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 degrees 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 m(1/2) compared to single crystalline Mo5SiB2 with a value of similar to 1.8 MPa m(1/2). 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 degrees C, the maximum increment (similar to 3.4 MPa m(1/2)) in toughness was found to be related to the brittle-to-ductile transition (BDT). At 1200 degrees C, the effects of extrinsic toughening mechanisms (microcracking, etc.) are limited, i.e. its toughness only increasing by similar to 1.1 MPa m(1/2). However, the cavities formed at triple junctions degraded toughness at 1400 degrees C. (c) 2013 Elsevier Ltd. All rights reserved.