Band 108, Heft 7

Constitutive analysis for hot working of Ti-6Al-4V alloy was carried out by using experimental stress–strain data from isothermal hot compression tests. A new kind of constitutive equation called a modified parallel constitutive model was proposed by considering the independent effects of strain, strain rate and temperature. The predicted flow stress data were compared with the experimental data. Statistical analysis was introduced to verify the validity of the developed constitutive equation. Subsequently, the accuracy of the proposed constitutive equations was evaluated by comparing with other constitutive models. The results showed that the developed modified parallel constitutive model based on multiple regression could predict flow stress of Ti-6Al-4V alloy with good correlation and generalization.

2D axi-symmetric transient simulation based on the thermo-plastic model was carried out for analyzing the thermal field of directional solidification (DS) furnaces during the solidification process. The simulation was carried out for two different furnaces: One conventional DS furnace and the other a modified DS furnace. In the modified DS furnace the bottom insulation was provided with a circular shape opening. The simulation was performed for conventional and modified furnaces which suggest that the lifting velocity of the side insulation in the conventional DS furnace significantly affects the solidification process of mc-Si ingot and the opening speed of the bottom insulation in the modified DS furnace has less influence on solidification and also gives optimal conditions to grow good quality mc-Si ingots. The results for conventional and modified DS grown mc-Si ingots were taken with two different insulation lifting profiles and their influence on thermally induced stress, dislocation density and growth rate were compared and analyzed.

The corrosion resistance of austenitic stainless steels is known to be hampered by the loss of chromium available for passive surface layer formation as a result of chromium carbide precipitation at austenite grain boundaries during annealing treatments. Although high-temperature annealing can promote carbide dissolution leading to better corrosion resistance, grain coarsening also results, which would lead to poorer mechanical properties. Processing methods to achieve both good corrosion resistance and mechanical properties are thus highly desirable for austenitic stainless steels. In the present study, we show that the corrosion resistance of AISI 304L stainless steel can be improved by grain refinement into the ultrafine-grained regime. Specifically, samples with different austenite grain sizes in the range of 0.65–12 μm were studied by potentiodynamic polarization and electrochemical impedance spectroscopy tests in a 3.5 wt.% NaCl solution. All samples showed a typical passive behavior with similar corrosion potential, but the corrosion current density decreased significantly with decreasing grain size. The results show that the sample with the finest grain size had the best corrosion resistance due to a higher resistance of the passive layer to pitting attacks. This study indicates that grain refinement which improves mechanical properties can also significantly improve the corrosion resistance of AISI 304L stainless steel.

Mechanical behavior and microstructural evolution of AZ31 sheet at high strain rates and elevated temperatures were investigated using a tensile Split-Hopkinson bar. Flow stress and elongation increased with increasing strain rate under high strain rate conditions, but temperature had little effect. Microstructural evolution was analyzed by optical and scanning electron microscopy. Twinning was observed at high strain rates, even at 200°C and 250°C, but not under quasi-static tension. Dynamic recrystallization was observed under quasi-static conditions with increasing temperature, but not at high strain rates. Dimples were observed over the entire temperature range at high strain rates. Increasing ductility was limited compared with that under quasi-static conditions. Dynamic recrystallization occurred at strain rates of 2 000 s −1 and 3 000 s −1 at 250 °C, but the recrystallized grains were very small.

Precision diffusion bonding of aluminum alloy requires good mechanical properties and very low deformation. However, oxidation lowers the quality of the joint. This paper used a self-assembled monolayer, 1-hexyl mercaptan, on the aluminum surface to prevent the secondary oxidation after pre-bonding treatment. An AA6063/Ni/AA6063 sandwich structure joint was intentionally designed and bonded to characterize the effect of the self-assembled monolayer. AA6063/AA6063 joints were diffusion bonded, in which three different pre-bonding surface treatments were used that were as-cleaned, ethanol adsorbed and self-assembled monolayer passivated. With the surface passivated by self-assembled monolayer, there were few voids at the bonding interface. Ductile fracture was observed during tensile testing. The joint had a bond ratio of 97.4% and elongation of 20.4%, which is very close to the base metal and is considered as equal-strength bonding.

In this paper, using industrial alumina, calcined magnesite, ground calcium carbonate as raw materials, spinel-containing calcium aluminate cements (CMA) were prepared at 1500 °C with different calcine holding times (3 h and 6 h). The mineral composition and microstructural characteristics of prepared cement samples were studied using X-ray diffraction and scanning electron microscopy, and their hydration and rheological behavior were also investigated. The results indicated that increasing holding time can promote growth of CaAl 2 O 4 grains and reduce setting time. The hydration mechanism of CMA follows the nucleation and crystal growth–diffusion (NG–D) process, and the hydration rate of CMA–3 h in the acceleratory period is higher than that of CMA–6 h, but there is no significant distinction in deceleration and stabilization periods. The slowdown of hydration rate directly affects the rheological behavior of CMA slurry, leading to decrease in yield stress, and thixotropic character. Also, the storage modulus ( G' ) growth rate of CMA–3 h slurry is significantly higher than for CMA–6 h slurry, indicating the CMA synthesized at 1500 °C for 6 h possesses lower hydration activity.

Hybrid copper matrix composites containing 5 wt.% of titanium dioxide and varying graphite content (0 wt.%, 2 wt.% and 4 wt.%) were synthesized using powder metallurgy. Metallurgical studies were carried out to examine the presence and distribution of reinforcements in the copper matrix. To investigate the forming behavior of the sintered composite preforms, cold upset tests were conducted from which the true axial stress, the true hoop stress, the true hydrostatic stress and the true effective stress were evaluated and their relationship with the true axial strain was analyzed and presented. It is observed that the increase in addition of weight percentage of graphite into the copper matrix increases the true axial, the true hoop, the true hydrostatic and the true effective stresses. The variation of hardness, strength coefficient and strain hardening with respect to the addition of graphite content is also evaluated and reported.

Binary bismuth–cadmium oxide nanorods have been synthesized by a simple hydrothermal process without templates and additives. X-ray diffraction and high-resolution transmission electron microscopy reveal that the nanorods possess single crystalline tetragonal Bi 2 CdO 4 phase. Scanning electron microscopy and transmission electron microscopy images show that the length and diameter of the nanorods are 20–300 nm and 5–10 μm, respectively. The formation of the binary bismuth–cadmium oxide nanorods is closely related to the hydrothermal parameters. The electrochemical sensing performance of the binary bismuth–cadmium oxide nanorods has been investigated using the nanorods as glassy carbon electrode modifiers. The detection limit is 0.19 μM with a linear range of 0.0005–2 mM. The nanorod-modified glassy carbon electrode exhibits good electrocatalytic activity toward L-cysteine and great application potential for electrochemical sensors.

The bubbling method has been refined to produce vaterite with both high productivity and purity for its promising applications. This involves: a) adding an adequate amount of ammonia water to achieve maximum precipitation of Ca 2+ , b) refining CO 2 bubble sizes into microns by a designed gas disperser, and c) using optimal final pH condition and CO 2 flow rate to obtain vaterite with both high productivity and purity. In addition, the conservation of vaterite was also investigated by exposing vaterite to two moist environments. The results show that 95.1 % pure vaterite can be produced with 88.8 % yield at a final pH value of 8.6 and CO 2 flow rate of 11 l min −1 . The corresponding vaterite particles mostly have a hollow structure. The conservation of vaterite is very sensitive to moisture, and it transforms into calcite gradually depending on the moisture content.

Crystal-packed, honeycomb-like, and partially developed spherical particles constituted of small crystals have been found in chips yielded in high-speed machining of alloy steel AerMet100. Scanning electron microscopy and X-ray energy dispersive spectroscopy examination of the chips were carried out to investigate the formation mechanism of the spherical particles. Results show that, depending on chip size and shape, chip melting completeness changes greatly, and the solidification process of the melted chips and the final microstructure of the spherical particles are controlled by heat dissipation rate.

ZnO whiskers were rapidly microwave synthesized from micron ZnO powder with Ti reducing agent. The compositions and microstructures of the products were studied using X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Results showed that white flocculent ZnO powder began forming at 600 °C and increased in amount with increasing temperature. In the sample obtained at 800 °C, the tetrapod ZnO whiskers had rod-like feet with lengths of 1.5–3 μm and diameters of approximately 0.2–0.5 μm. The reduction reaction was completed at 1 100°C. Coarser and irregularly shaped ZnO whiskers with thick roots and sharp tips formed at 1 100°C.