Mechanical properties of Mo-Nb-Si-B quaternary alloy fabricated by powder metallurgical method

Mo-Si-B alloys composed of two intermetallic compound phases (Mo₅SiB₂ and Mo₃Si) and a molybdenum solid solution matrix phase have been investigated for use as high-temperature structural materials due to their high melting point and good creep resistance. However, despite these advantages, Mo-Si-B alloys are difficult to use in practical applications because they have insufficient fracture toughness at room temperature. So, in many researches, microstructure control and the addition of other elements in the α-Mo matrix phase are conducted as an effective way to improve the fracture toughness.In this study, niobium (Nb) was added to a Mo-Si-B alloy by a powder metallurgical method to improve the mechanical properties. First, the Mo and Nb powders were pulverized by high-energy ball milling. Then, the synthesized intermetallic compound powders, which were fabricated by continuous heat treatment under a H₂ atmosphere, were mixed with ball-milled Mo and Nb powder. Pressureless sintering was conducted at 1400 °C for 3 h under a H₂ atmosphere. The Vickers hardness and fracture toughness were measured to investigate the mechanical properties of the sintered Mo-Si-B and Mo-Nb-Si-B alloy. The Vickers hardness was about 425 Hv for a Mo-Nb-Si-B alloy, which was lower value of 165 Hv compared to Mo-Si-B alloy (590 Hv). On the other hand, the fracture toughness of the Mo-Nb-Si-B alloy (14.5 MPa·√m) greatly increased compared to that of the Mo-Si-B alloy (12.6 MPa·√m).     

Fabrication of Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> Dispersion-Strengthened Mo-Si-B Alloy by Powder Metallurgical Method

In this study, we investigate the effect of oxide dispersion strengthening on mechanical properties by dispersion of nano-sized Ta₂O₅ particles in Mo-Si-B alloy. A Mo-Si-B core-shell powder consisting of two intermetallic compounds of Mo₅SiB₂ and Mo₃Si as the core and nano-sized Mo solid solution surrounding intermetallic compounds was fabricated by chemical vapor transport. And Mo-Si-B core-shell powder with uniformly dispersed nano-sized Ta₂O₅ particles on the surface of a Mo solid solution shell was produced by a wet blending process with TaCl₅ solution and heat treatment. Then, pressureless sintering was performed at 1400°C for 3 h under a H₂ atmosphere. The hardness and fracture toughness of the Ta₂O₅-dispersed Mo-Si-B alloy were measured using Vickers hardness and 3-point bending tests, respectively. The Vickers hardness and fracture toughness of the fabricated Mo-Si-B-Ta₂O₅ alloy were more improved than that of the Mo-Si-B alloy fabricated using core-shell powder with no addition of Ta₂O₅ particles (Mo-Si-B alloy: 353 Hv, 13.5 MPa·√m, Mo-Si-B-Ta₂O₅ alloy: 509 Hv, 15.1 MPa·√m).     

Mechanical properties of Mo-Si-B alloys fabricated by using core-shell powder with dispersion of yttria nanoparticles

In recent years, refractory materials with excellent high-temperature properties have been in the spotlight as a next generation’s high-temperature materials. Among these, Mo-Si-B alloys composed of two intermetallic compound phases (Mo₅SiB₂ and Mo₃Si) and a ductile α-Mo phase have shown an outstanding thermal properties. However, due to the brittleness of the intermetallic compound phases, Mo-Si-B alloys were restricted to apply for the structural materials. So, to enhance the mechanical properties of Mo-Si-B alloys, many efforts to add rare-earth oxide particles in the Mo-Si-B alloy were performed to induce the improvement of strength and fracture toughness. In this study, to investigate the effect of adding nano-sized Y₂O₃ particles in Mo-Si-B alloy, a core-shell powder consisting of intermetallic compound phases as the core and nano-sized α-Mo and Y₂O₃ particles surrounding the core was fabricated. Then pressureless sintering was carried out at 1400 °C for 3 h, and the mechanical properties of sintered bodies with different amounts of Y₂O₃ particles were evaluated by Vickers hardness and 3-point bending test. Vickers hardness was improved by dispersed Y₂O₃ particles in the Mo-Si-B alloy. Especially, Mo-3Si-1B-1.5Y₂O₃ alloy had the highest value, 589 Hv. The fracture toughness was measured using Mo-3Si-1B-1.5Y₂O₃ alloy and the value indicated as 13.5 MPa·√m.     

Effect of titanium addition on mechanical properties of Mo-Si-B alloys

In this study, we investigated the effect of titanium addition on microstructure and mechanical properties in Mo-Si-B alloys. The Mo-Ti-Si-B alloy (Mo-3.9Ti-3Si-1B, wt%), which has α-Mo, Mo₃Si, Mo₅SiB₂ and TiO₂ phases_, was fabricated by a powder metallurgy (PM) method. The starting materials were pulverized by using a high-energy ball milling and the resultant powder was subjected to a reduction process followed by cold isostatic pressing (CIP) compaction and pressureless sintering. In the microstructure, intermetallic compound phases were uniformly distributed in the α-Mo matrix. Some titanium atoms solved into the α-Mo matrix and the others formed a TiO₂ phase caused by reaction with oxygen at the grain boundary. Fracture toughness of the Mo-Ti-Si-B sintered body was recorded as 10.42 MPa·m^1/2, which is lower than that of the Mo-Si-B sintered body without addition of titanium. In the Mo-Ti-Si-B sintered body, the fracture mode is similar to the Mo-Si-B sintered body where intergranular fracture through the Mo grain boundary and transgranular fracture cross the intermetallic compound phase. The decrease of fracture toughness is due to the relatively large TiO₂ at the grain boundary, promoting intergranular fracture.     

The microstructural engineering of Mo-Si-B alloys produced by reaction synthesis

Mo-Si-B alloys are candidate materials for next-generation jet engine turbine blades and have the potential to increase the service temperature of the base metals 200°C higher than nickel superalloys. These refractory alloys form a composite microstructure of molybdenum solid Solution (Mo_ss and two intermetallic phases, Mo₃Bi and Mo₅SiB₂, where the Mo_ss phase enhances toughness and the intermetallic phases provide oxidation resistance. The properties of the alloys are highly dependent on the morphology of the microstructure. A powder processing approach has been developed to synthesize the three-phase alloys through the reaction of molybdenum, Si₃N₄ and BN powders. Electron backscatter diffraction imaging has been used to map the location of individual phases and provide a method for quantifying the Cluster size distribution of a secondary phase to examine the effect of BN reactant powders on the dispersion of the intermetallic phases.     

Volume and size effects of intermetallic compounds on the high-temperature oxidation behavior of Mo-Si-B alloys

In this study, we investigated the high-temperature oxidation behavior of Mo-Si-B alloys with different volume fractions or sizes of intermetallic compound phases. Mo-Si-B alloys with uniformly dispersed intermetallic compound phases (Mo₅SIB₂ and Mo₃Si) in Mo solid solution matrix phase were fabricated using a novel powder metallurgical route, as introduced in our previous study. An isothermal oxidation test was conducted at 1300 °C for up to 10 h. The high-temperature oxidation resistance of Mo-Si-B alloys improved by increasing the volume fraction of intermetallic compound phases; this was a result of the increased amount of protective oxidized layers, which protect the Mo_ss phase from oxidation by covering the surface. In addition, Mo-Si-B alloy with smaller intermetallic compound phases pulverized by high-energy ball milling had better high-temperature oxidation resistance compared to Mo-Si-B alloy with as-synthesized intermetallic compound phases.     

Mo-Si-B三元系中T2相合金制备及其室温力学性能

通过放电等离子烧结(SPS)制备T2(Mo5SiB2)相合金,并采用SEM、XRD及压痕、压缩和三点弯曲等实验对合金的微观组织和室温力学性能进行表征。结果表明:SPS法以独特的等离子活化烧结方式制备出纯度高、致密且晶粒细小的T2相合金。该合金在室温压缩下几乎没有塑性变形,抗压强度为2907MPa;维氏硬度为17.86GPa,压痕法测得的断裂韧性为3.23MPa·m1/2,与三点弯曲法测得的3.34MPa·m1/2接近,沿{001}面发生解理断裂。共价键交替排列,是T2相室温脆性、高强度、高硬度的根本原因。     

Microstructure,sintering behavior and mechanical properties of SiC/MoSi2 composites by spark plasma sintering

SiC/MoSi₂ composites were synthesized at different temperatures by spark plasma sintering using Mo, Si and SiC powders as raw materials. The phase composition, microstructure and mechanical properties of the as-prepared composites were investigated and the sintering behavior was also discussed. Results show that SiC/MoSi₂ composites are composed of MoSi₂, SiC and trace amount of Mo_4.8Si₃C_0.6 phase and exhibit a fine-grain texture. During the synthesis process, there was an evolution from solid phase sintering to liquid phase sintering. When sintered at 1600 °C, the SiC/MoSi₂ composites present the most favorable mechanical properties, the Vickers hardness, bending strength and fracture toughness are 13.4 GPa, 674 MPa and 5.1 MPa·m^1/2, respectively, higher 44%, 171%, 82% than those of monolithic MoSi₂. SiC can withstand the applied stress as hard phase and retard the rapid propagation of cracks as second phase, which are beneficial to the improved mechanical properties of SiC/MoSi₂ composites.     

球磨时间和热压温度对粉末冶金Nb-16Si-22Ti-2Al-2Hf-2Cr合金显微组织和室温力学性能的影响

采用粉末冶金法制备了Nb-16Si-22Ti-2Al-2Hf-2Cr合金,研究了粉末球磨时间（5、10、20 h）及热压烧结温度（1500、1600 ℃）对合金组织和室温力学性能的影响。结果表明：热压烧结后的合金由Nb基固溶体NbSS、Ti基固溶体TiSS和硅化物Nb5Si3三相组成。随着球磨时间的延长,Nb5Si3和TiSS的含量增加,而NbSS的含量减少。室温硬度随球磨时间延长和热压烧结温度的升高而提高,20 h/1600 ℃热压烧结合金硬度值最高,HV硬度达到11500 MPa。1500和1600 ℃热压烧结下合金的断裂韧性随着粉末球磨时间的延长均呈下降的趋势,5 h/1500 ℃热压烧结合金断裂韧性值最高,为10.14 MPa·m1/2。     

Comparative studies on densification behavior,microstructure, mechanical properties and oxidation resistance of Mo-12Si-10B and Mo3Si-free Mo-26Nb-12Si-10B alloys

Two bulk Mo-Si-B based alloys (Mo-12Si-10B and Mo-26Nb-12Si-10B (at.%), abbreviated as 0Nb and 26Nb alloy respectively) were fabricated by mechanical alloying and then hot pressing. Comparative studies were carried out on the densification behavior, microstructure, room-temperature fracture toughness, elevated temperature compression and oxidation resistance of these two alloys. The results showed that alloy 0Nb was composed of (Mo), Mo₃Si and Mo₅SiB₂, while alloy 26Nb was free of Mo₃Si and had higher (Mo) content and a little γNb₅Si₃. Compared to the alloy 0Nb, alloy 26Nb presented better compactibility, higher room-temperature fracture toughness (8.84 ± 0.17 vs. 6.77 ± 0.20 MPa·m^1/2) and elevated temperature compression strength (851.7 ± 11.7 vs. 644.2 ± 10.2 MPa) but worse oxidation resistance.     

Influence of consolidation process and sintering temperature on microstructure and mechanical properties of near nano- and nano-structured WC-Co cemented carbides

In this paper the influence of the consolidation process and sintering temperature on the properties of near nano- and nano-structured cemented carbides was researched. Samples were consolidated from a WC 9-Co mixture by two different powder metallurgy processes; conventional sintering in hydrogen and the sinter-HIP process. Two WC powders with different grain growth inhibitors were selected for the research. Both WC powders used were near nanoscaled and had a grain size of 150 nm and a specific surface area of 2.5 m²/g. Special emphasis was placed on microstructure and mechanical properties; hardness and fracture toughness of sintered samples. Consolidated samples are characterised by different microstructural and mechanical properties with respect to the sintering temperature, the consolidation process used and grain growth inhibitors in starting powders. Increasing sintering temperature leads to microstructure irregularities and inferior hardness, especially for samples sintered in hydrogen. The addition of Cr₃C₂ in the starting powder reduced a carbide grain growth during sintering, improved microstructural characteristics, increased Vickers hardness and fracture toughness. The relationship between hardness and fracture toughness is not linear. Palmqvist toughness does not change with regard to sintering temperature or the change of Vickers hardness.     

Synthesis of dense MoSi2 by high-frequency induction heated combustion and its mechanical properties

Dense MoSi₂ compound was synthesized with the high-frequency induction heated combustion synthesis method in one step from elemental powders of Mo and Si within 2 min. Simultaneous combustion synthesis and densification were carried out under the combined effects of induced current and mechanical pressure. A highly dense MoSi₂ with a relative density of up to 98% was produced with simultaneous application of 60 MPa pressure and induced current. The percentages of the total shrinkage occurring before and during the synthesis reaction were 16% and 53%, respectively. The average grain size was about 15 μm and a slight amount of Mo₅Si₃ was observed at the boundaries of the MoSi₂ grains. The fracture toughness and hardness values obtained were 3.5 MPa·m^1/2 and 1050 kg/mm², respectively. These values were similar to those of commercial ones.     

Correlation between microstructure and properties of fine grained Mo–Mo3Si–Mo5SiB2 alloys

Three phase α-Mo–Mo₃Si–Mo₅SiB₂ alloys of various compositions, namely Mo–6Si–5B, Mo–9Si–8B, Mo–10Si–10B and Mo–13Si–12B (at.%) were processed by a powder metallurgical (PM) route. Increasing the Si and B concentration in these Mo–Si–B alloys resulted in increasing volume fractions of the intermetallic phases Mo₃Si (A15) and Mo₅SiB₂ (T2) and the distribution of the three phases present in these alloys was dependent on the volume fractions of the individual phases. Above volume fractions of about fifty percent, bcc Mo solid solution (α-Mo) formed the matrix. Consequently, Mo–6Si–5B and Mo–9Si–8B alloys, which possessed a continuous α-Mo matrix provided increased fracture toughness at ambient temperatures. Additionally, a decreased BDTT of about 950 °C is caused by the homogeneous α-Mo matrix. In contrast, Mo–13Si–12B with 65 vol.% of the intermetallic phases that formed the matrix phase in this material had a BDTT value higher than 1100 °C, while the strength at elevated temperatures up to 1300 °C was significantly increased compared to alloys that have the α-Mo matrix. Alloy compositions with ≥50 vol.% of intermetallic phases (corresponding to alloys containing a minimum of 9 at.% Si and 8 at.% B) were oxidation resistant with minimal mass loss under cyclic conditions for 150 h at 1100 °C due to the formation of a dense borosilicate glass layer that protects the material surface.     

Microstructure and creep properties of a near-eutectic directionally solidified multiphase Mo–Si–B alloy

Due to their excellent creep behavior and acceptable oxidation resistance at ultrahigh temperatures multiphase Mo-based alloys are potential candidates for applications in aerospace engines and the power generating industry. The resulting materials properties, as well as the microstructure of Mo–Si–B materials, strongly depend on the manufacturing process. In the following paper we report on a new Mo–Si–B alloy which was processed by crucible-free zone melting (ZM) from cold pressed elemental powders. SEM investigations of the zone molten microstructure showed well-aligned arrangements of a three-phase microstructure consisting of a Mo solid solution (Mo_SS), and the two intermetallic phases Mo₃Si and Mo₅SiB₂. First, high temperature mechanical properties, such as the compressive strength and creep strength at about 1100 °C, were evaluated and compared with a commonly used Ni-based superalloy and a PM processed Mo–Si–B material. In comparison to the PM processed reference alloy, the creep resistance of ZM materials was found to be substantially improved due to the relatively coarse directionally solidified microstructure. Thus, ZM alloys show great potential for applications at targeted application temperatures of around 1200–1300 °C.     

Effects of sintering additives on microstructure and mechanical properties of TiB2-WC ceramic-metal composite tool materials

TiB₂-WC ceramic-metal composite tool materials were fabricated using Co, Ni and (Ni, Mo) as sintering additives by vacuum hot-pressing technique. The microstructure and mechanical properties of the composite were investigated. The composite was analyzed by the observations of scanning electron microscope (SEM), X-ray diffraction (XRD) and energy dispersive spectrometry (EDS). The microstructure of TiB₂-WC ceramic-metal composites consisted of the fine WC grains and uniform TiB₂ grains. The brittle phase of Ni₃B₄ and a few pores were found in TiB₂-WC-Ni ceramic-metal composite. A lot of pores and brittle phases such as W₂CoB₂ and Co₂B were formed in TiB₂-WC-Co ceramic-metal composite. The liquid phase of Co was consumed by the reaction which led to the formation of the pores and the coarse grains of TiB₂. The pores, brittle phases and coarse grains of TiB₂ were harmful to the improvement of the mechanical properties of the composite. The sintering additive of (Ni, Mo) had a significant effect on the density and the mechanical properties of TiB₂-WC ceramic-metal composite. The formation of intermetallic compound of MoNi₄ inhibited the consumption of liquid phase of (Ni, Mo). The liquid phase of (Ni, Mo) not only inhibited the formation of the pores and the coarse grains of TiB₂ but also strengthened the interface energy between WC and TiB₂ grains. The grain size was fine and the average relative density of TiB₂-WC-(Ni, Mo) ceramic-metal composite reached 99.1%. The flexural strength, fracture toughness and Vickers hardness of TiB₂-WC-(Ni, Mo) ceramic-metal composite were 1307.0 ± 121.4 MPa, 8.19 ± 0.29 MPa m^1/2 and 22.71 ± 0.82 GPa, respectively.     

2％C／MoSi2复合材料的组织结构与性能

采用热压烧结工艺制得了2％C／MoSi2（质量分数）复合材料,并测定了材料的显微组织和结构、室温和高温力学性能、耐磨性能以及电阻率。结果:C／MoSi2复合材料由大量的MoSi2、多量的Mo5Si3和少量的β－SiC组成,其硬度Hv为1060,抗弯强度为470MPa,断裂韧性为5．12MPa.m^1/2,800℃的硬度Hv为750,1200℃的抗压强度为450MPa,1400℃的抗压强度为142MPa;在Al2O3和SiC磨盘上表现出优异的耐磨性能,材料的电阻率为349n.m。与纯MoSi2相比,2％C／MoSi2复合材料在硬度、抗弯强度、断裂性、高温抗压强度、弹性模量和耐磨性能等方面都有较大的提高。     

Mechanical properties of spark plasma sintered Nb–Al compacts strengthened by dispersion of Nb2N phase and additions of Mo and W

Mechanically alloyed and blended Nb–Al–N powders were sintered by the spark plasma sintering process, and their microstructure and mechanical properties were investigated. All of the Nb–Al–N compacts consisted of phases in the Nb–Al system in which the Nb₂N phase was dispersed. The microstructure of blended powder compacts was much coarser than that of mechanically alloyed powder compacts. The compacts obtained by sintering powder produced by crushing blended powder compacts have finer microstructure, higher hardness, and higher fracture toughness than blended powder compacts. The strength of Nb–Al–N compacts increases with increasing the fraction of AlN added to the Nb powder, while their fracture toughness at room temperature decreases. As for the Nb–Al–Mo and Nb–Al–W system, the effect of solid-solution hardening of W was larger than that of Mo, and Nb–15Al–40Mo compact has the highest strength at room temperature and 1273 K among Nb–15Al–xMo compacts.     

A study on powder mixing for high fracture toughness and wear resistance of WC-Co-Cr coatings sprayed by HVOF

The effects of mixing powders with various particle sizes on the fracture toughness and wear resistance of thermally sprayed WC-10Co-4Cr coating layers fabricated by the HVOF (High-Velocity Oxygen Fuel) process on a S45C steel substrate were investigated. In order to obtain a high fracture toughness and wear resistance, the powder size and powder mixing ratio were varied. The microstructure and chemical composition of the phases in the coatings were characterized by means of the SEM and XRD techniques. Image analysis was used for the evaluation of the porosity of the coatings. Indentations tests were carried out on the cross sections of the coatings to evaluate the hardness and fracture toughness. The wear properties of the coatings were assessed using a pin-on-disk wear tester at ambient temperature without lubrication. The mixing of a small amount of coarse powders with fine powders resulted in the highest fracture toughness and wear resistance, due to the formation of coating layers having the lowest porosity.     

Synthesis of WC-Co composite powders with two-step carbonization and sintering performance study

In this study, WC-Co composite powder was synthesized by two-step carbonization method using W, Co and C as raw materials. X-ray diffraction (XRD) showed that the η phase (Co₆W₆C) was kept at 1100 °C for 1 h under vacuum, and it could be completely carbonized into WC-Co composite powders. The surface morphology of WC-Co composite powders was analyzed by scanning electron microscope (SEM). The effects of η phase and second phase (W phase) on WC morphology and Co phase distribution were investigated. Electron backscattered diffraction (EBSD) was used to analyze WC-10 wt% Co cemented carbide particle distribution. Comparison of transverse rupture strength, hardness and fracture toughness of two kinds of WC-10 wt% Co cemented carbides synthesized by WC-Co composite powders + WC and WC + Co respectively, the cemented carbide of composite powders + WC increases the fracture toughness from 11.4 ± 0.3 MPa·m^1/2 to 12.4 ± 0.3 MPa·m^1/2.     

TiN-SiC复合陶瓷材料的研制

以Ti、Si3N4、石墨和α-SiC粉体为原料,通过反应热压合成了TiN-SiC复合陶瓷材料。研究结果表明:在TiN-SiC复合陶瓷材料中,TiN和SiC晶粒细小均匀,无异常晶粒长大现象,TiN晶粒尺寸为1μm～7μm,SiC均匀分布于TiN之间;该复合材料的密度、维氏硬度和断裂韧性分别为4.41g/cm3、13.6GPa和6.89MPa·m1/2,其增韧机制主要为裂纹偏转和裂纹分叉机制。