Solid metal induced embrittlement of titanium alloys
Abstract: Titanium alloys were for a time believed to be highly resistant to environmentally assisted cracking because of their ability to form a protective oxide film on the surface. Their resistance can still be considered to be high, but when cracking resistance was originally defined to ensure reliable functionality of fracture-critical components, certain conditions that promote cracking were discovered. One of the environmental assisted cracking processes relevant to titanium alloys is solid metal induced embrittlement (SMIE). SMIE refers to the embrittlement that occurs in normally ductile materials under tensile stress in contact with solid metals with a lower melting temperature than titanium. Even though failures resulting from SMIE are rare, they do occur, partly because the industry is not aware of conditions under which SMIE may exist. Titanium alloys are frequently used in the aerospace industry where solid copper contact can be found in for instance, welding electrodes and fixtures in various manufacturing processes. The main scope of the present work has been to clarify the effect of copper in contact with titanium alloys with respect to SMIE and further to increase the understanding of SMIE. Three titanium alloys: Ti-8Al-1V-1Mo, Ti-6Al- 2Sn-4Zr-2Mo and Ti-6Al-4V have been evaluated in contact with copper, and in contact with gold for comparison. In order to be able to evaluate SMIE, a U-bend test method adapted from an aerospace recommended practice for stress-corrosion cracking (ARP SAE 1795A) was modified for SMIE evaluation. The acceptability of the test method was successfully established by using reference specimens that were intended to crack (or not to crack) when in contact with the embrittling environment. The results of the SMIE tests show that both Ti-8Al-1V-1Mo and Ti-6Al-2Sn-4Zr-2Mo are susceptible to SMIE in contact with copper and gold, whilst no SMIE was observed with Ti-6Al-4V. Based on these findings it is suggested that the SMIE susceptibility of titanium alloys is dependent on alloy composition. Furthermore, resistance welded Ti-8Al-1V-1Mo and Ti-6Al-2Sn-4Zr-2Mo were evaluated to investigate whether the presence of copper electrodes, (the welding operation itself) could lead to SMIE. No SMIE was found in the resistance welded specimens, which may be explained by the short time that the copper electrodes were in intimate contact with the titanium alloy, the magnitude of residual stresses after welding, or both, which were too low to initiate SMIE. In order to obtain a better understanding of the crack path characteristics and the mechanisms involved, one U-bent specimen showing SMIE (Ti-8Al-1V-1Mo with copper) was selected for further examination using electron backscatter diffraction (EBSD). The EBSD results indicated a preferable crack propagation path along high angle grain boundaries, which supports the suggested adsorption mechanism of the embrittling species at the crack tip. A tendency for favourable crack growth along grain boundaries adjacent to grains oriented close to  in the crack direction could also be seen, which indicates that there is a connection between the SMIE crack characteristics and the crystallographic orientation.
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