Kinetic phase predictions in High entropy alloys

Kinetic approach for phase predictions in High Entropy Alloys

Chinmoy Chattopadhyay

Phase prediction in high entropy alloys (HEA) is a prime challenge for materials engineers as the final phase determines the properties of the HEA. Several approaches depending on thermodynamic, topological or electronic properties of constituent elements have been explored so far without much success. Moreover, most importantly, the formation of phase has been observed to be greatly dependent on processing route. For ex. an alloy may form BCC by Cu-mold casting, however, it can result in amorphous phase formation by melt spinning. The kinetic dependence of phase transformation which basically predicts the phase formation at different cooling rates has not been studied so far for HEAs. A simple and completely predictive approach has been explored to predict whether an equiatomic combination of elements will form amorphous, BCC, FCC or HCP single phase or a combination of two/three of these or the combination with the presence of one or more intermetallic compounds (IM). This approach is based on the variation of viscosity of alloys or IMs as a function of temperature utilizing the viscosities of its constituting elements and suitably incorporating the crystal structure information. Some other parameters affecting viscosity of an alloy like atomic size, packing density of the unit cell, etc., are suitably incorporated in the model. The TTT diagrams for each probable phase are generated with the help of the viscosity data. The formation of the amorphous, BCC, FCC single phase in HEAs has been excellently predicted by this approach with utmost success. The most important part of the present approach is that it acts as an efficient guide about the processing route that should be adopted to form a particular phase or combination of phases with or without IMs in a particular alloy via the critical cooling rate Rc obtained through the predicted TTT diagrams.

The Figure (a) shows that the BCC phase is the most stable one in AlCoCrFeNi HEA followed by FCC. The amorphisation would happen at a cooling rate of nearly 108 K/s. Therefore, it is predicted that, even melt spinning with 106 K/s cooling rate cannot vitrify the alloy. The Fig. (b) shows the XRD results of arc melted AlCoCrFeNi sample having BCC phase with a minor presence of FCC whereas, the melt spinning results in BCC phase formation and not amorphisation, validating the model.