Formation of complex intermetallic phases in novel refractory high-entropy alloys NbMoCrTiAl and TaMoCrTiAl: Thermodynamic assessment and experimental validation

Abstract

In this work, we present the results of the thermodynamic assessment of two equiatomic refractory High Entropy Alloys (HEAs), namely TaMoCrTiAl and NbMoCrTiAl, in the temperature range between 700 and 1500°C. Particular attention is paid on the constitution of the intermetallic phases stable in these alloy systems. Thermodynamic calculations were performed using a self-developed thermodynamic database based on the CALPHAD (Calculation of Phase Diagram) approach. The details of the thermodynamic modelling and particular characteristics of the relevant phases within the Ta-Nb-Cr-Ti-Al system are presented. To verify the new database, the phase formation and stability of both quinary alloys in near-equilibrium conditions were studied experimentally by utilizing scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD) as well as X-ray powder diffraction (XRD). Both equiatomic alloys reveal a complex microstructure including several intermetallic phases at intermediate temperatures. The alloy NbMoCrTiAl consists of an ordered B2 phase, Al(Mo, Nb)3 and two polytypes (C14 and C15) of the Cr2Nb Laves phase. Precipitations of Cr2Ta Laves phase (C14, C15 and C36-type) in the B2 matrix were observed in the alloy TaMoCrTiAl. Based on the results of thermodynamic calculations, it was concluded that: (i) Nb stabilizes the AlMo3 A15 phase in the alloy NbMoCrTiAl, (ii) Al and Ti play a crucial role in the formation of the ordered B2 phase in both alloys and (iii) the concentrations of Cr and/or Ta/Nb should be dramatically reduced to decrease the Laves phase volume fraction.

The Effect of Al on the Formation of a CrTaO4 Layer in Refractory High Entropy Alloys Ta-Mo-Cr-Ti-xAl

“…Comprehensive information on the microstructure of the equiatomic Ta-Mo-Cr-Ti-20Al has already been reported in Ref. [8,14]. After homogenization, this alloy consists of large grains of the ordered B2 crystal structure as well as some Laves phase predominantly decorating the grain boundaries.…”

“…Thus, the microstructure of these alloys is not shown here. It should, however, be mentioned that all alloys are not in the thermodynamic equilibrium after heat treatment; therefore, it is expected that the amount of the Laves phase increases during oxidation [14].. The kinetic oxidation curves of the alloys Ta-Mo-Cr-Ti-xAl (x = 5; 10; 15; 20 at%) recorded during isothermal exposure to air at 1200 °C for 24 h are shown in Fig.…”

A review on phase prediction in high entropy alloys

“…Muller et al 122 synthesized TaMoCrTiAl and NbMoCrTiAl refractory high entropy alloys through arc melting technique. The thermodynamic calculations and experimental observations indicate that both the alloys NbMoCrTiAl and TaMoCrTiAl consist of single-phase structures.…”

Rapid screening of high-entropy alloys using neural networks and constituent elements

“…Cantor alloy -CoCrFeMnNi), it can be argued that they display overly complex morphologies to be classified as HEAs. However, many multi-phase alloys have been reported over the last decade that have been widely accepted as HEAs [42][43][44], as long as the behavior of the MPEA is consistent with HEA properties. Since such MPEAs have been classified as HEAs comprising (SS+IM) structures and encoded as "1" entries in the datasets, it is fully expected that the x = 1.0 and x = 1.3 compositions be predicted accordingly.…”