The alloy Beta 21S, with nominal chemical composition Ti-15Mo-2.7Nb-3.0Al-0.2Si (weight percent), was developed by TIMET USA in the year 1989. The alloy is being used in the engine plugs and nozzles of the Boeing 777 as well as the Airbus A-330. A major European effort is also underway to replace the heavy Nickelbase-Superalloys currently used in helicopter mufflers by lighter Titanium alloys (Beta 21S is one of the selected alloys).
The main objectives of this study were, firstly, to investigate the influence of variation in heat treatment parameters such as cooling rate after solution treatment, solution treatment temperature, aging temperature and time, and grain growth on the microstructure and mechanical properties; and secondly, to investigate the influence of hot forming operations on the microstructure and mechanical properties. The main microstructural feature of ß Titanium alloys is the preferential precipitation of the hardening α phase at ß grain boundaries forming a continuous α layer. The resulting PFZ (precipitate free zone) adjacent to this continuous α layer does not contain any precipitates, and is therefore soft with respect to the surrounding age-hardened matrix. Preferential plastic deformation in the soft zones can have a negative effect on some properties, like ductility and fatigue crack nucleation resistance. Variables such as cooling rate after solution treatment or hot forming, and aging temperature and time have a significant influence on the age-hardening kinetics of the alloy Beta 21S. These parameters determine the resulting microstructures and thus variation of the mechanical properties.
If sheet material is allowed to cool down at a slow rate below approximately 400°C after solution treatment or hot forming, the resulting microstructure (after subsequent aging treatment of 8h 600°C) consists of a uniform distribution of fine α plates. Fast cooling rates (air cooling and 50°C/min), on the other hand, resulted in inhomogeneous microstructures consisting of coarse α plates and interior of ß grains without α plates, after the same aging treatment of 8h 600°C. A homogeneous microstructure results for the slowly cooled material due to the ω phase precipitation during the slow cooling. The ω particles provide nucleation sites for α precipitation during heating to the aging temperature.
Aging temperature and time controls the size, spacing, and volume fraction of α precipitates. The width of grain boundary α layer is also influenced by the aging temperature. For starting material which was slowly cooled below 400°C during the production, single-step aging treatments performed at low temperatures (500°C-600°C) resulted in a homogeneous distribution of fine α precipitates. Aging treatments performed at high temperatures (above 600°C), on the other hand, resulted in an inhomogeneous distribution of coarse α plates. At low temperatures a high density of α precipitates is formed due to the presence of ω particles during heating to the aging temperature serving as precursors for α nucleation. At high temperatures complete dissolution of the ω particles occurs, i.e. the direct ω to α transformation is suppressed. It is possible to create a homogeneous microstructure of α plates even at very high aging temperatures (725°C) by using two-step aging treatments (e.g. 8h 500°C+24h 725°C). The homogeneous distribution of α platelets when combined with decreased age-hardening level, reduces the actual strength difference between the matrix and the grain boundary region to such a level that increasing plastic deformation of the matrix occurs before fracture within the soft zones takes place, and therefore the effect of the PFZ due to the continuous α layers as sites for early crack nucleation is considerably reduced.
There was a strong influence of the different microstructures, which resulted from the variation in heat treatment parameters, on the tensile properties, HCF resistance, creep resistance, and the resistance against fatigue crack propagation of macrocracks. In this study, a microstructure with a good combination of properties usable at high application temperatures has been created by using a two-step aging treatment. The microstructure created is more suitable for application at high temperatures than the standard aging treatments recommended by the alloy producer. It has also been possible to restore the loss of yield stress and HCF strength induced by hot forming operations by using the slow cooling method outlined above.