Intergranular Fracture in BCC Metals

The intergranular fracture (IGF) in high purity molybdenum and high purity iron are discussed together with the effects of interstitial and substitutional solutes on it.Recrystallized molybdenum is inherently susceptible to IGF. Purification does not improve it. Grain boundary segregation of carbon increases the grain boundary cohesion of general boundaries and that of oxygen decreases it.IGF occurs in recrystallized commercial high purity iron about 99.995% pure, but does not in specially prepared iron of about 99.999% or better purity. Segregation of oxygen decreases the boundary cohesion, but the effect is much smaller than usually believed. Segregation of carbon increases the grain boundary cohesion and reduces IGF caused by segregation of other impurities, such as hydrogen, phosphorus and sulfur. Molybdenum and chromium reduce IGF caused by the segregation of phosphorus as far as the alloy is in the solid solution. These elements interact with phosphorus to reduce the detrimental effect of phosphorus and also increase the boundary cohesion of iron. Nickel also reduces IGF, but the reduction is caused by the solid solution softening due to nickel.Also discussed are the chemical state of the segregated phosphorus and the dependence of the phosphorus segregation on the grain boundary character.