Mechanical properties of conventionally cast,directionally solidified,and single-crystal superalloys

Abstract

In this paper the authors compare the creep and low-cycle fatigue properties of conventional, directionally solidified, and single-crystal castings produced from nickel-base superalloys. A brief historical review describes the reasons for the evolution from wrought to cast product through directionally solidified to modern single-crystal (‘monocrystal’) castings. The influence of microstructural variations produced by the casting conditions, such as porosity and grain size, on creep and low-cycle fatigue properties are illustrated. The important aspects of postsolidification heat treatment, hot isostatic pressing, and the damaging effects of impurities are described for conventional castings. The results of controlling the microstructures produced by directional solidification especially by high temperature gradient solidification are demonstrated by comparing the creep properties of directionally solidified materials with those of the conventionally cast alloys in long-term tests. The creep and low-cycle fatigue properties depend on the stress direction relative to the crystallographic directions of the material for both directionally solidified and single-crystal castings. For single crystals, individual alloys show variable dependences of properties on the crystallographic directions. Directionally solidified materials show advantages in thin sections and are less sensitive to the effects of impurities compared to conventional castings.

MST/329     

低Re含量第二代柱晶高温合金DZ59的蠕变断裂性能

采用液态金属冷却(LMC)工艺和成分优化设计,制备出一种高温性能优异的低Re含量第二代柱晶高温合金DZ59,通过扫描电镜(SEM),透射电镜(TEM)等方法对其组织特征及蠕变断裂进行了表征,研究了合金的蠕变断裂性能.结果表明,DZ59合金的高温蠕变断裂性能超过一代单晶合金,并接近二代单晶合金的水平.发现柱晶高温合金的二次晶界反应(SGRZ)现象,表明SGRZ受温度和应力的控制,由于增加了合金横向晶界的受力界面,在高负荷下可能成为蠕变空洞萌生和扩展的位置.     

人工神经网络法在镍基变形合金蠕变断裂寿命预测中的应用

在试验数据的基础上,利用人工神经网络建立不同成分镍基变形合金的不同温度,外应力与蠕变断裂寿命之间关系模型,并进行网络训练,对合金的蠕变断裂寿命进行模拟.结果表明,模拟结果与实测结果吻合良好,采用人工神经网络方法可以为镍基变形合金蠕变断裂寿命的预测提供一种有效的手段.     

Constitutive modeling and failure mechanisms of anisotropic tensile and creep behaviors of nickel-base directionally solidified superalloy

A transversely isotropic continuum elasto-viscoplasticity model is formulated to capture the tensile and creep behaviors of a directionally solidified (DS) nickel-base superalloy. A fourth-order tensor is introduced to model material anisotropy. The Kachanov damage evolution equation is coupled with stress tensor to improve capability of modeling creep deformation. This model is implemented as an ABAQUS user material (UMAT) subroutine using a self-adaptive explicit integration scheme. A grouping optimization strategy is employed to identify the material parameters by fitting experimental curves of isothermal tension and creep loading at high temperature. Failure mechanisms are investigated by observing the fracture morphology by means of Scanning Electron Microscope (SEM) with the Energy Dispersive X-ray Spectrometer (EDXS). The results obtained showed that Chaboche constitutive model coupled with anisotropy and creep damage was able to characterize the rate-dependent anisotropic tensile and creep behaviors of DS superalloy and the simulation results agreed well with the experimental data. The tensile fracture surface of DS superalloy mainly contained a mixture of large cleavage planes and small amount of dimples. Meanwhile, the creep fracture mechanism of DS superalloy at 760 and 850 °C was transgranular fracture induced by the dimple accumulation. The morphology of the dimples and non-metallic inclusions at 760 °C was different from that at 850 °C.     

On the tensile creep behaviour of a directionally-solidified Ni3Al-based alloy

High temperature tensile creep behaviour of a directionally-solidified Ni₃Al-based alloy is presented. The study involved selection of nine alloy systems based on Ni₃Al. The alloys contained varying amounts of Cr and Ta, fixed amounts of 1·5 at.% Hf and 0·5 at.% Zr and doped with 0·2 at.% each of C and B. The alloys were vacuum arc-melted into buttons and homogenized at 1050°C for 68 h. The test pieces of the alloys were hot compression tested at 600, 700, 800 and 900°C. The yield strength data of some of the alloys were superior to conventionally cast Mar-M 200, a cast nickel-base superalloy widely used in gas turbine structural applications. The best alloy system was chosen based on consistent performance in the hot compression studies. The alloy so chosen was directionally solidified and vacuum-homogenization-treated for 20 h at various selected temperatures. Optimum creep properties were observed at 1120°C, 20 h treatment. The minimum creep rate data of the DS alloy showed relatively higher values even at lower temperatures and stress levels as compared to Mar-M 200. Hence, the alloy is less promising in replacing nickel-based superalloys used as structural materials in gas turbine applications.     

Abnormal precipitation behavior of Ni3Al phase

Evolution of Ni₃Al phase in a nickel-base directionally solidified superalloy has been investigated during creep rupture testing (1253 K/150 MPa). Besides the splitting, rafting and Ostwald ripening cases, many finer Ni₃Al particles have been firstly observed to precipitate from the rafted Ni₃Al particles. A simple qualitative interpretation has been proposed.     

A dramatic increase in dendrite number for directionally solidified superalloy DZ417G with a strong static magnetic field

The results show the per area dendrite number for directionally solidified superalloy DZ417 is increased with a strong static magnetic field at the different drawing speeds and temperature gradient. With enhancing the intensity of magnetic field, the per area dendrite number for directionally solidified superalloy DZ417 gradually increases. This is due to the interactive competition between the magnetic damping and thermoelectromagnetic convection. Therefore, the dendrite number for directionally solidified superalloys can be controlled by tuning the intensity of magnetic field, which may provide a new way for improving the properties of superalloys.     

„In Situ”︁ Composites for jet propulsion and stationary gas turbine applications

Metallurgists and casting engineers focused on new composites (DSE) for high temperature applications in jet engines. Some examples of reinforced superalloys are discussed. The phase stability of microstructures has been tested using a special high thermal gradient device. For stationary gas turbines efforts in alloy development, transfer of laboratory scale to precision casting technology, and first engine tests of directionally solidified Co-Cr₇C₃ eutectic alloy are being reported. Alloy development was designed to deliver better creep rupture life and surface stability. It centered on the use of elements such as Al and Y, Hf and Ta. The properties measured were creep behavior, fatigue, and oxidation resistance. They are being compared with corresponding data obtained for both the conventionally and directionally cast Ni-base superalloy IN 738. First engine tests (18 000 hrs) with unalloyed Co-Cr₇C₃ vanes indicated the importance of a fully eutectic microstructure and stability against carbide transformations.     

Mechanisms and modelling of creep in superalloys

The characteristics of creep deformation of nickel-base superalloys are reviewed and the implications for the micromechanisms controlling the behaviour are considered. The development of a model of the creep deformation that is compatible with the physical mechanisms is traced, first in an isotropic form and later incorporating full crystallographic anisotropy. The validity of the model and its ability to be extrapolated to more complex loading conditions are evaluated against a wide range of experimental measurements. Much of the work described in this review has been funded by the Defence Research Agency and creep data and specimens for examination were supplied by Dr M R Winstone of DRA.     

Comparative thermal fatigue resistance of several oxide dispersion strengthened alloys

The thermal fatigue resistance of several oxide dispersion strengthened (ODS) alloys has been evaluated through cyclic exposure in fluidized beds. The ODS nickel-base alloy MA 754 and ODS iron-base alloy MA 956 as well as four experimental ODS Ni-16Cr-4.5Al base alloys with and without Ta additions were examined. Both bare and coated alloys were subjected to up to 6000 cycles where each cycle consisted of a 3 minute immersion in fluidized bed at 1130°C followed by a 3 minute immersion in a bed at 357°C. Testing revealed that the thermal fatigue resistance of the ODS nickel-base alloys was excellent and about equal to that of directionally solidified superalloys. However, the thermal fatigue resistance of MA 956 was found to be poor. Metallographic examination of tested specimens revealed that, in general, the post-test microstructures can be rationalized on the basis of previous diffusion, mechanical property, and oxidation studies.     

微量元素对镍基高温合金微观组织与力学性能的影响

在高温环境中镍基高温合金具有良好的高温强度、抗氧化性能、抗腐蚀性能和抗疲劳性能,被广泛应用于航空航天等领域。镍基高温合金优异的综合性能与其微观组织紧密相关。综述了微量元素B, C, Y, Ce, Hf, Re, Ru, P对镍基高温合金微观组织及其力学性能的影响。针对不同的镍基高温合金,对微量元素的不同作用进行讨论分析。镍基高温合金微观组织及其力学性能与微量元素的含量及其分布有关。添加于镍基高温合金中的微量元素分布在合金基体或者其析出相中,通过偏聚于晶界处或者元素偏析等方式,改变合金的微观组织,从而影响其力学性能。     

Creep deformation and rupture behaviour of directionally solidified GTD 111 superalloy

The creep behaviour of directionally solidified (DS) Ni‐base superalloy GTD 111 has been investigated at various temperatures (649 °C to 982 °C) and stresses (124 MPa to 896 MPa). Specimens machined in longitudinal and transverse directions with respect to the grain orientation from three batches of the material were tested. The specimens in the longitudinal direction consistently exhibited higher creep rupture life and creep ductility than specimens from the transverse direction. There were some systematic variations in creep deformation and rupture behaviour among specimens from different batches. Optical and scanning electron microscopy investigations were conducted to understand the creep rupture behaviour. Various deformation and rupture models were evaluated for representing the creep behaviour of the alloy and a neural network model was applied to creep rupture data to assess its predictive capability.     

Effects of disoriented grains on the elastic constants of directionally solidified superalloys

Two models, namely, coaxial and non-coaxial, are proposed to estimate the elastic constants of directionally solidified superalloys, which behave like transversely isotropic materials and require five independent elastic constants. Coaxial model considers each grain as an individual and obtains the averaged values. Because of the longitudinal grain structure, three independent constants are carried over from original cubic single crystal and the other two are obtainable through the averaging process. For each disoriented direction in the non-coaxial model, a lumped grain, which behaves like a transverse isotropy is proposed. By assuming the disoriented angle follows Weibull distribution, non-coaxial model obtains the expectations of compliances from probability consideration. Disoriented effect could be simulated through the parameters of Weibull distribution. Experimental off-axis Young's modulus data are compared with numerical predictions by both models. Excellent agreement is observed between coaxial model and 90° off-axis experimental data. However, the coaxial model over predicts the 45° off-axis Young's modulus, because anisotropic coupling effect is very strong in the real off-axis specimens. As non-coaxial model considers the disoriented effect, excellent agreement is observed between non-coaxial model and 45° off-axis experiment data. Disoriented grain consideration reduces the anisotropic coupling effect and predicts better to the real off-axis specimens.     

Anisotropic viscoplasticity constitutive model for directionally solidified superalloy

Abstract

The macroscopic deformation behaviour of a Ni-based directionally solidified (DS) superalloy was experimentally investigated, and an anisotropic constitutive model of the material was developed. Monotonic and creep tests were performed on uniaxial test specimens machined from DS plates so that the angle between the loading direction and the solidified grain direction varied between 0 and 90°. Tension-torsion creep tests were also conducted to examine the anisotropic behaviour under multiaxial stress conditions. The material exhibited marked anisotropy under elastic and viscous deformation conditions, whereas it showed isotropy under plastic deformation conditions of high strain rates. Then crystal plasticity analyses were carried out to identify slip systems under creep loading conditions, assuming the anisotropic creep behaviour of the DS material. A viscoplastic constitutive model for expressing both the anisotropic elasticity-viscosity and the isotropic plasticity was proposed. The elastic constants were determined using a self-consistent approach, and viscous parameters were modelled by crystal plasticity analyses. The calculation results obtained using the constitutive model were compared with the experimental data to evaluate the validity of the model. It was demonstrated that the constitutive model could satisfactorily describe the anisotropic behaviour under uniaxial and multiaxial stress conditions with a given set of material parameters.     

Microstructure and Its Influence on Mechanical Behaviour of Directionally Solidified DZ38G Superalloy

The features of microstructure and theirinfluence on mechanical properties onstress-rupture as well as thermal fatigue ofdirectionally solidified DZ38G nickel-base su-peralloy have been investigated.It has beenfound that the contents of carbides on grainboundaries are increased and morphology of γ'-precipitates has been changed after testing.Particularly γ'-precipitates coalesced intoraft-like in morphology perpendicular to theapplied stress-axis have been observed thathas no detrimental effect on the stress-rupturebehaviour.The tendency of σ-formation hasalso been discussed.The σ-phase can be avoidedwhen the process parameters are chosenappropriately during solidification.     

Investigation of the crack growth behavior of Inconel 718 by high temperature Moiré interferometry

The effect of environment on creep crack growth behaviors of many nickel-base superalloys is a well-documented and serious problem. Stress accelerated grain boundary oxidation (SAGBO) is accepted as the prior mechanism of the environment effect. In this paper, the crack growth behavior of Inconel 718 was investigated by high temperature moiré interferometry (HTMI), coupled with SEM/EDAX. Based on the results obtained from this research, the mechanism is proposed to be caused by the segregated Nb, which couples with the oxygen diffusing into the grain boundaries in front of the crack tip and forms an NbO layer on the grain boundaries, thereby causing the brittle elastic cracking behavior.     

Creep analysis of directionally solidified GTD111 based on stress relaxation testing

A new approach to creep analysis based on short time high precision stress relaxation testing is applied to a directionally solidified nickel based superalloy, GTD111. For specimens taken in the longitudinal, transverse and diagonal orientations, it is shown that extensive data may be generated on a single specimen. The analysis, either in terms of stress vs creep rate, or in terms of stress vs predicted times for a specific creep strain, may be used directly in design. At low stresses and high temperatures the longitudinal orientation may offer the highest creep strength, but at high stresses and lower temperatures the diagonal and transverse orientations are stronger. This new methodology allows rapid evaluation of the consequences of microstructural evolution, rather than attempting to incorporate such changes in the actual test as in the conventional long time creep-rupture testing approach. For high strength cast superalloys, such as GTD111, the creep strength is not strongly affected by thermal exposures or moderate deformation at high temperatures. However, such exposures may lead to embrittlement which requires a separate test for evaluation. This decoupling of creep strength and fracture resistance allows enhanced efficiency in both alloy optimization and component life assessment.     

Creep fatigue of a directionally solidified Ni‐base superalloy – smooth and cylindrically notched specimens

Turbine blade life modelling is complicated by the presence of notches, dwells, high temperatures, thermal cycles and temperature gradients. Furthermore, directionally solidified (DS) Ni‐base superalloys are highly anisotropic. This work seeks to characterize the response of the DS Ni‐base superalloy CM247LC subjected to isothermal low cycle fatigue at either 750 or 950 °C. This study considers the effects of strain rate, dwells at the maximum temperature, and stress concentrations. Experiments were conducted under uniaxial loading on smooth and cylindrically notched round‐bar specimens in both longitudinal and transverse orientations. The location of the creep‐fatigue crack is at the maximum Hill's effective stress in the notched specimens. In addition, the notch behaviour is discussed in light of finite element analysis using an anisotropic elastic‐crystal viscoplastic material model.     

镍基单晶高温合金的再结晶

镍基单晶高温合金作为先进发动机叶片的主要用材,其再结晶问题日益受到重视.本文综述了热处理温度、热处理时间、变形程度及合金成分等多种因素对镍基单晶高温合金再结晶的影响规律,分析了镍基单晶高温合金再结晶对其蠕变和疲劳性能的影响,并讨论了回复处理及浸蚀直接去除表面变形层、渗碳和表面涂层等控制再结晶的方法.最后,指出了镍基单晶...     

Mechanical alloying – the development of strong alloys

Abstract

Mechanical alloying is a solid-state process for making alloys by high-energy milling, under conditions such that constituent powders are repeatedly fractured and welded together and ever more intimately mixed. After subsequent consolidation at elevated temperature, the alloys can be shaped by rolling, forging, and machining. The process is used to incorporate a fine dispersion of ceramic particles. Mechanically alloyed nickel-base superalloys, combining a dispersion of yttria with conventional precipitation strengthening, have achieved higher strength at 900–1100°C than directionally solidified and single-crystal alloys, and are being used for gas-turbine vanes and blades. Mechanically alloyed ferritic stainless steel, with outstanding strength and corrosion resistance at temperatures as high as 1300°C, has been produced as sheet, tube, plate, rings, and forgings. Mechanically alloyed aluminium alloys also offer higher strength, e.g. in as-forged thick sections of Al–Mg–Li alloy.

MST/567