Isothermal fatigue of an aluminide-coated single-crystal superalloy: Part II. effects of brittle precracking

The effect of brittle coating precracking on the fatigue behavior of a high-activity aluminide-coated single-crystal nickel-base superalloy has been studied using hollow cylindrical specimens at test temperatures of 600 °, 800 °, and 1000 °. Three types of precrack were studied: narrow precracks formed at room temperature, wide precracks formed at room temperature, and narrow precracks formed at elevated temperature. The effect of precracking on fatigue life at 600 ° was found to depend strongly on the type of precrack. No failure was observed for specimens with narrow room-temperature precracks because of crack arrestvia an oxidation-induced crack closure mechanism, while the behavior of wide precracks and precracks formed at elevated temperature mirrored the non-precracked behavior. Crack retardation also occurred for narrow room-temperature precracks tested at 800 °—in this case, fatigue cracks leading to failure initiated in a layer of recrystallized grains on the inside surface of the specimen. A significant reduction in fatigue life at 800 ° relative to non-precracked specimens was observed for wide precracks and elevated temperature precracks. The presence of precracks bypassed the initiation and growth of coating fatigue cracks necessary for failure in non-precracked material. No effect of precracking was observed at 1000 °. Formerly Research Student, Department of Materials Science and Metallurgy, University of Cambridge. Formerly Lecturer, Department of Materials Science and Metallurgy, University of Cambridge CB2 3QZ, United Kingdom.     

Fatigue behavior of SiC reinforced Ti(6AI-4V) at 650 °C

Axial, low cycle fatigue properties of 25 and 44 fiber vol pct SiC/Ti(6Al-4V) composites, measured at 650 °C, were compared with the fatigue properties of unreinforced Ti(6Al-4V) at the same temperature. A prior study of the fatigue behavior of this composite system at room temperature indicated that the SiC fiber reinforcement did not provide the anticipated improvement of fatigue resistance of this alloy. At 650 °C, the composite fatigue properties degraded somewhat from those at room temperature. However, these properties degraded more for the unreinforced matrix at 650 °C with the result that the composite fatigue strength was two to three times the fatigue strength of the matrix alloy. The reasons for this reversal are discussed in terms of crack initiation at broken fibers and residual matrix stresses.     

Tensile and fatigue properties of 17-4 PH stainless steel at high temperatures

The tensile and high-cycle fatigue properties for 17-4 PH* stainless steels in three different conditions were investigated at temperatures ranging from room temperature to 400 °C. Results indicated that the yield strength and fatigue strength for the three conditions at a given temperature took the following order: condition H900 > condition A> condition H1150. The yield strength of each condition decreased with increasing temperature except for condition A, which was tested at 400 °C with longer hold times, where a precipitation-hardening effect took place. The S-N curves showed that the fatigue strengths of each condition in the short-life regime were decreased with an increase in temperature. In the long-life regime, the fatigue strengths of condition A at 400 °C were greater than those at lower temperatures as a result of an in-situ precipitation-hardening effect. The fatigue strengths of condition H900 in the long life regime at 300 °C were superior to those at lower temperatures, due to the mechanisms of surface oxidation and thermal activation of dislocations. Fractography observations indicated that a shift of fatigue fracture from surface to internal crack initiation occurred at higher temperatures (300 °C and 400 °C) with long fatigue lives.     

The influence of overlay coatings on high cycle fatigue of an advanced tantalum carbide strengthened eutectic composite

High cycle fatigue of coated and uncoated NiTaC 3-116A2, a directionally solidified tantalum carbide strengthened eutectic, has been examined over a range of temperatures. In the uncoated condition, the fatigue strength of the alloy increased with temperature from room temperature to 600 °C, but was slightly lower at 900 °C than at 600 °C, which is consistent with the variation in 0.2 pct yield strength with temperature. At room temperature and 600 °C, the influence of several plasma sprayed coatings in reducing the fatigue life was found to be sensitive to the coating composition and alternating stress level. A Ni-20Cr coating resisted fatigue damage on the basis of its ductility and therefore performed better at high cyclic stress levels than at low cyclic stress levels. By contrast, Ni-20Cr-10Al-2Hf-0.1C. and Ni-20Co-20Cr-10Al-2Hf-0.1C coatings were stronger and so exhibited high fatigue strengths at long lives but, because of their limited ductility, cracked readily when tested at high stress levels. At 900 °C the coatings had little effect on fatigue life. On the basis of the present results and the metallographic observations of early crack growth through the coatings, it is concluded that the selection of a mechanically compatible coating requires a knowledge of the fatigue cycle. To ensure a minimum effect on fatigue lives for the anticipated stress cycle, it is necessary to match the coating and substrate mechanical properties as closely as possible. Formerly with Corporate Research and Development     

Fatigue of Ni-Ai-Mo aligned eutectics at elevated temperatures

The elevated-temperature mechanical behavior of two aligned eutectics (Ni-8.1 wt pct Al-26.4 wt pct Mo and Ni-6.3 wt pct Al-31.2 wt pct Mo) has been investigated utilizing monotonic and cyclic testing in vacuum. Tensile yield strength and fatigue resistance increased from 25 to 725 °C, but then were reduced at 825 °C. The fatigue lives of specimens tested at 725 °C decreased sharply with decreasing frequency. A shift from surface to internal crack initiation was observed upon increasing the test temperature from 725 to 825 °C. Stage II crack propagation was observed at both temperatures, in contrast to stage I cracking at 25 °C. The test results are compared to those for other nickel and cobalt-base aligned eutectics to show that the frequency effect on fatigue life is not limited to the Ni-AI-Mo system. formerly Graduate Assistant in the Department of Materials Engineering, Rensselaer Polytechnic Institute     

Isothermal fatigue of an aluminide-coated single-crystal superalloy: Part I

The isothermal fatigue behavior of a high-activity aluminide-coated single-crystal superalloy was studied in air at test temperatures of 600 °, 800 °, and 1000 °. Tests were performed using cylindrical specimens under strain control at ≈0.25 Hz; total strain ranges from 0.5 to 1.6 pct were investigated. At 600 °, crack initiation occurred at brittle coating cracks, which led to a significant reduction in fatigue life compared to the uncoated alloy. Fatigue cracks grew from the brittle coating cracks initially in a stage II manner with a subsequent transition to crystallographic stage I fatigue. At 800 ° and 1000 °, the coating failed quickly by a fatigue process due to the drastic reduction in strength above 750 °, the ductile-brittle transition temperature. These cracks were arrested or slowed by oxidation at the coating-substrate interface and only led to a detriment in life relative to the uncoated material for total strain ranges of 1.2 pct and above 800 °. The presence of the coating was beneficial at 800 ° for total strain ranges less than 1.2 pct. No effect of the coating was observed at 1000 °. Crack growth in the substrate at 800 ° was similar to 600 °; at 1000 °, greater plasticity and oxidation were observed and cracks grew exclusively in a stage II manner. Formerly Research Student, Department of Materials Science and Metallurgy, University of Cambridge. Formerly Lecturer, Department of Materials Science and Metallurgy, University of Cambridge CB2 3QZ, United Kingdom.     

Fatigue Strength and Crack Initiation Mechanism of Very-High-Cycle Fatigue for Low Alloy Steels

The fatigue strength and crack initiation mechanisms of very-high-cycle fatigue (VHCF) for two low alloy steels were investigated. Rotary bending tests at 52.5?Hz with hour-glass type specimens were carried out to obtain the fatigue propensity of the test steels, for which the failure occurred up to the VHCF regime of 10⁸ cycles with the S-N curves of stepwise tendency. Fractography observations show that the crack initiation of VHCF is at subsurface inclusion with ??fish-eye?? pattern. The fish-eye is of equiaxed shape and tends to tangent the specimen surface. The size of the fish-eye becomes large with the increasing depth of related inclusion from the surface. The fish-eye crack grows faster outward to the specimen surface than inward. The values of the stress intensity factor (K _I ) at different regions of fracture surface were calculated, indicating that the K _I value of fish-eye crack is close to the value of relevant fatigue threshold (??K _th ). A new parameter was proposed to interpret the competition mechanism of fatigue crack initiation at the specimen surface or at the subsurface. The simulation results indicate that large inclusion size, small grain size, and high strength of material will promote fatigue crack initiation at the specimen subsurface, which are in agreement with experimental observations.     

The thermal fatigue behavior of the combustor alloys In 617 and HAYNES 230 before and after welding

The nickel-base alloys IN 617 and HAYNES 230 for welded high-temperature components have been subjected to thermal fatigue (TF) loading. In a series of TF tests in air, single wedge specimens were induction heated and compressed-air cooled at the leading edge for various temperature cycles between 200 °C and either 850 °C, 950 °C, or 1050 °C. The test rigs permitted simultaneous measurements of temperature and total strain along the edge of specimen during TF cycling. Both materials have been tested in conditions relevant for hot path components in the gas turbines, e.g., “as delivered,” “welded,” and “welded + notched”. Under identical temperature cycles and thermal gradients, HAYNES 230 showed a higher TF strength than IN 617 in the as-delivered condition. It is suggested that this advantage of HAYNES 230 is primarily related to its lower value of the relevant combination of properties of this alloy: coefficient of thermal expansion, thermal conductivity, elastic modulus, ultimate tensile strength, taken at maximal operating temperature. In addition, the advantage of the HAYNES 230 is described by a lower plastic strain, which is induced at the wedge region during TF loading. Moreover, microstructural details of crack initiation, crack propagation, and reactions with the gaseous environment play an important role. Both alloys investigated in the present work showed plastic deformation with a maximum in the central zone of the wedge tip. In this zone, slip bands and grain distortion occurred, whereas both ends of the wedge tip free of visible plastic deformation. The TF cycles led to multiple transgranular crack initiation and propagation. In welded specimens of IN 617 and HAYNES 230, cracks appeared first in the center of the weld. The susceptibility of welds to TF cracking depends considerably on the weld filler and the surface quality. It was shown for HAYNES 230 that a mismatched weld could reduce the TF life to less than 50 pct of non-welded specimens. The lower TF-fatigue strength of the welded specimens can be explained by the difficulty of the cast alloy in the welded zone to accommodate the repeated thermal shocks by plastic deformation. Notches introduced in the heat-affected zone (depth about 0.1 mm) reduced the TF life of both alloys by a factor as high as 4. The thermal fatigue strength of the welded material can almost reach the values of the base alloy provided the use of matching electrodes, post-weld heat treatment, and grinding off the weld beads is carefully executed.     

Effect of broaching on high-temperature fatigue behavior in notched specimens of INCONEL 718

Notches were machined in specimens of INCONEL 718 by a broaching process, where differing broaching runs led to differing extents of subsurface deformation and surface roughness. Fatigue tests were carried out at 600 °C with a trapezoidal loading waveform at 0.25 Hz. The broaching process that led to the more severe subsurface deformation (but lower surface roughness) showed the worst fatigue performance. Analysis of total strain amplitude in the notch root with the aid of an elastoplastic finite-element (FE) model showed that the work hardening related to the subsurface deformation caused by the different broaching can account for the difference in fatigue lives. Differences in initiation and growth behavior were seen for the two broached finishes as well as for broached and subsequently polished samples. These differences are discussed in terms of a change in crack growth initiation and growth mechanisms due to the presence of the work-hardened layer.     

High temperature fatigue properties and crack initiation and its expansion mechanism of high nitrogen bearing steel

The flexural fatigue properties of 40Cr15Mo2VN high nitrogen bearing steel at 200?? was 883MPa, which was 17% lower than that at room temperature. The results show that the types of fatigue failure are surface failure initiation and internal non- metallic inclusion. Compared with room temperature, the threshold value of fatigue stress intensity factor ??Kth at 200?? decreases by 20%, leading to the reduction of the critical non- metallic inclusion size at the start of fatigue crack initiation. The interaction of small cracks near the fatigue source in the surface initiation cracks at 200?? increases the stress intensity factor KI and accelerates the initial surface crack propagation. At the same time, according to the ratio of the nominal stress amplitude and the ultimate fatigue strength of the non- metallic inclusions in the high- nitrogen bearing steel, the influence of the size, position and temperature of the non- metallic inclusions on the fatigue life was analyzed.     

Initiation and Early-Stage Growth of Internal Fatigue Cracking Under Very-High-Cycle Fatigue Regime at High Temperature

Metallurgical and Materials Transactions A - The initiation and early-stage crack growth under very-high-cycle fatigue (VHCF) at room temperature, 750 °C, and 850 °C on...     

Mechanical Behaviors and Failure Analysis of S38C Steel with Gradient Structure Fabricated by Induction Heating and Quenching

This article proposes a simple and fast method of induction heating and quenching to produce surface gradient structure for S38C steel, and its mechanical behavior and strengthening mechanism are revealed. The variation of the gradient structure from surface to interior is characterized by electron backscatter diffraction, and the tensile behavior of the gradient structure at different depths is acknowledged by the small-scale tensile tests. The gradient structure is tempered martensite microstructure, which significantly improves the hardness and tensile strength of surface and subsurface regions. Accordingly, with the strengthening of the gradient structure, the general tensile strength and fatigue behavior of the S38C steel are increased close to those of high-strength steel. Moreover, the fatigue crack initiation mechanism of the gradient structure is studied by energy dispersive spectroscopy, transmission Kikuchi diffraction, and transmission electron microscope characterization on the crack initiation regions. It reveals that the fatigue failure of the gradient structure can be due to stress concentration on the surface and around subsurface inclusions, and the crack initiation modes present surface crack initiation and internal crack initiation, respectively.     

Effect of residual magnesium content on thermal fatigue cracking behavior of high-silicon spheroidal graphite cast iron

This study investigates the thermal fatigue cracking behavior of high-silicon spheroidal graphite (SG) cast iron. Irons with different residual magnesium contents ranging from 0.038 to 0.066 wt pct are obtained by controlling the amount of spheroidizer. The repeated heating/cooling test is performed under cyclic heating in various temperatures ranging from 650 °C to 800 °C. Experimental results indicate that the thermal fatigue cracking resistance of high-silicon SG cast iron decreases with increasing residual magnesium content. The shortest period for crack initiation and the largest crack propagation rate of the specimens containing 0.054 and 0.060 wt pct residual magnesium contents are associated with heating temperatures of 700 °C and 750 °C. Heating temperatures outside this range can enhance the resistance to thermal fatigue crack initiation and propagation. When thermal fatigue cracking occurs, the cracks always initiate at the surface of the specimen. The major path of crack propagation is generally along the eutectic cell-wall region among the ferrite grain boundaries, which is the location of MgO inclusions agglomerating together. On the other hand, dynamic recrystallization of ferrite grains occurs when the thermal cycle exceeds a certain number after testing at 800 °C. Besides, dynamic recrystallization of the ferrite matrix suppresses the initiation and propagation of thermal fatigue cracking.     

Effects of load ratio and temperature on the near-threshold fatigue crack propagation behavior in a CrMoV steel

The influence of temperature in the range of 24 to 260 °C and load ratio on the near-threshold fatigue crack growth rate behavior of a CrMoV steel was characterized. At all temperatures investigated, the threshold stress intensity range, ΔK _th, for fatigue crack growth decreased with increasing load ratio. The near-threshold crack growth rates increased significantly at 149 °C when compared with the rates at room temperature. However, the crack growth rates at 260 °C were comparable to those at 149 °C. These observations are rationalized in terms of the concepts of roughness and oxide-induced crack closure. Extensive fracture surface characterization using SEM, oxide thickness measurements by Auger spectroscopy, and roughness measurements by light-section-microscopy were conducted to substantiate the explanations.     

The effect of defects on the fatigue crack initiation process in two p/m superalloys: part i. fatigue origins

Two high strength P/M nickel-base superalloys, AF-115 and AF2-1DA, with different defect populations, were tested to determine the effect of preexisting defects on the fatigue crack initiation process. Strain controlled continuous cycle fatigue tests were performed at room and at elevated temperature; these were followed by fractographic examination to characterize both the location and character of the fatigue origins. In most cases, particularly at elevated temperature, the initiation process was associated with a large pre-existing defect, either a pore or a nonmetallic inclusion. There was also a change in the location of the crack that caused failure as the strain range varied: at high strain ranges initiation occurred at or near the specimen’s surface, while at the lower strain ranges the failure originated in the specimen’s interior. The initiation mode for both alloys at room temperature was different than at elevated temperature. At room temperature, Stage I crystallographic cracking at or near the surface dominated the process in all strain range regimes. This difference was attributed, in part, to the differences in deformation mode for nickel-base superalloys at room and elevated temperature. Formerly with Metals and Ceramics Division, Air Force Materials Laboratory, Wright-Patterson AFB, OH.     

Microstructure and fatigue of a Ni,Cr, Al-TaC eutectic composite

The Ni,10Cr,5Al-TaC aligned eutectic has been heat treated subsequent to solidification to produce several microstructural conditions. High cycle fatigue tests have been conducted at 25 and 825°C, while creep tests were performed at 825 and 950°C. Experimental variables in the fatigue tests were stress level, test frequency, and environment. Post solidification heat treatments caused substantial improvements in fatigue properties at 825°C, but had little effect at room temperature. Elevated temperature fatigue lives were substantially reduced at low test frequencies in vacuum, accompanied by a shift from external to internal crack nucleation sites. Dislocation substructures produced in high temperature fatigue tests resembled those of creep specimens.     

Enhanced fatigue crack growth resistance at elevated temperature in TiC/Ti-6Al-4v composite: Microcrack-lnduced crack closure

The fatigue crack growth behavior of TiC/Ti-alloy composite was examined at 450 °C and compared to the room-temperature behavior. Contrary to the temperature-dependent fatigue crack growth behavior of the monolithic alloy, fatigue crack growth resistance of the composite was improved at the elevated temperature. At 450 °C, the fatigue threshold of the composite was found to be 50 Pct higher than at room temperature. Such an improved fatigue crack growth resistance is shown to result from extensive microcracking of reinforcing particles, which promotes fatigue crack closure at the elevated temperature.     

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.