High-pressure torsion (HPT) allows the preparation of novel nanocrystalline bulk and composite materials with enhanced functional properties. Such functional materials are essential for green technologies, like hydrogen production and storage. A promising candidate for hydrogen storage in the form of metal hydrides is the intermetallic FeTi system. Interestingly, the hydrogen sorption properties of FeTi can be improved by HPT processing. However, due to its high hardness and brittle nature, it is challenging to process, even by HPT, and a heterogeneous microstructure is obtained.
In this study we investigated HPT processing of the FeTi-Cu system with the goal of preparing a homogeneous (nano)composite, whereby Cu takes the role of a ductile phase for improving the deformability. The Cu content was reduced as much a possible, while still aiming to maintaining a percolating structure; therefore an optimized Cu content of 25 vol% was fixed.
At room temperature, a heterogeneous two-phase structure remained due to localization of plastic deformation in the softer Cu phase. The suppression of localization at elevated temperatures (>200 °C), due to increased strain-rate sensitively and possible strength convergence, resulted in the formation of an FeTi-Cu nanocomposite. While this nanocomposite exhibited the desired uniform structure, large FeTi grains remained unrefined. Below 250 °C this was attributed to slipping of the sample between the HPT anvils, while above 250 °C the formation of Cu-rich shear bands inside the nanocomposite occurred. The latter phenomenon highlights that plastic instabilities can reappear again at higher temperatures. Further increasing the temperature to 550 °C resulted in the most uniform microstructure, despite the Cu-rich shear band persisting. The improved homogeneity was attributed to a ductile-to-brittle transition happening in this temperature range, allowing more pronounced refinement of FeTi during severe plastic deformation.