GRAIN-BOUNDARY CRACKING

A chronological summary is given of the various types of grain boundar y fracture found in metals. In each case, there is an impurity that ad sorbs at the new (fracture) surface being formed. For the case of Fe-P alloys, a quantitative argument can show that adsorption of phosphoro us on the free surface greatly reduces the barrier to void nucleation compared to that in the absence of phosphorous. The same or larger red uction would appear for any other element, which adsorbs more strongly than phosphorous and displaces it at the surface. Such an argument is shown to explain a great many cases of dimpled grain boundary fractur e in strong alloys undergoing creep or hydrogen attack. The reduction in surface energy can also lead to a smooth grain boundary fracture (n o void nucleation), in which diffusion of solute to the new surface li mits crack growth. Numerous examples of this are also discussed.