Papers by Keyword: Phase Decomposition

Abstract: The change in hardness due to 475°C embrittlement was investigated in the melt-run GTA welds of type 329J4L duplex stainless steel. A ferritic phase was hardened with ageing at 673-773K due to the phase decomposition into Fe-rich and Cr-rich phases, while an austenitic phase was barely hardened with ageing. Hardness in a ferritic phase was rapidly increased with ageing in the base metal (BM) region, and the hardening rate was reduced in the order of BM, weld metal (WM) and heat affected zone (HAZ). The ferrite/austenite fractions in HAZ and WM were higher than that in BM, furthermore, Cr content in a ferritic phase was lowered and Ni content in it was contrarily heightened in the same order of BM, WM and HAZ. A computed phase diagram suggested that the chemical composition of a ferritic phase in each region was located in the nucleation/growth region not spinodal decomposition region, which was situated between the spinodal and binodal lines. Computer simulation of phase decomposition phenomena in a ferritic phase using phase field model revealed that phase decomposition was accelerated with an increase in Cr content and a decrease in Ni content. It followed that Cr would enhance the 475°C embrittlement and Ni would inhibit it, because of the increased/decreased driving force of the phase decomposition. The difference in 475°C embrittlement behaviour at each region could be attributed to the difference in the chemical composition of a ferritic phase caused by the ferrite/austenite phase transformation during welding.

Abstract: Dolomite is widely used in the construction, glass ceramics, iron and steel, pharmaceutical industries, as a source of CaO and MgO and as thermal energy storage material. Thermal decomposition analysis of natural dolomite of the so-called Jeddih limestone has been carried out. A thermogravimetry analysis (TGA) in the air evaluates the thermal decomposition of dolomite. The natural dolomite has been analyzed by x-ray flourescence (XRF) and x-ray diffraction (XRD) to test crystal structure and decomposition phase, fourier transform infra-red (FTIR) was utilized to identify the presence of functional groups. The particle morphology was observed by scanning electron microscopy. TGA curve shows that the thermal decomposition of dolomite occurs in two stages. The first stage is in temperature range of 600 - 779°C and the second one is at the temperature 779°C. The results are in line with the XRD and FTIR measurements. Which shows that calcite begins to grow at a temperature of 600°C and MgO phase is observed at 700 - 900°C. Moreover, CaO phase starts to be found at 800°C.

Abstract: In the present study, we investigate the performance of efficient pair potentials in comparison to accurate ab initio potentials as energy descriptions for Monte Carlo simulations of solid-state precipitation. As test scenario, we take the phase decomposition kinetics in binary Fe_1-xCu_x. In a first effort, we predict thermodynamic equilibrium properties of bcc-rich Cu precipitates in an Fe-rich solution with a temperature and composition dependent Cluster Expansion. For this Cluster Expansion, combined ab inito and phonon calculations for various configurations serve as input. Alternatively, we apply the Local Chemical Environment approach, where the energy is described by computationally efficient pair potentials, which are calibrated on the first principles cluster expansion results. We observe that these fundamentally different approaches provide similar information in terms of the precipitate radius, chemical composition and interface constitution, however, the computational effort for the Local Chemical environment approach is significantly lower.

Abstract: The influence of heat treatment on phase decomposition of Co-10 at. pct. Cu alloy was studied. The materials and phase compositions were studied by using energy dispersive spectrometry (EDS) and X-ray diffraction (XRD) techniques. XRD analysis showed that the samples contained Co, Cu, CuO and CoCu2O3 phases depending on the heat treatment regimes. Moreover it is found that the formation of dendrite Co phase render the spinodal decomposition even for deep long aging inside the miscibility gap. The crystal structural parameters were refined with FULLPROF program. 2-D computer simulation indicates that the morphology and the shape of the microstructure agree with experimental SEM micrographs for the Cu rich phase.

Abstract: The effect of Al-addition on the mechanism of phase decomposition from β₁ to α-phase in Cu-40at.%Zn alloy (base alloy) has been investigated using hardness measurement and HRTEM observation. The peak hardness during annealing and the number density of α-phase in Al-bearing alloy were the same as those in the base alloy. Many striations which have been considered as the periodicity of 9R structure were observed in the α-phase of the Al-bearing alloy. There were a lot of striations in the α-phase at the early annealing time, and the number of these striations decreased with increasing annealing time. The rate of decreasing of this striation in the α-phase of the Al-bearing alloy was the slower than that of the base alloy. According to the analysis of the selected area electron diffraction (SAED) patterns obtained for these α-phase in those alloys, transition time from the 9R structure to fcc structure t (9R-fcc) of has been detected by the changing of angles in SAED patterns. The t (9R-fcc) of α-phase in the Al-bearing alloy showed the slower than the base alloy.

Abstract: As high-temperature metallic materials, Co-base ODS alloys were produced by means of mechanical alloying, spark plasma sintering and hot rolling. Co-3wt%Al-1.2wt%Hf-ODS alloy was found to be an attractive composite like material, which is formed by spinodal like decomposition. The metastable phases were traced by hard phase containing dense oxide particles and soft one containing less oxide particles. Their tensile stress at 1,000 °C was improved by Hf addition that forms Y₂Hf₂O₇ type oxide particles and shortens their space distance.

Abstract: Structure and mechanical properties of high-strength alloys on the basis of the Fe-Cr-Co system with W, Ga, Cu and Al additives have been investigated by the Mossbauer technique. It is shown that the magnitude of yield strength is independent of the dopants, whereas the relative elongation is controlled by the process of phase separation in the alloys which is dependent on additions.

Abstract: The phase decomposition was investigated in Cu-Al alloys processed to a nanostructure condition by High Pressure Torsion (HPT). The microstructures are characterized by optical microscopy (OM), X-ray diffraction (XRD) and Atom Probe Tomography (APT). The results show that the’ → (1 + decomposition reaction begins in the early stage of annealing and it is much faster than in the coarse-grained state although there are similar phases after annealing.

Abstract: It is known that the phase-decomposition process of 60/40 Cu-Zn alloy is so-called the bainitic transformation, and decomposition of α-phase from the β’-phase is as follow: β’ → α9R → αfcc. In this work,decomposition of α-phase from the β’ single phase of Cu-40.26at.%Zn alloy has been investigated by high-resolution transmission electron microscopy (HRTEM) to understand the phase transformation of this alloy. Especially, striations in the α-phase has been focused on the special feature for the change of the structure and hardening of this alloy during annealing. The result of a comparison between this alloy and the Si added alloy is also reported.

Abstract: Although the γ"(D022) phase has been known as a strengthen phase for the turbine disk of wrought Ni-base superalloys, the computer simulation of the γ"(D022) precipitation is hardly performed. In this study, it is demonstrated that the phase-field modeling of the complex microstructure developments including γ"(D022) precipitation in Ni-V-X (X=Co,Nb,Fe) alloys. The simulation results obtained are as follows: (1) The complex morphologies of the γ(A1)+γ"(D022) two-phase microstructure, such as the maze-microstructure, the chessboard-microstructure, and the chessboard-like microstructure, in Ni-V-X (X=Co,Nb,Fe) alloys are simulated reasonably by using phase-field method. (2) The morphology of the microstructure is mainly controlled by the elastic strain energy induced from the lattice mismatch. In particular, the tetragonal distortion is a key parameter to control and understand the complex microstructure changes.