Volume 108, Issue 3

In this paper, a single crystal (SC) partition model, consisting of primary grains and grain defects, is proposed to simulate the weakening effect of grain defects generated at geometric discontinuities of SC materials. The plastic deformation of SC superalloy is described with the modified yield criterion, associated flow rule and hardening law. Then a bicrystal model containing only one group of misoriented grains under uniaxial loading is constructed and analyzed in the commercial finite element software ABAQUS. The simulation results indicate that the yield strength and elastic modulus of misoriented grains, which are determined by the crystallographic orientation, have a significant effect on the stress distribution of the bicrystal model. A critical stress, which is calculated by the stress state at critical regions, is proposed to evaluate the local stress rise at the sub-boundary of primary and misoriented grains.

This paper investigates the influence of different cryogenic treatments vis-à-vis conventional vacuum heat treatment on the evolution of static mechanical properties and microstructure of hot die steel grade AISI–H13. Deep cryogenic treatment was performed under two different temperatures (−154 °C and −184 °C) and varying soak duration of 6, 21, and 36 h. The resultant mechanical properties were characterized in order to understand the influence of cryogenic treatment vis-à-vis vacuum heat treatment and tempering on the hardness, toughness and the tensile strength. The results showed that samples cryogenically treated at −154°C for a soak duration of 6 h and tempered at 620°C for 2 h had 3.1% higher hardness, samples cryogenically treated at −184 °C for a soak duration of 6 h and tempered at 620 °C for 2 h showed 69 % higher percentage elongation, 36% higher toughness (Charpy V–notch) and showed 12.8% reduction in tensile strength in comparison to conventionally treated samples. X-ray diffraction and field emission scanning electron microscopy techniques were utilized for confirmation of various phases and complex carbides, morphology of microstructure and morphology of fractured surfaces, respectively.

In this study, magnesium was reinforced with micrometer size fine pure nickel particles using the ingot and powder metallurgy processes. The ingot metallurgy process induced a matrix–reinforcement reaction in the magnesium matrix, produced an enormous number of fine intermetallic compound particles and reduced the pure nickel particle size. The powder metallurgy process preserved the incorporated nickel particle intact. The ingot metallurgy made a remarkable improvement in the strength of the reinforced magnesium matrix with marginal strain hardening. The powder metallurgy process did not affect the yield strength of the reinforced magnesium matrix while inducing large strain hardening during plastic deformation. The fracture mode was similar for the ingot and powder metallurgy processed reinforced magnesium samples.

Monolithic ZK61 magnesium alloy and carbon nanotube (CNT)-reinforced ZK61 matrix composites were successfully prepared via spark plasma sintering. The effects of the sintering temperature on the microstructure and mechanical properties of monolithic ZK61 were studied, and the microstructural and mechanical properties of CNT/ZK61 composites were investigated as functions of the CNT content. The grain sizes of the CNT/ZK61 composites are smaller and more homogeneous than those of the monolithic ZK61 powder, and the width of the grain boundary is greater than those present in ZK61. The Vickers hardness and compressive yield strength of the CNT/ZK61 composites are observed to initially increase with increasing CNT content, reaching a maximum at 1.5 wt.% CNT, however, these values begin to decrease as the content increases further. We believe that this is owing to the grain refinement effect and load transfer mechanism of the CNT.

In this paper, Cu 43 Zr 48 Al 9 and (Cu 43 Zr 48 Al 9 ) 98 Y 2 bulk metallic glasses were prepared by casting them in a wedge-shaped copper mold. X-ray diffraction, optical microscopy, scanning electron microscopy and transmission electron microscopy were used to analyze their microstructures. Wedge casting is an effective and flexible technique for exploring alloy solidification microstructures. The results demonstrate that the development of microstructural transitions is related to the thickness of the wedge-shaped samples. These microstructural transitions are represented by the appearance of altered microstructural morphologies as well as changes from one product structure to another. It is found that the crystal/glass transition occurs as a U-shape change in Cu-based wedge-shaped alloys. Further, the critical cooling rate of (Cu 43 Zr 48 Al 9 ) 98 Y 2 became lower than that of Cu 43 Zr 48 Al 9 and that (Cu 43 Zr 48 Al 9 ) 98 Y 2 has a better glass forming ability. We developed practical strategies in microstructure control to extend our knowledge of phase competition leading to the successful synthesis of Cu-based metallic glass.

An oxide film was prepared on the surface of 7475 aluminum alloy by AO (anodic oxidation), and the corrosion behavior of the obtained AO film in 5 ± 0.5% NaCl solution was investigated. The surface–interface morphologies, chemical compositions and phases were analyzed by means of field emission scanning electron microscopy, energy dispersive X-ray spectrometry and X-ray diffraction, respectively, and the chemical elements of the surface and bonding interface were analyzed by a plane scan and line scan after salt spray corrosion. The results show severe corrosion on the original sample surface, the corrosion products are mainly composed of Al oxide after salt spray corrosion. The AO film is only slightly corroded, and the AO film–substrate is still metallurgically combined. Pitting corrosion in the aluminum alloy, primarily caused by Cl − , is the main mode of salt spray corrosion. There are stratified enrichments of Al and O in the AO film interface after the salt spray test, which are related to pitting corrosion and spalling phenomena, while the other elements were uniformly dispersed. The continuous Al 2 O 3 in the AO film effectively prevents Cl − from eroding the metal substrate to improve salt spray corrosion resistance of the 7475 aluminum alloy.

Composite powders of Ni-30SiC (vol.%) were fabricated by mechanical alloying. Scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy were used to analyze the microstructure, morphology and composition of the composite powders. Mixed powders and Ni coating layers on the surface of SiC particles were investigated. The Ni/SiC composite powder with the optimal coating was mixed with Cu powder to manufacture Cu matrix composites by powder metallurgy, and the relative density, hardness and electrical resistivity of the composites were measured. The results show that the Ni coating becomes increasingly prominent and that the mixed powders become increasingly homogeneous as the ball-milling time increases; however, an excessively long milling time has a negative impact on the coating. The milling speed also has an important effect on the coating. Too high of a rotational speed quickens the rate of work hardening of the coating layer, which causes the coating layer to fall off. When the milling time is 20 h, the milling speed is 300 r min −1 and the ball to powder weight ratio is 10 : 1, the SiC particles are well coated by Ni and the powders are uniformly mixed. The properties of the copper matrix composite processed with the optimal milling parameters are effectively improved.

In this study, Li 2 CO 3 -doped Bi 0.5 (Na 0.8 K 0.2 ) 0.5 TiO 3 (BNKT) ceramics were fabricated via the conventional solid-state reaction using ultrasound waves for preliminary milling. The milling time was shortened from 20 h to 1 h with ball milling. The phase structure of BNKT investigated by X-ray diffraction showed a single perovskite phase. With increasing Li 2 CO 3 content, the phase structure of the ceramics changed from rhombohedral to tetragonal. At a sintering temperature of 1100 °C and Li 2 CO 3 content of 0.4 wt.%, the best physical properties of the ceramics, such as density ( ρ ), 5.9 g cm −3 ; electromechanical coupling factors ( k p ), 0.32; ( k t ), 0.29; remanent polarization ( P r ), 12.0 μC cm −2 ; dielectric constant ( ∊ r ), 1 191; and highest dielectric constant ( ∊ max ), 5 800, were obtained.

Thixoforming processes depend upon three basic features of the semisolid slurry: the thermodynamics of the liquid-to-solid transition; the morphology of the microstructure; and the slurry's rheological properties. By studying the thermodynamics of the solid-to-liquid transition of a specific alloy, one can determine whether the entire thixoforming process can be controlled when the alloy is used as the raw material. The rheology of the semisolid material is determined not only by the morphology of the microstructure of the primary solid phase immersed in the liquid, but also by the relative amounts of liquid and solid. In thixoforming operations the raw material is heated up to the semisolid state and injected into the die. Hence, the heating procedure has a significant impact on the morphology of the remaining solid and, consequently, its viscous behaviour, as shown in this paper. A heating rate close to 50 K min −1 was found to produce the most suitable morphology for semisolid processing as the mechanisms involved (Ostwald ripening and coalescence) were present to an equal extent.

Feedstocks containing 54 and 60 vol.% powder loadings were prepared using a multi-component binder and different amounts of nano-alumina (0–9 wt.%). Feedstock containing 60 vol.% powder and 6 wt.% nano-alumina had the lowest shear rate sensitivity ( n ). The Al feedstocks containing nanoparticles were injected, de-bound and sintered. The rate of solvent de-binding for samples containing nano-particles was slightly lower than that of similar specimens. Results showed that nano-scale additive caused increased relative density, hardness and strength of manufactured composites, but decreased shrinkage and grain size. The composite with 54 vol.% powder loading and 9 wt.% nano-alumina had the best evaluations for mechanical properties. Particle clustering caused a decline in all evaluations for mechanical properties for 60 vol.% powder loading and 9 wt.% nano-alumina sample.

Here we describe a rapid approach for synthesizing pure vaterite calcium carbonate microspheres through a fast precipitation method. The precipitated CaCO 3 microspheres were produced in alkaline aqueous media with the addition of sodium citrate at room temperature. The obtained microspheres were investigated with scanning electron microscopy, dynamic light scattering, fourier transform infrared spectroscopy and powder X-ray diffraction. The characterization results reveal that the obtained microspheres are pure vaterite with an average diameter of 5 μm. The possible reason for the formation of vaterite CaCO 3 microspheres instead of calcite has also been discussed.

Powdered mixtures of Al 2 O 3 /SiO 2 /C/Al were heated together to synthesize Al 4 SiC 4 and the corrosion resistance of the Al 4 SiC 4 product was subsequently investigated. The results show that the addition of aluminium accelerates the synthesis of Al 4 SiC 4 . The slag corrosion resistance of the Al 4 SiC 4 product is excellent (as determined using the static-crucible method). At high temperatures, the surface of the Al 4 SiC 4 initially reacts with oxygen to form Al 2 O 3 and SiO 2 . The Al 2 O 3 formed dissolves in molten slag, and further reacts with MgO to form MgAl 2 O 4 . The above reaction processes can form a protective layer on the surface of the ternary carbide, which inhibits further corrosion of the Al 4 SiC 4 material.