Effect of Re Addition and Withdrawal Rate on the Solidification Behavior of Directionally Solidified Superalloy AM3

The influence of Re addition and withdrawal rate on the solidification behavior of the first generation single crystal superalloy AM3 was investigated by directional solidification and quenching experiments.The primary dendrite arm spacing and eutectic volume fraction were measured from directionally solidified superalloy AM3 with different Re contents.It is found that the primary dendrite arm spacing is determined by the withdrawal rate,and Re does not influence on the value.The eutectic fraction increases with increasing Re addition.Partition coeffcients of alloying elements were investigated with energy-dispersive X-ray spectrometry(EDS) analysis.The data was submitted to a statistical treatment to establish the solidification path,and the partition coeficients were measured by fitting the curve with a modified Scheil formula.It is shown that the addition of Re results in bigger microsegregation of alloying elements in directionally solidified AM3 superalloy.     

Microstructure and electrical conductivity of high volume Al2O3-reinforced copper matrix composites produced by plasma spray

A mixture of Cu/-Al₂O₃ (30:70 in vol.%) feedstock powder was plasma-sprayed onto a graphite substrate in order to investigate the electrical conductivity and microstructure of the deposits. As input power increased, the amount of phase transformed γ-Al₂O₃ and the volume fraction of Al₂O₃ in the plasma-sprayed composites increased, while the electrical conductivity decreased. Cuprite (Cu₂O) was also found in the deposits. The electrical conductivities of the plasma-sprayed Cu/Al₂O₃ composites were compared to those of the different predictive models. It was revealed that the two-phase self-consistent predictive model had the closest agreement with the measured values.     

Characterization of protective oxide layers formed on molten AZ91 alloy containing Ca and Be

AZ91 alloy in its molten state oxidizes rapidly in atmospheric conditions and begin to burn from the melt surface. In order to prevent the molten AZ91 alloy from burning, it is usually protected by a cover gas such as sulfur hexafluoride (SF₆). However, the use of SF₆ gas poses a serious threat to the environment as it is widely acknowledged to contribute to the global warming effect. Thus, several types of studies aimed at controlling the oxidation behavior of molten magnesium alloys have been carried out. This paper describes the characteristics of the oxide layers formed on molten AZ91 alloys containing Ca and Be. The surface analysis technique of GDS (Glow Discharge Spectroscopy) was used to investigate the oxide layers of AZ91 alloys containing Ca and Be. The GDS results showed that the oxide layer consists of CaO, MgO, BeO and Al₂O₃.     

Effect of precipitates on damping capacity and mechanical properties of Ti–6Al–4V alloy

The present study was concerned with the effects of over-aging on damping property and fracture toughness in Ti–6Al–4V alloy. Damping property and toughness become important factors for titanium implants, which have big modulus difference between bone and implant, and need high damping capacity for bone-implant compatability. Widmanstätten, equiaxed, and bimodal microstructures containing fine α₂ (Ti₃Al) particles were obtained by over-aging a Ti–6Al–4V alloy. Over-aging heat treatment was conducted for 200 h at 545 °C. Fracture toughness, Charpy impact, and bending vibration tests were conducted on the unaged and the over-aged six microstructures, respectively. Charpy absorption energy and apparent fracture toughness decreased as over-aging was done, even if the materials were strengthened by precipitation of very fine and strong α₂-Ti₃Al particles. On the other hand, damping properties were enhanced by over-aging in Widmanstätten and equiaxed microstructures, but was weakened in bimodal microstructure due to the softening of tempered martensite and the decreasing of elastic difference between tempered martensite and α phase contained α₂ particles, etc. These data can provide effective information to future work about internal damping and fracture properties of Ti–6Al–4V alloy.     

Elastic modulus and in vitro biocompatibility of Ti−xNb and Ti−xTa alloys

The effect of niobium and tantalum on the elastic modulus and the in vitro biocompatibility in binary titanium alloys was studied. The Young's modulus of titanium was effectively lowered with additions of Nb or Ta, depending significantly upon the microstructure. Martensitic microstructures such as α' and α" decreased the elastic modulus, while the ω phase increased it. Ti−10%Nb, Ti−30%Nb and Ti−30%Ta alloys exhibited very low elastic moduli of 74, 80, and 58 GPa, respectively. The corrosion resistance of Ti−xTa was slightly higher than that of Ti−xNb, which was comparable to that of CP Ti or Ti−6A−4V. No ion release was detected in Hank's solution, while Ti ions were released in 0.1% lactic acid ranging from 0.03 to 0.11 μg/ml for both the Ti−xNb and Ti−xTa alloys. MG63 osteoblast-like cell proliferation on Ti−30%Ta was less active compared with Ti−30%Nb, CP Ti or Ti−6Al−4V.     

Synthesis of snowman-shaped microparticles by monomer swelling and polymerization of crosslinked seed particles

Nonspherical snowman-shaped micro-sized particles were synthesized via monomer swelling and the polymerization of crosslinked seed particles. Monodispersed crosslinked polystyrene microspheres and methylmethacrylate were used as seed particles and the swelling monomer, respectively. Methylmethacrylate (MMA) induced crosslinked polystyrene microparticle swelling; however, compared to polystyrene, MMA is relatively hydrophilic. As a result, phase separation was observed, resulting in monomer-swollen, cross-linked particles protruding from the surface of the seed particles. By changing the monomer-to-particle weight ratio from 4 to 8, the ratio of the size of the head to the body of the snowman-shaped particles was varied from 0.3 to 0.7. The morphologies of the snowman-shaped particles were predicted using Surface Evolver software, and the simulation was applied to show the unique self-organization morphologies of snowman-shaped particles. We synthesized snowman-shaped microparticles by swelling and polymerizing cross-linked PS seed particles with methylmethacrylate. The monomer-swollen, cross-linked particles exhibited protrusions from the surface of the microparticles due to the phase separation of seeds from the particles. The size of the protrusion or head of the snowmanshaped particles was controlled by changing the monomer-to-particle weight ratio during the swelling process. Simulations were applied to estimate the aspect ratio of snowman-shaped particles and their self-assembled morphologies.     

Enhanced electromechanical performance of carbon nano-fiber reinforced sulfonated poly(styrene-b-[ethylene/butylene]-b-styrene) actuator

In this paper, the carbon nano-fibers (CNFs) were used to improve the performance of sulfonated poly(styrene-b-[ethylene/butylene]-b-styrene) (SSEBS) composite actuator. The SSEBS actuator, which was developed in our previous study, does not have good electromechanical performance because the SSEBS ionic polymer is too flexible and soft. CNFs as a reinforcement for the polymer membranes greatly increased the bending performance of the SSEBS actuators. The surface and cross-sectional morphologies of the CNF-SSEBS actuators were investigated by SEM observation. The results show that carbon nano-fibers were homogeneously dispersed in the SSEBS polymer matrix without local agglomeration. The bending responses of the CNF-SSEBS composite actuators under step inputs and sinusoidal excitations were compared with those of the pure SSEBS actuator. The tip displacement of the CNF-SSEBS actuator was much faster and larger than that of the pure SSEBS actuator.     

Microstructure and mechanical properties of Mg–4.5Al–1.0Zn alloy sheets produced by twin roll casting and sequential warm rolling

Microstructure and mechanical properties of Mg–4.5Al–1.0Zn (designated as AZ41M in short) alloy sheets produced by twin roll casting, sequential warm rolling and post annealing at 350 °C were studied in this paper. Microstructure of twin roll casting strip consisted of dendrite structure, eutectics and intermetallic compounds located in the interdendritic region. AZ41M alloy sheets showed higher strength and lower elongation after sequential warm rolling, while post annealing after warm rolling induced the decrease of strength and increase of elongation. This results in the balance of strength and elongation in AZ41M alloy sheets. The grain refinement during manufacturing processes was attributed to the formation of heavy shear bands, high dislocation density, twinning, and precipitates of Al₂Ca/Mg₂Ca or Al₈Mn₅ and the Ca dissolution into Mg₁₇Al₁₂ phase.     

Formation of nano-sized grains in Cu and Cu−Fe−P alloys by accumulative roll bonding process

Mechanical properties and formation of nano-sized grains in Cu and Cu−Fe−P alloys by the accumulative roll bonding (ARB) process were investigated. Nano-sized grains were successfully obtained in OFC and PMC-90 alloys by the ARB process after the third cycle. Once the 200 nm grains formed, further reduction in the grain size was not observed up to 8 ARB process cycles. For both alloys, the tensile strength values increased drastically in the initial stage of the ARB process. The tensile strength values of both alloys tended to saturate after the third ARB process cycle. The tensile elongation value greatly decreased by 1 cycle of the ARB process due to the strain hardening. After the third cycle of the ARB process, each alloy showed a gradual increase in tensile elongation due to the dynamic recovery. For PMC-90 alloy, the strength value was higher than that of OFC due to addition of the alloying elements. With increased annealing temperature, the nanosized grains tended to grow in OFC at 150°C, and after annealing at 200°C, coarse grains formed. On the other hand, in PMC-90 alloy, there was no grain growth up to 250°C due to the alloying elements (Fe, P).     

Plasma processing for surface modification of trivalent chromium as alternative to hexavalent chromium layer

Plasma source ion implantation (PSII) treatment was undertaken to improve the mechanical properties of electrodeposited trivalent chromium layers. Nitrogen ions were implanted, with energies of − 15 to − 25 keV and doses of 1, 5 and 10 × 10¹⁷ atoms cm^− 2, to modify the surface properties of Cr plating layer. The surface properties of the films were characterized by XRD, SEM, ruby-ball on disk type tribometer and nanoindenter. Polycrystalline CrN films with (200), (220) and (222) orientations were preferentially grown and numbers of surface cracks were increased by N⁺-PSII onto trivalent chromium layers. The surface hardness of the Cr³⁺ plating layer was increased from 16 to 25 GPa by N⁺-PSII. Severe wear and higher friction was observed on N⁺-PSII treated trivalent Cr plating. It seemed that the wear debris from hardened and cracked surface of the N⁺-PSII treated specimen prompted abrasive wear in the wear test. Roughness of the Cr³⁺ plating layers was smoothed with increasing implantation doses.