Fatigue and fracture of porous steels and Cu-infiltrated porous steels

The effects of Cu infiltration on the monotonic fracture resistance and fatigue crack growth behavior of a powder metallurgy (P/M) processed, porous plain carbon steel were examined after systematically changing the matrix strength via heat treatment. After austenitization and quenching, three tempering temperatures were chosen (177 degrees C, 428 degrees C, and 704 degrees C) to vary the strength level and steel microstructure. The reductions in strength which occurred after tempering at the highest temperature were accompanied by the coarsening of carbides in the tempered martensitic steel matrix, as confirmed by optical microscopy and by microhardness measurements of the steel. Each steel-Cu composite, containing approximately 10 vol pet infiltrated Cu, had superior fracture toughness and fatigue properties compared to the porous matrix material given the same heat treatment. Although the heat treatments given did not significantly change the fatigue behavior of the porous steel specimens, the fatigue curves (da/dN vs Delta K) and fracture properties were distinctly different for the steel-Cu composites given the same three heat treatments. The fracture toughness (K-IC and J(IC)), tearing modulus, and Delta K-TH,values for the composites were highest after tempering at 704 degrees C and lowest after tempering at 177 degrees C. In addition, the fracture morphology of both the fracture and fatigue specimens was affected by changes in strength level, toughness, and Delta K. These fractographic features in fatigue and overload are rationalized by comparing the size of the plastic zone to the microstructural scale in the composite.