Papers by Keyword: Metastable Phase

Abstract: The microstructure and mechanical properties of Al-Zn-Mg alloys with low Zn/Mg ratios have been studied.According to various researchers, the major strengthening is due to η-phase and T phase. There are many briefly research on the microstructure and interface of the η-phase types but not briefly information about the T-phase That’s why now our aim to work on T-phase. In actual our aim is to observe the T-phase interface. The MgZn₂ phase (η phase) and its metastable phase (η′ phase) were the most prominent precipitates. Another study revealed various Mg₃₂(Al, Zn)₄₉ phases (T phase) and their metastable phase (T′ phase) in Al-Zn-Mg alloys with low Zn and high Mg content. Al-Zn-Mg alloys with a Zn/Mg ratio of 0.71 were explored for this study. The alloy with a Zn/Mg ratio of 0.71 aged at 473k for 2000 minutes exhibited the highest hardness, according to the observations. The strengthening precipitates in the investigated alloy were totally T′ phase, according to TEM observation.

Abstract: The development of the transition metastable phases and the accompanying hardening of these transformations are studied for Al-20wt%Ag and Al-3wt%Cu-1wt%Mg supersaturated solid solutions. During the precipitation the GP and GPB zones form by a process of negative diffusion. The metastable phases are characterized by their hardness isotherms and their temperature range determined on the hardness isochronal.

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: The effects of the different heat-treatments on the eutectic carbides in HSS cast strip are studied. The microstructure of the high speed steel(HSS) is analysized by means of optical microscopy and SEM. The results show that the heat treatment can make M2C and M6C metastable phase decompose. After comparison we can find that the optimized heat treatment processing of HSS cast strip is at 1200°Cfor 6 h, under which the fine and uniform carbides can be obtained.

Abstract: An experimental study on the influence of cooling rate and manganese content on the structures and phase formation of Fe-Mn alloy was carried out during sub-rapid solidification process. BCC structures were obtained for samples with manganese content ranging from 2pct to 11pct, and a HCP phase was obtained when the manganese content is up to 15pct. The micro- hardness increases with cooling rate and manganese content increasing.

Abstract: Microstructure evolution of Fe-1.18%Cu high purity steels during solution and aging was investigated under high-resolution electron microscope (HREM). In addition, the aging strengthening mechanisms were discussed based on the microstructure observation. The results show that there were lots of Cu atom clusters in ferrite matrix during solid solution and aging initial stages, subsequently, Cu-rich metastable Fe-Cu particles precipitate at the aging strength peak. It is found that the intense strengthening is controlled by the coherency relationship of Fe-Cu metastable phase with matrix that forms the obstacle of the dislocation motion, while the decrease of strength after the peak is attributed to the loss of coherency, which should highly likely be the dominant reason of aging strengthening in Cu bearing high purity steels Thus our TEM observation results are in reasonably agreement with some previous assume.

Abstract: Coating and gradient structure can improve the quality of products, but growth-induced stress generates in the forming process of the coating and the gradient structure due to the existence of the defects and metastable phase. The existence of growth-induced stress can conversely affect the quality of the product, even lead to cracking and product failure. This paper shows the model of stress calculation according to the crystal structure’s defect theory.

Abstract: The precipitation process in an aged Cu-1.9wt%Ni-0.3wt%Be alloy has been examined by high-resolution transmission electron microscopy. The precipitation sequence found is: Guinier- Preston (G.P.) zones → γ'' → γ' → stable γ. The disk-shaped G.P. zones and the disk-shaped γ'', γ' and γ precipitated phases are composed of monolayers of Be atoms on {100}_α of the Cu matrix and alternative Be and Ni matrix layers parallel to {100}_α. The γ'' phases consisting of two to eight Be-layers has a body-centered tetragonal (bct) lattice with a=b=0.24 nm and c=0.28 nm. The γ' or γ phase is bct with a=b=0.24 nm and c=0.26 nm or a=b=0.26 nm and c=0.27 nm. The γ'', γ' or γ phase aligns with the matrix according to the Bain orientation relationship. The growth kinetics of disk-shaped γ precipitates on aging at 500°C has been also investigated. The {001}_α habit planes of the γ precipitates migrate by a ledge mechanism. The average thickness of the γ disks increases with aging time t as t^1/2. An analysis of experimental data using a kinetic model yields the diffusivity of solute in the Cu matrix, which is in agreement with the reported diffusivity of Ni in Cu.

Abstract: The present work focuses on a physical description of phenomena occurring during the heat treatment of a cold deformed aluminium sheet. The major practical interest lies in the prediction of microstructural changes and their impact on the yield strength. The material softening part was described due to thermally activated glide of dislocations. The precipitation kinetics are calculated by using the computational thermodynamics program MatCalc (MATerials CALCulator). The model was validated by comparison with experimental data from tensile tests of cold deformed and heat treated sheets. Finally, it was shown that the model can be used to predict the yield strength during heat treatment of Al-Mg-Si alloys.

Abstract: Metastable phase was observed in as-solidified microstructure of undercooled Fe-Co alloy, provided that the initial undercooling (T) of the melt exceeds the critical value. On this basis, the forming mechanism and stability of metastable phase were investigated. The forming mechanism of metastable phase in undercooled Fe-Co alloy was concluded as cooperative effect of “competitive nucleation—remelting—extensive growth—incomplete solid-state transformation” in this work. With the increase of annealing time, the number and dimension of metastable phase decreased at the same time. Moreover, metastable phase was transformed to stable phase completely when annealing time increased to three hours.