In situ synchrotron diffraction and modeling of non-equilibrium solidification of a MnFeCoNiCu alloy

The solidification mechanism and segregation behavior of laser-melted Mn35Fe5Co20Ni20Cu20 was firstly investigated via in situ synchrotron x-ray diffraction at millisecond temporal resolution. The transient composition evolution of the random solid solution during sequential solidification of dendritic and interdendritic regions complicates the analysis of synchrotron diffraction data via any single conventional tool, such as Rietveld refinement. Therefore, a novel approach combining a hard-sphere approximation model, thermodynamic simulation, thermal expansion measurement and microstructural characterization was developed to assist in a fundamental understanding of the evolution of local composition, lattice parameter, and dendrite volume fraction corresponding to the diffraction data. This methodology yields self-consistent results across different methods. Via this approach, four distinct stages were identified, including: (I) FCC dendrite solidification, (II) solidification of FCC interdendritic region, (III) solid-state interdiffusion and (IV) final cooling with marginal diffusion. It was found out that in Stage I, Cu and Mn were rejected into liquid as Mn35Fe5Co20Ni20Cu20 solidified dendritically. During Stage II, the lattice parameter disparity between dendrite and interdendritic region escalated as Cu and Mn continued segregating into the interdendritic region. After complete solidification, during Stage III, the lattice parameter disparity gradually decreases, demonstrating a degree of composition homogenization. The volume fraction of dendrites slightly grew from 58.3 to 65.5%, based on the evolving composition profile across a dendrite/interdendritic interface in diffusion calculations. Postmortem metallography further confirmed that dendrites have a volume fraction of 64.7% ± 5.3% in the final microstructure.

In-Situ Synchrotron Diffraction and Modeling of Non-Equilibrium Solidification of a MnFeCoNiCu Alloy

The solidification mechanism and segregation behavior of laser-melted Mn35Fe5Co20Ni20Cu20 was firstly investigated via in-situ synchrotron x-ray diffraction at millisecond temporal resolution. The transient composition evolution of the random solid solution during sequential solidification of dendritic and interdendritic regions complicates the analysis of synchrotron diffraction data via any single conventional tool, such as Rietveld refinement. Therefore, a novel approach combining a hard-sphere approximation model, thermodynamic simulation, thermal expansion measurement and microstructural characterization was developed to assist in a fundamental understanding of the evolution of local composition, lattice parameter, and dendrite volume fraction corresponding to the diffraction data. This methodology yields self-consistent results across different methods. Via this approach, four distinct stages were identified, including: (I) FCC dendrite solidification, (II) solidification of FCC interdendritic region, (III) solid-state interdiffusion and (IV) final cooling with marginal diffusion. It was found out that in Stage I, Cu and Mn were rejected into liquid as Mn35Fe5Co20Ni20Cu20 solidified dendritically. During Stage II, the lattice parameter disparity between dendrite and interdendritic region escalated as Cu and Mn continued segregating into the interdendritic region. After complete solidification, during Stage III, the lattice parameter disparity gradually decreases, demonstrating a degree of composition homogenization. The volume fraction of dendrites slightly grew from 58.3–65.5%, based on the evolving composition profile across a dendrite/interdendritic interface in diffusion calculations. Postmortem metallography further confirmed that dendrites have a volume fraction of 64.7% ± 5.3% in the final microstructure.

Service Temperature Evaluation of Cast K465 Superalloy Turbine Vane Based on Microstructural Evolution

The overtemperature in a vane made of cast K465 superalloy is investigated in this paper. The vane was taken from a K465 guide ring after the service for specific period of time. The relationship between the temperature and content of γ´ precipitates was established for the K465 superalloy, which was then used to make the microstructural evolution analysis on failed turbine vane associated with overtemperature. The content of γ´ precipitates in dendritic region and the volume fraction of melting zone in interdendritic region in K465 superalloy were used as the microstructural parameters to check the overtemperature. It is found that there is a sound relationship between the parameters and the temperature. On the basis of this relationship, the microstructural evolution along with temperature variation of the overtemperature K465 turbine vane can be analyzed and the overall service temperature which the K465 guide ring experienced can be evaluated.

Research on Microstructure and Properties of Co-Si Alloys by Vacuum Suction Casting

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 Co76.9Si23.1 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 Co60.3Si39.7 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.

Effects of Co on the Solidification and Precipitation Behaviors of IN 718 Alloy

The effects of Co from 0 to 11.60 % (in mass fraction) on the solidification and precipitation behaviors of IN 718 alloy had been investigated. The results showed that the volume fraction of the dendrite core increased with the addition of Co. In the alloys with 0-5.84 %Co, the addition of Co could restrain the precipitation of blocky Laves phase and promoted the formation of eutectic Laves phase. In the alloys with 9.00-11.60 % Co, the eutectic gray phase and small blocky Laves phase precipitated in the interdendritic region. The eutectic gray phase increased and small blocky Laves phase decreased with increasing Co. The parallel lath-like δ-Ni3Nb phase was observed to precipitate in some interdendritic region without the formation of gray phase and Laves phase in the 9.00-11.60 % Co alloys. Further research found that Co slightly segregated in the dendrite core and markedly raised the solubility of element Mo in the dendrite core which resulted in reduced Mo in the residual liquid, and consequently, restrained Laves phase while promoted the precipitation of Mo-depleted gray phase and δ-Ni3Nb phase. Furthermore, Co was seemed to elevate the solidification point of the γ matrix while decrease that of the Laves phase.

Effect of Hot Isostatic Pressing on the Microstructure of K417G Cast Turbine Discs

The present study focused on the effect of hot isostatic pressing (HIP) treatment on the microstructure of K417G superalloy. The experimental results showed that after the HIP treatment the size and volume fraction of the porosities significantly decreased. In addition, the dendritic structure and γ/γ' eutectics in the as-cast specimens became obscure after the HIP treatment due to the improvement of segregation. The γ′ phases in the dendrite core were smaller than those in the interdendritic region, whether in the as-cast or HIP specimens. The slow cooling at the end of the HIP treatment leads to the irregular morphology of the γ′ phases.

Microstructure Characteristics of TIG Welding Joint of 740H Pipe for Ultra-Supercritical Power Plant Boilers

Microstructure and mechanical property of tungsten inert gas (TIG) welding joint of 740H pipe were studied by optical microscope, scanning electron microscope and HV tester in this paper. The principal segregated elements in fusion zone of 740H weldment were Nb and Ti. The discontinuous MC-type phase with irregular shape and γ′ phase were distributed in the interdendritic region. The γ′, M23C6 and MC phases on grain boundary in heat affected zone (HAZ) dissolved into matrix. Hardness of weld metal and HAZ were apparantly lower than that of base metal. The previous weld bead was the heat affected zone of the subsequent weld metal, thus hardness was different in different area of the fusion zone (FZ). After the postweld heat treatment of 800°C/4h followed by air cooling, the volume fraction of γ′ in the interdendritic region was larger than that in the dendrite arm. The hardness of HAZ and FZ were approximately equal to that of base metal (BM).