Materials Science Forum Vols. 704-705

Abstract: 12Cr2MoV steel pipe and T91 steel pipe were joined by transient liquid phase diffusion bonding process (TLP) in argon atmosphere using FeNiCrSiB amorphous filler. TP304H steel pipe and 12Cr2MoV steel pipe were joined by the same method using Fe₇₈Si₉B₁₃ and BNi₂ amorphous filler. The qualified welding joint can be achieved by suitable adjustment of process parameters. The diffusion asymmetry in TLP of different metals has been analyzed. The research shows that there exists a diffusion asymmetry in TLP of different metals. The main reason is the difference of isothermal solidification rate in different materials, which results in a deviation of the bonding interface from the original centerline to T91 and TP304H side.

Abstract: TiAl-based alloy with a composition of Ti-47%Al-3%Cr (mole fraction) was prepared by high-energy ball milling and hot-pressing sintering. The relationship between microstructure and mechanical properties of Ti-47%Al-3%Cr alloy was studied by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and mechanical testing. The results showed that the TiAl-based alloy with high density and uniform microstructure could be obtained by high-energy ball milling and hot-pressing sintering. The compactibility and sintering densification of the element powder could be promoted efficiently by high-energy ball milling. The main phase TiAl and few phases Ti₃Al were observed in the hot pressing sintering bulk samples. In addition, the microstructure changed with ball milling times, as a result, the mechanical properties changed with the microstructure. The finer the microstructure was, the higher the strength at room temperature became. After the element powder was milled for 20 hours and hot-pressing sintered at 1300 °C for 2 hours, TiAl-based alloys were found to have good room temperature mechanical properties with the compressive strength of 2870 Mpa and the relative compressive ratio of 27.3%. Keywords: TiAl-based alloys; hot-pressing sintering; microstructure; mechanical properties

Abstract: Considering the problems and details of modeling process of cold rolling, it was constructed a theoretical model of deformation area based on a set of equations, which were practical testing. The model is based on cold-rolled numerical solution of differential equations of forces equilibrium in the deformation area by Karman. In the model there are included modules for calculating strip temperature and strip gage at the exit from the stand, considering elastic recovery of the strip. For adaptation of the equations to the specific conditions of cold rolling of HSLA steels in the continuous five stand mill 2030 and assess the deviation of calculated data from the actual, a procedure of correction for hardening curve of rolled steel has been developed. The model allows defining the energy-power parameters of rolling: average unit rolling force, the full force of rolling, rolling moment, rolling force, an average temperature of strip at the inlet and outlet of each of the mill stands, steel hardening curve when rolling. The model is implemented as software using object-oriented approach in the language of C++ Builder.

Abstract: The transmission properties of polycrystalline transparent ceramics are influenced by the chemical composition and the microstructure of the material. The fundamental optical mechanism of transparent ceramics and the influencing factors on transmission properties were discussed in this paper. The Mie theory for light scattering is applied to calculate scattering coefficients of residual pores and optical birefringence. The in-line transmission curves of the transparent alumina were calculated as a function of pore size, porosity and grain size. The results show that scattering by the residual pores is the dominant influencing factors on transmission properties. The scattering by the residual pores increases with an increase in porosity and the maximum of the scattering was observed when the pore size close to the optical wavelength. Optical birefringence in the transparent ceramics with non-cubic crystal structure has an important effect on the in-line transmission. The in-line transmission increases with a decrease in grain size. Keywords: transparent ceramics; microstructure; light scattering; birefringence

Abstract: The embrittlement of particular weather resistant steel has been investigated by thermal tensile tests using a Gleeble-1500D system at temperatures range from 600 to 1300°C and at a strain rate of 10^-3/s. The specimen was reheated and cooled to the test temperature before the tensile deformation in order to get the mechanical properties subject to the continuous casting process. To make clear the plastic flow behavior in Austenite and γ-Ferrite temperature range of the weather resistant steel, the tensile tests were performed and the results indicate that the stress-strain curves as a function of temperature and with the temperature increased the stress became less, while the hot ductility changed significantly due to the work hardening and recrystallization. All of the strength indices (including yield strength, tensile strength and fracture strength) of the steel increased gradually with decreasing temperature, except for small fluctuations during the γ→α transformation occurred. Special emphases were placed on the hot ductility to clarify the sensitivity of surface cracking during unbending operation and the embrittlement zone occurred in the temperature range between 750 and 1050°C, in which the minimum %R of A was around 22% at 850°C. In the embrittlement zone, the specimens were fractured with little plastic deformation by either grain boundary sliding or by localization of strain in the film proeutectoid ferrite produced by the γ→α transformation. Keywords: Weather Resistant Steel, Embrittlement Zone, Stress-Strain Analysis, Hot Ductility, Fracture Surface.

Abstract: An installation, consisting of an explosive-driven planar flyer and a cylindrical chamber, was designed to synthesize γ-Si₃N₄ with cubic spinel structure using the mixtures of impure α-Si₃N₄ and copper powders as starting materials. Sintering of γ-Si₃N₄ with Y₂O₃-Al₂O₃-La₂O₃ as additives were studied under pressures of 5.7 GPa and temperatures of 1370K. The corrosion resistance of γ-Si₃N₄ sintered compact to hydrofluoric acid was investigated at temperature between 490K and 500K, and its thermal stability of γ-Si₃N₄ powder prepared by shock wave synthesis was studied at temperature between 1670K and 1690K in a nitrogen atmosphere. The results show that γ-Si₃N₄ sintered compact could not react with hydrofluoric acid at 490K. At 500K, γ-Si₃N₄ sintered compact could reaction with hydrofluoric acid slowly, and 0.2g γ-Si₃N₄ sintered compact quality reduced 0.013g after 6h. The γ-Si₃N₄ sintered compact could not react with hydrochloric acid, nitric acid, sulfuric acid and universal solvent at 500K. The γ-Si₃N₄ powder was quite stable and without phase transformation at 1670K, and after soaking at 1680K for 10 min and 30 min, the ratio of transformation from γ-Si₃N₄ to β-Si₃N₄ is about 0% and 80%, respectively. The γ-Si₃N₄ powder could be completely transformed into β-Si₃N₄ after soaking at 1690K for 30 min. Keywords: cubic silicon nitride; shock wave synthesis; phase transformation; corrosion

Abstract: The effect of ultrasonic vibration grinding for mechanical properties of Al₂O₃/ZrO₂ ceramics composite was studied in this work. The phase compositions and the microstructures of Al₂O₃/ZrO₂ ceramics were investigated by XRD, SEM and TEM. Results indicated that the hardness and fracture toughness of Al₂O₃/ZrO₂ ceramics could reach 1496Mpa and 9.44 MPa.m^1/2 respectively, which increase 60% and 12% compared to that without ultrasonic vibration grinding.

Abstract: The present studies are to investigate the microstructure features during transformation from austenite to ferrite without and with magnetic field on Fe-0.76%C alloy. It is found that the area fraction and numbers of proeutectoid ferrite grain as well as the lamellar spacing of pearlite in Fe-0.76%C alloy increased considerably with the increase of magnetic field intensity. The reason is that, the magnetic field increases the driving force of proeutectoid ferrite nuclei and shifts the eutectoid point to the side of high carbon content and high temperature, which increases the starting-temperature of the transformation from austenite to ferrite. The proeutectoid ferrite grains are elongated along the magnetic field direction, which can be explained as follows: the proeutectoid ferrite becomes the magnetic dipolar under high magnetic field, and then the polarized austenite atoms are much easier to diffuse into ferrite grains along the magnetic field direction. Key words: high magnetic field; Fe-0.76%C alloy; microstructure

Abstract: In the present study, the goatskin collagen matrices were tanned by chrome powder and chestnut extract. The samples were then thermocycled in a container at different temperature ranges with a dwell time of 5 h for different cycles. The thermal degradation behaviors of the collagen matrices before and after the thermocycling were investigated. The Coates-Redfern Method was applied to study the thermal degradation kinetics of different samples. The results indicate that the thermocycling exerted more dramatic effect on the native collagen matrices than on tanned samples. The tanning process endows the samples with extra crosslinking structure, and therefore, less sensitivity to thermocycling treatment. It was found that the thermal treatment at moderate temperature (0~50˚̓˹˰̵̵̵̴͈͂̈́˰̵̼̹̼̈́̈́˰̶̵̵̹̼̳̾̾ͅ˰̿̾˰̸̵̈́˰̸̵̱̼̈́͂̽˰̴̵̷̴̱̱̹͂̈́̿̾˰̱̳̹̱̹̈́͆̈́̿̾˰̵̵̷̵̹̾͂̓˰˸ΔE) of the sample. However, for the samples thermocycled at high upper limit temperature (0~150˚̓˹˼˰ΔE was decreased significantly.

Abstract: Abstract: In this article, the high-speed dry sliding tribological behaviors of CrNiMo steel against brass in nitrogen and oxygen atmospheres are investigated using a pin-on-disc tribometer. The worn surface is characterized by scanning electron microscopy and electron dispersion spectrums analysis. The wear mechanisms of CrNiMo steel are also analyzed. The results indicate that the tribological properties of CrNiMo steel are coincidental with the law of dry sliding of metal, where the friction coefficients decreases with an increase in sliding speed and with normal load. However, the atmosphere has obvious effects on the tribological properties of CrNiMo steel. In the sliding process, friction heat plays an important role on the tribological properties of materials in high-speed dry friction. The high-speed wear mechanism of CrNiMo steel varies at different atmospheres. In a nitrogen atmosphere, the wear mechanism of CrNiMo steel is mainly characterized by adhesion at a lower speed and load. When the speed and load are increased, melting trace is found in the worn surface accompanied by an abrasive wear. In an oxygen atmosphere, the mechanism is characterized by adhesion at a lower speed and load; with an increase in speed and load, it gradually transformed into oxidation wear and abrasive wear. The difference of the wear mechanisms in the different atmospheres and test parameters is primarily due to the transfer films formed on the contact surfaces of the sliding pairs. In our experimental conditions, the surface film is mainly the metal film in nitrogen, whereas, it is the oxide film in oxygen.