Effect of the microstructure of Al 7050-T7451 on anodic oxide formation in sulfuric acid

The effect of the microstructure of an Al 7050-T7451 substrate on the anodic oxide formation in sulfuric acid was studied in this article. The microstructure of the substrate was assessed by optical microscope (OM) and transmission electron microscope (TEM). The surface and cross-section morphologies of the oxide films were examined by scanning electron microscope (SEM). The chemical composition of intermetallic particles in the alloys and films was investigated using energy dispersive spectroscope (EDS). The roles of intermetallic phases and grain or subgrain boundaries on the oxide film formation were researched using the potentiodynamic and potentiostatic polarization technique in sulfuric acid solution. The results show that the transition of coarse intermetallic particles or grain (subgrain) boundaries at the surface of Al alloys can be characterized by potentiodynamic polarization curves. The surface and cross-section micrographs of the anodic layer seem to preserve the microstructure of the substrate. Large cavities in the anodic films are caused by the preferential dissolution of coarse AItCuMg particles and the entrance of Cu-rich remnants into the electrolyte during anodizing. The Al₇Cu₂Fe particles tend to be occluded in the oxide layer or lose from the oxide surface because of peripheral trenching. Small pores in the films are induced by the dissolution of precipitates in grain or subgrain boundaries. The film surface of recrystallized grain bodies is smooth and homogeneous.     

Fabrication of Fe–TiC–Al2O3 composites on the surface of steel using a TiO2–Al–C–Fe combustion reaction induced by gas tungsten arc cladding

The aim of the present study was to fabricate Fe–TiC–Al₂O₃ composites on the surface of medium carbon steel. For this purpose, TiO₂–3C and 3TiO₂–4Al–3C–xFe (0 ≤ x ≤ 4.6 by mole) mixtures were pre-placed on the surface of a medium carbon steel plate. The mixtures and substrate were then melted using a gas tungsten arc cladding process. The results show that the martensite forms in the layer produced by the TiO₂–3C mixture. However, ferrite–Fe₃C–TiC phases are the main phases in the microstructure of the clad layer produced by the 3TiO₂–4Al–3C mixture. The addition of Fe to the TiO₂–4Al–3C reactants with the content from 0 to 20wt% increases the volume fraction of particles, and a composite containing approximately 9vol% TiC and Al₂O₃ particles forms. This composite substantially improves the substrate hardness. The mechanism by which Fe particles enhance the TiC + Al₂O₃ volume fraction in the composite is determined.     

Evaluation of precipitation hardening in TiC-reinforced Ti<Subscript>2</Subscript>AlNb-based alloys

Ti₂AlNb-based alloys with 0.0wt%, 0.6wt%, and 2.0wt% carbon nanotube (CNT) addition were fabricated from spherical Ti-22Al-25Nb powder by sintering in the B2 single-phase region. Phase identification and microstructural examination were performed to evaluate the effect of carbon addition on the hardness of the alloys. Carbon was either in a soluble state or in carbide form depending on its concentration. The acicular carbides formed around 1050℃ were identified as TiC and facilitated the transformation of α₂+B2 → O. The TiC was located within the acicular O phase. The surrounding O phase was distributed in certain orientations with angles of 65° or 90° O phase particles. The obtained alloy was composed of acicular O, Widmanstatten B2+O, and acicular TiC. As a result of the precipitation of carbides as well as the O phase, the hardness of the alloy with 2.0wt% CNT addition increased to HV 429±9.     

Microstructure and mechanical properties of spark plasma sintered Ti-Mo alloys for dental applications

Ti-Mo alloys with various Mo contents from 6wt% to 14wt% were processed by spark plasma sintering based on elemental powders. The influence of sintering temperature and Mo content on the microstructure and mechanical properties of the resulting alloys were investigated. For each Mo concentration, the optimum sintering temperature was determined, resulting in a fully dense and uniform microstructure of the alloy. The optimized sintering temperature gradually increases in the range of 1100–1300℃ with the increase in Mo content. The microstructure of the Ti-(6–12)Mo alloy consists of acicular α phase surrounded by equiaxed grains of β phase, while the Ti-14Mo alloy only contains single β phase. A small amount of fine α lath precipitated from β phase contributes to the improvement in strength and hardness of the alloys. Under the sintering condition at 1250℃, the Ti-12Mo alloy is found to possess superior mechanical properties with the Vickers hardness of Hv 472, the compressive yield strength of 2182 MPa, the compression rate of 32.7%, and the elastic modulus of 72.1 GPa. These results demonstrate that Ti-Mo alloys fabricated via spark plasma sintering are indeed a perspective candidate alloy for dental applications.     

Microstructure and corrosion behavior of NiTi shape memory alloys sintered in the SPS process

NiTi shape memory alloys (SMAs) was developed using the spark-plasma sintering (SPS) process with different average particle size (45 µm and 10 µm) under various temperature. The influence of particle size and temperature on the density, microstructure, and corrosion behavior of the NiTi in simulated body fluid was examined. The porosity decreased with increasing sintering temperature and decreasing particle size, which resulted in an increase in density of the alloy. Increasing the sintering temperature led to the formation of Ni- and Ti-rich intermetallic such as Ni₃Ti and NiTi₂. The formation of these secondary phases influenced the corrosion behavior of NiTi by changing its chemical composition. The planar structure of NiTi was transformed into a dendritic structure at 900℃, which resulted in the formation of uniform oxide and phosphate layers on the entire surface. A high corrosion potential and low corrosion current density were achieved with NiTi prepared with 10 µm particles at 900℃, which exhibited superior corrosion resistance.     

Sintering Behavior and Microstructure of diphasic β-Sialon/Al2O3 Composites

Six compositions with different ratio of β-Sialon/Al₂O₃ were synthesized from Al₂O₃, Si₃N₄ and SiO₂ by sintering with 3%Y₂O₃(mass fraction) as addition under the cover with powders of SiC+C and at nitrogen atmosphere. Theeffects of atmosphere, sintering temperature and composition on the sintering behavior were studied. The results showedthat the composites reached the best sintering behavior with the highest density about 92% at 1 650℃ under the weakreduction atmosphere. Finally the relative density of diphasic β-Sialon and β-Sialon/Al₂O₃ composites were studied andpredicted using ANN (Artificial Neural Networks) method and the results were experimental examined by fore randomsamples.     

Microstructure and wear properties of the electroslag remelting layer reinforced by TiC particles

The electroslag remelting (ESR) layer reinforced by TiC particles was obtained by electroslag remelting. The microstructure and wear properties of the ESR layer were studied by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), and wear test. The results indicate that TiC particles are synthesized by self-propagating high-temperature synthesis (SHS) reaction during the electroslag remelting process. The size of TiC particles is in the range of 1-10 μm, and the distribution of TiC particles is uniform, from outside to inside of the ESR layer, and the volume fraction and the size of TiC particles decrease gradually. Molten iron and slag flow into porosity due to the SHS process leading to rapid densification and the elimination of porosity in the ESR layer during the ESR process. TiC particles enhance the wear resistance of the ESR layer, whereas CaF₂ can improve the high temperature lubricating property of the ESR layer.     

Precipitation characteristic of high strength steels microalloyed with titanium produced by compact strip production

Transmission electron microscopy (TEM) and physics-chemical phase analysis were employed to investigate the precipitates in high strength steels microalloyed with Ti produced by compact strip production (CSP). It was seen that precipitates in Ti microalloyed steels mainly included TiN, Ti₄C₂S₂, and TiC. The size of TiN particles varied from 50 to 500 nm, and they could precipitate during or before soaking. The Ti₄C₂S₂ with the size of 40-100 nm might precipitate before rolling, and the TiC particles with the size of 5-50 nm precipitated heterogeneously. High Ti content would lead to the presence of bigger TiC particles that precipitated in austenite, and by contrast, TiC particles that precipitated in ferrite and the transformation of austenite to ferrite was smaller. They were less than 30 nm and mainly responsible for precipitate strengthening. It should be noted that the TiC particles in higher Ti content were generally smaller than those in the steel with a lower Ti content.     

CaO-Al2O3-SiO2系烧结微晶玻璃的结晶过程

To control the quatity, grain size and morphology of crystals in glass-ceramics, relative crystallinity of CaO-Al₂O₃-SiO₂ sintered glass-ceramics was determined using XRD method. The crystallization at different temperature range was studied by SEM. The results indicate that sintering at low temperature (below 900　℃), crystals originate at the interface of glass particles and grow toward the inside of particles. There was no crystallization taken place within particles and the crystallinity was relatively low. Higher crystallinity can be obtained at mid temperature range (900～1　050　℃) when crystallization takes place both at and within the interface of glass particles. The crystallinity tends to decrease at high temperature range (1　050～1　100　℃) due to the rapid sintering and drop of driving force for phase transformation.     

Effect of sintering temperature on the preparation of Cu–Ti3SiC2 metal matrix composite

Ti3SiC2 has the potential to replace graphite as reinforcing particles in Cu matrix composites for applications in brush,electrical contacts and electrode materials.In this paper the fabrication of Cu-Ti3SiC2 metal matrix composites prepared by warm compaction powder metallurgy forming and spark plasma sintering(SPS) was studied.The stability of Ti3SiC2 at different sintering temperatures was also studied.The present experimental results indicate that the reinforcing particles in Cu-Ti3SiC2 composites are not stable at and above 800℃.The decomposition of Ti3SiC2 will lead to the formation of TiC and/or other carbides and TiSi2.If purity is the major concern,the processing and servicing temperatures of the Cu-Ti3SiC2 composite should be limited to 750℃ or lower.The composites prepared by warm compaction forming and SPS sintering at 750℃ have lower density when compared with the composites prepared by SPS sintering at 950℃,but their electrical resistivity values are very close to each other and even lower.     

Microstructure and mechanical properties of Nb–Mo–ZrB2 composites prepared by hot-pressing sintering

Nb–Mo–ZrB₂ composites (V(Nb)/V(Mo)=1) with 15vol% or 30vol% of ZrB₂ were fabricated by hot-pressing sintering at 2000℃. The phases, microstructure, and mechanical properties were then investigated. The composites contain Nb-Mo solid solution (denoted as (Nb, Mo)ss hereafter), ZrB, MoB, and NbB phases. Compressive strength test results suggest that the strength of Nb–Mo–ZrB₂ composites increases with increasing ZrB₂ content; Nb–Mo–30vol%ZrB₂ had the highest compressive strength (1905.1 MPa). The improvement in the compressive strength of the Nb–Mo–ZrB₂ composites is mainly attributed to the secondary phase strengthening of the stiffer ZrB phase, solid-solution strengthening of the (Nb, Mo)ss matrix as well as fine-grain strengthening. The fracture toughness decreases with increasing ZrB₂ content. Finally, the fracture modes of the Nb–Mo–ZrB₂ composites are also discussed in detail.     

Spark plasma and microwave sintering of Al6061 and Al2124 alloys

Despite the importance of aluminum alloys as candidate materials for applications in aerospace and automotive industries, very little work has been published on spark plasma and microwave processing of aluminum alloys. In the present work, the possibility was explored to process Al2124 and Al6061 alloys by spark plasma and microwave sintering techniques, and the microstructures and properties were compared. The alloys were sintered for 20 min at 400, 450, and 500℃. It is found that compared to microwave sintering, spark plasma sintering is an effective way to obtain homogenous, dense, and hard alloys. Fully dense (100%) Al6061 and Al2124 alloys were obtained by spark plasma sintering for 20 min at 450 and 500℃, respectively. Maximum relative densities were achieved for Al6061 (92.52%) and Al2124 (93.52%) alloys by microwave sintering at 500℃ for 20 min. The Vickers microhardness of spark plasma sintered samples increases with the increase of sintering temperature from 400 to 500℃, and reaches the values of Hv 70.16 and Hv 117.10 for Al6061 and Al2124 alloys, respectively. For microwave sintered samples, the microhardness increases with the increase of sintering temperature from 400 to 450℃, and then decreases with the further increase of sintering temperature to 500℃.     

Effects of metal binder on the microstructure and mechanical properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-based micro-nanocomposite ceramic tool material

The Al₂O₃-(W,Ti)C composites with Ni and Mo additions varying from 0vol% to 12vol% were prepared via hot pressing sintering under 30 MPa. The microstructure was investigated via X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS). Mechanical properties such as flexural strength, fracture toughness, and Vickers hardness were also measured. Results show that the main phases Al₂O₃ and (W,Ti)C were detected by XRD. Compound MoNi also existed in sintered nanocomposites. The fracture modes of the nanocomposites were both intergranular and transgranular fractures. The plastic deformation of metal particles and crack bridging were the main toughening mechanisms. The maximum flexural strength and fracture toughness were obtained for 9vol% and 12vol% additions of Ni and Mo, respectively. The hardness of the composites reduced gradually with increasing content of metals Ni and Mo.     

Evaluation of precipitation hardening in TiC-reinforced Ti_2AlNb-based alloys

Ti_2AlNb-based alloys with 0.0 wt%, 0.6 wt%, and 2.0 wt% carbon nanotube(CNT) addition were fabricated from spherical Ti–22 Al–25 Nb powder by sintering in the B2 single-phase region. Phase identification and microstructural examination were performed to evaluate the effect of carbon addition on the hardness of the alloys. Carbon was either in a soluble state or in carbide form depending on its concentration. The acicular carbides formed around 1050℃ were identified as TiC and facilitated the transformation of α_2 + B2 → O. The TiC was located within the acicular O phase. The surrounding O phase was distributed in certain orientations with angles of 65° or 90° O phase particles. The obtained alloy was composed of acicular O, Widmanstatten B2 +O, and acicular TiC. As a result of the precipitation of carbides as well as the O phase, the hardness of the alloy with 2.0 wt% CNT addition increased to HV 429 ± 9.     

Microstructure and electrolysis behavior of self-healing Cu-Ni-Fe composite inert anodes for aluminum electrowinning

The microstructure evolution and electrolysis behavior of (Cu₅₂Ni₃₀Fe₁₈)-xNiFe₂O₄ (x=40wt%, 50wt%, 60wt%, and 70wt%) composite inert anodes for aluminum electrowinning were studied. NiFe₂O₄ was synthesized by solid-state reaction at 950℃. The dense anode blocks were prepared by ball-milling followed by sintering under a N₂ atmosphere. The phase evolution of the anodes after sintering was determined by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The results indicate that a substitution reaction between Fe in the alloy phase and Ni in the oxide phase occurs during the sintering process. The samples were also examined as inert anodes for aluminum electrowinning in the low-temperature KF-NaF-AlF₃ molten electrolyte for 24 h. The cell voltage during electrolysis and the corrosion scale on the anodes were analyzed. The results confirm that the scale has a self-repairing function because of the synergistic reaction between the alloy phase with Fe added and the oxide phase. The estimated wear rate of the (Cu₅₂Ni₃₀Fe₁₈)-50NiFe₂O₄ composite anode is 2.02 cm·a-1.     

Preparation of in situ and ex situ reinforced Fe–10Cr–1Cu–1Ni–1Mo–2C containing NbC particles by milling and hot pressing

An in situ and ex situ reinforced powder metallurgy(PM) steel was prepared by the combination of high-energy ball milling and subsequent hot pressing of elemental mixed powders of Fe–10Cr–1Cu–1Ni–1Mo–2C by mass with the addition of Nb C particles. A 40-h milling pretreatment makes the powder particles nearly equiaxed with an average diameter of ~8 μm, and the ferrite grain size is refined to ~6 nm. The sintered density reaches 99.0%–99.7% of the theoretical value when the sintering is conducted at temperatures greater than 1000°C for 30 min. In the sintered bulk specimens, the formation of an in situ M7C3(M = Cr, Fe, Mo) phase is confirmed. M7C3 carbides with several hundred nanometers in size are uniformly distributed in the matrix. Some ultra-fine second phases of 50–200 nm form around the ex situ Nb C and in situ M7C3 particles. The sintered steel exhibits an excellent combination of hardness( Hv 500) and compressive strength(2100–2420 MPa).     

Corrosion behavior of Ni–P/nano-TiC composite coating prepared in electroless baths containing different types of surfactant

In current research, in order to enhance the incorporation of nano-sized TiC particles into electroless Ni–P (EN) coating, different types of surfactant (cationic, anionic, and polymeric) were added to the plating bath. The effects of addition of the surfactants on surface morphology, deposition rate, TiC and P contents of the prepared coatings were investigated. The surface morphology was evaluated by scanning electron microscopy (SEM). It was demonstrated that in the presence of the anionic, polymeric and somehow cationic surfactants, TiC nano-particles were embedded in the matrix which influenced the surface morphology. The effect of surfactant types on the corrosion properties of Ni–P/TiC coated steel was also studied. Corrosion behavior of the coated steel was evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) which affected by the incorporation of TiC particles into the Ni–P matrix. The level of corrosion resistance improvement depended largely on the phosphorous and TiC concentration of the applied coating.     

Porous α-Al2O3 thermal barrier coatings with dispersed Pt particles prepared by cathode plasma electrolytic deposition

Porous α-Al₂O₃ thermal barrier coatings (TBCs) containing dispersed Pt particles were prepared by cathode plasma electrolytic deposition (CPED). The influence of the Pt particles on the microstructure of the coatings and the CPED process were studied. The prepared coatings were mainly composed of α-Al₂O₃. The average thickness of the coatings was approximately 100 μm. Such single-layer TBCs exhibited not only excellent high-temperature cyclic oxidation and spallation resistance, but also good thermal insulation properties. Porous α-Al₂O₃ TBCs inhibit further oxidation of alloy substrates because of their extremely low oxygen diffusion rate, provide good thermal insulation because of their porous structure, and exhibit excellent mechanical properties because of the toughening effect of the Pt particles and because of stress relaxation induced by deformation of the porous structure.     

Microstructure and mechanical properties of a hot-extruded Al-based composite reinforced with core-shell-structured Ti/Al3Ti

An Al-based composite reinforced with core-shell-structured Ti/Al₃Ti was fabricated through a powder metallurgy route followed by hot extrusion and was found to exhibit promising mechanical properties. The ultimate tensile strength and elongation of the composite sintered at 620℃ for 5h and extruded at a mass ratio of 12.75:1 reached 304 MPa and 14%, respectively, and its compressive deformation reached 60%. The promising mechanical properties are due to the core-shell-structured reinforcement, which is mainly composed of Al₃Ti and Ti and is bonded strongly with the Al matrix, and to the reduced crack sensitivity of Al₃Ti. The refined grains after hot extrusion also contribute to the mechanical properties of this composite. The mechanical properties might be further improved through regulating the relative thickness of Al-Ti intermetallics and Ti metal layers by adjusting the sintering time and the subsequent extrusion process.     

Sintering behavior,microstructure and properties of TiC-FeCr hard alloy

TiC based cermets were produced with FeCr, as a binder, by conventional P/M (powder metallurgy) to near 〉97% of the theoretical density. Sintering temperature significantly affects the mechanical properties of the composite. The sintering temperature of 〉1360℃ caused severe chemical reaction between TiC particles and the binder phase. In the TiC-FeCr cermets, the mechanical properties did not vary linearly with the carbide content. Optimum mechanical properties were found in the composite containing 57wt% TiC reinforcement, when sintered at 1360℃ for 1 h. Use of carbon as an additive enhanced the mechanical properties of the composites. Cermets containing carbon as an additive with 49wt% TiC exhibited attractive mechanical properties. The microstructure of the developed composite contained less or no debonding, representing good wettabifity of the binder with TiC particles. Homogeneous distribution of the TiC particles ensured the presence of isotropic mechanical properties and homogeneous distribution of stresses in the composite. Preliminary experiments for evaluation of the oxidation resistance of FeCr bonded TiC cermets indicate that they are more resistant than WC-Co hardmetals.