Materials Science Forum Vol. 850

Abstract: In situ synchrotron X-ray diffraction was used to study a deformed Cu-0.88 Fe-0.24 P alloy during heating process. The measurements were performed at room temperature and also at high temperatures up to 893 K in order to determine the recovery, ageing and recrystallization process. With the increase of temperature, the angles of copper matrix peaks moved left and the FWHM (full width at half maximum) decreased slightly. Fe₃P precipitates were first detected at 533 K, reached the maximum at 673 K, and re-dissolved into matrix at 853 K. A dramatic decrease in FWHM was observed accompanied by the precipitation of Fe₃P phases, indicating the reduction of lattice distortion of copper matrix.

Abstract: The wear performance and wear mechanism under two-body abrasion of five advanced high strength steels, i.e. Nanobainite (NB) steel, Tempered Martensitic (TM) steel, Dual Phase (DP) steel, Transformation Induced Plasticity (TRIP) Steel and Twining Induced Plasticity (TWIP) steel were studied. By using the scanning electron microscopy (SEM), we investigated the wearing surface. Phase transformation strengthening behavior was also be discussed by analyzing the surface and sub-surface after abrasion. The results showed that micro-cutting was the major role of wear mode in the condition of two-body abrasion. In the circumstance of two-body abrasion, hardness was an important factor, the property of wear resistance enhanced while the hardness increased except for TM steel. NB steel possessed the best wear resistance which was 1.71 times higher than that of TWIP steel. The retained austenite transformed into martensite which can improve the hardness so that it enhanced the wear resistance of NB steel.

Abstract: Rapid solidification of Co-Si alloys was investigated by using vacuum suction casting in this study. Different microstructures and intermediate phases were obtained. Eutectic εCo phase and eutectoid εCo + αCo₂Si structures were obtained in the first eutectic Co_76.9Si_23.1 alloy. The microstructures of hypereutectic Co₇₀Si₃₀ alloys were composed of primary αCo₂Si phase and interdendritic lamellar eutectoid εCo + αCo₂Si. While for hypoeutectic Co₆₃Si₃₇ alloy at the second eutectic point, CoSi dendrites were the primary phase, and αCo₂Si+CoSi eutectoid structures can be seen at the interdendritic region. Especially, a metastable CoSi₂ phase was found in Co₆₃Si₃₇ alloy. This indicates that eutectoid decomposition βCo₂Si→ CoSi+αCo₂Si is restrained in eutectic Co_60.3Si_39.7 alloy. For rapid solidified Co₅₅Si₄₅ and Co₅₂Si₄₈ alloys, αCo₂Si+CoSi eutectoid structures were not observed, while metastable CoSi₂ were obtained. The higher hardness achieved in Co-Si alloys at the second eutectic point, for the reason of the higher volume fractions of compound phases.

Abstract: Flow behavior and microstructure evolution of 2297 Al-Cu-Li alloy were investigated by isothermal compression tests conducted at the deformation temperature of 300-500°C and strain rates of 0.001-10s-1. The results demonstrate that the characteristics of stress-strain curves depended on the interaction of work hardening and dynamic softening. The true stress increased with the decreasing of temperature and the increasing of the strain rate. At a given deformation condition, the flow curve consisted of three stages: stage I (work hardening stage), stage II (softening stage) and stage III (steady stage). Deformation temperature and strain rate had a great influence on microstructure evolution. 2297 alloy deformed at low temperature (300°C) and high strain rate (10s-1) showed a DRV characteristic. As deformed at high temperature (500°C) and low strain rate (0.001s-1), DRX gradually become the main softening mechanism. The measured flow stress was friction corrected and then employed to develop constitutive equations on the basis of the Arrhenius-type equation by considering the effect of the strain on material constants by a sixth orders polynomials. Flow stress value of 2297 alloy predicted by the proposed constitutive equations shows a good agreement with experimental results, thereby confirming the validity of the developed constitutive relation.

Abstract: In the present investigation a batch of impellers made of 319S aluminum alloy with about 300 pieces and produced by semi-solid thixocasting process were subjected the ultrasonic testing (UT). The experimental results revealed that the equivalent size of the defects in the impellers was not more than φ0.4mm FBH+12dB. And there were two main types of defects in 12 mm depth area from the machined surface, where defect was apt to form. One type was single defect and the other was intensive defect. Moreover, other nondestructive testing (NDT) was used to test some impellers containing typical defect, and the result suggests that UT was more sensitive than x-ray testing. The defects existed in the impellers were investigated by Optical Microscope (OM) and Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectrometer (EDS). The observation showed that the both types of defects existed in eutectic zone. The single defect derived from billet and the intensive defect came from die casting process.

Abstract: A new detection method based on the ultrafast refractive index change of semiconductor was used for X-rays detection. Since temporal and spatial resolutions are important parameters of the ultrafast X-rays detector, the electron cascades in gallium arsenide and cadmium selenide were studied using Monte Carlo method. According to the calculations, the energy deposition time and scale are similar in gallium arsenide and cadmium selenide at low energy region, but different at high energy region. Electron cascades don’t have much impact on getting picosecond time resolution and high spatial resolution.

Abstract: In this paper, the application of 9% Ni steel in natural gas storage tanks and the welding thermal crack tendency of 9% Ni steel were studied. For the section shrinkage and breaking strength of tensile specimens at different temperature, the high temperature tensile test (600°C~1450°C) of 9%Ni steel was carried out by Gleeble 3500 thermal simulation machine. The nil-ductility transition temperature (NDT), nil-ductility stength temperature (NST) and ductility recovery temperature (DRT) of 9%Ni steel were identified. The result showed that the NDT for 9% Ni steel was 1400°C, the NST was 1430°C, the DRT is 1370°C, and the brittle temperature range (BTR) was 60°C. In this study, the welding hot crack of 9% Ni steel was a little sensitive.

Abstract: A comprehensive study was carried out on the determination of mechanical parameters in the weld zone by using the digital image correlation (DIC) method. The DIC method provided quantitative two-dimensional strain maps of the deformation field across the weld joint throughout the tensile test. Then the local stress-strain response was extracted based on these strain maps on the uniform stress hypothesis. The strain distribution evolution during loading performed a good consistency with the indentation test results and the cracking position of the specimen. The deformation related parameters of local areas in weld joint were successfully extracted by DIC method. These parameters are very valuable for the structure design of the weld components, especially in the case of the structure in which the allowable stress is controlled by the deformation feature but not the strength of the material such as the high temperature applications.

Abstract: The orientation effects of the organic functional groups CH₃NH₃⁺ along [100], [110], [111] and [210] on the stability and photoelectric properties of CH₃NH₃PbI₃ were investigated using first-principles calculations. The results showed that the system energies when C-N bond was along [100]/[210] directions were lower than those while C-N bond was along [110]/[111] directions. The band gap while C-N bond was along [100]/[210] direction was larger than that while C-N bond was along [110]/111] direction. The system energy changed within the range of 0.8 eV, and the band gap changed within the range of 0.05 eV as CH₃NH₃⁺ moving along the same crystal direction. The optical properties of CH₃NH₃PbI₃ with the C-N bond along [100] direction were different from those with the C-N bond along [111] direction, while the differences were not obvious. The change trends of optical properties with CH₃NH₃⁺ moving for two structures were in agreement with each other. The optical properties indicate that CH₃NH₃PbI₃ is a good light absorber material for thin film solar cells.

Abstract: Oxygen adsorption, aluminium segregation and interface adhesion on the surface of NiPtAl and MCrAlY bond coats (BC) in EB-PVD TBC system were investigated using first-principles calculations within the density functional theory (DFT). Examination of oxygen adsorption and aluminium segregation indicated that the addition of Pt always obstructed the growth of alumina. In addition, NiPtAl, as bond coats in EB-PVD TBC System, had less lattice variation and stronger interface adhesion than MCrAlY when alumina was produced. It is found that Pt is an important factor that affects the Al₂O₃ growth in thermal barrier coating. It is proved that Pt improves the bonding performance of Al₂O₃ and lifetime of thermal barrier coating. This offsets the high cost of Pt in industry application.