2020 Vol. 27, No. 5

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Invited Review
Interactive effect of minerals on complex ore flotation: A brief review
Wan-zhong Yin and  Yuan Tang
2020, vol. 27, no. 5, pp. 571-583. https://doi.org/10.1007/s12613-020-1999-y
Abstract:

Froth flotation is the most effective industrial method used to separate fine-grained minerals. The main problem of complex ore flotation is the negative effect of interactions among minerals in slurry, leading to variation in surface properties during separation. In this review, studies on the interactive effect among minerals on the flotation of iron ores, magnesite ores, and scheelite ores are summarized, and the main problems and mechanisms that diminish the separation efficiency of minerals are revealed in detail. Recent research progress on the flotation of these ores has confirmed that mineral aggregation, coating, and dissolution, as well as other factors caused by interacting behavior, explain the depressing effects of fine particles on mineral separation. Solvable methods for these effects are further discussed. Novel flotation processes and more selective reagents are critical for further investigations on various approaches to improve the beneficiation efficiency of these ores. This review aims to provide a good reference for conducting studies related to complex ore flotation.

Invited Review
Anodized metal oxide nanostructures for photoelectrochemical water splitting
Ying-zhi Chen, Dong-jian Jiang, Zheng-qi Gong, Jing-yuan Li, and  Lu-ning Wang
2020, vol. 27, no. 5, pp. 584-601. https://doi.org/10.1007/s12613-020-1983-6
Abstract:

Photoelectrochemical (PEC) water splitting offers the capability of harvesting, storing, and converting solar energy into clean and sustainable hydrogen energy. Metal oxides are appealing photoelectrode materials because of their easy manufacturing and relatively high stability. In particular, metal oxides prepared by electrochemical anodization are typical of ordered nanostructures, which are beneficial for light harvesting, charge transfer and transport, and the adsorption and desorption of reactive species due to their high specific surface area and rich channels. However, bare anodic oxides still suffer from low charge separation and sunlight absorption efficiencies. Accordingly, many strategies of modifying anodic oxides have been explored and investigated. In this review, we attempt to summarize the recent advances in the rational design and modifications of these oxides from processes before, during, and after anodization. Rational design strategies are thoroughly addressed for each part with an aim to boost overall PEC performance. The ongoing efforts and challenges for future development of practical PEC electrodes are also presented.

Research Article
Suspension calcination and alkali leaching of low-grade high-sulfur bauxite: Desulfurization, mineralogical evolution and desilication
Hong-fei Wu, Jun-qi Li, Chao-yi Chen, Fei-long Xia, and  Zhen-shan Xie
2020, vol. 27, no. 5, pp. 602-610. https://doi.org/10.1007/s12613-019-1941-3
Abstract:

To enable the utilization of low-grade and high-sulfur bauxite, the suspension calcination was used to remove the sulfur and the activate silica minerals, and the calcinated bauxite was subjected to a desilication process in NaOH solution under atmospheric pressure. The desulfurization and desilication properties and mineralogical evolution were studied by X-ray diffraction, thermogravimetry–differential thermal analysis, scanning electron microscopy, and FactSage methods. The results demonstrate that the suspension calcination method is efficient for sulfur removal: 84.21% of S was removed after calcination at 1000°C for 2 min. During the calcination process, diaspore and pyrite were transferred to α-Al2O3, magnetite, and hematite. The phase transformation of pyrite follows the order FeS2 → Fe3O4 → Fe2O3, and the iron oxides and silica were converted into iron silicate. In the alkali-soluble desilication process, the optimum condition was an alkali solution concentration of 110 g/L, a reaction time of 20 min, and a reaction temperature of 95°C. The corresponding desilication ratio and alumina loss ratio were 44.9% and 2.4%, respectively, and the alumina-to-silica mass ratio of the concentrate was 7.9. The Al2O3·2SiO2, SiO2, and Al2O3 formed during the calcination process could react with NaOH solution, and their activity decreased in the order of Al2O3·2SiO2, SiO2, and Al2O3.

Research Article
Reaction mechanisms between molten CaF2-based slags and molten 9CrMoCoB steel
Lei-zhen Peng, Zhou-hua Jiang, and  Xin Geng
2020, vol. 27, no. 5, pp. 611-619. https://doi.org/10.1007/s12613-020-1976-5
Abstract:

Investigating the reaction mechanism between slag and 9CrMoCoB steel is important to develop the proper slag and produce qualified ingots in the electroslag remelting (ESR) process. Equilibrium reaction experiments between molten 9CrMoCoB steel and the slags of 55wt%CaF2–20wt%CaO–3wt%MgO–22wt%Al2O3xwt%B2O3 (x = 0.0, 0.5, 1.0, 1.5, 2.0, 3.0) were conducted. The reaction mechanisms between molten 9CrMoCoB steel and the slags with different B2O3 contents were deduced based on the composition of the steel and slag samples at different reaction times. Results show that B content in the steel can be controlled within the target range when the B2O3 content is 0.5wt% and the FeO content ranges from 0.18wt% to 0.22wt% in the slag. When the B2O3 content is ≥1wt%, the reaction between Si and B2O3 leads to the increase of the B content of steel. The additions of SiO2 and B2O3 to the slag should accord to the mass ratio of [B]/[Si] in the electrode, and SiO2 addition inhibits the reaction between Si and Al2O3.

Research Article
In situ observation of the effect of AlN particles on bainitic transformation in a carbide-free medium carbon steel
Xiao-jie Zhao, Zhi-nan Yang, and  Fu-cheng Zhang
2020, vol. 27, no. 5, pp. 620-629. https://doi.org/10.1007/s12613-019-1911-9
Abstract:

The bainitic transformation of the steels with different mass fractions of N, ~0.002% and 0.021%, was observed in situ by using high-temperature metalloscope. Micrometer- and nanometer-sized aluminum nitride (AlN) particles were found in the steel with 0.021% N. Grain boundaries, the interior of the grains, and AlN particles were used as initial nucleation sites of bainitic ferrite, and bainitic ferrite subunits served as new nucleation sites to induce secondary nucleation. The lengthening rate of bainitic ferrite varied at different nucleation sites, which was controlled by the repeated nucleation and growth of bainitic subunits. The AlN particles not only provided several nucleation sites, but also increased the autocatalytic effect on the transformation, further shortening the incubation period, promoting the bainitic transformation, and refining the bainitic microstructure.

Research Article
Effect of process parameters on the microstructure and properties of laser-clad FeNiCoCrTi0.5 high-entropy alloy coating
Ying Zhang, Teng-fei Han, Meng Xiao, and  Yi-fu Shen
2020, vol. 27, no. 5, pp. 630-639. https://doi.org/10.1007/s12613-019-1958-7
Abstract:

FeNiCoCrTi0.5 coatings with different process parameters were fabricated by laser cladding. The macro-morphology, phase, microstructure, hardness, and wear resistance of each coating were studied. The smoothness and dilution rate of the FeNiCoCrTi0.5 coating generally increased with the increase of specific energy (Es), which is the laser irradiation energy received by a unit area. FeNiCoCrTi0.5 coatings at different parameters had bcc, fcc, and Ti-rich phases as well as equiaxed, dendritic, and columnar structures. When Es increased, the size of each structure increased and the distribution area of the columnar and dendritic structures changed. The prepared FeNiCoCrTi0.5 coating with the Es of 72.22 J·mm–2 had the highest hardness and the best wear resistance, the highest hardness of the coating reached HV 498.37, which is twice the substrate hardness. The average hardness of the FeNiCoCrTi0.5 coating with the Es of 72.22 J·mm–2 was 15.8% higher than the lowest average hardness of the coating with the Es of 108.33 J·mm–2. The worn surface morphologies indicate that the FeNiCoCrTi0.5 coatings exhibited abrasive wear.

Research ArticleOpen Access
Optimization of the heat treatment of additively manufactured Ni-base superalloy IN718
Benedikt Diepold, Nora Vorlaufer, Steffen Neumeier, Thomas Gartner, and  Mathias Göken
2020, vol. 27, no. 5, pp. 640-648. https://doi.org/10.1007/s12613-020-1991-6
Abstract:

Additive manufacturing (AM) of Ni-base superalloy components can lead to a significant reduction of weight in aerospace applications. AM of IN718 by selective laser melting results in a very fine dendritic microstructure with a high dislocation density due to the fast solidification process. The complex phase composition of this alloy, with three different types of precipitates and high residual stresses, necessitates adjustment of the conventional heat treatment for AM parts. To find an optimized heat treatment, the microstructures and mechanical properties of differently solution heat-treated samples were investigated by transmission and scanning electron microscopy, including electron backscatter diffraction, and compression tests. After a solution heat treatment (SHT), the Nb-rich Laves phase dissolves and the dislocation density is reduced, which eliminates the dendritic substructure. SHT at 930 or 954°C leads to the precipitation of the δ-phase, which reduces the volume fraction of the strengthening γ′- and γ′′-phases formed during the subsequent two stage aging treatment. With a higher SHT temperature of 1000°C, where no δ-phase is precipitated, higher γ′ and γ′′ volume fractions are achieved, which results in the optimum strength of all of the solution heat treated conditions.

Research Article
Comparative studies on the hot corrosion behavior of air plasma spray and high velocity oxygen fuel coated Co-based L605 superalloys in a gas turbine environment
Kuzhipadath Jithesh and  Moganraj Arivarasu
2020, vol. 27, no. 5, pp. 649-659. https://doi.org/10.1007/s12613-019-1943-1
Abstract:

An improvement in the corrosion resistance of alloys at elevated temperature is a factor for their potential use in gas turbines. In this study, CoNiCrAlY has been coated on the L605 alloy using air plasma spray (APS) and high-velocity oxygen fuel (HVOF) coating techniques to enhance its corrosion resistance. Hot corrosion studies were conducted on uncoated and coated samples in a molten salt environment at 850°C under cyclic conditions. Thermogravimetric analysis was used to determine the corrosion kinetics. The samples were subjected to scanning electron microscopy, energy-dispersive spectroscopy, and X-ray diffraction for further investigation. In coated samples, the formation of Al2O3 and Cr2O3 in the coating acts as a diffusion barrier that could resists the inward movement of the corrosive species present in the molten salt. Coated samples showed very less spallation, lower weight gain, less porosity, and internal oxidation as compared to uncoated sample. HVOF-coated sample showed greater corrosion resistance and inferred that this is the best technique under these conditions.

Research Article
Microstructural analyses of aluminum–magnesium–silicon alloys welded by pulsed Nd: YAG laser welding
Hossain Ebrahimzadeh, Hassan Farhangi, Seyed Ali Asghar Akbari Mousavi, and  Arman Ghahramani
2020, vol. 27, no. 5, pp. 660-668. https://doi.org/10.1007/s12613-020-2027-y
Abstract:

Revealing grains and very fine dendrites in a solidified weld metal of aluminum–magnesium–silicon alloys is difficult and thus, there is no evidence to validate the micro- and meso-scale physical models for hot cracks. In this research, the effect of preheating on the microstructure and hot crack creation in the pulsed laser welding of AA 6061 was investigated by an optical microscope and field emission electron microscopy. Etching was carried out in the gas phase using fresh Keller’s reagent for 600 s. The results showed that the grain size of the weld metal was proportional to the grain size of the base metal and was independent of the preheating temperature. Hot cracks passed the grain boundaries of the weld and the base metal. Lower solidification rates in the preheated samples led to coarser arm spacing; therefore, a lower cooling rate. Despite the results predicted by the micro and meso-scale models, lower cooling rates resulted in increased hot cracks. The cracks could grow in the weld metal after solidification; therefore, hot cracks were larger than predicted by the hot crack prediction models.

Research Article
Processing of AM60 magnesium alloy by hydrostatic cyclic expansion extrusion at elevated temperature as a new severe plastic deformation method
Farshad Samadpour, Ghader Faraji, and  Armin Siahsarani
2020, vol. 27, no. 5, pp. 669-677. https://doi.org/10.1007/s12613-019-1921-7
Abstract:

Hydrostatic cyclic expansion extrusion (HCEE) process at elevated temperatures is proposed as a method for processing less deformable materials such as magnesium and for producing long ultrafine-grained rods. In the HCEE process at elevated temperatures, high-pressure molten linear low-density polyethylene (LLDPE) was used as a fluid to eliminate frictional forces. To study the capability of the process, AM60 magnesium rods were processed and the properties were investigated. The mechanical properties were found to improve significantly after the HCEE process. The yield and ultimate strengths increased from initial values of 138 and 221 MPa to 212 and 317 MPa, respectively. Moreover, the elongation was enhanced due to the refined grains and the existence of high hydrostatic pressure. Furthermore, the microhardness was increased from HV 55.0 to HV 72.5. The microstructural analysis revealed that ultrafine-grained structure could be produced by the HCEE process. Moreover, the size of the particles decreased, and these particles thoroughly scattered between the grains. Finite element analysis showed that the HCEE was independent of the length of the sample, which makes the process suitable for industrial applications.

Research Article
Reinforcing effect of laminate structure on the fracture toughness of Al3Ti intermetallic
Yang Cao, Dan-dan Zhang, Pei-jun Zhou, Kun Liu, Wu-yi Ming, and  Jun Ma
2020, vol. 27, no. 5, pp. 678-686. https://doi.org/10.1007/s12613-019-1899-1
Abstract:

Metal/intermetallic laminate composites can improve the mechanical properties of intermetallic materials using metal layers. In recent years, titanium aluminide intermetallics have received increasing attention due to their excellent performance properties, such as high melting point, high specific strength and stiffness, and good corrosion resistance. However, the low fracture toughness of Al3Ti alloys at room temperature has greatly limited their application, and fiber or particle reinforcement has not shown a significant toughening effect. Research into the reinforcing effects of the interface and near-interface zone on the fracture behavior of Al3Ti is lacking. Ti/Al3Ti metal/intermetallic laminate composite was synthesized from titanium and aluminum foils using vacuum hot-pressed sintering technology. The microstructure of the prepared material was analyzed by scanning electron microscope and electron backscattered diffraction. Results illustrate that both Ti and Al3Ti were single-phase and there was a noticeable stress concentration on the interface. To obtain indentation and cracks, loads were applied to different locations of the composite by a microhardness tester. The growth path of the cracks was then observed under microscope, showing that crack propagation was prevented by the interface between the Ti and Al3Ti layers, and the cracks that propagated parallel to the laminate shifted to the interface. Fracture toughness of the different areas, including Al3Ti layers, interface, and near-interface zone, were measured by the indentation fracture method. The fracture toughness at and near the interface was 1.7 and 2 times that of the Al3Ti layers, respectively. Results indicate that crack blunting and crack front convolution by the laminate structure was primarily responsible for increased toughness.

Research Article
Luminescence properties of nitrogen-rich Ca-SiAlON:Eu2+ phosphors prepared by freeze-drying assisted combustion synthesis
Zhang-lin Chen, Kai Liu, Xuan-yi Yuan, and  Ke-xin Chen
2020, vol. 27, no. 5, pp. 687-692. https://doi.org/10.1007/s12613-019-1934-2
Abstract:
Nitrogen-rich Eu2+-doped Ca-α-SiAlON phosphors (Cam/2−​​​​​​​xSi12−​​​​​​​​​​​​​​m−​​​​​​​​​​​​​​nAlm+nOnN16−​​​​​​​​​​​​​​n:xEu) were synthesized by a freeze-drying assisted combustion synthesis (CS) route. Fast-synthesized products with high purity and uniform particle morphology were confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The analysis of lattice parameters by comparison with empirical equations showed that the as-prepared phosphors had low oxygen content. A series of samples were prepared according to the stoichiometry of Cam/2−​​​​​​​0.08Si12−​​​​​​​​​​​​​​mAlmN16:0.08Eu for further research. The influences of m value on the luminescence properties were investigated in detail. As m increased, a redshift phenomenon was observed in both the excitation and emission spectra. First-principle electronic structure calculations showed that the 3d energy level of Ca played an important role in the occurrence of the redshift phenomenon.
Research Article
Effects of chelating agents on the sol−gel synthesis of nano-zirconia: Comparison of the Pechini and sugar-based methods
Faramarz Kazemi, Farzin Arianpour, Mahdiar Taheri, Ali Saberi, and  Hamid Reza Rezaie
2020, vol. 27, no. 5, pp. 693-702. https://doi.org/10.1007/s12613-019-1933-3
Abstract:

This study focused on the comparison of the Pechini and sugar-based combustion synthesis methods to produce nano-zirconia. Zirconium hydroxide was utilized as metal precursor and citric acid, sucrose, and fructose were used as chelating agents, followed by calcination at 500, 600, and 700°C in air, respectively. Characterization was conducted by thermal analysis, specific surface area measurement, Fourier transform infrared spectroscopy, X-ray diffraction, and scanning and transmission electron microscopy. When sucrose and citric acid were used as chelating agents during synthesis, mixtures of monoclinic and tetragonal phases were formed after calcination at 600 and 700°C. In the fructose samples, the tetragonal structure was the unique characterized phase. The tetragonal parameters in the fructose samples were determined using the diffraction data and the lattice parameter ratio was proven to increase with the temperature increase. Compared with the citrate and sucrose samples, the largest specific surface area (27 m2·g−1) and smallest particle size (39.1 nm) were obtained for the fructose sample after calcination at 700°C. The study revealed the formation of single-phase stabilized tetragonal zirconia using fructose as chelating agent after calcination at 500°C, and the presence and formation mechanism of stabilized tetragonal phase were also discussed on the basis of the X-ray and electron diffraction studies.

Research Article
Comparison of α particle detectors based on single-crystal diamond films grown in two types of gas atmospheres by microwave plasma-assisted chemical vapor deposition
Yan-zhao Guo, Jin-long Liu, Jiang-wei Liu, Yu-ting Zheng, Yun Zhao, Xiao-lu Yuan, Zi-hao Guo, Li-fu Hei, Liang-xian Chen, Jun-jun Wei, Jian-peng Xing, and  Cheng-ming Li
2020, vol. 27, no. 5, pp. 703-712. https://doi.org/10.1007/s12613-019-1944-0
Abstract:

Chemical vapor deposition (CVD)-grown diamond films have been developed as irradiation-resistant materials to replace or upgrade current detectors for use in extreme radiation environments. However, their sensitivity in practical applications has been inhibited by space charge stability issues caused by defects and impurities in pure diamond crystal materials. In this study, two high-quality CVD-grown single-crystal diamond (SCD) detectors with low content of nitrogen impurities were fabricated and characterized. The intrinsic properties of the SCD samples were characterized using Raman spectroscopy, stereomicroscopy, and X-ray diffraction with the rocking curve mode, cathode luminescence (CL), and infrared and ultraviolet-visible-near infrared spectroscopies. After packaging the detectors, the dark current and energy resolution under α particle irradiation were investigated. Dark currents of less than 5 pA at 100 V were obtained after annealing the electrodes, which is comparable with the optimal value previously reported. The detector that uses a diamond film with higher nitrogen content showed poor energy resolution, whereas the detector with more dislocations showed poor charge collection efficiency (CCE). This demonstrates that the nitrogen content in diamond has a significant effect on the energy resolution of detectors, while the dislocations in diamond largely contribute to the poor CCE of detectors.