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Abstract
Free volume and medium-range order (MRO) present in rapidly solidified ribbons (RSRs) and bulk metallic glasses (BMGs) of Zr52Ti6Al10Cu18Ni14 have been probed by high resolution electron microscopy, fluctuation microscopy, positron annihilation and differential scanning calorimetry. In the as-solidified condition, RSRs showed higher free volume and lower MRO in comparison to BMGs. Within BMGs, the central regions showed higher MRO and lower free volume than the peripheral regions. Uniform deformation of BMGs and RSRs modified their structures, where in, free volume increased in the former and reduced in the latter. These changes in structures led to work hardening in RSRs and work softening in BMGs. Such behaviour could be explained by invoking a concept of critical free volume in the glass phase. For samples (in as-solidified condition) having free volume higher than the critical value, free volume decreased with deformation and showed work-hardening behaviour. In contrast, the work softening behaviour was noticed in samples having free volume lower than the critical free volume.
Acknowledgements
The authors are grateful to Dr A.K. Suri, Director, Materials Group, BARC for his consistent encouragement and guidance. Special thanks to Dr D. Srivastava of Materials Science Division, BARC for their help in positron annihilation experiments and in analysing the results. The authors are also thankful to Mr Chatterjee H of Refuelling Technology Division, BARC for his help in estimating cooling rates in metallic glasses using Finite Element Method-based software.