EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

In recent years the uncommon behaviour of nanocrystalline systems to increase in hardness or plastic yield onset upon annealing treatments has been observed more and more frequently. This is counterintuitive to the common argument that heat treatments lead to changes which would decrease any resistance to plastic flow, e.g. decrease in dislocation density, increase in grain size, growth of precipitates. Thus, this ‘hardening by annealing’ phenomenon is usually attributed to local changes in grain boundaries (GBs) as their high content in nanocrystalline materials would show a major impact in comparison with coarse grained systems. However, a solid quantifiable proof remains ambitious as the necessary atomic resolution imaging of highly distorted GBs is still challenging. Therefore, it seems feasible to search for alternative methods to investigate such problems.
In the present work, ultra-fine grained tantalum, processed by high pressure torsion and annealed up to 400°C, was studied by macroscopic hardness testing, micropillar compression and a novel method utilizing mechanical spectroscopy on micron sized specimens (µMS). The µMS-technique is based on the oscillating system of a Hysitron PI-85 with a NanoDMA III upgrade, which has a resonance frequency of ~114Hz. Changes in resonance frequency as well as damping capability in contact with the microcantilever shaped specimens can be studied and correlated with established techniques. For the investigated tantalum, an increase in hardness as well as yield onset and flow level during micropillar compression was observed upon annealing, while µMS showed a decrease in damping capability. Considering established models for dislocation damping as well as GB structure, the µMS data leads to the conclusion that local changes in GBs are the origin for this ‘hardening by annealing’ phenomenon.