helium. A notch was cut in the ribbons which were tested at a strain rate of 1.66x10-4 s-1 in a Hounsfield tensometer. The fracture toughness was obtained from the resultant failure load using: K C a I σ π = (2) where σ is the fracture strength, a is the crack length and C is given by: 4 3 2 W a 30.39 W a 21.72 W a 10.55 W a C 1.12 0.231 ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ⎟ + ⎠ ⎞ ⎜ ⎝ ⎛ ⎟ − ⎠ ⎞ ⎜ ⎝ ⎛ ⎟+ ⎠ ⎞ ⎜ ⎝ ⎛ = − (3) where W is the ribbon width. The fracture surfaces of the wire and ribbon samples were examined by SEM operated at 20 kV. RESULTS AND DISCUSSION Tensile strength and Vickers hardness Figure 3 shows the microhardness, Hv, and tensile strength, σf, for the Fe78-xCrxSi10B12 alloy wires and HT steel wires. It can be seen that both Hv and σf increase with increasing Cr content. These increases in Hv and σf are broadly consistent with those observed previously [3] for the Fe75-xCrxSi10B15 alloy series, although the absolute values are significantly smaller, consistent with the lower B content in the present case. The Cr containing alloy wires proved to be easier to cast with a uniform cross section than the Fe78Si10B12 base alloys because, it is considered, of the formation of a surface film of Cr/Si hydrated oxide (confirmed by XPS studies [3,5]), which aided in stabilising the liquid jet. The Fe78Si10B12 wire was subsequently characterised by a shallow necking which could adversely influence the measured strength and fatigue life. The fracture toughness, KI, for Fe78-xCrxSi10B12 system is almost constant over the range of Cr content (Figure 4). The values are similar to those reported for NiCrB glassy ribbon and X-200 steel [7]. Fracture in both wires and ribbons occurred by shear-induced deformation following inhomogeneous flow, similar to previously reported fractures in these glassy alloys [7-9]. The fracture occurred at ~45° to the tensile axis (i.e. plane of maximum critical resolved shear stress), implying that fracture was shear induced. The fracture morphologies for wires and ribbons consisted of two distinctive regions; one uniform, and the other veined, as has been reported previously [9-11], (figure 5). The smooth regions represent the initial shear displacement. The fracture veins resulted from the existence of a “liquid-like” or softened layer formed during shear displacement, involving extensive plastic flow, corresponding to instability phenomena characteristic of ductile glassy alloys [8]. It has been suggested [10] that the thin layer of softened material must result from local heating occurring under close to adiabatic conditions. The fracture morphology in the Figure 3.- Tensile strength and Vickers microhardness results for the Fe78-xCrxSi10B12 wires tested. 0 1 2 3 4 0 2 4 6 8 X at% Cr 500 1000 1500 2000 σf (GPa) Hv (VHN) σf HT steel wire Hv HT steel wire σf Hv
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