Fatigue Crack Propagation in Single Crystal and Polycrystalline, age-hardenable TiV Alloys

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Department of Materials Science and Engineering


The relationships between age hardening and fatigue crack propagation behavior have been studied in oriented Ti-27V-alloy single crystals and Ti-30V polycrystalline specimens with a metastable β-phase (b.c.c.) structure. Samples have been tested after the following heat treatments: as-quenched (solid solution β), aged at 300°C (β + ω-phase), or aged at either 400 or 450 °C (β + α-phase). The resulting age hardening and slip behavior have been correlated with microstructure and are related to the fatigue crack propagation rates and fracture behavior. Except for the specimens in which plate-like α-phase appears to enhance crack growth, the single crystal samples exhibit ductile fatigue fracture with only a small influence of microstructure on the fatigue crack propagation rates. In these conditions, crack growth is adequately described by the crack-tip opening models for crack propagation. Unlike the single crystals, age hardening the polycrystalline samples, especially by ω-phase formation, can significantly increase the crack propagation rate. The faster growth rates are associated directly with the presence of second phase particles, as in the case of grain boundary α-phase causing intergranular fracture after aging at 400°C, or indirectly, when the presence of ω-phase particles affects polycrystalline slip behavior sufficiently to induce a low energy fracture process.

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© 1978

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Materials Science and Engineering