Microstructural Features Causing an Increase in High-Temperature (700–800 °C) Fracture Toughness of Ti–Cr–Al–X Composite

Lightweight titanium composites are considered high-temperature materials for producing various components in aircraft and power generation industries. They can operate in a temperature range of 20–700 °C, which is higher by 100–300 °C than the operating temperature of conventional titanium alloys. In addition, these composites exhibit perfect temperature stability at elevated temperatures. The purpose of this work was to investigate the influence of test temperature on fracture toughness of Ti–Cr–Al–X composite and estimate changes in corresponding fracture micromechanisms. Ingots of Ti–Cr–Al–X composite containing Cr (41 wt%), Al (1.5 wt%), C (5 wt%), and Ti (balance) were prepared by melting using an electric arc furnace. Bar specimens were cut from ingots, ground, and polished. Then, a sharp edge notch was machined in each specimen. Fracture toughness tests of specimens under three-point bending were carried out in a temperature range of 20 to 800 °C. The SEM microstructure and fractography analyses of examined specimens were performed. The X-ray diffraction analysis of the material was carried out and phase balance was evaluated. At temperatures up to 600 °C, no discernible change in fracture toughness of Ti–Cr–Al–X composite was found (the average value of this parameter was about 5 MPa m1/2). A steep increase in fracture toughness up to 19.5 MPa m1/2 at 800 °C was found to be a result of a change in the fracture micromechanism. Microstructural features causing an increase in high-temperature (700–800 °C) fracture toughness of Ti–Cr–Al–X composite were suggested. In particular, the impact of the MAX phase and carbide phases was discussed.