Effect Of Nd And Reciprocating Extrusion On Microstructure And High Temperature Creep Resistance Of Al₂O_3f/AZ91D Composite

Al2O3f/AZ91D composites with different Nd contents (0.0%,0.2%,0.5%and 0.8%) were prepared by pressureless infiltration method, and the volume fraction was 20%. Reciprocating extrusion was carried on the samples with 0.2%,0.5%and 0.8%Nd. The effect of Nd and reciprocating extrusion on the microstructure, phase composition and fracture mechanism were analyzed by OM, SEM, XRD and EDS. Effect of Nd on the hardness and high temperature creep resistance were studied, and the creep resistance of Al2O3f/AZ91D was studied by the high temperature compression tests. Creep process and creep mechanism were revealed by creep curves and creep stress exponent of composites, and the evolution of microstructure and dynamic recrystallization mechanism during the creep process were also discussed.The granular and rod shaped Al-Nd phase were formed and matrix microstructure was refined by adding Nd. The addition of Nd made Mg2Si phase become fine and dispersive. The hardness and the creep resistance of composites added with Nd were improved significantly, and the optimum content of the additives was 0.8%. The microhardness of Mg2Si phase increased first and decreased afterwards by adding different contents of Nd, and reached maximum at 0.5%.The matrix microstructure of Al2O3f/AZ91D composite was refined, the segregation of fiber was eliminated and the homogeneity of fiber distribution was improved after 2 passes of reciprocating extrusion. The creep resistance of composites after reciprocating extrusion was better than the composites as cast at temperature 250℃under pressure 90MPa. The hardness of composite was improved although the microhardness of Mg2Si phase was reduced by reciprocating extrusion.After high temperature compression creep test, the distribution of fiber became even, matrix microstructure was refined and deformation twins and cross sliding lines could be observed, and recrystallized grains were formed in fiber segregation area. According to the lnε-lnσcurve at temperature 250℃, stress exponent n is calculated to be 6.6, which indicates that the controlled high temperature creep mechanism is dislocation climb controlled by grain boundary diffusion and a constant substructure model. According to n theε1/n-σcan be plotted and the threshold stress is calculated to be 1.32MPa at temperature 250℃.