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

Chevron-notched beam (CNB) fracture toughness testing was conducted to exam ine the effects of environment such as air, vacuum (similar to1.3x10(-4) Pa ) and dry oxygen on fracture toughness of binary and ternary nickel silicid e intermetallic alloys in the Ni-Si-Mg system. The results of testing indic ate that environment appears to have little effect on the fracture toughnes s of these alloys. Regardless of environment the fracture toughness of sing le and multiphase alloys consisting of binary (Ni2Si. Ni3Si. Ni31Si12 (Ni5S i2). Ni3Si2) and ternary (eta -Mg6Si7Ni16 and kappa -Mg2Si10Ni13) silicides exhibit low fracture toughness (within the range of 2-8 MPam(1/2)), associ ated with predominantly cleavage fracture mode. Fracture toughness of a sin gle-phase Ni3Si with grain size 58 +/- 7 mum is quite high (similar to 40 M Pam(1/2)) in both air and dry oxygen. Fracture mode of Ni3Si is predominant ly intergranular failure, irrespective of environment, with only a small am ount of transgranular quasi-cleavage. In air, fracture toughness of Ni3Si d ecreases with increasing volume fraction of fine-grained (Ni(Si) + Ni3Si) c olonies (where Ni(Si) is a solid solution of Si in Ni) as well as with decr easing grain size of Ni3Si. Very fine, unidentified precipitates are observ ed on both intergranular and transgranular fracture surfaces of a single-ph ase 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 low est for the specimens tested in air., intermediate for the specimens tested in vacuum and the highest for the specimens tested in dry oxygen. (C) 2001 Elsevier Science Ltd. All rights reserved.