Strengthening effect of grain and twin boundaries in zirconium bi-crystal micropillars

ABSTRACT

In this work, the effect of crystallographic orientations and the strengthening due to grain/twin boundaries were investigated by uniaxial compression of single- and bi-crystal micropillars of pure zirconium (Zr) with ≥99.9% purity. Micropillars fabricated using focused ion beam (FIB) milling were compressed using a flat punch tip in a nanoindenter to obtain stress–strain curves for individual orientations of Zr. Only near-basal-oriented samples were deformed by multiple slip, while samples with other orientations were deformed by a single slip. Bi-crystal samples had significantly higher flow stress than single-crystal samples – the increment in flow stress arising from grain boundaries being higher than that arising from twin boundaries. Our analysis shows that while twin-boundary strengthening can be explained based on a size effect according to the Hall-Petch equation, the higher strengthening effect of grain boundaries appears to arise from a combination of a sample-size effect as well as dislocation accumulation at boundaries.

KEYWORDS:

• Zirconium
• micropillar compression
• orientation effect
• boundary strengthening
• bi-crystal deformation

Acknowledgements

The authors acknowledge helpful discussions with Dr. Dinesh Srivastava and Dr. G. K. Dey from BARC. The authors also acknowledge support from the National Facility of Texture and OIM – a DST-IRPHA facility, and the Nanoindentation central facility at IIT Bombay, Mumbai, India.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by Board of Research in Nuclear Sciences [grant number 09BRNS010].