Influence of the Porosity on the Damage Evolution of Ni-Cu-Mo Steels Sintered by the Conventional Method and Spark Plasma Sintering

Abstract

The damage evolution of Ni-Cu-Mo conventionally sintered (CS) steels was investigated and compared to the damage resistance of a fully dense equivalent model material obtained by spark plasma sintering (SPS). The complex microstructure of the CS material—comprising martensite, bainite, and nickel-rich austenitic areas—was successfully reproduced via SPS followed by an additional post-sintering heat treatment. A residual porosity of less than 1% was achieved with SPS, compared to 8–14% with conventional sintering. Consequently, the steel produced by SPS exhibited higher ductility and tensile strength than the CS samples. CS and SPS samples both displayed a sharp decrease in Young’s modulus with strain, even at the onset of loading, indicating early damage nucleation and growth. A quantitative analysis of the porosity in fractured CS samples revealed no significant evolution in the pore dimensions and number. Concerning the SPS samples, the number and volume fraction of porosities were significantly higher in the fractured specimens compared to pristine ones. Two damage models derived from the continuum damage mechanics, one purely elastic, the other fully plastic, were compared with experimental data. It is shown that the elastic damage model could be successfully applied to the CS material, whereas, for the SPS process route, the steel underwent plastic straining assisted by plastic damage mechanisms.