Effects of environment on fracture toughness of binary and ternary nickel silicide-based intermetallics

Chevron-notched beam (CNB) fracture toughness testing was conducted to examine the effects of environment such as air, vacuum (∼1.3×10−4 Pa) and dry oxygen on fracture toughness of binary and ternary nickel silicide intermetallic alloys in the Ni–Si–Mg system. The results of testing indicate that environment appears to have little effect on the fracture toughness of these alloys. Regardless of environment the fracture toughness of single and multiphase alloys consisting of binary (Ni2Si, Ni3Si, Ni31Si12 (Ni5Si2), Ni3Si2) and ternary (η-Mg6Si7Ni16 and κ-Mg2Si10Ni13) silicides exhibit low fracture toughness (within the range of 2–8 MPam1/2), associated with predominantly cleavage fracture mode. Fracture toughness of a single-phase Ni3Si with grain size 58±7 μm is quite high (∼40 MPam1/2) in both air and dry oxygen. Fracture mode of Ni3Si is predominantly intergranular failure, irrespective of environment, with only a small amount of transgranular quasi-cleavage. In air, fracture toughness of Ni3Si decreases with increasing volume fraction of fine-grained (Ni(Si)+Ni3Si) colonies (where Ni(Si) is a solid solution of Si in Ni) as well as with decreasing grain size of Ni3Si. Very fine, unidentified precipitates are observed on both intergranular and transgranular fracture surfaces of a single-phase Ni3Si. In the fixed test environment the probability of the presence of precipitates increases with increasing fracture toughness values for Ni3Si. The threshold level of fracture toughness to form precipitates is the lowest for the specimens tested in air, intermediate for the specimens tested in vacuum and the highest for the specimens tested in dry oxygen.