Shear band formation and fracture behavior of nanocrystalline (Co,Fe)-based alloys

Abstract

The crystal structure, crystallization, fracture behavior and mechanical properties of (Co1 − x Fe x )89Zr7B4 (x = 0–0.7) nanocrystalline ribbons were investigated. The crystallization peaks of the amorphous ribbons tend to shift to higher temperatures with increasing Fe content. After annealing at 475°C for 3600 s, the main crystallization product is hcp-(Co,Fe) for the Co-rich composition (x = 0), bcc-(Co,Fe) for high Fe contents (x ≥ 0.3) and a mix of bcc, and fcc for intermediate compositions (0.025 ≤ x ≤ 0.15). The relative strain at fracture decreases dramatically (εf < 0.01) for x ≥ 0.15, whereas for lower Fe content it has a maximum (εf > 0.037) at x = 0.025 and 0.050 resulting in excellent resistance against fracture. The brittle ribbons (x ≥ 0.15) showed smooth fracture surface with dimples less than 230 nm in diameter, small localized or absent shear bands and large Vickers hardness (>1200 kg mm−2). On the other hand, the Co-rich ribbons with greater ductility (x = 0.025, 0.05) exhibit a vein pattern filled with voids (features ∼2–11 µm), extensive shear banding and lower Vickers hardness (<1050 kg mm−2).