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.     

Effect of high-temperature hot corrosion on the low cycle fatigue behavior of a directionally solidified nickel-base superalloy

The low cycle fatigue behavior of a directionally solidified nickel-based superalloy DZ125 was examined at 850 °C in air using bare and salt-coated specimens. Experimental results show that the salt-coated specimen showed relatively low fatigue life compared with the bare specimen, and this effect accelerated with the increased applied maximum stress. Damage of hot corrosion in fatigue life was found to be associated with the reduction of the bare area and the early crack initiation from the weaken grain boundaries of recrystallized grains.     

Microsegregation in directionally solidified dendritic-cellular structure of superalloy CMSX-4

A directionally solidified sample of superalloy CMSX-4 was investigated to show the effect of crystal orientation on the segregation distribution. The solute distribution of alloying elements across a dendritic cell was measured. Due to the preferred crystal growth in <100> orientation the segregation profiles in this direction is much flatter than that in <110> orientation.     

The development of the “necklace” structure in a powder-produced nickel-base superalloy

This paper describes the nucleation and propagation of recrystallization in a powder-produced nickel-base superalloy. It is shown that recrystallization is initiated by a sub-grain coalescence mechanism, on coarse γ′ precipitates associated with pre-existing grain boundaries, leading to the formation of the so-called “necklace” structure. No intragranular nucleation of recrystallization is observed, owing to the inherent stability of the recovered matrix regions. It is also shown that the networks of MC-type carbides which often delineate the prior-particle boundaries do not radically affect the recrystallization process.     

Microstructural characterization of rapidly solidified nickel-base superalloys

Several Ni-Al-Mo-based eutectic superalloys were rapidly solidified using a chilled block melt spinning process. The effects of rapid solidification on the microstructure were studied using optical microscopy, transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM). Results showed, except for the alloy containing chromium, that the microstructure varied as a function of ribbon thickness from segregationless solidification at the wheel side of the ribbon to dendritic solidification at the free side. In addition, alloys with the same solidification rate showed a large variation in microstructure depending upon the solid state cooling rate. The rapidly solidified eutectic Ni-Al-Mo alloy with a small amount of rhenium and vanadium did not show any improvement on delaying or prohibiting the formation of the embrittling-NiMo phase on ageing at 1000 C. This was determined from microstructural as well as chemical analysis using STEM. Differential thermal analysis was used to obtain melting temperature,-Ni₃Al solvus, and heat of formation for the alloys.     

Research and Improvement on structure stability and corrosion resistance of nickel-base superalloy INCONEL alloy 740

INCONEL alloy 740 is a newly developed Ni–Cr–Co–Mo–Nb–Ti–Al superalloy in the application to ultra-supercritical boilers with steam temperatures up to 700 °C. By means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), micro-chemical phase analyses, and corrosion-resisting test, this paper investigates the structure stability of the alloy at elevated temperature and concentrates on coal ash corrosion performance of the alloy under the simulated coal ash/flue gas condition. Experimental results show that the most important structure instabilities of the alloy during prolonged aging are γ′ coarsening, γ′ to η transformation and G phase formation at grain boundary. The performance of corrosion resistance of the alloy would meet the requirement of ultra-supercritical boiler tubes. The phase computation by means of Thermo-Calc has been adopted in chemical composition modification for structure stability improvement. Two suggested new modified alloys in adjustment of the Al and Ti contents and in control of Si level, and also in maintenance of Cr content of the alloy were designed and melted for experimental investigation. These two modified alloys exhibit more stable microstructure during 760 °C long time aging.     

Temperature effects on fatigue thresholds and structure sensitive crack growth in a nickel-base superalloy

Fatigue thresholds and slow crack growth rates have been measured in a powder formed nickel-base superalloy from room temperature to 600°C. Two grain sizes were investigated: 5–12 μm and 50 μm. It is shown that the threshold increases with grain size, and the difference is most pronounced at room temperature. Although crack growth rates increase with temperature in both microstructures, the threshold is only temperature dependent in the material with the larger grain size. It is also only in the latter that the room temperature threshold falls when the load ratio is increased from 0.1 to 0.5. At 600°C the higher load ratio causes a 20% reduction in the threshold irrespective of grain size.The results are discussed in terms of surface roughness and oxide-induced crack closure, the former being critically related to the type of crystallographic crack growth, which is in turn shown to be both temperature and stress intensity dependent.     

Analysis of second phase particles in a powder metallurgy HIP nickel-base superalloy

The nature of the second phase particles associated with LC Astroloy prepared using powder metallurgy techniques has been examined. The individual particles have been identified using energy dispersive X-ray analysis and convergent beam electron diffraction. Four distinct types of particles have been observed: a cubic MC carbide in which M is either titanium or titanium plus molybdenum, a monoclinic phase ZrO₂, a trigonalα-Al₂O₃ and a tetragonal M₃B₂ phase in which M is molybdenum or molybdenum and chromium. The observations indicate that, although some MC carbides are associated with the ZrO₂ phase, the majority of the prior particle boundary precipitates are separate entities. Hot isostatic pressing or subsequent heat treatments above or below the γ′ solvus were observed to have little effect on the incidence or distribution of the precipitation associated with the prior particle boundaries. In contrast, heat treatments above the γ′ solvus resulted in the dissolution of the M₂B₃ phase and its preferential precipitation on the grain boundaries.     

Study on the castability and mechanical properties of thin-walled nickel-base superalloy

The castability and mechanical properties of thin-walled nickel-base superalloy castings fabricated by gravity casting and centrifugal casting were investigated. It is shown that, despite its microporosity has slight change, the casting fabricated by centrifugal casting presents fewer misruns and less Laves phase than that of gravity casting. The ultimate tensile strength, yield strength and stress rupture lives of centrifugal casting are improved by about 2.2, 7.4 and 41.1%, respectively. However, the elongation (EL) and reduction in area are sharply decreased by about 10.9 and 25.0%, respectively. Paradoxically, the stress ELs of both castings are similar. The seemingly contradictory results in mechanical properties stem from the opposite effects of centrifugal force on hardening elements macrosegregation and melt turbulence.     

Thermomechanical processing of a nickel-base superalloy powder compact

Some means by which the grain size and flow strength of nickel-base superalloy powder .compacts can be controlled during processing have been examined. The compacts were prepared by hot isostatic pressing and their flow behaviour studied by constant true strain rate compression testing under conditions simulating isothermal forging. It was found that the selection of a finer starting powder results in slight lowering of flow strength at forging temperatures. Prior cold working of the compact by hydrostatic extrusion also leads to small reductions in strength at these temperatures. Both the . grain size and flow strength may change substantially during plastic deformation at forging temperatures. This depends on the conditions of strain rate and temperature. Under all the testing conditions examined this transient flow behaviour is followed by a stesdy state regime of flow during which neither the grain size nor the flow strength continue to change. These observations and their practical implications are discussed.     

High temperature fatigue of the nickel-base single-crystal superalloy CMSX-10

Push–pull fatigue tests were conducted under a sinusoidal stress waveform with a frequency of 1 Hz and a trapezoidal one with a hold time in both tension and compression at 300 MPa-amplitude. Tests were conducted at a temperature of 1273 K using smooth bar specimens of the nickel-base single-crystal superalloy CMSX-10. Small cracks were observed on the surface of the interrupted specimens by means of optical and scanning electron microscopes and their number and length were measured. The fatigue behaviour was characterized as follows: (1) A number of small cracks were initiated at a relatively early stage on the grain boundaries of the surface oxide which were perpendicularly to the tensile stress axis direction. (2) Some of these cracks grew inside and reached the base metal. Their growth brought about final fracture of the specimen. (3) The creep strain during the stress hold period accelerated the growth rate of the small cracks and shortened the fatigue life.     

Creep of PE-10 nickel-base superalloy at 973 K

Creep data, at 973 K and stresses between 355 and 512 MPa, in flat specimens of PE-10 nickel-base superalloy are reported. The data have been interpreted in terms of a constitutive equation based on a creep model involving dislocation climb and cross-slip over the strengthening phase. Strain-rate sensitivity and apparent activation energy have also been measured and analysed in the frame of the proposed model.     

镍基铸造高温合金K24的切削温度实验研究

通过实验,分析了使用YW1刀具切削K24时,切削用量对切削温度的影响。对切削温度影响最大的是切削速度,但是,随着切削速度的增加切削温度的增加量是下降的。这是因为在切削过程中,速度的增加使得副后刀面磨损加剧,切削深度减小,切削力减小,切削温度也随着相对降低。切削用量对切削温度的影响其次是进给量,影响最小的是切削深度。通过实验所得的切削用量与切削温度的经验公式,将这3个公式合并可得到切削用量对切削温度的总的经验公式。     

The creep fracture characteristics of nickel-base superalloy sheet samples

Center notched sheet samples of INCONEL alloy 718 with thicknesses from 0.5 to 3.1 mm were creep tested in the 595–704°C temperature range. No significant effect of thickness was observed in terms of failure times, minimum stress intensities needed to initiate crack growth, or fracture mode. Flat, intergranular fractures with no shear lips and no detectable change in thickness were observed in all cases. The results indicate that with materials such as nickel-base superalloys tested at half their melting point, creep fractures can be analyzed by fracture mechanics techniques.     

Statistical modeling of fatigue-crack growth in a nickel-base superalloy

Two fracture mechanics-based statistical models for the fatigue crack propagation of engine materials are developed. The models are applied to fatigue crack growth rate data for IN 100, a nickel-based superalloy used in F100 engine disks, at various elevated temperatures, loading frequencies and stress ratios. The lognormal and the randomized parameter models allow the incorporation of the statistical variability associated with crack growth data into the life prediction process. The statistical distributions of (i) the crack growth rate, (ii) the propagation life to reach a given crack size (iii) the crack size at any given service life have been derived. A correlation analysis is performed to compare the results of the statistical models with test data. The correlation is demonstrated to be very good.