Transgranular fracture

HK31A-T6 tended to fracture intergranularly, while the remaining five alloys fractured transgranularly. 

Microstructural examinations revealed that the cracks originated on the external surface (Fig. 9.15). The cracks were highly branched and transgranular. The branched, transgranular character of these cracks is typical of stress-corrosion cracking of austenitic stainless steels. The thick-walled fracture faces are also typical of cracking by this mode. 

Intergranular fracture along pre-existing paths Transgranular fracture along straingenerated paths Mixed crack paths by adsorption, decohesion or fracture of brittle phase ... 

The fracture mode of stress-corrosion cracks in austenitic stainless steels can be transgranular, intergranular or a mixture of both. One of the earliest environments found to cause problems was solutions containing chlorides or other halides and the data due to Copson (Fig. 8.30) is very informative. The test solution for that data was magnesium chloride at 154°C the alloys contained 18-20alloy with a composition of approximately 18Cr-8Ni has the least resistance to cracking in this environment. 

Some liquid metals have been observed to cause embrittlement in many titanium alloys. In mercury, for example, Ti-8Al-lMo-lV exhibits both intergranular and transgranular fracture with velocities as high as lOcm/s. Heat treatment affects this behaviour in a manner similar to that observed in aqueous and methanolic solutions. Some alloys are embrittled by liquid cadmium and zinc. More surprising, perhaps, is the observed solid metal embrittlement which has been found on titanium alloy components coated with cadmium, silver or zinc Service failures of cadmium-plated Ti-6A1-4V fasteners have been reported , and cracking of this alloy and... 

No mechanism for cracking in N2O4 has been established . In organic media crack velocities are similar to those obtained in distilled water. Lowering the water content results in lower velocities. Not all authors attribute failures in organic liquids to the residual moisture . Furthermore, part of the fracture may be transgranular . Water additions to methanol increase crack velocities as do halide additions. In oils velocities are similar to those in organic liquids and distilled water. 

B. The direct identification of such microscopic fracture modes as transgranular cleavage, radial fracture, intergranular separation or ductile dimpled rupture, was shown to be considerably more dependable by SEM than by OM... 

Auger specimens 10 x 2 x 1 mm were heat-treated and then notched to control the location of fracture. Two of these notched samples at a time were loaded into specimen grips such that they could be broken independently under UHV and then examined by a SAM instrument. Scanning electron micrographs allowed different grain boundaries and transgranular cleavage areas to be selected for analysis. 

Figure 6. SEM micrographs showing the reduced ductility observed at high hydrogen fugacities (region 3 of Figure 4(b)) in the parent metal of the (a) HY-100 and (b) HSLA-100 alloys and the transgranular cleavage-like (TCL) fracture morphology in (c) HY-100 and (d) HSAL-100.

---