Effects of Thermal Residual Stress on Mechanical Properties of SiC Whisker Reinforced Al–Mg and Al–Li Alloys Composites

Abstract

Effects of residual stresses measured by X-ray diffraction on mechanical properties in the extruded pure Al, age-hardenable Al-Li, Al-Li-Cu alloys and solution-hardening Al-Mg alloy matrices composites reinforced with SiC whiskers have been investigated as a function of whisker volume fraction and heat treatment process. The tensile residual stress in the matrix decreased remarkably by liquid nitrogen (L.N.) treatment, and dislocation density in the L.N.- treated composite is higher than that in the water-quenched composite. The tensile residual stress in the matrix decreases the tensile yield strength and increases the compressive one, and many dislocations increase also both at the yield strengths. Consequently, the tensile yield strength of the L.N.-treated composites is considerably increased by double positive effects, which one is due to the reduction of tensile residual stress and the other is the increase of the average dislocation density in the matrix. Finally, an application of the positive effects of L.N. treatment for the age hardenable Al-Li-Cu alloy composites leads further increase of the tensile strength compared to the conventionally aged composites.