Mechanical milling of gas-atomized Al-Ni-Mm (Mm = misch metal) alloy powders

Al-14Ni-14Mm (Mm = misch metal) alloy powders rapidly solidified by the gas atomization method were subjected to mechanical milling (MM). The microstr ucture, hardness, and thermal stability of the powders were investigated as a function of milling time using X-ray diffraction (XRD), transmission ele ctron microscopy (TEM), and differential scanning calorimetry (DSC) methods . In the early stages of milling, a cold-welded layer with a fine microstru cture formed along the edge of the milled powder (zone A). The interior of the powder remained unworked (zone B), resulting in a two-zone microstructu re, reminiscent of the microstructures in rapidly solidified ribbons contai ning zones A and B. With increasing milling time, the crystallite size decr eased gradually reaching a size of about 10 to 15 nm and the lattice strain increased reaching a maximum value of about 0.7 pet for a milling time of 200 hours. The microhardness of the mechanically milled powder was 132 kg/m m(2) after milling for 72 hours and it increased to 290 kg/mm(2) after mill ing for 200 hours. This increase in microhardness is attributed to a signif icant refinement of microstructure, presence of lattice strain, and presenc e of a mixture of phases in the alloy. Details of the microstructural devel opment as a function of milling time and its effect on the microhardness of the alloy are discussed.