An Investigation of Atomic Interaction between Ag and Ti2AlC under the Processing Temperature of 1080 °C

Ti2AlC is a typical MAX (M: early transition metal, A: main group element, and X: carbon and/or nitrogen) phase with ceramic and metallic properties due to its unique nano-layered structure. In order to investigate the interaction behavior between Ag and Ti2AlC, a sessile drop experiment was conducted at 1080 °C for 5 min. The atomic rearrangement occurred at the Ag–Ti2AlC interface was revealed using high-angle annular dark-field scanning transmission electron microscopy coupled with high-resolution transmission electron microscopy analysis. The results show that Ag nanoclusters generally appeared in most of the Ag–Ti2AlC interaction regions thermally processed at 1080 °C. In addition, Ag can also substitute for Al and Ti atoms in the Ti2AlC, promoting local structural decomposition of the Ti2AlC and producing 4H–Ag with a hexagonal close-packed (hcp) structure. Additionally, Al atoms released from the Ti2AlC lattices can dissolve locally into the liquid Ag, particularly at the grain boundaries. When the loss concentration of Al exceeded the critical level, the Ti2AlC started to decompose and the residual Ti6C octahedrons and Al atoms recombined, giving rise to the production of anti-perovskite Ti3AlC with a cubic structure. Lastly, the discrepancy in substitution behavior of Ag in the Ti2AlC was compared when thermally processed at different temperatures (1030 °C and 1080 °C). This work contributes to the understanding of the intrinsic stability of Ti2AlC MAX ceramics under high-temperature treatment.


Introduction
Ti 2 AlC ceramics, as a representative 211-type MAX phase, have attracted increased attention recently [1][2][3]. The unique nano-layered structure and diverse bonding characteristics enable them to possess both ceramic and metallic properties [4][5][6]. However, studies on Ti 2 AlC ceramics are relatively few, which limits the understanding toward their intrinsic properties and structural characteristics. Among the MAX-phase materials, Ti 2 AlC has a low density (4.11 g·cm −3 ) [7], high thermal conductivity (46 W/m·K) [8], and excellent electrical conductivity (4.42 × 10 6 S·m −1 ) [9]. In particular, the ceramic has a coefficient of thermal expansion of 8.8 × 10 6 • C −1 , which means that it exhibits outstanding high-temperature oxidation resistance [10,11]. Based on this, Ti 2 AlC will be expected to be used in applications with frequent alternation of thermal cycles, such as the heating bodies of furnaces [12], coating materials of atomic energy reactors [13,14], and current-carrying friction devices [15,16]. However, it is difficult to prepare pure Ti 2 AlC with a large size and complex shape because its compositional range is rather narrow [17]. The joining of the Ti 2 AlC to itself or to metals, as well as dissimilar materials, will enable the production and assembly of components with complex shapes and will also improve the capabilities and properties of such materials. For example, heat sink components, automotive commutators, and electric locomotive pantograph pans require the joining of C-based materials (such as graphite) with Cu [18][19][20]. Compared with C-based materials, Ti 2 AlC has greater strength, better high-temperature stability, self-lubrication capability, and better conductivity [21]. Replacement of the C-based materials with the Ti 2 AlC will enhance the component performance and extend the service life.
Brazing is commonly used to join non-metallic materials to metallic materials [22,23]. The design and selection of brazing filler materials are very important, and are directly related to the joining quality. Generally, Ag and Cu based brazing filler alloys are commonly used for joining ceramics [24,25]. Considering the fact that the elements in the brazing filler materials will interact with substrates, it is critical to understand their interaction behavior during brazing. The Ag-Cu brazing filler alloys have been introduced to join Ti 2 AlC and Cu, with strong joints being achieved [26,27]. The strong affinity of Cu (from the Ag-Cu filler alloys and Cu substrates) toward Ti and Al (from the Ti 2 AlC) gave rise to the formation of AlCu 2 Ti in the braze seam, leading to the structural decomposition of Ti 2 AlC. The dissolution of Cu substrates beside the braze seam provided sufficient Cu for the reaction. Compared with Cu, the interaction of Ag with Ti 2 AlC was relatively weaker at the processing temperature of 1030 • C [28]. Based on these Ag nanoclusters, nanotwins and polycrystals were distributed in the Ti 2 AlC lattices after joining. No decomposition of the Ti 2 AlC was detected. Unfortunately, the interaction mechanism between Ag and Ti 2 AlC under higher temperatures (over 1300 • C) has yet to be reported. An investigation of the interaction of Ag with Ti 2 AlC over a wide temperature range could help in screening alloying elements for the design of brazing filler materials, promoting the understanding toward the intrinsic structure, bonding characteristics, and crystal stability of the Ti 2 AlC. The basic theory about the atomic-scale structure of Ti 2 AlC will, therefore, be enriched, creating a prerequisite condition to promote its engineering applications. For this, the presented work was focused on studying the substitution behavior of Ag in the Ti 2 AlC and the structural stability of this kind of ceramic at high temperatures (for example 1080 • C). The sessile drop experiment was performed to investigate the interaction behavior in the Ag-Ti 2 AlC system under isothermal conditions at 1080 • C for 5 min and the asobtained interface was examined by transmission electron microscope (TEM). The spherical aberration-corrected scanning transmission electron microscope analysis was conducted for the TEM foils of the Ag-Ti 2 AlC interaction zones.

Materials and Methods
The Ti 2 AlC ceramics synthesized by self-propagating high-temperature synthesis were purchased from Wuhan University of Technology. They were cut into 3 × 4 × 3 mm 3 samples for microstructural observation. The microstructure of the Ti 2 AlC used in the current work is shown in Figure 1a, in which two regions with different contrasts can be distinguished. The energy-dispersive spectroscopy (EDS) analysis indicates that the two regions share a similar composition. The X-ray diffraction (XRD) results for the Ti 2 AlC is presented in Figure 1b. It can also be shown from the picture that in addition to the diffraction peaks of the Ti 2 AlC, no other diffraction information can be described, suggesting the Ti 2 AlC is pure.
The high-temperature interactions between Ag and Ti 2 AlC were evaluated by a sessile drop experiment. First, pure Ag powders were placed in the mold and then they were pressed into cylindrical pellets using a uniaxial two-sided compaction method. Next, the cylindrical Ag pellet was placed on the surface of the Ti 2 AlC and put into a vacuum furnace. The contact angle value measurement instrument coupled with the vacuum furnace was introduced to inspect the contact angle during the heating stage. The highest temperature was 1080 • C and isothermal soaking was performed for 5 min. During heating, the samples were first heated up to 900 • C with a heating rate of 20 • C/min and held for 10 min to achieve temperature homogenization. The Ag pellet still preserved a cylindrical shape at 900 • C, since the melting point of Ag (~961 • C) had not been reached, as shown in Figure 2b. The temperature was increased up to 1080 • C at a rate of 5 • C/min. The Ag pellet started to melt and spread out to form a spherical droplet, as described in Figure 2c. The wetting angle was found to be 21 • , suggesting very good wetting at the Ag-Ti 2 AlC triple line. After heating, the samples were cooled to room temperature. In order to conduct the TEM examination, the middle part of the sample was cross-cut to expose interaction zones. A wafer with a diameter of~3 mm was then machined from the interaction zones. The thickness of the wafer was mechanically polished to 40 µm, followed by ion milling. It should be noted that the central region of the wafer was polished to nearly 10 µm in thickness, which was achieved using a RES101-type ion-beam thinner (Leica, Wetzlar, Germany) working at 5 kV. Finally, the spherical aberration-corrected transmission electron microscope (ARM200F, JEOL, Tokyo, Japan) equipped with an X-ray spectrometer and a high-angle annular dark-field (HAADF) probe (operating at 200 kV) was performed for the TEM thin foils to reveal atomic-scale information.   Figure 3a shows the HAADF-STEM (High-angle Annular Dark-field Scanning Transmission Electron Microscopy) morphology of the Ag-Ti 2 AlC interaction zone along with the crystallographic orientation of [1120] Ti 2 AlC . Apart from the Ag nanoclusters, another atomic arrangement can also be revealed from the figure. This zone measures~40 × 100 nm, which is much larger than the critical size of Ag nanoclusters (9.072 nm) in the Ti 2 AlC [28]. The boundary of this zone is parallel to basal plane of the Ti 2 AlC. The magnified rectangular box in Figure 3a is indicated in Figure 3b, which exhibits two representative atomic arrangements. The upper part in Figure 3b shows the atomic arrangement of Ag under the zone axis of [1101] Ag , while the lower part exhibits the atomic arrangement of Ti 2 AlC under the zone axis of [1120] Ti 2 AlC , in which a parallel relationship can be found between them. Figure 3c,d presents their fast Fourier transformation (FFT) results. It has been mentioned that Ag occurred in the Ti 2 AlC in the form of nanoclusters, nanotwins, or polycrystals under a lower processing temperature of 1030 • C [28]. In the current work, Ag with a hexagonal close-packed (hcp) structure was first detected in the Ti 2 AlC. Similar to the Ti 2 AlC, the 4H-Ag (ICDD PDF #01-087-0598) [29] with the hcp structure has a space group of P63/mmc through measuring the interplanar spacing. It can be seen from Figure 3f that both the 4H-Ag and Ti 2 AlC share a similar long-periodic crystal structure, differing in the length of c-axis slightly. It is also demonstrated in Figure 3b that the crystal lattice of Ag is parallel to that of the Ti 2 AlC in one direction. Figure 3e presents the FFT results of the interaction zone in Figure 3b. A specific crystallographic relationship can be derived between the 4H-Ag and Ti 2 AlC: [1101] Ag || [1120] Ti 2 AlC (1) (1102) Ag ||(0002) Ti 2 AlC (2) It has been reported that Al atoms can separate from the Ti 2 AlC lattices and migrate along the crystallographic plane of (0001) under high temperature (for example 1080 • C), producing vacancies and diffusion channels [30][31][32][33]. Coupled with the outward migration of Al atoms, Ti atoms would also break away from the Ti 2 AlC and likewise give rise to the vacancies. However, the Ti 2 AlC still preserved its normal crystal structure despite losing parts of Al and Ti atoms. The Ag atoms entered into the Ti 2 AlC lattices along the diffusion channels and occupied part of the vacancies. The occupation of Al vacancies by Ag was beneficial in reducing the vacancy formation energy of Al, which means that the substitution behavior of Ag promoted the generation of more Al vacancies in the Ti 2 AlC. To maintain integrity, the Ti 2 AlC could withstand vacancy concentrations of up to 50% [34,35], far greater than the limit of the Ag concentration replacing Al (~12.5%). It can be speculated that redundant vacancies would be filled by Ag atoms when the vacancy concentration of Al is greater than the substitution ratio of Ag. Upon cooling from 1080 • C, Ag nucleated and grew up epitaxially along the basal planes of the Ti 2 AlC (namely atomic-diffusiondriven channels), generating the 4H-Ag with the hcp structure; that is, Ag could replace Al and Ti atoms in the Ti 2 AlC. The 4H-Ag was, therefore, precipitated from the Ti 2 AlC locally, leading to the instability of the Ti 2 AlC.

Phenomenon 2: Structural Transformation of the Ti 2 AlC at the Grain Boundaries
It should be mentioned that no clear crystallographic orientation relationship between the Ag and Ti 2 AlC was inspected for most of the interaction zones thermally processed at 1080 • C for 5 min. However, a specific crystallographic relationship can still be detected locally. Figure 4 indicates the TEM results of the Ag and Ti 2 AlC at the grain boundaries. It can be seen from Figure 4a that the white phase belongs to the Ag, while the gray phase is actually the Ti 2 AlC. The yellow box in Figure 4a shows the morphology of the Ag-Ti 2 AlC interface, with a high-resolution TEM image presented in Figure 4b. Figure 4c,d describes the FFT results of the two phases in shown in Figure 4b. It can be demonstrated from Figure 4c that Ag in Figure 4a has a cubic structure. Interestingly, the gray phase with a cubic structure does not belong to the Ti 2 AlC according to its diffraction pattern in Figure 4d. Based on the structural information, the gray phase may belong to following phases: Ag (Ti, Al), TiC, and Ti 3 AlC. It can be excluded that the phase belongs to Ag (Ti, Al) because there is a significant discrepancy in contrast (Figure 4a) between this phase and Ag. Table 1 provides the theoretical and experimental lattice parameters of the three phases. The theoretical values of the lattice parameters were obtained from [36], while the experimental values were calculated from the diffraction patterns in Figures 4 and 5. According to the presented results, the experimental lattice parameters of the gray phase approach the theoretical value of the Ti 3 AlC. It can then be inferred that the gray phase in the yellow box (Figure 4a) should be determined to be the Ti 3 AlC.    Figure 5b shows the atomic arrangement of the white dashed box in shown in Figure 5a. Observing the investigated zone axis, it can be shown that the Ti 3 AlC lattices are composed of regular square grids with three atoms at each side and one atom in the center. The FFT in Figure 5b yields the diffraction patterns shown in Figure 5c. In general, diffraction does not occur easily on the crystallographic planes of (010) and (100) under the zone axis of [001] for the cubic structure. However, for the Ti 3 AlC with an anti-perovskite cubic structure, the diffraction could happen at such a position [37][38][39]. Actually, the anti-perovskite Ti 3 AlC belongs to a metallic carbide, as presented in Figure 5d. A single cell contains Ti atoms at the face-centered position, Ag atoms at the corner position, and a C atom at the body-centered position. Compared to the Ti 2 AlC, the Ti 3 AlC can be described as a Ti 6 C octahedron placed in a cubic Al cell. The lattice arrangement of the Ti 3 AlC obtained by projection under the zone axis of [001] corresponds to the lattice arrangement in Figure 5b perfectly; that is, the face-centered position in the Ti 3 AlC contains Ti atoms, and their projection position is located at the center of the square gird and side length. The vertex position contains Al atoms, corresponding to the vertex position of the square grid in Figure 5b. The body-centered position contains the C atom, while its projection position overlaps the position of the Ti atom at the face-centered position. This demonstrates that the gray phase in the yellow box (Figure 4a) should belong to the Ti 3 AlC. It has been mentioned in Figure 1 that no Ti 3 AlC was observed in the Ti 2 AlC substrates. Therefore, the Ti 3 AlC (Figures 4 and 5) should be produced through the interaction of Ag with Ti 2 AlC during high-temperature treatment. The high-resolution TEM of the Ag-Ti 3 AlC interface is shown in Figure 6a and its inverse FFT result is shown in Figure 6b. The Ag and Ti 3 AlC can be distinguished from the figure because the size of the Ag lattice is one-quarter of the Ti 3 AlC. Despite this, the interface of Ag and Ti 3 AlC shares a completely coherent relationship. It can also be detected that a small lattice mismatch occurred at their phase boundaries. This lattice mismatch accumulation resulted in apparent lattice distortion. The lattice-distortion-driven stacking faults can be seen in Figure 6c (magnified white dashed box shown in Figure 6b). The FFT result of Figure 6a is shown in Figure 6d. It is worth noting that only a set of diffraction patterns can be detected, which hints that the diffraction patterns of the Ag and Ti 3 AlC should overlap. The overlapping phenomenon arises because they have a similar crystal structure and share a coherent relationship at the interface. Based on this, the crystallographic orientation relationship between the Ag and Ti 3 AlC can be described below:  is arranged along the crystallographic plane of (001) symmetrically, leading to a coherent relationship. Based on this, the formation mechanism of the Ti 3 AlC can be inferred (Figure 8). It is worth noting that the Ti 2 AlC lattice was formed through the alternating distribution of Ti-C octahedrons and Al atom layers. When the heating temperature exceeded the melting temperature of Ag (~961 • C), Al separated from the Ti 2 AlC and dissolved into the liquid Ag at the grain boundaries. When the loss of Al concentration reached the critical level, Ti 2 AlC destabilized, leading to the rearrangement of Ti-C octahedrons and Al atoms. For this, the Ti 2 AlC with the hcp structure was transformed into the anti-perovskite Ti 3 AlC with a face-centered cubic (fcc) structure locally. The cubic Ag was solidified along the Ti 3 AlC epitaxially during cooling and a coherent relationship was shared between them.
The interaction of Ag with Ti 2 AlC has been investigated at 1030 • C [28]. It has been reported that Ag atoms can substitute for Al and Ti atoms in the Ti 2 AlC. By increasing the amount of Ag, Ag nanoclusters, nanotwins, and polycrystals nucleated and grew up in the Ti 2 AlC lattices without destroying the integrity. When the temperature was increased to 1080 • C, Ag generally appeared in the Ti 2 AlC in the form of nanoclusters. However, the Ti 2 AlC started to decompose locally when the substitution ratio of Ag exceeded the limiting ratio. For example, Ag atoms could replace Al and Ti atoms in the Ti 2 AlC, triggering the formation of the 4H-Ag with the hcp structure along the specific crystallographic plane and orientation of the Ti 2 AlC. Apart from this, parts of Al atoms could also de-intercalate from the Ti 2 AlC lattices at the grain boundaries to promote the transformation from the Ti 2 AlC to Ti 3 AlC.

Conclusions
The interaction of Ag with Ti 2 AlC was investigated to better understand the basic rule of atomic rearrangement at high temperatures (for example, 1080 • C). The Ag-Ti 2 AlC interaction zones were experimentally investigated via the sessile drop experiment conducted under isothermal conditions at T = 1080 • C for 5 min. The substitution behavior of Ag atoms in the Ti 2 AlC was investigated using the TEM analysis. Observing along the crystallographic orientation of [1120] Ti 2 AlC , Ti 2 AlC was found to decompose upon in contact with molten Ag. Ag nucleated along the basal plane of Ti 2 AlC during cooling and grew into the 4H-Ag with the hcp structure, which shared a specific crystallographic orientation relationship with the original Ti 2 AlC. In local regions, particularly at the Ag-Ti 2 AlC grain boundaries, parts of Al atoms broke away from the Ti 2 AlC lattices, while the Ti 6 C octahedrons coupled with the remaining Al-atomic layers gave rise to the generation of the Ti 3 AlC. A specific crystallographic orientation relationship was also produced between the Ag and Ti 3 AlC. To sum up, Ag appeared in the Ti 2 AlC in the form of nanoclusters for most of the Ag-Ti 2 AlC interaction zones thermally processed at 1080 • C. When the substitution ratio of Ag far exceeded the limiting ratio, Ti 2 AlC was locally decomposed.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
There are no conflicts to declare.