Inverse Hall–Petch like behaviour in a mechanically milled nanocrystalline Al₅Fe₂ intermetallic phase

Abstract

In the present investigation, the indentation study on the high-energy ball-milled nanocrystalline Al₅Fe₂ intermetallic compound has established the inverse Hall–Petch (IHP) behaviour. The structural characterisation of the milled powder particles by X-ray diffraction (XRD) and transmission electron microscopy has shown the evolution of nanocrystalline phase. Micro-indentation measurements have revealed the increase in hardness with decreasing grain size, reaching to a maximum of 9.0 ± 0.3 GPa up to a grain size of 32 ± 4 nm, followed by a decrease. The decrease in hardness with further refinement, an indication of grain size softening, demonstrates the IHP-like behaviour. The deviation from the Hall–Petch behaviour has been discussed using various models based on the dislocations and grain boundary-mediated processes. From the analysis, it appears that the model based on mesocopic grain boundary sliding phenomena is more appropriate to account for the observed grain size softening.

Keywords:

• High-energy ball milling
• Al₅Fe₂
• nanocrystalline
• intermetallics
• grain size
• hardness

Acknowledgements

The authors are grateful to Professor K. A. Padmanabhan for the fruitful discussion and suggestions; and Dr S. Srikanth, Director, CSIR-National Metallurgical Laboratory (Jamshedpur, India) for his encouragement and support during this work. One of the authors (NKM) would like to thank the Indian National Academy of Engineering (INAE) for providing him the support under INAE Chair Professorship. Authors would like to record their sincere gratitude to the anonymous reviewers for their critical comments and constructive suggestions for improving the quality of the manuscript.