Chemically complex double solid solution MAX phase-based ceramics in the (Ti,Zr,Hf,V,Nb)-(Al,Sn)-C system

Chemically complex MAX phase-based ceramics in the (Ti,Zr,Hf,V,Nb)-(Al,Sn)-C system were synthesised by reactive hot pressing for 30 min at 1250–1450°C under a load of 30 MPa for the first time in this work. The dense bulk ceramics contained chemically complex double solid solution MAX phases, each comprising five M-elements and two A-elements. The predominant 211 (Ti0.23,Zr0.18,Hf0.20,V0.11,Nb0.28)2(Al0.42,Sn0.58)C MAX phase characterised all ceramics, while the 312 (Ti0.23,Zr0.31,Hf0.31,V0.08,Nb0.08)3(Al0.36,Sn0.64)C2 and 211 (Ti0.26,Zr0.07,Hf0.07,V0.47,Nb0.13)2(Al0.66,Sn0.34)C MAX phases were only present in the ceramics sintered at 1350–1450°C. A limited amount (4–5 vol%) of parasitic phases (mainly, binary intermetallics) was found in the pseudo-binary carbide-free ceramics sintered at 1350–1450°C. GRAPHICAL ABSTRACT IMPACT STATEMENT Chemically complex 211 & 312 double solid solution MAX phase-based ceramics with a MAX phase content up to 96 vol% were synthesised in the (Ti,Zr,Hf,V,Nb)-(Al,Sn)-C system for the first time.


Introduction
The MAX phases are ternary carbides or nitrides with the M n+1 AX n stoichiometry, where M is an early transition metal, A is an A-group element (mainly groups 13-16 in the periodic table), X is C or N, and n = 1, 2 or 3 (corresponding to the 211, 312, or 413 subgroups, respectively). Their nanolaminated unit cell, consisting of M 6 X ceramic blocks separated by metallic Alayers, imparts to this family of materials a unique set of properties, some characteristic of metals and some of ceramics. Their compatibility with inherently corrosive reactor coolants [1] and their radiation tolerance [2] makes the MAX phases promising candidate materials for advanced nuclear systems. However, the properties of the MAX phases are compound-specific; for example, Zr 2 AlC interacts with liquid lead and leadbismuth eutectic [3] even though most MAX phases are characterised by complete immunity to liquid metal corrosion attack. On the other hand, the commonly CONTACT Bensu Tunca bensu.tunca@kuleuven.be Department of Materials Engineering, Katholieke Universiteit Leuven, Leuven, Belgium observed stability under ion irradiation is not observed in Cr 2 AlC, which tends to amorphize [4]. Synthesising solid solutions on the M-, Aand/or X-site improved the oxidation resistance [5][6][7], fracture toughness [8,9], strength [6,8], and/or phase purity [10,11] of various ternary MAX phases. 211 MAX phase-based ceramics with increased phase purity were experimentally synthesised by using elements with similar average atomic radii to produce double solid solutions on both Mand A-sites [10,11]. Alloying is commonly used to improve material properties and produce single-phase concentrated solid solutions, such as high-entropy alloys (HEAs) characterised by high strength-to-weight ratios, thermal stability, hardness, high-temperature strength, fatigue and corrosion resistance [12,13], as well as radiation tolerance [14][15][16][17]. Typically, HEAs contain at least five principal elements with 5-35 at% each.
Designing radiation-tolerant materials typically strives for materials with (1) numerous point defect sinks or recombination centres, (2) limited point defect mobility, and/or (3) intrinsically low defect accumulation [17]. The A-layers in the MAX phases act as sinks for radiation-induced point defects, inherently satisfying the first condition [18]. On the other hand, the enhanced chemical complexity of HEAs is responsible for high stresses on the atomic scale, reducing defect mobility and promoting point defect recombination [19]. Moreover, a number of HEAs investigated for nuclear applications were found to possess appealing properties, such as significantly reduced radiation swelling, low neutron crosssection, reduced activation, high-temperature strength and oxidation resistance [19][20][21][22][23]. Similarly, high-entropy (HE) ceramics show higher hardness [24], oxidation resistance [25], and radiation tolerance [26] as compared to ZrC, a candidate material for Gen-IV fission reactors [26].
In light of these advances, a chemically complex, highentropy MAX phase material concept is very appealing for nuclear applications, therefore, its study was recommended as a promising future research path [27]. To our knowledge, the few prior studies on multi-elemental MAX phases were associated with V 2 (A x ,Sn 1−x )C with A = Fe, Co, Ni, Mn or binary/ternary/quaternary combinations thereof [28], 'high-entropy' M 2 AlC-MC composite (M = Ti, Zr, Hf, Nb, Ta) [29], 'medium-entropy' (Ti,Zr,Hf) 2 SC MAX phase (90 wt.% phase purity) [30], and 'compositionally complex' (Ti,Nb,Ta) 4 AlC 3 and (Ti,Nb,Ta,V) 4 AlC 3 MAX phases [31]. Inspired by previous success in synthesising high-purity double solid solution MAX phases by adding Sn and carefully adjusting the relative atomic radii of the Mand Aelements in order to minimise crystal lattice distortions [10,11], this work synthesised chemically complex MAX phases with five M-elements and two A-elements in the (Ti,Zr,Hf,V,Nb)-(Al,Sn)-C system.

Materials and methods
HfH 2 , ZrH 2 , Nb, V, TiH 2 , Sn, Al, and C raw powders (Table 1) were mixed at an M:A:C ratio of 2:1.1:0.95 with equiatomic fractions of M-elements (Hf, Zr, Nb, V, Ti; 20 at% each) and A-elements (Al, Sn; 50 at% each). The powders were dry-mixed on a multidirectional Turbula mixer for 24 h in argon, and the powder mixtures were reactively hot pressed (RHP; W100/150-2200-50LAX, FCT System) in a graphite die (Ø 30 mm) under 30 MPa, for 30 min at 1250°C, 1350°C, or 1450°C (heating rate: 20°C/min). Density was measured by the Archimedes' method and residual porosity was calculated from the theoretical density obtained by X-ray diffraction (XRD) and Rietveld refinement. Vickers hardness (Model FV700,  Simulated powder XRD patterns were calculated by the VESTA version 3 software [32], while the Materials Analysis Using Diffraction (MAUD) software was used for Rietveld refinement of the XRD patterns [33]. Ternary MAX phase crystal structure (P6 3 /mmc) input files were modified to include 5 elements on the Mand 2 elements on the A-site, assuming a random distribution. MAX phase compounds with distinctly different compositions were identified by EDS inspection and introduced to the Rietveld refinement software. The elemental occupancies of each site in the MAX phase structure were determined using elemental ratios measured by SEM/EDS, however, these occupancies were not refined further due to the high chemical complexity of the solid solutions. Thin foils made by focused ion beam (FIB; Nova NanoLab 600 DualBeam, FEI) were studied by Cs-corrected scanning transmission electron microscopy (S/TEM, ARM200F JEOL) at 200 kV, using selected area diffraction (SAED), bright field and high-angle annular dark field (HAADF) STEM imaging, and EDS mapping. Selected area quantification of STEM/EDS elemental maps was used to determine individual grain compositions and averages over 18 grains. The average grain size was determined by measuring the width and length of ∼ 270 MAX phase laths using the ImageJ software [34].

Results and discussion
The density of the produced ceramics was 6.43 g/cm 3 (RHP 1250°C), 7.19 g/cm 3 (RHP 1350°C), and Table 2. Phase assembly (vol%), chemical composition, lattice parameters (Å), density (g/cm 3 ), porosity (%), hardness (GPa), and grain size in length/width (both in μm) of the produced ceramics. Uncertainties: ∼ 1 for vol%, ∼ 10 −3 Å for lattice parameters and z. SP and DP stand for 'single-phase' and 'dual-phase'. Numbers in parenthesis indicate uncertainties in the last digit.  (4) 0.14(4) 0.11 (5) Table 2) characterised the material that was sintered at the lowest temperature. BSE microstructural overview images of the produced ceramics are provided in Figure 1(a-c). RHP 1250°C consisted of a matrix of sub-micrometre 211 MAX phase grains, MC grains with varying composition (some Vrich, some (Zr,Hf)-rich), and dark alumina (Al 2 O 3 ) grains (Figure 1(g, h)). This ceramic also contained large ( ≤ 40 μm) 'islands' of intermetallic compounds (IMCs) with Nb 2 Al cores (Figure 1(a), point-II) and Al 3 Nb rims (Figure 1(a), point-I); the Al 3 Nb rims contained traces of undissolved Sn, while Sn dissolved in the Nb 2 Al cores. The binary-carbide-free RHP 1350°C and RHP 1450°C featured a dual microstructure made of two distinct regions, herein referred to as 'single-phase' (Figure 1(d, i)) and 'dual-phase' (Figure 1(d, j)). The XRD patterns were characterised by broad MAX phase (hkl) peaks, possibly due to peak overlap of MAX phase grains/clusters with slightly inhomogeneous distributions of elements leading to lattice variations. In contrast to the broad MAX phase peaks, the IMC peaks in RHP 1250°C are sharp (Figure 2(a)). Broad peaks of pseudo-binary MC (M = Ti, Zr, Hf, V, Nb) were also observed in the XRD pattern of RHP 1250°C (Figure 2(a)), possibly due to the presence of different MC compositions with varying lattice parameters, some of which were V-rich and some (Hf,Zr)-rich according to EDS. With increasing sintering temperature, the MAX phase peaks became sharper (Figure 2(b,  c)). Although not visible in the XRD patterns due to its low fraction and peak overlap with other phases, Al 2 O 3 was invariably present in the produced ceramics due to the aluminothermal reduction of Nb 2 O 5 in the Nb raw powder and the possible oxygen contamination of the other powders [10]. 00l peaks belonging to MAX phase compounds had particularly low intensity, as also seen in the simulated powder XRD peak intensities of the SP211 phase (Figure 2  in SP211 could be associated with a reduced Nb content in the main (18 vol%) MC secondary phase in RHP 1250°C. Although the exact MC phase compositions are not reported here due to their complexity, the Nb content of MC phases was invariably small (almost zero in V-rich carbides, < 10 at% in (Hf,Zr)-rich carbides). The rate-limiting factor for the formation of high-entropy carbides is the M-element diffusivity, which is proportional to the ease of M-site vacancy formation in MC carbides [35]. Since Nb has the lowest vacancy formation energy on the M-site among MC carbides (Nb < V < Ti < Hf < Zr [36]), NbC is likely to become the preferred 'host compound' for the easy inward diffusion of other M-elements. Such NbC-based, chemically complex 'host' carbides could aid the nucleation of SP211, explaining the formation of the Nb-rich MAX phase. The reduced V incorporation in SP211 is discussed further in the text. Considering the fractions of DP312  suggest a MAX phase synthesis route that involves reactions between pseudo-binary carbides and IMCs. Likewise, the presence of IMCs and MC carbides indicate an incomplete MAX phase formation reaction at 1250°C. SEM/EDS compositions and refined lattice parameters of the constituent phases, along with density and hardness values, are given in Table 2. Figure 1(e) shows plastic deformation without crack formation at the corner of a Vickers indentation in the 'single-phase' region of RHP 1350°C. On the other hand, Figure 1(f) shows strong intergranular crack deflection, indicative of high fracture toughness, at the corner of an indentation in the 'dual-phase' region of RHP 1450°C. The hardness of the produced ceramics falls within the 2-8 GPa range, typical for the MAX phases [37]. The slightly more porous RHP 1250°C (3% porosity) was substantially softer (3.6 ± 0.5 GPa) than RHP 1350°C and RHP 1450°C (7.1 ± 0.4 and 6.8 ± 0.2 GPa, respectively), even though it contained 9 vol% of the harder Al 3 Nb IMC ( ∼ 6 GPa) and 27 vol% pseudo-binary MC carbides ( Table 2). The hardness of ternary MAX phases comprising elements used in this work varied between 2.5 GPa for V 2 AlC [38] and 6.5 GPa for Nb 2 AlC [39]. (Zr,Ti) 2 (Al 0.5 ,Sn 0.5 )C quinary compounds were harder (∼ 5.9 GPa) than the related Zr 2 (Al 0.5 ,Sn 0.5 )C (5.3 GPa) and Ti 2 (Al 0.5 ,Sn 0.5 )C (3.7 GPa) quaternary compounds, a fact that was tentatively attributed to solid solution hardening [11]. Similarly, the (Ti,Zr,Hf,V,Nb) n+1 (Al,Sn)C n MAX phase double solid solutions were harder than all known ternary, quaternary, and quinary MAX phase solid solutions made of the same elements [11,38,39].  [10]. The formation of DP312 and DP211 at higher temperatures in ceramics with similar SP211 contents suggests that a transformation/decomposition of SP211 is not really responsible for the nucleation of these MAX phases. In fact, the disappearance of the (Hf,Zr)-rich and V-rich MC carbides detected at 1250°C and the concurrent formation of (Hf,Zr)-rich (DP312) and V-rich (DP211) MAX phases indicate that reactions between pre-existing MCs and IMCs form MAX phases with compositions similar to those of the 'precursor' MC phases.
The dominant SP211 phase in RHP 1350°C was studied in-depth by TEM/STEM/EDS. Figure 4 presents STEM/EDS data collected from an SP211 'single-phase' region with uniform chemical composition. In alignment with XRD results, the identified secondary phases in this region were metallic V (bcc) with some dissolved Al, and Al 3 V IMC in triple junctions and grain boundaries. The presence of V-rich secondary phases agrees with the reduced V content of the dominant SP211 phase, which might possibly reflect a low V solid solubility in the MAX phase lattice at 1350°C. Moreover, some of the added V resided in sporadic, large ( ∼ 500 μm) V-rich and Al-containing areas (not shown here) that presumably resulted from non-optimal raw powder mixing.
TEM confirmed that the chemically complex 211 MAX phases have the typical MAX phase crystal structure, space group P6 3 /mmc, as proven by the SAED patterns of the [2110] and [1100] zone axes, Z.A., in Figure 5(d-e). Figure 5(f-g) show SAED and Fast Fourier Transformation (FFT) patterns belonging to the V and Al 3 V secondary phases. Figure 5(b-c) are highresolution (HR) HAADF-STEM images of a SP211 grain with slightly brighter single-atom rows (A) between double-atom rows (M) (Figure 5(c)). The Z (atomic number) contrast induced by HAADF-STEM imaging is difficult to observe, likely due to similar average Z values in the M (41 ± 1) and A (39 ± 1) layers of SP211, and an intensity profile is overlaid with the HRSTEM image for confirmation ( Figure 5(c)). Even though STEM/EDS results from a small area (Figure 4(j)) are slightly different from SEM/EDS results from a large area (Table 2), they are probably more representative of the examined region due to the higher spatial resolution of STEM/EDS. Based on the SAED pattern of Figure 5(d), the SP211 lattice parameters were a ≈ 3.20 Å and c ≈ 14.85 Å, differing from the values obtained by XRD Rietveld refinement. Although XRD typically yields accurate lattice parameter data, the lattice parameter values derived by SAED might be more representative for the specific SP211 grains, as the XRD values are affected by the overlap of multiple phases, peak broadening, and chemical complexity.

Conclusions
Fine-grained ( < 5 μm) MAX phase ceramics were produced in the (Ti,Zr,Hf,V,Nb)-(Al,Sn)-C system by reactive hot pressing of transition metal hydride and elemental powders at 1250-1450°C for 30 min under a load of 30 MPa. All ceramics contained chemically complex double solid solution MAX phases, each comprising five Mand two A-elements. All produced ceramics contained a predominant 211 phase with the (Ti 0.23 ,Zr 0.18 ,Hf 0.20 ,V 0.11 ,Nb 0.28 ) 2 (Al 0.42 ,Sn 0.58 )C average composition. The binary-carbide-free ceramics sintered at 1350-1450°C contained two more chemically complex MAX phases, i.e. a 312 phase with the (Ti 0.23 ,Zr 0.31 ,Hf 0.31 ,V 0.08 ,Nb 0.08 ) 3 (Al 0.36 ,Sn 0.64 )C 2 average composition and a 211 phase with the (Ti 0.26 ,Zr 0.07 , Hf 0.07 ,V 0.47 ,Nb 0.13 ) 2 (Al 0.66 ,Sn 0.34 )C average composition. This work demonstrates that ultra-pure, dense, chemically complex MAX phase ceramics can be synthesised in the (Ti,Zr,Hf,V,Nb)-(Al,Sn)-C system via the double solid solution approach. Considering the inherent radiation tolerance of the nanolaminated MAX phases and the expected reduction in defect mobility due to the high chemical complexity, these materials are promising candidates for nuclear applications close to the reactor core (e.g. fuel cladding applications).