B2 Solid Solution Research Articles

In-situ solid solution of Mo or Cr in WB2: Densification, microstructure, and properties

Pure tungsten diboride (WB2) ceramic prepared by the boro/carbothermal reduction combined with spark plasma sintering (SPS) of WB2 can reach a relative density >90%. However, the final sintered density still needs further improvement to meet the application requirement. In this study, the WB2 ceramics doped with Mo or Cr (W0.95Mo0.05B2 and W0.95Cr0.05B2) were prepared and the effects of Mo or Cr doping on powder characters, sintering behavior, microstructure, mechanical and physical properties of WB2 were systematically investigated. The results showed that the average particle size of the WB2 powders was increased significantly after adding either dopant, and the relative density of WB2 ceramics was improved from ∼93% to ∼95% and 96% with Cr and Mo addition, respectively. However, the Vickers hardness of WB2 with either dopant was reduced. On the other hand, the W0.95Mo0.05B2 solid solution exhibited slight improvement on both of the thermal and electrical conductivity as compared with WB2. But, both of the thermal and electrical conductivity of W0.95Cr0.05B2 solid solution decreased significantly to about 50% of the values obtained for WB2. The decrease in thermal and electrical conductivity of W0.95Cr0.05B2 solid solution was mainly attributed to the formation of high concentration of atomic defects, i.e. dislocations.

Effect of Nb on microstructure and properties of AlCoCrFeNi2.1 high entropy alloy

To overcome the strength limitation of eutectic high entropy alloys (EHEAs) in industrial application, it is necessary to develop new EHEA materials with high strength. In this work, the AlCoCrFeNi2.1Nbx (0 ≤ x ≤ 0.5) high entropy alloys (HEAs) were synthesized by arc-melting introducing heterogeneous structures by adding to the Nb element. The microstructures and mechanical properties of the AlCoCrFeNi2.1Nbx alloys were investigated. The results show that all the AlCoCrFeNi2.1Nbx (x > 0) HEAs are composed of FCC solid solution, B2 solid solution and Fe2Nb-type Laves. The microstructures of the AlCoCrFeNi2.1Nbx alloys change from FCC + B2 dual-phase eutectic structure (x = 0) to FCC + B2/Laves bimodal eutectic structure (x = 0.1), then to the mixed microstructures of FCC + B2 dendrite and FCC + Laves eutectic (0.1 < x ≤ 0.4), and finally to the hypoeutectic structure with BCC primary phase (x = 0.5). With the addition of Nb, the Vickers hardness and compressive yield strength of the AlCoCrFeNi2.1Nbx alloys increase continuously from 294.8 HV to 618.9 HV and 626 MPa–1998 MPa, respectively. Wherein, the AlCoCrFeNi2.1Nb0.2 alloy exhibits the best comprehensive mechanical properties with the compressive yield strength of 1158 MPa and ductility of 29.7%. In addition, the precipitation strengthening of the hard Laves phase and solid solution strengthening is the main strengthening mechanism in the AlCoCrFeNi2.1Nb0.2 alloys.

High‐temperature reactive synthesis of the Zr–Ta multiboride with a supercomposite structure

AbstractThis study reports the formation of a ceramic composite consisting only of boride phases with an artificially created hierarchical superstructure. Such a composite was formed during the reaction‐driven consolidation process using a mixture of ZrB2, tantalum, and amorphous boron powders. The homogeneity of the reaction among these powders at 1900°C allowed the forming of a highly reproducible and repetitive superstructure where Ta3B4 forms a chain‐like mesh that entraps the ZrB2, ZrB, TaB, and (Zr,Ta)B2 phases. The multiboride ceramic composite exhibited extremely high hardness: 28.6 ± 3.2 or 22.6 ± 0.6 GPa. These were estimated using 98‐ and 196‐N loads, respectively. At 2000°C, the multiboride composite showed a strength of nearly 400 MPa and fractured in an elastic manner at the loading rate of 2.5 mm/min. This level of strength is usually observed for the bulk zirconium diboride at room temperature. The main factors leading to the improvement of the mechanical properties were considered to be the formation of the (Zr,Ta)B2 solid solution as well the composite structure.

On the reactive spark plasma sinterability of ZrB2–SiC–TiN composite

In the present research, the influence of TiN addition on the ZrB2–SiC composite was studied. Hereof, TiN-doped ZrB2–SiC composite was manufactured via the route of reactive spark plasma sintering under an applied load of 40 MPa at the peak temperature of 1850 °C for 360 s. Mechanical properties, microstructure evaluation, and sinterability were investigated and compared to the non-doped ZrB2–SiC. X-ray diffraction analysis, thermodynamic appraisal, energy dispersive spectroscopy, and scanning electron microscopy divulged the formation of in-situ h-BN, ZrO2, and ZrN as well as the (Zr,Ti)B2 solid solution in TiN-doped ZrB2–SiC. Comparing the relative density of 95% for the TiN-free sample with the relative density of 94.1% for the sample containing TiN, the addition of TiN increased porosity content owing to the formation of in-situ BN with layered structure. Flexural strength decreased from 460.2 MPa for TiN-free ZrB2–SiC to 435.9 MPa for TiN-doped ZrB2–SiC, due to matrix grain growth and relative density drop. In contrast, indentation fracture toughness and Vickers hardness increased to 6.1 MPa.m1/2 and 29.1 GPa, respectively, as the result of TiN addition.

Evolution of the structure and properties of (Zr1-xHfx)B2 solid solution ceramics from first-principle theory

As typical ultra-high temperature ceramics, both ZrB2 and HfB2 have unique properties such as excellent mechanical performance and oxidation resistance, are promising candidates for the aerospace industry. In this work, the phase stability, electronic, mechanical and thermodynamic properties of (Zr1-xHfx)B2 solid solution ceramics were systematically studied performing by first-principles calculations. The calculated results of structural optimization and elastic constants are in good agreement with the experimental values, which confirms the reliability of the proposed model. The lattice parameters of the optimized (Zr1-xHfx)B2 solid solution ceramics are linearly related to the Hf content, while a decrease is detected as the increasing Hf content. The calculated elastic constants show that the addition of Hf is beneficial in improving the mechanical properties of the (Zr1-xHfx)B2 solid solution, including the elastic modulus, hardness and fracture toughness. In addition, the (Zr1-xHfx)B2 solid solution was found to be kinetically stable. From the trend of the calculated value of the Debye temperature, it can be found that the addition of Hf can effectively reduce the thermal conductivity of solid solution materials. Our work introduced an important theoretical framework, which is of great significance for the development of solid solution diboride material systems with adjustable composition and properties.

Microstructures and mechanical properties of reactive spark plasma-sintered ZrB2–SiC–MoSi2 composites

Ternary ZrB2–SiC–MoSi2 composites with different nominal volume fractions of SiC (10–25 vol%) and MoSi2 (5–20 vol%) were sintered by reactive spark plasma sintering (RSPS) at 1650 °C and 35 MPa from the starting powders of ZrB2, Si, Mo and activated charcoal. The densification behaviour, microstructures and mechanical properties were comprehensively investigated. SiC clusters and MoSi2 were formed by in-situ reactions and the improved sintering efficiency was attributed to the mass transfer by liquid Si. The formation of a core–shell (Zr, Mo)B2 solid solution was confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). With the addition of the nominal MoSi2 content up to 15 vol%, the molar ratio of MoB2 in the solid solution shell increased owing to the increase of Mo; the densification and mechanical properties decreased owing to the softness of MoSi2, the cracks, and the large size of SiC clusters. The relative density, Vickers hardness, and fracture toughness of ZrB2–SiC–MoSi2 composites were significantly better than those of bulk ZrB2 owing to the combined effect of phase enhancement and solid solution strengthening.

Hot-pressed ZrB2–SiC composite ceramics: Effect of various Ta-containing additives on the microstructure and mechanical properties

The ZrB2–SiC composites have been commercially used at ultrahigh temperatures, but it often failed due to their poor toughness. In order to solve this problem, four types of Ta-containing additives (Ta, TaC, TaB2 and TaSi2) were used as the “third phase” to regulate the microstructure, so as to enhance the mechanical properties of hot-pressed ZrB2–20SiC-based ceramics (in vol. %). The incorporation of the additives generated a core–shell structure, which comprised of a ZrB2 core and a (Zr, Ta)B2 solid solution shell. The additives helped refine the ZrB2 grains in addition to the metallic Ta and release the internal stress field generated by the thermal misfit. The interfacial structure was modified by the formation of the coherent core/shell interface and the semi-coherent interface of adjacent ZrB2 grains and the semi-coherent ZrB2/(Zr, Ta)C interface in the TaC-additive composite. The addition of TaB2 or TaC hardened the ZrB2–20SiC ceramics, whereas the addition of Ta or TaSi2 reduced the hardness. The fracture toughness was enhanced by the formation of the Ta-containing phases. These phases reduced the stress intensity factor of the crack tip, which was proportional to the intrinsic residual stress. However, the crack-propagation mechanism would be changed by the incorporation of various Ta-containing additives. The decrease in the crack deflection, which was induced by the stronger interfacial bonding force and the significant consumption of SiC, resulted in relatively low toughness in the Ta- and TaC-included samples. The weaker interfacial bonding force in the TaB2- and TaSi2-included samples caused an increase in deflection and generated branching, which enhanced the toughness of the TaSi2-included composites to ∼4.72 MPa⸱m1/2.

Oxidation-affected zone in the sintered ZrB2–SiC–HfB2 composites

Understanding the behavior of ultra-high temperature ceramics (UHTCs) against oxidation is of particular importance in high-temperature applications. In this study, ZrB2–SiC–HfB2 UHTC composites were fabricated by spark plasma sintering (SPS) method at different temperatures, times, and pressures to investigate the effects of sintering process variables on their oxidation resistance. Before the oxidation tests, the as-sintered samples contained ZrB2 and SiC phases with (Zr,Hf)B2 solid solution. The samples were subjected to oxidative conditions at 1400 °C and their relative mass changes were measured as a function of oxidation time up to 20 hours. FESEM and EDS equipment were used for microstructural and elemental analyzes of cross-sections of different oxide layers. Due to the oxygen diffusion, ZrO2 and SiO2 phases appeared alongside (Zr, Hf)O2 in the surface layers. After identifying the several oxides and SiC-depleted layers in the oxidation-affected zone, a schematic model for the arrangement of such layers was proposed.

Advancements in carbon fibre reinforced ultra-refractory ceramic composites: Effect of rare earth oxides addition

Continuous carbon fibre ceramic matrix composites capable of tolerating multiple thermal-shock cycles and resisting ablation are needed for aerospace and hypersonic systems. Carbon fibre around 50 vol% and ultra-refractory matrices are fundamental parameters. The influence of rare earth (RE) oxides on the microstructure and mechanical properties of carbon fibre-ZrB2/SiC composites was investigated. Materials were produced by slurry infiltration and hot pressing. The addition of Y2O3, La2O3 and CeO2 led to the formation of lamellar boro-carbides that improved the densification, while Sc2O3 promoted the formation of (Zr,Sc)B2 solid solutions in the matrix. All these composites exhibited improved mechanical properties compared to a RE-free baseline, with room temperature strengths and toughness above 330 MPa and 9 MPa m0.5, respectively, and strengths above 600 MPa at 1500 °C. The lamellar phase was identified as a fibre by-product with general formula REB2C2. Only CeO2 was detrimental on the long run due to its high reactivity with humidity which induced swelling and jeopardized the structural stability of the composite.This study revealed new fundamental insights into the microstructure evolution of carbon-fibre refractory composites and its impact on the mechanical properties, which will contribute to the development of new generation of reusable ceramic matrix composites for harsh environments.

Kinetics of precipitate formation in (Ti x W y Cr z )B2 solid solutions: influence of Cr concentration and Co impurities

Abstract Supersaturated (Ti0.5 – x W0.5Cr x )B2 (x = 0, 0.1, 0,2) solid solutions were produced by reaction sintering of transition metal diboride powders at a pressure of 60 MPa at 1800 °C. The formation kinetics of W2B5 precipitates were investigated at lower temperatures by X-ray diffractometry (XRD) and secondary electron microscopy (SEM). The precipitation rate constants as a function of reciprocal temperature show non-monotonic behaviour in the range between 1450 and 1700 °C with a maximum at about 1650 °C. At low temperatures, Arrhenius behaviour is approximately valid, where the precipitation kinetics are dominated by the diffusion of the transition metals. In contrast, at temperatures higher than 1650 °C, the precipitation rates decrease rapidly, since supersaturation, and consequently nucleation, is strongly reduced. The rate constants of precipitation are identical within error limits for Cr concentrations of x = 0.1 and x = 0.2, however, they are significantly lower for x = 0. This means that the addition of small amounts of Cr accelerates the precipitate formation, while further addition has no further influence. Samples resulting from powders which were high-energy milled with WC/Co hard balls before reaction sintering show an increased density of precipitated crystallites but no increased rate constants of precipitation. This result can be explained under the assumption that for these samples a significant amount of additional pre-existing nuclei was present before precipitation starts, which were formed by heterogeneous nucleation at Co impurities, introduced by the milling process.

Fabrication of (Zr,Ti)B2–ZrN–BN composites through reactive spark plasma sintering of ZrB2 and TiN

Mechanical properties and sintering behavior of additive-free and TiN-doped ZrB2 ceramics were studied. Reactive spark plasma sintering method was applied for manufacturing of ceramics at 1850 °C for 6 min under 40 MPa. The impact of TiN addition on the microstructure evaluation, densification, and mechanical feathers was investigated. A porous monolithic ZrB2 with a relative density of 76.5 % was manufactured, while the introduction of 5 wt% TiN resulted in enhancement of relative density to 93.1 %. The formation of (Zr,Ti)B2 solid solution and in-situ h-BN and ZrN phases was proven by microstructural assessments and X-ray diffractometry. Minimizing the grain growth and improving the densification, as the results of TiN addition, led to enhancement in mechanical properties. The values of bending strength, fracture toughness, and Vickers hardness boosted from 187.6 MPa, 1.9 MPa.m½, and 10.1 GPa for additive-free ZrB2 to 606.5 MPa, 4.5 MPa.m½, and 18.8 GPa for (Zr,Ti)B2–ZrN–BN composite.

Design of ultra-high temperature ceramic nano-composites from multi-scale length microstructure approach

The evolution of the multi-scale microstructure of a (Zr,Ta)B2 solid solution was studied as a function of time and temperature. The ceramics were produced by hot pressing a mixture of ZrB2 with 15 vol% TaSi2 followed by annealing at 2100 °C. Formation of a super-saturated solid solution led to the precipitation of TaC nano-needles within the micron-sized boride grain matrix. Phase stability diagrams were used to define the conditions of partial pressure within the sintering chamber that drove precipitation of nano-inclusions in the form of either metal or carbide. Through this approach, other systems containing various transition metals were explored to design other formulations for in-situ nano-composites with unprecedented strength at ultra-elevated temperatures.

Effects of graphene nanoplatelets and hexagonal boron nitride on spark plasma sintered (Zr,Nb)B2 solid solutions

Bulk (ZrxNb1−x)B2 solid solution systems with x ranging from 0 to 1 with intervals of 0.25, were consolidated by spark plasma sintering (SPS) at 1850 °C under 50 MPa with a holding time of 5 min. In addition, 1 vol% graphene nanoplatelets (GNPs) and 1 vol% hexagonal boron nitride (h-BN) were added to (Zr,Nb)B2 solid solutions to investigate their effects on densification, mechanical properties, and optical properties. Transmission electron microscopy analysis, phase analysis, and lattice parameters confirmed the formation of a complete (Zr,Nb)B2 solid solution. Raman analysis confirmed the structural integrity of both the GNPs and h-BN in the solid solution. The additives modified different properties of the (Zr,Nb)B2 solid solution. Namely, the addition of GNPs increased the relative density by ~2% by removing oxide impurities. The highest Vickers hardness was obtained for h-BN-modified Zr0.25Nb0.75B2, which had the lowest grain size. The addition of GNPs and h-BN contributed to the improvement in toughness by activating toughening mechanisms. The h-BN-modified (Zr,Nb)B2 exhibited the best oxidation performance. The optical absorbance of the (Zr,Nb)B2 solid solution was higher than that of the GNP- and h-BN-modified samples.

Compositional pathways and anisotropic thermal expansion of high-entropy transition metal diborides

The recent discovery of high entropy transition metal diborides (HEBs) has sparked renewed interest in ultra-high temperature ceramics (UHTCs). Presently, transition metal (Me) oxides based boro-carbo/thermal reduction (BCTR) syntheses show great promise as relatively cheap production methods, but also may present limits to attain single phase pure HEBs. Herein, by selectively tuning the concentration of boron and carbon, the reducing agents of Me oxide mixture (Me = Ti, Ta, Nb, Zr and Hf), and exploiting high-resolution synchrotron X-ray powder diffraction, we first identified and quantified the formation of intermediate phases during the BCTR synthesis, with the ultimate intent to achieve a full dense (Ti,Ta,Nb,Zr,Hf)B2 solid solution (SS). Additional insight was obtained by temperature dependent diffraction, which highlighted, for the first time in this class of materials, anisotropic thermal expansion, most likely at the origin of the SS micro-cracking, as was also observed by electron microscopy.

Formation of core-shell structure in W-added B4C-based composites

B4C-TiB2-SiC composites with W addition were successfully prepared by spark plasma sintering with B4C, TiB2, SiC, and W as raw materials. The XRD results indicate that the main phases of the composites are B4C, TiB2, SiC, and W2B5. A large number of core-shell structures were observed in the composites with the introduction of W. It is determined that the core is the TiB2 phase and the shell is a (Ti, W) B2 solid solution by EDS, EBSD, and TEM characterization methods. The core-shell structure is conducive to the deflection of cracks, thereby improving the fracture toughness of the composites. As a result, with the increase of W addition from 0 to 5 vol%, the fracture toughness of composites increased from 3.96±0.25 to 5.14±0.23 MPa m1/2, the Vickers hardness improved from 20.24±0.22 to 28.35±0.22 GPa.

Ablation behavior of (Zr,Ta)B2-SiC coating on carbon/carbon composites at 2300 °C

Zr-Ta-B-SiC multiphase coating was prepared on carbon/carbon composites by pack cementation and slurry method. The (Zr,Ta)B2 solid solution was found to be the key antiablative component. The ablation behavior suggested that the synthesized Zr0.70Ta0.30B2-SiC coating exhibited the best ablation resistance at 2300 °C. It is asserted that the micromorphology and distribution of Zr-Ta-O in the coating played a vital role in the evolution of the protective layer and ablation performance. A coating with ∼30 % Ta was beneficial for stabilizing the t-ZrO2 phase and alleviating the volume change at high temperature. The formation of a thermal barrier and low volatility of Zr-Ta-O layer were responsible for the excellent ablation resistance.

Role of TiCN addition on the characteristics of reactive spark plasma sintered ZrB2-based novel composites

The present investigation was carried out to illustrate the effect of TiCN on the microstructure development of spark plasma sintered fully dense ZrB2-based ceramics. The microstructural observations, X-ray diffraction patterns, and field emission-electron probe microanalysis revealed the presence of the (Zr,Ti)B2 and ZrCN phases in the final microstructures of the prepared composites. The advancement of a chemical reaction between TiCN and ZrB2 was suggested over the sintering process through the ZrCN and TiB2 phases formation. Although the ZrCN compound was stable during the sintering process, the generated TiB2 was dissolved into the ZrB2 matrix, forming a (Zr,Ti)B2 solid solution. The composition of this solid solution was roughly uniform in the ternary samples, thanks to the role of SiC in grain-refining, contrary to the ZrB2-TiCN composite with a heterogenous composition. Ultimately, the Vickers hardness and elastic modulus of the (Zr,Ti)B2 matrix in the ternary system were measured to be 29.3 GPa and 450.8 GPa, respectively.

Theoretical and experimental study of combustion synthesis of microgradient ULTRA high-temperature ceramics in Zr-Ta-Si-B system

The macrokinetic features and phase and structure formation mechanisms are investigated in the Zr-Ta-Si-B system in the framework of the fractal theory of combustion. The reaction mechanisms are explored using ab initio grand potential modeling and Ellingham diagrams and then verified via direct in situ time-resolved XRD and quenching of combustion fronts in a copper block. The tendency of the Zr-TaSi-B system to form tantalum boride Ta3B4 and a transient tantalum silicide Ta5Si3 and release elemental Si and Zr in the combustion front is suggested based on the developed models and verified using a quenched combustion front and timeresolved XRD analysis. Investigation of hot-pressed combustion products shows that despite slightly higher relative density, TaSi2-rich ceramics have relatively lower mechanical properties and oxidation resistance under plasma torch as compared to single-phase (Zr,Ta)B2 solid solution.

Effect of tantalum solid solution additions on the mechanical behavior of ZrB2

Mechanical properties and microstructure were compared for zirconium diboride and two zirconium diboride solid solutions containing 3 and 6 at% tantalum diboride. X-ray diffraction indicated that the ceramics were nearly phase-pure and that tantalum dissolved into the ZrB2 lattice to form (Zr,Ta)B2 solid solutions. Microstructural analysis indicated that samples achieved nearly full relative density with average grain sizes that ranged from 3−5 μm. The three compositions had similar values of Young’s modulus (510−531 GPa), shear modulus (225−228 GPa), Vickers hardness (15.2–16.4 GPa), and flexural strength (391−452 MPa). Fracture toughness ranged from 2.6 to 3.7 MPa m1/2 and with increasing tantalum content, the fracture mode changed from predominantly intergranular to predominantly transgranular. Diboride solid solution materials had comparable properties to the single metal diboride, but differences in microstructure, secondary phases, and strain state among the three ceramics partially obscured the actual effects of the solid solution on fracture behavior.

Synthesis and flash sintering of (Hf1-xZrx)B2 solid solution powders

(Hf1-xZrx)B2 solid solution powders were synthesized by two methods. First, solution-based processing of HfCl4, ZrCl4, sucrose, and H3BO3 was conducted followed by heat treatment in Argon to carry out the carbothermal reduction (CTR) reaction to form the diboride powders. Alternatively, in the so-called borohydride reduction (BHR) method, HfCl4, ZrCl4 and NaBH4 were mixed in an Argon glove box followed by heat treatment in Argon at 700−1500 °C. The synthesized powders were characterized by XRD, SEM, TEM, EDS, and TGA, and the influence of different parameters such as starting composition, heat treatment temperature and time on products characteristics were revealed. Both CTR and BHR solid solution powders were then consolidated within ∼5 min in a homemade flash sintering (FS) setup. The composition, microstructure, hardness, and thermal-oxidation properties of flash sintered ceramics were characterized, and the implication of this study and directions for future research were discussed.