Fabrication of Fullerene-Reinforced Aluminum Matrix Nanocomposites

Fullerene-reinforced A1 matrix nanocomposites were fabricated by high-energy mechanical milling followed by consolidation through hot extrusion or high-pressure torsion(HPT). The results indicate that a relatively homogeneous microstructure consisting of elongated, micrometer-sized A1 grains is formed in the hot-extruded specimens. However, the microstructure is not uniform along the radius of the HPT disks, which includes coarse grains near the center of the disk and ultrafine grains in the middle and along the edge of the specimen. Microstructural evaluations of the HPT disks indicate that A1 grain refinement occurs due to the addition of fullerene, as grain size is reduced to 60 nm from 118 nm. The formation of the harmful aluminum carbide phase is not detected during the fabrication of Al/C_(60) nanocomposites. The hardness, yield stress, and ultimate tensile strength of the Al-2 vol.% C_(60) nanocomposites are about 27-160% higher than those of the monolithic A1 samples, revealing the effective strengthening of fullerene particles in A1 matrix. Moreover,mechanical properties of the Al/fullerene nanocomposites are significantly enhanced(59-272%) by utilizing HPT in comparison to hot-extruded specimens due to their much finer A1 grain structure. The reduction in the number and the size of the dimples, as well as the formation of smooth regions on the tensile fracture surface of Al/C_(60), results in their overall lower ductility compared to monolithic Al.     

Fabrication of CuZr(Al) bulk metallic glasses by high pressure torsion

In this paper, pure Cu, Zr and Al metal sheets were alternatively stacked and, then, cold roll-bonded. The overall compositions of the stacks were Cu₇₁Zr_29, Cu₆₂Zr_38, Cu₅₃Zr₄₇ and Cu₅₂Zr₃₈Al₁₀. The bonded materials were subsequently sliced into disks and, then, used for high pressure torsion (HPT) process. After HPT process up to maximum equivalent strain of about 1200 at ambient temperature, bulk materials with 10 mm in diameter and 0.30 mm in thickness were obtained. The phase constitution and microstructural evolutions of the HPTed samples with various rotations ranging from 0.5 to 20 were studied by XRD, FESEM and TEM. Thermal properties of the HPTed samples were analyzed by DSC. It was found that CuZr(Al) BMGs could be fabricated with a wide composition range by the HPT process. Tensile tests were used to measure the mechanical properties of the HPT processed samples. The mechanism of the solid-state amorphization during the HPT process was put forward and discussed.     

Electrochemical corrosion behaviour of Sn–Ag–Cu (SAC) eutectic alloy in a chloride containing environment

The corrosion behaviour of the Sn_94.5Ag_3.8Cu_1.5 (SAC) eutectic alloy was investigated in 0.1 M NaCl solution by potentiodynamic polarization and impedance spectroscopy measurements and compared with that of the conventional Sn_73.9Pb_23.1 eutectic solder employed for a long time in the packaging of microelectronic components and devices. Scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) were used to characterize the SAC eutectic alloy prior to and after the electrochemical tests. The electrochemical results indicated that the Sn–Ag–Cu eutectic alloy exhibits better corrosion behaviour than the Sn–Pb eutectic solder in NaCl solution. The presence of a corrosion products layer constituted by tin oxy‐chloride was detected at the surface of both alloys investigated after the electrochemical tests. The better corrosion behaviour of SAC eutectic alloy compared to Sn–Pb eutectic solder is ascribed to the formation of a more compact surface film of corrosion products with improved protective properties owing to the presence of copper and silver, as revealed by EPMA.     

Microstructure formation in Sn-Cu-Ni solder alloys

A systematic study of the solidification mechanisms of binary Sn-Cu and ternary Sn-Cu-Ni alloys has been carried out. It is found that Sn-Cu is a weakly-irregular eutectic system with Cu₆Sn₅ as the leading phase. Interestingly, two different eutectic morphologies (coarse and fine) are found to grow simultaneously during eutectic solidification. When the growth rate or composition is changed, the eutectic interface breaks down into a cellular eutectic with the fine eutectic in the center of the cells and the coarse one at the cell boundaries. The mechanisms responsible for these phenomena are discussed in conjunction with the beneficial effect of Ni additions to the Sn-Cu solder.     

Thermodynamic analysis of aluminate stability in the eutectic bonding of copper with alumina

The interfacial reactions between Cu and Al₂O₃ which occur during the eutectic bonding process have been examined. A thermodynamic analysis of phase equilibria in the Cu–Al–O system shows that a five-phase equilibrium exists among solid copper with oxygen in solution, liquid copper with oxygen in solution, solid CaAlO₂, Al₂O₃ and oxygen gas at the invariant state T=1348 K, p_O2=5.6×10⁻⁷ atm (0.055 Pa). The existence of this invariant state has been confirmed experimentally by heating slightly oxidized copper disks placed in contact with alumina disks. The experimentally determined invariant state was found to be in good agreement with that calculated. During eutectic bonding the compound CuAlO₂ forms at the interface between copper and the alumina in specimens containing solid copper only at temperatures lower than 1348±3 K.     

Effects of laser processing parameters on solidification microstructures of ternary Al2O3/YAG/ZrO2 eutectic in situ composite and its thermal property

Rapid surface resolidification with a high powered CO₂-laser was performed in preparing directionally solidified Al₂O₃/YAG/ZrO₂ ternary eutectic ceramic in situ composite. The effects of laser processing parameters on the solidification microstructure characteristics and thermal properties were studied by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffractometry (XRD) and synthetically thermal analysis (STA). Detailed investigations of the influence of laser power and scanning rate on the preparation and microstructural parameters of the ternary eutectic were presented. Moreover, the eutectic phase separation rule at high temperature was discussed. The results indicate that solidification microstructure of the ternary eutectic composite is greatly influenced by the laser processing parameters. The synthetically thermal analysis shows that the eutectic temperature of ternary Al₂O₃/YAG/ZrO₂ composite is 1 738 °C, well matching the phase diagram of Al₂O₃-Y₂O₃-ZrO₂.     

Microstructure and tensile properties of nanocrystalline (FeNiCoCu)1−xTixAlx high entropy alloys processed by high pressure torsion

In this study, an equal-atomic FeNiCoCu high entropy alloy (HEA) and a Ti and Al added (FeNiCoCu)₈₆Ti₇Al₇ HEA were subjected for high pressure torsion (HPT) up to 10 rotations. Microstructure observation and mechanical properties test revealed that significant grain refinement as well as enhanced strength could be obtained in both HPT processed alloys. The HPT processed FeNiCoCu HEA alloy shows nanocrystalline structure consisting of FCC matrix (grain size ∼100 nm) and FeCo-riched BCC phase. The HPT processed (FeNiCoCu)₈₆Ti₇Al₇ HEA alloy shows nanocrystalline structured FCC matrix (mean grain size ∼50 nm) and refined NiCoTiAl-riched particles (mean particle size ∼0.71 μm). The ultimate tensile strength of the HPT processed FeNiCoCu and (FeNiCoCu)₈₆Ti₇Al₇ alloys are 1402 MPa and 1849 MPa, respectively. The microstructure evolution during HPT and strengthening mechanisms of the HPT processed specimens were discussed.     

Evolution of eutectic structures in Al-Zn-Mg-Cu alloys during heat treatment

1 Introduction The solute redistribution during solidification leads to microsegregation and the formation of coarse intermetallic particles, which can significantly influence the properties and productivity of the 7000 series aluminium alloys[1]. In ord…     

Phase transformation behavior of Al9(Mn,Ni)2 eutectic phase during heat treatment at 600 °C in Al-4Ni-2Mn alloy

The morphology evolution and phase transformation of Al₉(Mn,Ni)₂ eutectic phase in an Al-4Ni-2Mn alloy during heat treatment at 600 °C were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results show that nearly all of the eutectic fibers change into prolate ellipsoid and spherical particles in the process of heat treatment, and Ostwald ripening phenomenon occurs in the eutectic region with the increase of the heat treatment time. Besides, a phase transformation from Al₉(Mn,Ni)₂ to O-phase is confirmed. The morphologies of the transformed particles indicate that the O-phase preferentially nucleates on the specific crystal plane of the Al₉(Mn,Ni)₂ eutectic phase and grows in a certain direction. During the phase transformation, the (010)[001] slip system in O-phase is activated, and the resultant slip traces appear on the surface of some O-phase particles.     

Onset of rod eutectic morphology in directional solidification

Abstract

Initial dynamics of morphological selection in a succinonitrile-(D)camphor organic transparent rod eutectic system is investigated experimentally using directional solidification with specimen thicknesses of 20 and 200 μm. The shape of the solid/liquid interface, the specimen thickness, the initial single-phase boundary thickness, and the grain boundaries are all observed to influence the onset of the eutectic morphology in a geometrically constrained system. Additionally, initiation of eutectic growth within a thin liquid layer at the specimen slide wall and lateral propagation of the array is observed, as suggested in a previous study by the present authors.     

High-pressure torsion for enhanced atomic diffusion and promoting solid-state reactions in the aluminum–copper system

This study reports that solid-state reactions occur by the application of high-pressure torsion (HPT) to the Al–Cu system even at low homologous temperature. A bulk form of disc consisting of two separate half-discs of pure Al and pure Cu are processed by HPT at ambient temperature under a pressure of 6 GPa. X-ray diffraction analysis and high-resolution transmission electron microscopy confirm the formation of different intermetallic phases such as Al₂Cu, AlCu and Al₄Cu₉, as well as the dissolution and supersaturation of Al and Cu in each matrix. It is shown that the diffusion coefficient is enhanced by 10¹²–10²² times during the HPT processing in comparison with the lattice diffusion and becomes comparable to the surface diffusion. The enhanced diffusion is attributed to the presence of a high density of lattice defects such as vacancies, dislocations and grain boundaries produced by HPT processing.     

Commercial AM60 alloy for semisolid processing: Alloy optimization and thermodynamic analysis

Semisolid processing is now a commercially successful manufacturing route to produce net-shape parts in automotive industry. The conspicuous results of alloy optimization with thermodynamic simulations for semisolid processing of commercial AM60 alloy were present. The results indicate that the available processing temperature range of AM60 alloy is 170 °C. The temperature sensitivity of solid fraction decreases with increasing solid fraction or with decreasing temperature above eutectic reaction temperature of AM60 alloy. When the solid fraction ?_s is 0.4, corresponding processing temperature is 603.8 °C and the sensitivity -d?_s/dT is 0.0184. The effects of various alloying elements on the solidification behavior and SSM processability of AM60 alloy were calculated with Pandat software.     

Synthesis and characterization of Ba<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>Co<Subscript>0.8</Subscript>Fe<Subscript>0.2</Subscript>O<Subscript>3−σ</Subscript>

Perovskite-type Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO) powders were synthesized using two methods, solid-state reaction (SSR) method and citrate-EDTA complexing method (CC-EDTA). Then the powders were pressed to green disks of 19 mm in diameter and sintered at 1140°C for 5 h. The shrinkage rate and relative density of the membranes prepared from the perovskite-type powders were determined and calculated, and the powders and derived membranes were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that the shrinkage rates of the two kinds of disks are nearly the same (about 10%). The disks prepared by the SSR method had a bigger grain size and lower relative density than those prepared by the CC-EDTA method. The conductivity of the membranes prepared by the SSR method was about 38 S/cm, higher than that of the membranes prepared by the CC-EDTA method, which was about 30 S/cm, at the same temperature of 600°C.     

Synthesis and protection of AM50 magnesium alloy and its composites using environmentally pretreatment electrolyte

This article details with the synthesis and surface modification of AM50 magnesium alloy and its composite (AM50-5%ZrO₂ and AM50-10% ZrO₂) substrate to improve the corrosion resistance. Environmentally friendly chemical conversion coating based on permanganate-phosphate solution was used in this study. Deposition of electroless Ni-P alloy and electrodeposition Ni onto AM50 magnesium alloys have been carried out. The influences of pretreatment steps on the surface morphology and composition analysis have been studied using scanning electron microscope/energy dispersed X-ray system (SEM/EDS). Additionally, the surface morphology and phase composition of the coatings were determined with (SEM/EDS) and X-ray diffraction (XRD), respectively. Moreover, potentiodynamic polarization tests have been used to estimate the corrosion behavior of unprotected and protected AM50 Mg and its composite alloys in 5% NaCl solutions.     

Severe plastic deformation driven nanostructure and phase evolution in a Al0.5CoCrFeMnNi dual phase high entropy alloy

The effect of severe plastic deformation on microstructure and phase evolution was investigated in a dual phase Al_0.5CoCrFeMnNi high entropy alloy (HEA). For this purpose, the as-cast HEA was subjected to initial thermo-mechanical processing by warm-rolling and annealing. The annealed dual phase alloy showed FCC and B2 phases. The B2 phase was enriched with Ni and Al, while the converse held good for the FCC phase. These annealed HEA specimens were severely deformed by high pressure torsion (HPT) up to five complete rotations (R). The severely deformed HEA revealed nanostructured FCC grains containing nano-twinned regions and coarser B2 phase. The nanostructure formation in the softer FCC phase was attributed to greater strain partitioning and propensity for the formation of nano-twins. Although with increasing rotations, the hardness difference between the edge and centre region was reduced, the 5R HPT processed specimens showed inhomogeneity featured by intermittent hardness spikes. Upon annealing, recrystallized dual phase microstructure was confirmed in the 5R HPT processed specimen. Microstructural differences between centre and edge regions were revealed by way of large B2 clusters (5 μm-10 μm) at the centre region. Remarkably, annealing resulted in the formation of a (Fe,Cr) rich σ-phase. The formation of σ-phase resulted in much greater hardness inhomogeneity in the annealed material as compared to the 5R HPT processed material.     

Printability of gas atomized Mo-Si-B powders by laser metal deposition

Mo-Si-B alloys, as a more and more frequently considered high-temperature material, face the challenge of machining complex shapes. In the present work, the feasibility of printing pre-alloyed Mo-Si-B powder materials via laser metal deposition (LMD) process was firstly demonstrated. Mo-Si-B powder was manufactured via gas atomization (GA) process out of solid raw materials meeting the requirements for additive manufacturing (AM) regarding flowability and particle size. Investigations of the powder particles after GA and detailed analyses of the printability and microstructural evolution of the multi-phase Mo_ss-Mo₃Si-Mo₅SiB₂ built are presented. As a result, distinct zones resulting from the layer-wise LMD process were observed next to a microstructure of primarily solidified Mo_ss phases embedded in fine dispersed eutectic regions. The hardness of the LMD processed material is shown to be comparable with Mo-Si-B alloys produced by ingot metallurgy (IM).     

High-pressure-torsion deformation of melt-spun Nd<Subscript>9</Subscript>Fe<Subscript>85</Subscript>B<Subscript>6</Subscript> alloy

The effect of severe plastic deformation by high-pressure torsion (HPT) at room temperature and subsequent annealing on the magnetic properties and structural transformations of the melt-spun alloy (MSA) Nd₉Fe₈₅B₆ is studied. The melt-spun ribbons in three structural states, such as nanocrystalline, mixed amorphous-nanocrystalline, and quasi-amorphous, have been subjected to deformation. In the nanocrystalline alloy, HPT leads to the decomposition of part of the Nd₂Fe₁₄B phase into the amorphous phase and α-Fe nanocrystals. The deformation of the alloy in the quasi-amorphous state leads also to the precipitation of a great amount of α-Fe nanocrystals; in this case, the amorphous matrix is depleted of iron. During the HPT of the MSA in the mixed amorphous + nanocrystaline state, both structural transformations occur. The annealing of deformed samples at above 500°C restores the two-phase (Nd₂Fe₁₄B + α-Fe) nanocrystalline state. This is accompanied by increasing magnetic hysteretic properties. The HPT has been found to suppress the formation of nonequilibrium magnetically soft phases, such as NdFe₇ and Nd₂Fe₂₃B₃, that precipitate upon annealing of the melt-spun amorphous alloy. This promotes the formation of an optimum nanocrystalline structure of the α-Fe/Nd₂Fe₁₄B composite material and an increase in its magnetic hysteretic properties because of enhancement of the intergranular exchange interaction. Compact micromagnets 6–15 mm in diameter and 0.2 mm thick, which were prepared from the Nd₉Fe₈₅B₆ alloy using HPT and subsequent annealing, exhibit the following characteristics: B _r = 11.4 kG, H _c = 5.4 kOe, and (BH)_max = 17.1 MG Oe.     

Hardness,Electrical Conductivity and Thermal Stability of Externally Oxidized Cu-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Composite Processed by SPD

Cu-Al₂O₃ composites prepared by external oxidation method were further enhanced by severe plastic deformation (SPD) processing, including equal-channel angular pressing (ECAP) and high-pressure torsion (HPT) methods. The HV hardness and electrical conductivity of the samples before and after SPD processing were tested. Results revealed that ECAP samples (with an equivalent strain of about 5.34) showed a relative small increase in hardness, whereas a significant decrease in electrical conductivity. The HPT samples (with an equivalent strain of about 6.94) showed not only a much improved hardness but also a higher electrical conductivity. Thermal stability of the SPD-processed Cu-Al₂O₃ composites was tested, and the HPT samples maintained good HV hardness together with high electrical conductivity even at 600 °C. The combination of external oxidation method and HPT processing resulted in enhanced mechanical properties, good electrical conductivity, acceptable thermal stability, and much simplified oxidation process.     

Using ring samples to evaluate the processing characteristics in high-pressure torsion

An Al–3% Mg–0.2% Sc alloy was processed by high-pressure torsion (HPT) using samples in the form of solid disks and rings. Following HPT, all values of the measured Vickers microhardness fall onto a single curve when plotted against the equivalent strain, such that there are increasing values of hardness at the lower strains and hardness saturation above equivalent strains of ～40. This saturation level is independent of the number of revolutions and the applied pressure. The grain size following HPT is ～220 nm, and tensile tests show that the material is superplastic at a testing temperature of 573 K.