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Processing and characterization of MoAl1-xSixB solid solutions
Journal of Alloys and Compounds 814 (2020) 152290
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Processing and characterization of MoAl1-xSixB solid solutions Pengfei Ma, Shibo Li*, Jie Hu, Xiaogang Lu, Wenbo Yu, Yang Zhou Center of Materials Science and Engineering, School of Mechanical and Electronic Control Engineering, Beijing Jiaotong University, Beijing, 100044, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 May 2019 Received in revised form 2 September 2019 Accepted 13 September 2019 Available online 14 September 2019
The MAB phase MoAlB has attractive properties both at room and high temperatures. To further improve its properties, a possible way is to form solid solutions by partially substituting one of constituting elements in MoAlB. This work reports on the synthesis of MoAl1-xSixB solid solutions by powder metallurgy. Influences of sintering temperature, dwelling time, and composition of starting mixture on the solubility limit of Si in MoAlB have been investigated. The maximum solubility of Si was 0.91 at.%, corresponding to x ¼ 0.03. Increase of Si content in the starting mixtures caused the formation of other competing phases. Phase composition and microstructure were characterized by x-ray diffraction method and scanning electron microscopy. A dense MoAl0$97Si0$03B solid solution has been achieved by hot-pressing a MoB/1.3Al/0.1Si mixture at 1200 C with 25 MPa for 1 h, and its mechanical properties have been measured. An increase in strength and hardness is achieved by solid-solution strengthening. The flexural strength and the hardness of the solid solution are higher than those of MoAlB, viz. 385 MPa vs 307 MPa, and 14.89 GPa vs 8.43 GPa. © 2019 Elsevier B.V. All rights reserved.
Keywords: Ceramics Powder metallurgy Sintering Mechanical properties Microstructure
1. Introduction The ternary MoAlB phase with an orthorhomic unit cell having the lattice constants a ¼ 0.3212 nm, b ¼ 1.3985 nm, c ¼ 0.3102 nm was reported in 1966 [1]. The MoAlB compound is an attractive phase in a new group of MAB phases (where M is a transition metal, A is Al or Zn, and B is boron [2]). The crystal structure is composed of “MoeB00 slabs interleaved by two layers of Al atoms [2e4]. Its nanolaminated structure and attractive properties are similar to the so-called MAX phases [5e7]. MoAlB has attractive high temperature properties, such as good resistance to oxidation, thermal shock and ablation [8e12]. The good oxidation resistance of MoAlB is comparable to Ti2AlC and Cr2AlC MAX phases [13,14], due to the formation of an adherent and protective a-Al2O3 scale at high temperatures. In addition, MoAlB has desirable properties at room temperature. The electrical conductivity of MoAlB is about 1.49e2.86 106 U1m1 [8,15], and the thermal conductivity is 35 W/m.K at 300 K [8]. The polycrystalline MoAlB bulk ceramic has a Vickers hardness of 9e14 GPa [2,8,9,15], higher than that of MAX phases [5,6]. This is attributed to the fact that MoAlB has higher bonding strength within MoeB slabs, and between the MoeB slab and Al layers [2]. MoAlB has a fracture toughness of 4e5 MPa m1/2
* Corresponding author. E-mail address: [email protected] (S. Li). https://doi.org/10.1016/j.jallcom.2019.152290 0925-8388/© 2019 Elsevier B.V. All rights reserved.
and a flexural strength of 307e456 MPa [9,11]. Although the high temperature properties make MoAlB promising for applications in high temperature environments, yet the relatively low strength and toughness limit its potential for use in high temperature structural components calling for a combination of high strength and toughness, good damage tolerance, and excellent oxidation resistance. To improve the mechanical properties of ceramics, solid solution strengthening as one of the most effective methods has always been adopted. Solid solution materials not only exhibit superior performance compared to their counterparts, but also reveal tunable properties by adjusting the percentage of substitutional atoms. MAX solid solutions have been studied extensively through substitution on the M, A, and X sites. There are over seventies of MAX solid solutions, such as (Ti,V)3AlC2, (Nb,V)2AlC, (Cr,Mn)2AlC, Ti3(Si,Al)C2, Ti3(Al,Sn)C2, and Ti2Al(C,N) [16e23]. Their mechanical properties, thermal properties, electronic properties, and magnetic properties have been successfully tailored. However, work on MAB solid solutions has been much less focused. Only MoxCrl-xA1B and MoxWl-xA1B solid solutions by substitution on the M site have been reported, together with the discussion of hardness, oxidation resistance and electrical conductivity [15,24]. The present work is to prepare new MAB solid solutions by selective replacing of atoms on the A sites. For example, Al atoms on
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the A sites in MoAlB are partially replaced by Si to form MoAl1-xSixB solid solutions. Selection of Si as a solute is due to the following reasons. First, it has been found that Si easily replace Al in Alcontaining MAX phases to form a variety of solid solutions which have improved mechanical properties and oxidation resistance [16,19,21]. Second, it is reasonably expected that solid solution strengthening makes MoAl1-xSixB solid solutions harder and stronger. In the present study, the effect of composition of starting mixtures on the solubility limit of Si in MoAlB has also been investigated, and the mechanical properties of MoAlB and MoAl1xSixB have been compared.
SENB bars was machined with a thin diamond blade. The span size and the crosshead speed were 20 mm and 0.05 mm/min, respectively. The Vickers hardness was measured in a TH700 hardness tester in the load range 1e20 kg with a dwelling time of 15 s. Six measurements in different areas were performed for each sample to obtain an average value.
2. Experimental details
3.1.1. Effects of Al content and starting mixture on the formation of solid solutions To synthesize Al-containing MAB phases with high purity, extraAl was always used in the starting mixtures to compensate the loss of Al at high sintering temperatures and to inhibit the formation of impurities in the final products. Our previous work showed that Fe2AlB2 and MoAlB MAB phases with high purity have been successfully obtained from mixtures containing extra Al [11,25]. To synthesize MoAl1-xSixB solid solutions, the effect of Al content in the starting mixtures on their purity was firstly investigated. Two mixtures, Mo/(1e1.3)Al/B/0.1Si and MoB/(1e1.3)Al/0.1Si, were used in the present study. Fig. 1 presents the XRD patterns for the Mo/(1e1.3)Al/B/0.1Si mixtures after sintering at 1200 C for 1 h in Ar atmosphere, together with the XRD pattern of MoAlB synthesized from a Mo/ 1.3Al/B mixture at the same conditions for comparison. It was found that 1.3 mol of extra-Al in the Mo/1.3Al/B mixture leads to the high purity of MoAlB (Fig. 1a). However, except for MoAlB, MoB,
2.1. Material preparation and synthesis Mo (particle size: 300 mesh, 99.5% purity, General Research Institute for Nonferrous Metals, GRINM, China), Al (particle size: 300 mesh, 99.5% purity, Beijing Reagent Company, China), and B (particle size: 300 mesh, 99% purity, GRINM, China), Si (particle size: 300 mesh, 99.9% purity, GRINM, China), and MoB (particle size: ~10 mm, 99.9% purity, GRINM, China) powders were used as starting materials. MoAl1-xSixB solid solutions were fabricated from two kinds of mixtures, one containing Mo, Al, B, and Si powders with a molar ratio of 1:(1e1.3):1:x (x ¼ 0.1, 0.3, and 0.5), denoted as Mo/(1e1.3)Al/B/xSi; and the other containing MoB, Al and Si powders with a molar ratio of 1:(1e1.3):x (x ¼ 0.1, 0.3, and 0.5), denoted as MoB/(1e1.3)Al/xSi. The mixtures were mixed with agate balls in containers for 10 h in a rotary drum type ball-miller. The rotate speed is 150 rpm. The mixtures were cold-pressed in a stainless steel mold with 20 MPa to form F50 5 mm compacts. The compacts were pressurelessly sintered at temperatures ranging from 1100 C to 1300 C for 30e120 min in Ar atmosphere in a high temperature furnace. The heating rate was 30 C/min. Based on the above results, an optimized mixture was hotpressed at 1200 C under 25 MPa for 1 h in Ar to prepare dense bulk samples. The density of the hot-pressed samples was determined using Archimedes’ method.
3. Results and discussion 3.1. Pressureless sintering and characterization of MoAl1-xSixB
2.2. Material characterization The synthesized sample was pulverized and then sieved with a 200 mesh. The powders were used to identify the phase composition with by X-ray diffractometer (XRD) with using Cu Ka radiation (D/Max 2200 PC). The microstructure of the synthesized samples was characterized with a ZEISS EVO 18 scanning electron microscope (SEM) equipped with an energy-dispersive spectrometer system (EDS). The accelerating voltage for the EDS measurement was 20 kV, and the counting time was 60 s. 2.3. Measurement of mechanical properties The dense bulk samples were cut into bars by diamond wheel. The bar specimens with dimensions of 3 mm 4 mm 36 mm and 2 mm 4 mm 24 mm were used to measure the flexural strength and fracture toughness, respectively. All bars were ground with SiC sand papers up to 2000 grit and then ultrasonically cleaned in the ethanol bath. However, the tensile surfaces of the bars for flexural strength measurement were polished to 0.5 mm by diamond paste. The flexural strength was measured by the three-point bending test. The span size and the crosshead speed were 30 mm and 0.5 mm/min, respectively. The fracture toughness was determined by using the single edge notched beam (SENB) test. A notch of about 0.2 mm in width and about 2 mm in length in the center of the
Fig. 1. XRD patterns of (a) Mo/1.3Al/B, (b) Mo/Al/B/0.1Si, (c) Mo/1.1Al/B/0.1Si, and (d) Mo/1.3Al/B/0.1Si mixtures after sintering at 1200 C for 1 h in Ar.
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mixture to produce MoAlB, due to the reaction paths in the MoBeAleSi system are simpler than those in the MoeBeAleSi system. The considerable drop in the peak intensities of MoB with increasing Al content (Fig. 2aec) is due to the fact that MoB is consumed to form the MoAl1-xSixB solid solution. The above result further confirms that 1.3 mol of extra-Al is beneficial to produce the desirable phases of MoAlB and its solid solutions (see Table 1). Hence, the mixtures containing 1.3 mol Al were used in the following study.
Fig. 2. XRD patterns of (a) MoB/Al/0.1Si, (b) MoB/1.1Al/0.1Si, and (c) MoB/1.3Al/0.1Si mixtures after sintering at 1200 C for 1 h in Ar.
Al8Mo3, Al2O3, and SiB6 phases were detected in the samples sintered from the Mo/Al/B/0.1Si mixture (Fig. 1b). This result indicates that 1 mol of Al in the Mo/Al/B/0.1Si mixture is impossible to obtain a single solid solution phase. With increasing Al content, the peak intensities of binary phases substantially decreased and those of MoAlB correspondingly increased (Fig. 1c and d), indicating formation of a relatively high purity MoAl1-xSixB solid solution. The XRD results of the MoB/(1e1.3)Al/0.1Si mixture after sintering at 1200 C for 1 h are presented in Fig. 2. By carefully compared with Fig. 1b, Fig. 2a shows the strong peak intensities of MoAlB, but without Al8Mo3 peaks. This suggests that the direct use of MoB is more favorable than the use of Mo and B in the starting
3.1.2. Effects of Si content and starting mixture on the formation of solid solutions To determine the maximum solubility of Si in the MoAlB solid solutions, different contents of Si were used in the starting mixtures of Mo/1.3Al/B/xSi and MoB/1.3Al/xSi (x ¼ 0, 0.1, 0.3, and 0.5 mol). The XRD analysis results are presented in Fig. 3. It can be found that MoAl1-xSixB is a predominant phase after sintering of the two mixtures with 0.1 mol of Si (Fig. 3a and b). As the Si content increased from 0.3 mol to 0.5 mol, peaks belonging to Mo(Al,Si)2 and Mo2B5 appeared. Their intensities become stronger with increasing Si content, indicating the increase in content of the two phases (Fig. 3a and b). It should be noted that the peaks of MoAlB slightly shift to high angles even the Si content is up to 0.5 mol (Fig. 3c). This feature illustrates the limit solubility of Si in the MoAlB. To determine the solubility of Si, EDS point analysis was performed on at least 10 MoAlB grains with a Mo:(Al þ Si) ratio of 1:1 in different areas. The content of the light B element can't be determined accurately by EDS, hence, the measured Al/Si ratio was used to calculate the solubility of Si. If (Al þ Si) is 33.3 at. %, the Si concentration is calculated to be 0.91 at. % from the measured Al/Si ratio of 36.5, corresponding to a MoAl0$97Si0$03B solid solution. Based on the EDS results, the solubility of Si in MoAlB doesn't change as the Si powder increases from 0.1 to 0.3 mol in the starting mixtures. This suggests a limit solubility of Si in the MoAl1-xSixB solid solution. According to the XRD results, 0.1 mol of Si in the starting mixture produces an almost single MoAl0$97Si0$03B solid solution. Hence, the following study mainly investigates the effects of temperature and dwelling time on the formation of MoAl0$97Si0$03B from the Mo/1.3Al/B/0.1Si and MoB/1.3Al/0.1Si mixtures. 3.1.3. Effect of temperature on the formation of solid solutions Fig. 4 depicts the XRD patterns of the MoB/1.3Al/0.1Si mixture after sintering at various temperatures for 1 h in Ar. After sintering at 1100 C, except for MoAl0$97Si0$03B, there are an unreacted phase
Table 1 Summary of composition and processing conditions of prepared samples. Starting mixture
Molar ratio
Temperature (ºC)
Time (h)
Resulting phases
Mo/Al/B Mo/Al/B/Si
1:1.3:1 1:1:1:0.1 1:1.1:1:0.1 1:1.3:1:0.1 1:1.3:1:0.3 1:1.3:1:0.5 1:1:0.1 1:1.1:0.1 1:1.3:0.1 1:1.3:0.3 1:1.3:0.5 1:1.3:0.1 1:1.3:0.1 1:1.3:0.1 1:1.3:0.1
1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1100 1300
1 1 1 1 1 1 1 1 1 1 1 0.5 2 1 1
MoAlB(s), Al2O3(vw) SS(m), MoB(s), Al8Mo3(s), SiB6(w), Al2O3(w) SS(s), MoB(m), Al8Mo3(m), Al2O3(w) SS(s), Al8Mo3(vw), Al2O3(vw) SS(s), Mo(Al,Si)2(m), Mo2B5 (w), Al2O3(w) SS(s), Mo(Al,Si)2(s), Mo2B5 (s), Al2O3(w) SS(s), MoB(s), Al2O3(w), SiB6(w) SS(s), MoB(m), Al2O3(w), SiB6(w) SS(s), MoB(vw), Al2O3(vw) SS(s), Mo(Al,Si)2(m), Mo2B(m), Al2O3(w) SS(s), Mo(Al,Si)2(s), Mo2B(s), Al2O3(w) SS(s), MoB(vw), Al2O3(vw) SS(s), MoB(vw), Al2O3(vw) SS(s), MoB(s), Al8Mo3(m), Al2O3(w) SS(s), MoB(vw), Al2O3(vw)
MoB/Al/Si
Symbols mean: “SS” MoAl0$97Si0$03B Solid Solution; “s” strong; “m” medium; “w” weak; and “vw” very weak.
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Fig. 3. XRD patterns of (a) Mo/1.3Al/B/xSi, and (b) MoB/1.3Al/xSi (x ¼ 0, 0.1, 0.3, and 0.5) mixtures after sintering at 1200 C for 1 h in Ar, (c) Enlarged XRD patterns taken from (a).
of MoB and a newly formed intermetallic phase of Al8Mo3. By contrast, sintering at 1200 C and 1300 C results in the formation of MoAl0$97Si0$03B as the predominant phase, with minor
impurities. The XRD patterns of the Mo/1.3Al/B/0.1Si mixture after sintering at the same conditions as for MoB/1.3Al/0.1Si showed similar phase constitutions.
Fig. 4. XRD patterns of MoB/1.3Al/0.1Si mixture after sintering at temperatures ranging from 1100 C to 1300 C for 1 h in Ar.
Fig. 5. XRD patterns of MoB/1.3Al/0.1Si mixture after sintering at 1200 C for 30e120 min in Ar.
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The above results confirmed that the MoAl0$97Si0$03B solid solution can be obtained by pressureless sintering the Mo/1.3Al/B/ 0.1Si and MoB/1.3Al/0.1Si mixtures at both 1200 C and 1300 C for 1 h (Table 1). 3.1.4. Effect of dwelling time on the formation of solid solutions The effect of dwelling time on the solid solution formation has also been investigated. After sintering of the MoB/1.3Al/0.1Si mixture at 1200 C for only 0.5 h, a predominant phase of MoAl0$97Si0$03B was obtained, together with minor Al2O3 and MoB (Fig. 5). As the sintering period was prolonged from 1 to 2 h, the phase composition didn't change so much (Fig. 5). EDS results demonstrated that the content of Si in the samples sintered at different temperatures (1200e1300 C) and times (0.5e2 h) changed slightly, indicating the limit solubility of Si in MoAlB. Table 1 summarizes the composition and processing conditions of the prepared samples. On the basis of the above results, the MoAl0$97Si0$03B solid solution can be obtained by sintering of the Mo/1.3Al/0.1Si and MoB/1.3Al/0.1Si mixtures at 1200 C for 1 h in Ar. With consideration of the low cost of product, the latter mixture is preferred. 3.1.5. Microstructural characterization SEM observation on the polished surface of the sample sintered at 1200 C for 1 h from the MoB/1.3Al/0.1Si mixture reveals that the MoAl0$97Si0$03B solid solution grains are less than 10 mm in size (Fig. 6a). It was found that the samples prepared by pressurelessly
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sintering of Mo/1.3Al/B/0.1Si and MoB/1.3Al/0.1Si mixtures have small grains, indicating the slow growth of MoAl0$97Si0$03B at 1200 C. Undamaged grains in the pore areas exhibit round or plate-like shapes and smooth surfaces (Fig. 6a). The backscattered SEM observation (Fig. 6b) shows that small black areas are mainly composed of Al2O3, identified by EDS. The Al2O3 impurity was always found in the Al-containing MAX and MAB phases due to the reaction of Al with O absorbed on the powder surfaces [8,11,18,20,21,26]. The EDS line analysis demonstrates the presence of Mo, Al, Si, and B elements in the analyzed area (right-hand inset of Fig. 6c). The elemental composition of Si along a line (left-hand inset of Fig. 6c) keeps almost constant, suggesting the uniform distribution of Si in the solid solution grain. Fig. 7 presents the crystal structure of MoAlB on the (100) plane, in which MoeB slabs are interleaved by two layers of Al atoms. Some Si atoms replace Al atoms and occupy their positions to form a solid solution. The MoAlB MAB phase and the MAX phases have a similar layered structure. Theoretically, the solubility of Si in MoAlB should be larger than that in MAX phases, due to the crystal structure of MoAlB with bilayers of Al atoms while that of MAX with only one layer of Al atoms interleaving MX blocks. However, the experimental results showed that the maximum solubility of Si was only 0.91 at. % in the present study. Moreover, impurities were found after sintering of the MoeAleB mixture even with 0.1 mol of Si. By contrast, the solubility of Si in Al-containing MAX solid solutions can be tunable. For example, a Ti3Al0$75Si0$25C2 solid solution with 4.2% of Si was achieved without any impurities after hot-
Fig. 6. (a) SEM micrograph of the polished surface of sample sintered at 1200 C for 1 h in Ar, (b) Enlarged backscattered SEM micrograph of the polished surface, (c) EDS line analysis for the elemental distribution along a line shown in the left-hand inset. The right-hand inset shows the presence of only Mo, Al, Si and B.
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pressing of a 3Ti/0.75Al/0.25Si/2C mixture [16]. The solubility of Si in the MoAlB MAB phase and the MAX phases may be dependent on their crystal structures and bonding energies. MoAlB has an orthorhombic crystal structure, while the MAX phases have a hexagonal crystal structure. Theoretical calculations showed that the bond lengths of MoeAl and AleAl in MoAlB are about 0.2693 nm and 0.2647 nm, respectively [2], shorter than 0.2885 nm of TieAl in the Ti3AlC2 MAX phase [27], suggesting the higher bonding energy in MoAlB. It has also been proved that the solubility of Si in Cr2AlC and Ti3AlC2, even both belonging to the same MAX phase family, is different. The former can dissolve a small amount of Si due to the higher bonding strength between the CreC blocks and Al layer in Cr2AlC [21]. On the basis of the above discussion, we argue that powder metallurgy is not a suitable process to improve the solubility of Si in MoAlB due to the thermodynamically governed solubility limit and the formation of other competing phases in the MoeAleSieB system at high sintering temperatures. Thin-film processing such as physical vapor deposition (PVD) and chemical vapor deposition (CVD) methods can be considered to obtain desirable MoAlB solid solutions at relatively low temperatures because the processing proceeds far from thermodynamic equilibrium, resulting in enhanced solubility limits. Fig. 7. Crystal structure of MoAlB viewed on the (100) plane.
Fig. 8. (a) SEM micrograph of fracture surface of dense MoAl0$97Si0$03B material, (b) Comparison of the flexural strength and fracture toughness of MoAlB and MoAl0$97Si0$03B, (c) Vickers hardness as a function of load for MoAl0$97Si0$03B and MoAlB.
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3.2. Preparation and mechanical properties of a dense MoAl0·97Si0·03B material To investigate the mechanical properties, a MoAl0$97Si0$03B bulk material has been prepared by hot pressing of the MoB/1.3Al/0.1Si mixture at 1200 C under 25 MPa for 1 h in Ar. The measured density of MoAl0$97Si0$03B bulk samples was about 93% of theoretical (6.39 g/cm3 [4]). Fig. 8a shows the microstructure of the prepared material. MoAl0.97Si0.03B has a grain size of less than 35 mm, close to that of MoAlB [11]. The XRD analysis result confirmed that MoAl0$97Si0$03B is the predominant phase, together with minor Al2O3 and MoB as impurities (XRD pattern not shown here). A histogram depicts the mechanical properties of MoAl0$97Si0$03B and MoAlB (Fig. 8b). The measured flexural strength and fracture toughness of MoAl0$97Si0$03B were about 385 MPa and 4.77 MPa m1/2, respectively. The flexural strength of the solid solution is higher than that of MoAlB (viz. 385 MPa vs 307 MPa), but the fracture toughness is slightly lower than that of MoAlB (viz. 4.77 MPa m1/2 vs 4.9 MPa m1/2). The solid solution strengthening is achieved in MoAl0$97Si0$03B material. It should be noted that each SENB sample for MoAl0$97Si0$03B and MoAlB was not fractured into two pieces after 3-piont bending test, suggesting that MoAlB and its solid solution are damage tolerant. The fracture surface of the dense sample clearly illustrates that the transgranular fracture is predominant, and the layered structure can be observed (Fig. 8a). This feature suggests that the crack paths were deflected as cracks propagated in the nanolaminated MoAl0$97Si0$03B grain, and the crack propagation energy was gradually consumed, contributing to the improvement of strength. However, grain buckling, bending, delamination, and kink bands which are always observed in the fracture surface of MAX phases [18e21,27e29], were not found in the MoAlB and MoAl0$97Si0$03B, indicating their damage tolerance is not so good as he MAX phases. Fig. 8c presents the Vickers hardness of MoAl0$97Si0$03B in the load range 1e20 kg, along with that of MoAlB for comparison. The average hardness value of MoAl0$97Si0$03B was about 14.89 GPa, higher than 8.43 GPa for MoAlB, confirming the solid solution hardening mechanism. Dominant cracks emanating from the corners of indentations were found in the MoAl0$97Si0$03B, further indicating the relative brittleness characteristic of the MoAl0$97Si0$03B material as compared with MAX phases. Enhanced strength and hardness have been achieved by the solid solution strengthening effect in the MoAl0$97Si0.03B. The Mo(Al,Si)B solid solutions are expected to improve the oxidation resistance of MoAlB at moderate temperatures and other properties at high temperatures. Future work on the investigation of such properties will be performed. 4. Conclusions MoAl1-xSixB solid solutions have been prepared by optimizing sintering conditions and starting mixtures. The influence of Si content in the starting mixture on its solubility in MoAlB has been investigated. The sintering temperatures (1100 ºC-1300 C), dwelling times (30e120 min), the type of mixtures (Mo/Al/Si/B and MoB/ Al/Si), contents of Al and Si in the starting mixtures all influence the purity of the MoAlB solid solution. The maximum solubility of Si is up to 0.91 at. %, corresponding to a solid solution of MoAl0$97Si0.03B. A nearly single phase of MoAl0.97Si0.03B can be obtained by sintering of the Mo/1.3Al/0.1Si/B and MoB/1.3Al/0.1Si mixtures at temperatures of 1200 C and 1300 C for 1 h in Ar. But the latter mixture is preferred to prepare solid solution materials, due to the reasons that MoB powder is cheaper than Mo and B powders, and reaction routes in the MoBeAleSi system are simpler than in the
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MoeAleSieB system. A dense MoAl0$97Si0$03B material with enhanced mechanical properties due to the solid solution strengthening effect has been achieved by hot pressing of the MoB/ 1.3Al/0.1Si mixture at 1200 C under 25 MPa for 1 h in Ar. Acknowledgements This work was supported by Beijing Natural Science Foundation (2182058), National Natural Science Foundation of China under Grant no. 51772020, and Beijing Government Funds for the Constructive Project of Central Universities. References [1] W. Jeitschko, Die kristallstruktur von MoA1B, Monatsch. Chem. 97 (1966) 1472e1476. [2] M. Ade, H. Hillebrecht, Ternary borides Cr2AlB2, Cr3AlB4 and Cr4AlB6: the first members of the series (CrB2)nCrAl with n ¼ 1, 2, 3 and a unifying concept for ternary borides as MAB-phases, Inorg. Chem. 54 (2015) 6122e6135. [3] J. Lu, S. Kota, M.W. Barsoum, L. Hultman, Atomic structure and lattice defects in nanolaminated ternary transition metal borides, Mater Res Lett 5 (2017) 235e241. [4] Y. Bai, X. Qi, A. Duff, N. Li, F. Kong, X. He, R. Wang, W.E. Lee, Density functional theory insights into ternary layered boride MoAlB, Acta Mater. 132 (2017) 69e81. [5] M.W. Barsoum, The Mnþ1AXn phases: a new class of solids; thermodynamically stable nanolaminates, Prog. Solid State Chem. 28 (2000) 201e281. [6] Z.M. Sun, Progress in research and development on MAX phases: a family of layered ternary compounds, Int. Mater. Rev. 56 (2011) 143e166. [7] J.Y. Wang, Y.C. Zhou, Recent progress in theoretical prediction, preparation, and characterization of layered ternary transition-metal carbides, Annu. Rev. Mater. Res. 39 (2009) 10.1e29. [8] S. Kota, E. Zapata-Solvas, A. Ly, J. Lu, O. Elkassabany, A. Huon, W.E. Lee, L. Hultman, S.J. May, M.W. Barsoum, Synthesis and characterization of an alumina forming nanolaminated boride: MoAlB, Sci. Rep. 6 (2016) 26475e26483. [9] L. Xu, O. Shi, C. Liu, D. Zhu, S. Grasso, C. Hu, Synthesis, microstructure and properties of MoAlB ceramics, Ceram. Int. 44 (2018) 13396e13401. [10] S. Kota, E. Zapata-Solvas, Y. Chen, M. Radovic, W.E. Lee, M.W. Barsoum, Isothermal and cyclic oxidation of MoAlB in air from 1100 ºC to 1400 ºC, J. Electrochem. Soc. 164 (2017) C930eC938. [11] X.G. Lu, S.B. Li, W.W. Zhang, W.B. Yu, Y. Zhou, Thermal shock behavior of a nanolaminated ternary boride: MoAlB, Ceram. Int. 45 (2019) 9386e9389. [12] G.P. Bei, S. van der Zwaag, S. Kotac, M.W. Barsoum, W.G. Sloof, Ultra-high temperature ablation behavior of MoAlB ceramics under an oxyacetylene flame, J. Eur. Ceram. Soc. 39 (2019) 2010e2017. [13] S. Basu, N. Obando, A. Gowdy, I. Karaman, M. Radovic, Long-term oxidation of Ti2AlC in air and water vapor at 1000-1300 C temperature range, J. Electrochem. Soc. 159 (2012) C90eC96. [14] S.B. Li, X.D. Chen, Y. Zhou, G.M. Song, Influence of grain size on high temperature oxidation behaviour of Cr2AlC ceramics, Ceram. Int. 39 (2013) 2715e2721. [15] S. Okada, K. Iizumi, K. Kudaka, K. Kudou, M. Miyamoto, Y. Yu, T. Lundstrӧm, Single crystal growth of (MoxCr1-x)AlB and (MoxW1-x)AlB by metal Al solutions and properties of the crystals, J. Solid State Chem. 133 (1997) 36e43. [16] Y.C. Zhou, J.X. Chen, J.Y. Wang, Strengthening of Ti3AlC2 by incorporation of Si to form Ti3Al1-xSixC2 solid solutions, Acta Mater. 54 (2006) 1317e1322. [17] M. Naguib, G.W. Bentzel, J. Shah, J. Halim, E.N. Caspi, J. Lu, L. Hultman, M.W. Barsoum, New solid solution MAX phases: (Ti0.5,V0.5)3AlC2, (Nb0.5,V0.5)2AlC, (Nb0.5,V0.5)4AlC3 and (Nb0.8,Zr0.2)2AlC, Mater. Res. Lett. 2 (2014) 233e240. [18] M.W. Barsoum, M. Ali, T. El-Raghy, Processing and characterization of Ti2AlC, Ti2AlN, and Ti2AlC0.5N0.5, Metall. Mater. Trans. A 31 (2000) 1857e1865. [19] S.B. Li, G.P. Bei, C.W. Li, M.X. Ai, H.X. Zhai, Y. Zhou, Synthesis and deformation microstructure of Ti3SiAl0.2C1.8 solid solution, Mater. Sci. Eng. A 441 (2006) 202e205. [20] H. Li, S. Li, H. Mao, Y. Zhou, Synthesis and mechanical and thermal properties of (Cr,Mn)2AlC solid solutions, Adv. Appl. Ceram. 116 (2017) 165e172. [21] W.B. Yu, S.B. Li, W.G. Sloof, Microstructure and mechanical properties of a Cr2Al(Si)C solid solution, Mater. Sci. Eng. A 527 (2010) 5997e6001. [22] G.P. Bei, V. Gauthier-Brunet, C. Tromas, S. Dubois, Synthesis, characterization, and intrinsic hardness of layered nanolaminate Ti3AlC2 and Ti3Al0.8Sn0.2C2 solid solution, J. Am. Ceram. Soc. 95 (2012) 102e107. [23] Z. Huang, H. Xu, H. Zhai, Y. Wang, Y. Zhou, Strengthening and tribological surface self-adaptability of Ti3AlC2 by incorporation of Sn to form Ti3Al(Sn)C2 solid solutions, Ceram. Int. 41 (2015) 3701e3709. €m, Crystal growth and structural investigation of the new [24] Y. Yu, T. Lundstro quaternary compound Mol-xCrxA1B with x¼ 0.39, J. Alloy. Comp. 226 (1995) 5e9. [25] J. Liu, S.B. Li, B.X. Yao, S.J. Hu, J. Zhang, Y. Zhou, Rapid synthesis and
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characterization of a nanolaminated Fe2AlB2 compound, J. Alloy. Comp. 766 (2018) 488e497. [26] M.X. Ai, H.X. Zhai, Y. Zhou, Z.Y. Tang, Z.Y. Huang, Z.L. Zhang, S.B. Li, Synthesis of Ti3AlC2 powders using Sn as an additive, J. Am. Ceram. Soc. 89 (2006) 1114e1117. [27] M. Magnuson, M. Mattesini, Chemical bonding and electronic-structure in
MAX phases as viewed by X-ray spectroscopy and density functional theory, Thin Solid Films 621 (2017) 108e130. [28] S.B. Li, H.L. Li, Y. Zhou, H.X. Zhai, Mechanism for abnormal thermal shock behavior of Cr2AlC, J. Eur. Ceram. Soc. 34 (2014) 1083e1088. [29] M.W. Barsoum, M. Radovic, Elastic and mechanical properties of the MAX phases, Annu. Rev. Mater. Res. 41 (2011) 195e227.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Processing and characterization of MoAl1-xSixB solid solutions Pengfei Ma, Shibo Li*, Jie Hu, Xiaogang Lu, Wenbo Yu, Yang Zhou Center of Materials Science and Engineering, School of Mechanical and Electronic Control Engineering, Beijing Jiaotong University, Beijing, 100044, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 May 2019 Received in revised form 2 September 2019 Accepted 13 September 2019 Available online 14 September 2019
The MAB phase MoAlB has attractive properties both at room and high temperatures. To further improve its properties, a possible way is to form solid solutions by partially substituting one of constituting elements in MoAlB. This work reports on the synthesis of MoAl1-xSixB solid solutions by powder metallurgy. Influences of sintering temperature, dwelling time, and composition of starting mixture on the solubility limit of Si in MoAlB have been investigated. The maximum solubility of Si was 0.91 at.%, corresponding to x ¼ 0.03. Increase of Si content in the starting mixtures caused the formation of other competing phases. Phase composition and microstructure were characterized by x-ray diffraction method and scanning electron microscopy. A dense MoAl0$97Si0$03B solid solution has been achieved by hot-pressing a MoB/1.3Al/0.1Si mixture at 1200 C with 25 MPa for 1 h, and its mechanical properties have been measured. An increase in strength and hardness is achieved by solid-solution strengthening. The flexural strength and the hardness of the solid solution are higher than those of MoAlB, viz. 385 MPa vs 307 MPa, and 14.89 GPa vs 8.43 GPa. © 2019 Elsevier B.V. All rights reserved.
Keywords: Ceramics Powder metallurgy Sintering Mechanical properties Microstructure
1. Introduction The ternary MoAlB phase with an orthorhomic unit cell having the lattice constants a ¼ 0.3212 nm, b ¼ 1.3985 nm, c ¼ 0.3102 nm was reported in 1966 [1]. The MoAlB compound is an attractive phase in a new group of MAB phases (where M is a transition metal, A is Al or Zn, and B is boron [2]). The crystal structure is composed of “MoeB00 slabs interleaved by two layers of Al atoms [2e4]. Its nanolaminated structure and attractive properties are similar to the so-called MAX phases [5e7]. MoAlB has attractive high temperature properties, such as good resistance to oxidation, thermal shock and ablation [8e12]. The good oxidation resistance of MoAlB is comparable to Ti2AlC and Cr2AlC MAX phases [13,14], due to the formation of an adherent and protective a-Al2O3 scale at high temperatures. In addition, MoAlB has desirable properties at room temperature. The electrical conductivity of MoAlB is about 1.49e2.86 106 U1m1 [8,15], and the thermal conductivity is 35 W/m.K at 300 K [8]. The polycrystalline MoAlB bulk ceramic has a Vickers hardness of 9e14 GPa [2,8,9,15], higher than that of MAX phases [5,6]. This is attributed to the fact that MoAlB has higher bonding strength within MoeB slabs, and between the MoeB slab and Al layers [2]. MoAlB has a fracture toughness of 4e5 MPa m1/2
* Corresponding author. E-mail address: [email protected] (S. Li). https://doi.org/10.1016/j.jallcom.2019.152290 0925-8388/© 2019 Elsevier B.V. All rights reserved.
and a flexural strength of 307e456 MPa [9,11]. Although the high temperature properties make MoAlB promising for applications in high temperature environments, yet the relatively low strength and toughness limit its potential for use in high temperature structural components calling for a combination of high strength and toughness, good damage tolerance, and excellent oxidation resistance. To improve the mechanical properties of ceramics, solid solution strengthening as one of the most effective methods has always been adopted. Solid solution materials not only exhibit superior performance compared to their counterparts, but also reveal tunable properties by adjusting the percentage of substitutional atoms. MAX solid solutions have been studied extensively through substitution on the M, A, and X sites. There are over seventies of MAX solid solutions, such as (Ti,V)3AlC2, (Nb,V)2AlC, (Cr,Mn)2AlC, Ti3(Si,Al)C2, Ti3(Al,Sn)C2, and Ti2Al(C,N) [16e23]. Their mechanical properties, thermal properties, electronic properties, and magnetic properties have been successfully tailored. However, work on MAB solid solutions has been much less focused. Only MoxCrl-xA1B and MoxWl-xA1B solid solutions by substitution on the M site have been reported, together with the discussion of hardness, oxidation resistance and electrical conductivity [15,24]. The present work is to prepare new MAB solid solutions by selective replacing of atoms on the A sites. For example, Al atoms on
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the A sites in MoAlB are partially replaced by Si to form MoAl1-xSixB solid solutions. Selection of Si as a solute is due to the following reasons. First, it has been found that Si easily replace Al in Alcontaining MAX phases to form a variety of solid solutions which have improved mechanical properties and oxidation resistance [16,19,21]. Second, it is reasonably expected that solid solution strengthening makes MoAl1-xSixB solid solutions harder and stronger. In the present study, the effect of composition of starting mixtures on the solubility limit of Si in MoAlB has also been investigated, and the mechanical properties of MoAlB and MoAl1xSixB have been compared.
SENB bars was machined with a thin diamond blade. The span size and the crosshead speed were 20 mm and 0.05 mm/min, respectively. The Vickers hardness was measured in a TH700 hardness tester in the load range 1e20 kg with a dwelling time of 15 s. Six measurements in different areas were performed for each sample to obtain an average value.
2. Experimental details
3.1.1. Effects of Al content and starting mixture on the formation of solid solutions To synthesize Al-containing MAB phases with high purity, extraAl was always used in the starting mixtures to compensate the loss of Al at high sintering temperatures and to inhibit the formation of impurities in the final products. Our previous work showed that Fe2AlB2 and MoAlB MAB phases with high purity have been successfully obtained from mixtures containing extra Al [11,25]. To synthesize MoAl1-xSixB solid solutions, the effect of Al content in the starting mixtures on their purity was firstly investigated. Two mixtures, Mo/(1e1.3)Al/B/0.1Si and MoB/(1e1.3)Al/0.1Si, were used in the present study. Fig. 1 presents the XRD patterns for the Mo/(1e1.3)Al/B/0.1Si mixtures after sintering at 1200 C for 1 h in Ar atmosphere, together with the XRD pattern of MoAlB synthesized from a Mo/ 1.3Al/B mixture at the same conditions for comparison. It was found that 1.3 mol of extra-Al in the Mo/1.3Al/B mixture leads to the high purity of MoAlB (Fig. 1a). However, except for MoAlB, MoB,
2.1. Material preparation and synthesis Mo (particle size: 300 mesh, 99.5% purity, General Research Institute for Nonferrous Metals, GRINM, China), Al (particle size: 300 mesh, 99.5% purity, Beijing Reagent Company, China), and B (particle size: 300 mesh, 99% purity, GRINM, China), Si (particle size: 300 mesh, 99.9% purity, GRINM, China), and MoB (particle size: ~10 mm, 99.9% purity, GRINM, China) powders were used as starting materials. MoAl1-xSixB solid solutions were fabricated from two kinds of mixtures, one containing Mo, Al, B, and Si powders with a molar ratio of 1:(1e1.3):1:x (x ¼ 0.1, 0.3, and 0.5), denoted as Mo/(1e1.3)Al/B/xSi; and the other containing MoB, Al and Si powders with a molar ratio of 1:(1e1.3):x (x ¼ 0.1, 0.3, and 0.5), denoted as MoB/(1e1.3)Al/xSi. The mixtures were mixed with agate balls in containers for 10 h in a rotary drum type ball-miller. The rotate speed is 150 rpm. The mixtures were cold-pressed in a stainless steel mold with 20 MPa to form F50 5 mm compacts. The compacts were pressurelessly sintered at temperatures ranging from 1100 C to 1300 C for 30e120 min in Ar atmosphere in a high temperature furnace. The heating rate was 30 C/min. Based on the above results, an optimized mixture was hotpressed at 1200 C under 25 MPa for 1 h in Ar to prepare dense bulk samples. The density of the hot-pressed samples was determined using Archimedes’ method.
3. Results and discussion 3.1. Pressureless sintering and characterization of MoAl1-xSixB
2.2. Material characterization The synthesized sample was pulverized and then sieved with a 200 mesh. The powders were used to identify the phase composition with by X-ray diffractometer (XRD) with using Cu Ka radiation (D/Max 2200 PC). The microstructure of the synthesized samples was characterized with a ZEISS EVO 18 scanning electron microscope (SEM) equipped with an energy-dispersive spectrometer system (EDS). The accelerating voltage for the EDS measurement was 20 kV, and the counting time was 60 s. 2.3. Measurement of mechanical properties The dense bulk samples were cut into bars by diamond wheel. The bar specimens with dimensions of 3 mm 4 mm 36 mm and 2 mm 4 mm 24 mm were used to measure the flexural strength and fracture toughness, respectively. All bars were ground with SiC sand papers up to 2000 grit and then ultrasonically cleaned in the ethanol bath. However, the tensile surfaces of the bars for flexural strength measurement were polished to 0.5 mm by diamond paste. The flexural strength was measured by the three-point bending test. The span size and the crosshead speed were 30 mm and 0.5 mm/min, respectively. The fracture toughness was determined by using the single edge notched beam (SENB) test. A notch of about 0.2 mm in width and about 2 mm in length in the center of the
Fig. 1. XRD patterns of (a) Mo/1.3Al/B, (b) Mo/Al/B/0.1Si, (c) Mo/1.1Al/B/0.1Si, and (d) Mo/1.3Al/B/0.1Si mixtures after sintering at 1200 C for 1 h in Ar.
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mixture to produce MoAlB, due to the reaction paths in the MoBeAleSi system are simpler than those in the MoeBeAleSi system. The considerable drop in the peak intensities of MoB with increasing Al content (Fig. 2aec) is due to the fact that MoB is consumed to form the MoAl1-xSixB solid solution. The above result further confirms that 1.3 mol of extra-Al is beneficial to produce the desirable phases of MoAlB and its solid solutions (see Table 1). Hence, the mixtures containing 1.3 mol Al were used in the following study.
Fig. 2. XRD patterns of (a) MoB/Al/0.1Si, (b) MoB/1.1Al/0.1Si, and (c) MoB/1.3Al/0.1Si mixtures after sintering at 1200 C for 1 h in Ar.
Al8Mo3, Al2O3, and SiB6 phases were detected in the samples sintered from the Mo/Al/B/0.1Si mixture (Fig. 1b). This result indicates that 1 mol of Al in the Mo/Al/B/0.1Si mixture is impossible to obtain a single solid solution phase. With increasing Al content, the peak intensities of binary phases substantially decreased and those of MoAlB correspondingly increased (Fig. 1c and d), indicating formation of a relatively high purity MoAl1-xSixB solid solution. The XRD results of the MoB/(1e1.3)Al/0.1Si mixture after sintering at 1200 C for 1 h are presented in Fig. 2. By carefully compared with Fig. 1b, Fig. 2a shows the strong peak intensities of MoAlB, but without Al8Mo3 peaks. This suggests that the direct use of MoB is more favorable than the use of Mo and B in the starting
3.1.2. Effects of Si content and starting mixture on the formation of solid solutions To determine the maximum solubility of Si in the MoAlB solid solutions, different contents of Si were used in the starting mixtures of Mo/1.3Al/B/xSi and MoB/1.3Al/xSi (x ¼ 0, 0.1, 0.3, and 0.5 mol). The XRD analysis results are presented in Fig. 3. It can be found that MoAl1-xSixB is a predominant phase after sintering of the two mixtures with 0.1 mol of Si (Fig. 3a and b). As the Si content increased from 0.3 mol to 0.5 mol, peaks belonging to Mo(Al,Si)2 and Mo2B5 appeared. Their intensities become stronger with increasing Si content, indicating the increase in content of the two phases (Fig. 3a and b). It should be noted that the peaks of MoAlB slightly shift to high angles even the Si content is up to 0.5 mol (Fig. 3c). This feature illustrates the limit solubility of Si in the MoAlB. To determine the solubility of Si, EDS point analysis was performed on at least 10 MoAlB grains with a Mo:(Al þ Si) ratio of 1:1 in different areas. The content of the light B element can't be determined accurately by EDS, hence, the measured Al/Si ratio was used to calculate the solubility of Si. If (Al þ Si) is 33.3 at. %, the Si concentration is calculated to be 0.91 at. % from the measured Al/Si ratio of 36.5, corresponding to a MoAl0$97Si0$03B solid solution. Based on the EDS results, the solubility of Si in MoAlB doesn't change as the Si powder increases from 0.1 to 0.3 mol in the starting mixtures. This suggests a limit solubility of Si in the MoAl1-xSixB solid solution. According to the XRD results, 0.1 mol of Si in the starting mixture produces an almost single MoAl0$97Si0$03B solid solution. Hence, the following study mainly investigates the effects of temperature and dwelling time on the formation of MoAl0$97Si0$03B from the Mo/1.3Al/B/0.1Si and MoB/1.3Al/0.1Si mixtures. 3.1.3. Effect of temperature on the formation of solid solutions Fig. 4 depicts the XRD patterns of the MoB/1.3Al/0.1Si mixture after sintering at various temperatures for 1 h in Ar. After sintering at 1100 C, except for MoAl0$97Si0$03B, there are an unreacted phase
Table 1 Summary of composition and processing conditions of prepared samples. Starting mixture
Molar ratio
Temperature (ºC)
Time (h)
Resulting phases
Mo/Al/B Mo/Al/B/Si
1:1.3:1 1:1:1:0.1 1:1.1:1:0.1 1:1.3:1:0.1 1:1.3:1:0.3 1:1.3:1:0.5 1:1:0.1 1:1.1:0.1 1:1.3:0.1 1:1.3:0.3 1:1.3:0.5 1:1.3:0.1 1:1.3:0.1 1:1.3:0.1 1:1.3:0.1
1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1100 1300
1 1 1 1 1 1 1 1 1 1 1 0.5 2 1 1
MoAlB(s), Al2O3(vw) SS(m), MoB(s), Al8Mo3(s), SiB6(w), Al2O3(w) SS(s), MoB(m), Al8Mo3(m), Al2O3(w) SS(s), Al8Mo3(vw), Al2O3(vw) SS(s), Mo(Al,Si)2(m), Mo2B5 (w), Al2O3(w) SS(s), Mo(Al,Si)2(s), Mo2B5 (s), Al2O3(w) SS(s), MoB(s), Al2O3(w), SiB6(w) SS(s), MoB(m), Al2O3(w), SiB6(w) SS(s), MoB(vw), Al2O3(vw) SS(s), Mo(Al,Si)2(m), Mo2B(m), Al2O3(w) SS(s), Mo(Al,Si)2(s), Mo2B(s), Al2O3(w) SS(s), MoB(vw), Al2O3(vw) SS(s), MoB(vw), Al2O3(vw) SS(s), MoB(s), Al8Mo3(m), Al2O3(w) SS(s), MoB(vw), Al2O3(vw)
MoB/Al/Si
Symbols mean: “SS” MoAl0$97Si0$03B Solid Solution; “s” strong; “m” medium; “w” weak; and “vw” very weak.
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Fig. 3. XRD patterns of (a) Mo/1.3Al/B/xSi, and (b) MoB/1.3Al/xSi (x ¼ 0, 0.1, 0.3, and 0.5) mixtures after sintering at 1200 C for 1 h in Ar, (c) Enlarged XRD patterns taken from (a).
of MoB and a newly formed intermetallic phase of Al8Mo3. By contrast, sintering at 1200 C and 1300 C results in the formation of MoAl0$97Si0$03B as the predominant phase, with minor
impurities. The XRD patterns of the Mo/1.3Al/B/0.1Si mixture after sintering at the same conditions as for MoB/1.3Al/0.1Si showed similar phase constitutions.
Fig. 4. XRD patterns of MoB/1.3Al/0.1Si mixture after sintering at temperatures ranging from 1100 C to 1300 C for 1 h in Ar.
Fig. 5. XRD patterns of MoB/1.3Al/0.1Si mixture after sintering at 1200 C for 30e120 min in Ar.
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The above results confirmed that the MoAl0$97Si0$03B solid solution can be obtained by pressureless sintering the Mo/1.3Al/B/ 0.1Si and MoB/1.3Al/0.1Si mixtures at both 1200 C and 1300 C for 1 h (Table 1). 3.1.4. Effect of dwelling time on the formation of solid solutions The effect of dwelling time on the solid solution formation has also been investigated. After sintering of the MoB/1.3Al/0.1Si mixture at 1200 C for only 0.5 h, a predominant phase of MoAl0$97Si0$03B was obtained, together with minor Al2O3 and MoB (Fig. 5). As the sintering period was prolonged from 1 to 2 h, the phase composition didn't change so much (Fig. 5). EDS results demonstrated that the content of Si in the samples sintered at different temperatures (1200e1300 C) and times (0.5e2 h) changed slightly, indicating the limit solubility of Si in MoAlB. Table 1 summarizes the composition and processing conditions of the prepared samples. On the basis of the above results, the MoAl0$97Si0$03B solid solution can be obtained by sintering of the Mo/1.3Al/0.1Si and MoB/1.3Al/0.1Si mixtures at 1200 C for 1 h in Ar. With consideration of the low cost of product, the latter mixture is preferred. 3.1.5. Microstructural characterization SEM observation on the polished surface of the sample sintered at 1200 C for 1 h from the MoB/1.3Al/0.1Si mixture reveals that the MoAl0$97Si0$03B solid solution grains are less than 10 mm in size (Fig. 6a). It was found that the samples prepared by pressurelessly
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sintering of Mo/1.3Al/B/0.1Si and MoB/1.3Al/0.1Si mixtures have small grains, indicating the slow growth of MoAl0$97Si0$03B at 1200 C. Undamaged grains in the pore areas exhibit round or plate-like shapes and smooth surfaces (Fig. 6a). The backscattered SEM observation (Fig. 6b) shows that small black areas are mainly composed of Al2O3, identified by EDS. The Al2O3 impurity was always found in the Al-containing MAX and MAB phases due to the reaction of Al with O absorbed on the powder surfaces [8,11,18,20,21,26]. The EDS line analysis demonstrates the presence of Mo, Al, Si, and B elements in the analyzed area (right-hand inset of Fig. 6c). The elemental composition of Si along a line (left-hand inset of Fig. 6c) keeps almost constant, suggesting the uniform distribution of Si in the solid solution grain. Fig. 7 presents the crystal structure of MoAlB on the (100) plane, in which MoeB slabs are interleaved by two layers of Al atoms. Some Si atoms replace Al atoms and occupy their positions to form a solid solution. The MoAlB MAB phase and the MAX phases have a similar layered structure. Theoretically, the solubility of Si in MoAlB should be larger than that in MAX phases, due to the crystal structure of MoAlB with bilayers of Al atoms while that of MAX with only one layer of Al atoms interleaving MX blocks. However, the experimental results showed that the maximum solubility of Si was only 0.91 at. % in the present study. Moreover, impurities were found after sintering of the MoeAleB mixture even with 0.1 mol of Si. By contrast, the solubility of Si in Al-containing MAX solid solutions can be tunable. For example, a Ti3Al0$75Si0$25C2 solid solution with 4.2% of Si was achieved without any impurities after hot-
Fig. 6. (a) SEM micrograph of the polished surface of sample sintered at 1200 C for 1 h in Ar, (b) Enlarged backscattered SEM micrograph of the polished surface, (c) EDS line analysis for the elemental distribution along a line shown in the left-hand inset. The right-hand inset shows the presence of only Mo, Al, Si and B.
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pressing of a 3Ti/0.75Al/0.25Si/2C mixture [16]. The solubility of Si in the MoAlB MAB phase and the MAX phases may be dependent on their crystal structures and bonding energies. MoAlB has an orthorhombic crystal structure, while the MAX phases have a hexagonal crystal structure. Theoretical calculations showed that the bond lengths of MoeAl and AleAl in MoAlB are about 0.2693 nm and 0.2647 nm, respectively [2], shorter than 0.2885 nm of TieAl in the Ti3AlC2 MAX phase [27], suggesting the higher bonding energy in MoAlB. It has also been proved that the solubility of Si in Cr2AlC and Ti3AlC2, even both belonging to the same MAX phase family, is different. The former can dissolve a small amount of Si due to the higher bonding strength between the CreC blocks and Al layer in Cr2AlC [21]. On the basis of the above discussion, we argue that powder metallurgy is not a suitable process to improve the solubility of Si in MoAlB due to the thermodynamically governed solubility limit and the formation of other competing phases in the MoeAleSieB system at high sintering temperatures. Thin-film processing such as physical vapor deposition (PVD) and chemical vapor deposition (CVD) methods can be considered to obtain desirable MoAlB solid solutions at relatively low temperatures because the processing proceeds far from thermodynamic equilibrium, resulting in enhanced solubility limits. Fig. 7. Crystal structure of MoAlB viewed on the (100) plane.
Fig. 8. (a) SEM micrograph of fracture surface of dense MoAl0$97Si0$03B material, (b) Comparison of the flexural strength and fracture toughness of MoAlB and MoAl0$97Si0$03B, (c) Vickers hardness as a function of load for MoAl0$97Si0$03B and MoAlB.
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3.2. Preparation and mechanical properties of a dense MoAl0·97Si0·03B material To investigate the mechanical properties, a MoAl0$97Si0$03B bulk material has been prepared by hot pressing of the MoB/1.3Al/0.1Si mixture at 1200 C under 25 MPa for 1 h in Ar. The measured density of MoAl0$97Si0$03B bulk samples was about 93% of theoretical (6.39 g/cm3 [4]). Fig. 8a shows the microstructure of the prepared material. MoAl0.97Si0.03B has a grain size of less than 35 mm, close to that of MoAlB [11]. The XRD analysis result confirmed that MoAl0$97Si0$03B is the predominant phase, together with minor Al2O3 and MoB as impurities (XRD pattern not shown here). A histogram depicts the mechanical properties of MoAl0$97Si0$03B and MoAlB (Fig. 8b). The measured flexural strength and fracture toughness of MoAl0$97Si0$03B were about 385 MPa and 4.77 MPa m1/2, respectively. The flexural strength of the solid solution is higher than that of MoAlB (viz. 385 MPa vs 307 MPa), but the fracture toughness is slightly lower than that of MoAlB (viz. 4.77 MPa m1/2 vs 4.9 MPa m1/2). The solid solution strengthening is achieved in MoAl0$97Si0$03B material. It should be noted that each SENB sample for MoAl0$97Si0$03B and MoAlB was not fractured into two pieces after 3-piont bending test, suggesting that MoAlB and its solid solution are damage tolerant. The fracture surface of the dense sample clearly illustrates that the transgranular fracture is predominant, and the layered structure can be observed (Fig. 8a). This feature suggests that the crack paths were deflected as cracks propagated in the nanolaminated MoAl0$97Si0$03B grain, and the crack propagation energy was gradually consumed, contributing to the improvement of strength. However, grain buckling, bending, delamination, and kink bands which are always observed in the fracture surface of MAX phases [18e21,27e29], were not found in the MoAlB and MoAl0$97Si0$03B, indicating their damage tolerance is not so good as he MAX phases. Fig. 8c presents the Vickers hardness of MoAl0$97Si0$03B in the load range 1e20 kg, along with that of MoAlB for comparison. The average hardness value of MoAl0$97Si0$03B was about 14.89 GPa, higher than 8.43 GPa for MoAlB, confirming the solid solution hardening mechanism. Dominant cracks emanating from the corners of indentations were found in the MoAl0$97Si0$03B, further indicating the relative brittleness characteristic of the MoAl0$97Si0$03B material as compared with MAX phases. Enhanced strength and hardness have been achieved by the solid solution strengthening effect in the MoAl0$97Si0.03B. The Mo(Al,Si)B solid solutions are expected to improve the oxidation resistance of MoAlB at moderate temperatures and other properties at high temperatures. Future work on the investigation of such properties will be performed. 4. Conclusions MoAl1-xSixB solid solutions have been prepared by optimizing sintering conditions and starting mixtures. The influence of Si content in the starting mixture on its solubility in MoAlB has been investigated. The sintering temperatures (1100 ºC-1300 C), dwelling times (30e120 min), the type of mixtures (Mo/Al/Si/B and MoB/ Al/Si), contents of Al and Si in the starting mixtures all influence the purity of the MoAlB solid solution. The maximum solubility of Si is up to 0.91 at. %, corresponding to a solid solution of MoAl0$97Si0.03B. A nearly single phase of MoAl0.97Si0.03B can be obtained by sintering of the Mo/1.3Al/0.1Si/B and MoB/1.3Al/0.1Si mixtures at temperatures of 1200 C and 1300 C for 1 h in Ar. But the latter mixture is preferred to prepare solid solution materials, due to the reasons that MoB powder is cheaper than Mo and B powders, and reaction routes in the MoBeAleSi system are simpler than in the
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MoeAleSieB system. A dense MoAl0$97Si0$03B material with enhanced mechanical properties due to the solid solution strengthening effect has been achieved by hot pressing of the MoB/ 1.3Al/0.1Si mixture at 1200 C under 25 MPa for 1 h in Ar. Acknowledgements This work was supported by Beijing Natural Science Foundation (2182058), National Natural Science Foundation of China under Grant no. 51772020, and Beijing Government Funds for the Constructive Project of Central Universities. References [1] W. Jeitschko, Die kristallstruktur von MoA1B, Monatsch. Chem. 97 (1966) 1472e1476. [2] M. Ade, H. Hillebrecht, Ternary borides Cr2AlB2, Cr3AlB4 and Cr4AlB6: the first members of the series (CrB2)nCrAl with n ¼ 1, 2, 3 and a unifying concept for ternary borides as MAB-phases, Inorg. Chem. 54 (2015) 6122e6135. [3] J. Lu, S. Kota, M.W. Barsoum, L. Hultman, Atomic structure and lattice defects in nanolaminated ternary transition metal borides, Mater Res Lett 5 (2017) 235e241. [4] Y. Bai, X. Qi, A. Duff, N. Li, F. Kong, X. He, R. Wang, W.E. Lee, Density functional theory insights into ternary layered boride MoAlB, Acta Mater. 132 (2017) 69e81. [5] M.W. Barsoum, The Mnþ1AXn phases: a new class of solids; thermodynamically stable nanolaminates, Prog. Solid State Chem. 28 (2000) 201e281. [6] Z.M. Sun, Progress in research and development on MAX phases: a family of layered ternary compounds, Int. Mater. Rev. 56 (2011) 143e166. [7] J.Y. Wang, Y.C. Zhou, Recent progress in theoretical prediction, preparation, and characterization of layered ternary transition-metal carbides, Annu. Rev. Mater. Res. 39 (2009) 10.1e29. [8] S. Kota, E. Zapata-Solvas, A. Ly, J. Lu, O. Elkassabany, A. Huon, W.E. Lee, L. Hultman, S.J. May, M.W. Barsoum, Synthesis and characterization of an alumina forming nanolaminated boride: MoAlB, Sci. Rep. 6 (2016) 26475e26483. [9] L. Xu, O. Shi, C. Liu, D. Zhu, S. Grasso, C. Hu, Synthesis, microstructure and properties of MoAlB ceramics, Ceram. Int. 44 (2018) 13396e13401. [10] S. Kota, E. Zapata-Solvas, Y. Chen, M. Radovic, W.E. Lee, M.W. Barsoum, Isothermal and cyclic oxidation of MoAlB in air from 1100 ºC to 1400 ºC, J. Electrochem. Soc. 164 (2017) C930eC938. [11] X.G. Lu, S.B. Li, W.W. Zhang, W.B. Yu, Y. Zhou, Thermal shock behavior of a nanolaminated ternary boride: MoAlB, Ceram. Int. 45 (2019) 9386e9389. [12] G.P. Bei, S. van der Zwaag, S. Kotac, M.W. Barsoum, W.G. Sloof, Ultra-high temperature ablation behavior of MoAlB ceramics under an oxyacetylene flame, J. Eur. Ceram. Soc. 39 (2019) 2010e2017. [13] S. Basu, N. Obando, A. Gowdy, I. Karaman, M. Radovic, Long-term oxidation of Ti2AlC in air and water vapor at 1000-1300 C temperature range, J. Electrochem. Soc. 159 (2012) C90eC96. [14] S.B. Li, X.D. Chen, Y. Zhou, G.M. Song, Influence of grain size on high temperature oxidation behaviour of Cr2AlC ceramics, Ceram. Int. 39 (2013) 2715e2721. [15] S. Okada, K. Iizumi, K. Kudaka, K. Kudou, M. Miyamoto, Y. Yu, T. Lundstrӧm, Single crystal growth of (MoxCr1-x)AlB and (MoxW1-x)AlB by metal Al solutions and properties of the crystals, J. Solid State Chem. 133 (1997) 36e43. [16] Y.C. Zhou, J.X. Chen, J.Y. Wang, Strengthening of Ti3AlC2 by incorporation of Si to form Ti3Al1-xSixC2 solid solutions, Acta Mater. 54 (2006) 1317e1322. [17] M. Naguib, G.W. Bentzel, J. Shah, J. Halim, E.N. Caspi, J. Lu, L. Hultman, M.W. Barsoum, New solid solution MAX phases: (Ti0.5,V0.5)3AlC2, (Nb0.5,V0.5)2AlC, (Nb0.5,V0.5)4AlC3 and (Nb0.8,Zr0.2)2AlC, Mater. Res. Lett. 2 (2014) 233e240. [18] M.W. Barsoum, M. Ali, T. El-Raghy, Processing and characterization of Ti2AlC, Ti2AlN, and Ti2AlC0.5N0.5, Metall. Mater. Trans. A 31 (2000) 1857e1865. [19] S.B. Li, G.P. Bei, C.W. Li, M.X. Ai, H.X. Zhai, Y. Zhou, Synthesis and deformation microstructure of Ti3SiAl0.2C1.8 solid solution, Mater. Sci. Eng. A 441 (2006) 202e205. [20] H. Li, S. Li, H. Mao, Y. Zhou, Synthesis and mechanical and thermal properties of (Cr,Mn)2AlC solid solutions, Adv. Appl. Ceram. 116 (2017) 165e172. [21] W.B. Yu, S.B. Li, W.G. Sloof, Microstructure and mechanical properties of a Cr2Al(Si)C solid solution, Mater. Sci. Eng. A 527 (2010) 5997e6001. [22] G.P. Bei, V. Gauthier-Brunet, C. Tromas, S. Dubois, Synthesis, characterization, and intrinsic hardness of layered nanolaminate Ti3AlC2 and Ti3Al0.8Sn0.2C2 solid solution, J. Am. Ceram. Soc. 95 (2012) 102e107. [23] Z. Huang, H. Xu, H. Zhai, Y. Wang, Y. Zhou, Strengthening and tribological surface self-adaptability of Ti3AlC2 by incorporation of Sn to form Ti3Al(Sn)C2 solid solutions, Ceram. Int. 41 (2015) 3701e3709. €m, Crystal growth and structural investigation of the new [24] Y. Yu, T. Lundstro quaternary compound Mol-xCrxA1B with x¼ 0.39, J. Alloy. Comp. 226 (1995) 5e9. [25] J. Liu, S.B. Li, B.X. Yao, S.J. Hu, J. Zhang, Y. Zhou, Rapid synthesis and
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