Low temperature carbide precipitation in a nickel base superalloy

A M₂₃C₆ carbide phase has been observed to precipitate at relatively low temperatures (732 to 760 °C) in a nickel base superalloy.* Transmission Electron Microscopy shows the low temperature carbide to reside at the grain boundaries in a continuous morphology. The continuous carbide has a typical width of 25 to 40 nm with aspect ratios on the order of 30:1. The structure of the carbide is face-centered cubic with a lattice parameter (α₀) of approximately 1.063 nm, which is typical of the M₂₃C₆ carbides that form at higher temperatures. STEM analysis indicates the carbide to have a typical M₂₃C₆ chemistry, enriched in chromium with lesser amounts of molybdenum, cobalt, and nickel. The formation of the continuous carbide occurs readily around 760 °C; however, at temperatures 55 °C lower the precipitation kinetics are significantly reduced. The extent of the low temperature carbide reaction is observed to be dependent upon the duration of the low temperature exposure and the degree of prior M₂₃C₆ stabilization at an intermediate temperature. Alloy modifications, involving hafnium additions and lower carbon levels, were studied with the aim of reducing the extent of this carbide reaction. Despite these chemistry modifications, the low temperature carbide was still observed to form to an appreciable extent. The presence of the continuous carbide is also observed to reduce the stress-rupture life of the alloy.     

Creep resistance of CMSX-3 nickel base superalloy single crystals

Creep deformation in 〈001〉 oriented nickel base superalloy single crystals has been studied in an effort to assess the factors which contribute to the overall creep resistance of superalloys with high volume fractions of γ′ phase. Detailed observations of three dimensional dislocation arrangements produced by creep have been made with the use of stereo electron microscopy. In the temperature range of 800–900°C at stresses of 552 MPa or lower, the dislocation-free γ′ precipitates are resistant to shearing by dislocations. As a result, creep deformation occurs by forced bowing of dislocations through the narrow γ matrix channels on {111} planes. At moderate levels of temperature and stress there are incubation periods in virgin crystals prior to the onset of primary creep. The incubations arise because of the difficult process of filling the initially dislocation starved material with creep dislocations from widely spaced sources. When the newly generated dislocations percolate through the cross section, incubation comes to an end and primary creep begins. In primary creep neither work hardening nor any type of recovery plays an important role. The creep rate decelerates because the favorable initial thermal misfit stresses between γ and γ′ phases are relieved by creep flow. Continued creep leads to a build-up of a three-dimensional nodal network of dislocations. This three-dimensional network fills the γ matrix channels during steady state creep and achieves a quasi-stationary structure in time. In situ annealing experiments show that static recovery is ineffective at causing rearrangements in the three-dimensional network at temperatures of 850°C or lower. The kinematical dislocation replacement processes which maintain the quasi-stationary dislocation network structures during apparent steady state creep are not understood and require further study. Because of the impenetrability of the γ′ precipitates, dislocations move through the γ matrix by forced Orowan bowing, and this accounts for a major component of the creep resistance. In addition, the frictional constraint of the coherent or semi-coherent precipitates leads to the build-up of pressure gradients in the microstructure, and this provides load carrying capacity. There is also a smaller component of solid solution strengthening. Work hardening is comparatively unimportant. Finite element analysis shows that the non-deforming precipitates are increasingly stressed as creep deformation accumulates in the matrix. In the later stages of steady state creep and during tertiary creep the stresses in the precipitates rise to high enough levels to cause shearing of the γ′ particles by dislocations entering from the γ matrix. The recovery resistance of the material is in part due to a very low effective diffusion constant and in another part due to the fact that the three-dimensional dislocation networks formed in the γ matrix serve to neutralize the misfit between the γ and γ′ phases.     

FGH95镍基合金的组织结构与蠕变行为

通过蠕变曲线测定和组织形貌观察,研究了FGH95合金的蠕变特征与变形机制.结果表明:经高温固溶及"盐浴"冷却后,FGH95合金的组织结构由细小γ'相及粒状碳化物弥散分布于γ基体所组成,由于沿晶界不连续析出的粒状(Ti,Nb)C相可提高合金的晶界强度,并抑制晶界滑移,故使其在650℃、1 034MPa条件下有较小的应变速率和较长的蠕变寿命.合金在蠕变期间的变形机制是位错切割γ或γ'相,其中,当(1/2)<110>位错切入γ相,或<110>超位错切入γ'相后,可分解形成(1/6)<112>肖克莱不全位错或(1/3)<112>超肖克莱不全位错+层错的位错组态;蠕变后期,合金的变形特征是晶内发生单取向和双取向滑移,随蠕变进行位错在晶界处塞积,其引起的应力集中致使裂纹在晶界处萌生及扩展是合金的蠕变断裂机制.     

Processing and evaluation of 15Cr-15Mo nickel base superalloy

A Nickel base solid solution strengthened alloy has been made with a nominal chemical composition of 15Cr-15Mo (balance Ni) through vacuum melting route and was characterized in hot worked and heat-treated conditions. Processing parameters have been optimized to get the desired microstructure and mechanical properties. The alloy has shown very good mechanical properties at elevated temperatures as well as at cryogenic temperatures. Role of solution treatment temperature and correspondingly role of grain size on mechanical properties at different temperatures has also been evaluated and reported. Heat treatment cycle for application of alloy upto 700°C has been optimized. Properties of the alloy have been compared with similar class of alloys.     

一种[011]取向镍基单晶合金的蠕变断裂

研究了[011]取向的镍基单晶高温合金在750~980℃温度范围和200~680 MPa应力下的蠕变断裂特征.在扫描电镜上对各种实验状态下的蠕变断口和纵向剖面进行了详细观察.研究发现：在低温750℃和中温870℃不同初始蠕变应力条件下,枝晶间区亚晶界处不规则γ＇/γ界面是裂纹主要萌生场所,这些已萌生的裂纹在与外加应力轴垂直的（011）面上沿〈110〉和〈100〉两个方向扩展;980℃不同初始应力条件下,裂纹主要在合金中显微疏松孔洞处萌生,沿与外应力轴垂直的方向扩展.观察750℃和870℃不同应力状态蠕变试样的纵向剖面,对亚晶界区不规则γ＇相面积分数的测量和计算表明,用面积分数表征该合金[011]取向在中低温状态下的蠕变损伤程度是可行的.     

Investigation of techniques for measuring lattice mismatch in a rhenium containing nickel base superalloy

Three techniques for measuring γ/γ′ lattice mismatch have been examined in a Ni-base superalloy subjected to two different aging heat treatments. The techniques used for measuring lattice mismatch were X-ray diffraction, convergent beam electron diffraction (CBED) and interface dislocation analysis. Additionally, a scanning transmission electron microscope equipped with an X-ray energy dispersive spectroscopy (EDS) system has been used to examine phase compositions. From this study, it has been determined that the X-ray diffraction and CBED yield similar results for room temperature lattice mismatch, although care must be taken in applying the CBED technique due to the complex strain fields present in high volume %γ′ alloys. The dislocation analysis technique gives larger negative values of mismatch. It is believed that these latter values represent those which exist at the aging temperature.     

High temperature elastic properties of polycrystalline mar-m200 (a nickel base superalloy)

A sonic resonance technique has been used to measure the variation in Young’s modulus and shear modulus of MAR-M200 at elevated temperatures to 1125 K. The slopes of these moduli with temperature are found to change abruptly at 915 and 970 K, respectively. The reason for the above noted changes appears to be related to the elastic behavior of the γ phase of MAR-M200.     

Mechanisms of damage and fracture in high-temperature,low-cycle fatigue of a cast nickel-based superalloy

In cast Udimet 500 subjected to high-temperature, low-cycle fatigue, localized oxidation at grain boundaries plays an important role in crack nucleation and propagation. Evidence is presented of a surface ridging and pronounced grain boundary penetration due to oxidation, a denudedγ′ zone adjacent to the oxide, and cracking of the oxide. The ridging is selective, and is presumed to occur on those boundaries where high stress exists. The phenomenon is viewed as analogous to stress-corrosion cracking.     

[011] Creep in a single crystal nickel base superalloy at 1033K

TEM investigation are performed on [011] single crystals of SRR99 after at 1033 K under a load at 680 MPa. At the creep rate minimum, multiple slip in the matrix occurs on four octahedral slip systems. Octahedral cross slip or cubic slip is not activated. Due to the superposition of coherency and external stressein small matrix channels, deformation is concentratedned in the “roof” matrix channels. During secondary creep, screw interfaial dislocations cross slip in the cube γ/gg' interfaces. In this stage, γ' precipitates are sheared by Shocley super parrials. Common shereing of matrix and γ' particles by Shockley super partials is observed. The high creep rate in the [011] orientation is considered to be caused by the stress concentration in roof matrix channels and the few actiated slip sysems.     

Effect of strain rate on damage evolution in a cast Al-Si-Mg base alloy

An important aspect of damage evolution in cast Al-Si-Mg base alloys is fracture/cracking of Si particles. This microstructural damage is quantitatively characterized as a function of strain rate in the range 10⁻⁴ to 3.7 × 10⁺³, at an approximately constant uniaxial compressive strain level (20 to 25 pct). It is shown that the fraction of damaged silicon particles, their average size, and size distribution do not vary significantly with the strain rate, and at all strain rates studied, larger Si particles are more likely to crack than the smaller ones. However, the stress-strain curves are sensitive to the strain rate. These observations have implications for modeling of deformation and fracture of cast components under high strain rate crash conditions.     

Microstructural investigation of the growth of large grains in prealloyed powder extrusions of a nickel base superalloy

The metallurgical conditions controlling the growth of large grains in consolidated Rene 95 superalloy powder were investigated. The starting material was -35 mesh powder produced by the rotating electrode process from low carbon, (0.08 wt pct) vacuum remelt stock. The powder was batch consolidated at temperatures between 1228 K (1750°F) and 1561 K (2350°F) and extruded to 10:1 ratio at temperatures between 1339 K (1950°F) and 1450 K (2150°F). High temperature conventional and gradient anneals to 1505 K (2250°F) employing both slow and fast heating rates were used to investigate the large grain growth phenomena. Results indicated that growth of large grains, 2 to 3 mm in length, occurs when extrusion temperatures were above theγ′ solvus temperature and the tendency for large grain growth increased with decreasing heating rate. The grain boundaryγ′ is believed to be responsible for the prevention of large grain growth but these precipitates are less effective when a slow heating rate anneal is used. The results are found to conform to the theoretical formulation of Hillert.     

Microstructural investigation of the growth of large grains in prealloyed powder extrusions of a nickel base superalloy

The metallurgical conditions controlling the growth of large grains in consolidated Rene 95 superalloy powder were investigated. The starting material was — 35 mesh powder produced by the rotating electrode process from low carbon, (0.08 wt pct) vacuum remelt stock. The powder was batch consolidated at temperatures between 1228 K (1750°F) and 1561 K (2350°F) and extruded to 10∶1 ratio at temperatures between 1339 K (1950°F) and 1450 K (2150°F). High temperature conventional and gradient anneals to 1505 K (2250°F) employing both slow and fast heating rates were used to investigate the large grain growth phenomena. Results indicated that growth of large grains, 2 to 3 mm in length, occurs when extrusion temperatures were above the γ′ solvus temperature and the tendency for large grain growth increased with decreasing heating rate. The grain boundary γ′ is believed to be responsible for the prevention of large grain growth but these precipitates are less effective when a slow heating rate anneal is used. The results are found to conform to the theoretical formulation of Hillert.     

W和Ta对新型镍基粉末高温合金显微组织的影响

摘要：采用相同的粉末冶金工艺与热处理方法设计3种W和Ta含量不同的合金,利用金相显微镜（OM）、扫描电镜（SEM）、场发射扫描电镜（FE SEM）、X射线衍射仪（XRD）等多种研究手段,研究W和Ta元素对新型镍基粉末高温合金显微组织的影响。结果表明：改变W和Ta含量对合金晶粒组织和MC型碳化物的析出没有明显影响;随着合金中Ta含量的减少和W含量的增加,γ/γ′相错配度逐渐减小;随着合金中Ta含量和γ/γ′相错配度的增加,γ′相含量和尺寸均增加,二次γ′相粗化长大和达到分裂的时间缩短,尺寸分布越来越不均匀。     

Effect of oxidation kinetics on the near threshold fatigue crack growth behavior of a nickel base superalloy

The influence of oxidation kinetics on the near threshold fatigue crack growth behavior of a nickel base precipitation hardened superalloy was studied in air from 427° to 649 °C. The tests were conducted at 100 Hz and at load ratios of 0.1 and 0.5. The threshold ΔK values were found to increase with temperature. This behavior is attributed to oxide deposits that form on the freshly created fracture surfaces which enhance crack closure. As determined from secondary ion mass spectrometry, the oxide thickness was uniform over the crack length and was of the order of the maximum crack tip opening displacement at threshold. Oxidation kinetics were important in thickening the oxide on the fracture surfaces at elevated temperatures, whereas at room temperature, the oxide deposits at near threshold fatigue crack growth rates and at low load ratios were thickened by an oxide fretting mechanism. The effect of fracture surface roughness-induced crack closure on the near threshold fatigue crack growth behavior is also discussed. Formerly with General Electric Company, Advanced Nuclear Technology Operation, Sunnyvale, CA 94086.     

Phase composition and chemical order in the single crystal nickel base superalloy MC2

The time evolution of phase composition at 1050°C in a new single crystal nickel base superalloy (MC2) recently developed, is studied by means of atom-probe techniques. The results indicate that the composition of both γ and γ′ phases evolves drastically during ageing. The composition of the μ phase was estimated for 100 h at 1050°C. Atom-probe data demonstrate that μ is a Ta, W, Mo enriched-phase. No impoverishment effect of the γ solid solution related to the μ phase precipitation was detected. The spatial distribution of μ-forming elements in the γ phase was investigated in order to find out whether clustering occurs. Ordering phenomena related to chromium were observed in γ. The long range order in the γ′ phase was also studied. The microanalysis of the material on a (001) superstructure plane allowed the preferential sites of various species to be determined.     

一种定向凝固镍基铸造高温合金的物理化学相分析

系统研究了一种定向凝固镍基铸造高温合金的物理化学相分析方法。对3种热处理制度下的试验合金中各析出相的结构、化学组成和含量及γ′相+微量相的粒度分布进行了测定,揭示了该合金中析出相在不同状态下的变化规律。该合金的析出相为:γ′,(Ti,Nb)C,HfC,M23C6,W3.2Cr1.8B3,WB,M6C(痕)等,没发现μ和M3B2相析出。粒度分布结果表明,在900℃时效3 000 h,γ′相略有长大。