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The microstructure of cast products, for example, in the Ti-6Al-4V alloy, consists of large P grains, extensive grain boundary a, and elongated coarse intragranular a, which can occur in colonies (of similarly aligned plates) or in a Widmanstatten morphology. This leads to strength, fracture toughness, fatigue crack growth rate, and creep behavior, which are at a relatively high level (Table 8.9) [26]. However, ductility and S-N fatigue are lower than cast and wrought product (Fig. 8.17) [12]. Both ductility and S-N fatigue can be enhanced by use of either innovative heat treatments or the use of hydrogen as a temporary alloying element (thermohydrogen processing, THP) to refine the microstructure (Fig. 8.18) [28, 29]. The high-cycle fatigue of alloys such as Ti-6Al-4V can be enhanced by hot isostatic pressing (HIPing) [7, 30, 31].

Casting of titanium alloys other than the conventional Ti-6Al-4V alloy is also possible. An example is the Ti-3Al-8V-6Cr-4Zr-4Mo alloy (38-6-44 or beta C), which exhibits excellent tensile properties and impressive fatigue behavior, with an endurance limit 85% above the average value typical of the Ti-6Al-4V alloy [12]. Recently, cast y alloys have been successfully produced and could see use in automobile and advanced gas turbine jet engines [3-17].

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