Phase transformation in Ti–48Al–6Nb porous alloys and its influence on pore properties

Ti–48Al–6Nb porous alloys were synthesized by the powder metallurgy (PM) method, and the associated phase transformation and pore parameter were investigated in order to reveal the pore-formation mechanism. The present results indicate that the Nb–Al and Ti–Al phase transformations contribute to the pore-formation. It was found that the five-step phase transformations for the Ti–48Al–6Nb porous alloys occur as follows: (1) Ti + Al → TiAl₃ at 600–700 °C; (2) Nb + Al → NbAl₃ at 700–900 °C; (3) TiAl₃ + Ti → TiAl at 900–1100 °C; (4) TiAl + Ti → Ti₃Al/TiAl at 1100–1350 °C; (5) NbAl₃ + Nb → Nb₂Al and the Ti₃Al turns to the major phase at 1350 °C. These phase transformations made the pore-diameter increasing continuously from 1.71 μm to 12.10 μm and also made the pore volume distributing widely. At the second step of 700–900 °C, the Nb–Al phase transformation leads to 5% more volume expansion compared to the Ti–Al based porous alloys. Meanwhile, the porosity and total pore area initially increase and then decrease at this step, but they increase intensely at the final step, which is needed as a catalytic carrier.     

The optimization of friction spot welding process parameters in AA6181-T4 and Ti6Al4V dissimilar joints

Friction spot welding is a relatively new solid-state joining process able to produce overlap joints between similar and dissimilar materials. In this study, the effect of the process parameters on the lap shear strength of AA6181-T4/Ti6Al4V single joints was investigated using full-factorial design of experiment and analyses of variance. Sound joints with lap shear strength from 4769 N to 6449 N were achieved and the influence of the main process parameters on joint performance was evaluated. Tool rotational speed was the parameter with the largest influence on the joint shear resistance, followed by its interaction with dwell time. Based on the experimental results following response surface methodology, a mathematical model to predict lap shear strength was developed using a second order polynomial function. The initial prediction results indicated that the established model could adequately estimate joint strength within the range of welding parameters being used. The model was then used to optimize welding parameters in order satisfy engineering demands.     

Numerical investigation on steel fibre reinforced cementitious composite panels subjected to high velocity impact loading

Steel fibre reinforced cementitious composite (SFRCC) panels are numerically investigated for their performances under high velocity impact of short projectiles. Numerical responses are obtained using advanced constitutive material model of Riedel–Hiermaier–Thoma (RHT) for cementitious materials and adopting appropriate modelling techniques. Effects of steel fibre volume and the thickness of panels on the impact performance are mainly highlighted in this paper. Various characteristics phenomenon during impact on cementitious composite panels namely, spalling, cracking, scabbing and perforation, are captured which is a difficult task. Scabbing is likely to occur when tensile stresses at the back face of the panel exceed dynamic tensile strength of the material. Various critical aspects in numerical modelling like boundary conditions, material input parameters, and handling severe distortion of the Lagrangian based finite elements are appropriately explained. Design chart is also developed to determine optimum fibre volume and thickness for an impact energy level up to 2.2 kJ. The numerically predicted impact responses are found to corroborate well with experimental results.     

Synthesis and properties of metal matrix composite foams based on austenitic stainless steels –titanium carbonitrides

Metal matrix composite foams based on 316L stainless steel and reinforced with TiC0.7N0.3 were produced by the replication method using polyurethane sponge as a template. The rheological properties of the slurry appeared to be the key issue in the preparation of the composite foams. A homogeneous distribution of TiC0.7N0.3 particles throughout the 316L matrix and a good interaction between the 316L matrix and TiC0.7N0.3 reinforcement particles were obtained. Compression strength results showed that TiC0.7N0.3 particles acted as the real reinforcement medium. The values of the compressive yield strength and the elastic modulus of the metal matrix composite foams increased significantly with increasing TiC0.7N0.3 content when compared to the open cell 316L stainless steel foams.     

Crystal growth,structure, infrared spectroscopy,and luminescent properties of rare-earth gallium borates RGa3(BO3)4, R = Nd,Sm–Er,Y

Crystals of the rare-earth gallium borates RGa₃(BO₃)₄, where R = Nd, Sm–Er, or Y, were grown by the flux method. The crystal structures of RGa₃(BO₃)₄ (R = Eu, Ho) were studied on the basis of single-crystal X-ray diffraction measurements. The hexagonal unit-cell parameters are a = 9.4657(1) Å, c = 7.4667(1) Å and a = 9.4394(2) Å, c = 7.4322(1) Å for EuGa₃(BO₃)₄ and HoGa₃(BO₃)₄, respectively, space group R32. Structure model was determined by “charge flipping” method and refined to R = 1.93% [EuGa₃(BO₃)₄] and R = 1.89% [HoGa₃(BO₃)₄] in anisotropic approximation. All grown gallium borates were investigated by infrared (IR) spectroscopy technique in a middle and far IR region. IR spectra of rare-earth gallium borates correspond to a pure rhombohedral (R32) polytype structure. Small inclusions of a monoclinic phase were detected only in Eu and Nd compounds. Luminescence of Eu and Ho gallium borates was studied at room temperature. The measured decay times for the most intensive emission lines of EuGa₃(BO₃)₄ (∼614 nm) and HoGa₃(BO₃)₄ (434 nm) are 940 μs and 140 μs, respectively. The scheme of crystal-field energy levels of Eu³⁺ in EuGa₃(BO₃)₄ was built on the basis of the temperature-dependent optical transmission measurements combined with the luminescence data. The measured UV absorption edge for RGa₃(BO₃)₄ is at about 300 nm.     

Immobilization of chromite ore processing residue with alkali-activated blast furnace slag-based geopolymer

Chromite ore processing residue (COPR) is an industrial waste produced in the chromic salts production process and contains a small portion of leached Cr(VI), which is highly toxic and is listed as a hazardous waste. The immobilization of COPR using a blast furnace slag-based geopolymer has been investigated in this study. The optimum parameters for preparing the blast furnace slag-based geopolymer using an orthogonal experiment were obtained. COPR was used to replace the amount of blast furnace slag for the preparation of the geopolymer. The COPR-bearing blast furnace slag-based geopolymer has potential application as a construction material and for geological disposal. The combined effect of physical fixation, adsorption and ion exchange in the geopolymeric and CSH (calcium silicate hydrate) gel is considered to be the main mechanism, and the reduction of S²⁻ in the blast furnace slag played a significant role in the solidification of the COPR.     

Fast microwave syntheses of fly ash based porous geopolymers in the presence of high alkali concentration

Microwave synthesis of porous fly ash geopolymers was achieved using a household microwave oven. Fly ash paste containing SiO₂ and Al₂O₃ component was mixed with sodium silicate (Na₂SiO₃) solutions at different concentrations of sodium hydroxide (NaOH) of 2, 5, 10, and 15 M, which were used as NaOH activators of geopolymerization. The mass ratio of Na₂SiO₃/NaOH was fixed at 2.5 with SiO₂/Al₂O₃ at 2.69. After the fly ash and alkali activators were mixed for 1 min until homogeneous, the geopolymer paste was cured for 1 min using household microwave oven at different output powers of 200, 500, 700, and 850 W. Porous geopolymers were formed immediately. Micro X-ray CT and SEM results showed that the porous structure of the geopolymers was developed at higher NaOH concentrations when using 850 W power of the microwave oven. These results derive from the immediate increase of the temperature in the geopolymer paste at higher NaOH concentrations, meaning that aluminosilicate bonds formed easily in the geopolymers within 1 min.     

Effect of the crystallinity of diamond coatings on cemented carbide inserts on their cutting performance in milling

Micro-crystalline diamond (MCD) coatings were deposited on cemented carbide inserts at different temperatures using hot filament chemical vapor deposition technique. For investigating the effect of the developed diamond crystallinity on the fatigue strength and wear behaviour of the prepared MCD coated inserts, inclined impact tests and milling investigations were conducted correspondingly. Raman spectra were recorded for capturing the crystalline phases after the film deposition and their potential changes after the impact and milling experiments induced by the mechanical and thermal loads. Thus, the explanation of the cutting performance of the employed diamond coated inserts with various crystalline phases was enabled.     

Effect of high temperature annealing on ultrahard polycrystalline diamond in air and vacuum conditions

The effect of high temperature annealing on ultrahard polycrystalline diamond (UHPCD) has been investigated in air and vacuum conditions up to 1500 °C. The thermal stability, carbon bonds, morphologies and wear resistance of UHPCD were evaluated by thermal gravimetric-differential scanning calorimetry (TG-DSC), Raman spectroscopy, scanning electron microscopy (SEM) and wear tester. The thermal analysis results indicated that the thermal stability of chemical vapor deposited (CVD) diamond was better than that of polycrystalline diamond (PCD) even though it was weakened by high pressure high temperature treatment, while no graphitization was observed on UHPCD in flowing argon up to 1500 °C. When the UHPCD annealed in air, the oxidation damage with the extension of cracks and spalling holes was observed on CVD diamond as the evolution of temperature. The result confirmed by the changes of diamond peak position and full width at half maximum (FWHM) in Raman spectra curves. The PCD had shown the damage with cracks induced exfoliation of binder regions and cracks ruined diamond grains. However, the diamond peak position and FWHM of CVD diamond and PCD showed slight reduction as a function of vacuum annealing temperature with no detectable change of morphologies. The high temperature annealing has strong impact on the wear resistance of UHPCD in air while slightly in vacuum.     

Microstructural evolution,mechanical and thermal properties of TiC-ZrC-Cr3C2 composites

Dense TiC-ZrC-Cr₃C₂ composites with various TiC content from 19.6 mol% to 78.4 mol% have been fabricated by hot-pressing sintering at 1950 °C using 2.0 mol% Cr₃C₂ as sintering aid. The effect of TiC content on the microstructure, mechanical and thermal properties of TiC-ZrC-Cr₃C₂ composites are investigated systematically. The single (Zr, Ti, Cr)C solid solution is obtained when TiC content is 19.6 mol%, while with increasing TiC content, the composites begin to consist of Zr-rich (Zr, Ti)C solid solution and Ti-rich (Ti, Zr, Cr)C solid solution phase. SEM and EDS analysis confirm that Cr element is not favorable to diffuse into ZrC lattice to form (Zr, Cr)C solid solution. Flexural strength and Vickers hardness increase gradually with increasing TiC content, but fracture toughness does not improve significantly. Fracture toughness are in the range of 3.34–4.01 MPa∙m^1/2 for all composites, and the optimum value reaches 4.01 MPa·m^1/2 with 49.0 mol% TiC. Experimental results of the thermal expansion coefficient reveal that the addition of TiC raises the thermal expansivity of TiC-ZrC-Cr₃C₂ composites. Noticeably, the thermal conductivities of TiC-ZrC-Cr₃C₂ composites show a decrement trend with increasing TiC content, not as theoretical predicting by the rule of mixtures. For instance, the thermal conductivity at 25 °C ranges from 18.0 W/m∙K for 8Z2T2C composite down to 10.6 W/m∙K for 2Z8T2C composite.