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

Abstract

The crystal structure, crystallization, fracture behavior and mechanical properties of (Co_{1 − 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 (ε_f < 0.01) for x ≥ 0.15, whereas for lower Fe content it has a maximum (ε_f > 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).

Keywords:

• nanocrystalline alloy
• nanostructure
• fracture
• fractography
• shear band

Acknowledgements

This work was support in part by the Office of Naval Research. MD and MAW would like to thank Dr Todd M. Heil (Technology Assessment and Transfer Inc.) for writing the Mathematica™ program used for bend test data analysis, Mr Leroy Levenberry (NRL) for assistance with ribbon thickness measurements, and Prof. John Lewandowski (Case Western Reserve Univ.) for helpful conversations. MEM and PRO gratefully acknowledge helpful conversations and support from V. Keylin and J. Huth at Magnetics, A Divison of Spang and Co. This material is based in part upon work supported by USAF/AFRL Air Force Office of Scientific Research under contract number FA-9550-09-C-0024.