INFLUENCE OF SOLUTE ADDITIONS ON THE FRACTURE-BEHAVIOR OF ARMCO IRON

The influence of solutes, namely cobalt, molybdenum, nickel and silico n, on the deformation and fracture behaviour of Armco iron has been st udied. The J-integral method was used for the measurement of ductile i nitiation 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 in crease 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 o f crack initiation. The large decrease in J(IC) with silicon and highe r molybdenum concentration is explained on the basis of a change in fr acture mode from ductile to cleavage as a result of stress concentrati on ahead of the crack tip reaching the cleavage fracture stress. The l oss in fracture toughness with nickel addition is attributed to sulphu r segregation at grain boundaries which results in pockets of intergra nular fracture. Cobalt addition leads to alloy softening. Through seco ndary ion mass spectroscopy (SIMS), experimental evidence has been obt ained for the first time for the suggestion that the 'scavenging' of i nterstitials leads to alloy softening. It was observed through and yie ld stress dependence on grain size that the Hall-Fetch constants sigma (0) and k(y) substantiate SIMS observations.