The effects of interfacial reaction zones on the mechanical properties of magnesium metal matrix composites

Four magnesium alloy particle reinforced metal matrix composites; Z6 / SiC, EM31 / SiC, AZ91 /SiC, and AZ91 / Al{dollar}sb2{dollar}O{dollar}sb3{dollar}; supplied by Dow Chemical Inc. were studied to determine the effects of reaction zone formation on mechanical properties. The composites were exposed at 773 K for extended periods of time to allow for formation of possible interfacial reaction zones. These reaction zones were observed using transmission electron microscopy. The AZ91 / Al{dollar}sb2{dollar}O{dollar}sb3{dollar} composite showed no evidence of a reaction zone while the SiC reinforced composites formed Mg{dollar}sb2{dollar}Si at the interface. The alloying additions of Al and Ce dramatically increased the extent of the reaction zones, through formation of Al and Ce carbides.; The composites' mechanical properties were adversely affected by reaction zone formation. The Young's modulus of the composites was modeled using the theories of isostress and isostrain, Hashin and Shtrikman upper and lower bounds, and Halpin and Tsai equations. Modeling indicated that the observed decreases in modulus could not be attributed to formation of the reaction zone, some additional mechanism was involved. Microcracking parallel to the reaction zone was found to be a theoretically feasible mechanism to account for the observed decreases in modulus. Such microcracking of the interface region was observed through surface replicas of the samples with reaction zones.; The interfacial energy was determined using Young's equation and the void particle contact angles observed after cold rolling. The interfacial energy increased substantially during reaction zone formation. The location of interfacial void formation changed from particle-matrix debonding to particle-reaction zone debonding. The increases in interfacial energy and changes in debonding behavior were used to explain decreases in tensile properties of the composites including yield stress, ultimate tensile strength and strain to void nucleation.