Update on Bifilms—The Fundamental Defect in Metals

Abstract

The concept of the bifilm is now nearly 20 years old. It has been clearly shown to control structural defects in castings such as porosity and hot tearing , plus, in solid wrought products, mechanical properties such as ductility and fatigue . It appears to explain for the first time the structures of Al–Si alloys and cast irons. Furthermore, although there are a number of proposed mechanisms for crack initiation in metals, it seems most probable that these are invalid. It follows that there is probably no intrinsic crack failure mechanism for most engineering metals; metals should never crack. The only crack-initiating feature appears to be the bifilm , introduced accidentally during casting . By appropriate casting techniques, the introduction of bifilms can be avoided. For this reason, failure by fatigue and creep may be eliminated, together with invasive corrosion such as pitting, stress corrosion cracking and possibly other failure modes.