4D-STEM is an electron microscopy technique that enables obtaining a convergent beam electron diffraction pattern corresponding to every pixel position from the region of interest in a TEM sample. This technique is highly effective in determining the local structure, strain mapping, dislocation substructures, and live dislocation interaction during in-situ tensile testing in the TEM. Therefore, this technique is very suitable to investigate high entropy alloys synthesized by mechanical alloying (MA) that have very high defect concentration and nanocrystalline grain size. These defect structures are dominated by dislocation entanglement and lead to sub-grain structure formation. These attributes are prevalent in the as-milled MA-powders of HEAs.

These as-milled HEA powders can be consolidated by spark plasma sintering (SPS), wherein extremely high densification is achieved by the application of high pressure (40-80 MPa) under vacuum and corresponding exposure to high temperatures in the range of 800-1200℃ for a very short duration of 5-10 mins. This leads to annealing twins and negligible grain growth in the lattice defect-intensive as-milled HEA powder microstructure. Mechanical alloying is a non-equilibrium processing technique leading to metastable phases and bimodal grain structure in this case post-SPS.

However, the post-SPS microstructure of HEAs has very high defect concentration and also possesses in-situ oxide nano-dispersoids obtained concomitantly during the wet milling stage. These nanocrystalline HEAs possess exceptionally high hardness which can be attributed to the interaction between the dislocations as well as the oxide dispersoid's interaction with the dislocations. In this present research, 4D-STEM technique has been effectively utilized to reveal the mechanism of these nanoscale dislocation interactions leading to exceptional strengthening and hardness obtained in MA-SPS HEAs.