Super-solvus Temperature Research Articles

Dynamic deformation mechanism for distinct flow behaviors in FGH4113A superalloy with high γ′ content during isothermal compression at sub-/near-/super-solvus temperatures

This study examines the flow behavior and deformation mechanism of a FGH4113A superalloy with high γ′ content for different strain rates (0.01–1 s−1) at sub-solvus (1030, 1080 °C), near-solvus (1130 °C), and sup-solvus temperatures (1180 °C). The results indicated that the alloy exhibits four different flow behaviors: rapid softening after reaching peak stress (at 1030–1080 °C), slow softening after reaching peak stress (at 1130–1180 °C/0.1–1 s−1), steady state flow (at 1130 °C/0.01 s−1), and continuous hardening (at 1180 °C/0.01 s−1). The strain rate sensitivity exponent decreases as the flow behavior transforms from dynamic softening to continuous hardening. Moreover, the dynamic recrystallization (DRX) fraction decreases with increasing strain rate at sub-solvus temperatures, where an inverse trend was observed at near- or sup-solvus temperatures. These observations are influenced by the interplay of continuous original grain boundary migration (COBM), DRX, and grain growth. Meanwhile, the primary γ′ phase has different roles under distinct deformation conditions. At sub-solvus temperature, the primary γ′ phase facilitates DRX, grain refinement, and coordinated plastic deformation. At near-solve temperature, the primary γ′ phase predominantly contributes to coordinated plastic deformation. At the sup-solvus temperature, the γ′ phase entirely dissolves, with the grains coarsened by COBM continuously undergoing compressed, exhibiting continuous hardening.

Microstructure evolution, hot deformation behaviour and processing map of Inconel X-750 superalloy in sub-solvus and super-solvus temperature ranges

Hot deformation behaviour of Inconel X-750 superalloy was investigated using the hot compression test in the temperature range of 950–1150 °C and strain rate of 0.001–1 s−1. After excluding the effect of friction and adiabatic heating from obtained flow curves, three constitutive equations and processing maps were developed. The hyperbolic sine with an activation energy of 461 kJ. mol−1 could confidently predict the flow stress pattern of the alloy with a correlation coefficient (R) of 0.99 for both sub and super-solvus temperatures. The processing map also accurately depicts two domains with high values of power efficiency η greater than 34%. Among these, the temperature range of 1090–1150 °C and strain rates between 0.01–0.1 s−1 are optimum and safe to obtain a fully dynamically recrystallized (DRXed) structure. The microstructure of the other domain with a high η value has not depicted a recrystallized structure and the high-power efficiency value was associated with dynamic precipitation of gamma prime (γ′) at temperatures within the range of 980–1100 °C and strain rates lower than 0.01 s−1. Finally, the correlation between processing maps, flow curves and microstructural observations was analyzed authoritatively. The MC precipitation takes deformation into trouble in the temperature range of 1050–1100 °C and strain rates higher than 0.01 s−1 due to Zener drag pining effect.

Effect of C content on microstructure and hot deformation behavior of a Ni-based superalloy

In this work, isothermal compression tests of a Ni-based superalloy were carried out at γ′ sub-solvus (1140 ℃) and γ′ super-solvus (1170 ℃) temperatures under a true strain 0.693. The effect of C contents (0.01 wt%, 0.04 wt% and 0.08 wt%) on the microstructural evolution and hot deformation behavior of the alloy was investigated. The formation of MC carbide is closely related to C content. Besides, the initial grain size decreased slightly with the increase of C content due to the effect MC carbides. During compression deformation at 1140 ℃, the peak stress increased with the increase of C content. When deformed at 1170 ℃, the 0.04 wt% C alloy has the lowest peak stress, because of the acceleration effect on dynamic recrystallization DRX by MC carbides. However, the 0.08 wt% C alloy has the highest peak stress, the dislocation density also increases with the number of MC carbides increase，which caused high working hardening rate and increased the peak stress in 0.08 wt% C alloy. The discontinuous dynamic recrystallization (DDRX) is the dominant mechanism during deformation at 1140 ℃, while the DDRX and continuous dynamic recrystallization (CDRX) occurred at the same time when deformed at 1170 ℃.

Evolution of Fe-Ti oxides within the Tiechagou carbonatite-related iron deposit: Insights from texture and in situ chemical compositions of magnetite and ilmenite

Ilmenite oxy-exsolution in magnetite recording different textures provides information on the Fe-Ti oxide cooling history. Magnetite-ilmenite intergrowths in carbonatites have the potential to offer important insights into the petrogenesis of the hosted ore deposits, whereas they have received limited attention. In this contribution, investigations have been conducted on different types of ilmenite oxy-exsolution hosted by magnetite within the Tiechagou carbonatite-associated iron deposit. Based on textural features, the magmatic magnetite-ilmenite intergrowths have been identified as thin lamellae (Type1), thick lamellae (Type2), fine-grained granular (Type3) and coarse-grained granular oxy-exsolutions (Type4). Major, minor and trace elements of the bulk oxide, magnetite and ilmenite were determined using EMPA and LA-ICP-MS. Bulk MgO and TiO2 generally decrease from Type1 to Type2 and Type3 to Type4, with Type1 oxides recording the highest. Bulk minor- and trace-element concentrations of all magmatic magnetite-ilmenite varieties exhibit similar trends in continental crust-normalized diagrams. Different magnetite and ilmenite groups are characterized by variable minor and trace element compositions, and have been successfully discriminated using Principal Component Analysis with the most discriminant elements of V, Cr, Ni, Co, Zn, Mn and Mg. Temperature and oxygen fugacity have been determined using WinMIgob, software developed by Yavuz (2021). Textural and chemical features of magnetite and ilmenite in the Tiechagou carbonatite combined with their re-equilibration temperature and oxygen fugacity highlight the evolution history, which is presented into two petrogenetic models. The first model records a significant decrease in temperature from 683 to 338 °C and oxygen fugacity (logfO2) from −14.58 to −34.27, which results in a progressive evolution of ilmenite oxy-exsolution from thin to thick lamellae starting at a supersolvus temperature. The second records a continuing evolution of the magnetite-ilmenite intergrowths from fine-grained to coarse-grained granular oxy-exsolutions through subsolvus oxidation, which is constrained by increase of the average oxygen fugacity (logfO2) from −28.38 to −26.93 at an intermediate temperature of 500–400 °C. Carbon in the oxidized form (e.g., CO32-) probably plays a significant role as the oxidizing agent especially when the oxygen fugacity is decreasing and/or low in carbonatite systems.

Deformation behavior of a new Ni-Co base superalloy GH4251 during hot compression

The deformation behavior of a new Ni-Co base superalloy GH4251 under hot compression tests within the deformation strain window of 0.36 to 1.2 was investigated in the temperature range of 1050 ∼ 1170 °C and strain rate range of 0.001 ∼ 1 s−1. Based on the analysis of true stress-strain curves, constitutive equations were established to describe the rheological behavior during hot compression. Microstructure evolution was investigated by transmission electron microscope (TEM), scanning electron microscope (SEM) and optical metallography (OM). The results show that flow behavior of GH4251 alloy is combinedly determined by the effect of work hardening and dynamic recrystallization (DRX). The deformation activation energies at strain of 0.36 to 1.2 are calculated to be 311 ∼ 536 kJ mol−1 in the super-solvus temperature region, and 796 ∼ 1064 kJ mol−1 in the sub-solvus temperature region. The recrystallization nucleation mechanism of GH4251 alloy is strain induced grain boundary migration (SIBGM). The occurrence and expansion of recrystallization are strongly promoted by high deformation temperatures and high strain rates, while the DRX grain size increases with elevated deformation temperature. When the deformation temperature is below 1090°C, the recrystallized grain can be extremely small (<17μm), which is rather independent on strain and strain rate. However, above 1110 °C the grain size at strain rate of 0.001s−1 is significantly larger than that of higher strain rates. The difference can be ascribed to the presence of γ′ phase, with which the development of dynamic recrystallization is postponed, while the growth of recrystallized grains is inhibited as well.

Evolution of γ′ Precipitation During the Early Stages of Industrial Forging of a Nickel-Based Superalloy

A sample of the Ni-based superalloy AD730 was heat treated at a supersolvus temperature (1160 °C) then slowly cooled through the solvus temperature (1110 °C) at 10 °C/hr down to 1080 °C, i.e., a rate representative of the cooling conditions of an industrial-scale billet undergoing controlled cooling. The γ′ precipitate distribution which forms during this cooling was investigated, and a mix of continuous and discontinuous precipitation was found. The discontinuous γ′ precipitates were imaged using 3D tomography, and were shown to present very different sizes, morphologies, and aspect ratios when observed in different 2D imaging planes. The interaction between different populations of γ′ precipitate and recrystallization was investigated, and it was found that the discontinuous precipitates present more of a barrier to recrystallization than the continuous ones. This has been explained based on the different inter-precipitate spacings observed for the two populations. In addition to these γ′ precipitates which form during slow cooling, a fine and dense distribution of approximately spherical γ′ precipitates was found to form dynamically, during subsequent subsolvus forging, within unrecrystallized grains.

Effects of the solution and first aging treatment applied to as-built and post-HIP CM247 produced via laser powder bed fusion (LPBF)

In this work CM247LC, a low weldable Ni-Based alloy, was produced using selective laser melting (SLM). Despite the initial process parameter optimization, the low defect volume fraction was still uncompliant with manufacturing standards. This condition is principally caused by the high γ’ volume fraction which strongly affects the alloy weldability. Nonetheless, a crack free condition was eventually achieved applying a γ’-sub-solvus Hot Isostatic Pressing Cycle (HIP) which lowered the defects fraction down to 0.04%. The HIP cycle also demonstrated to play an important role in the stabilization of the microstructure, considerably limiting the carbides coarsening during the following heat treatment. Apart from the effectiveness of the healing process brought by HIP, the material microstructure still needs an optimization process which will be described along this paper. In fact, the Initial microstructure obtained after the printing process (the as-built condition) as well as the one obtained after HIP (post-HIP) won’t meet the desired requirements. Namely, the dendritic and γ’ free microstructure of the as-built material or the one with coarse and disordered particles obtained right after HIP, still need a tailored homogenization process. This paper will show how the combined effect of the solution and first aging treatment will profoundly alter the γ’ precipitation. More specifically, here, a new heat treatment recipe was developed to promote the precipitation of ordered cuboidal primary γ’ so as to improve creep and high temperature fatigue resistance. Moreover, the use of a γ’ super-solvus temperature allowed to achieve a γ’ volume fraction as high as 73% reducing its average size to 520 nm. At the same time, such heat treatment caused a profound alteration of the crystalline structures of the material promoting a general grain coarsening and the formation of equiaxial grains.

Influence of Cooling Scenarios on the Evolution of Microstructures in Nickel-Based Single Crystal Superalloys

The reprecipitation and evolution of γ’ precipitates during various cooling approaches from supersolvus temperature are studied experimentally and via phase field simulation in nickel-based single crystal superalloys. The focus of this paper is to explore the influence of cooling methods on the evolution of the morphology and the distribution of γ’ precipitates. It is demonstrated that small and uniform spherical shape γ’ particles formed with air cooling method. When the average cooling rate decreases, the particle number decreases while the average matrix and precipitate channel widths increase. The shape of γ’ precipitates which changed from spherical to cubic and irregular characteristics due to the elastic interaction and elements diffusion are observed with the decrease of the average cooling rate. The phase field simulation results are in good agreement with the experimental results in this paper. The research is a benefit for the study of the rejuvenation heat treatment in re-service nickel-based superalloys.

Microstructural origin and control mechanism of the mixed grain structure in Ni-based superalloys

Mixed grain structure, as an incomplete recrystallization, should be homogenized and refined by subsequent processing to improve the mechanical properties of the aeroengine components. The microstructural origin and control mechanism is quite necessary for the microstructure homogenizing and grain refining through thermo-mechanical processing. In present research, microstructural origin and control mechanism of the mixed grain structure have been conducted deeply. With the consideration of temperature drop in practical industrial forging, the optimum processing parameters can be determined as 1110–1140 °C/1.0 s−1 to obtain homogeneous equiaxed grains. The microstructure evolution corresponding to the identified optimal parameters, viz., deformation temperature and strain rate, were systematically investigated. Higher dynamic recrystallization (DRX) fraction and coarser DRX grains can be observed at super-solvus temperature deformation (1140 °C). In addition, sub-solvus deformation (1110 °C) shows random orientated distribution while near< 001 > compression texture can be detected in super-solvus deformation. Particular attention was paid to the DRX mechanisms during hot deformation. Discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) occur simultaneously, and the synergistic effect of them contributes to microstructure control. Moreover, DDRX characterized by grain boundary bulging dominants the microstructure control. Progressive subgrain rotation labeled as CDRX can be regarded as an important assistant mechanism. The findings would provide theoretical support for microstructure control of Ni-based superalloys with mixed grain structure, which is of vital importance for components to obtain excellent performance.

Hot compression behaviors and deformation mechanisms of a Ni–Co-based superalloy with columnar grains

In this work, hot compression behaviors and deformation mechanisms of a Ni–Co-based superalloy prepared using directional solidification (DS) were investigated by carrying out the isothermal compression tests within the temperature range of 1080–1190 °C and the strain rate range of 0.001–1 s-1 under the true strain of 0.693. The effects of temperature and strain rate on dynamic recrystallization (DRX) were analyzed, and constitutive equations were established. The activation energies for γ+γʹ dual-phase and the γ single-phase regions of Ni–Co-based superalloy with columnar grains were 1190 kJ mol-1 and 415 kJ mol-1, respectively. The results showed that the dominant deformation mechanisms were closely related to the strain rate and temperature. DRX was accelerated at low strain rate conditions, while the DRX process was apparently sluggish at strain rates higher than 1 s-1. The undissolved γʹ phase interacted with dislocations and accelerated the DRX of the matrix during γʹ sub-solvus temperature (1080–1120 °C) deformation. Continuous dynamic recrystallization (CDRX) nucleation proved to play a dominant role. During γʹ super-solvus temperature (1150–1190 °C) deformation, the dominant dynamic softening mechanism of the alloy is discontinuous dynamic recrystallization (DDRX). Local migration of grain boundaries (GBs) causes an increase of misorientation within the deformed grains and induced the formation of subgrain boundaries near the GBs, which subsequently evolved into high angle grain boundaries (HAGBs) and participated in DDRX. Furthermore, during high-temperature deformation in Ni–Co-based superalloys, the relatively rare microtwins (MTs) were detected under compression at 1100 °C/1 s-1.

Hot deformation behavior and flow stress modeling of a novel CoNi-based wrought superalloy

The thermal deformation behavior and flow stress modeling of a novel CoNi-based wrought superalloy after forging was investigated at γ' sub-solvus temperature (1050 °C and 1075 °C) and γ' super-solvus temperature (1100 °C and 1125 °C) with strain rate range from 0.001 s−1 to 0.1 s−1 under a 50% strain. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) techniques were used to characterize the microstructures and explain the flow behavior. The results show that the flow stress increases with decreasing temperature and increasing strain rate. Dynamic recrystallization (DRX) occurs under all deformation conditions. When the alloy was deformed sub-solvusly at 1075 °C/0.01 s−1 and 0.001 s−1 conditions, the γ' phase dissolved resulting in substantial grain growth. The constitutive equations were established via the flow stress data with the respect to the γ' phase dissolution. The activation energies for the γ + γ' dual-phase region and the γ single-phase region of the studied alloy are about 650 and 390 kJ/mol, respectively, which is comparable with traditional Ni-based superalloys.

Irregular growth of the γ′ phase in a Ni-based superalloys under slow cooling rate

The effects of cooling rate on the evolution of γ′ phase were investigated by homogenization treatments performed at the γ′ super-solvus temperature (1160 ℃) with the soaking time of 30 h, following cooled by furnace cooling. Irregular growth and morphological instability of the γ′ phase under slow cooling condition was identified. The shape of γ′ phase was mixture of cubic, octets and dendrites in this process. The growth of the γ′ phases during cooling was consistent with a diffusion-controlled mechanism. Therefore, chemical composition of γ′ phase and γ matrix was analyses, it is found that the γ′ phase was enriched in Al, Ti, Ni and poor in Co, Cr, Fe. Based on the experimental results, the irregular growth process of γ′ phases was discussed, and supersaturation and sufficient diffusivity are the dominated mechanisms of this process.

Effect of heat treatment on the microstructure characteristics and microhardness of a novel γ′ nickel-based superalloy by laser powder bed fusion

The fabrication of gamma prime (γ′) strengthened nickel-based superalloys by additive manufacturing (AM) techniques is of huge interest from the industrial and research community owing to their excellent high-temperature properties. The effect of post-AM-processing heat treatment on the microstructural characteristics and microhardness response of a laser powder bed fused (LPBF) γ′ strengthened nickel-based superalloy, MAD542, is systematically investigated. Post-processing heat treatment shows the significant importance of tailoring the γ′ morphology. With insufficient solutioning duration time, coarse γ′ formed in the interdendritic region heterogeneously, due to the lack of chemical composition homogenization. The cooling rate from the super-solvus solutioning plays an important role in controlling the γ′ size and morphology. Spherical γ′ is formed during the air cooling while irregularly shaped γ′ formed during the furnace cooling. The following aging heat treatment further tunes the γ′ morphology and γ channel width. After two-step aging, cuboidal γ′ is developed in the air-cooled sample, while in contrast, bi-modally distributed γ′ is developed in the furnace cooled sample with fine spherical γ′ embedded in the wide γ channel between coarse irregular shaped secondary γ′. More than 90% of the grains recrystallized during solutioning treatment at the super-solvus temperature for 30 min. The rapid recrystallization kinetics are attributed to the formation of annealing twins which significantly reduced the stored energy. Microhardness responses from different heat-treated conditions were examined.

Creep of single-crystals of nickel-base γ-alloy at temperatures between 1150 °C and 1288 °C

A γ-analogue of the superalloy CMSX-4 that does not contain the strengthening γ′-phase and only consists of the γ-solid solution of nickel has been designed, solidified as single-crystals of different orientations, and tested under creep conditions in the temperature range between 1150 and 1288 °C. The tests have revealed a very high creep anisotropy of this alloy, as was previously found for CMSX-4 at supersolvus temperature of 1288 °C. This creep anisotropy could be explained by the dominance of 011111 octahedral slip. Furthermore, the analysis of the creep data has yielded a high value of the creep activation energy, Qc≈442 kJ/mol, which correlates with the high activation energy of Re diffusion in Ni. This supports the hypothesis that dislocation motion in the γ-matrix of Re-containing superalloys is controlled by the diffusion of the Re atoms segregating at the dislocation core. The Norton stress exponent n is close to 5, which is a typical value for pure metals and their alloys. The absence of γ′-reprecipitation after high-temperature creep tests facilitates microstructural investigations. It has been shown by EBSD that creep deformation results in an increasing misorientation of the existing low angle boundaries. In addition, according to TEM, new low angle boundaries appear due to reactions of the a/2011 mobile dislocations and knitting of new networks.

Effects of hot processes on microstructure evolution and tensile properties of FGH4096 Ni-based superalloy processed by Laser Powder Bed Fusion

Hot Isostatic Pressing (HIP) is one of the most important manufacturing processes to improve the performance of powder metallurgy nickel-based superalloys produced by Laser Powder Bed Fusion (LPBF), which can effectively reduce the pores and cracks in the LPBF alloys. However, there are limited studies on the effects of tuning HIP parameters on the LPBF alloys microstructure and mechanical performance. In this paper, we systematically studied the effects of hot processes on the microstructural evolution and tensile properties of an LPBF prepared typical second generation of powder metallurgy (PM) nickel-based superalloy which was designated as FGH4096. The as-deposited alloy was HIP treated at sub-solvus, solvus, and super-solvus temperature, respectively, followed by an optimized heat treatment (HT). The effects of HIP temperature on the microstructures and mechanical properties of the LPBF alloys were studied by Optical Microscope (OM), Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM) and Electron Backscattered Diffraction (EBSD). The results indicated that both HIP and HT processes led to recovery and recrystallization in the as-deposited alloy. During HIP, the increase of HIP temperature gradually eliminated the dendritic and equiaxed structures in the alloy while refined the grains along the long axis simultaneously. However, the columnar crystal morphology in the vertical section, together with straight grain boundaries, was still maintained during HIP. In all the HIPed alloys, the secondary γ′ precipitates were sparsely distributed, and due to the existence of continuous pressure, some grains with large strains remained. After HIP + HT, the dendritic and equiaxed structures basically disappeared. The grains with more curved grain boundaries tended to be equiaxed and irregularly shaped. Compared with other state alloys, the LPBF + HIP (solvus) + HT alloy has a more uniform and dense distribution of secondary γ′ precipitates, together with a chain distribution of block primary γ′ precipitates at the grain boundaries, which contributes mostly to the highest room and high-temperature tensile strengths. Moreover, the LPBF + HIP (solvus) + HT alloy has the lowest internal strain among the treated alloys, leading to the highest elongation.

Microstructural evolution of a Ni-Co based superalloy during hot compression at γ′ sub-/super-solvus temperatures

The effects of strain rate on the microstructural evolution and deformation mechanism of a Ni-Co based superalloy were investigated by isothermal compression tests performed at γ′ sub-solvus (1090 °C) and γ′ super-solvus temperatures (1150 °C) with a wide strain rate range from 0.001 to 10 s-1 under a true strain of 0.693. Electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and transmission electron microscope (TEM) techniques were used to characterize the microstructures. The results revealed that the dynamic recrystallization (DRX) volume fraction increased and stored energy of the γ matrix grains decreased with increasing the strain rate during γ′ sub-solvus temperature deformation, while the opposite phenomena were observed during γ′ super-solvus temperature deformation. The comprehensive effect of initial grain size, primary γ′ phase, twins and adiabatic temperature rise led to these results. The primary γ′ particles undergone the deformation behavior within itself and obviously accelerated the DRX of the matrix. The microstructural evolution proved that discontinuous dynamic recrystallization (DDRX) was the dominant mechanism during the hot deformation carried out at both γ′ sub-solvus and γ′ super-solvus temperatures. Primary γ′ particles obviously accelerated the nucleation step and retarded the growth step of DDRX during γ′ sub-solvus temperature deformation. Besides, the acceleration effect of primary γ′ particles on DDRX increased with the increase of strain rate. Continuous dynamic recrystallization (CDRX) was confirmed to be an assistant mechanism during γ′ super-solvus temperature deformation and was promoted with the increase of strain rate.

Characterization of Hot Deformation Behavior and Dislocation Structure Evolution of an Advanced Nickel-Based Superalloy

The hot deformation behavior of an advanced nickel-based Haynes282 superalloy was systematically investigated employing isothermal compression tests in the sub-solvus and super-solvus temperature with various strain rates. The influence of deformation temperature and strain rate on the microstructure was studied by transmission electron microscope. The results reveal that the interaction between work hardening and dynamic softening did not reach equilibrium under lower deformation temperature and higher strain rate. The active energy of alloy is around 537.12 kJ/mol and its hot deformation constitutive relationship equation was expressed. According to the processing map and microstructure observations, two unsafe flow instability domains should be avoided. The optimum hot processing condition for homogeneous and fine dynamic recrystallization grains are obtained. TEM micrograph observations indicated that deformation temperature and strain rate affected recrystallization by affecting the evolution of dislocation substructures within the alloy. The nucleation and growth of DRX grains can be promoted by the relatively high deformation temperature and low strain rate. The main mechanism of dynamic recrystallization nucleation preferred to discontinuous dynamic recrystallization and the typical feature of discontinuous dynamic recrystallization showed grain boundary migration nucleation. The findings improve the understanding of hot deformation behavior and dislocation substructures evolution of the superalloy, which benefits the accurate control of microstructures of nickel-based superalloys, and tailors the properties of final components used in the land-based gas turbine.

The Mechanism of Grain Boundary Serration and Fan-Type Structure Formation in Ni-Based Superalloys

A model of discontinuous precipitation of γ′ phase at grain boundaries (GBs) in polycrystalline Ni-based superalloys during continuous cooling from supersolvus temperature was developed. The model calculates the size and γ′ phase fraction in GB precipitates as a function of cooling rate and GB mobility. The model is based on the classical mechanism of diffusion-controlled precipitation. It considers fast diffusion of γ′-forming elements along the GB and the motion of the GBs under driving force induced by alloy decomposition. The model predicts either discrete GB gamma-prime particles and GB serration or the growth of fan-type structures depending on the cooling rate and GB mobility. We conclude from the model predictions that the GB mobility is the key factor controlling the type and morphology of GB precipitates. High GB mobility results in formation of fan-type structures, while the lower GB mobility leads to GB serration. The predicted dependence of the size of GB precipitates on the cooling rate is in good agreement with the experimental observations.

Microstructure evolution and underlying mechanisms during the hot deformation of 718Plus superalloy

718Plus superalloy was investigated to obtain a detailed and fundamental understanding of the microstructure evolution and the underlying mechanisms. Isothermal compression tests were conducted, and different strains were utilized to trace the whole deformation process at the selected deformation conditions. The results showed that no yield drop phenomenon (YDP) occurred at all deformation conditions involved in this research except a very slight one at 1020 °C with the strain rate of 0.1 s−1 The heterogeneity of initial microstructure was responsible for the absence or weakening of YDP. Deformation conditions and the Taylor factor differences between adjacent grains were found both strongly affected the distribution of dislocation density and local misorientation, as well as the formation of low angle grain boundaries (LAGBs), which finally influenced the dynamic recrystallization (DRX) behaviors and the underlying mechanisms. Moreover, discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) behaviors as well as the underlying mechanisms were discussed in detail. Subgrain rotation would also be observed during the DDRX process, which may lead to a misjudgment of CDRX if only focused on misorientation gradient. Hence, based on the morphology examination, misorientation gradient and misorientation angle evolution, progressive subgrain rotation labeled as CDRX was confirmed to be an important assistant nucleation mechanism during the deformation of the studied 718Plus superalloy at both sub-solvus and super-solvus temperatures.

Effect of phosphorus content and grain size on the long-term phase stability of Ni-base superalloys

The effect of grain size on the phase stability of two nominally identical high refractory content powder-processed Ni-base superalloy with varying levels of P additions (0.013 wt% for alloy P1 and 0.026 wt% for alloy P3) was studied. Solutioning of the alloys at either sub-solvus or super-solvus temperatures were used to vary the grain size of the starting microstructure prior to long-term thermal exposures at 800 °C for up to 1000 h. EBSD analyses revealed that the sub-solvus solutioned samples had an average grain size of 10 μm and possessed a high angle grain boundary length density that was approximately 2.5 times greater than that found in the super-solvus solutioned samples with an average grain size of 16 μm. Differences in both the initial grain boundary character distribution and P content present in these alloys resulted in varying behavior with respect to the grain boundary precipitation of Laves and sigma phase during aging. Since P additions to Ni-base superalloys are known to promote the formation of Laves phases and segregate predominately to grain boundaries, sub-solvus heat treated samples exhibited a lower susceptibility of Laves phase precipitation compared with super-solvus heat treated samples for both P1 and P3 alloys. Sub-solvus heat treated P1 samples were found to be resistant to the formation of Laves phase after 1000 h exposure while the coarser grained super-solvus heat treated P3 samples exhibited extensive formation of intertwined Sigma-Laves along the grain boundaries just after 100 h exposure. The improvement in phase stability could be attributed to the reduced concentration of P along the grain boundary as increasing the length density of random high-angle grain boundary resulted in a more dilute spatial distribution of segregated P atoms.