Dependence of impact deformation behaviour of Fe–2Ni sintered alloy on strain rate and sintered density

Abstract

The present study investigates the impact deformation behaviour of Fe–2Ni sintered alloy as a function of the strain rate and sintered density using an MTS 810 servohydraulic machine and a compressive split Hopkinson bar. Impact deformation is conducted at room temperature at strain rates ranging from 10⁻³ to 8·1 × 10³ s⁻¹ for specimens with sintered densities of 85, 90 and 95% respectively. The impact flow response is found to be sensitive to both the strain rate and the sintered density. Changes of the strain rate and sintered density induce corresponding changes in the flow strength, work hardening rate, strain rate sensitivity and activation volume. A constitutive law based on Gurson's plastic potential and the flow rule of Khan, Huang and Liang (KHL) provides a reasonable description of the flow behaviour of Fe–2Ni sintered alloy. Specimen fracture occurs when the strain rate exceeds 4 × 10³ s⁻¹ and is dominated by a localised shearing mode. The degree of cracking increases with increasing strain rate or decreasing sintered density. The microstructural characteristics of the undeformed specimens vary with the sintered density. Following impact, the structural phase is unchanged from the undeformed condition, but the grain size is found to decrease with increasing strain rate for each sintered density. The reduction in grain size induces an increase in the flow stress. At a constant strain rate, the lower flow stress and smaller grain size observed in the specimens with a lower sintered density are attributed primarily to a higher overall porosity.