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

Abstract

Al-14Ni-14Mm (Mm = misch metal) alloy powders rapidly solidified by the gas atomization method were subjected to mechanical milling (MM). The microstructure, hardness, and thermal stability of the powders were investigated as a function of milling time using X-ray diffraction (XRD), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC) methods. In the early stages of milling, a cold-welded layer with a fine microstructure formed along the edge of the milled powder (zone A). The interior of the powder remained unworked (zone B), resulting in a two-zone microstructure, reminiscent of the microstructures in rapidly solidified ribbons containing zones A and B. With increasing milling time, the crystallite size decreased gradually reaching a size of about 10 to 15 nm and the lattice strain increased reaching a maximum value of about 0.7 pct for a milling time of 200 hours. The microhardness of the mechanically milled powder was 132 kg/mm2 after milling for 72 hours and it increased to 290 kg/mm2 after milling for 200 hours. This increase in microhardness is attributed to a significant refinement of microstructure, presence of lattice strain, and presence of a mixture of phases in the alloy. Details of the microstructural development as a function of milling time and its effect on the microhardness of the alloy are discussed.