Voids high-temperature alloys

Figure 5-13. Cross-sectional optical micrograph showing the subsurface region in a commercial, high-temperature alloy after long-term oxidation. The subsurface region consists of large voids and smaller oxide precipitates (courtesy of Haynes International).

At elevated temperatures where titanium alloys could be the adherend of choice, a different failure mechanism becomes important. The solubility of oxygen is very high in titanium at high temperatures (up to 25 at.%), so the oxygen in a CAA or other surface oxide can and does dissolve into the metal (Fig. 12). This diffusion leaves voids or microcracks at the metal-oxide interface and embrittles the surface region of the metal (Fig. 13). Consequently, bondline stresses are concentrated at small areas at the interface and the joint fails at low stress levels [51,52]. Such phenomena have been observed for adherends exposed to 600°C for as little as 1 h or 300°C for 710 h prior to bonding [52] and for bonds using... 

Major issues of radiation effects on V-aUoys are radiation embrittlement at relatively low temperature, and irradiation creep at intermediate temperature. Void swelling is known to be quite small if the alloy contains Ti. He embrittlement is a key issue determining the high-temperature operation limit in fusion neutron environments where 5—10 appm He are produced by transmutation during the irradiation to 1 dpa. However this may be a minor issue for fission neutron environments where the production rate is much lower because the cross-section of He-producing reactions is small when the neutron energy is below 10 MeV. 

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