Comparative irradiation response of an austenitic stainless steel with its high-entropy alloy counterpart
Graphical abstract
Introduction
The evolution of physical metallurgy as the main branch of science addressing the relationship between microstructure, properties and processing of metals and alloys (and later of materials science, addressing all classes of materials) has brought – over the past century – unprecedented advances in several fields of human technology. Lightweight Al-, Mg- and Ti-based alloys entirely revolutionised modern industry with applications ranging from recyclable and high strength-to-weight ratio parts for both automotive and aerospace industries to the infant industry of biocompatible and biodegradable implants [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]]. Fe-based alloys – in particular steels – have formed the backbone of our technology thus far [14,15] and are still just as relevant in modern technology as structural materials. The combination of metallic alloys such as steels with Zr-based alloys has led to the success of both thermal- and fast-neutron reactors [[16], [17], [18]], providing clean electricity making up 10% of the world's energy supply [19].
Throughout history, the great majority of metallic alloys were developed based on available raw materials [[20], [21], [22]]. The occurrences of such raw materials, in most cases, consist of minerals of a given element and require particular technologies to be exploited. This led to the development of specific metal industries, each one characterized by its own knowledge set. Traditionally, alloys developed within one of these sets were named after the particular metal, so, the Cu industry developed Cu alloys, the Fe industry developed steels and cast irons, the Al industry develops Al alloys, and so on. Naturally in most cases that particular metal is predominant in the alloy, but not necessarily.
This resulted in the notion of the terminal solid solution principle which would consist of alloying a base metal with a small content of either major or minor solutes, which today are supposedly selected following the formalisation of the Hume-Rothery rules [23,24], but in fact empirical experience is often used. Using the perspective of the thermodynamics of alloy systems, terminal solid solution alloys (sometimes referred to as dilute alloys) are those engineered considering the solute-rich corner of phase diagrams [24]. Naturally this is a gross simplification of the technical alloys’ universe and there are plenty of examples which are neither based on the terminal solid solution (e.g the so called beta bronzes), nor can be described as dilute alloys (e.g stainless steels).
Most recently, the science of metallurgy has been impacted with the advent of a new class of metallic alloys: the high-entropy alloys (or HEAs) [25]. As opposed to terminal solid solutions, HEAs are metallic alloys designed at or closer to the highly concentrated centre of phase diagrams. Although HEAs attracted the attention of the materials science and metallurgy communities only in the past 15 years, the idea of casting highly concentrated alloys is far from new; in fact, it dates back to the eighteenth century. In 1778, Achard reported [26] his discoveries after casting and characterising around 1000 metallic alloys with Ag, Pt, Co, Cu, Fe, Pb, Sn, Zn, Bi, Sb and As. According to Murty et al. [25], Achard was probably the first metallurgist to synthesise equiatomic multicomponent alloys. His work was revisited two centuries later by the research groups of Cantor and Yeh working independently with focus on the quinary equiatomic alloy FeCrMnNiCo [25,[27], [28], [29], [30], [31], [32], [33], [34]].
The distinct properties of HEAs compared with conventional diluted alloys were reviewed recently by Murty et al. [25]. It is believed that alloying in the highly-concentrated region of the multicomponent metallic systems results in a strong thermodynamic stabilisation of a single-phase structure with a random site-occupancy for substitutional solutes: this is an important core-effect of HEAs known as the “high-entropy effect” [33]. At the foundation of this core-effect lies the hypothesis that the Boltzmann configurational entropy [[35], [36], [37]] – as shown in equation (1) – dominates the entropic contribution in promoting the thermodynamic stabilisation as an equiatomic alloy composition (the composition of each i-th constituent of the alloy is denoted as in equation (1)) will maximises the whole entropy of the system, thus reducing its Gibbs free energy even at high temperatures [25,[27], [28], [29], [30], [31], [32], [33], [34]].
Severe lattice distortion and sluggish diffusion are two important core-effects of HEAs recently under intense research. The first occurs as a direct consequence of the random solid solution principle which generates a distorted crystal structure as each lattice equilibrium position has an element with different atomic radius. The second core-effect (hypothesised by Yeh et al. [38] and Chang et al. [39]) suggests that solid-state diffusion and phase transformation kinetics are both suppressed and/or retarded in HEAs when compared with diluted solid solution alloys. The “sluggish” solid-state diffusion in HEAs would presumably occur due to the large fluctuation of the lattice potential energy and high activation energy barriers, both closely related to the local chemical environment [25,[40], [41], [42], [43]]. Experiments recently indicated that the sluggish diffusion core-effect might affect several synthesis and processing parameters in HEAs, for example nucleation and growth of precipitates, reduced grain coarsening rate and increased creep resistance [25].
Both high-entropy and sluggish diffusion core-effects are of interest to the nuclear materials community as a (more) thermodynamically stable material with suppressed diffusion of point defects induced by irradiation would result in an alloy with superior radiation tolerance when compared with the conventional diluted alloys often used in nuclear materials such as Fe- and Zr-based alloys. In fact, several scientists have already reported that HEAs are of superior radiation resistance [[44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55]]. Despite such reported stability under irradiation, some questions remain to be answered: are these constitutive HEA core-effects real? Is the configurational entropy behind the single-phase random solid solution stabilisation and sluggish diffusion, thus leading to radiation tolerance? In fact, criticism of the constitutive hypotheses of the HEAs was presented by Bhadeshia in an editorial article in 2015 [56]. Four critical points were raised by Bhadeshia regarding the emerging concept of HEAs and are summarised below:
- 1.
The concept of ideal solid solutions has been used to shed light on the core-effects of HEAs although ideal solid solutions simply do not exist;
- 2.
The dominance of the configurational entropy factor in the Gibbs free energy assumes that the enthalpy of mixing in a HEA is negligible. But as the HEAs should not be considered ideal solid solutions, the quasi-chemical approach [57,58] should be used instead where the enthalpy is not (always) negligible;
- 3.
HEAs are in fact already satisfying existing metallurgical principles and they should not be considered different than conventional dilute alloys. For example, the addition of Cr to equimolar concentrations results in increased corrosion resistance simply because Cr itself improves corrosion resistance [59] (unless precipitation is favoured in a certain metallic system);
- 4.
Regarding the mechanical properties, it is often mentioned [60] that the atomic misfit causing lattice strain would restrict the motion of dislocations, resulting in unique mechanical strength for HEAs. But the concept of lattice strain induced by atomic misfit is not clear considering that in an HEA the distinction between solute and solvent does not exist.
Along with Bhadeshia [56], recent criticism of both sluggish diffusion [42,[61], [62], [63]] and high-entropy core-effects [[64], [65], [66]] indicate that the constitutive hypothesis of HEAs is still awaiting a more definitive confirmation, especially whether they are attributed as the main reason for such a superior resistance to energetic particle irradiation. In this present paper, we target the hypothesis of configurational entropy behind higher alloy thermodynamic stability, thus high radiation resistance. Heavy ion irradiation in situ within a Transmission Electron Microscope (TEM) is used for real-time monitoring of the effects of irradiation in two similar alloys, the austenitic stainless steel grade 348 (71.2Fe-17.5Cr-1.8Mn-9.5Ni wt.%) as a “low-entropy” alloy and its equivalent high-entropy alloy (26.8Fe-18.4Cr-27.3Mn-27.5Ni wt.%). In order to compare the response to irradiation in both alloys, the microstructural characterisation within the TEM was focused on analysing any defects in the alloys under bright-field conditions and the stability of the matrix phases using selected-area electron diffraction.
Section snippets
Alloys synthesis and processing
A plasma arc melting furnace and drop casting were used to synthesise the FeCrMnNi HEA bulk at the Oak Ridge National Laboratory and more specific details on the procedure have been published elsewhere [50]. The AISI-348 steel belongs to the 300-series of austenitic stainless steels and it was commercially obtained (AISI is the designation of the American Iron and Steel Institute). The grade 348 contains Nb as a minor solute and according to Padilha et al. [67] these austenitic stainless steels
Results and discussion
We report the results obtained with the pre-irradiation characterisation and the heavy ion irradiations in situ within a TEM of the two alloys investigated in this work: the AISI-348 steel and the FeCrMnNi HEA. Although a brief discussion on the results will be given in this section, a more comprehensive assessment of the radiation response of both alloys is in section 4. For each irradiation case investigated and for each alloy studied, two (or more than two in some cases) independent
Comprehensive discussion
A summary of the results obtained with low- and medium-energy heavy ion irradiations on both the FeCrMnNi HEA and AISI-348 steel is presented in Table 3 and it is evident that among all the cases analysed in this present research, the FeCrMnNi HEA has a superior radiation tolerance than its “low-entropy” counterpart, the AISI-348 steel. The results are more interesting when the common operational temperature of light-water reactors (LWRs) is taken into consideration, i.e. at a temperature of
Conclusions
Low- and medium-energy heavy ion irradiations were performed in this work in a FeCrMnNi high-entropy alloy and in the AISI-348 steel as its “low-entropy” counterpart. Within the wide range of irradiation configurations analysed, the FeCrMnNi HEA was more stable to irradiation than the steel in terms of the stability of the matrix phase. A trend for RIP to occur in the AISI-348 steel was observed in almost all the conditions studied.
A surprising metallurgical finding is that by simply tuning the
CRediT author statement
MAT: Conceptualization, Formal Analysis, Investigation and Writing – Original Draft.
GG: Conceptualization, Formal Analysis, Investigation and Writing – Review and Editing.
HB: Conceptualization, Formal Analysis, Investigation and Writing – Review and Editing.
PDE: Conceptualization, Formal Analysis, Investigation, Supervision, Project Funding and Writing – Review and Editing.
YZ: Conceptualization, Formal Analysis, Investigation, Supervision, Project Funding and Writing – Review and Editing.
SED:
Data availability
The raw data required to reproduce these findings are available upon reasonable request.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
All the authors are grateful to the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom for funding of MIAMI-2 under grant number EP/M028283/1 and also for funding from the United Kingdom National Ion Beam Centre (UKNIBC) of which the MIAMI facility is a part. MAT is grateful for a previous funding through the ASTRO fellowship, a United States Department of Energy workforce development program implemented at Oak Ridge National Laboratory through the Oak Ridge
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2022, Materials Science and Engineering: ACitation Excerpt :The alloys proposed for the structural materials of next-generation nuclear reactors are austenitic stainless steels, reduced activation ferritic/martensitic (RAFM) steels, oxide dispersion strengthened (ODS) alloys, V alloys, etc. [6–11]. Austenitic stainless steels often have low volume swelling resistance under high temperature irradiation [6]. RAFM steels are prone to irradiation embrittlement and with high ductile-to-brittle transition temperatures after irradiation [7,8].
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Present address: Materials Science and Technology Division, Los Alamos National Laboratory, United States of America.