Radiation damage tolerance of a novel metastable refractory high entropy alloy V2.5Cr1.2WMoCo0.04

Abstract

A novel multicomponent alloy, V2.5Cr1.2WMoCo0.04, produced from elements expected to favour a BCC crystal structure, and to be suitable for high temperature environments, was fabricated by arc melting and found to exhibit a multiphase dendritic microstructure with W-rich dendrites and V-Cr segregated to the inter-dendritic cores. The as-cast alloy displayed an apparent single-phase XRD pattern. Following heat treatment at 1187 °C for 500 hours the alloy transformed into three different distinct phases -BCC, orthorhombic, and tetragonal in crystal structure. This attests to the BCC crystal structure observed in the as-cast state being metastable. The radiation damage response was investigated through room temperature 5 MeV Au + ion irradiation studies. Metastable as-cast V2.5Cr1.2WMoCo0.04 shows good resistance to radiation induced damage up to 40 displacements per atom (dpa). 96 wt% of the as-cast single-phase BCC crystal structure remained intact, as exhibited by grazing incidence Xray diffraction (GI-XRD) patterns, whilst the remainder of the alloy transformed into an additional BCC crystal structure with a similar lattice parameter. The exceptional phase stability seen here is attributed to a combination of self-healing processes and the BCC structure, rather than a high configurational entropy, as has been suggested for some of these multicomponent "High Entropy Alloy" types. The importance of the stability of metastable high entropy alloy phases for behaviour under irradiation is for the first time highlighted and the findings thus challenge the current understanding of phase stability after irradiation of systems like the HEAs.

High-Entropy Alloys for Advanced Nuclear Applications

“…Most refractory metals have high DBTTs in their pure form, and although efforts have been made to 'ductilise' some of them (for instance, adding Re to W [151]), the issue remains endemic and in most cases BCC refractory metals are found to be hard and brittle at room temperature. It is not surprising, therefore, that BCC refractory HEAs have typically been found to have high hardnesses in comparison to most conventional alloys, for example, References [16,[19][20][21]24,28,31,124], and the results of tensile tests are very rarely reported.…”

“…A number of studies have found that some HEAs possess good phase stability under irradiation [ 49 , 51 , 52 , 54 , 55 , 67 , 93 , 94 , 95 , 110 , 122 , 123 , 124 ]. This behaviour has been attributed to a number of factors.…”

“…Interestingly, they also found that there was no loss of ductility following irradiation, suggesting good damage tolerance (although the alloy was in a nanocrystalline condition to allow for improved small-scale testing). Some have even found significant softening following irradiation [ 124 ]. Comparisons to pure refractory BCC metals irradiated under the same conditions again demonstrate the apparent improved resistance of concentrated multicomponent compositions, Figure 5 .…”

“…Irradiation damage in alloys that comprise significant fractions of multiple phases has often been found to result in preferential damage (including amorphisation) or precipitation of one phase, resulting in mechanical property degradation. It may only be tolerable if the precipitating phase is also a solid solution of the same crystal structure [ 124 ].…”

Influencing factors and mechanism of high-temperature oxidation of high-entropy alloys: A review

“…The high mixing entropy of high-entropy alloys tends to form a solid solution; however, it is not easy to form an intermediate phase, which is important for the excellent properties of high-entropy alloys. High-entropy alloys have excellent thermal stability [18][19][20][21][22], radiation resistance [23][24][25][26], corrosion resistance [27][28][29][30][31][32][33][34][35], and mechanical properties [36][37][38][39][40][41][42]. High-entropy alloys are expected to be used in turbine blades and high-speed cutting tools, and lightweight high-entropy alloys [43] may be used in energy and transportation fields.…”