An equiatomic CoCrFeMnNi high-entropy alloy (HEA), produced by arc melting and drop casting, was subjected to severe plastic deformation (SPD) using high-pressure torsion. This process induced substantial grain refinement in the coarse-grained casting leading to a grain size of approximately 50nm. As a result, strength increased significantly to 1950MPa, and hardness to ∼520HV. Analyses using transmission electron microscopy (TEM) and 3-dimensional atom probe tomography (3D-APT) showed that, after SPD, the alloy remained a true single-phase solid solution down to the atomic scale. Subsequent investigations characterized the evolution of mechanical properties and microstructure of this nanocrystalline HEA upon annealing. Isochronal (for 1h) and isothermal heat treatments were performed followed by microhardness and tensile tests. The isochronal anneals led to a marked hardness increase with a maximum hardness of ∼630HV at about 450°C before softening set in at higher temperatures. The isothermal anneals, performed at this peak hardness temperature, revealed an additional hardness rise to a maximum of about 910HV after 100h. To clarify this unexpected annealing response, comprehensive microstructural analyses were performed using TEM and 3D-APT. New nano-scale phases were observed to form in the originally single-phase HEA. After times as short as 5min at 450°C, a NiMn phase and Cr-rich phase formed. With increasing annealing time, their volume fractions increased and a third phase, FeCo, also formed. It appears that the surfeit of grain boundaries in the nanocrystalline HEA offer many fast diffusion pathways and nucleation sites to facilitate this phase decomposition. The hardness increase, especially for the longer annealing times, can be attributed to these nano-scaled phases embedded in the HEA matrix. The present results give new valuable insights into the phase stability of single-phase high-entropy alloys as well as the mechanisms controlling the mechanical properties of nanostructured multiphase composites

After more than 30 years of fundamental research and development activities intermetallic titanium aluminides based on the ordered γ-TiAl phase have found applications in aerospace and automotive industries. The advantages of this class of innovative high-temperature lightweight materials are their low density, their good strength and creep properties, as well as their oxidation resistance up to 750 °C. A drawback, however, is their limited ductility at room temperature, which is reflected by a low plastic strain at fracture. Advanced engineering TiAl alloys, such as the β-solidifying so-called TNM alloy with a nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in atomic percent), are complex multi-phase materials which can be processed by ingot or powder metallurgy, precision casting methods as well as additive manufacturing. Each production process leads to specific microstructures which can be altered and optimised by thermomechanical processing and/or subsequent heat treatments, whereby the knowledge of the occurring solidification processes and phase transformation sequences is essential. Therefore, thermodynamic calculations were conducted to predict the phase fraction diagrams. After experimental verification, these phase diagrams provided the base for the development of heat treatments to adjust balanced mechanical properties. To determine the influence of deformation and kinetic aspects, sophisticated ex- and in situ methods have been employed. Finally, the application of TiAl alloys in aerospace is reported.