Most of the failure analysis we have been talking about on this blog so far is about fractography and, specifically, about fractography that relates to the macro scale. This means it is often something that can be seen with just a decent pair of eyeballs, or at low magnification using an optical microscope. For very tiny parts, higher magnification using a scanning electron microscope may be necessary, but we might still call the activity MACRO fractography, since the main things we have been learning to look for have to do with the loading geometry. In this week’s discussion, we’ll look back at the same very basic micro-fractography features we introduced on Oct. 2, 2008.
To review and clarify, if we are dealing with a polycrystalline metallic material that is nominally anisotropic (has randomly oriented grains without a preferred orientation) and has been broken all at once (i.e. is a result of a monotonic event, not fatigue), there is a choice of three classical types of features that will be visible when a fracture surface is examined in a scanning electron microscope (SEM). These classical features are microvoids (Figure 1) if the material is behaving in a ductile manner, and either cleavage, transgranular (Figure 2) or intergranular (Figure 3) if the material is behaving in a brittle manner. Fatigue features, oriented structures (cold-drawn material) and any features in materials with coarse or inhomogeneous microstructures complicate the situation a lot. But just about anyone can learn to recognize the three basic types of features, as they are clearly different from each other. The features shown here were all found on heat-treated (quenched-and-tempered or austempered) steels.
The size of the features can vary a lot from material to material, lot to lot or even part to part. Note that the original magnifications of these three photos varied somewhat. Note also that the specific features vary from location to location within the same image.
Note that in Figure 1, the microvoids are finer in area A than in B, and there are elongated features in area C. Area C happened to be in the center of the thickness of a piece of relatively thin hot-rolled steel. There might have been some microsegregation that made the steel behave a little differently at that location. The factors which resulted in a finer void size in A compared to B are less obvious. They may have to do with the previously speculated microsegregation, or microscopically thin “bands” of higher and lower carbon and manganese, for example. The microvoids, or “ductile dimples” form around microscopic particles that may be contaminants (sulfide or oxide inclusions) or carbides that are intentionally present to strengthen the steel. Everything else being equal, finer microvoids are correlated with higher strength. The edges of the microvoids form in a similar way to how taffy deforms as it is being pulled to mix the ingredients. Microvoids take time to form then, and when you see them they are an indication that the fracture process did not happen instantaneously. Had the load been removed before the crack was complete, the crack might have been able to stop itself.
Check back for more on this subject next week.