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.     

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.     

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.     

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 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.     

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.     

Microstructural study: an aid to determination of failure mechanism in nickel base superalloy blades

Cast nickel base superalloys are extensively used for high temperature gas turbine blade applications. The elevated temperature properties in these alloys are optimized through engineered microstructure, which is a combination of (a) γ-solid solution of Ni with elements such as W, Mo, Cr, Ta, Re etc, (b) γ′ precipitates, and (c) dispersed carbides. Over the years, the demand for high engine efficiency has resulted in the development of new generation superalloys with improved elevated temperature properties, especially creep resistance. These superalloys are highly alloyed with solid-solution strengthening elements and hence, impose greater technological challenges in fabrication. Due to high alloy contents, these alloys are prone to formation of detrimental phases such as the topologically close packed (TCP) phases. These phases may appear in the microstructure during the blade fabrication stages or precipitate out during exposure to high temperature and stress. In addition, the γ′ precipitates can change in morphology, shape and size during applications resulting in deterioration in high temperature mechanical properties, in general. These unfavorable microstructural changes often lead to premature failure in gas turbine engines. While analyzing these failures, the microstructural study provides important information in identifying whether the blades had faulty microstructure to start with or the abnormalities observed have resulted during exposure to service conditions. This in turn can be related to engine operating conditions. In the present paper, this has been demonstrated through analysis of two service failure cases wherein the high pressure turbine blades had failed in flight leading to aircraft accidents.     

Discontinuous γ′ coarsening in a Ni-Al-Mo base superalloy

Discontinuous coarsening in a Ni-AI-Mo base superalloy was studied emphasizing the influence of grain boundary misorientation. Nucleation and growth rates were determined as a function of temperature, time, and degree of misorientation for (001) twist boundaries. Both nucleation and growth were controlled by grain boundary diffusion and increased with twist angle and temperature. Grain boundary diffusivities (calculated assuming reduced γ′ surface energy as the driving force) were more than one order of magnitude greater than those reported for static boundaries of similar misorientation. Consideration of atomic jump frequencies in static and dynamic boundaries indicated that the high diffusivities cannot be attributed to motion enhanced grain boundary diffusion. Rather, the high calculated diffusivities are interpreted to indicate the presence of additional driving forces for discontinuous coarsening in the Ni-AI-Mo base superalloys. Potential sources of additional driving force are presented.     

Structure and initial precipitation in a rapidly solidified nickel superalloy

A nickel base superalloy (Nimonic 80A) has been rapidly solidified at cooling rates of between 10⁵ to 10⁶ K.S^-1 by pendant drop melt extraction and by chill block melt spinning in an evacuated chamber backfilled with helium or argon. The internal structure is described in terms of process variables pertaining to rotating chill block quenching techniques. Both transmission electron microscopy and atom-probe field-ion microscopy have been employed to give structural and constitutional data on quenched and aged specimens. The as-quenched structure is homogeneous apart from fluctuations in titanium concentration which upon aging undergoes a spinodal phase decomposition to form disordered Ni₃(Ti,Al,Cr) precipitates in the matrix, which after prolonged aging produces ordered γ (Ni₃(Ti,Al)). inin6 particles form readily on grain boundaries and also appear in conjunction with ordered γ, via a discontinuous reaction, after short aging times.     

Effects of B,C, and Zr on the structure and properties of a P/M nickel base superalloy

The boron and carbon levels of a P/M nickel base superalloy were systematically varied in order to determine the mechanisms by which these elements strengthen the alloy, and their optimum concentration. Carbon levels were reduced to 20 ppm while the boron level was varied from 0.02 to 0.10 wt pct. Carbon levels of 0.002 and 0.05 wt pct were also studied, while maintaining a boron concentration of 0.02 wt pct. Zirconium levels were maintained at 0.06 wt pct. The resulting alloys were subjected to identical heat treatments and examinedvia SEM, TEM, and STEM microscopy. The alloys were also subjected to tensile, creep, stress-rupture, and fatigue crack growth tests. Results show that both carbon and boron have a strong influence on the formation of grain boundary precipitates, as expected. Carbon was present as the MC and M₂₃C₆ type carbides, while boron combined to form an intergranular M₃B₂ boride. Boron and zirconium were observed to be critical to the alloys' mechanical properties, although boron levels above the solubility limit resulted in no further improvement or debit in strength. Carbon additions resulted in no improvement in properties, indicating the feasibility of a carbon-free P/M superalloy. The role of the minor element additions is discussed in terms of both microstructural features and related strengthening mechanisms.     

Influence of microstructure on the thermal fatigue behavior of a cast cobalt-base superalloy

The influence of microstructure on the thermal fatigue (TF) behavior of MAR-M 509, a cast cobalt-base superalloy, was investigated using a burner rig and wedge-type specimens which were submitted to thermal shock from 200 °C to 1100 °C. Two microstructures were studied: a coarse microstructure using specimens machined from bulk castings and a fine microstructure using cast-to-size specimens. Metallographic observations showed that cast-to-size specimens display a gradient in microstructure since the size of secondary dendrites increases with the distance to the thin edge. As high-temperature fatigue in this superalloy is controlled by oxidation-fatigue interactions, the kinetics of interdendritic oxidation was studied at 900 °C. Interdendritic oxidation was found to be inhibited by a refinement of dendritic microstructure. A fine microstructure was shown to give a much longer TF life-to-crack initiation and a much lower crack growth rate. This behavior was mainly related to differences in interdendritic oxidation since interdendritic areas act as crack initiation sites as well as easy crack propagation paths. The influence of microstructure on crack growth rates was accounted for by using a previously proposed oxidation-fatigue crack growth model and interdendritic oxidation kinetic data. Formerly with the Centre des Matériaux, Ecole des Mines de Paris     

Driving force for discontinuous coarsening in a Ni-Al-Mo base superalloy

The driving force and mechanism for discontinuous coarsening (DC) in a Ni-Al-Mo base superalloy were studied. Samples were solutionized at 1300 °C, cooled, and aged at 1150 °C to produce DC. Both the intragranular and DC-produced microstructure were studied using optical and scanning electron microscopy of metallographically prepared surfaces and scanning transmission electron microscopy (STEM) of thin foils. Resulting microstructural and microchemical information were analyzed to elucidate the DC process. Grain boundary diffusion of Mo appeared to control the DC growth rate. The principal contributor to the DC driving force was determined to be coarsening of semicoherentγ′ with coarsening of the Mo-rich a phase makingα minor contribution. Using the estimated driving force, grain boundary diffusivities were calculated which were within a factor of 3 of published values for stationary boundaries of comparable misorientation.     

Environmental effects on the creep behavior of a nickel-base superalloy

Environmental effects on the 760 °C creep behavior of a nickel-base superalloy are isolated by testing at varying stress levels, in laboratory air and vacuum, specimens of different gage diameters and grain sizes, and in a few cases, different grain boundary microstructures. For all specimens receiving a standard heat treatment that results in grain boundaries free of discrete carbides, the minimum creep rate is greater in air than in vacuum for a given specimen and grain size. In contrast, for specimens aged to precipitate carbides along the grain boundaries, the creep rate is lower in air than in vacuum. The minimum creep rate and the sensitivity of this rate to applied stress also are found to be functions of grain size, specimen size, and grain boundary microstructure to extents dependent on the test environments. Rationalizations of these environmental creep behaviors are suggested in terms of the apparently competing phenomena of the relative weakening of the alloy grain boundaries in the air environment, and the alloy creep strengthening in air due to the external surface oxide scale. Formerly Graduate Research Assistant, Henry Krumb School of Mines, Columbia University     

Effect of nitrogen content on the microstructure and mechanical properties of a cast nickel-base superalloy

The effects of nitrogen content on the microstructure and the mechanical properties of a cast nickel-base superalloy (CNS) have been investigated experimentally. Experimental results demonstrated that the grain structure of CNS samples was refined by increasing the nitrogen content, but the area percentage of microporosity has been augmented with increased nitrogen content. Increasing the nitrogen content resulted in the morphology evolution of carbide from an acicular or ‘Chinese hieroglyphs’ type to blocky one, while negligible change of the morphology of γ′ precipitates was observed. Finally, it was found that the tensile strength has no obvious variation as the nitrogen content increases from 5 to 26?ppm, but it reduces sharply when the nitrogen content is raised to 34?ppm. The elongation decreases gradually with increasing nitrogen content.     

Low-cycle fatigue of nickel superalloy single crystals at elevated temperatures

Single-crystal samples of nickel superalloys containing rhenium or rhenium plus ruthenium are subjected to low-cycle fatigue (LCF) tests under rigid cycle conditions at temperatures of 850 and 1050°C. It is found that the single crystals of the alloy containing rhenium with ruthenium have higher LCF resistance at 10⁴ cycles as compared to the alloy containing only rhenium. At a test temperature of 850°C, volume stress concentrators in the form of pores or their clusters represent fatigue crack nucleation zones; at 1050°C, surface corrosion cracks are the main fracture zones. The fatigue microcrack growth rate is anisotropic: it is higher in the [001] direction and lower in the [011] direction.