Fatigue of metallic glasses

Abstract

The fatigue behaviour of Ni₄₉Fe₂₉P₁₄B₆Si₂, Ni₄₈Fe₂₉P₁₄B₆Al₃ and Pd_77.5Cu₆Si_16.5 metallic glasses is examined. In the finite lifetime regime the relationship between stress amplitude (σ_a), fracture stress (σ_f), mean stress (σ_m) and cycles to failure (N_f) isσ_a=A(σ_f−σ_m) (2N_f) ^b , whereA andb are 16.9 and −0.40 respectively for reduced gauge section Ni₄₉ strips (for σ_m ≲ 140 kg mm⁻²) and 27.0 and −0.44 for Pd base wires. These results are unusual in thatA ≫ 1. Consequently, a sharp discontinuity exists nearσ_a(σ_f−σ_m) −1. In a simple tensile test failure occurs at σ_f(=σ_y) and 2N_f=1; for peak stresses only a percent or so less thanσ_f the sample will withstand hundreds of cycles of stress. For uniform cross-section glassy metal filaments, a fatigue limit is observed at stress ratios (σ_a/σ_f) in the vicinity of 0.07 to 0.15. The fatigue limit for reduced section specimens is a factor of ∼ 2 higher. Fatigue failure of the Ni-Fe strips may occur under partially or fully plane stress or plane strain conditions, depending on sample thickness and stress. Final failure of the Pd_77.5Cu₆Si_16.5 wires always occurs by general yielding of the remaining section.