Indentation fracture toughness behavior of FeCo-based bulk metallic glass intrinsic composites

Indentation fracture toughness of (Fe0·5Co0.5)69−xNb6B25+x (x=0, 2, 4) bulk metallic glass alloys in the partially crystalline (intrinsic composite) state has been studied. Corner cracking was observed on indentation in all the compositions above a certain critical load. High hardness with values in excess of 1100HV was obtained for all the alloy compositions. Indentation fracture toughness measured through various empirical relations was found to be <5MPa√m for all the alloys irrespective of crack morphology. Palmqvist crack morphology was observed within the materials, for the applied indentation loads upto 9.8N. The change of crack morphology from Palmqvist to half-penny is generally expected to happen at a c/a (crack length plus half diagonal length to half diagonal length) value of 2.5 but the Plamqvist crack morphology prevailed even to a c/a of 3. The values of crack length to half diagonal length ratio (l/a) remained below 2 for all the loadings. Indentation fracture toughness values of 1.85MPa√m, 2.1MPa√m, 2.9MPa√m obtained were based on the observed crack morphology. The values of energy release rate (GC), surface energy (γS) and brittle index are also given.