THE INFLUENCE OF 2ND-PHASE DISPERSIONS ON SHEAR INSTABILITY AND FRACTURE-TOUGHNESS OF ULTRAHIGH STRENGTH AISI 4340 STEEL

The resistance to shear instability and subsequent flow localization in ultrahigh strength (UHS) steels is dependent upon second-phase particle dispersions and the matrix strain hardening. The effect of the interparticle spacing lambda to the geometric mean particle radius R ratio on the shear instability strain of UHS AISI 4340 steel is discussed. Experimental results indicate that a linear relationship exists between shear instability strain and this lambda/R ratio. Microvoid nucleation softening associated with second-phase particles appears to be the dominant destabilizing event leading to fracture. The effect of the hydrostatic stress is also discussed. Experimental results of mode I and II fracture toughness testing are compared. A high hydrostatic tension field was found to be the cause for the lower mode I critical stress intensity factor (K(Ic)) than mode II (K(IIc)). The high hydrostatic tensile stress field induced early microvoid nucleation which promoted flow localization leading to fracture. However, both mode I (K(Ic)) and mode II (K(IIc)) critical stress intensity factors directly relate to the critical particle parameter lambda/R1/2.