Fracture toughness of f.c.c. nickel and strain ageing b.c.c. iron in the temperature range 77-773 K

Abstract

Ductile initiation fracture toughness J_IC of b.c.c. Armco iron and f.c.c. nickel has been measured in the temperature range 77-773 K. Armco iron exhibits dynamic strain ageing (DSA) in the temperature range 383-573 K while nickel of the purity used does not evince DSA. Load vs load line displacement (LLD) plots during fracture toughness testing of Armco iron show serrations in the temperature range 383-573 K similar to those observed in the tensile stress-strain curves. DSA is found to have a beneficial effect on the fracture toughness J_IC. A marked increase in tensile strength and fracture toughness occurs in Armco iron in the DSA regime. The strain hardening exponent, known to have a bearing on the plastic zone size and the void growth rate, seems a clear parameter in terms of which the observed J_IC variation with temperature can be understood. Remarkably, the variation of n with temperature is found to closely follow the observed trend in_IC. However, the slope of the J-R curve, dJ/da, decreases in the DSA regime with a minimum at 423 K. The decrease has been related to the fracture propagation process which is shown to occur by an alternate fast fracture and the ductile dimpled mode in the DSA regime. In the case of nickel, free from DSA, J_IC or dJ/da are found to be largely unaffected by the test temperature. At room temperature f.c.c. nickel, at closely matching strength levels, possesses higher fracture toughness as compared to b.c.c. Armco iron. The crystal structure effect is more pronounced at temperatures below the ambient. At 77 K, the fracture toughness of iron is drastically reduced due to the onset of cleavage while nickel, not prone to a change in the fracture mode, maintains the same level of J_IC, as at the ambient.