Modeling Interdiffusion Processes in CMSX-10/Ni Diffusion Couple

A diffusion couple between directionally solidified nickel and the single crystal Ni-base superalloy CMSX-10 was produced by hot pressing in vacuum. The diffusion couples were heat treated at temperatures between 1050 and 1250 °C. The exposed samples were characterized by SEM/EBSD/EPMA. The interdiffusion results in dissolution of the γ′-Ni₃Al in the superalloy and in growth of nickel grains towards CMSX-10. Rapid diffusion of aluminum from the superalloy into pure nickel leads to a significant formation of pores in the superalloy. The interdiffusion processes were modelled using the finite-element simulation software DICTRA with the databases TCNi5 and MobNi2, tailored specially for Ni-base superalloys. The effect of alloying elements on the interdiffusion profiles is discussed in terms of alloy thermodynamics. The calculated element concentration profiles are in good agreement with the EPMA measurements. The interdiffusion modeling correctly predicts the shapes of the concentration profiles, e.g. kinks on the Al and Ti profiles in the vicinity of the original interface in the joint. The calculation predicts with reasonable accuracy the extent and the location of the Kirkendall porosity.     

镍基铸造高温合金的热等静压处理

评述了镍基铸造高温合金的热等静压（HIP）处理对组织和力学性能的影响。镍基铸造高温合金由于存在着铸造工艺难以消除的气孔类缺陷，严重影响着合金的使用可靠性和成品率，通过HIP处理后的合金，不仅可有效地消除合金中的缺陷，获得致密合金，而且还可改善合金的显微组织，提高合金的拉伸、持久和疲劳性能，显著地减小性能分散度。     

Isothermal Oxidation Behaviour of NiCoCrAlYTa Coatings Produced by HVOF Spraying and Tribomet™ Process

Protective NiCoCrAlYTa coatings are used on gas turbine single crystal superalloy blades to provide environmental resistance. They can be deposited by several processes. In this study, isothermal oxidation behaviour of NiCoCrAlYTa coatings produced by HVOF spraying and Tribomet? process and deposited on single crystal nickel-based superalloy CMSX-4 were compared between 950 and 1,150 °C for several exposure durations. Microstructure and chemical composition of both coatings were examined before and after oxidation testing and quite similar observations were made for both coating processes. The combination of phase and chemical analyses allowed the establishment of an occurrence diagram of phases for both coating processes, according to temperature and duration of exposure. The obtained diagrams seemed similar for both processes. Finally both processes appeared to be equivalent for the protection of CMSX-4 superalloy in isothermal oxidation conditions.     

Grain structure and texture evolutions during single crystal casting of the Ni-base superalloy CMSX-4

The solidification grain structure and texture evolutions during single crystal (SX) casting of the advanced Ni-base superalloy CMSX-4 have been investigated. In order to understand the development of the solidification grain structure, SX casting experiments were carried out with a specially designed grain selector in a Bridgman directional solidification (DS) furnace. In addition to casting trials, the SX casting process was simulated by a 3-D cellular automaton-finite element (CAFE) model. The predicted solidification grain structure and the texture evolutions were validated by comparison with the microstructural observation and the electron back scattered diffraction (EBSD) results. It was shown that the overall grain structure, crystallographic texture evolution, and the location where the final selection of the single crystal occurs can be predicted well by the present CAFE model. The axial texture evolution of the single crystal was found to be significantly influenced by the grain density at the chill surface. The CAFE predictions also revealed that the geometry of the grain selector plays a significant role in the final selection of the single crystal.     

DD3单晶高温合金的高温蠕变断裂行为

通过对DD3单晶高温合金试棒进行1030℃／196MPa、1040℃／177MPa和1050℃／140MPa蠕变实验,并对试样断口形貌及其组织进行观察和分析,研究了第一代单晶高温合金DD3的高温蠕变断裂行为。蠕变试验结果表明：在1030～1050℃条件下,DD3合金具有优良的蠕变性能;蠕变开始不到10h,合金即进入稳态蠕变阶段。断口形貌及组织分析表明：高温条件下的DD3单晶高温合金蠕变断裂裂纹萌生于微孔,断裂模式为微孔聚集型断裂。     

Substrate Effect on the High Temperature Oxidation Behavior of a Pt-modified Aluminide Coating. Part II: Long-term Cyclic-oxidation Tests at 1,050 °C

This second part of a two-part study is devoted to the effect of the substrate on the long-term, cyclic-oxidation behavior at 1,050 °C of RT22 industrial coating deposited on three Ni-base superalloys (CMSX-4, SCB, and IN792). Cyclic-oxidation tests at 1,050 °C were performed for up to 58 cycles of 300 h (i.e., 17,400 h of heating at 1,050 °C). For such test conditions, interdiffusion between the coating and its substrate plays a larger role in the damage process of the system than during isothermal tests at 900, 1,050, and 1,150 °C for 100 h and cyclic-oxidation tests at 900 °C which were reported in part I [N. Vialas and D. Monceau, Oxidation of Metals 66, 155 (2006)]. The results reported in the present paper show that interdiffusion has an important effect on long-term, cyclic-oxidation resistance, so that clear differences can be observed between different superalloys protected with the same aluminide coating. Net-mass-change (NMC) curves show the better cyclic-oxidation behavior of the RT22/IN792 system whereas uncoated CMSX-4 has the best cyclic-oxidation resistance among the three superalloys studied. The importance of the interactions between the superalloy substrate and its coating is then demonstrated. The effect of the substrate on cyclic-oxidation behavior is related to the extent of oxide scale spalling and to the evolution of microstructural features of the coatings tested. SEM examinations of coating surfaces and cross sections show that spalling on RT22/CMSX-4 and RT22/SCB was favored by the presence of deep voids localized at the coating/oxide interface. Some of these voids can act as nucleation sites for scale spallation. The formation of such interfacial voids was always observed when the β to γ′ transformation leads to the formation of a two-phase β/γ′ layer in contact with the alumina scale. On the contrary, no voids were observed in RT22/IN792, since this β to γ′ transformation occurs gradually by an inward transformation of β leading to the formation of a continuous layer of γ′ phase, parallel to the metal/scale interface.     

Influence of service-induced microstructural changes on the aging kinetics of rejuvenated Ni-based superalloy gas turbine blades

Rejuvenation of Ni-based superalloy gas turbine blades is widely and successfully employed in order to restore the material microstructure and properties after service at high temperature and stresses. Application of hot isostatic pressing (HIP) and re-heat treatment can restore even a severely overaged blade microstructure to practically “as-new” condition. However, certain service-induced microstructural changes might affect an alloy’s behavior after the rejuvenated blades are returned to service. It was found that advanced service-induced decomposition of primary MC carbides, and the consequent changes of the γ-matrix chemical composition during the rejuvenation, can cause a considerable acceleration of the aging process in the next service cycle. The paper will discuss the influence of the previous microstructural deterioration on the aging kinetics of rejuvenated gas turbine blades made from IN-738 and conventionally cast GTD-111 alloys.     

Kinetics of γ’ precipitation and its influence on fatigue crack growth behavior of a new single-crystal nickel- based superalloy (cmsx-4g) at room temperature

An investigation was carried out to determine the growth kinetics of γ precipitates in a newly developed single-crystal nickel-base superalloy containing rhenium (CMSX- 4G). The investigation also examined the influence of γ’ precipitates (size and distribution) on fatigue crack growth behavior of the material in a room-temperature ambient atmosphere. The influence of load ratio on fatigue threshold of the material and crack growth mechanisms in fatigue was also studied. Compact tension specimens were prepared from a single-crystal nickel-base superalloy, CMSX-4G, with the (001) crystallographic direction. These specimens were given two different heat treatments to produce two different γ’ size precipitates. Fatigue crack growth behavior of these materials was studied at three different load ratios (R = 0.10, 0.50, and 0.90) in room-temperature ambient atmosphere. The results of the present investigation demonstrate that rhenium additions in CMSX-4G substantially lowers the γ coarsening kinetics of this alloy. The smaller γ’ precipitate size was found to be beneficial for fatigue resistance and has resulted in a higher fatigue threshold and lower fatigue crack growth rate in the threshold region. The fatigue threshold was found to decrease with an increase in load ratio. The crack growth mechanism in the threshold region was found to occur by a combination of microvoid coalescence and striations.     

热暴露下预应变对CMSX-4单晶高温合金的显微组织演变的影响

在室温下,对经完全热处理的第二代单晶高温合金CMSX-4实施压缩和拉伸预应变。压缩和拉伸预应变在单晶CMSX-4中产生了剪切带。单晶CMSX-4在950℃下热暴露10h,沿剪切带产生了γ′粒子择优粗化。剪切带上的γ′粒子逐渐侵入γ通道。最后,γ通道沿着剪切带消失。TCP状粒子伴随着γ通道的消失而出现。然而,热暴露10h的普通单晶CMSX-4没有产生TCP沉淀,也没有γ′粒子择优粗化。热暴露100h的预应变CMSX-4沿剪切带产生了γ′粒子和TCP相粒子择优粗化,基体中也有γ′粒子粗化。     

High temperature mechanical properities and creep crack initiation of DS CM186LC for nozzle guide vane

The second generation DS alloy, CM186LC is used in the as-cast and double aged condition which has creep-rupture properities equivalent to the first generation single crystal alloys CMSX-2 and CMSX-3. In production, cast vane components have to be subjected to a brazing treatment for joining into pairs. The effect of the brazing treatment and modified brazing treatment (heat treatment) on mechanical properities at high temperature was studied in accordance with microstructure. Brazing treatments gave no effect on tensile properities and creep failure mode of DS CM186LC, although a small decrease in stress-rupture life was observed. Creep failure was related to the solidified microstructure. Creep cracks began at the grain boundary normal to the applied stress, especially at the γ/γ’ eutectic phase on grain boundaries. Most of γ/γ’ eutectics which had solidified at the last stage of casting, had microporosity which became a crack initiation site during creep. MC carbide reaction with the matrix γ was observed in the creep failed specimens.     

两种热处理对镍基单晶高温合金CMSX-4微观组织和持久性能的影响

研究了第2代镍基单晶高温合金CMSX-4的标准多级预处理+固溶热处理(多级固溶)和一种超初熔点固溶热处理工艺对其组织、元素偏析和持久性能的影响。结果表明:合金经1280℃/1 h+1290℃/2 h+1300℃/6 h,AC+1140℃/4h,AC+870℃/16 h,AC多级固溶热处理(工艺1)后,元素偏析得到较明显改善,在980℃/250 MPa和1070℃/140 MPa条件下的持久寿命分别为207.50 h和280.84 h。工艺1的固溶热处理过程中形成较多固溶微孔,铸态微孔尺寸增大,这些显微孔洞在持久实验中成为合金的裂纹源,使合金发生微孔聚集型断裂,是CMSX-4合金发生持久断裂的关键原因。合金经1320℃/3 h,AC+1140℃/4 h,AC+870℃/16 h,AC超初熔点热处理(工艺2)后,改善合金元素偏析情况不如工艺1,但采用的固溶处理制度形成的微孔数量较少,持久寿命更高,在980℃/250 MPa和1070℃/140 MPa条件下的持久寿命分别为293.08 h和310.10 h。     

Influence of Bridgman process conditions on microstructure and porosity of single crystal Ni-base superalloy CMSX-4

Abstract

Synchrotron radiation micro-X-ray fluorescence and micro-X-ray diffraction analysis, micro-computed tomography and transmission electron microscopy were used to investigate the microstructure of turbine blades made of a second generation nickel based superalloy CMSX-4 manufactured by investment casting process via Bridgman technique. The dependence of microstructure on process parameters, among others, on the withdrawal rate was analysed. The results set the stage for optimising the microstructure of the blades in terms of their mechanical properties.     

A high thermal gradient directional solidification method for growing superalloy single crystals

The experiments here were conducted at withdrawal rates of 3 mm/min and 1 mm/min using a CMSX-6 and a CMSX-4 superalloy, respectively. The process was assessed in terms of the thermal gradient (G_L), structural refinement, microsegregation and porosity distribution, and compared to those using a Bridgman process. The G_L of the process was 200–236 K/cm, which was 10–12 times higher than that in the Bridgman process. A more refined microstructure was produced having average primary and secondary dendrite arm spacing values as low as 243 μm and 72 μm, as well as 272 μm and 76 μm in the CMSX-6 and the CMSX-4 castings, respectively. The diameter of γ′ phase in the dendrite core of CMSX-6 and CMSX-4 castings was reduced from 0.8 μm to 0.3 μm and from 1.2 μm to 0.6 μm, respectively. The average areas of (γ′ + γ) eutectic pools became smaller and more homogeneously distributed. The mean pore sizes in the castings were reduced by 57% and 43% for the CMSX-6 and CMSX-4 superalloys, respectively, and the area fractions of the pores in the CMSX-6 and CMSX-4 samples were 16% and 12% of those produced in the Bridgman samples. The segregation coefficients of the major alloying elements were closer to unity than those in the Bridgman process, which indicates that the composition distribution is more uniform.     

CMSX-2单晶高温合金高梯度定向凝固下过渡区的组织演化特征

用高温度梯度定向凝固装置研究了CMSX－2单晶高温合金初始过渡区的组织演化特征，获得了不同条件下过渡区单晶高温合金的平－胞－枝组织结构，以及相应的一次胞枝晶间距大小，发现初始过渡区的凝固过程影响到最终获得的凝固组织。采用的匀加速抽拉方式有利于提高单晶的引晶率和单晶凝固组织的完整性。     

热等静压冷速对SLM成形K536合金力学性能的影响

采用激光选区熔化工艺制备K536航空发动机零件,研究了不同冷速的热等静压HIP工艺对制件组织及力学性能的影响。结果表明,激光选区熔化K536高温合金制件经HIP处理后,制件内部产生完全再结晶,金相组织为奥氏体基体及晶内和晶界碳化物,合金强度降低、塑性上升,数据分散性下降;随HIP冷速的增大,组织中晶界碳化物减少,晶内出现少量颗粒状碳化物,有效提升了合金在815℃的高温屈服强度。     

Oxidation of Simple and Pt-Modified Aluminide Diffusion Coatings on Ni-Base Superalloys—II. Oxide Scale Failure

The spallation behavior of oxides formed during isothermal oxidation at 1050°C of one simple (PWA73) and three Pt-modified (RT22, SS82A and MDC150L) aluminide diffusion coatings on the same Ni-base, single-crystalline superalloy (CMSX-4) was investigated by scanning electron microscopy (SEM) and transmission-electron microscopy (TEM). It was found that the main spallation mechanism was the formation of large Kirkendall voids at the oxide–coating interface. It is believed that the void formation was caused by counter-current flow of vacancies to the diffusion of Ni away from the interface as Al is consumed by the oxide. The magnitude of the vacancy current was determined by the oxidation rate. The properties of the void-formation mechanism are discussed in view of previous data on the microstructure of the oxide scales.     

Improvement of creep strength of a third generation, single-crystal Ni-base superalloy by solution heat treatment

The standard solution heat treatment of the third generation, single-crystal Ni-base superalloy, CMSX-10, requires multiple steps at temperatures up to 1365 °C and lasts a total of approximately 45 hours. These high temperatures and long hours result in a heat treatment that is costly. Reducing the temperature and/or time of the heat treatment to reduce the cost of the heat treatment would result in residual segregation of W and Re to the dendrite cores, a locally unstable microstructure. Therefore, a modified heat treatment was evaluated that increased the maximum heat-treatment temperature and the amount of time at the higher heat-treatment temperatures. The microstructures and mechanical properties were evaluated for samples given both the standard and the modified solution heat treatment.     

Effect of Heat Treatment on Chemical Segregation in CMSX-4 Nickel-Base Superalloy

Superalloys display a strong tendency toward chemical segregation during solidification. Therefore, it is of great importance to develop appropriate techniques for the melting and casting of superalloys. Elements partitioning between the γ and γ′ phases in single crystal superalloys have been investigated by several authors using electron probe microanalysis (Hemmersmeier and Feller-Kniepmeier Mater Sci Eng A 248:87-97, 1998; Kearsey et al. Intermetallics 12:903-910, 2004; Kearsey et al. Superalloys 2004, pp 801-810, 2004; D’Souza et al. Mater Sci Eng A 490:258-265, 2008). We examined the effect of the particular stages of standard heat treatment (solution treatment and ageing) applied to CMSX-4 single crystal superalloy on chemical segregation that occurs between dendrites and interdendritic areas. Dendritic structures were observed using a scanning electron microscope. Analyses of the chemical composition were performed using energy dispersive x-ray spectroscopy. The obtained qualitative and quantitative results for the concentrations of elements enabled us to confirm the dendritic segregation in as-cast CMSX-4 superalloy. The concentrations of some refractory elements (tungsten, rhenium) were much greater in dendrites than in interdendritic areas. However, these differences in chemical composition gradually decreased during heat treatment. The results obtained in this study warrant further examination of the diffusion processes of elements during heat treatment of the investigated superalloy, and of the kinetics of diffusion.     

Calculating solidification and transformation in As-Cast CMSX-10

Advanced single-crystal nickel-based superalloy s contain significant levels of refractory metal alloying additions and exhibit severe solidification segregation that must be subsequently removed by solution heat treatment. The solidification partitioning and transformation temperatures of the third-generation single-crystal nickel-base superalloy, CMSX-10, was calculated using Thermo-Calc™. All of the calculated values were compared to the experimentally determined results of this study.     

Metallurgical characterisations of CMSX-4 vacuum-brazed with BNi-2 and BNi-9

High temperature brazing of nickel-based superalloys often produces joints containing hard, brittle micro-constituents that can be detrimental to mechanical properties and challenging to characterise consistently. In this study, techniques including low angle micro-sectioning, image analysis with ImageJ and electron probe micro-analysis were used to determine the composition, hardness and dispersion parameters of phases in single crystal superalloy CMSX-4, vacuum furnace brazed with BNi-2 and BNi-9 filler metals (FMs). Both FMs produced similar joints with hard centreline eutectic phases, a soft isothermally solidified zone and boron diffusion-affected zone in the CMSX-4. The volume fraction, particle size distribution and inter-particle spacing data generated will provide a framework for future metallurgical characterisations and assist in the development of microstructure–mechanical property relationships.