A scanning method based on diffusion couples has been set up to experimentally explore phase diagrams and to observe the stable phases in quinary alloys. This method enables to realize a fast, experimental and reliable scanning of composition and then determine promising alloy compositions for potential structural application. Aiming at developing promising refractory complex concentrated alloys (RCCAs), the method was applied on the Ti-Nb-Al-Si system with an initial two phase, body-centered cubic (bcc) + orthorhombic (O), microstructure and focuses on the effect of Mo, Ta, V and Zr additions on this microstructure. The evolution of the orthorhombic phase fraction according to the content of additional elements was quantified, as well as the maximum concentration of those elements in order to stabilize the duplex microstructure, and maintain it above 800 • C. In such framework, it appears that Mo, Ta and V show the most promising effects on the Ti-Nb-Al-Si system. The comparisons between experimental results and calculated phase diagrams using current thermodynamic database showed that these calculations applied to RCCAs are good qualitative tools but are not accurate enough to be used as predictive means.
The effect of Tungsten (W), Tantalum (Ta) and simultaneous addition of Germanium (Ge) and Silicon (Si) on the microstructure evolution, tensile and creep properties of the near-alpha alloy Ti-5.7Al-3.9Sn-3.7Zr-0.7Nb-0.5Mo-0.35Si-0.05C have been investigated at high temperatures up to 650°C. Microstructural characterizations following solution treatment at 1050°C for 2 hours with oil quenching and aging treatment at 700°C for 2 hours followed by air cooling, highlighted that the additions of refractory elements such as W and Ta led to a decrease of both the volume fraction of the primary alpha phase (ap) and its average size. Tensile tests performed up to 650°C revealed a significant improvement in tensile strength with additions of W and Ta, even though a decrease of ductility has been also detected. Creep tests carried out at 600°C under a constant stress of 200 MPa pointed out that, refractory elements, Ge and Si have a beneficial effect on both primary and steady-state creep strain rates.
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