Volume 104, Issue 4

A modeling approach to calculate the kinetics of non-isothermal bainite formation in low carbon microalloyed steels is presented. The model development is based on experimental data obtained by means of deformation dilatometry and electron back scattering diffraction. It is suggested that the misorientations inherited from the plastically deformed austenite make a significant contribution to the misorientation distribution in the final microstructure. Taking the inheritance phenomenon into consideration, the model allows prediction of the transformation kinetics and the effective grain size of bainitic microstructure as a function of the austenite deformation level and the cooling rate.

In this research, the formation and transformation mechanisms of η (Ni 3 Ti) phase in an Fe-Ni-based superalloy at different Ti/Al ratios were investigated. In addition to Ti content, Ti/Al ratio also affects the η phase. So alloys with different Ti/Al ratios were prepared and the microstructures were analysed using optical and scanning electron microscopy and X-ray diffraction. The alloy with lower Ti but higher Ti/Al ratio has a higher η volume fraction. Time-temperature-precipitation diagrams of η phase in the superalloys with different Ti/Al ratios are also presented. Based on the experimental results, when the Ti/Al ratio was decreased from 20 to 3, the time and temperature of η precipitation shifted to longer and higher values, respectively, and the η volume fraction decreased. Ti/Al of 3 could significantly retard the formation of η phase. Depending on the Ti/Al ratio, and the time and temperature of ageing, η was precipitated at twin and grain boundaries or inside the grains, either by γ′ → η transformation or directly by formation of austenite. It was also shown that a high Ti/Al ratio in the alloy would reduce the fraction of twin boundaries after solution annealing which affects the η phase nucleation.

The growth kinetics and morphology of intermetallic compounds at the interface of H13 tool steel and A380 molten aluminum alloy have been investigated through immersion experiments in the temperature range of 680 to 740 °C. The effect of nitride coating of the surface of H13 steel on the formation and growth of intermetallic layers has also been studied. The results of scanning electron microscopy indicated that three intermetallic layers are formed at the interface, Al 8 Fe 2 Si, Al 5 FeSi, and Al 12 Fe 5 Si. Nitride coating decreased the overall intermetallic layer thickness at temperatures between 680 and 740 °C. It decreased the number of layers at 680 °C; the Al 12 Fe 5 Si layer was not formed. Kinetics study of the interfacial reaction between H13 steel samples and A380 aluminum alloy revealed parabolic growth. The growth rate of the intermetallic layer was evaluated and the activation energy was found to be 40 and 158 kJ mol −1 for H13 steel and nitride-coated H13 steel, respectively.

On the basis of the thermochemical and phase equilibrium experimental data, the phase diagram of the Ba–Mg binary system has been assessed by means of the calculation of phase diagrams technique. The liquid phase is of unlimited solubility and modeled as a solution phase using the Redlich–Kister equation. The intermetallic compounds, Mg 17 Ba 2 , Mg 23 Ba 6 and Mg 2 Ba, with no solubility ranges are treated as strict stoichiometric compounds with the formula Mg m Ba n . Two terminal phases, Bcc_Ba and Hcp_Mg, are kept as solution phases, since the solubilities of the two phases are of considerable importance. After optimization, a set of self-consistent thermodynamic parameters has been obtained. The calculated values agree well with the available experimental data.

The partial isothermal section of the Dy–Co–Ga ternary system (below 33.3 at.% Dy) at 500 °C was investigated and constructed mainly using X-ray powder diffraction analysis with the aid of scanning electron microscopy. The 8 binary compounds, CoGa, CoGa 3 , Dy 2 Co 17 , Dy 2 Co 7 , DyCo 3 , DyCo 2 , DyGa 3 and DyGa 2 , and the 9 previously reported ternary compounds DyCoGa (γ), Dy 5 Co 12 Ga 3 (δ), DyCo 3 Ga 2 (κ), Dy 6 Co 7 Ga 21 (ν), Dy 2 CoGa 8 (ο), Dy 2 Co 3 Ga 9 (θ), DyCoGa 5 (ρ), DyCo 5 Ga 7 (λ) and Dy 2 Co 13 – 11 Ga 4 – 6 (α) have been confirmed in the partial isothermal section at 500 °C. Five new ternary compounds, DyCo 4.41 – 3.35 · Ga 0.59 – 1.65 (β), DyCo 1.33 Ga 0.67 (∊), DyCo 0.67 – 0.35 Ga 1.33 – 1.65 (η), DyCo 0.25 – 0.15 Ga 1.75 – 1.85 (ϕ), Dy 3 Co 3 Ga 8 (μ), were found in this partial isothermal section. The structures of all compounds have been confirmed or determined. The nine compounds, Dy 2 Co 17 , Dy 2 Co 13.5 – 9.10 Ga 3.5 – 7. 6 (α), DyCo 4.41 – 3.35 · Ga 0.59 – 1.65 (β), Dy 2 Co 7 , DyCo 3 , DyCo 2. 55 – 1.8 Ga 0. 45 – 1.2 (δ), DyGa 2 , DyCo 0.67 – 0.35 Ga 1.33 – 1.65 (η) and DyCo 0.25 – 0.15 · Ga 1.75 – 1.85 (ϕ), form solid solutions by the substitution of Ga atoms for Co atoms in the compounds. The solid solution ranges for these solid solutions have been obtained from their powder X-ray diffraction data.

In this study isothermal equilibration experiments were done at 866–1 166 K for the copper–lead (Cu-Pb) binary system. The experiments, conducted in a quartz ampoule, enabled the solubility limits at each end of the phase diagram to be obtained. In addition to the experiments, a literature survey and compilation of the experimental data of the solid solubilities was done. Critically evaluated and selected literature data and our own experimental values were used to optimise the fcc phase interaction parameters. According to the experiments the lead solubility in solid copper at the copper-rich end is smaller than previously reported. The liquidus curve at the lead-rich end was validated with our own experimental results, which agree well with previous experimental data. The thermodynamic assessment of the Cu-Pb system, in the area of fcc_A1 solid solutions, has been improved.

By combination of electromagnetic levitation and copper-plate quenching, deep undercooling and non-equilibrium solidification of Al-50%Si melts were realized. Through repeatedly heating and cooling, a maximum undercooling of 310 K was reached. Several different morphologies of primary Si solidified at different undercoolings were found on the quenched surfaces. With increasing of undercooling, the primary Si crystals solidified gradually changed to fine clumpy crystals from coarse faceted dendrites. When the undercooling reached the maximum value of 310 K, the microstructures contained fine granular crystals. The gain size of primary Si was refined significantly.

The microstructure and mechanical properties of as-cast and solution-treated Mg- x Zn- y Gd ( x / y = 6, at.%) alloys reinforced with quasicrystals have been investigated. The results indicate that with the increase in the alloying elements contents, the dendritic microstructure is refined, the corresponding strength gradually increased, and the elongation gradually decreased as the increase in the second phase sizes leads to stress concentration under as-cast and solution-treated conditions. For as-cast alloys, micro-cracks originated in the α-Mg matrix and quasicrystals interface. The fracture mode exhibits intergranular fracture characteristics. After solution treatment, the yield strength decreased, and the ultimate tensile strength and elongation increased clearly due to the Mg 2 Zn 3 phase dissolution and the second phase sizes decreasing. The fracture mode changes to transgranular fracture.

Al–Ni energetic composites, applicable to localized heating, were fabricated from atomized powders and hammer-milled flakes by means of ultrasonic powder consolidation, a novel low-temperature rapid powder consolidation technique. Full-density consolidation was achieved in 1 s of consolidation time at temperatures as low as 573 K without causing appreciable reactions between Al and Ni. The consolidated composite structure consists of Ni powder particles or flakes uniformly distributed in a metallurgically consolidated Al matrix. Consolidated materials were evaluated using spark and continuous-heating ignition tests. Ultrasonic powder consolidation is potentially applicable to the fabrication of a wide range of metastable materials that cannot be processed at elevated temperatures.

Conventional high-pressure die casting aluminium components present certain limitations in terms of mechanical properties attainable due to the intrinsic porosity of the castings as well as the presence of iron-based brittle intermetallic phases. The present work approaches the increase in ductility and tensile strength through the analysis of the effect of the alloying elements of Al-Si alloys used for high-pressure die casting. The combination of alloying elements providing the best results in terms of ductility and tensile strength were eventually selected to produce a batch of components that were thoroughly tested. The final alloy had a composition of Si 8.21, Fe 0.78, Cu 1.53, Mn 0.64, Mg 0.46, Ni 0.07, Zn 3.37, Pb 0.34, Sn 0.27, Ti 0.18 and Cr 0.04 wt.%. The selected alloy performance was compared to that of the commercial Al-Si9Cu3 and Silafont ® 36 alloys.

The main objective of the present work is to process porous silicon–silicon carbide (Si–SiC) ceramic by the oxidation of carbon–carbon silicon carbide (C/C–SiC) composites. Phase studies are performed on the oxidized porous composite to examine the changes due to the high temperature oxidation. Further, various characterization techniques are performed on Si–SiC ceramics in order to study the material's microstructure. The effects of various parameters such as fiber alignment (twill weave and short/chopped fiber) and phenolic resin type (resol and novolak) are characterized.

CeB 6 powder was prepared via combustion synthesis (CS) and mechanochemical processing (MCP) methods starting from CeO 2 , B 2 O 3 and Mg powder mixtures. In CS, reactant mixtures were ignited in a preheated pot furnace under argon atmosphere. Products contained CeB 6 , MgO and Mg 3 B 2 O 6 , as revealed by X-ray diffraction analysis. After leaching in 1 M HCl for 15 h, MgO was removed but Mg 3 B 2 O 6 could not be removed from the products. Ball milling of products in ethanol prior to leaching made the removal of Mg 3 B 2 O 6 possible by leaching. Yield of CeB 6 was 68.6 % in CS. MCP was performed in a stainless steel vial with a planetary ball mill at 300 rpm for 30 h. MCP products contained CeB 6 , MgO and small amount of Fe. Leaching in 1 M HCl for 30 min was sufficient to remove MgO. Yield of CeB 6 was 84.4 % in MCP. According to scanning electron microscopy examinations, particles of CeB 6 prepared by CS and MCP had submicrometer size. Average particle sizes were determined as 290 nm and 240 nm, respectively.

Bilayer microelectromechanical components such as microcantilevers, microbridges or micromembranes are usually used in microtransduction for actuation and sensing. One layer achieves the structural and elastic recovery function and the other layer acts as the active part by deforming under actuations. This paper describes the studies of mechanical characteristics of flexible bilayer microcantilevers fabricated from the polymer SU8 with a reflective nano-metallic layer on the top. The mechanical characteristics investigated are stiffness, modulus of elasticity, resonant frequency, bending strain and stress. The analytical relationships for bending stiffness of bilayer microcantilevers are determined by using Castigliano's second theorem. The first bending resonant frequency is computed based on the lumped-parameter model. Experimental tests of mechanical characteristics and materials characterization are developed using atomic force microscopy and nanoidentation. Finite element analysis is used to determine the maximum stress in the sample layers and their mechanical responses.