Effects of dopants with various valences on densification behavior and phase composition of a ZrO2–SiO2 nanocrystalline glass-ceramic

Effects of dopants with different valences on the densification behavior and phase composition of a ZrO₂–SiO₂ nanocrystalline glass-ceramic (NCGC) during pressureless sintering were investigated in this study. The raw powder of Ca²⁺, La³⁺, Ce⁴⁺ and Ta⁵⁺ ions doped ZrO₂–SiO₂ (referred to as Ca-ZS, La-ZS, Ce-ZS, Ta-ZS, respectively) and pure ZrO₂–SiO₂ (PZS) sample were synthesized by sol-gel method, followed by pressureless sintering. Compared with the PZS sample, doping of Ca²⁺ and La³⁺ ions significantly promoted the densification of the NCGCs. The “densification promotion” effect was attributed to the formation of oxygen vacancies and the decrease of SiO₂ viscosity due to doping of aliovalent cations. The dopants with various valences showed significant effects on the phase compositions of the NCGCs during sintering. Doping of Ca²⁺ ion accelerated the reaction kinetics between ZrO₂ nanocrystallites and amorphous SiO₂ to yield ZrSiO₄. The La³⁺ ion acted as destabilizer of t-ZrO₂, which resulted in a rapid tetragonal (t) to monoclinic (m) ZrO₂ phase transformation during sintering, while in the Ta⁵⁺ and Ce⁴⁺ ions doped sample, the phase transformation occurred gradually. All the doping ions increased the lattice parameters and the volume of t-ZrO₂ unit cell, while the effects of the doping ions on the lattice parameters of m-ZrO₂ unit cell were more complex.     

Isostructural phase transition (IPT) in CaMoO4 with Scheelite structure

The ceramic compound CaMoO₄ is synthesized via a solid-state reaction technique. Rietveld refinement studies were done on the powder X-ray diffraction data of CaMoO₄ and revealed that the compound is crystallized in the tetragonal Scheelite structure with I4₁/a space group. The differential scanning calorimetry (DSC) studies on CaMoO₄ divulged an anomaly around 440 °C. This anomaly is further probed using the temperature-dependent Raman and dielectric spectroscopic measurements and are corroborating with the results obtained from DSC. A detailed investigation on the temperature-dependent Raman spectroscopic data revealed that the A_1g mode of CaMoO₄ showed a soft phonon behavior up to the phase transition temperature. It is observed that the A_1g mode displayed phonon hardening behavior with further increasing the temperature. The anomaly is attributed to an isostructural phase transition (IPT), a rarely observed phenomenon in the compounds with Scheelite structure. The IPT in CaMoO₄ is elucidated with a phonon softening mechanism.     

Microstructure,mechanical and tribological properties of Mo–V–Cu–N coatings prepared by HIPIMS technique

A combination of high wear-resistance and low-friction is crucial for improving the wear performance of self-lubricating coatings, which is generally determined by an excellent lubricating effect and mechanical strength. In this study, the Mo–V–Cu–N coatings were prepared by HIPIMS technique with a spliced target of Mo–V–Cu at various charge voltages. The results revealed that Mo–V–Cu–N coatings presented a solid solution phase of B1–MoVN with (200) preferred orientation, and the preferred orientation was enhanced at high charge voltages. Whereas the Cu atoms formed an amorphous phase in Mo–V–Cu–N coatings due to a low Cu content of 2.3–3.6 at.%. As the charge voltage increased to 750 V, more charged metallic ions were accelerated and bombarded substrate surface efficiently, forming smooth and dense Mo–V–Cu–N coatings with a high hardness of 31.0 GPa. All the coatings presented a low friction coefficient of 0.34–0.39 due to the formation of MoO₂, VO₂ and CuO mixed oxides, and the wear mechanism was dominated by abrasive and tribo-oxidation wear at room temperature.     

Defect control and ionic conductivity of oxynitride perovskite Sr0.83Li0.17Ta0.83O1.88N0.74

Perovskite lattice was tailored by introducing site vacancies and mixed anion composition, to produce Sr_0.83Li_0.17Ta_0.83O_1.88N_0.74 (Li02N). Further, Li02N was converted to a defect oxide Sr_0.83Li_0.17Ta_0.83O₃ (Li02O) by applying an optimized treatment: heating in air at 1173 K for 2 h. According to the neutron Rietveld refinement, Li02N and Li02O are tetragonal and orthorhombic, respectively, where the lattice volume of Li02O is significantly smaller than that of Li02N. The ionic conductivity (σ_ion) of Li02N and Li02O was evaluated by the ac impedance spectroscopy and the equivalent circuit analysis. Both Li02N (σ_ion = 10^?5.5 S/cm at 671 K) and Li02O (σ_ion = 10^?6.2 S/cm at 667 K) exhibited an Arrhenius behavior of ionic conductivity with activation energies of 0.87 eV and 0.75 eV, respectively. It is interpreted that the nitride component enhances the ionic conduction of Li02N, while the vacancy of the anion lattice makes an opposite effect.     

Reconstruction and hydration of hydrotalcite response to thermal activation temperature: Enhancement of properties for magnesia castables

Optimization binding system for refractory castables is significant to enhance the service performance. Hydrotalcite has been considered a promoter for high-temperature performance of basic castables, however, its binding property remains to be improved before practical application. In this work, the thermal activated Mg–Al hydrotalcites were incorporated in magnesia castables, and the mutual influence of pre-calcination temperature on the hydration, microstructure, and strength of castables was investigated. The obtained results indicated that the reconstruction of calcined hydrotalcite took place in the hydration process and effectively motivated the hydrolysis. Hydrate was thus promoted and a relatively dense microstructure of magnesia castables was confirmed by X-ray computed tomography analysis. Hydrotalcite pre-calcinated at 300 °C contributed to the highest early strength for castable, and the high-temperature properties also performed better than that of other pre-calcinated hydrotalcite-adding. The enhancement mechanisms of calcined hydrotalcite were attributed to the two following reasons: (ⅰ) the modified microstructure of magnesia castables from the early stage by hydration process, (ⅱ) the further enhanced sinterability inspired by the appropriate thermal activation effect.     

Development of mullite fibers and novel zirconia-toughened mullite fibers for high temperature applications

Polycrystalline mullite fibers and novel zirconia-toughened mullite (ZTM) fibers with average diameters between 9.7 and 10.3 μm containing 3, 7 and 15 wt.-% tetragonal ZrO₂ (ZTM3, ZTM7, ZTM15) in the final ceramic were prepared via dry spinning followed by continuous calcination and sintering in air. A shift in the formation of transient alumina phases and tetragonal ZrO₂ to higher temperatures with increasing amounts of ZrO₂ was observed. Concomitantly, the mullite formation temperature was lowered to 1229 °C for ZTM15 fibers. X-ray diffraction revealed formation of the desired tetragonal crystal structure of ZrO₂ directly from the amorphous precursor. Room temperature Weibull strengths of 1320, 1390 and 1740 MPa and Weibull moduli of 9.5, 7.1 and 9.0 were determined for mullite, ZTM3 and ZTM15 fibers, respectively. Average Young’s moduli ranged from 190 to 220 GPa. SEM images revealed crack-free fiber surfaces and compact microstructures independent of the amount of ZrO₂.     

Microstructure and mechanical behavior of laser aided additive manufactured low carbon interstitial Fe49.5Mn30Co10Cr10C0.5 multicomponent alloy

Laser aided additive manufacturing(LAAM)was used to fabricate bulk Fe49.5Mn30Co10Cr10C0.5 interstitial multicomponent alloy using pre-alloyed powder.The room temperature yield strength(σy),ultimate tensile strength(σUTS)and elongation(εUST)were 645 MPa,917 MPa and 27.0％respectively.The as-built sample consisted of equiaxed and dendritic cellular structures formed by elemental segregation.These cellular structures together with oxide particle inclusions were deemed to strengthen the material.The other contributing components include dislocation strengthening,friction stress and grain bound-ary strengthening.The high εUTS was attributed to dislocation motion and activation of both twinning and transformation-induced plasticity(TWIP and TRIP).Tensile tests performed at-40℃and-130℃demonstrated superior tensile strength of 1041 MPa and 1267 MPa respectively.However,almost no twinning was observed in the fractured sample tested at-40℃and-130℃.Instead,higher fraction of strain-induced hexagonal close-packed(HCP)ε phase transformation of 21.2％were observed for fractured sample tested at-40℃,compared with 6.3％in fractured room temperature sample.     

Methods and mechanisms for uniformly refining deformed mixed and coarse grains inside a solution-treated Ni-based superalloy by two-stage heat treatment

The uniform refinement mechanisms and methods of deformed mixed and coarse grains inside a solution-treatment Ni-based superalloy during two-stage annealing treatment have been investigated.The two-stage heat treatment experiments include an aging annealing treatment(AT)and a subsequent recrystallization annealing treatment(RT).The object of AT is to precipitate some δ phases and consume part of storage energy to inhibit the grain growth during RT,while the RT is to refine mixed and coarse grains by recrystallization.It can be found that the recrystallization grains will quickly grow up to a large size when the AT time is too low or the RT temperature is too high,while the deformed coarse grains cannot be eliminated when the AT time is too long or the RT temperature is too low.In addition,the mixed microstructure composed of some abnormal coarse recrystallization grains(ACRGs)and a large number of fine grains can be observed in the annealed specimen when the AT time is 3 h and RT tem-perature is 980℃.The phenomenon attributes to the uneven distribution of δ phase resulted from the heterogeneous deformation energy when the AT time is too short.In the regions with a large number of δ phases,the recrystallization nucleation rate is promoted and the growth of grains is limited,which results in fine grains.However,in the regions with few δ phases,the recrystallization grains around grain boundaries can easily grow up,and the new recrystallization nucleus is difficult to form inside grain,which leads to ACRGs.Thus,in order to obtain uniform and fine annealed microstructure,it is a prereq-uisite to precipitate even-distributed δ phase by choosing a suitable AT time,such as 12 h.Moreover,a relative high RT temperature is also needed to promote the recrystallization nucleation around δ phase.The optimal annealing parameters range for uniformly refining mixed crystal can be summarized as:900℃×12 h+990℃×(40-60 min)and 900℃×12 h+1000℃×(10-15 min).     

Effect of Nb content on the phase composition,densification, microstructure,and mechanical properties of high-entropy boride ceramics

Dense high-entropy (Hf,Zr,Ti,Ta,Nb)B₂ ceramics with Nb contents ranging from 0 to 20 at% were produced by a two-step spark plasma sintering process. X-ray diffraction indicated that a single-phase with hexagonal structure was detected in the composition without Nb. In contrast, two phases with the same hexagonal structure, but slightly different lattice parameters were present in compositions containing Nb. The addition of Nb resulted in the presence of a Nb-rich second phase and the amount of the second phase increased as the Nb content increased. The relative densities were all >99.5 %, but decreased from ～100 % to ～99.5 % as the Nb content increased from 0 to 20 at%. The average grain size decreased from 13.9 ± 5.5 μm for the composition without Nb additions to 5.2 ± 2.0 μm for the composition containing 20 at% Nb. The reduction of grain size with increasing Nb content was due to the suppression of grain growth by the Nb-rich second phase. The addition of Nb increased Young’s modulus and Vickers hardness, but decreased shear modulus. While some Nb dissolved into the main phase, a Nb-rich second phase was formed in all Nb-containing compositions.     

Fabrication and characterization of in-situ Al3Ni intermetallic and CeO2 particulate-reinforced aluminum matrix composites

Al-xNi-yCeO₂ (x = 6, 10, 15, 20 and y = 0, 5, 10 wt%) composites were produced by a powder metallurgical production route. Powder mixtures of Al, Ni and CeO₂ were fabricated via mechanical alloying (MA) for 4 h in a Spex-type high-energy ball mill. Both the mechanically alloyed (MAed) and non-MAed (as-blended mixtures) powders were pre-compacted in a hydraulic press under 650 MPa and then pressurelessly consolidated at 630 °C for 2 h under an inert atmosphere. The effects of MA process and the amounts of Ni and CeO₂ on the microstructural, mechanical and tribological properties of the sintered composites were determined. Based on the SEM and XRD investigations, the MAed powders illustrated a homogenous structure, comprising flaky particles with smaller crystallite sizes and greater lattice strain. According to the XRD analysis, Ni formed Al–Ni intermetallic compounds in the matrix of sintered composites that act as secondary reinforcement phases. The SEM observations conducted on the MAed samples demonstrated more uniformly and finely distributed Al₃Ni and CeO₂ phases in the microstructure of the MAed samples, unlike the non-MAed ones. The hardness values of sintered composites increased due to the MA process and increasing Ni and CeO₂ amounts, and the hardness value of the MAed Al20Ni–10CeO₂ sample reached 179 HV. The ultimate compressive strength and failure strain of the MAed Al6Ni–10CeO₂ sample were 441 MPa and 11.3%. In the Al20Ni–10CeO₂ sample, the compressive strength and failure strain were 391 MPa and 5.5%, respectively. Additionally, the reciprocating wear test results illustrated that both wear resistance and hardness values of the composites increased as the amounts of Ni and CeO₂ increased, and the Al20Ni–10CeO₂ sample exhibited the highest wear resistance as 0.175 × 10^-3 mm³/Nm.