Time-Dependent Failure Mechanisms of Metals; The Role of Precipitation Cleavage

New proposals include the precipitation of new phases and intermetallics in light alloys occur on bifilms rather than on grain boundaries (confusion is understandable because, commonly, it seems that as many as 50 per cent of grain boundaries contain bifilms). The formation and growth on bifilms result from the saving of plastic work to accommodate the volume and shape change of the new phase, which can be orders of magnitude higher than the saving of interfacial energy on grain boundaries. The bifilm, if previously closed, is forced to open by this process, called ‘Precipitation Cleavage’. The term ‘cleavage’ refers to the cleaving open of the bifilm, dispers- ing the volume change elastically over a relatively extensive area. This is the mechanism explain- ing the sensitisation heat treatments for (i) embrittlement, (ii) invasive corrosion as in stress corro- sion cracking, and (iii) invasion of hydrogen into opened bifilms as ‘sinks’, leading to surface blis- tering and hydrogen embrittlement of the matrix. Direct visual evidence is provided by (i) surface blisters during hydrogen charging, and (ii) fracture surfaces displaying quasi cleavage facets, and fisheyes observed in steels and more recently also in light alloys.