Effect of fabrication parameters on the microstructure and mechanical properties of unidirectional Mo-fiber reinforced TiAl matrix composites

Effect of the fabrication parameters on the microstructure and mechanical properties of unidirectional Mo-fiber reinforced TiAl matrix composites was investigated. The composites were fabricated through the following two steps. The first is the preparation of the composite preforms by powder slurry casting; the second is degassing the preforms at 380 °C for 0.5 h, 1 h and 2 h and then hot pressing them at different temperatures between 1000 °C and 1380 °C for 0.5 h, 1 h and 2 h. In all the synthesized composites, Mo fibers distribute homogeneously and the matrices consist of two phases, i.e. γ-TiAl and α2-Ti3Al. Two reaction layers, i.e. δ-(Mo,Ti)3Al inner layer and β‘-(Mo,Al)Ti outer layer, were formed between the Mo fibers and matrix. With the hot pressing temperature and time increasing, the two reaction layers become thicker by consuming the Mo fibers and the matrix becomes denser, which is harmful and beneficial to the strengthening of the composites, respectively. The composite fabricated by hot pressing at 1100 °C for 1 h has good balance between the fiber degradation and matrix densification, and its flexural strength in the direction parallel to the fibers is 735.5 MPa at room temperature and 762.9 MPa at 800 °C; whereas the room temperature flexural strength in the direction perpendicular to the fibers is 249.3 MPa and increases with the degassing time increasing. After bending test, a main macro-crack together with a large number of micro-ones was observed within the matrix of the fractured samples, and the fractured Mo fibers showed obvious necking and localized debonding with the inner reaction layer. All these can absorb a great amount of energy before fracture, and thus the composite is highly toughened.