MAX Phases: Understanding of Erosion, Corrosion and Oxidation Resistance Properties in TiAlSiCN and TiCrSiCN Composition
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Purity (wt.%) 99.5 99.5 99.5
Powder standard
Al Ti Cr SiC Si3N4
AL-M-0251M-P0.5UM TI-M-0.251M-P.05UM CR-M-0251M-P.05UM H.C. STARK: Grade UF-5 Alpha-Silicon Carbide CAS 409-21-2 H.C. STARCK: Grade B 7 CAS 12033-89-5 Table II. Studied compositions.
Name of compositions
Initial Powder Components (wt.%)
Ti 65 50
TiAlSiCN TiCrSiCN
Al 15
Cr
SiC 10 10
30
Si3N4 10 10
Sintering parameters The conditions employed for the SPS sintering of the two studied compositions are given in Table III. A EF-SPSF SPS system was used for this. Table III. Sintering parameters. Composition
Current (kA)
Voltage (V)
TiAlSiCN
1.46
4.1
60
TiCrSiCN
1.71
4.1
60
Load Heating (MPa) (°C/min) 200 300 100
Vacuum (atm)
Temperature (°C)
Time (min)
Density (g/cm3)
Relative density
1.9x10-4
1550
6
4.32
92%
1.5x10-4
1700
8
5.12
94%
Characterization techniques A Carl ZEISS EVO MA and a LS Series Scanning Electron Microscope with SmartSEM software were used for microstructural analysis. A QUANTAX CrystAlign 400i modular and user-friendly electron backscatter diffraction (EBSD) system allowed us to determine the crystallographic characteristics of the specimens with local crystal orientations. Embedded in the graphic user interface, the EBSD pattern could be simultaneously indexed. DISCUSSION TiAlSiCN composition Si was located in the “A” position of the formula M(n+1)AXn of composition TiAlSiCN. Al is not a transition metal and forms intermetallic compounds with Ti. It is also well-known that Al forms a solid solution with Si. The EBSD analysis revealed the presence of Al either in Ti3Al
intermetallic phases or as Al2O3 (Table IV). The MAX structure was formed with Ti, Al and SiC or Si3N4. The EBSD analysis indicated the presence of Ti3Si, SiC, TiC and TiN compounds, plus the presence of Si3N4 and moissainite SiC (Table IV). Similar phases were identified by Thermocalc simulations, see Figure 1. It is known that the MAX structure forms TiC and TiN, and has two octahedral constructions embedded in the orthogonal prisms layered by Si planar zones [2]. In our case, a considerable amount of titanium silicide was present in Table IV. Traces of Si3N4 and SiC were present in the sintered products. The presence of titanium silicide could be explained by the considerable amount of titanium present in the material (60 wt.%), which reacted with silicon during SPS. The Ti4Si3 is a MAX promoter. Figure 2 shows the microstructure of TiAlSiCN Spark Plasma Sintered sample. It has been mentioned in the literature [3] that the MAX structure should reveal the presence of parallel layered structures. Such structures have not been identified in the investigated samples. Most probably, Ti and Al reacted with silicon and oxygen, respectively, based on the existing conditions, and as a result, the structure of the MAX phases could not be formed. This could not take place because the main participants in its construction, titanium and aluminum, had already reacted. The role of aluminum is still not evident. This element is amphoteric and it i
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