Abstract
Aluminum based cryomilled ultrafine-grained metal matrix composites (UFG-MMCs) are of interest for their high specific strength when compared to conventional Al alloys - sometimes in excess of three times the strength on an unreinforced conventional alloy. These MMCs present unique challenges to understanding the effect of thermomechanical processing on microstructure, which is essential to maintaining desirable mechanical properties in the UFG-MMC. Trimodal UFG-MMCs, consisting of cryomilled Al 5083, B4C, and unmilled Al 5083, were vacuum hot pressed and extruded for this investigation. Extrusions at various temperatures and ratios were studied, with powders cryomilled at different times to change the contribution of nitrogen to strength. Microstructural evidence reveals a change in grain size scaled with extrusion ratio attributed to stress assisted grain growth, and wet chemical analysis shows variations in the nitrogen content that scale directly with strength. Zener-Holloman and Hall-Petch parameters of the UFG-MMCs are discussed.