The role of surface oxide composition on the fatigue strength of metallic glass wire

Abstract

The application of cyclic bending has been the most practical way in evaluating the fatigue behaviour of thin sub-millimetre wires. For metallic glass wires, the fatigue behaviour is typically expressed in the form of strain–cycle relationship because conversions of applied bend strain to stress require the accurate measure of the modulus of elasticity (E). Here, we have applied the use of ultra-nano indentation testing (UNHT) to determine modulus of elasticity of metallic glass wire of compositions Fe77.5−x CrXSi7.5B15 (x=0, 4, 8). It is found that the modulus of elasticity averaged 187±5GPa and do not show any marked dependence on the Cr substitution for Fe. The value of E so measured was used to represent fatigue data in stress–cycle configuration. It is shown that fatigue limits were both composition and atmosphere dependent. The ternary Fe77.5 Si7.5B15 alloy wire had fatigue limit dependent on humidity ranging from 1GPa in ambient (60–70% RH) to 1.86GPa at reduced humidity (10% RH) and no limit in aqueous environment. It is also evident that Cr substitution of Fe in the ternary alloy wire, improved the fatigue behaviour, reaching a fatigue strength of 2.32GPa for alloy wire with 8at.% Cr. The effects of composition and atmosphere on fatigue limits are related to the role played by the composition of oxide film on the wire surface. XPS studies indicated that Cr containing wires had established Cr/Si oxides that were responsible for protection against an environmentally induced fatigue failure. The drop in fatigue strength for Cr containing wire in the aqueous medium is due to the reduced protection as a result of the deterioration of the high temperature oxide that is not reformed at room temperature because of low Si chemical activity.