Volume 33, Issue 6

Aluminum coating on 1Cr18Ni9Ti stainless steel was prepared by magnetron sputtering method. The specimens were treated with pre-oxidation (PO) or vacuum diffusion annealing (VA). Hot corrosion resistance of the coatings beneath the deposits of Na 2 SO 4 at 1050 °C was investigated. Corrosion products were analyzed by XRD and SEM. Results show that the presence of coating could improve the corrosion resistance of stainless steel. FeAl phase appeared after VA at 600 °C, which enhanced cohesive force between the coating and the substrate, and reduced the oxidation and sulfidation rate. PO treatment can protect the substrate more effectively than VA treatment for metastable Al 2 O 3 formed during PO treatment can be translated to stable Al 2 O 3 more quickly at high temperatures. The corrosion products of the two kinds of specimens with aluminum coating were both composed of Al 2 O 3 , a little amount of FeS and Fe 2 O 3 after 24 h corrosion. Al 2 O 3 was formed mainly in the coatings, FeS was mainly distributed in the interface between coating and substrate of the specimens, and a small amount of FeS was distributed in the substrate. Al 2 O 3 film remained intact after 24 h corrosion, and kept its protective effect on the substrate.

In this work, the microstructure evolution of the Nb-12Si-22Ti-14Cr-2Al-2Hf alloy (at.%) fabricated by liquidmetal-cooled directional solidification (LMC) was investigated and the solidification behaviors were discussed. The results revealed that the growth rate was of crucial in controlling the microstructure of this alloy and the higher growth rate was beneficial to the formation of metastable phase Nb 3 Si. The microstructure of non-melted region consisted of Nb SS dendrites, Nb SS /Nb 5 Si 3 eutectics and Ti-rich Nb SS /Cr 2 Nb eutectics while the microstructure of quasi-steady-state growth region of this alloy consisted of Nb SS dendrites, Nb 3 Si laths and Ti-rich Nb SS /Cr 2 Nb eutectic. Nb 3 Si had the advantage of nucleation at medium/high growth rate. Therefore, the microstructure evolution occurred in this study was owing to the increase of growth rate.

In this study, a new constitutive model is established for AZ31B magnesium alloy sheet at elevated temperatures and strain rates in order to describe two competing mechanisms for deformation, i.e. both work-hardening and softening stage of AZ31B magnesium alloy sheet. Stress-strain curves obtained by conducting uni-axial tensile tests at elevated and strain rates were first separated at the maximum stress and corresponding strain values. Voce's law [25] was then employed to fit separated hardening and softening stage. A MATLAB tool is used to determine material parameters by using least square fitting method at various temperatures and strain rate. The mergence of separated work-hardening and softening equations is in good agreement with experimental data. The parameters of fitting curves are utilized to determine them as a function of temperature and strain rate using a surface fitting method. The final equation is then implemented to predict stress-strain curves at various temperatures and strain rates. The proposed equation showed the good comparability between the simulation results and the corresponding experiments.

Thermal Energy Storage (TES) is the key for a stable electricity production in future Concentrated Solar Power (CSP) plants. This work presents a study on the thermal protection of the central receiver of CSP plant using a tower which is subject to considerable thermal stresses in case of cloudy events. The very high temperatures, 800 °C at design point, impose the use of special materials which are able to resist at high temperature and high mechanical constraints and high level of concentrated solar flux. In this paper we investigate a TES coupling a metallic matrix drilled with tubes of Phase Change Material (PCM) in order to store a large amount of thermal energy and release it in a short time. A numerical model is developed to optimize the arrangement of tubes into the TES. Then a methodology is given, based from the need in terms of thermal capacity, in order to help the choice of the geometry.

In the present study microstructural evolution in transient liquid phase (TLP) bonded Ti 3 Al-Nb alloy joints using a pure copper as interlayer was investigated. TLP bonded Ti 3 Al-Nb alloy joints composed of intermetallic compound layers were produced. Microstructural evolution of joints depended on both bonding time and bonding temperature. With increasing bonding time and bonding temperature, the joint width increased and amount of compounds in the joint decreased. The joint microstructure at 1173 K × 1 min mainly consisted of Ti (solid solution) + Ti 2 Cu + TiCu + Ti 3 Cu 4  + Ti 2 Cu 3  + TiCu 4  + Cu (solid solution) phase and it changed to Ti (solid solution) + Ti 2 Cu + TiCu at 1223 K × 60 min. Compounds formed on cooling from the bonding temperature by liquid phase were eliminated from the joint at 1223 K × 60 min due to isothermal solidification of liquid phase. The increase of the width of joint is attributed to the composition difference between the isothermal solidification production and its adjacent base material.

The hot ductility of Fe-36Ni invar alloy doped with and without yttrium was investigated using a Gleeble-3800 thermal-mechanical simulator over the temperature range 850–1050 °C and the improvement mechanism of the hot ductility was analysed with a combination of SEM, EDS and OM. The results showed that Fe-36Ni invar alloy had a poor hot ductility below 1050 °C, which was mainly attributed to the presence of the grain boundary sliding and weak grain boundaries. The addition of 0.048% yttrium had a substantial improvement in the hot ductility of Fe-36Ni invar alloy over the whole testing temperature range especially at 950–1000 °C. At 850–900 °C, the improvement of the hot ductility was mainly associated with the grain boundary strengthening and the restriction of the grain boundary sliding because the addition of yttrium could reduce the segregation of sulfur at grain boundaries and refine the grain structure. At 950–1000 °C, the hot ductility was highly improved, which was owed to the acceleration and occurrence of dynamic recrystallization as a result of the refinement of the grain structure by addition of yttrium.

In this work, dielectric and tunability properties of glass spheres containing elastomer based materials have been investigated with dielectric spectroscopy as a function of frequency, composition and DC bias field. Real and imaginary part of dielectric constant and dielectric loss values of the gs1 (glass spheres), gs2, and gs3 were investigated in the frequency range of 100 to 40 × 10 6 Hz. Dielectric tunabilities of 69%, 87% and 33% were measured on gs1, gs2, and gs3 at 10 kHz and at room temperature. The degrees of anharmonic contributions to the polarization are very high for gs2 and gs3 as 4.53 × 10 −8 and 1.77 × 10 −8 (mm/kV) 2 . Also, dielectric constant, FOM (%) and absorption coefficient a values were calculated at 10 × 10 3 Hz and 20 × 10 3 V/mm depending for different glass spheres.

In this work of the material investigation, electrical parameters, which are real part and imaginary part of modulus, dielectric constant, dissipation factor, and conductivity, in the bulk pellet of probertite sample is presented. Electrical properties were investigated via temperature and frequency dependent dielectric spectroscopy. Real and imaginary part of dielectric parameter properties of the probertite were measured at frequencies from 100 to 15M Hz in the temperature range of 25 to 150 °C. Temperature dependence of the real part of the dielectric constant suggests that these compounds exhibit strong electromagnetic absorption and broadband electrical behavior.

The hot ductility of eco-friendly Bi-S based free cutting steels with different Mn/S ratios was studied using a Gleeble-1500 thermal–mechanical simulator. The hot ductility of the steel was found to depend on the Mn/S ratio, and the Mn/S ratio of the steel should be greater than 3.5 for hot rolling of billets without crack development. The low Mn/S ratio would inhibit the occurrence of the dynamic recrystallization and cause the formation of the low melting point sulfide Fe-rich (Fe,Mn)S as secondary phases, which could obviously reduce the strength of the grain boundary and resulted in the formation of cracks along the grain boundary. The higher the Mn/S ratio in the steel, the lower the Fe content in the Fe-rich (Fe,Mn)S phases. When the Mn/S ratio in the steel was high enough, the sulfide phases in the steel were mainly MnS as primary inclusions and the low melting point sulfide phases could be effectively avoided forming. While the Mn/S ratio could influence the hot ductility of the steel over the whole temperature range of 900–1200 °C, the segregation of bismuth along grain boundary could be harmful to the hot ductility in addition to the lower Mn/S ratio for the temperature was no more than 1050 °C.

In this paper, a hyperbranched polyborocarbosilane (HPBCS) was prepared by a one-pot synthesis with Cl 2 Si(CH 3 )CH 2 Cl, Cl 3 SiCH 2 Cl and BCl 3 as the starting materials. The obtained HPBCS was characterized by GPC, FT-IR and NMR, and was confirmed to have hyperbranched structures. The thermal property of the resulting HPBCS was investigated by TGA. The ceramic yield of the HPBCS is about 84% and that of the counterpart hyperbranched hydridopolycarbosilane is only 45%, indicating that the introduction of boron into the preceramic polymer significantly improved the ceramic yield. With the polymer-derived ceramic route, the final ceramics were annealed at 1800 °C in argon atmosphere for 2 h in order to characterize the microstructure and to evaluate the high-temperature behavior. The final ceramic microstructure was studied by XRD and SEM, indicating that the introduction of boron dramatically inhibits SiC crystallization. The boron-containing SiC ceramic shows excellent high-temperature behavior against decomposition and crystallization at 1800 °C.

Titanium carbide is used as reinforcement particles in composites due to its hardness, wear resistance and stability. This work is a part of the study in which titanium carbides are formed in stainless steel castings in the mold to improve the wear resistance of a certain surface of the casting. Such local reinforcement is a very potential method but it is a quite demanding task requiring profound knowledge of interfacial phenomena in the system, wettability, stability, dissolution and precipitation of new phases in production of these materials. Good wetting between different constituents in the material is a key factor to attain maximal positive effects. Mo is used with TiC or Ti(C,N) reinforcement in composites to improve wettability. In this work the effect of Mo on the phenomena in Fe/stainless steel–TiC systems was examined by wetting experiments between the substrate and the alloy. Wetting was not significantly improved by adding Mo to the systems. Core-rim type carbides as well as more homogenous carbide particles were observed. Overall the carbide particles are very complex regarding to their chemistry, size and shape which aspects have to be taken into account in the development of these materials and manufacturing processes.

Conducting experimental studies on the impact compressive mechanical properties of rock under the high temperature environment is of both theoretical value and practical significance to understanding the relationship between the rock under the effect of impact loads and the high temperature environment. Based on the Φ100 mm SHPB and the self-developed Φ100 mm high-temperature SHPB test devices, the impact compressive tests on the sandstone, whether cooling after high temperatures or under real-time high temperatures are carried out. As the test results indicate that since the two high-temperature ways of loading are different from each other, the impact compressive properties of sandstone, after as well as under high temperatures, show different variations along with changes in temperature. Under the effect of the same impact loading rate, there exists a clear critical temperature range in the impact compressive mechanical properties of sandstone after high temperature, and, near the critical temperature, there occurs a significant mutation in the impact compressive mechanical properties. Under high temperatures, however, the impact compressive mechanical properties follow an overall continuity of change except that there are slight fluctuations at individual temperatures.