Ni-base Superalloy CM247LC Research Articles

Comparison of erosion rate of EB-PVD and plasma sprayed TBC

ABSTRACT A thermal barrier coating (TBC) consisting of platinum-aluminide bond coat and 7wt-% yttria stabilized zirconia (7YSZ) ceramic top coat deposited by electron beam physical vapour deposition (EB-PVD) system cast Ni-base superalloy CM-247LC. The erosion behaviour of the EB-PVD and air plasma sprayed (APS) TBCs are compared. The APS TBC consists of 7YSZ top coat and CoNiCrAlY bond coat on Inconel 738 substrate. Erosion responses of TBCs have been examined as a function of temperature. The erosion rate of EB-PVD TBC was significantly lower than that of APS TBC. Erosion rate of EB-PVD TBC decreased with test temperature. APS TBC exhibits mostly temperature independent erosion rate. The material removal mechanism of EB-PVD TBC was by the formation of parallel cracks in the ceramic layer close to its surface and their subsequent joining. The material is removed by propagation of subsurface cracks along the splat boundaries in case of APS TBC.

Extraordinary Deformation Modes in Ni-Base Superalloy CM247LC at Room Temperature

Extraordinary Deformation Modes in Ni-Base Superalloy CM247LC at Room Temperature

Strain-rate insensitive yield strength and deformation mechanisms of Ni-base superalloy CM247LC at 600 °C

The compression deformation behavior and deformation mechanisms of [001]-oriented single crystal CM247LC are investigated at 600 °C and at the strain rate of 5 × 10−6 to 5 × 10−3 s−1. We found that although the Protevin-Le Châtelier effect appears during plastic deformation under the investigated strain rates, its yield strength is strain-rate insensitive. Transmission electron microscopy observations on the slightly deformed specimens reveal that in contrast to previous findings, shearing of γ′ precipitates is accomplished by pairs of a/2 <110>dislocations as well as partial dislocations, which result in the formation of superlattice stacking faults and extended stacking faults, in the experimental alloy at the early stage of plastic deformation at all the strain rates. Meanwhile, it is also found that the frequency of occurrence of the two shear processes is not influenced by the strain rate. Based on these observations, the formation of stacking faults and the relationship between operative deformation mechanisms and the yield strength of the experimental alloy are discussed.

Influence of Hf on the heat treatment response of additively manufactured Ni-base superalloy CM247LC

The influence of Hf on the heat treatment response of a Laser Powder Bed Fusion (L-PBF) fabricated Ni-base superalloy was investigated. A Hf-free version of CM247LC, CM247LC NHf, and CM247LC (~1.4 wt% Hf) were subjected to identical thermal treatments and characterized. After solutionizing at 1260 °C for 2 h, both alloys displayed recrystallization and grain growth. The CM247LC NHf had numerous annealing twins suggesting that Hf inhibits twin formation. The CM247LC NHf alloy displayed a lower peak hardness than CM247LC (5000 MPa and 5200 MPa, respectively) and this was attributed to a ~6% point reduction (Calphad calculated) in γ′ phase fraction due to the removal of Hf. Despite the lower hardness values, the CM247LC NHf displayed a higher yield strength than CM247LC at 760 °C (969 MPa and 926 MPa, respectively). Elongation at fracture at 760 °C was similar (~1%) for both CM247LC and CM247LC NHf. The removal of Hf was shown to be effective at mitigating the strain age cracking susceptibility of CM247LC, likely due to the decrease in γ’ phase fraction and lattice mismatch.

Characterization of microstructural changes due to prolonged thermal exposure of directionally solidified Ni-base super alloy CM 247LC using ultrasonic

The high temperature strength of directionally solidified Ni-base super alloy CM 247LC strongly depends on the morphology, volume fraction, size and size distribution of γ′ precipitate (Ni3Al) in the FCC γ matrix. The microstructure of the alloy is engineered to achieve the right combination of these parameters that provides the required high temperature strength and creep resistance. The alloy contains high volume fraction of coherent γ′ precipitates having near cubic shape. High temperature exposure of gas turbine components made out of the alloy leads to coarsening of the γ′ precipitates and broadening of the γ matrix channel. This in turn, adversely affects the high temperature mechanical properties of the alloy. The present study endeavours to non-destructively characterize such detrimental changes in the microstructure that controls the mechanical properties and limits the life of components. The microstructural changes of the fully heat treated alloy exposed at 980 °C for different hours (100–1200) of thermal exposure have been characterized using ultrasonic methods. Changes in microstructural parameters due to different hours of thermal exposure have been correlated with changes in ultrasonic velocity, ultrasonic attenuation coefficient and second order acoustic nonlinearity parameter. It is observed that the change in attenuation is predominantly by absorption of the ultrasonic wave due to dislocation damping in the γ channels. Nonlinear ultrasonic parameter changes with thermal exposure predominantly due to the alteration of dislocation precipitate interaction. A dislocation precipitation interaction model for ultrasonic wave distortion has been used to explain the observed variation in nonlinear parameter. A microstructural parameter has been identified that varies in a similar way as ultrasonic attenuation and second order ultrasonic parameter. It is shown that variations in the acoustic non-linearity parameter follow the trend more closely with the identified microstructural parameter.

Law of fracture life under creep–fatigue interactive conditions for Ni-base directionally solidified superalloy based on non-equilibrium science (the effect of stress holding time)

Ni-base directionally solidified superalloy are used under complex conditions of high temperature creep and fatigue interaction. Under these conditions, the characteristics of load frequency for fracture life show a nonlinear behavior, and these characteristics show various features depending on material, temperature and stress holding time. In this study, crack growth tests were conducted under creep-fatigue interactive conditions with stress holding time, tH using the in-situ observational system for directionally solidified Ni-base superalloy CM247LC. To characterize the load frequency of fracture life for CM247LC, the separate estimation of cyclic dependent from time dependent mechanisms was conducted. Additionally, a developed representation of the characteristic of fracture life under the conditions of creep fatigue interaction based on non-equilibrium science and chaos theory was proposed. Furthermore, verification of the established law of predicting the fracture life under creep-fatigue interactive conditions was conducted.

니켈계 초내열합금 CM247LC의 일방향응고 시 미세조직 형성거동 분석

The Ni-base superalloy CM247LC was directionally solidified (DS) using the Bridgman-type furnace to understand the effect of the chill plate on the microstructural evolution, such as dendrite arm spacing, microporosity, and MC-type carbide. The DS process was also modeled by the PROCAST to predict the solidification rate, thermal gradient, and resultant cooling rate in the entire length of the DS specimen. Due to the quenching effects of chill plate, four distinct areas were found to form in the specimen, in which the solidification rate was changed, during DS at a given withdrawal rate of 0.083 mm/s. Among the microstructural features investigated, the dendrite arm spacings and average size of the MC-type carbide near the chill plate were found to be influenced by the quenching effect of the chill plate. However, no significant influence was found on the size and volume fraction of microporosity, and the volume fraction of the MC-type carbide. The relationship between the microstructural features and the solidification variables was also analyzed and discussed on the basis of a combination of experimental and modeling results.

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 so- lidified (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.

Evaluation of Thermal Aging Embrittlement in Directionally Solidified Ni-Base Superalloy by Small Punch Test

In order to develop a procedure for evaluating the degradation of impact toughness in directionally solidified Ni-base superalloy CM247LC, which is commonly used for advanced gas turbine blades, the change in small punch (SP) fracture energy due to thermal aging has been investigated. The SP testing technique has been applied to materials aged under various aging conditions, and correlation with the results of Charpy V-notch impact tests has been examined. The experimental results reveal that SP fracture energy at room temperature decreases with aging at 800 °C and is uniquely correlated with high-temperature Charpy impact toughness. The current experiment has shown that the SP testing technique is useful in evaluating the degree of thermal aging embrittlement, one of the parameters required for remaining-life prediction of aged components.

Creep Life Prediction of Ni-Base Superalloy Used in Advanced Gas Turbine Blades by Electrochemical Method.

In order to develop a methodology for creep life assessment of directionally solidifed Ni-base superalloy CM247LC, commonly used in advanced gas turbine blades, changes in electrochemical property of the alloy caused by creep have been investigated. Experimental results on electrochemical polarization measurements revealed that the peak current densities Ip and Ipr, which appeared at a specific potential during potentiodynamic polarization reactivation measurements in a dilute glyceregia solution, increased linearly with the life fraction at an early stage of creep life and were uniquely correlated with a newly proposed Arrhenius-type parameter (t/tr) exp (-Qc/RT). The creep life fraction can be nondestructively evaluated by electrochemical polarization measurements and the above parameter.

The role of HCl-impurities in the environment and Y for the high temperature corrosion of CoNiCrAl(Y)-coatings on a Ni-base-superalloy under complex loading conditions

The influence of small concentrations of HCl in synthetic air on the high-temperature corrosion of an Y-free and an Y-modified CoNiCrAl-coating on the directly solidified Ni-base superalloy CM247LC DS was investigated at 1,000°C under isothermal and thermocyclic conditions, accompanied by acoustic emission measurements. Furthermore, thermomechanical fatigue tests were carried out. Both coatings formed an Al2O3-layer. Y had a positive influence on the scale adherence and changed the growth direction of the Al2O3-scale. Y-modified coatings showed substantial internal oxidation. Pore formation at the oxide/metal interface took place in the case of the Y-free coating during oxidation in synthetic air as well during oxidation in HCl-containing atmospheres leading to a decrease of scale adherance. No crack closure by oxidation was observed under thermomechanical fatigue conditions in the case of Y-modified coatings on the alloy CM247LC DS. For the Y-free coating in HCl-containing atmospheres scale damage could be detected by accoustic emission measurements even under isothermal oxidation.

Creep Life Prediction of Ni-Base Superalloy Used in Advanced Gas Turbine Blades by Electrochemical Method.

In order to develop a methodology for creep life assessment of directionally solidifed Ni-base superalloy CM247LC, commonly used in advanced gas turbine blades, changes in electrochemical property of the alloy caused by creep have been investigated. Experimental results on electrochemical polarization measurements revealed that the peak current densities Ip and Ipr, which appeared at a specific potential during potentiodynamic polarization reactivation measurements in a dilute glyceregia solution, increased linearly with the life fraction at an early stage of creep life and were uniquely correlated with a newly proposed Arrhenius-type parameter (t/tr) exp (-Qc/RT). The creep life fraction can be nondestructively evaluated by electrochemical polarization measurements and the above parameter.