Strengthening of MMCs by interfacial reaction
Reactions between the matrix and reinforcement generally deteriorate the properties of the composites and should, therefore, be avoided. However, interfacial reactions are not always detrimental. For example, in the Al/Al-Cu-Fe quasicrystal (QC) composite system, we have used the interfacial reaction between the Al matrix and the QC reinforcement to further enhance the room temperature strength of the composites through the formation of a new microstructure consisting of the Al matrix reinforced with Al7Cu2Fe ω-phase particles . The mechanical behavior of these transformation-strengthened composites is remarkably improved as the QC-to-ω phase transformation progresses: the yield strength increases from 195 MPa for the starting material reinforced exclusively with QC particles to 400 MPa for the material where the QC-to-ω reaction is complete. The reduction of the matrix ligament size resulting from the increased volume fraction of the reinforcing phase during the transformation can account for most of the observed improvement in strength, whereas the additional strengthening can be ascribed to the possible presence of nanosized ω-phase particles as well as to the improved interfacial bonding between matrix and particles caused by the compressive stresses arising in the matrix.