| Summary: | Near β titanium alloy is widely used in automotive and aerospace industries due to their high strength-to-weight ratio and high corrosion resistance. The near β titanium alloy can precipitate <i>ω</i> phase and <i>α</i> phase after solution and ageing treatment, the strength of which can be remarkably increased, usually at the expense of ductility. It is one of the most important structural components of load-bearing that usually as aircraft skin, shell plating, main frame, linker and special fastener. The alloy used in this paper is a self-developed Ti-Al-V-Mo-Cr-Zr-Fe-Nb ultra-high-strength β titanium alloy, which is a typical near β titanium alloy. The characteristic of isothermal phase transformation of near β titanium alloys is diversity and complexity, which is sensitive to temperature and directly affects the mechanical properties after ageing. In this paper, the microstructural evolution and mechanical properties of a Ti-Al-V-Mo-Cr-Zr-Fe-Nb ultra-high strength β titanium alloy after isothermal treatment were investigated by scanning electron microscopy(SEM),transmission electron microscopy(TEM) and micro-hardness tester. The results show that only the isothermal <i>ω</i> precipitates are formed during ageing at 300℃, and the size of isothermal <i>ω</i> phases increase with the ageing time. The isothermal <i>ω</i> precipitates are first precipitated during ageing at 400℃. With the extension of the ageing time, the <i>α</i> phase nucleation occurs near the <i>ω</i>/β interface. No <i>α</i> precipitates are obtained in the alloy aged at 500℃, and needle-like <i>α</i> precipitates are directly precipitated from the β matrix, which is evenly distributed in the β matrix in a "V" shape. Tensile test shows that the tensile strength of the alloy is 1716.1 MPa and the elongation is 2% after ageing at 400℃ for 12 h. The tensile strength of the alloy is 1439.8 MPa and the elongation is 9.84% after ageing at 500℃ for 12 h, and has a good combination of strength and toughness.
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