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Abstract In a previous work by the present authors (Acta Mater. 148, 2018, 263–279), the thermodynamics of V–Nb–Ta–Mo–W highly concentrated alloy at equiatomic concentration was investigated using the cluster variation method in the irregular tetrahedron approximation. It was shown that the disordered solid solution is stabilized in this highly concentrated area of the system as prescribed by the high entropy alloy (HEA) paradigm. The configurational entropy, however, has only a minor (and sometimes contrary) influence in this stabilization. It was then postulated that configurational frustration, i.e. the competition between conflicting interactions, is the more relevant factor in the stabilization of the highly-concentrated alloys. The present work elaborates this idea by further considering the cases of V–Nb–Mo–Al and V–Nb–W–Al quaternary systemas and the corresponding quinary systems with Ta addition. The quaternary system are characterized by strong Al – M pair interactions that could enhance configurational frustration. The introduction of Ta, with weaker Ta – Al and practically non-existing Ta – Nb interactions, is meant to disrupt the frustration, hence leading to a decrease of the stability of the disordered alloys. It was shown that V–Nb–Mo–Al and its quinary do not exhibit configurational frustration as expected, since a stable compound with stoichiometry VMoNbAl is formed, and the stability of their disordered alloys (A2) is low. On the contrary, V–Nb–W–Al and its quinary demonstrated strong configurational frustration and correspondingly a stronger A2 stabilization. Deviations from equiatomic composition were also investigated and even stronger A2 stabilization was observed in this case. These results show that the stabilization of the disordered phase can only be attributed to configurational frustration, corroborating the previous work's conclusion that the effect of configurational entropy is only marginal in this stabilization. Graphical abstract Image 1 Highlights • Systems V – Nb – (Ta) – Mo – Al and V – Nb – (Ta) – W – Al behave differently. • Thermodynamics of disordered highly concentrated alloys is defined by the enthalpy. • Deviations of the ideal activity are large and even non-linear both in x and T. • Short-range order parameters are large and even non-linear in T. • Role of the entropy is secondary and thus no need to restrain to equiatomic alloys. [ABSTRACT FROM AUTHOR]

Abstract B2–L2 1 configurational order resulting from thermal processing of Ni–Co–Mn–In Heusler alloys is one of the main factors defining the characteristics of the martensitic transformation experienced by these alloys. While the effects of the quenched-in disorder on the martensitic transformation have been extensively investigated, the kinetics of the ordering process is not well known and mostly inferred from the indirect results such as the dependence of the martensitic transformation or magnetic transition temperatures to thermal processing. However, recent results suggested that the kinetics of B2– L 2 1 ordering could have a more significant role than expected in the final state of the material and its functional properties. In the current work, an attempt has been made to investigate the kinetics of B2–L2 1 ordering in order to evaluate the dependence of the quenched-in disorder on the different variables of the thermal procedures. Results showed that ordering rate has a strong dependence on annealing temperature. Below 600 K, slow kinetics is forcing the crystal to always contain a certain amount of quenched-in disorder at room temperature. Between 600 K and 750 K, kinetics is fast enough to reach thermal equilibrium state within tens of minutes during the annealing process. Above 750 K, kinetics is very fast and thermal equilibrium can be reached during heating, so no isothermal holding is required. These predictions have been confirmed using differential scanning calorimetry and in-situ high-energy X-Ray diffraction measurements for non-direct estimations of the degree of order. Graphical abstract Image 1 Highlights • Ordering rate has a strong dependence on annealing temperature. • Below 600 K, the alloys to contain some quenched-in disorder at room temperature. • From 600 K to 750 K, ordering reaches equilibrium in tens of minutes when annealing. • Above 750 K, ordering can reach equilibrium during heating process. [ABSTRACT FROM AUTHOR]