The notched tensile strength (NTS) of Ti-4.5Al-3V-2Fe-2Mo specimens with distinct heat treatment conditions was investigated and correlated to microstructures. The specimens were solution-treated at either 980 or 870 C (above or below the -transus) and then waterquenched prior to aging treatments. After aging at 482, 593, and 704 C, titanium martensite ( 0 or 00 ) decomposed into and with distinct sizes. Additionally, the specimens aged at 482 C were particularly susceptible to notch brittleness, regardless of the solution temperature. As the aging temperature increased, the lower hardness associated with coarsened þ structures resulted in decreased notch brittleness of the alloy. In general, the specimens containing primary in microstructure were prone to brittle fracture and reduced NTS. The alignment of martensite packets (or needles) with respect to the crack growth direction could affect the fracture appearance of the specimens, particularly for the coarsegrained specimens which were solution-treated at 980 C. Furthermore, the existence of grain boundary for the specimens aged at 593 C or higher promoted grain boundary sliding during notched tensile tests.
Fatigue crack growth tests of Ti-4.5Al-3V-2Fe-2Mo (SP-700) laser welds after various postweld heat treatments (PWHTs) were investigated. The welds and the mill-annealed base metal had similar fatigue crack growth rates (FCGRs) at a stress ratio (R) of 0.1. After increasing the stress ratio to 0.5, the peak-aged (482°C) weld exhibited higher FCGRs due to increased notch brittleness of the material. The tough microstructure as well as tortuous crack path of the overaged (704°C) weld could account for the reduced FCGRs, particularly at a higher R. The fatigue fracture appearance of the welds varied from transgranular to intergranular failures, depending on the stress intensity factor ranges and PWHTs. Experimental results also demonstrated that the 704°C-aged weld with coarsened a + b structures had better impact toughness than the base metal with banded structures.
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