Grain boundary effects on fatigue damage and material properties: Macro- and micro-considerations

The cyclic deformation behavior of single- and poly-crystals are well documented. There exists a great difference in the fatigue damage mechanisms between single- and poly-crystals. It can be mainly attributed to the effects of grain boundaries (GBs) and the crystallographic orientations. This research is concerned with copper bicrystals with various types of GBs and different component crystals, emphasizing on the macroscopic cyclic stress-strain response and fatigue damage mechanisms at the micro-scale. Direct evidence is offered to show the obvious strengthening effect caused by the large-angle GBs during cyclic deformation. The data of cyclic stress-strain responses will also be presented to show the effects of the GBs and the macro-scale crystallographic orientation. The influence of various GBs on fatigue cracking behavior will also be considered for the crystalline materials. It is shown that the interactions of persistent slip bands (PSBs) with various GBs play a decisive role in intergranular fatigue cracking. Explained are the conditions under which GBs would enhance or impede fatigue cracking. It is found that intergranular fatigue cracking depends strongly on the interactions of PSBs with GBs in fatigued crystals, rather than the GB structure itself. In the sequel, the fatigue damage mechanisms will be referred to the micro-scale.