Power, torque, and rotation speed of the impeller of an attritor mill were measured during preparation of metal matrix composite powders. Compositions of Al center dot MgO and Al center dot B4C were selected to include ductile (Al) and brittle components. Brittle components, MgO, and B4C, were respectively softer and harder than the milling media (hardened steel). Yield strength of pressed powder compacts and x-ray diffraction peak width were treated as indicators of the achieved structural refinement. Both peak width and yield strength increased for both materials at longer milling times. Torque and power increased for Al center dot MgO but decreased for Al center dot B4C at longer milling times. The results were interpreted considering differences in hardness of MgO and B4C. Amplitudes of rapid oscillations of the milling process parameters were also monitored and were observed to correlate with the material refinement. Utility of real-time milling progress indicators for different types of materials is demonstrated. (C) 2012 American Institute of Chemical Engineers AIChE J, 59: 1088-1095, 2013