Pre-existing crack populations in metals, John Campbell

Professor John Campbell, a talk given to the Warwick Manufacturing Group, Warwick University, organised by Prakash Srirangam. During casting of our engineering metals, including light alloys, steels and superalloys (but with a few interesting exceptions) it is not widely known that the turbulence of pouring in air (and ‘vacuum’) folds over the oxide on the surface of the melt, forming doubled-over films in suspension. The double films do not bond, therefore forming effective cracks. Turbulent pouring fills the liquid with cracks. The cracks, known as ‘bifilms’, are subsequently frozen into the solid. The bifilms remain as a population of pre-existing cracks, mainly located in grain boundaries. The precipitation of carbides in steels can force open the bifilms to form enlarged pathway and sinks, encouraging SCC and HE. For power generation there is exciting evidence that low bifilm-containing metals have improved creep resistance, promising better steels for heat exchangers. Fatigue is improved to the point that it has been eliminated in some applications, and toughness in super duplex stainless improved several hundred percent. Bifilms seem to represent a new era in metallurgy.