High Temperature and Ion Implantation-Induced Phase Transformations in Novel Reduced Activation Si-Fe-V-Cr (-Mo) High Entropy Alloys

Abstract

For fusion to be realized as a safe, sustainable source of power, new structural materials need to be developed which can withstand high temperatures and the unique fusion radiation environment. An attractive aspect of fusion is that no long-lived radioactive wastes will be produced, but to achieve this structural materials must comprise reduced activation elements. Compositionally complex alloys (CCAs) (also called high entropy alloys, HEAs) are promising candidates for use in extreme environments, including fusion, but few reported to date have low activation. To address these material challenges, we have produced novel, reduced activation, HEAs by arc-melting, and investigated their thermal stability, and radiation damage resistance using 5 MeV Au 2+ ion implantation. Whilst the alloys were designed to form single phase BCC, using room temperature and non-ambient in situ X-ray diffraction we have revealed the thermodynamically stable structure of these alloys is in fact a sigma phase. We propose that a BCC phase is formed in these alloys, but at high temperatures (>1000 • C). A BCC phase was also formed during heavy ion implantation, which we propose to be due to the rapid heating and cooling that occurs during the thermal spike, effectively freezing in the BCC phase produced by an implantation induced phase transformation. The BCC phase was found to have high hardness and a degree of ductility, making these new alloys attractive in the development of reduced activation HEAs for nuclear applications.

High-Entropy Alloys for Advanced Nuclear Applications

“…We can see that many of the elements that we use in many engineering alloys are precluded, including Ni, Zr, Nb and Mo. A number of studies have considered HEAs based on low-activation elements like Ti, V, Cr, Mn, Fe, Ta and W [16][17][18][19][20][21][22][23][24], although their development remains in its infancy. Despite increasing the ease of alloy design efforts (there are fewer elements and combinations to choose from), there are some serious challenges associated with the use of these elements to form HEAs, as will be discussed later in Sections 5.1.2 and 5.2.…”

“…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.…”

“…Other HEAs, however, have been found to exhibit phase changes on irradiation, with some exhibiting extensive microstructural evolution, especially precipitation [ 21 , 22 , 84 , 113 , 122 , 125 , 126 , 127 , 128 ]. Owing to the propensity of most HEAs to decompose during thermal ageing [ 108 , 112 ], it is not always clear if the HEAs in these studies would have decomposed simply under thermal annealing, since many do not make a suitable comparison.…”

“…Interestingly, they still concluded this was likely a case of radiation-enhanced precipitation (specifically, radiation-enhanced diffusion), rather than radiation-induced precipitation—that is, that the precipitates were likely to be stable phases under equilibrium conditions, and the precipitation was accelerated by enhanced diffusion. In a more likely case of irradiation-induced phase change, a sigma to BCC transition at room temperature has been observed in some alloys, which is very unlikely to have been favourable in the non-irradiated state [ 21 , 55 ].…”

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

“…In the current work, to reduce the elemental segregation displayed by the alloys fabricated by Ikeuchi et al [22] (formed by large differences in the melting temperatures of the components), refinement of the alloy composition and amendment of the arc-melting process were undertaken. The alloy design process was achieved by utilising a combination of the CALPHAD method and the thermodynamic parameters used for prediction of solid solution for high entropy alloys (Gandy et al, 2019) [23] resulting in the prediction of a novel alloy of nominal composition V2.5Cr1.2WMo. To avoid the only partial melting of the MoVW phase observed in the CrMoVW alloy, all constituents were melted together with the highest melting temperature elements covering the lowest melting temperature elements.…”