The effect of processing and microstructure on the mechanical properties of aluminum alloy 339-T5

The influence of six microstructural variations of 339-T5 on tensile behavior, high cycle fatigue crack initiation (HCF) and fatigue crack propagation (FCP) was determined. Specimens were tested at room temperature, 177{dollar}spcirc{dollar}C, and 343{dollar}spcirc{dollar}C, representing extremes of service conditions of diesel pistons. Specimens were cast (1) conventionally in permanent molds, (2) in permanent molds plus water quenching following solidification, and (3) in a squeeze-cast press. One series of the above processing conditions was inoculated with phosphorus and the other, with strontium, both silicon modifying agents.; Phosphorus-inoculated specimens surpassed strontium inoculation in tensile strength and crack initiation resistance. Water quenching after casting improved tensile properties but effects on crack initiation were trivial. Squeeze casting produced ambiguous results, attributed to modest microstructural refinement (versus the other conditions) and inconsistent thermal history. Fatigue crack propagation was not sensitive to the microstructural differences.; Tensile failure was controlled by the strength of the interdendritic region. Closely-spaced interdendritic constituents generated a triaxial stress state and a brittle fracture mode. At 343{dollar}spcirc{dollar}C dendritic ligaments remained after interdendritic failure and sustained decreasing loads.; Stress concentrations at constituent particles and precipitate free zones at phase boundaries led to interface decohesion and HCF crack initiation. Microporosity, when present, facilitated crack initiation. Cracks initiated quite early but left the initiation site much later, the result of decreasing stress gradients surrounding hard particles. Constituent particles in Sr-inoculated specimens were more closely spaced which diminished stress gradients and allowed crack propagation to start sooner. Crack initiation at 343{dollar}spcirc{dollar}C was accompanied by accumulation of extensive surface damage due to strain incompatibility between the matrix and constituent phase particles.; The insensitivity of FCP to the microstructural variations was credited to absence of differences in basic deformation modes. A conceptual model was proposed that considers contributions of independently growing secondary cracks within the plastic zone. This model explains observations of multitudinous striation orientations and the general low resistance to FCP.