Fracture toughness and fracture mechanisms in Mo5SiB2 at ambient to elevated temperatures

The fracture behavior of the polycrystalline Mo5SiB2 alloy was investigated by three-point bending tests, in vacuum, in the range of 25–1400 °C at a crosshead speed of 10−3 mm/s. At room temperature (RT), the alloy exhibited a higher fracture toughness of 3.34 MPa m1/2 compared to single crystalline Mo5SiB2 with a value of ∼1.8 MPa m1/2. The improved toughness is attributed to crack overcoming the barrier effects of grain boundaries. With increasing temperature, the fracture mode underwent a conversion from transgranular cleavage at RT to a mix of transgranular cleavage and intergranular failure at elevated temperatures. At 1000 °C, the maximum increment (∼3.4 MPa m1/2) in toughness was found to be related to the brittle-to-ductile transition (BDT). At 1200 °C, the effects of extrinsic toughening mechanisms (microcracking, etc.) are limited, i.e. its toughness only increasing by ∼1.1 MPa m1/2. However, the cavities formed at triple junctions degraded toughness at 1400 °C.