Influence of solute additions on the fracture behaviour of Armco iron

Abstract

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.

Grain boundary segregation and fracture resistance of engineering steels

“…In a recent study on the influence of solute addition on the fracture behavior of Armco iron, NiSiCr and NiSiCrCo alloys, the increase in fracture toughness with cobalt addition was attributed to grain boundary segregation of carbon. 64 which increased the intergranular cohesive strength and rendered the crack initiation process difficult. 64 It was observed that in the presence of cobalt there was displacement of sulfur from the grain boundaries by carbon through a site-competitive process.…”

“…64 which increased the intergranular cohesive strength and rendered the crack initiation process difficult. 64 It was observed that in the presence of cobalt there was displacement of sulfur from the grain boundaries by carbon through a site-competitive process. The increase in the intergranular cohesive strength by carbon segregation was substantiated through yield stress dependence on grain size that the Hall-Petch constants 0 and k y were influenced.…”

“…The increase in the intergranular cohesive strength by carbon segregation was substantiated through yield stress dependence on grain size that the Hall-Petch constants 0 and k y were influenced. There was an increase of k y and a decrease of 0 , 64 where 0 is a measure of friction stress resisting dislocation movement across a grain and ky is a measure of grain boundary strength.…”

Enhancement of structural and mechanical properties of Fe + 0.5 % C steel powder alloy via incorporation of Ni and Co nanoparticles

“…Thus, it is advisable not to use Co MPs in the formation of the Fe + 0.5 % C alloy as it may reduce the strength of the material. Srinivas et al found that the addition of 0.5 wt.% Co MPs led to changes in the structure and lowered the yield strength of Armco iron by about 70 MPa [21].…”

Effect of Cobalt in Thin Wall Ductile Iron and Solid Solution Strengthened Ferritic Ductile Iron

“…• Srinivas, Malakondaiah, and Rama Rao found that Fe-Co alloys (0.5 wt.% and 5 wt.% Co) resulted in lower yield strength but no effect on the tensile strength compared to the base material unalloyed ARMCO Iron. In contrast to molybdenum, nickel, and silicon, cobalt significantly increased the fracture toughness (JIC) [54].…”