Microstructural Evolution of TiAl-Intermetallic Alloys Containing W and B

The TiAl alloys have been considered as promising candidates for structural-materials applications at around 8000C. 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 colony size and lamellar spacing, were analyzed. It is important to point out that fine uniform microstructures (with the colony size smaller than 50 mm) can be conveniently developed after Hot-Isostatic Pressing (HIP) the as-cast alloys at 1,2500C and 150 MPa for 4 h without the deformation process. It was found that tungsten prefers to react with boron to form borides, and disperses mainly along grain boundaries, and occasionally inside grains. With the increase of the tungsten content, the microstructure can be further refined. Heat treatments at temperatures ranging from 9000C to 1,3100C were conducted. 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 ductile phase, forms when the tungsten content exceeds 0.4 atomic percent (at.%). The α phase transus temperature, Tα, 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. Mechanical testing, such as hardness experiments, has been conducted on the alloys. The addition of the alloying element, tungsten, increases the hardness of TiAl alloys by the solution strengthening and refinement of grain sizes