Microstructural Instability of an Experimental Nickel-Based Single-Crystal Superalloy

Microstructural instability with the precipitation of topologically close-packed (TCP) phases of an experimental nickel-based single-crystal superalloy has been investigated. A significant amount of σ phases are distinguished in the interdendritic region of the as-cast samples after thermal exposure at 900 °C for 1000 h. The σ phases are preferentially precipitated at the periphery of coarse γ/γ′ eutectic, and their morphological evolution from needles to granules is observed. Microstructural analysis suggests that the local segregation of Cr and Ti at the periphery of coarse γ/γ′ eutectic accounts for the formation of σ phases in the as-cast samples. After heat treatment with low solution temperature and short holding time, the dendritic segregation of alloying elements (i.e., W, Re, Ti and Ta) and the volume fraction of γ′ phase in the interdendritic region are similar to that of the as-cast samples. However, no TCP phases are present in the interdendritic region of the heat-treated samples after thermal exposure, which is primarily ascribed to the elimination of local segregation of Cr and Ti near the coarse γ/γ′ eutectic. Moreover, small quantities of μ phases are precipitated in the secondary dendrite arm near the interdendritic region after thermal exposure, due to the increased volume fraction of γ′ phase and the concomitant enrichment of W and Re in the γ matrix.     

Re-recognition of the Effects of Phosphorus and Boron on the γ’’ and γ’ Phases in IN718 Alloy

The effects of P and B on the matrix strength and precipitations of γ’ and γ’’ phases in the grain interior were re-recognized in this study. The combination addition of P and B markedly accelerated the precipitations of γ’ and γ’’ phases and strengthened the matrix of IN718 alloy when air-cooled from high temperature, while made no difference when waterquenched from high temperature. The effect of single addition of P on the precipitations of γ’’ and γ’ phases was the same with that of the combination addition of P and B, while the single addition of B had no effect on the precipitations of the two phases. Therefore, it was P rather than B which accelerated the precipitations of γ’ and γ’’ phases. P could take part in the precipitations of γ’ and γ’’ phases, which was revealed by electrochemical extraction and quantitative analysis of chemical composition. It also revealed that P atoms were dissolved in the c matrix to a relatively high degree at the temperature that γ’ and γ’’ phases began to precipitate, and consequently the precipitations of γ’ and γ’’ phases were accelerated. The first-principle calculation indicated that P decreased the formation enthalpies of γ’ and γ’’ phases when it occupied the Ni lattice sites in the two phases, which explained the effect of P on the γ’ and γ’’ phases.     

Element Segregation and Solidification Behavior of a Nb,Ti, Al Co-Strengthened Superalloy ЭК151

The as-cast microstructure, element segregation and solidification behavior of a multi-alloyed superalloy ЭК151 have been investigated. The results show that the severe element segregation leads to the complicated precipitations at the inter-dendritic region, including η-Ni_3(Ti, Nb), eutectic(γ + γ') and Laves, which shows the characteristics of both Ti, Al-strengthened and Nb-strengthened alloys. Differential thermal analysis, heating and quenching tests reveal the solidification sequence as follows: Liquids →γ matrix →(Nb, Ti)C →η-Ni 3(Ti, Nb) →eutectic( γ+γ') → Laves. The melting points are between 1250 and 1260 °C for(Nb, Ti)C, between 1200 and 1210 °C for η phase, between 1180 and 1190 °C for eutectic(γ+γ') and Laves. γ' initially precipitates from matrix at 1150 °C and achieves the maximum precipitation at 1130 °C. According to the microstructure evolution captured during solidification and composition analysis by an energy dispersive spectrometer and electron probe microanalyzer,(Nb, Ti)/Al ratio is put forward to explain the formation of η-Ni_3(Ti, Nb) and eutectic( γ+γ'). The solidification of γ matrix increased the Nb, Ti concentration in the residual liquids, so the high(Nb, Ti)/Al ratio would result in the formation of η-Ni_3(Ti, Nb); the precipitation of the phase consumed Nb and Ti and decreased the(Nb, Ti)/Al ratio in the liquid, which led to the precipitation of eutectic(γ + γ'). Laves formed by the sides of η-Ni_3(Ti, Nb) and in front of the eutectic( γ + γ') after Al, Ti were further depleted by the two phases and Cr, Co, Mo were rejected to liquids.     

Effect of Co on Microstructure and Stress Rupture Properties of K4750 Alloy

The effects of substituting Co for Fe on the microstructure and stress rupture properties of K4750 alloy were studied.The microstructure of the alloy without Co(K4750 alloy) and the alloy containing Co(K4750-Co alloy) were analyzed.Substitution of Co for Fe inhibited the decomposition of MC carbide and the precipitation of η phase during long-term aging treatment.In K4750-Co alloy,the morphology of MC carbide at the grain boundary(GB) remained dispersed blocky shape and no η phase was observed after aging at 750℃for 3000 h.However,in K4750 alloy,almost all the MC carbides at GBs broke down into granular M_(23)C_6 carbide and needle-like η phase.The addition of cobalt could delay the decomposition of MC carbides,which accordingly restricted the elemental supply for the formation of η phase.The stress rupture tests were conducted on two alloys at 750℃/430 MPa.When Co is substituted for Fe in K4750 alloy,the stress rupture life increased from 164.10 to 264.67 h after standard heat treatment.This was mainly attributed to increased concentration of Al,Ti and Nb in γ' phase in K4750-Co alloy,which further enhanced the strengthening effect of γ' phase.After aging at 750℃for 3000 h,substitution of Co for Fe can also cause the stress rupture life at 750℃/430 MPa to increase from 48.72 to 208.18 h.The reason was mainly because MC carbide degradation and η phase precipitation in K4750 alloy,which promoted the initiation and propagation of micro-crack during stress rupture testing.     

热处理对Inconel718合金组织及力学性能的影响

针对Inconel 718合金的不同用途,分析研究了4种常用热处理工艺对Inconel 718合金组织和力学性能的影响。结果表明:固溶温度超过1020℃时,奥氏体晶粒显著长大。合金中主要析出相有MC、δ、γ’和γ″相。δ相沿晶界分布,1025℃固溶时呈颗粒状少量析出;950℃固溶时呈块状大量析出;直接时效时呈网状不连续分布。同时,δ相对合金的晶粒度影响较大,且其析出数量和形态决定了合金的韧塑性,γ″、γ’相的析出量和尺寸与晶粒尺寸决定了合金的强度变化。     

Role of Ru on the Microstructural Evolution During Long-Term Aging of Ni-Based Single Crystal Superalloys

Two experimental alloys containing different contents of Ru were investigated to study the effect of Ru on the microstructural evolution during long-term thermal exposure. The increase in Ru promoted the formation of cubical, tiny, and even γ′ phase after full heat treatment. Moreover, the samples after full heat treatment were exposed at 1100 °C for different time. Based on the classical model by Lifshitz, Slyozov, and Wagner, the coarsening of γ′ phase of the alloy containing 2.5 and 3.5 wt.% Ru during the long-term aging was controlled by the interface reaction and diffusion, respectively. The γ/γ′ lattice misfit was more negative with the increment of Ru addition, which induced the formation of stable rafted γ′ phase in the alloy containing 3.5 wt.% Ru at the initiation of long-term aging. Besides, the increase in Ru reduced the diffusion coefficient, which could restrain the γ′ phase coarsening. The lower γ/γ′ lattice misfit of the alloy containing 2.5 wt.% Ru promoted the interface reaction, which induced the rapid coarsening of γ′ phase. Therefore, the increase in Ru improved the microstructural stability of the alloys. On the other hand, the raise of Ru induced “reverse partitioning” behavior, which was effective in suppressing the emergence of the topologically close-packed phase (TCP phase). The TCP phase occasionally occurred in the alloy containing 2.5 wt.% Ru, which was attributed to the high temperature and the supersaturation of the γ matrix. Moreover, the TCP phase was determined as μ phase, which had a high concentration of Co, Re, Mo, and W. A sketch map describing the evolution of the TCP phase was also constructed.     

Precipitation Behavior of σ Phase in Ultra-Supercritical Boiler Applied HR3C Heat-Resistant Steel

The precipitation kinetics of σ phase in commercial HR3C heat-resistant steel during aging at 650–800 °C was studied in the paper. Through morphology, composition and structural analyzing on the second phase in the HR3C steel, it was confi rmed that the precipitations after aging were mainly NbCrN, M_23C_6 and FeCr type σ phase. The time-dependent mass change of the three precipitated phases showed that the linearly increased σ phase after aging at 750 °C–2000 h was transformed from NbCrN phase or M_23C_6 phase. According to the calculation on the volume fraction of electrolytically dyed σ phase, the time–temperature transformation(TTT) curve for σ phase at 1 vol% in two kinds of commercial HR3C steel(different in grain size) was obtained and analyzed. The nose of the TTT curve was located at around 750 °C for the two kinds of HR3C steel, and the larger grain size HR3C steel displayed a inhibit effect on the precipitation of σ phase. The impact energy of the HR3C steel after aging at 700 °C decreased obviously with the fracture mechanism changing from ductile fracture to intergranular brittle fracture, which was considered to be related to the density of σ-brittle phase after aging.     

Effects of Ti and Cu on the Microstructure Evolution of AlCoCrFeNi High-Entropy Alloy During Heat Treatment

The microstructure evolution of AlCoCrFeNiTi_(0.5) alloy and AlCoCrFeNiCu alloy during heat treatment was systematically studied,to reveal the influence rules of chemical activity of adding element on the microstructure evolution of AlCoCrFeNi system.Owing to the negative mixing enthalpy with the constituent elements,Ti element was mainly dissolved in the Al-Nirich phases,and aggravated the lattice distortion of B2 phase.The structure variation of BCC phase by adding Ti inhibited the formation of FCC phase and enhanced the precipitation of σ phase during heat treatment.Owing to the positive mixing enthalpy with constituent elements,Cu element tended to be repelled to the ID region and formed metastable Cu-rich FCC1 phase which would transform into Cu-Al-Ni-rich FCC2 phase with increasing temperature.The addition of Cu inhibited the precipitation of σ phase during heat treatment.Adding Ti maintained the stable dendritic morphology,while adding Cu reduced the thermal stability of microstructure.Two dramatic morphology changes occurred at 1000~℃ and 1100~℃ in the AlCoCrFeNiCu alloy.The lattice distortion of phase in AlCoCrFeNiTi_(0.5) alloy was aggravated with increasing temperature up to 800~℃,then relaxed together with the dissolution of σ phase when temperature was above 900~℃.The variation in lattice distortion dominated the hardness of AlCoCrFeNiTi_(0.5) alloy.With increasing heating temperature,the increasing volume fraction of region with FCC structure due to the transformation between FCC phases,and the pronounced coarsening in microstructure due to the reduced thermal stability,resulted in the mainly decreasing trend in the hardness of AlCoCrFeNiCu alloy.     

Effects of PWHT on Microstructure and Mechanical Properties of Weld Metals of Ni-Based Superalloy 617 and 263 for Hyper-Supercritical Power Plants

It is well known that solid solution-strengthened alloy 617 and γ' precipitation-strengthened alloy 263 have excellent mechanical properties and corrosion resistance at high temperatures.Hyper-supercritical power plants work at temperatures above 700 ℃,and these superalloys are considered candidate materials for steam turbines components of these power plants.In this study,gas tungsten arc weldability of these superalloys was evaluated,and the effects of postweld heat treatment(PWHT) on the microstructural characteristics and the mechanical properties of their weld metals were investigated.Scanning transmission electron microscopy,energy-dispersive spectroscopy and electron probe microanalysis were utilized for the investigation.The experimental results confirmed that these weld metals had different characteristics in microstructure and mechanical properties.PWHT resulted in the precipitation of intergranular carbides,γ' particles and an increase in tensile strength of these superalloy weld metals.Furthermore,fine γ' particles,which were not detected in the as-welded metal of alloy 263,were precipitated after PWHT and those particles were the reason for the drastic increase in tensile strength.     

Tensile Properties of Cast Alloy IN625 in Relation to d Phase Precipitation

The relationship between the tensile properties and δ (delta) phase precipitation in cast alloy IN625 was investigated in this paper. Although the influences of δ phase on the mechanical properties have been pointed out in our previous work, the relationship is still not directly determined due to the coexistence of γ″ (gamma double prime) and δ precipitates. In order to exclude the effect from γ″ phase, various fractions of δ precipitates with few γ″ precipitates were obtained by a set of experimental alloys aging at 750 °C, and tensile tests were conducted in parallel. The results showed that both yield strength and ultimate tensile strength increased nearly in linear with increasing δ phase fraction, while the elongation was relatively and limitedly affected by δ precipitation when the area fraction of δ phase was above 10%.     

Evolutions of Microstructure and Mechanical Properties in Mg-5Li-1Zn-0.5Ag-0.5Zr-xGd Alloy

The evolution of the microstructure and mechanical properties of alloy system with nominally composition Mg-5Li-1Zn-0.5Ag-0.5Zr-xGd (x = 0, 1.2, 2.4, 3.6, 4.8, 6) is evaluated based on computational phase diagram and corresponding experimental studies. The results show that grains are significantly refined with the increase of Gd content. The main phases of as-cast alloys are α-Mg, β-Li, AgLi₂Mg, and Mg₃Gd. With the increase of Gd content, the amounts of Mg₃Gd phase and β-Li phase have been increased. When the Gd content exceeds 3.6 wt%, Mg₃Gd phase precipitates in a form of the network at the grain boundaries. The precipitation of β-Li can be attributed to the competitive dissolution of Zn, Gd, and Li in Mg. Meanwhile, γ″ is formed after the addition of Gd, which grows and transforms into γ′ with the increase of Gd content. In solidification process, stacking faults are formed by solid transformation of partial α-Mg and Mg₃Gd. Eventually, with the synergistic effect of Mg₃Gd, β-Li, and γ″ (or γ′), as the Gd content increasing, the tensile strength of the alloy first increases, then decreases, and the elongation decreases. When the content of Gd is 4.8 wt%, the ultimate tensile strength and yield strength reach the maximum values of 227 MPa and 139 MPa, and the elongation is 18.1%, respectively.     

Effect of Different Ageing Processes on Microstructure and Mechanical Properties of Cast Al–3Li–2Cu–0.2Zr Alloy

The effect of different ageing processes on microstructure and mechanical properties of cast Al–3 Li–2 Cu–0.2 Zr alloy was investigated using transmission electron microscopy and tensile tests. The results showed that the mean size of δ′-Al_3 Li particles and the number density of Cu-rich precipitates both increased with increasing ageing temperature from 150 to 190 °C for 24 h, resulting in increasingly high strength. In contrast, the ductility deteriorated with the increase in ageing temperature as a result of the intensified planar slip. The duplex low-to-high ageing treatment(120 °C for 6 h followed by 160 °C for 24 h) was shown to be beneficial to the ductility compared with the corresponding single-stage ageing treatment(160 °C for 24 h). The reduced slip length induced by the precipitation of θ′-Al 2 Cu phases was found to be mainly responsible for this ductility improvement.     

Effect of δ Phase on Microstructure and Hardness of Heat-Affected Zone in TIG-Welded GH4169 Superalloy

The GH4169 superalloy with different content of δ-Ni₃Nb phase was welded by tungsten inert gas welding. A detailed study of microstructure and hardness of heat-affected zone (HAZ) was performed in both as-welded and aged state. The results show that the precipitation of δ phase, especially the intergranular δ phase, can lead to the enrichment of Nb and Mo elements, which promote the formation of γ/Laves eutectic constituent at grain boundaries in HAZ. In as-welded state, the hardness decreases first and then increases (exhibiting a “V” shape) with distance away from fusion line in HAZ, which is governed by grain size. After aging treatment, however, the γ″ phase plays a key role in hardness and leads to the “Λ” shape profiles of hardness in HAZ.     

Microstructure Evolution of Primary γ'Phase in Ni3Al-Based Superalloy

In this work, water cooling, air cooling (AC) and furnace cooling (FC) were applied to investigate the effect of cooling rate on microstructure evolution of primary γ′ in a newly designed Ni₃Al-based alloy. The results showed that nucleation rate of primary γ′ increased with increasing cooling rate. In addition, higher cooling rate shortened growth period of primary γ′, which made its morphology close to the initial precipitated γ′. For AC and FC specimens, due to the lower cooling rate, primary γ′ possessed longer growth period and its morphology was mainly due to the evolution of lattice misfit between γ and primary γ′. Meanwhile, growth of primary γ′ depended on lattice misfit distribution between its corner and edge area. Moreover, primary γ′ morphologies of sphere, cube and concave cube with tip corners were illustrated by considering interaction between elemental diffusion and elastic strain energy.     

Effect of grit blasting and subsequent heat treatment on stress rupture property of a Ni-based single-crystal superalloy SGX3

In the present study, the effect of grit blasting and subsequent heat treatment on the stress rupture properties of a third-generation nickel-based single-crystal superalloy SGX3 sheet was studied. It was found that the stress rupture life of alloy SGX3 sheet at 980 °C/250 MPa was reduced by about 60% by only vacuum heat treatment at 1100 °C for 200 h and further reduced by 20% and 70% respectively with grit blasting of 0.3 MPa/1 min and 0.5 MPa/2 min before heat treatment. The microstructure analysis results indicated that the degradation of stress rupture life of alloy SGX3 sheet by vacuum heat treatment was mainly attributed to the variation of γ/γ′ microstructure, i.e., the decrease in γ′ volume fraction and the coarsening of γ′ precipitates. Furthermore, such degradation by grit blasting and subsequent vacuum heat treatment should be attributed to the formation of cellular recrystallization with different thicknesses at the surface of alloy SGX3 sheet, which not only acts as the vulnerable site for cracks to initiate and propagate but also reduces the effective loading area.     

Evolution of TCP Phase During Long Term Thermal Exposure in Several Re-Containing Single Crystal Superalloys

The application and component designs of single crystal superalloys are restricted by the precipitation of topologically closed packed(TCP) phases,which can deteriorate the microstructural stability of the alloys severely.Limited researches concerning the type and morphology evolution of TCP phases under elevated temperature conditions have been reported previously.In the present work,three Re-containing single crystal alloys were designed to investigate TCP phase evolution via long term isothermal exposure tests at 1120℃ while the effects of Re on the microstructural characteristic and elements segregation were also clarified.The results showed that the addition of Re increased the instability of the alloys and the volume fraction of the TCP phases exceeded 5 vol% when the Re content reached 3 wt%.The increasing Re content had also raised the precipitation temperature of TCP phases but it did not change the type of them after long term aging;all the TCP particles were identified as μ phase in this study.Moreover,the elements segregation became considerably serious as Re addition increased constantly,which brought about various morphologies of the μ phase in the experimental alloys.In particular,the rod-like and needle-like μ phases demonstrated the typical orientation within γ matrix while the blocky μphase was dispersedly distributed in the space.No specific orientation relationship could be observed in the μ phase when the addition of Re exceeded certain threshold value.     

Distribution of Phosphorus and Its Effects on Precipitation Behaviors and Tensile Properties of IN718C Cast Superalloy

The effect of phosphorus on the precipitations of γ",γ' and δ phases and associated tensile properties in IN718C alloy are investigated in this study.It is revealed that P atoms are dissolved in the grain interior to a relatively high degree and hence influence the precipitation behaviors in the grain interior and improve the tensile strength of IN718C alloy.γ" and γ' phases did not precipitate in the alloy without P addition during air cooling,while γ" and γ' phases precipitated in the grain interior during air cooling in the alloys with P addition,and the amounts of γ" and γ' phases increased with increasing P content.Therefore,the Vickers micro-hardness in the as-cast state increased gradually with increasing P content.In double-aging state,the sizes of γ" and γ' phases in the alloys with P addition were larger than that in the alloy without P addition,while the sizes were invariable when the P content(wt%)was higher than 0.015.Therefore,the micro-hardness and tensile strength of IN718C alloy treated by double aging increased first and then kept invariable with increasing P content.The precipitations of δ phases both in the grain interior and on grain boundaries were inhibited by P markedly.The inhibitory effect of P on δ phase enhanced gradually with increasing content of P,but the plasticity increased first and then decreased.What is more,the crack tended to propagate into the matrix around the particles(Laves phases and NbC carbides)in the alloys without P addition at the beginning of the tensile fracture,while it tended to propagate along the interfaces between the matrix and those particles in the alloys with P addition,which resulted from the synthetical effect of P on γ" γ' and δ phases.     

Effect of Interactions Among Elements on Diffusion Process Associated with γ′ Coarsening in a Ni-Based Single-Crystal Superalloy

Coarsening of cuboidal γ' precipitates and relevant diffusion process in Ni-based single-crystal superalloy CMSX-4 were investigated at 1000,1020 and 1040℃ for specific times.The y' coarsening kinetics followed a cubic rate law with time and was presumably controlled by bulk diffusion of elements in y matrix.The associated diffusion activation energy was experimentally determined to be about 300 kJ/mol when it is considered the temperature-dependent thermo-physical parameters in modified Lifshitz-Slyozov-Wagner theory.The influence of temperature on γ/γ' microstructure is briefly discussed based on pseudo-binary [Ni]-[Al] phase diagram.Interactions among elements can effectively raise the local vacancy formation and vacancy-atom exchange barriers close to γ-and γ'-partitioning elements,respectively.Thus,it can significantly reduce the inter-coupling migrations of atoms during the macroscopic cross-diffusion process associated with γ' coarsening of Ni-based superalloys.     

Hot Deformation Behavior and Microstructural Characteristics of Ti–46Al–8Nb Alloy

Hot deformation behavior,microstructural evolution and flow softening mechanism were investigated in Ti–46Al–8Nb alloy via isothermal compression approach.The true stress–strain curves exhibited typical work hardening and flow softening,in which the dependence of the peak stress on temperature and strain rate was obtained by hyperbolic sine equation with Zener–Hollomon(Z)parameter,and the activation energy was calculated to be 446.9 k J/mol.The microstructural analysis shows that the alternate dark and light deformed ribbons of Al-rich and Nb-rich regions appeared and were associated with local flow involving solute segregation.The Al segregation promoted flow softening mainly arising from the recrystallization of γ phase with low stacking fault energy.The coarse recrystallized γ and several massive γ phase were observed at grain boundaries.While in the case of Nb segregation,β/B2 phase harmonized bending of lamellae,combined with the growth of recrystallized γ grains and α+β+γ→α+γ transition under conditions of temperature and stress,leading to the breakdown of α_2/γ lamellar colony.During the hot compression process,gliding and dissociation of dislocations occurred in γ phase that acted as the main softening mechanism,leading to extensive c twins and cross twins in α/γ lamellae and at grain boundaries.In general,homogeneous microstructure during the hot deformation process can be obtained in Ti Al alloy with high Nb addition and low Al segregation.The deformation substructures intrinsically promote the formability of Ti–46Al–8Nb alloy.     

Typical Microstructural Characteristics of Ti-5Al-5Mo-5V-3Cr-1Fe Metastable βTi Alloy Forged in α+β Region

In this study, typical microstructural characteristics of a metastable β Ti alloy (Ti-5Al-5Mo-5V-3Cr-1Fe) forged in a dual-phase region (strain of 54% at 820 °C) were investigated in detail by the combined use of X-ray diffraction, energy dispersive spectroscopy, electron channeling contrast imaging and electron backscatter diffraction techniques. Results show that the microstructure of the forged alloy is composed of bulk α grains, α plates and β matrix. The bulk α grains correspond to retained primary α phase (α_p, average grain size~2.4 μm), while the α plates are secondary α phase (α_s, width~70 nm) precipitated from the β matrix during air cooling. During forging, the β matrix experiences dynamic recovery with many subgrains and significant orientation gradients formed. Analyses of the orientation relationship between the α and β phases show that the Burgers orientation relationship is not maintained between some α_p and β phases, which should be related to thermal deformation-induced changes of their orientations. In contrast, all of the α_s plates are found to maintain well the Burgers orientation relationship with the β phase.