AbstractA method is described for designing powder metallurgy rapidly solidified aluminium alloys using experimental and/or calculated nucleation maps which give the microstructure of gas atomised powders as a function of powder particle size and alloy composition. This method was used to predict the compositions of Al–Cr–Zr–Mn alloys for which the<45μm sizefraction of the gas atomised powders exhibits a microstructure with or without Al13Cr2intermetallic particles. Powders were produced by high pressure gas atomisation and were examined using analytical electron microscopy. The microstructures observed were in excellent agreement with those predicted. The powders exhibited four distinct microstructures with increasing powder particle diameter: (i) segregation free, (ii) cellularαaluminium, (iii)αaluminium plus fine spherical precipitates rich in chromium and manganese, and (iv)αaluminium plus Al13Cr2primary intermetallic particles. The solidification of these powders is discussed in terms of solidification front velocity controlled by external heat flow and by the initial undercooling. Particles less than 10μm in diameter undercool significantly before solidification. Segregation free microstructures occur in the fine<1μm) particles, where the solidification front velocity exceeds the absolute stability velocity.MST/1247a