Microstructural Control and Alloy Design of the Ti-Al-Nb-W-B Alloys

The TiAl alloys have been considered as promising candidates for structural-materials applications at around 800⁰C. The major concern for the structural use of the TiAl alloys is their low ductility and poor fracture resistance at ambient temperature. Refining the grain size of the TiAl alloys can be an effective way to improve the mechanical properties of the alloys. In this work, new TiAl alloys, containing tungsten (W) and boron (B), have been developed. Using the scanning-electron microscopy (SEM), electron-microprobe, and transmission-electron microscopy (TEM), the effects of W and B on the microstructural evolution of TiAl alloys, including the grain size and lamellar spacing, were analyzed. It is important to point out that fine uniform microstructures (with the grain size smaller than 50 μm) can be conveniently developed after Hot-Isostatic Pressing (HIP) the as-cast alloys at 1,250⁰C and 130 MPa for 5 h, produced through arc-melting.

With the increase of the tungsten content, the microstructure of the TiAl-based alloy can be refined. The addition of tungsten can restrain the grain coarsening and stabilize the microstructure up to 1,280⁰C by hindering the migration of grain boundaries at high temperatures. It is also noteworthy that the beta phase, a high-temperature residual phase, forms when the tungsten content exceeds 0.4 atomic percent (at.%). The α-phase transus temperature, T_α, has been determined through differential-thermal analyses (DTA) and further proved by the investigation of the microstructural changes during various heat treatments. Different microstructures meeting desirable needs can be developed through heat treatments beyond and below the α-phase transus temperature. The mass production of the TiAl-based alloy, with the optimal composition developed through arc-melting, has been made through a magnetic-floatation-melting method.

A comparison in the microstructures of the mass production and arc-melting small production has been made. A larger grain size, with a significant amount of the β phase, has been observed in the large ingot. Heat treatments have been conducted in order to obtain desirable microstructures and to minimize the amount of the β phase in the alloy. Hot forging is another effective method chosen to refine the grain size and eliminate the β phase in the alloy. Hot simulation has been conducted to the alloy in order to obtain the optimal parameters for the hot deformation of the TiAl-based alloy. Mechanical testing, such as hardness measurements and tensile tests, have been performed on the alloys.

The addition of the alloying element, such as tungsten, increases the hardness of TiAl alloys by the solution strengthening and refinement of grain sizes. The room-temperature ductility and yield strength of the alloy have been enhanced through the alloy development and heat treatments. A ductility as high as 1.9% has been obtained in the newly-developed TiAl-based alloy at ambient temperature in the heat-treated cast samples. This result demonstrates the potential use of the cast material for structural applications of TiAl-based alloys with controlled compositions and optimal heat treatments.