PSI - Issue 23

Václav Paidar / Procedia Structural Integrity 23 (2019) 402–406

403

Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

Multicomponent alloys, so called high entropy alloys, are not often homogeneous solid solution (see e.g. Miracle and Senkov (2017)) and the interfaces between various phases have a significant impact on the properties of materials in general, and in particular, on mechanical properties (see e.g.Basu, Ocelik et al. (2018)). A question arises whether the interfaces between the regions with the same crystal structure but different composition can have similar effect on mechanical properties as that described by Hall-Patch relationship, Hall (1951), Petch (1953). In addition to standard grain boundaries, also the twin boundaries can have a similar effect, Laplanche, Kostka et al. (2016). The aim of this paper is to find quantitative estimates of element separation in refractory high-entropy alloys. After the first element separation during solidification that is controlled by the temperature differences between liquidus and solidus, the miscibility gap leads to the creation of zones with lower and higher concentrations in solid solutions. The information on those processes is summarized in the phase diagrams. Several cases can be distinguished: For example, a large concentration gap that separates the zirconium and tantalum rich solid solution regions can be found in Zr-Ta system. The width of the gap can be characterized by the concentration difference at the temperature of hexagonal phase appearance that we denote here as a critical temperature T c . On the other hand, the occurrence of an intermetallic phase divides the regions of solid solutions and consequently, there are two temperatures of largest solid solution concentrations of the two elements from the two sides of the binary phase diagram what is the case, for example, of Zr-Mo system. The width of the solid solution concentration gap can be defined in such cases as the composition interval at the temperature of hexagonal phase appearance reduced by the composition interval in between the two solid solution regions. In fact, the composition gap is composed of two parts in this case, the first corresponding to the BCC solid solution close the hexagonal element and the second close to the BCC transition metal element. 2. Multi-element alloys Primary source of the data on element separation in multi-element alloys can be found in the binary phase diagrams. Let us consider the alloys of BCC and HCP transition metals where the coexistence of two BCC regions differing in composition is likely to be found. The BCC-phase occurs at higher temperatures also in such metals with the HCP phase at low temperatures. The beta BCC solid solution can cover at certain temperature interval the entire composition range as inTi-Ta, Ti-Nb, Ti-VTi-W, Ti-Mo or Zr-Ta, Zr-Nb, However, there may be an intermetallic phase that persists up to almost liquid zone in certain systems as in Zr-W, Zr-Mo, Zr-V and thus the beta BCC solid solution phase is then limited only to two regions close to the pure elements. The element separation at solidification in binary alloys is essentially determined by the difference between the melting temperatures when the dendrites are formed. Diffusion in beta zones results in element separation that depends on the width and height of the miscibility gap above the monotectoid temperature below which the hexagonal phase can be formed. In the case of binary alloys with the intermetallic regions in between the beta zones, the borders of the two phase mixtures of intermetallic and BCC-phases can be taken as branches of miscibility gap. We selected in this paper among various structures of high entropy alloys, Senkov, Miracle et al.(2018), only those with two BCC-phases differing in local compositions. In particular, let us compare a septenary TiZrVHfNbMoTa(A7) and an octonary TiZrVHfNbMoTaW(A8) alloys with the compositions published by Gao, Carney et al. (2015).

The strongest expulsion from D regions (see Table 1) was found for zirconium and the next was hafnium.