Dendrite Core Research Articles

Heat treatment impacts the micro-structure and mechanical properties of AlCoCrFeNi high entropy alloy

AlCoCrFeNi alloy was prepared by arc melting and examined as-cast and after various heat treatments using XRD, SEM, micro-hardness and compression tests. It was found that the alloy solidified dendritically with an Al- and Ni-rich dendrite core and inter-dendritic regions rich in Co, Cr, and Fe. The dendrite core of the as-cast material presented a relatively soft matrix with nano-sized precipitates, while the inter-dendritic regions consisted of a relatively hard matrix with nano-sized particles larger than those in the dendrite core. Heat treatment did not change the dendritic morphology, however, different phase transformations occurred, especially in the inter-dendritic regions. Heat treatment between 650 and 975 °C caused transformation of the BCC matrix in the inter-dendritic regions to the σ phase, leading to a further increase in hardness. Heat treatment at 1100 °C caused transformation of the σ phase in the inter-dendritic matrix to a BCC structure and softening of the alloy. Heat treatment at 1200 °C caused partial phase dissolving and homogenization, which in turn enabled the alloy to re-enter the miscibility gap and decomposition to a BCC matrix, with B2 precipitation in the dendrite core and inter-dendritic regions. The impact of these changes on morphology and phase composition upon heat treatment in terms of mechanical behavior was discussed.

니켈기 초내열 합금 GTD 111에서 편석에 의한 합금원소 분포 및 미세조직 변화

Segregation during solidification and homogenization during thermal exposure in GTD 111 were investigated. The microstructures of as-cast, standard heat-treated, and thermally exposed specimens were observed by SEM. A compositional analysis of each specimen was conducted by EDS. The dendrite core was enriched in W and Co, though lower levels of Ti and Ta were observed. An unexpected phase, in this case like the phase, was observed due to segregation near the eutectic in the standard heat-treated specimen. Segregation also induced microstructural evolution near the eutectic during the standard heat treatment. A quantitative analysis and microstructural observations showed that the thermal exposure at a high temperature enhanced the chemical homogeneity of the alloy.

Influence of Ru on Solidification Behavior, Microstructure and Hardness of Re-free Ni-based Equiaxed Superalloys with High Cr Content

The effect of Ru addition on solidification behavior, microstructure and hardness of Re-free Ni-based equiaxed superalloys with high Cr content has been investigated. With the increase of Ru, the solidus temperature of the alloys and the γ′ solvus temperature decreased, respectively. However, the liquidus temperatures of the alloys had no obvious change. The microstructure of the as-cast alloys was mainly composed of γ, γ′, γ/γ′ eutectic and MC carbides. The γ/γ′ eutectic was completely dissolved after the heat treatment. The morphology of γ′ was more cuboidal in heat-treated alloys with increasing Ru. Furthermore, the volume fraction of γ′ in the as-cast and heat-treated alloys diminished upon the increase of Ru. It was noted that Ru addition changed the segregation behaviors of Cr and Mo in the alloys from positive segregation element to negative segregation element and promotes the segregation degree of W. As the Ru content increased, the magnitude of segregation of the positive segregation elements Ta and Ti increased accordingly. Meanwhile, the magnitude of Al segregation decreased and Ru tended to segregate in the dendrite core. In addition, the hardness of the alloys improved and their porosity reduced with increasing amount of Ru.

Rejuvenation Heat Treatment of the Second-Generation Single-Crystal Superalloy DD6

The second-generation single-crystal superalloy DD6 with [001] orientation was prepared by screw selecting method in the directionally solidified furnace. The long-term aging of the alloy after full heat treatment was performed at 1100 °C for 400 h. Then the rejuvenation heat treatment 1300 °C/4 h/AC + 1120 °C/4 h/AC + 870 °C/24 h/AC was carried out. The stress rupture properties were investigated at 760 °C/800 MPa, 850 °C/550 MPa, 980 °C/250 MPa and 1100 °C/140 MPa after different heat treatments. The microstructures of the alloy at different conditions were studied by SEM. The results show that γ′ phase of the alloy became very irregular and larger after long-term aging at 1100 °C for 400 h. A very small amount of needle-shaped TCP phase precipitated in the dendrite core. The coarsened γ′ phase and TCP phase dissolved entirely after rejuvenation heat treatment. The microstructure was restored and almost same with the original microstructure. The stress rupture life of the alloy decreased in different degrees at various test conditions after long-term aging. The stress rupture life of the alloy after rejuvenation heat treatment all restores to the original specimen more than 80% at different conditions. The microstructure degradation of the alloy during long-term aging includes coarsening of the γ′ phase, P-type raft and precipitation of TCP phase, which results in the degeneration of stress rupture property. The rejuvenation heat treatment succeeds in restoring the original microstructure and stress rupture properties of the alloy.

Effect of phosphorus segregation on as-cast microstructure and homogenization treatment of IN706 alloy

The distribution of phosphorus in IN706 alloy was studied, with particular emphasis on the as-cast microstructure and homogenization treatment. It is found that phosphorus is segregated in Laves phase and markedly influences the as-cast microstructure of IN706 alloy. Phosphorus retards the formation of Laves phase with the eutectic form, but promotes the Laves formation with the blocky form. Phosphorus refines the dendritic structure and noticeably enlarges the interdendritic area. The reason for phosphorus influencing the as-cast microstructure is attributed to its extremely low solubility in γ matrix and great retarding effect on γ matrix solidification. In addition, phosphorus slightly decreases the melting temperature of Laves phase, and retards the dissolution of Laves phase and the element diffusion between dendrite core and interdendritic area during the homogenization treatment. And it is thought that phosphorus delays the homogenization process by inhibiting the element diffusion.

Electron microscopy study of direct laser deposited IN718

The microstructure of direct laser deposited (DLD) IN718 has been investigated in detail using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results confirm that the dendrite core microstructure can be linked to the cooling rate experienced during the deposition. A ~100μm wide δ partially dissolved region in the IN718 substrate was observed close to the substrate/deposit boundary. In the deposited IN718, γ/Laves eutectic constituent is the predominant minor microconstituent. Irregular and regular (small) (Nb,Ti)C carbides and a mixture of the carbides and Laves were observed. Most M3B2 borides were nucleated around a (Nb,Ti)C carbide. Needles of δ phase precipitated from the Laves phase were also observed. A complex constituent (of Laves, δ, α-Cr, γ″, and γ matrix) is reported in IN718 for the first time. The formation of α-Cr particles could be related to Cr rejection during the formation and growth of Cr-depleted δ phase.

Phase-field modeling of microstructure formation during rapid solidification in Inconel 718 superalloy

The prediction of the equilibrium and metastable phase/grain morphologies during selective laser melting (SLM) of Ni-base superalloys can be accomplished using phase-field modeling (PF). In this paper, a model for binary and multi-component systems is presented and validated via the steady-state growth problem by comparison to theoretical predictions of the Green-function calculations and the Kurz–Fisher model. By means of the PF simulations, a phenomenological dependency of the growth velocity on undercooling during the rapid solidification is evaluated. It is found that the growth velocity does not achieve steady-state values for real process and material parameters. The final microstructure predicted by the PF is in good agreement with the experimental observations. Additionally, the simulations provide concentration profiles, which show a pronounced segregation of solutes to the dendrite core. The simulated microstructures and concentration fields can be used as inputs for the simulation of the precipitation of secondary phases.

Electron Microscopy Investigation of Inconel 625 Weld Overlay on Boiler Steel

The investigation was focused on the microstructure characterization as well as changes in chemical composition and hardness of water wall tubing weld overlaid with Inconel 625. The analysis comprised studies in a light and electron microscopy scale that included the evaluation of weld overlays microstructure and microsegregation of alloying elements across the overlay and base metal interface. The particular attention was turned to the distribution of the main element content (Fe, Ni, Mo, Nb, Cr) in the base metal fusion zone as well as in the weld overlay itself. It was shown that the solidification process resulted in significant segregation in alloying elements giving rise to the substantial differences in chemical composition between dendrite cores and interdendritic spaces. It is believed that the microsegregation together with precipitation of secondary phases may contribute to the deterioration of corrosion resistance and overall mechanical properties of weld overlay including ductility and fracture toughness.

Response of the solidification microstructure of a high Nb containing TiAl alloy to an isothermal high-temperature heat treatment

Vacuum induction melting device combined with temperature control system was employed to investigate the effect of isothermal heat treatment (IHT) during solidification process on the microstructure evolution of a high Nb containing TiAl alloy. The microstructures of the alloy in as-cast condition and after IHT were studied. The results show that the as-cast microstructures exhibit a significant microstructural inhomogeneity with fine grains in dendrite core and coarse grains in interdendritic region. A new treatment approach by means of a short-term IHT within the β phase field during solidification process is proposed to obtain a uniform and refined microstructure. Compared with the as-cast alloy, IHT can reduce the tendency for crack. This phenomenon is attributed to the improvement of microstructural homogeneity by the elimination of peritectic α phase and the microstructure refinement by β → α transformation.

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.

Nanoindentation studies of the mechanical properties of the μ phase in a creep deformed Re containing nickel-based superalloy

Addition of Re in nickel-based superalloys results in an increase of the creep life. However, Re is also known to segregate to the dendrite core and to promote the formation of topologically closed packed (TCP) phases. In the present work, the local segregation of Re was studied in the heat treated and creep deformed state of a nickel-based superalloy. Tensile creep deformation at 1050°C resulted in the formation of TCP phases in the dendritic regions. Characterization using TEM confirmed the presence of μ phase that grows on {111} planes. Measurements with a nanoindenting AFM show that the μ phase is harder and shows less work hardening than both the γ and the γ′ phases. Furthermore, in the creep deformed state the hardness of the matrix phase is very similar in the dendrite core and in interdendritic areas, although Re is still enriched in the dendrite core. It is shown that Re is consumed in the dendrite core by the TCP phases.

Effects of Solutioning on the Dissolution and Coarsening of γ′ Precipitates in a Nickel-Based Superalloy

The dissolution and the coarsening of the γ′ precipitates in a nickel-based superalloy GTD-111 solutionized under various solution heat treatment conditions were investigated. The γ′ solvus temperature for the GTD-111 superalloy was about 1180.79 °C obtained by differential scanning calorimetry test. The dissolution and the coarsening of γ′ in the dendrite core were simultaneously observed, but the γ′ precipitates in the interdendritics only occurred to coarsen under the condition of 1125 °C/2 h. The γ′ dissolution, including dendrite core and interdendritics, gradually played a dominant role in the competition between the dissolution and the coarsening of γ′ during the solutioning with the increase of solution temperature and holding time, indicating that the elastic strain field of the alloy gradually reduced. The solution condition of 1225 °C/6 h or 1250 °C/2 h was the optimal solutioning schedule than the other schedules. For a lower solution temperature, the volume fraction of primary γ′ precipitates can faster reach its equilibrium value which is larger than that for a higher solution temperature. With the increase of holding time, the γ′ dissolution rate continuously decreased, and the dissolution activation energy of γ′ gradually increased.

The deformation and the recrystallization initiation in the dendrite core and interdendritic regions of a directionally solidified nickel-based superalloy

The deformation and the recrystallization initiation in the dendrite core and interdendritic regions were investigated by employing microhardness tester, optical microscope, scanning electron microscope and electron backscatter diffraction technique. It was found that different recrystallization nucleation mechanisms operated in the dendrite core and interdendritic regions. Recrystallization nucleation may take place by strain induced boundary migration in the dendrite cores, while particle stimulated nucleation occurs in the interdendritic regions. The deformation and the recrystallization initiation in the dendrite core and interdendritic regions were compared and discussed. The role of the carbides and eutectics in particle stimulated nucleation was also clarified.

Effect of Melting Temperature on the Microstructure Stability of a Ni-based Single Crystal Superalloy

The single crystal superalloy with [001] orientation were prepared by screw selecting method in the directionally solidified furnace. Three different melting temperature 1570°C, 1620°C and 1670°C were used in the melting process, respectively. The long term aging of the specimens after full heat treatment were performed at 1100°C for 800h. The effect of melting temperature on the microstructure stability of the alloy was investigated. The results showed that the primary dendrite arm spacing of alloy decreased with the increasing of melting temperature. The size of γ′ phase particles in these two regions decreases and cubic shape of those turns a little regular with increasing of melting temperature. The difference of γ′ size between the dendrite core and the interdendritic region reduces. The size of γ′ phase particles after heat treatment decreases with increasing of melting temperature. The extent of elements segregation decreases with increasing of melting temperature. The needle shaped TCP phases have already been observed in all of specimens and the amount of TCP phases decreases with increasing of melting temperature. Increase melting temperature of the single crystal superalloy with high refractory content can increase high temperature phase stability.

The effect of cast microstructure and crystallography on rafting, dislocation plasticity and creep anisotropy of single crystal Ni-base superalloys

In the present work we investigate three mechanical and microstructural aspects of high temperature and low stress creep of the single crystal superalloy LEK 94. First, we compare the tensile creep behavior of specimens loaded in precise [001] and [110] directions and show that tensile creep specimens with precise [110] directions show significantly lower minimum creep rates. However, small deviations from precise [110] orientations result in a significant increase of creep rate. Second, we use a novel SEM technique to measure dislocation densities. We show that after short periods of creep, dislocation densities in dendritic regions are always higher than in interdendritic regions. This finding is probably associated with wider γ-channels, higher concentrations of W and Re and higher misfit stresses in the γ-channels of dendrites. Finally, we show that internal stresses associated with solidification can drive complex rafting processes during high temperature exposure, which differ between dendrite cores and interdendritic regions.

One-step synthesis of monodisperse gold dendrite@polypyrrole core-shell nanoparticles and their enhanced catalytic durability

Colloidal noble metal-conducting polymer core–shell nanoparticles show promising applications in a wide diversity of areas, but it remains a great challenge to develop a convenient synthetic method allowing strict control over their size/dimension and facile modulation of their nanostructures. In this work, we report the synthesis of monodisperse gold dendrite@polypyrrole (AuD@PPy) core–shell nanoparticles with an eco-friendly one-step solution reaction by triggering oxidative polymerization of pyrrole with tetrachloroauric acid. As-prepared AuD@PPy nanoparticle is composed of a gold dendrite core coated by a thin PPy shell; the size and the thickness of the PPy shell could be adjusted by simply changing the pyrrole concentration. Benefiting from the protective effect of the PPy shell, the nanoparticles demonstrate enhanced catalytic durability toward the reductive reaction of methylene blue (MB) compared with the citrate-capped gold nanoparticles (AuNPs).

Microstructure and Microsegregation of an Inconel 625 Weld Overlay Produced on Steel Pipes by the Cold Metal Transfer Technique

Abstract The aim of this work was to investigate the development of microstructure and variations in chemical composition in commercial Inconel 625 coatings on a ferritic-pearlitic steel overlaid by the CMT method.The investigation showed that microsegregation occurring during the weld overlay solidification makes the dendrite cores to be richer in Ni, Fe and Cr and in the between dendrite arms in Mo and Nb. Niobium shows the strongest tendency to segregation during solidification; molybdenum tends to segregate less and chromium has the lowest tendency to segregation. Although Inconel 625 is a solid solution strengthened alloy, Nb and Mo-rich phases are formed in the between dendrite arms of weld overlays.

Microstructural evolution and solidification cracking susceptibility of Fe–18Mn–0.6C–xAl steel welds

In this work, the solidification behavior and solidification cracking of Fe–18Mn–0.6C–xAl (x = 1.49, 2.37, 4.79, 6.04 wt%) alloys were investigated. A longitudinal Varestraint test was applied to evaluate the solidification cracking tendency of Al-added high-Mn steel welds. In terms of total crack length and maximum crack length at 4 % applied strain, the solidification cracking susceptibility of high-Mn steel decreased with increasing Al content. Addition of Al suppressed the formation of low melting point eutectics (γ + (Fe,Mn)3C) along the grain boundaries during the final stage of solidification, which resulted in the decrease of solidification cracking tendency. The Al segregated extensively to the dendrite core opposite to Mn and C during solidification, which promoted the formation of δ ferrite. Further, the transition of the solidification sequence from the primary austenitic to primary ferritic mode provided a noticeable improvement in solidification cracking resistance in high-Mn steel welds similar to austenitic stainless steel welds.

Effect of thermal exposure on degradation of compressor blades in aero-jet engines

The environmental resistance of two single crystal nickel based superalloys, CMSX-4 and CMSX-10, used as blading materials in aero-jet engines has been studied at temperatures in the range of 800–1000°C, representative of that encountered in the high compression section of the engines. Mapping of residual stresses in ex-service blades using ruby luminescence provide an insight into the thermomechanical loading history of these blades. Comparison with laboratory test samples revealed higher compressive stresses in the blades due to extensive thermal cycling. Oxidation of CMSX-10 showed predominant NiO formation on the outer surface, whereas CMSX-4 displayed disparate oxidation regimes between the dendrite core and interdendritic regions.