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An investigation into the microstructure and mechanical properties of V4AlC3 MAX phase prepared by spark plasma sintering
Author’s Accepted Manuscript An Investigation into the microstructure and mechanical properties of V4AlC3 MAX phase prepared by spark plasma sintering Mohsen Hossein-Zadeh, Omid Mirzaee, Hamidreza Mohammadian-Semnani www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(19)30040-9 https://doi.org/10.1016/j.ceramint.2019.01.036 CERI20491
To appear in: Ceramics International Received date: 8 December 2018 Revised date: 28 December 2018 Accepted date: 5 January 2019 Cite this article as: Mohsen Hossein-Zadeh, Omid Mirzaee and Hamidreza Mohammadian-Semnani, An Investigation into the microstructure and mechanical properties of V4AlC3 MAX phase prepared by spark plasma sintering, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.01.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
An Investigation into the microstructure and mechanical properties of V4AlC3 MAX phase prepared by spark plasma sintering
Mohsen Hossein-Zadeh, Omid Mirzaee, Hamidreza Mohammadian-Semnani1*
Department of Materials Engineering, Semnan University, P.O. Box 35195-363, Semnan, Iran [email protected] [email protected] *
Corresponding author. Tel.: +98 9123317550
Abstract The present work investigated the effect of spark plasma sintering of V4AlC3 MAX Phase on the microstructure and mechanical properties. Accordingly, Vanadium, Aluminum and carbon-black powders were mixed through a high energy mixer mill for 10 min in an ethanol medium. The direct inserted mixture of Al-V-C systems in a graphite mold was spark plasma sintered at 1350°C with the initial and final applied pressure of 10 and 30 MPa at the vacuum condition of 20 Pa. The XRD investigations demonstrated the V4AlC3 crystalline phase along VC and Al2O3 as the side-effect reaction products. The microstructure analyses further indicated a perfect lathlike MAX phase with an almost proper growth. The FESEM micrographs of the prepared sample also indicated the formation of the V4AlC3 MAX phase with the lath-like shape. The fracture surface of the specimen also displayed deformed and separate lath-like phases as a result of the bending test. The different loads used for the hardness test demonstrated a deformation zone around the mark indentation. By increasing the applied load in the hardness test, the deformed area was enhanced. The prepared FESEM images from the deformed area showed different crack propagation mechanisms such as separation and branching, deflection, bridging, particles pulling out and breaking. 389±19 MPa of bending strength and 6.74±0.12 GPa of Vickers hardness were obtained for the prepared sample along the almost full densification as 99 % of theoretical density.
Keywords: Aluminum, Vanadium, MAX phase, Spark plasma sintering.
1. Introduction Layered ternary compounds, also called MAX phases, have shown outstanding properties as the novel engineering materials, such as shock resistance, proper thermal and electrical conductivity, machinability, and promising high temperature applications. The MAX phases are known with the general formula of Mn+1AXn, where M exhibits an early transition metal, A is the main group metal and X is C or N (n=1-3) [1,2]. However, the maximum amount of n=3 is usually considered for the MAX phases; recently, new members with n=5, 6 and 7 as the high order have been added to the MAX phases by the researchers [3–6]. Generally, MAX phases offer a combination of strong covalent M-X bondings and comparatively weaker M-A bonds, presenting the exclusive and remarkable blend of hybrid metallic and ceramic characteristics [7,8]. Moreover, the MAX phases possess a laminar structure with the hexagonal symmetry in its crystals; due to this unique feature, they show anisotropic properties [9,10]. The literature review indicates more than 50, 6 and 8 of M2AX, M3AX2 and M4AX3 have been indentified and studied, respectively, as groups of MAX phases; also, a new family of MAX phases has emerged [11–13]. A simple comparison of
the investigated MAX phases
demonstrates that comparatively few studies have been done by increasing n, which seems that the successful preparation of the monolithic M4AX3 faces more challenges due to the higher chance of MX formation as the reaction product [14,15]. The theoretical crystallographic investigations on the MAX phases have depicted that, normally, there are two, three and four layers of MX in between every two a layers. Therefore, the differences between orders of MAX phases can result in different atomic configurations and hence, different physical and mechanical properties [16,17]. Moreover, in the same chemical system, the 413 subgroup is more interesting due to its characteristically superior temperature stability, as compared to those other order sequences of the MAX phases [18]. MAX phases can be produced by the
intense (wide-range) category of methods, such as
combustion synthesis [19], chemical and physical vapor deposition [20,21], self propagating high-temperature synthesis (SHS) [22], arc melting [23], mechanical alloying and sintering processes [24,25], etc. Among all preparation methods, producing the bulk MAX phases with almost the lowest impurities as the reaction products has become a major challenge in the investigations [26]. Spark plasma sintering (SPS) as a new sintering method is capable of producing a wide range of materials due to its unique advantages, such as Joules heating,
applied pressure, vacuum condition, local sparks between particles, etc [27–29].The SPS has several advantages compared to conventional methods of MAX phases as follows; (1) sintering time and temperature can be reduced, and thus lower grain growth occurs, (2) residual porosity can be reduced, (3) lower residual porosity and lower grain growth result in higher strength, and (4) densification at low temperature can be improved. As previously explained, the investigation on the preparation of 413 MAX phases is limited. C. Hu et al [30], however, investigated the crystal structure of V4AlC3 by the reactive hot pressing of the V-Al-C system with the stoichimetric molar ratio of V:Al:C=4:1:3 at 1700°C. The results obtained by them almost demonstrated that using the stoichiometric composition of V, Al and C mixture with heat treatment at 1700 for 60 min led to the formation of the dominant phase of VC alongside V4AlC3. Christin M. Hamm and et al.[31] also prepared the V4AlC3 MAX phase using microwave heating at 1000 W for 60 min and post densification by SPS at 1350°C. They used the elemental molar ratio of V:Al:C=4:5.2:3, finding that microwave heating could lead to the formation of V4AlC3. But the XRD patterns taken after the DSC analysis revealed only V2AlC formation at 1200°C. However, while Christin M. Hamm et al. [31] studied the formation of V4AlC3 with microwave heating, they did not mention the final microwave heating temperature and the reason for using such a molar ratio of starting elements with the ultra high amount of Al; also, their investigation did not display the successful synthesis of V4AlC3 by applying SPS , as this methid was only used for the densification purpose of the synthesized powders. The preparation of Nb4AlC3 by SPS using mechanically activated Nb, Al and C powders was also investigated by C. Hu et al.[32]. They used sintering temperatures between 800-1600°C and also, different compositions of starting powders with the excess aluminum due to its evaporation at the high temperature. The results obtained by C. Hu et al. [32] also revealed that Nb4AlC3 started to appear at 1400°C by XRD investigation and also,
at the optimum sintering
temperature of 1600°C. To the best of authors’ knowledge, there are not any researches on the preparation of the V 4AlC3 MAX phase through the direct reaction of the elemental powders without any mechanical activation and/or post heat treatment. Therefore, this present research investigated the production of the bulk V4AlC3 MAX phase by SPS; also, the microstructure, mechanical properties and the observed toughening mechanism were studied in details. 2. Experimental procedures
Vanadium (Riedel-de Haen
Art. no. 10461 vanadium powder, 250 mesh, 99.9% purity),
aluminum (Merck Art. no. 1056 aluminum powder, 250 mesh, 99% purity), and carbon black (99.9% purity,
particle size <30 μm) powders were used as the
starting materials. The
composition of the V:Al:C elemental mixture was 4:1.5:3. The mixing procedure was performed using a high energy mixer mill (Spex mixer mill -8000D model) in an Ethanol medium for 10 min with alumina balls. Then, the mixture was dried at 70°C on the hot plate. The mixed powders were directly loaded into a graphite die (30 mm diameter) of SPS (SPS-20T-10, China) using a graphite foil to separate the powder from the die. The sintering process was carried out at 1350°C with the initial and final pressure of 10 MPa (with increasing the temperature) and 30 MPa (at the temperature of 1350°C) in the vacuum condition of 20 Pa, respectively. The phase identification was performed using XRD (Philips) with a Cu kα radiation source and an imageplate detector over the 2θ range 10-80° in the reflection geometry. The bulk density of the sintered samples was measured using the Archimedes’ principle. The three-point flexural strength test was used to measure the strength of the sintered samples. The bending strength specimens with the bar shape (3×5×25 mm) prepared by SPS were cut from the sintered disc with the diameter of 30 mm. Vickers hardness was tested by the MKV-h21 hardness testers (Islamic Republic of Iran) at the loads of 3, 5, 10 and 30 Kgf, and the induced cracks under a load of 30 kgf were observed by FESEM. The microstructure of the specimen after the grinding and polishing procedure was examined using FESEM (MIRA 3 TESCAN, Czech Republic) equipped with an energy dispersive spectrometer (EDS).
3. Results and discussion Fig. 1 represents the back scattered FESEM images and EDS elemental mapping of the mixed powders after the 10 min high energy milling. Approximately, some simple mixing without any size reduction of powders could be observed in Fig. 1. As expected, due to the low mixing time, mechanical activation could normally lead to better results in the synthesis of compounds and prepare some part of the activation energy [33,34]; however, the present work investigated the capability of the SPS process in the formation of MAX phases because of omitting the milling process results in cost and energy saving, regardless of offering impurities to the system as a consequence of balls and cup contiguity.
Fig. 1 FESEM images and corresponding EDS elemental mapping analysis
Fig. 2 Displacement/displacement rate- temperature-time curves during spark plasma sintering
Fig. 2 shows the displacement/displacement rate and temperature changes versus the sintering time during the SPS process. SPS can offer unique advantages in comparison to other sintering methods by monitoring the shrinkages as the criterion of densification. Therefore, the thickness of the final product could be estimated as a function of densification by monitoring the total displacement. This means that raising temperature along the applied pressure could normally lead to increasing the shrinkage; so, by knowing the densification behavior of the loaded powders, selecting the regime of heating and applying pressure could be easier to produce the almost fully dense sample. At the present work, the aluminum part of system could react with carbon or vanadium to form Al4C3 carbide or Al3V intermetallic, respectively. But the most possible change during heating at the temperature of 600-700°C could be the melting of Al.
Therefore, the first attempt was made to control Al melting and prevent its exhausting from the mold. As can be seen in Fig. 2, the first dominate shrinkage happened at temperatures between 600 and 700°C (times of 30-35 min). The DSC results obtained from the investigations by HAMM et al. [31] also confirmed the melting phenomenon of Al by the first endothermic peak at 667°C. Moreover, the collected information about the reactions during the heating of the VAl-C system by Hamm et al. [31] revealed three exothermic peaks at 677, 733 and 1090°C which were related to the formation of Al3V, Al8V5 and V2AlC formation, respectively. The second major of displacement change took place at temperatures between 900 and 1000°C (55-58 min), which seemed to be related to V2AlC formation (Fig. 2). C. Hu et al. [35] proposed the following reactions for the formation of V4AlC3: V+3Al=Al3V
(1)
8Al3V+7V=3Al8V5
(2)
Al8V5+3C=2V2AlC+VC+6Al V2AlC+2VC=V4AlC3
(3)
(4)
According to the proposed reactions, the third major displacement change at temperatures between 1200 and 1300°C (64-66 min) belongs to Eq. 4 and the formation of V4AlC3. By monitoring the displacement change during the sintering process and the slight amounts of changes at 1300°C, the final part of shrinkage was forced to the system as the pressure was increased from 10 to 30 MPa, leading to the final shrinkage area (73-76 min). After that, by observing the constant amounts of displacement at the maximum temperature (soaking time), the finishing of the sintering process could be concluded by considering the selected regime. This means that the used temperature and applied pressure acted on densification based on their own rules. Consequently, the higher sintering temperature and applied pressure would be required or the maximum shrinkage obtained, and the specimen could not shrink more. It is worth mentioning that lots of other parameters can affect the monitoring densification, such as reactions with shrinkage or expansion, the amounts of the loaded powders, the thickness of the graphite foils, thermal expansion of the engaged parts of the SPS machine, etc. However, the above notes can be helpful to prepare the almost fully dense sample with the highest possibility.
Fig. 3 XRD pattern of spark plasma sintered V4AlC3 sample at 1350°C
Fig. 3 displays the XRD pattern of the spark plasma sintered sample at 1350°C. It is worth mentioning that the phase detection was done based on the literature review [30,31]. As can be concluded from Fig. 3, the main peaks were related to the V4AlC3 compound with some other peaks (comparatively lower intensities) of Al2O3 and VC. The origination of Al2O3 in the Al-VC system after the sintering process could be attributed to the presence of oxygen at the surface of the initially used Al and V powders [36]. Moreover, the processing conditions such as mixing and sintering process can be regarded as another part of the procedure that can introduce O to the system, finally leading to the formation of Al2O3. According to the Ellingham’s diagram, Al2O3 has become the more stable phase in comparison to V2O5. Therefore, the reduction of oxides from the V source is thermodynamically favored by Al [37,38]. In the case of VC formation, it seems that evaporation of Al and its oxidation and diffusion to spaces between the mold can lead to the remaining VC according to the Eqs. (3) and (4). It has been approved that the reaction between Al and VC at high temperatures (>650°C) can produce Al3V and Al4C3 [39,40]. These reaction products can also react with each other to produce V2AlC [41].
Fig. 4 FESEM micrographs of polished surface of spark plasma sintered V4AlC3.
Fig. 5 Secondary and back-scattered electron (SE and BSE) FESEM images of polished surface of V4AlC3
Al2O3
Fig. 6 FESEM images of polished surface of V4AlC3 at high magnifications
Figs 4-6 demonstrate the FESEM images of the polished surface of the V4AlC3 prepared sample. As can be seen from Fig. 4, the bright region in the back-scattered mode represents the V4AlC3 phase surrounding some gray particles. At the low magnification, it seemed that the dark area was related to Al2O3. For more detailed investigation, the EDS elemental mapping analysis of SE and BSE related FESEM images has been brought in Fig. 5. According to the corresponding EDS elemental mapping in Fig. 5, the presence of Al, V and C could be observed at the bright phase, while at the gray phases, the absence of V element with the concentration of Al could denote the presence of Al2O3 in these regions. Fig. 6 depicts high magnification FESEM micrographs of the polished surface of the V4AlC3 sample. As can be seen, the layered structure of the MAX phase was recognized even at the polished surface. Also, the gray phase as Al2O3 almost showed interfaces without pores by the main phase of microstructure. Fig. 7 reveals the SE-FESEM images of the fracture surface of the spark plasma sintered V4AlC3 sample at different magnifications. As can be observed, many lath-like and layered V4AlC3 phases were distributed at the 3 dimensional directions. The images with the higher magnification in Fig. 7 show the formation of the layered V4AlC3 along particles pulling out. By increasing the magnification on the lath-like V4AlC3 phase, delamination and plastic deformation of V4AlC3 layered could be seen. The kink deformation as the applied stress and fracture have been previously reported in the literature [42], showing the plastic deformation of the MAX phase as the inherent properties of these materials [43]. Furthermore, the formed layer with nanosize in thickness could improve the toughness of the prepared sample.
Fig. 7 SE-FESEM images of fracture surface of prepared V4AlC3 sample at different magnifications
Fig. 8 BSE-FESEM image, EDS spectra and quantitative analyses of V4AlC3 prepared sample
Fig. 8 shows BSE-FESEM images along with the corresponding EDS spectra and the quantitative analyses from different areas of the microstructure. By considering the abovementioned detected phases through the XRD pattern (Fig. 3) as V4AlC3, VC and Al2O3, Fig. 8 reveals three different contrasts examined by the quantitative EDS analysis. As can be seen, spot A at the dark gray phase shows Al and O elements, which represent a composition close to that of Al2O3. Spot B displays the high amounts of V and C along with the low content of Al, which
could be considered as VC (gray phase); finally, the spot C shows the presence of V, Al and C together as V4AlC3 compounds (the bright layered phase). It is important to note that the picture taken at BSE mode shows V4AlC3 and VC as the bright and gray phases, while the density of VC has been reported to be 5.77 g/cm3 [44]. However, the available data regarding the density of V4AlC3 represented the value of 5.24 g/cm3 [42]. It is worth mentioning that the reported data about V4AlC3 was calculated not based on the experimental. C. Hu et el. [30] prepared VC compounds with the V4AlC3 MAX phase. As observed and approved by EDS quantitative analyses, the darker phase represented VC-based compounds more than V4AlC3 regions did. It seemed that there were contradictions regarding the theoretical and experimental density of the MAX phase due to the fact that preparation of the ultra pure MAX phases is still one of problems not yet solved in this research area. Fig. 9 demonstrates the FESEM image of the mark of indentation at different loads of 3, 5, 10 and 30 Kgf after the Vickers hardness test. A simple look at Fig. 9 almost reveals that by increasing the indentation loads, the mark of indentation was increased, as expected. Also, the corresponding insets belonging to different indentation loads depicted almost the isotropic deformation and relatively equal diagonals at the marks. It seemed that this feature was as a result of the SPS process and the uniformly applied pressure along the C axis during the heating process. C. Hu et al. [13] also investigated the highly textured polycrystalline Nb4AlC3 MAX phase, finding that increasing the indentation loads led to decreasing the hardness values; this phenomenon could be considered as the indentation size effect [45]. on the other hand, H.B. Zhang et al. prepared the Ti3AlC2 MAX phase through spark plasma sintering and examined the properties of the prepared sample parallel and perpendicular to c axis ( c axis is parallel to the applied pressure direction). They found that the hardness changes in the parallel direction were minor quantities, but for those perpendicular to the c axis, the decrease of hardness as a result increasing indentation was high. In the present work, the calculated hardness values demonstrated low changes by increasing the indentation leads and reported the value based on the average of them. Consequently, it seems that the sintering process in this present study led to preparing the sample with an almost low anisotropy at the surface of the specimen.
Fig. 9 FESEM images of mark of indentation at different loads after hardness test and corresponding deformation zone (low magnification).
Fig. 10 SE and BSE FESEM images of deformation zone at corner of indention mark at 30 Kg (right mark)
Fig. 11SE high magnification deformation zone as result of at 30Kgf. As Fig. 10 shows and BSE FESEMFESEM imagesimages of the of corner of the indentation mark indentation at load of can be seen, besides the deformation zone at the corner of the indentation mark, the applied load
of 30 Kgf led to cracks at this corner. The toughening mechanisms could be studied by crack propagation at the corner of the indentation load, as shown in Fig. 10. As can be seen, breaking on the lath-like V4AlC3 max phase appeared due to the formation of the crack at the corner of the indentation mark. By considering the distance from the corner and following the crack path, bridging, particles pulling out, deflection and separation could be detected as other toughening mechanisms for this prepared sample [46]. Fig. 11 displays the high magnification FESEM images of the deformation zone at the corner of the indentation mark. Fig. 11 a shows delamination and plastic deformation of the kink and bowed layer of the V4AlC3 phase as a result of deformation at the corner of the indentation mark. Moreover, the breaking of the layer of V4AlC3 as crack propagation could be observed at the high magnification FESEM image. On the other hand, the prepared FESEM image in Fig. 11 b could confirm the slipped lath-like V4AlC3 phases on each other as a result of plastic deformation. Table.1 and Fig. 12 reveal the physical and mechanical properties and the load-extension curve during the bending strength test of the prepared V4AlC3 sample, respectively. As can be seen, the Vickers hardness and bending strength of 389±19 MPa and 6.74±0.12 GPa were obtained for the spark plasma sintered sample; this was in a full agreement with the literature [47].
Table.1 Physical and mechanical properties of V4AlC3 sample. sample
Relative density (%)
Bending strength (MPa)
Hardness (GPa)
V4AlC3
99.66±0.14
389±19
6.74±0.12
In the case of relative density, an almost fully dense sample was prepared through spark plasma sintering. It is worth mentioning that theoretical density in relative density measurement accounted for the calculated data in other works and the amounts of Al2O3 and VC were not considered. Furthermore, the fracture toughness was not reported in this present study due to the uncertain amounts of crack length.
Force (N)
Extension (mm) Fig. 12 load-extension curve during bending strength test of V4AlC3 sample
Conclusion In the present study, some successful single-step spark plasma sintering was applied without any mechanical activation to produce the V4AlC3 MAX phase. The displacement-temperature-time curves of the sintering process revealed four dominant shrinkage which were probably related to the melting of Al, V2AlC formation, synthesis of V4AlC3, and the pressure increase from 10 to 30 MPa, respectively. The XRD patterns confirmed the formation of V4AlC3 as the dominant phase along some tiny amounts of Al2O3 and VC impurities. The FESEM images of the fracture surface of the prepared sample after the bending strength test also demonstrated delamination and plastic deformation of the layered V4AlC3 phase with particles pulling out of VC and Al2O3. Investigation on the crack path at the corner of the indentation mark at 30 Kgf also exhibited cracks separation, bridging, deflection, particles pulling out and breaking as the toughening mechanisms in the bulk V4AlC3 MAX phase. The almost fully dense sample with the Vickers hardness of 6.74±0.12 GPa and the bending strength of 389±19 MPa was obtained for the spark plasma sintered sample at 1350°C.
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PII: DOI: Reference:
S0272-8842(19)30040-9 https://doi.org/10.1016/j.ceramint.2019.01.036 CERI20491
To appear in: Ceramics International Received date: 8 December 2018 Revised date: 28 December 2018 Accepted date: 5 January 2019 Cite this article as: Mohsen Hossein-Zadeh, Omid Mirzaee and Hamidreza Mohammadian-Semnani, An Investigation into the microstructure and mechanical properties of V4AlC3 MAX phase prepared by spark plasma sintering, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.01.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
An Investigation into the microstructure and mechanical properties of V4AlC3 MAX phase prepared by spark plasma sintering
Mohsen Hossein-Zadeh, Omid Mirzaee, Hamidreza Mohammadian-Semnani1*
Department of Materials Engineering, Semnan University, P.O. Box 35195-363, Semnan, Iran [email protected] [email protected] *
Corresponding author. Tel.: +98 9123317550
Abstract The present work investigated the effect of spark plasma sintering of V4AlC3 MAX Phase on the microstructure and mechanical properties. Accordingly, Vanadium, Aluminum and carbon-black powders were mixed through a high energy mixer mill for 10 min in an ethanol medium. The direct inserted mixture of Al-V-C systems in a graphite mold was spark plasma sintered at 1350°C with the initial and final applied pressure of 10 and 30 MPa at the vacuum condition of 20 Pa. The XRD investigations demonstrated the V4AlC3 crystalline phase along VC and Al2O3 as the side-effect reaction products. The microstructure analyses further indicated a perfect lathlike MAX phase with an almost proper growth. The FESEM micrographs of the prepared sample also indicated the formation of the V4AlC3 MAX phase with the lath-like shape. The fracture surface of the specimen also displayed deformed and separate lath-like phases as a result of the bending test. The different loads used for the hardness test demonstrated a deformation zone around the mark indentation. By increasing the applied load in the hardness test, the deformed area was enhanced. The prepared FESEM images from the deformed area showed different crack propagation mechanisms such as separation and branching, deflection, bridging, particles pulling out and breaking. 389±19 MPa of bending strength and 6.74±0.12 GPa of Vickers hardness were obtained for the prepared sample along the almost full densification as 99 % of theoretical density.
Keywords: Aluminum, Vanadium, MAX phase, Spark plasma sintering.
1. Introduction Layered ternary compounds, also called MAX phases, have shown outstanding properties as the novel engineering materials, such as shock resistance, proper thermal and electrical conductivity, machinability, and promising high temperature applications. The MAX phases are known with the general formula of Mn+1AXn, where M exhibits an early transition metal, A is the main group metal and X is C or N (n=1-3) [1,2]. However, the maximum amount of n=3 is usually considered for the MAX phases; recently, new members with n=5, 6 and 7 as the high order have been added to the MAX phases by the researchers [3–6]. Generally, MAX phases offer a combination of strong covalent M-X bondings and comparatively weaker M-A bonds, presenting the exclusive and remarkable blend of hybrid metallic and ceramic characteristics [7,8]. Moreover, the MAX phases possess a laminar structure with the hexagonal symmetry in its crystals; due to this unique feature, they show anisotropic properties [9,10]. The literature review indicates more than 50, 6 and 8 of M2AX, M3AX2 and M4AX3 have been indentified and studied, respectively, as groups of MAX phases; also, a new family of MAX phases has emerged [11–13]. A simple comparison of
the investigated MAX phases
demonstrates that comparatively few studies have been done by increasing n, which seems that the successful preparation of the monolithic M4AX3 faces more challenges due to the higher chance of MX formation as the reaction product [14,15]. The theoretical crystallographic investigations on the MAX phases have depicted that, normally, there are two, three and four layers of MX in between every two a layers. Therefore, the differences between orders of MAX phases can result in different atomic configurations and hence, different physical and mechanical properties [16,17]. Moreover, in the same chemical system, the 413 subgroup is more interesting due to its characteristically superior temperature stability, as compared to those other order sequences of the MAX phases [18]. MAX phases can be produced by the
intense (wide-range) category of methods, such as
combustion synthesis [19], chemical and physical vapor deposition [20,21], self propagating high-temperature synthesis (SHS) [22], arc melting [23], mechanical alloying and sintering processes [24,25], etc. Among all preparation methods, producing the bulk MAX phases with almost the lowest impurities as the reaction products has become a major challenge in the investigations [26]. Spark plasma sintering (SPS) as a new sintering method is capable of producing a wide range of materials due to its unique advantages, such as Joules heating,
applied pressure, vacuum condition, local sparks between particles, etc [27–29].The SPS has several advantages compared to conventional methods of MAX phases as follows; (1) sintering time and temperature can be reduced, and thus lower grain growth occurs, (2) residual porosity can be reduced, (3) lower residual porosity and lower grain growth result in higher strength, and (4) densification at low temperature can be improved. As previously explained, the investigation on the preparation of 413 MAX phases is limited. C. Hu et al [30], however, investigated the crystal structure of V4AlC3 by the reactive hot pressing of the V-Al-C system with the stoichimetric molar ratio of V:Al:C=4:1:3 at 1700°C. The results obtained by them almost demonstrated that using the stoichiometric composition of V, Al and C mixture with heat treatment at 1700 for 60 min led to the formation of the dominant phase of VC alongside V4AlC3. Christin M. Hamm and et al.[31] also prepared the V4AlC3 MAX phase using microwave heating at 1000 W for 60 min and post densification by SPS at 1350°C. They used the elemental molar ratio of V:Al:C=4:5.2:3, finding that microwave heating could lead to the formation of V4AlC3. But the XRD patterns taken after the DSC analysis revealed only V2AlC formation at 1200°C. However, while Christin M. Hamm et al. [31] studied the formation of V4AlC3 with microwave heating, they did not mention the final microwave heating temperature and the reason for using such a molar ratio of starting elements with the ultra high amount of Al; also, their investigation did not display the successful synthesis of V4AlC3 by applying SPS , as this methid was only used for the densification purpose of the synthesized powders. The preparation of Nb4AlC3 by SPS using mechanically activated Nb, Al and C powders was also investigated by C. Hu et al.[32]. They used sintering temperatures between 800-1600°C and also, different compositions of starting powders with the excess aluminum due to its evaporation at the high temperature. The results obtained by C. Hu et al. [32] also revealed that Nb4AlC3 started to appear at 1400°C by XRD investigation and also,
at the optimum sintering
temperature of 1600°C. To the best of authors’ knowledge, there are not any researches on the preparation of the V 4AlC3 MAX phase through the direct reaction of the elemental powders without any mechanical activation and/or post heat treatment. Therefore, this present research investigated the production of the bulk V4AlC3 MAX phase by SPS; also, the microstructure, mechanical properties and the observed toughening mechanism were studied in details. 2. Experimental procedures
Vanadium (Riedel-de Haen
Art. no. 10461 vanadium powder, 250 mesh, 99.9% purity),
aluminum (Merck Art. no. 1056 aluminum powder, 250 mesh, 99% purity), and carbon black (99.9% purity,
particle size <30 μm) powders were used as the
starting materials. The
composition of the V:Al:C elemental mixture was 4:1.5:3. The mixing procedure was performed using a high energy mixer mill (Spex mixer mill -8000D model) in an Ethanol medium for 10 min with alumina balls. Then, the mixture was dried at 70°C on the hot plate. The mixed powders were directly loaded into a graphite die (30 mm diameter) of SPS (SPS-20T-10, China) using a graphite foil to separate the powder from the die. The sintering process was carried out at 1350°C with the initial and final pressure of 10 MPa (with increasing the temperature) and 30 MPa (at the temperature of 1350°C) in the vacuum condition of 20 Pa, respectively. The phase identification was performed using XRD (Philips) with a Cu kα radiation source and an imageplate detector over the 2θ range 10-80° in the reflection geometry. The bulk density of the sintered samples was measured using the Archimedes’ principle. The three-point flexural strength test was used to measure the strength of the sintered samples. The bending strength specimens with the bar shape (3×5×25 mm) prepared by SPS were cut from the sintered disc with the diameter of 30 mm. Vickers hardness was tested by the MKV-h21 hardness testers (Islamic Republic of Iran) at the loads of 3, 5, 10 and 30 Kgf, and the induced cracks under a load of 30 kgf were observed by FESEM. The microstructure of the specimen after the grinding and polishing procedure was examined using FESEM (MIRA 3 TESCAN, Czech Republic) equipped with an energy dispersive spectrometer (EDS).
3. Results and discussion Fig. 1 represents the back scattered FESEM images and EDS elemental mapping of the mixed powders after the 10 min high energy milling. Approximately, some simple mixing without any size reduction of powders could be observed in Fig. 1. As expected, due to the low mixing time, mechanical activation could normally lead to better results in the synthesis of compounds and prepare some part of the activation energy [33,34]; however, the present work investigated the capability of the SPS process in the formation of MAX phases because of omitting the milling process results in cost and energy saving, regardless of offering impurities to the system as a consequence of balls and cup contiguity.
Fig. 1 FESEM images and corresponding EDS elemental mapping analysis
Fig. 2 Displacement/displacement rate- temperature-time curves during spark plasma sintering
Fig. 2 shows the displacement/displacement rate and temperature changes versus the sintering time during the SPS process. SPS can offer unique advantages in comparison to other sintering methods by monitoring the shrinkages as the criterion of densification. Therefore, the thickness of the final product could be estimated as a function of densification by monitoring the total displacement. This means that raising temperature along the applied pressure could normally lead to increasing the shrinkage; so, by knowing the densification behavior of the loaded powders, selecting the regime of heating and applying pressure could be easier to produce the almost fully dense sample. At the present work, the aluminum part of system could react with carbon or vanadium to form Al4C3 carbide or Al3V intermetallic, respectively. But the most possible change during heating at the temperature of 600-700°C could be the melting of Al.
Therefore, the first attempt was made to control Al melting and prevent its exhausting from the mold. As can be seen in Fig. 2, the first dominate shrinkage happened at temperatures between 600 and 700°C (times of 30-35 min). The DSC results obtained from the investigations by HAMM et al. [31] also confirmed the melting phenomenon of Al by the first endothermic peak at 667°C. Moreover, the collected information about the reactions during the heating of the VAl-C system by Hamm et al. [31] revealed three exothermic peaks at 677, 733 and 1090°C which were related to the formation of Al3V, Al8V5 and V2AlC formation, respectively. The second major of displacement change took place at temperatures between 900 and 1000°C (55-58 min), which seemed to be related to V2AlC formation (Fig. 2). C. Hu et al. [35] proposed the following reactions for the formation of V4AlC3: V+3Al=Al3V
(1)
8Al3V+7V=3Al8V5
(2)
Al8V5+3C=2V2AlC+VC+6Al V2AlC+2VC=V4AlC3
(3)
(4)
According to the proposed reactions, the third major displacement change at temperatures between 1200 and 1300°C (64-66 min) belongs to Eq. 4 and the formation of V4AlC3. By monitoring the displacement change during the sintering process and the slight amounts of changes at 1300°C, the final part of shrinkage was forced to the system as the pressure was increased from 10 to 30 MPa, leading to the final shrinkage area (73-76 min). After that, by observing the constant amounts of displacement at the maximum temperature (soaking time), the finishing of the sintering process could be concluded by considering the selected regime. This means that the used temperature and applied pressure acted on densification based on their own rules. Consequently, the higher sintering temperature and applied pressure would be required or the maximum shrinkage obtained, and the specimen could not shrink more. It is worth mentioning that lots of other parameters can affect the monitoring densification, such as reactions with shrinkage or expansion, the amounts of the loaded powders, the thickness of the graphite foils, thermal expansion of the engaged parts of the SPS machine, etc. However, the above notes can be helpful to prepare the almost fully dense sample with the highest possibility.
Fig. 3 XRD pattern of spark plasma sintered V4AlC3 sample at 1350°C
Fig. 3 displays the XRD pattern of the spark plasma sintered sample at 1350°C. It is worth mentioning that the phase detection was done based on the literature review [30,31]. As can be concluded from Fig. 3, the main peaks were related to the V4AlC3 compound with some other peaks (comparatively lower intensities) of Al2O3 and VC. The origination of Al2O3 in the Al-VC system after the sintering process could be attributed to the presence of oxygen at the surface of the initially used Al and V powders [36]. Moreover, the processing conditions such as mixing and sintering process can be regarded as another part of the procedure that can introduce O to the system, finally leading to the formation of Al2O3. According to the Ellingham’s diagram, Al2O3 has become the more stable phase in comparison to V2O5. Therefore, the reduction of oxides from the V source is thermodynamically favored by Al [37,38]. In the case of VC formation, it seems that evaporation of Al and its oxidation and diffusion to spaces between the mold can lead to the remaining VC according to the Eqs. (3) and (4). It has been approved that the reaction between Al and VC at high temperatures (>650°C) can produce Al3V and Al4C3 [39,40]. These reaction products can also react with each other to produce V2AlC [41].
Fig. 4 FESEM micrographs of polished surface of spark plasma sintered V4AlC3.
Fig. 5 Secondary and back-scattered electron (SE and BSE) FESEM images of polished surface of V4AlC3
Al2O3
Fig. 6 FESEM images of polished surface of V4AlC3 at high magnifications
Figs 4-6 demonstrate the FESEM images of the polished surface of the V4AlC3 prepared sample. As can be seen from Fig. 4, the bright region in the back-scattered mode represents the V4AlC3 phase surrounding some gray particles. At the low magnification, it seemed that the dark area was related to Al2O3. For more detailed investigation, the EDS elemental mapping analysis of SE and BSE related FESEM images has been brought in Fig. 5. According to the corresponding EDS elemental mapping in Fig. 5, the presence of Al, V and C could be observed at the bright phase, while at the gray phases, the absence of V element with the concentration of Al could denote the presence of Al2O3 in these regions. Fig. 6 depicts high magnification FESEM micrographs of the polished surface of the V4AlC3 sample. As can be seen, the layered structure of the MAX phase was recognized even at the polished surface. Also, the gray phase as Al2O3 almost showed interfaces without pores by the main phase of microstructure. Fig. 7 reveals the SE-FESEM images of the fracture surface of the spark plasma sintered V4AlC3 sample at different magnifications. As can be observed, many lath-like and layered V4AlC3 phases were distributed at the 3 dimensional directions. The images with the higher magnification in Fig. 7 show the formation of the layered V4AlC3 along particles pulling out. By increasing the magnification on the lath-like V4AlC3 phase, delamination and plastic deformation of V4AlC3 layered could be seen. The kink deformation as the applied stress and fracture have been previously reported in the literature [42], showing the plastic deformation of the MAX phase as the inherent properties of these materials [43]. Furthermore, the formed layer with nanosize in thickness could improve the toughness of the prepared sample.
Fig. 7 SE-FESEM images of fracture surface of prepared V4AlC3 sample at different magnifications
Fig. 8 BSE-FESEM image, EDS spectra and quantitative analyses of V4AlC3 prepared sample
Fig. 8 shows BSE-FESEM images along with the corresponding EDS spectra and the quantitative analyses from different areas of the microstructure. By considering the abovementioned detected phases through the XRD pattern (Fig. 3) as V4AlC3, VC and Al2O3, Fig. 8 reveals three different contrasts examined by the quantitative EDS analysis. As can be seen, spot A at the dark gray phase shows Al and O elements, which represent a composition close to that of Al2O3. Spot B displays the high amounts of V and C along with the low content of Al, which
could be considered as VC (gray phase); finally, the spot C shows the presence of V, Al and C together as V4AlC3 compounds (the bright layered phase). It is important to note that the picture taken at BSE mode shows V4AlC3 and VC as the bright and gray phases, while the density of VC has been reported to be 5.77 g/cm3 [44]. However, the available data regarding the density of V4AlC3 represented the value of 5.24 g/cm3 [42]. It is worth mentioning that the reported data about V4AlC3 was calculated not based on the experimental. C. Hu et el. [30] prepared VC compounds with the V4AlC3 MAX phase. As observed and approved by EDS quantitative analyses, the darker phase represented VC-based compounds more than V4AlC3 regions did. It seemed that there were contradictions regarding the theoretical and experimental density of the MAX phase due to the fact that preparation of the ultra pure MAX phases is still one of problems not yet solved in this research area. Fig. 9 demonstrates the FESEM image of the mark of indentation at different loads of 3, 5, 10 and 30 Kgf after the Vickers hardness test. A simple look at Fig. 9 almost reveals that by increasing the indentation loads, the mark of indentation was increased, as expected. Also, the corresponding insets belonging to different indentation loads depicted almost the isotropic deformation and relatively equal diagonals at the marks. It seemed that this feature was as a result of the SPS process and the uniformly applied pressure along the C axis during the heating process. C. Hu et al. [13] also investigated the highly textured polycrystalline Nb4AlC3 MAX phase, finding that increasing the indentation loads led to decreasing the hardness values; this phenomenon could be considered as the indentation size effect [45]. on the other hand, H.B. Zhang et al. prepared the Ti3AlC2 MAX phase through spark plasma sintering and examined the properties of the prepared sample parallel and perpendicular to c axis ( c axis is parallel to the applied pressure direction). They found that the hardness changes in the parallel direction were minor quantities, but for those perpendicular to the c axis, the decrease of hardness as a result increasing indentation was high. In the present work, the calculated hardness values demonstrated low changes by increasing the indentation leads and reported the value based on the average of them. Consequently, it seems that the sintering process in this present study led to preparing the sample with an almost low anisotropy at the surface of the specimen.
Fig. 9 FESEM images of mark of indentation at different loads after hardness test and corresponding deformation zone (low magnification).
Fig. 10 SE and BSE FESEM images of deformation zone at corner of indention mark at 30 Kg (right mark)
Fig. 11SE high magnification deformation zone as result of at 30Kgf. As Fig. 10 shows and BSE FESEMFESEM imagesimages of the of corner of the indentation mark indentation at load of can be seen, besides the deformation zone at the corner of the indentation mark, the applied load
of 30 Kgf led to cracks at this corner. The toughening mechanisms could be studied by crack propagation at the corner of the indentation load, as shown in Fig. 10. As can be seen, breaking on the lath-like V4AlC3 max phase appeared due to the formation of the crack at the corner of the indentation mark. By considering the distance from the corner and following the crack path, bridging, particles pulling out, deflection and separation could be detected as other toughening mechanisms for this prepared sample [46]. Fig. 11 displays the high magnification FESEM images of the deformation zone at the corner of the indentation mark. Fig. 11 a shows delamination and plastic deformation of the kink and bowed layer of the V4AlC3 phase as a result of deformation at the corner of the indentation mark. Moreover, the breaking of the layer of V4AlC3 as crack propagation could be observed at the high magnification FESEM image. On the other hand, the prepared FESEM image in Fig. 11 b could confirm the slipped lath-like V4AlC3 phases on each other as a result of plastic deformation. Table.1 and Fig. 12 reveal the physical and mechanical properties and the load-extension curve during the bending strength test of the prepared V4AlC3 sample, respectively. As can be seen, the Vickers hardness and bending strength of 389±19 MPa and 6.74±0.12 GPa were obtained for the spark plasma sintered sample; this was in a full agreement with the literature [47].
Table.1 Physical and mechanical properties of V4AlC3 sample. sample
Relative density (%)
Bending strength (MPa)
Hardness (GPa)
V4AlC3
99.66±0.14
389±19
6.74±0.12
In the case of relative density, an almost fully dense sample was prepared through spark plasma sintering. It is worth mentioning that theoretical density in relative density measurement accounted for the calculated data in other works and the amounts of Al2O3 and VC were not considered. Furthermore, the fracture toughness was not reported in this present study due to the uncertain amounts of crack length.
Force (N)
Extension (mm) Fig. 12 load-extension curve during bending strength test of V4AlC3 sample
Conclusion In the present study, some successful single-step spark plasma sintering was applied without any mechanical activation to produce the V4AlC3 MAX phase. The displacement-temperature-time curves of the sintering process revealed four dominant shrinkage which were probably related to the melting of Al, V2AlC formation, synthesis of V4AlC3, and the pressure increase from 10 to 30 MPa, respectively. The XRD patterns confirmed the formation of V4AlC3 as the dominant phase along some tiny amounts of Al2O3 and VC impurities. The FESEM images of the fracture surface of the prepared sample after the bending strength test also demonstrated delamination and plastic deformation of the layered V4AlC3 phase with particles pulling out of VC and Al2O3. Investigation on the crack path at the corner of the indentation mark at 30 Kgf also exhibited cracks separation, bridging, deflection, particles pulling out and breaking as the toughening mechanisms in the bulk V4AlC3 MAX phase. The almost fully dense sample with the Vickers hardness of 6.74±0.12 GPa and the bending strength of 389±19 MPa was obtained for the spark plasma sintered sample at 1350°C.
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