Volume 110, Issue 9

On the basis of the recently proposed physically-based segmented model, the descriptions for the Gibbs energy of pure Cr and Ta down to 0 K were first established. After that, a thermodynamic re-assessment of the binary Cr–Ta system down to 0 K was then performed by taking into account all the critically reviewed experimental phase equilibria and thermodynamic properties. Especially, the experimental heat capacities of Cr 2 Ta at low temperatures ignored in previous assessments were utilized in the present assessment. The calculated phase equilibria and thermodynamic properties according to the presently obtained thermodynamic descriptions of the binary Cr–Ta system agree well with most of the experimental data, and show better agreement than the previous assessment.

The Time–Temperature–Precipitation diagram of M 23 C 6 carbide formation was established for a Nimonic 105 Ni-based superalloy by means of microstructural observation and mathematical-thermodynamic analysis. The results showed that during heat treatment at 750 °C, M 23 C 6 carbide usually forms at grain boundaries due to the decomposition of M C carbides, indicating that the grain boundaries promote M 23 C 6 carbide precipitation. Also, by increasing temperature or time, diffusion rate increases and new diffusional paths (such as γ matrix and twinning) are provided leading to the nucleation of these phases inside the matrix by means of γ→ M 23 C 6 + γ′ reaction. According to thermodynamic analysis, the interactions between M 23 C 6 formation Gibbs free energy and diffusion activation energy of elements, especially carbon, defines M 23 C 6 precipitation behavior. The peak of the Time–Temperature–Precipitation diagram was calculated to be about 900 °C based on experimental results, and about 927 °C based on thermodynamic results. These two temperatures can be considered as approximately equal.

Tensile tests at 450 °C and 1 × 10 −3 s −1 were conducted on an AZ31 alloy with an average grain size of 7.28 μm. At strains of 0.15, 0.30, and 0.45, microstructures and textures were examined and analyzed to investigate grain and texture evolution. The experimental results show that with the strain increase, the average grain size slightly increases; the initial basal texture gradually weakens, while the prismatic and pyramidal textures develop. Schmid factors were calculated for the representative slip systems based on their Euler angles and orientation. Some atypical textures show up at the low strain of 0.15. The initial basal texture becomes discontinuous and a twin texture of (011¯1)[01¯12]$\left( {01\bar 11} \right)\left[ {0\bar 112} \right]$ appears at the strain of 0.30. At the high strain of 0.45, the initial basal texture significantly weakens, and a strong pyramidal texture of (022¯5)[96¯3¯1]$\left( {02\bar 25} \right)\left[ {9\bar 6\bar 31} \right]$ appears.

In this study, a reciprocating extrusion process (REP) was employed on Mg-5Sn-1Si-0.8Y magnesium alloy and the effect of the REP temperature (in the range of 340 °C to 400 °C) and number of REP passes (in the range of 2 to 8 at 340 °C) on the microstructural evolution of the alloy were investigated. The results show that the microstructure observed varied from edge to centre on the cross and longitudinal sections of the specimen. Upon increasing the extrusion temperature from 340 °C to 400 °C, the grain size distribution became uneven; the Mg 2 Si phase was concentrated at the grain boundaries, and the amount of Mg 2 Sn precipitated was reduced. In comparison with changing temperature, increasing the number of REP passes was more effective for improving the homogeneity of the alloy. With an increasing number of extrusion passes, the distributions for the grain size and second phases became increasingly uniform. In this study, an alloy subject to 8-REP passes and extrusion at 340 °C showed the finest and most homogeneous microstructure. Finally, a quantitative relationship between the REPed grain size and the Zener–Hollomon parameter was obtained.

CuZn39Pb2 is a typical forged copper alloy widely used in various hot-forged valve bodies because of its super plasticity, high strength, and corrosion resistance. In this work, the deformation behavior of CuZn39Pb2 alloy at temperatures and strain rates of 650–750 °C and from 0.1 s −1 to 10 s −1 are investigated by isothermal hot compression tests on a Gleeble 3800 thermomechanical simulator. The experimental data are used to develop constitutive equations based on the Arrhenius-type equation. In the constitutive equations, the effects of true strain, deformation temperature, and strain rate on flow stress are incorporated and represented by the Zener–Hollomon parameter. The flow stress predicted by the constitutive equations agrees well with the experimental stress, which can be used in the finite-element simulation study of hot-forged CuZn39Pb2 alloy valve body.

The effects of austempering heat treatment on the microstructure and mechanical properties of sintered steel containing 3 % Cu and 3 % Ni powders with addition of 0.8 % graphite powders were examined. For this purpose, the powder mixtures containing 3 % Cu, 3 % Ni, and 0.8 % graphite powder were compacted under 700 MPa of pressure within molds in accordance with the standard tensile and impact test specimen dimensions. All of the green compacts were sintered under an argon (99.99 %) gas atmosphere for 20 min at 1 150 °C. The austempering heat treatments were applied to the sintered specimens for different periods ranging from 15 to 120 min in a salt bath at 350 °C in order to obtain a bainitic structure after the austenitization process at 850 °C. The hardness, tensile, and charpy impact tests were carried out at room temperature. It was found that there was a decrease in the hardness value depending on the austempering conditions; moreover, a significant increase was observed in the tensile and impact toughness values. Remarkable strength and ductility values were obtained with the austempering applied for 60–120 min.

A gas atomization process has been explored to enhance the microstructure and grain refining performance of Al-5Ti-1B master alloy. The results indicate that a gas atomization process has evidently alleviated the aggregation tendency of TiB 2 particles and refined TiAl 3 particles, resulting in a fine uniform dispersion of TiB 2 and TiAl 3 particles. Grain refining performance tests showed that the gas atomized Al-5Ti-1B master alloy effectively refined the average grain size of commercial purity Al from 2500 μm to 183 μm after 2 min of its inoculation, and the average grain size remained at 229 μm after inoculation for 180 min. The gas atomized Al-5Ti-1B master alloy demonstrated a much better grain refining efficiency and higher anti-fading ability than that of the as-cast Al-5Ti-1B master alloy.

Suitable pressure levels and heat treatment of casting wrought aluminum alloy are key process parameters in making components with better metallurgical properties. For this purpose, the effect of applied pressure and aging treatment on the microstructure and the mechanical properties of an industrial 2017 aluminum alloy manufactured by gravity and direct squeeze casting processes are investigated in this paper. After treating to various aging states, the correlation between these parameters and the characteristics of the cast alloy has been analyzed and discussed.

Al alloys with improved thermal conductivity and similar castability compared to the most commonly used Al–Si alloying system (ALDC12) were developed. Starting with the ALDC3 composition, we modified the alloying elements to investigate the effect on thermal conductivity and fluidity. The modified alloys had slightly higher thermal conductivity of ∼110–116 W m −1 K −1 compared to ALDC12 (92 W m −1 K −1 ). The largest improvement in the thermal conductivity compared to ALDC12 was 26 %. Moreover, the developed alloys showed a relative fluidity of ∼80–88% (depending on the composition) compared to ALDC12. Simulations confirmed that the heat generated from the internal parts of an electronic device could be effectively absorbed and discharged to the developed alloys as a heat sink.

Friction stir processing (FSP) was used to fabricate an Al3003 metal matrix surface composite reinforced by in-situ formed Al 3 Zr particles. Zirconium elemental powders were incorporated into the surface of Al3003-H14 alloy base metal and subjected to six FSP passes. Microstructural properties, wear behavior and corrosion resistance of the surface composite were investigated and compared with the FSPed and un-FSPed base metal. Results showed that aluminide particles formed at the interface between the aluminum matrix and zirconium particles, and thereafter fragmented and distributed throughout the matrix on stirring. Microstructural modification induced by FSP accompanied with in-situ formed particles improved hardness and wear resistance but reduced the corrosion resistance of the alloy.

In this work, porous ceramic substrates based on magnesium aluminate spinel (MAS), Mg 2 Al 2 O 4 , were infiltrated with a lanthanum-rich glass for applications as dental ceramic material. The substrates were fabricated by uniaxial compaction of the spinel powder at 100 MPa for 60 s and sintering at 1 550 °C, 1 600 °C or 1 650 °C for 2 h. The porosity of the substrates after sintering varied between 17 vol.% and 24 vol.%. The substrates were then infiltrated with a lanthanum-rich glass at 1 140 °C for 2 h. After infiltration, dense ceramics were obtained, while hardness, fracture toughness and flexural strength varied from 850 to 1 000 HV, 2.8 to 3.5 MPa · m 1/2 and 235 to 305 MPa, respectively, as a function of glass content. Theoretical calculations indicate that the amount of infiltrated secondary glassy phase should be about 17 vol.%, in order to obtain the highest crack propagation resistance.

Pre-rolling was performed along the transverse direction at room temperature to investigate the microstructural evolution of non-basal textured AZ61 Mg alloy. Microstructural characterization was used to evaluate grain boundary effects related to texture orientation. The microstructure and texture evolution were examined by means of optical microscopy, X-ray diffraction, and electron-backscatter diffraction. The results revealed that a mass of {101¯2}$\left\{ {10\bar 12} \right\}$ extension twins were introduced by pre-rolling with a small thickness reduction of 3 %. The c -axis of the grains tended to rotate toward the normal direction after the pre-rolling process. The deformation mechanism of the extension twin lamellae is discussed.