degrees C, and the Al-19(Ni,Co)(5)Y-3 plates coarsen slowly. At higher temperatures, abnormal coarsening is observed with the development of a secondary population of much larger Al-19(Ni,Co)(5)Y-3 plates. An analysis of the coarsening kinetics gives a constant coarsening exponent of 3, but a distinct transition in the activation energies. These values suggest that the normal coarsening at lower temperatures occurs by short-circuit diffusion, whereas the abnormal coarsening at higher temperatures involves lattice diffusion. The Al grain size is dictated by the Al-19(Ni,Co)(5)Y-3 inter-plate separation, and grain growth is limited by the extent of plate coarsening. Such systems could form the basis of new high-strength high-temperature Al alloys for structural applications.