Abstract
The development of aluminium alloys that can be processed by Rapid Solidification (RS) techniques for use in high temperature applications has recently been an area of intense study. One of the alloy systems of interest is Al-Cr-Zr-Mn. This work comprises a study of the microstructure and tensile properties of alloys of this system processed by melt spinning, high pressure gas atomisation (HPGA) and chill casting. The RS microstructures of Al-Cr and Al-Zr binary alloys were also compared with those of the quaternary alloys. The variety of microstructures observed in the powders of the quaternary alloys was consistent with the different cooling rates and nucleation temperatures experienced by droplets of different sizes, A cubic phase not previously reported was observed in the finer powder. The transition from a partitionless to a cellular microstructure occurred at estimated solidification front velocities similar to those predicted by morphological stability theory. The distribution of discrete Al[13]Cr[2] intermetallic particles within Al-Cr gas atomised powders of different sizes was found to be consistent with a probabilistic model of nucleants distributed in the volume of the alloy melt. Based on these results the original Al-5.2Cr-1.4Zr-1.3Mn (wt%) alloy was diluted to give an Al-3.3Cr-0.7Zr-0.7Mn (wt%) alloy so that the bulk of the powder (the sub-45mum size fraction) did not contain coarse intermetallic particles but exhibited a mainly cellular microstructure. A relationship has been determined between the thickness of wedge shaped chill castings and powder diameters for. similar microstructures. Prediction of alloy compositions designed to give a particular microstructure in a specified powder size can therefore be tested by a simple casting technique. The mechanical properties of the original and optimised quaternary alloy powders consolidated by Conform and extrusion have been determined and related to the as-consolidated and aged microstructures. The extruded powders of both alloys exhibited better properties than the Conformed powder. A large contribution to the strength of the extruded materials is made by their stabilised fine grain size. The optimised alloy had a consistently better ductility. Neither of the alloys retained its strength after prolonged treatment at 400°C, but the results suggest that a service temperature of 300°C may be possible.