Influence of Solute Additions on the Fracture Behaviour of Armco Iron

The influence of solutes, namely cobalt, molybdenum, nickel and silicon, on the deformation and fracture behaviour of Armco iron has been studied. The J-integral method was used for the measurement of ductile initiation fracture toughness J_IC. Although cobalt is seen to enhance significantly J_IC of the starting material (Armco iron), molybdenum, nickel and silicon are observed to have a deleterious effect. The increase in J_IC with cobalt addition can be understood in terms of the enhanced strain hardening exponent, which is known to have a bearing on the plastic zone size around the crack tip and on micromechanisms of crack initiation. The large decrease in J_IC with silicon and higher molybdenum concentration is explained on the basis of a change in fracture mode from ductile to cleavage as a result of stress concentration ahead of the crack tip reaching the cleavage fracture stress. The loss in fracture toughness with nickel addition is attributed to sulphur segregation at grain boundaries which results in pockets of intergranular fracture. Cobalt addition leads to alloy softening. Through secondary ion mass spectroscopy (SIMS), experimental evidence has been obtained for the first time for the suggestion that the `scavenging' of interstitials leads to alloy softening. It was observed through yield stress dependence on grain size that the Hall-Petch constants σ ₀ and k_y substantiate SIMS observations.