Microstructure and mechanical properties of HIP-ZTC4 during thermal exposure and tensile deformation

Casting titanium alloy TC4(ZTC4) after hot isostatic pressing(HIP) is one of the preferred materials in the field of aerospace manufacturing. In this paper, HIPZTC4 after a long-term thermal exposure was investigated.In order to evaluate the variation of mechanical properties with service time, the tensile properties of this alloy after exposure at 400 °C for 100, 500, and 1,000 h were investigated. Microstructure of samples was observed by the means of optical microscopy(OM), scanning electron microscopy(SEM), and transmission electron microscopy(TEM). Tensile test was carried out under different test temperatures and fracture appearance was studied. The results show that the strength increases with exposure time due to the harder a2(Ti3Al) phase precipitated in the a phase and superficial oxygen layer formed, which results in the fact that the resistance of crack propagation could be increased and cracks first initiate between surface oxidation and the matrix. Besides, the tensile temperature also has a significant effect on the mechanical properties of HIPZTC4. The yield strength and tensile strength decrease with the increase of tensile temperature, while the total elongation increases.

Effect of carbon addition on creep behavior of cast TiAl alloy with hard-oriented directional lamellar microstructure

Two TiAl alloys,Ti-47.5Al-3.7(Cr,V,Zr)and Ti-47.5Al-3.7(Cr,V,Zr)-0.1C(at.%),were prepared by cold crucible levitation melting to couple the hard-oriented directional lamellar microstructure with carbon microalloying strengthening.The creep behavior and mechanism for the improvement in creep properties by carbon addition were investigated by mechanical tests and electron microscopy characterizations.The results show that obvious improvements on the creep properties at 760°C and 276 MPa are achieved by 0.1 at.%C addition into TiAl alloy with directional lamellar microstructure,which promotes the creep strain and minimum creep rate decreasing with a large content.The minimum creep rate is reduced from 4.37×10^(-8) to 3.97×10^(-9) s^(-1),and the duration entering into creep acceleration is prolonged for more than 10 times.The mechanism for creep property improvement by 0.1%C addition is attributed to two aspects.The first one is that Ti_(2) AlC is found to be strong obstacles of 1/2[110]dislocations when moving across the lamellar interface in the carbon containing alloy.The other one is that the in terfacial dislocatio ns are effectively impeded and the release process is hindered by dynamic precipitation of Ti_(3) AlC,which is proposed to be the special mechanism for creep resistance improvement of this hard-oriented directional lamellar microstructure.