Nickel-base Superalloy MAR-M247 Research Articles

Assessment of dwell-fatigue properties of nickel-based superalloy coated with a multi-layered thermal and environmental barrier coating

A three-layered experimental thermal and environmental barrier coating (TEBC) was deposited using air plasma spraying technology on cylindrical specimens of nickel-based superalloy MAR-M247. TEBC consists of mullite (Al6Si2O13) and hexacelsian (BaAl2Si2O8) upper layer, Y2O3 stabilised ZrO2 interlayer and CoNiCrAlY bond coat deposited on the grit-blasted surface of MAR-M247. The cyclic plastic response and damage mechanisms in uncoated and TEBC-coated MAR-M247 have been studied in isothermal dwell-fatigue tests conducted under strain control with constant strain amplitude at 900 °C. In each cycle, 5-minute dwells were introduced in both tensile and compression peaks of the hysteresis loop. Fatigue hardening/softening curves, stress relaxation curves, cyclic stress-strain curves and fatigue life curves are reported. Ceaseless mild softening has been found in both uncoated and TEBC-coated MAR-M247. TEBC-coated MAR-M247 showed an improved lifetime in the whole range of tested strain amplitudes. Data obtained from stress relaxation curves were used to assess the fraction of creep damage. The generalised damage accumulation rule was used to evaluate damage due to fatigue-creep-environment interaction. A study of the surface relief and internal microstructure using SEM and TEM helped to interpret the specifics of fatigue behaviour of uncoated and TEBC-coated material. The effectiveness of this newly developed TEBC, together with the dwell sensitivity of MAR-M247, were discussed from the perspective relevant to dwell-fatigue cyclic straining.

Thermomechanical and isothermal fatigue properties of MAR-M247 superalloy

Thermomechanical low-cycle fatigue (TMF) tests were performed on polycrystalline cast nickel-based superalloy MAR-M247 under in-phase (IP) and out-of phase (OP) loading in the temperature range of 500–900 °C. A constant heating and cooling rate of 5 °C.s−1 was selected. Furthermore, isothermal low-cycle fatigue (ILCF) tests with the same cycle period as TMF cycle were carried out at the maximal temperature of TMF cycle (900 °C). All tests ran under strain control and fully reversed cycle on solid cylindrical specimens in laboratory air. High resolution scanning electron microscopy and transmission electron microscopy were utilized to characterize the damage mechanism occurring during TMF and ILCF loading. The cyclic deformation and lifetime behaviour as well as damage mechanisms dependent on loading regime were examined. Results show that the TMF loading has a detrimental effect on the lifetime of MAR-M247. Lifetime reduction is most significant under IP loading. The fracture surfaces are characterized by typical striation fields, where the largest spacing between striations was observed for IP loading with prevailing intercrystalline cracking followed by OP and isothermal ILCF loading with transcrystalline crack propagation. To improve discussion of obtained results, experimental data were compared with damage and life prediction models.

Effect of Cooling Rate on the Microstructure and Mechanical Property of Nickel-Based Superalloy MAR-M247.

Heat treatment is an important process for optimizing the microstructures of superalloys, and the cooling rate after solid solution treatment is one of the most critical parameters. In this work, we treated solid solution MAR-M247 alloys with water quenching, air cooling, and furnace cooling. Microstructure characterization, hardness, and room temperature tensile tests were conducted to investigate the effect of cooling rate on the microstructure and mechanical properties of MAR-M247 alloys. The results showed that the cooling rate after solid solution treatment mainly affected the precipitation behavior of the secondary γ' phase, but it had few effects on other microstructure characterizations, including grain size, γ/γ' eutectic, and MC carbide. The water-quenched sample had the highest cooling rate (400 °C/s) and hardness (400 HV) but suffered from premature fracture because of quenching cracks. A further decrease in cooling rate from 1.5 °C/s to 0.1 °C/s deteriorated hardness (384 HV to 364 HV) and yield strength (960 MPa to 771 MPa) but increased elongation (8.5% to 13.5%). Moreover, the deformation mechanism was transformed from dislocation shearing to Orowan bypassing. The decreased yield strength was mainly due to the weakened precipitation strengthening resulting from γ'-phase coarsening. The improved elongation was attributed to not only the higher work-hardening index caused by interface dislocation networks but also the more uniform deformation, which delayed necking.

The low cycle fatigue behaviour of MAR-M247 superalloy under different strain rates and cycle shapes at 750 °C

High-temperature low-cycle fatigue experiments were performed at different strain rates on nickel-based superalloy MAR-M247. Furthermore, slow-fast, fast-slow and dwell-fatigue tests were conducted. The deformation, damage and lifetime behaviour as a function of strain rate and cycle shape were observed. Results show that the lifetime decreases with decreasing strain rate, indicating the negative influence of time-dependent creep and oxidation mechanisms. The lifetimes of slow-fast and dwell-fatigue tests are lower than those for fast-slow tests, although the mean stress is compressive in slow-fast and dwell-fatigue tests and tensile in fast-slow tests. Slow-fast and dwell-fatigue tests lead to grain boundary cavitation.

Densification, microstructure, and mechanical properties of heat-treated MAR-M247 fabricated by Binder Jetting

Abstract Additive Manufacturing (AM) enables the design of complex part geometries for high-temperature applications. Laser Powder Bed Fusion or Direct Energy Deposition of the nickel-based superalloy MAR-M247 poses a challenge in AM due to its poor weldability. Binder Jetting does not utilize a heat source and interaction with a liquid metal during build-up and thus has the potential to overcome this limitation. In this study, MAR-M247 was manufactured by Binder Jetting and subsequently characterized regarding density and microstructure. Combinations of two different Hot-Isostatic-Pressing (HIP) treatments (T = 1120–1180 °C, p = 1000–1500 bar, t = 4 h) and four different heat-treatments involving solution treatment (T = 1250 °C, t = 4 h) and different aging steps (T = 700–1000 °C, t = 12–24 h) were applied to study the densification and microstructural evolution of binder-jetted MAR-M247. The influence of the build direction in combination with HIP is studied concerning the resulting density and mechanical properties at room temperature. The results show that close-to-full densification can be achieved after HIP. Subsequent solution treatment and double-aging after HIP lead to a favorable bimodal microstructure. A process chain for binder-jetted MAR-M247 is presented, which yields tensile properties comparable to those of analogously post-processed cast material. Further possibilities of microstructural optimization and the design philosophy are discussed in the light of the Binder Jetting process-chain.

Fatigue Crack Initiation in Nickel-Based Superalloy MAR-M247 at High Temperature

The present work is focused on the study of crack initiation during low cycle fatigue (LCF) loading of the second generation nickel-based superalloy MAR-M247 treated with hot isostatic pressing. LCF tests were conducted on cylindrical specimens in symmetrical push-pull cycle under strain control with constant total strain amplitude and strain rate at 800 °C in air atmosphere. Selected specimens were electrolytically polished to facilitate surface relief observations. Crack initiation sites were studied by means of scanning electron microscopy (SEM) in dual beam microscope TESCAN LYRA 3 XMU FESEM equipped with focus ion beam (FIB). The microstructure of the material is characterised by coarse dendritic grains with numerous carbides and small casting defects. The average grain size was 2.1 ± 0.3 mm. Fractographic analysis revealed the fatigue crack initiation sites and their relation to the casting defects and material microstructure. Casting defects, carbide inclusions and interdendritic areas were found to be important crack nucleation sites. Specimens’ surface observations revealed the formation of pronounced surface relief with short worm-like markings. Fatigue crack initiation in these places is documented and discussed.

Effect of tensile dwell on high-temperature low-cycle fatigue and fracture behaviour of cast superalloy MAR-M247

Hot isostatically-pressed, nickel-based superalloy MAR-M247 was tested under fatigue and creep-fatigue conditions. Continuous cycling and cycling with 10min tensile dwells in each cycle were applied in symmetrical strain control cycle with constant total strain amplitude at 900°C in air. Cyclic hardening/softening curves, cyclic stress-strain curves and fatigue life curves were obtained. The introduction of tensile dwells resulted in lower fatigue life in total strain representation. Surface relief, specimen sections and fracture surface observations revealed crack initiation sites, crack density and crack paths, and help to clarify the effect of dwells on fatigue behaviour.

Microstructure of nickel-base superalloy MAR-M247 additively manufactured through scanning laser epitaxy (SLE)

Abstract Scanning laser epitaxy (SLE) is a laser powder bed fusion (LPBF) based additive manufacturing (AM) process developed for the repair and manufacture of gas turbine hot-section components made of nickel-base superalloys. In the present study, the single-pass fabrication of more than 1500 μm thick deposits of MAR-M247, a non-weldable superalloy atop similar chemistry substrates using a high-power laser beam is demonstrated. Metallurgical continuity is achieved across the entire deposit-substrate interface and the samples show little or no warpage across a broad range of processing parameters. In order to relieve the residual stresses and enable precipitation of the strengthening phases, the SLE-deposited MAR-M247 samples are subjected to a commercially available heat treatment process. Microstructures of the as-deposited and the heat-treated MAR-M247 samples are investigated using optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and Vickers microhardness measurements. The crack-free deposits obtained here for MAR-M247 represent one of the first few successes reported for a non-weldable alloy of its kind. The results demonstrate that the SLE process has significant potential for the AM-based repair of existing and fabrication of entirely new gas turbine hot-section components utilizing feedstocks of high γ′ content nickel-base superalloy powders. The results also establish that MAR-M247 is an attractive material for the LPBF-based AM processes.

Characterization of shear stresses in nickel-based superalloy Mar-M247 when orthogonal machining with coated carbide tools

Mar-M247 is a nickel-based alloy which is well known as difficult-to-machine material due to its characteristics of high strength, poor thermal diffusion and work hardening. Calculation of shear stress by an analytical force model to indicate the effect of coating material, cutting speed, feed rate on tool life and surface roughness was conducted experimentally. Cutting tests were performed using round inserts, with cutting speeds ranging from 50 to 300 m/min, and feed rates from 0.1 to 0.4 mm/tooth, without using cooling liquids. The behavior of the TiN and TiCN layers using various cutting conditions was analyzed with orthogonal machining force model. Cutting results indicate that different coated tools, together with cutting variables, play a significant role in determining the machinability when milling Mar-M247.

Lifetime, Cyclic Deformation and Damage Behaviour of MAR-M247 LC under Thermo-Mechanical Fatigue Loading with 0°, 180°, -90° and +90° Phase Shift between Strain and Temperature

Thermo-mechanical fatigue tests under 0° (in-phase), +180° (out-of-phase), +90° (clockwise diamond) and -90° (counterclockwise diamond) phase shift between mechanical loading and temperature were conducted on nickel-based superalloy Mar-M247 LC. Tests were carried out under total strain control with a temperature range of 100 – 850°C and a heating and cooling rate of 5K/s. Mechanical strain amplitudes were 0.3 to 0.6% with a strain ratio of Rɛ = -1. Results show, that for the strain amplitudes tested, lifetime varies significantly with strain-temperature phase. In-phase loading gives the shortest lives followed by out-of-phase and the diamond-phase loadings which show comparable lifetimes. Metallographic examination indicates that the dominant damage mechanisms are creep damage at higher temperatures and early cracking of oxide layers at lower temperatures. Both mechanisms occur primarily under tensile stress. Hence, the portion of each mechanism varies with the phase angle.

Microstructure and Oxidation Resistance of an Aluminide Coating on the Nickel Based Superalloy Mar M247 Deposited by the CVD Aluminizing Process

Abstract An investigation was conducted to synthesize βNiAl coating on the nickel based superalloy Mar M247 in a chemical vapor deposition process (CVD). The low activity process of aluminizing was carried out for 8 hours at the temperature 1050°C. Surface morphology and cross-section microstructure of the diffusion coating were studied and compared by using an optical microscope, a scanning electron microscope (SEM) equipped with energy dispersive spectroscopy and an X-ray diffractometer. It was found that aluminide coating with the thickness of 37 μm consisted of two layers: an outer one and the inner interdiffusion one. The outer layer consists of single phase βNiAl. The inner one, consisted of βNiAl phase and carbides: MC and M23C6 types which were originally present in the substrate. Cyclic oxidation test was performed at 1000°C for 1000h in the air atmosphere. The aluminized samples exhibited a small mass increase and the α- Al2O3 oxide formed during oxidation test had a good adherence to the coating. The decrease of aluminum content in the coating with the prolongation of the oxidation time and the phase transformation of βNiAl to γ’ Ni3Al and to γNi solid solution were observed. The samples without the coating showed a strong mass decrease in comparison to the coated samples.

Research on the Cutting Characteristics of Coated Tools on End Milling Nickel-Based Superalloy Mar-M247

Mar-M247 is widely used in industry for its excellent mechanical properties at high temperatures, but it has the shortcoming of difficulty manufactured. In order to obtain the cutting characteristics of Mar-M247, firstly, an end milling experiment was set up accordingly, where three types of cutting tools coated respectively by TiN, TiCN and TiAlN were employed. Then the parameters of cutting speed and feed rate were defined as the tool cutting variables. Finally, based on different cutting variables, the performance of tool wear, tool life, and workpiece surface roughness were analyzed and discussed. The results indicate when the tool coated by TiAlN, cutting speed of range 1600 ~ 3200 rpm and feed rate of range 0.06 ~ 0.08 mm / tooth are chosen together, the integrated states manufactured of the tool and the workpiece would be best, the method of this research can provide some references for studying others Nickel-based superalloys.

Quantitative evaluation of carbides in nickel-base superalloy MAR-M247

It has been established that carbides in superalloys serve three functions. Fine carbides precipitated in the matrix give strengthening results. Carbides also can tie up certain elements that would otherwise promote phase instability during service. Grain boundary carbides prevent or retard grain-boundary sliding and strengthen the grain boundary, which depends significantly on carbide shape, size and distribution. Various types of carbides are possible, depending on superalloy composition and processing. In the paper optical and scanning electron microscopy investigations of carbides occurring in specimens of the polycrystalline nickel-base superalloy MAR-M247 were carried out. Conditions of carbides revealing and microstructure images acquisition have been described. Taking into consideration distribution and morphology of the carbides in matrix a method of quantitative description of Chinese script-like and blocky primary carbides on the basis of image analysis was proposed.

Nonlocal Cyclic Life Prediction for Gas Turbine Components With Sharply Notched Geometries

Abstract The safe and efficient operation of modern heavy duty gas turbines requires a reliable prediction of fatigue behavior of turbine components. Fatigue damage is located in areas where cyclic stress and strain amplitudes are highest. Thus, geometrical notches associated with stress/strain concentrations and stress/strain gradients appear to be the most important sites for fatigue crack initiation. The paper addresses a nonlocal concept for cyclic life prediction of notched components. Contrary to various local approaches in the field, the proposed method explicitly accounts for stress and strain gradients associated with notches arising from grooves, cooling holes, fillets, and other design features with stress raising effect. As a result, empirical analytical expressions for considering either strain or stress gradients for cyclic life prediction are obtained. The method has been developed from cyclic test data on smooth and notched specimens made of a ferritic 1.5CrNiMo rotor steel. The analytical formulations obtained have then been applied to test data on the nickel base superalloy MAR-M247 CC showing a good agreement between prediction and measurement. Moreover, the proposed nonlocal lifing concept has been validated by component tests on turbine blade firtrees. The predicted number of cycles to failure correlates well with the experimental results showing the applicability of the proposed method to complex engineering designs.

Influence of solidification variables on the microstructure, macrosegregation, and porosity of directionally solidified Mar-M247

The solidification microstructure is critical in determining the amount and distribution of porosity that develops during the freezing of castings. As the solidification velocity V s increases, the microstructural length scales (primary and secondary dendrite arm spacings) decrease; thus (1) the mushy zone permeability would be expected to decrease; and (2) nucleated pores would be increasingly isolated. Although the first effect would tend to increase the observed porosity, the second effect would tend to decrease the porosity. To better understand these competitive mechanisms, a series of controlled unidirectional solidification experiments were performed on bars of nickel-base superalloy Mar-M247. Samples were produced with constant dendrite arm spacing throughout an extended length of each cast bar. The axial thermal gradient and withdrawal velocity imposed on each casting were varied between castings to produce a range of microstructures from aligned cellular dendritic to aligned dendritic to mis-aligned dendritic. Macrosegregation effects along the lengths of the bars were evaluated and the resultant impact upon the density along the length of each casting was also characterized. The density measurements were found to be very sensitive to both (1) compositional macrosegregation in these castings and (2) internal porosity. Statistical analyses of microporosity in the castings were based upon metallographic measurements. The development of microporosity in the unidirectionally solidified castings is shown to be dependent upon the hydrogen gas content of the samples and the imposed solidification velocity through the sample's cast microstructures. An optimum intermediate withdrawal velocity of 0.005–0.01 cm s −1 was found, which led to closely spaced dendrite arms, a large number of very small pores and a minimum total porosity level. Lower velocities lead to increased porosity from larger pores whereas higher velocities lead to macroporosity due to centerline shrinkage.

The effect of pack-aluminisation on the microstructure of MCrAlY and the performance of thermal barrier coatings

Abstract Plasma spraying and pack-aluminising processes were combined and applied to the nickel-base superalloy Mar-M247 to improve its cyclic oxidation resistance. The performance tests of duplex ZrO 2 -8 wt.%Y 2 O 3 /MCrAlY thermal barrier coatings (TBCs) were conducted at 1050 °C, 1075 °C, 1100 °C, 1150 °C and 1200 °C. The results of the experiments in this study showed that TBC specimens with the aluminised MCrAlY bond coat exhibited higher cyclic lives (except for the Ni-22Cr-10Al-1Y bond coat), at all the temperatures tested, than specimens on which the bond coat was not aluminised. The microstructures of the Co-29Cr-6Al-1Y, Co-32Ni-21Cr-8Al-0.5Y and Ni-22Cr-10Al-1Y bond coats with or without aluminising treatment were examined in detail using a scanning electron microscope equipped with an electron probe microanalyzer.

The morphology and formation of gamma prime in nickel-base superalloy

The major aim of this work was concentrated on studying the formation and morphology of γ'-particles under solution treatment. Microstructural observations were made mainly with the use of scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM). The experimental material was commercially available nickel-base superalloy MAR-M247 with the following chemical composition (in wt%): 8.4 chromium, 10.0 cobalt, 10.0 tungsten, 0.7 molybdenum, 3.0 tantalum, 5.5 aluminium, 1.0 titanium, 1.5 hafnium, 0.15 carbon, 0.05 zirconium, balance nickel

The influence of cobalt on the tensile and stress-rupture properties of the nickel-base superalloy mar-m247

The influence of cobalt on the mechanical properties of MAR-M247, a cast nickel-base superalloy, was investigated. Nickel was substituted for cobalt to produce 0, 5, and the standard 10 pct cobalt versions of MAR-M247. Tensile tests were performed between 649 and 982 dgC; stress-rupture tests were conducted at temperatures ranging from 760 to 982 dgC. The tensile properties were not significantly affected by cobalt level, but a slight peak in strength at 5 pct Co was apparent. A -80 °C shift in the peak yield strength temperature as Co level was reduced from 10 to 0 pct was also evident. This behavior was related to a reduction in the γ′ volume fraction, an increase in γ′ particle size, an increase in W and Ti concentrations in the γ′, and a decrease in Cr and Al concentrations in the γ as Co level in MAR-M247 was reduced. Stress-rupture properties, however, were more significantly affected by Co level. The 10 pct Co alloy exhibited rupture lives typically 1.2 times greater than that of the 5 pct Co alloy and 3 times greater than that of the 0 pct Co alloy. The steady state creep rate of the 10 pct Co alloy was generally equal to that of the 5 pct Co alloy, but was only one third as large as the creep rate of the 0 pct Co alloy. This behavior was associated with a decrease in γ′ volume fraction and the formation of a grain boundary carbide film as cobalt level was reduced.