Search

The recently developed refractory metal high entropy superalloys (RSA) have been proposed as novel materials for high temperature service. A key requirement of any material used in such applications is to retain its properties throughout the life of a component. Consequently, it is critical that the microstructures that give rise to these properties are stable across the temperature ranges experienced in service, yet, at present, very little is known about the microstructural stability of RSA. To address this issue, here we report on the microstructural evolution of two alloys from the AlMoNbTaTiZr RSA system following long duration thermal exposures of 1000 h at 1200, 1000 and 800 ˚C. At these temperatures, the initial microstructures were found to be unstable, forming new intragranular precipitates. In AlNbTa0.5TiZr0.5, both Al-Zr hexagonal and Al-Nb rich orthorhombic phases were observed following exposure at 1200 and 1000˚C, whilst additional fine scale phases were found to have formed following exposure at 800˚C. In AlMo0.5NbTa0.5TiZr0.5, the Al-Zr phase was observed to precipitate within the grains following exposure at both 1200 and 1000˚C. A refractory metal-rich secondary bcc phase was observed following exposure at 1000˚C and 800˚C and a Zr-rich phase precipitated at 800˚C. The formation of these additional phases would undoubtedly affect the properties of these alloys, altering their in-service performance. Consequently, these results highlight the need to consider microstructural stability in the future development of RSA and to gain an enhanced understanding of the phase equilibria of these systems.

The development of refractory metal high entropy superalloys has received great interest due to their potential for high temperature structural applications. The formation of a two phase nanoscale microstructure, comprising bcc and ordered B2 phases, is thought to be fundamentally linked to the presence of Al. As such, determining the influence of Al concentration on the microstructural formation and thermal stability of these novel materials is critical for future alloy development. To elucidate this effect, the microstructural evolution of a series of alloys with systematically varying compositions from the Ti-Ta-Zr-Alx system have been studied following homogenisation and long term exposures at 1100, 900 and 700˚C. Nanoscale cuboidal morphologies were observed in all alloys, but ordering was only observed when the bulk Al content was > 5 at.%. These results indicated that a common formation mechanism, thought to be spinodal decomposition, occurred prior to any B2 ordering. The results also showed that the B2 phase was only stable to relatively moderate temperatures, < 900˚C, eliminating order hardening as a high temperature strengthening mechanism. In addition, for the alloy with the greatest Al content, deleterious intragranular Al-Zr-rich intermetallics were observed following the long duration thermal exposures.

The lattice parameters and misfit of the &gamma, and &gamma, &prime, phases in a series of model quaternary Ni-based superalloys with and without Mo additions have been determined using neutron diffraction between room temperature and 700 &deg, C. Despite the fact that Mo is typically expected to partition almost exclusively to the &gamma, phase and thereby increase the lattice parameter of that phase alone, the lattice parameters of both the &gamma, phases were observed to increase with Mo addition. Nevertheless, the effect on the &gamma, lattice parameter was more pronounced, leading to an overall decrease in the lattice misfit with increasing Mo content. Alloys with the lowest Mo content were found to be positively misfitting, whilst additions of 5 at.% Mo produced a negative lattice misfit. A general decrease in the lattice misfit with increasing temperature was also observed.

© 2016 Acta Materialia Inc. Cu 3 Au is often cited as a case example of a metallic system exhibiting both short-range order in the solid solution phase and a long-range order-disorder transition. In this work, X-ray total scattering data obtained from the in situ heating of a gas-atomised powder sample of Cu 3 Au are used to demonstrate the suitability of total scattering, in conjunction with large-box modelling, for the analysis of short-range order in alloys. The existence of an ordering transition at c. 400° is confirmed, and the development of short-range order reminiscent of the L1 2 long-range ordered structure is observed prior to this transition. Furthermore, it is found that a degree of short-range order is present even in quenched samples (usually assumed to be completely random) which throws into question the identification of short-range order in previous ex situ studies. It is demonstrated that total scattering can be used successfully to identify the type and degree of ordering, differences in the bond length distributions in the first coordination shell and to suggest a likely mechanism for the formation of order in the system.

In spite of its influence on a number of physical properties, short-range order in crystalline alloys has received little recent attention, largely due to the complexity of the experimental methods involved. In this work, a novel approach that could be used for the analysis of ordering transitions and short-range order in crystalline alloys using total scattering and reverse Monte Carlo (RMC) refinements is presented. Calculated pair distribution functions representative of different types of short-range order are used to illustrate the level of information contained within these experimentally accessible functions and the insight into ordering which may be obtained using this new method. Key considerations in the acquisition of data of sufficient quality for successful analysis are also discussed. It is shown that the atomistic models obtained from RMC refinements may be analysed to identify directly the Clapp configurations that are present. It is further shown how these configurations can be enhanced compared with a random structure, and how their degradation pathways and the distribution of Warren-Cowley parameters, can then be used to obtain a detailed, quantitative structural description of the short-range order occurring in crystalline alloys.