Comparative irradiation response of an austenitic stainless steel with its high-entropy alloy counterpart

Highlights

• Two similar alloys from the Fe–Cr–Mn–Ni system are herein investigated.

• The alloys are a FeCrMnNi High-Entropy Alloy (HEA) and the AISI-348 steel.

• HEA theory: higher configurational entropy, higher phase stability.

• Heavy ion irradiation in situ within a TEM was carried out.

• In 80% of the cases: HEA presents high matrix phase stability; the steel does not.

Abstract

Two metallic alloys in the quaternary system Fe–Cr–Mn–Ni were irradiated in situ within a transmission electron microscope (TEM) using Xe⁺ heavy ions in the temperature range of 293–873 K and in the regime of low- (30 keV) and medium-energies (300 keV) with respective maximum doses of around 40 and 140 dpa. The first alloy is the FeCrMnNi high-entropy alloy (HEA) synthesised with the alloying elements close to equimolar composition. The second alloy is a commercial austenitic stainless steel AISI-348 (70.5Fe-17.5Cr-1.8Mn-9.5Ni wt.%), selected as the “low-entropy” counterpart of the FeCrMnNi HEA. Microstructural characterisation was carried out in the TEM with in situ heavy ion irradiation to investigate the role of entropy on radiation induced segregation and precipitation (RIS and RIP). The results demonstrated that among all the irradiation cases investigated, the FeCrMnNi HEA had its random solid solution matrix phase preserved in 80% of the experiments whilst the austenite matrix of the AISI-348 steel underwent RIP in 80% of the cases. It is therefore demonstrated that small differences between two alloys can lead to different radiation responses, confirming the trend that, by tuning the elemental composition superior radiation resistance can be achieved in metallic alloy systems, but emphasising that some of the constitutive core-effects of HEAs are still in need of further confirmation especially when the application of HEAs in energetic particle irradiation environments is considered.

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 $x_i$ 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]].

$$\text{Δ}{S}_{conf}=-k_B{N}_A\sum _{i=1}^N{x}_i\text{ln}{x}_i$$

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:

• 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;

• 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;

• 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);

• 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.