Abstract
Four alloys with nominal compositions Al-8Fe-4Ni, Al-8Fe-4Ni-2,5Zr, Al-8Fe-4Ni-2.5Zr-1Mo and Al-8Fe-4Ni-2.5Zr-2V-1Mo (wt%) were produced in the form of wedge shaped ingots by chill casting in a copper mould and in powder form by high pressure gas atomisation using Helium as the atomising gas. Sub-45, sub-50 or +50-100 mum powder size fractions were canned and degassed at 573K. The first two alloys were consolidated by hot extrusion at 573K and ER=18:1 and 25:1. The third alloy could be extruded at 698K and ER=5:1 while the powders of the fourth alloy were hot pressed at 623K because of difficulties with extrusion. The microstructures at the tips of the wedge castings could be related to those of the atomised powder particles. Rapid solidification suppressed the formation of Al3Fe, Al12Mo and A111V in the tips of the wedge castings and m powder particles up to 200 mum diameter, where the microstructures consisted only of a-Al and Al9FeNi, with some week evidence for possible formation of Ll2-Al3Zr in the larger powder particles. Two types of microstructures were observed in the powder particles: Zone A, consisting of mu-Al solid solution or microcellular mu-Al+Al9FeNi eutectic structures or Zone B consisting of mu -Al+Al9FeNi dendritic structures. The transition from Zone A to Zone B microstructres shifted to smaller powder particle sizes (from 20 mum to less than 10 mum) or smaller thickness (<300mum) of the wedge shaped ingots with increasing solute addition to the first alloy. In all cases the Al9FeNi phase formed under RS conditions had Fe/Ni=2 with solubility for the TM alloying additions. The microstructure of the extruded first alloy exhibited bands of finer and coarser structure without any remaining undeformed powder particles. The banding effect was far less pronounced m the extrusions of the second alloy, where the distribution of the Al9FeNi phase was more homogeneous but there was some evidence of non-fully deformed powder particles (<5vol%), which were always small (<10 mum diameter) and hard particles exhibiting the Zone A microstructure of the RS powders. In addition to mu-Al and Al9FeNi, there was a very small volume fraction of Many undeformed particles, which were associated with extended porosity, were also present in the extrusion of the third alloy but bands of finer and coarser structure were not visible. The hot pressed powders of the fourth alloy were at best partially deformed and had retained their RS microstructure with some present in the as pressed microstructure. The microstructures of the consolidated alloys were stable after heat treatment at 673K consisting of mu-Al, Al9FeNi and Ll2-Al3Zr and with the Al9FeNi phase experiencing some coarsening after 1000h, especially in the first alloy, where also the banded structure disappeared after prolonged treatment. The Zone A microstructure was significantly harder than Zone B. The additions of Zr, Mo and V resulted to increases in hardness of 20%, 60% and 110% over the first alloy in the as extruded or hot pressed conditions. After 1000h at 673K the hardness of the first alloy had dropped by 28% and 24% for the 18:1 and 25:1 extruded materials respectively, with the corresponding values for the second alloy being 17% and 11%. The hardnesses of the third and fourth alloys had dropped by about 15%. Higher tensile properties were achieved in the alloys extruded with ER=18:1. The ductilities of all alloys were poor owing to the high volume fractions of Al9FeNi. The mu-Al matrix of the first alloy extruded at ER=18:1 and 25:1 had a fibre texture with crystallographic directions parallel to the axis of the bar. The texture became stronger after heat treatment and consisted of a major fibre. Other minor components such as were also detected. Then- intensity was however fairly small when compared to . On the contrary, although the processing parameters were similar, the texture in the as-extruded bar of the second alloy (ER=18:1 and 25:1) was almost random.