Tensile/Compressive Fatigue and Interface Characterization of Al2O3 Fiber/Al-2.5% Li Alloy Metal Matrix Composite.

摘要

The fatigue response to cyclic roads of a continuous A12O3 fiber/aluminum-lithium alloy metal matrix composite was successfully characterized at room temperature and 175 C. Low cycle fatigue was dominated by crack propagation perpendicular to the axial fiber direction at both temperatures. Alternately, high cycle fatigue was governed by crack propagation parallel to the axial fiber direction; at room temperature this crack growth was believed to have occurred through the matrix, whereas at 175 C it propagated through weakened composite interfaces. The interfacial reaction product was also thoroughly characterized for both as-received composites and composites subjected to 10 million cycles at 175 C (defined as runout). In the as-received composite, interfaces ranged in thickness from 0.05 to 0.8 microns. Its growth was non-homogeneous and preferential along the grain boundaries of Al203. The reaction product was composed of two phases: (1) LiA1508 (a cubic spinel) and (2) alpha-LiA102. The cubic spinel was determined to be the major phase. The combination of cyclic loads (as they affect compressive and tensile strength) and elevated temperatures caused an exaggerated growth of the interfacial reaction zone; the layer thickness after runout ranged from 0.05 to 6.0 microns. This growth process corresponded to a weakening of the interface, which decreased fatigue strength at the elevated test temperature of 175 C.