Study of creep crack growth in 2618 and 8009 aluminum alloys

Creep crack growth (CCG) has been investigated in an 8009 (Al-Fe-V-S) P/M alloy at 175 °, 250 °, and 316 ° and in a 2618 ingot alloy at 150 °, 175 °, and 200 °. Under sustained load, subcritical crack growth is observed at stress intensity levels lower thanK _ic ; for 2618, at 200 °, crack growth is observed at stress intensities more than 40 pct lower thanK _ic . Alloys 8009 and 2618 exhibit creep brittle behavior,i.e., very limited creep deformation, during CCG. The CCG rates do not correlate with CCG parameters C* and C but correlate with the stress intensity factor,K, and theJ integral. Generally, crack growth rates increase with increasing temperature. Micromechanisms of CCG have been studied with regard to microstructural deg-radation, environmental attack, and creep damage. Although theoretical estimation indicates that CCG resistance decreases with second-phase coarsening, such coarsening has not been observed at the crack tip. Also, no evidence is found for hydrogen- or oxygen-induced crack growth in comparing test results in moist air and in vacuum. Creep deformation and cavitation ahead of crack tip are responsible for observed time-dependent crack growth. Based on the cavitation damage in the elastic field, a micromechanical model is proposed which semiquantitatively explains the correlations between the creep crack growth rate and stress intensity factor,K.     

Crack growth in a nearly fully-lamellar gamma TiAl alloy at 650 °C and 800 °C under constant load conditions

The crack growth behavior of a gamma titanium aluminide alloy, K5S, was investigated at 650 °C and 800 °C under constant load conditions in a nearly fully-lamellar microstructural form. Crack growth at both temperatures ensues at stress intensities (K) much higher than anticipated from the R curves. At 650 °C, creep crack extension occurs through the formation of microcracks (interlamellar (IL) separation) and their joining to the main crack tip through ligament fracture. This results in a mainly transgranular (TG) fracture with occasional IL separation. This process features a rapid initial crack growth but at decreasing growth rate, followed by a nearly no-growth stage. At 800 °C, crack extension is accompanied by extensive plastic deformation and consists of an initial rapid transition period and a subsequent steady state. For similar K’s, crack extension and growth rate are greater at 800 °C than at 650 °C, but even these are very slow processes for this alloy. The resistance to crack propagation at 650 °C is explained in terms of work hardening that arises during the extended primary creep deformation occurring ahead of the crack tip. Increased crack propagation at 800 °C is accredited to grain boundary and lamellar-interface weakening and extensive post primary creep damage in the plastic zone. The resulting fracture at 800 °C is mainly boundary fracture, which consists of IG fracture involving formation and coalescence of voids, and IL separation.     

Effects of temperature and environment on the tensile and fatigue crack growth behavior of a Ni3AI-base alloy

The effects of temperature, frequency, and environment on the tensile and cyclic deformation behavior of a nickel aluminide alloy, Ni-9.0 wt pct Al-7.97 pct Cr-1.77 pct Zr (IC-221), have been determined. The tensile properties were obtained in vacuum at elevated temperatures and in air at room temperature. The alloy was not notch sensitive at room temperature or at 600 °C, unlike Cr-free Ni₃Al + B alloys. In general, crack growth rates of IC-221 increased with increasing temperature, decreasing frequency, exposure to air, or testing at higherR ratios. At 25 °C, crack growth rates were slightly higher than for a previously investigated Cr-free Ni₃Al alloy. However, at 600 °C, the crack growth rates for IC-221 were lower than for the Cr-free alloy. Substantial frequency effects were noted on crack growth of IC-221 at both 600 °C and 800 °C in both air and vacuum, especially at highK. The relative contributions of creep and environmental interactions to fatigue crack growth are discussed.     

Creep crack growth behavior of several structural alloys

Creep crack growth behavior of several high temperature alloys, Inconel 600, Inconel 625, Inconel X-750, Hastelloy X, Nimonic PE-16, Incoloy 800, and Haynes 25 (HS-25) was examined at 540, 650, 760, and 870 °C. Crack growth rates were analyzed in terms of both linear elastic stress intensity factor and J*-integral parameter. Among the alloys Inconel 600 and Hastelloy X did not show any observable crack growth. Instead, they deformed at a rapid rate resulting in severe blunting of the crack tip. The other alloys, Inconel 625, Inconel X-750, Incoloy 800, HS-25, and PE-16 showed crack growth at one or two temperatures and deformed continuously at other temperatures. Crack growth rates of the above alloys in terms ofJ* parameter were compared with the growth rates of other alloys published in the literature. Alloys such as Inconel X-750, Alloy 718, and IN-100 show very high growth rates as a result of their sensitivity to an air environment. Based on detailed fracture surface analysis, it is proposed that creep crack growth occurs by the nucleation and growth of wedge-type cracks at triple point junctions due to grain boundary sliding or by the formation and growth of cavities at the boundaries. Crack growth in the above alloys occurs only in some critical range of strain rates or temperatures. Since the service conditions for these alloys usually fall within this critical range, knowledge and understanding of creep crack growth behavior of the structural alloys are important.     

Creep and stress rupture of a mechanically alloyed oxide dispersion and precipitation strengthened nickel-base superalloy

The creep and stress rupture behavior of a mechanically alloyed oxide dispersion strengthened (ODS) and γ′ precipitation strengthened nickel-base alloy (alloy MA 6000E) was studied at intermediate and elevated temperatures. At 760 °C, MA 6000E exhibits the high creep strength characteristic of nickel-base superalloys and at 1093 °C the creep strength is superior to other ODS nickel-base alloys. The stress dependence of the creep rate is very sharp at both test temperatures and the apparent creep activation energy measured around 760 °C is high, much larger in magnitude than the self-diffusion energy. Stress rupture in this large grain size material is transgranular and crystallographic cracking is observed. The rupture ductility is dependent on creep strain rate, but usually is low. These and accompanying microstructural results are discussed with respect to other ODS alloys and superalloys and the creep behavior is rationalized by invoking a recently-developed resisting stress model of creep in materials strengthened by second phase particles. The analysis indicates that at the intermediate temperature the creep strength is controlled by the high volume fraction of γ′ precipitates and the contribution to the creep strength from the oxide dispersion is small. At the elevated temperature, the creep strength is derived mainly from the inert oxide dispersoids. Formerly at Columbia University.     

High-temperature fracture and fatigue-crack growth behavior of an XD gamma-based titanium aluminide intermetallic alloy

A study has been made of the effect of temperature (between 25 °C and 800 °C) on fracture toughness and fatigue-crack propagation behavior in an XD-processed, γ-based titanium aluminide intermetallic alloy, reinforced with a fine dispersion of ∼1 vol pct TiB₂ particles. It was found that, whereas crack-initiation toughness increased with increasing temperature, the crack-growth toughness on the resistance curve was highest just below the ductile-to-brittle transition temperature (DBTT) at 600 °C; indeed, above the DBTT, at 800 °C, no rising resistance curve was seen. Such behavior is attributed to the ease of microcrack nucleation above and below the DBTT, which, in turn, governs the extent of uncracked ligament bridging in the crack wake as the primary toughening mechanism. The corresponding fatigue-crack growth behavior was also found to vary inconsistently with temperature. The fastest crack growth rates (and lowest fatigue thresholds) were seen at 600 °C, while the slowest crack growth rates (and highest thresholds) were seen at 800 °C; the behavior at 25 °C was intermediate. Previous explanations for this “anomalous temperature effect” in γ-TiAl alloys have focused on the existence of some unspecified environmental embrittlement at intermediate temperatures or on the development of excessive crack closure at 800 °C; no evidence supporting these explanations could be found. The effect is now explained in terms of the mutual competition of two processes, namely, the intrinsic microstructural damage/crack-advance mechanism, which promotes crack growth, and the propensity for crack-tip blunting, which impedes crack growth, both of which are markedly enhanced by increasing temperature.     

Fatigue crack growth through

Fatigue cracks were grown at 25 °C and 800 °C in a titanium aluminide alloy heat-treated to give a γ+ α ₂ lamellar microstructure. These lamellae, having widths of =0.5 to 2 μm, were in colonies approximately 1.2 mm across. Crack growth was observed and photographed under high resolution conditions using a loading and heating cyclic stage for the scanning electron microscope. Stereoimaging was used to measure displacements around crack tips, from which crack opening displacements and strains were derived. Cracks were found to grow about 10 times faster at 25 °C than at 800 °C, and the threshold stress intensity for fatigue crack growth was lower at 25 °C. Strain to fracture the lamellae was determined as ≈0.08, while fatigue crack tips could sustain up to 0.3 strain at 25 °C and 0.5 strain at 800 °C. The lamellar micro- structure was found to have a strong influence on crack tip behavior.     

Creep and fatigue crack propagation in a directionally-solidified carbide eutectic alloy

Creep and fatigue crack growth rates and threshold stress intensity amplitudes have been measured for a directionally solidified carbide-eutectic alloy, C73. Fatigue testing temperatures have been ambient, 750 and 950°C for cracking perpendicular and parallel to the solidification direction. In the former cracking direction comparative propagation rates may be understood in terms of the properties of the matrix, which shows a phase transformation from hexagonal to cubic above ~900°C. A situation where crack growth rates decrease with increasing apparent stress intensity amplitudes (ΔK) has been found to exist for propagation parallel to the solidification direction at low ΔK values and high temperatures only. This phenomenon can be related to the occurrence of crack branching and multiple cracking of the carbide fibers. Considerations of plastic zone sizes and critical defect sizes for crack propagation are consistent with the conditions necessary for such crack deceleration to occur. Transformation of the M₇C₃ fibers, present in the as-cast condition, to M₂₃C₆ at cell boundaries of the solidification structure occurs at a temperature of 950°C. Although M₂₃C₆ carbides are easily cracked and therefore probably reduce propagation rates by causing secondary cracking, their presence is known to be detrimental to creep properties. High cycle fatigue threshold stress intensity amplitudes for C73 in either loading direction at room temperature, 750 and 950°C are ~20 pct lower than for the cast nickel-base alloy, EST 738LC,i.e. critical defect sizes are ~10 pct smaller in C73. Despite the known sensitivity of cracking rates and threshold values to factors such as minor fluctuations in loading amplitude it is believed that these differences are significant.     

High Temperature Oxidation Behavior of Conventional and Nb-Modified MAR-M246 Ni-Based Superalloy

The present investigations focused on the thermal oxidation of two variants of MAR-M246 alloy having the same contents of Ta and Nb in at. pct, considering the effects of total replacement of Ta by Nb. The alloys were produced by investment casting using high purity elements in induction furnace under vacuum atmosphere. The alloys were oxidized pseudo-isothermally at 800 °C, 900 °C and 1000 °C up to 1000 hours under lab air. Protective oxidation products growing on the surface of the oxidized samples were mainly Al₂O₃, Cr₂O₃. Other less protective oxide such as spinels (NiCr₂O₄ and CoCr₂O₄) and TiO₂ were also detected as oxidation products. The conventional alloy exhibited slight internal oxidation at 800 °C and an enhanced resistance at 900 °C and 1000 °C. The Nb-modified alloy presented an exacerbated internal oxidation and nitridation at 900 °C and 1000 °C and an enhanced resistance at 800 °C. At 1000 °C, Nb-modified alloy was particularly affected by excessive spalling as the main damage mechanisms. From a kinetic point of view, both alloys exhibit the same behavior at 800 °C and 900 °C, with k_p values typical of alumina forming alloys (2 × 10⁻¹⁴ to 3.6 × 10⁻¹³ g² cm⁻⁴ s⁻¹). However, Ta modified alloys exhibited superior oxidation resistance at 1000 °C when compared to the Nb modified alloy due to better adherence of the protective oxide scale.

     

Effect of Environment on Creep Crack Growth in PM/HIP René-95

The creep crack growth rates (CCGR) of PM/HIP René-95 were measured from 10^-9 m per second to 10^-4 m per second in air and in high purity argon at 760°C and 650°C using single edge notched (SEN) specimens. The crack length was monitored by the D.C. potential difference technique. The data were reported asda/dt vs the elastic stress intensity factor,K,, since PM/HIP René-95 is a creep-brittle material. The CCGR were shown to be strongly environment sensitive. The CCGR were up to 1000 times faster in air than in argon for a given value ofK _I . The temperature andK _I dependence of CCGR in air were shown to correlate with a modified formulation of the Larson-Miller parameter. Notched stress rupture (NSR) tests were performed at 650°C in air in order to study the effect of notch root radius on the time to initiate a creep crack. A comparison of the NSR data with SEN data for the same values of initialK, shows that the crack initiation times are a strong function of the notch root radius. It was observed that ninety percent of the rupture time is spent in crack initiation when the notch root radius is finite, while no incubation time was observed for creep crack growth from fatigue precracked specimens. Formerly with Massachusetts Institute of Technology, Cambridge, MA     

Tensile properties and creep behavior of a new Ti-Al-Nb intermetallic alloy with an O+<Emphasis Type="Italic">α</Emphasis><Subscript>2</Subscript> microstructure

A new Ti-Al-Nb alloy with a composition of Ti-27.5Al-13Nb (at. pct) was proposed. The density of this alloy was 4.7 g/cm³, which is about 13 pct lower than that for O+B2 alloys. After hot processing, the alloy was heat treated under two conditions: directly aged at 850 °C (DA treatment), or cooled from above the β-transus temperature with a cooling rate of 3 °C/min and then aged at 850 °C (BCA treatment). Under the present heat-treatment conditions, the phase constitution was primarily O+α ₂. A very fine Widmanstätten microstructure was obtained after the DA treatment, while a microstructure with coarse O plates was obtained after the BCA treatment. The tensile properties were investigated at 20 °C to 950 °C, and the creep behavior was investigated at 650 °C to 750 °C/90 to 380 MPa. The elongation to fracture at room temperature for the DA-treated tensile specimen was as high as 2.6 pct, despite the high Al content in this alloy. In comparison with the O+B2 ternary alloys, this alloy showed higher specific proof stress at temperatures over 800 °C and higher creep strength. The stress exponent and the apparent activation energy for creep were calculated. The fracture mechanism was discussed.     

Stress-assisted hydrogen attack cracking in 2.25Cr-1Mo steels at elevated temperatures

Crack growth in 2.25Cr-lMo steels exposed to 3000 psi hydrogen has been investigated in the temperature range 440 °C to 500 °C, using modified wedge-opening loaded specimens to vary stress intensity. Under conditions of temperature and hydrogen pressure, where general hydrogen attack does not occur, the crack propagated by the growth and coalescence of a high density of methane bubbles on grain boundaries, driven by the synergistic influence of internal methane pressure and applied stress. Crack growth rates were measured in base metal, and the heat-affected zones (HAZs) of welds were tempered to different strength levels. The crack growth rate increased with material strength. Above a threshold of about K_l = 20 MPa√m (at 480 °C), the crack growth rate increased rapidly with stress intensity, increasing as roughly K_l ^6.5. Because of better creep resistance, stronger materials can sustain higher levels of stress intensity to drive crack growth and nucleate the high density of voids necessary for crack growth. Stress relaxation by creep reduces the stress intensity, and thus the growth rate, especially in weaker materials. The crack growth rate in the heat-affected zone was found to be substantially faster than in the base metal of the welds. Analysis indicates that K_l rather than C* is the appropriate crack-tip loading parameter in the specimen used here and in a thick-walled pressure vessel. The DC potential drop technique met with limited success in this application due to the spatially discontinuous manner of crack growth and limited crack-tip opening displacement. Formerly Graduate Student, Materials Science and Engineering Department, The Ohio State University     

Fatigue crack growth behavior of an oxide dispersion strengthened MA 956 alloy

Fatigue crack growth behavior of oxide dispersion strengthened ferritic MA 956 alloy was studied at 25 °C and 1000 °C in air at 0.17 Hz. The growth rates were analyzed using the linear elastic parameter ΔK and the elastic-plastic parameter ΔJ. Crack growth, although transgranular at both temperatures, increased by nearly three orders of magnitude with increase in temperature from 25 to 1000 °C. The growth rates were essentially the same in terms of either ΔK or ΔJ parameters indicating that plasticity effects are small even at 1000 °C. Detailed fractographic analysis revealed the presence of ductile striations in the ΔK range of 25 to 40 MPa√m at 25 °C and in a much narrower range at 1000 °C. Presence of voids could be detected at 1000 °C. Using the measured load-displacement hysteresis energies for a unit increment in crack length, crack growth rates were calculated using cumulative damage models and were compared with the experimental data. At 1000 °C the predicted and the experimental values agree within a factor of two and it is concluded that the growth occurs essentially by a damage accumulation process except in a narrow range of ΔK where the plastic blunting process is superimposed, resulting in ductile striations that were observed. At 25 °C the predicted and the experimental value reasonably agree for ΔK values greater than 40 MPa√m, and below this value the two diverge with predicted values being much lower. This divergence is related to occurrence of the plastic blunting process in this ΔK range as confirmed by fractographic evidence. The cumulative damage process at 1000 °C was related to the environmentally assisted void formation at dispersoid-matrix interfaces. At 25 °C the damage is related to the formation of microcracks ahead of the crack tip. These results and interrelation between alloy microstructure and fatigue fracture path are discussed in detail.     

Creep deformation and crack growth behavior of a single-crystal nickel-base superalloy

Uniaxial creep deformation and crack growth data are presented on the single-crystal nickel-base superalloy SC16, which is a candidate material for industrial gas turbine applications. All testing was performed at 900 °C. The uniaxial experiments were conducted with the loading direction aligned approximately along the [001] crystallographic axis of the material. Under these conditions, a small primary region followed by mainly tertiary creep was obtained, and failure initiated from cracks at interdendritic pores. The crack growth experiments were performed on single-edge notch tension specimens and compact tension test pieces containing deep side grooves to examine state-of-stress effects. A selection of crystallographic orientations was also examined. Little effect of stress state and orientation was obtained. It has been found that the creep crack growth characteristics of the alloy can be predicted satisfactorily from a model of the accumulation of damage at a crack tip using the creep fracture mechanics parameter C* and assuming plane stress conditions. This article is based on a presentation made at the “High Temperature Fracture Mechanisms in Advanced Materials” symposium, as a part of the 1994 Fall meeting of TMS, October 2-6, 1994, in Rosemont, Illinois, under the auspices of the ASM/SMD Flow and Fracture Committee.     

Investigations on the influence of internal nitridation on creep crack growth in alloy 800 H

For two batches of Alloy 800 H with Al contents of 0.02 and 0.34 pct, creep crack growth was investigated at 1000 °C in air and in an Ar-5 pct H₂ atmosphere. TheK concept, the net-section stress concept, and theC ^* concept of creep fracture mechanics were applied when plotting the experimental results. TheC ^* concept proved to give the best correlation between load parameter and crack growth rate. A good agreement was observed between the experimental results and the model calculations for crack extension by constrained diffusive cavity growth. Strong internal nitridation, which occurred in the air tests and which had been shown to increase the creep strength in creep rupture tests, did not show any significant influence on the creep crack growth rate in comparison with tests in an Ar-5 pct H₂ atmosphere, in which no internal nitridation was observed. Also, the differences in the Al contents of the two batches did not play a role. This behavior is explained by the fact that neither the nitride particles nor the particle matrix interface is a particularly weak site in the material. It also becomes obvious that theC ^* concept can be rather insensitive to precipitation strengthening effects, if these only affect the parameterB in Norton's creep law. M. WELKER, formerly with DECHEMA     

Fatigue crack growth mechanisms in Ti6242 lamellar microstructures: Influence of loading frequency and temperature

Fatigue crack growth experiments were carried out on Ti6242 alloy with large colony size. The alloy was heat treated to provide three different lamella size; fine, coarse, and extra coarse. Tests were conducted at two temperatures, 520 °C and 595 °C, using two loading frequencies, 10 and 0.05 Hz. The latter frequency was examined with and without a 300-second hold time. All tests were performed in air environment and at a stress ratio of 0.1. This study shows that at 520 °C, the Fatigue crack growth rate (FCGR) is not significantly influenced by changes in the microstructure. For 0.05 Hz/low ΔK, however, the FCGR is higher in the fine lamellar microstructure and is accompanied by- the appearance of a plateau, which disappears in the extra large lamella microstructure. Furthermore, the addition of a 300-second hold time does not alter the crack growth rate. At 595 °C, while the general level of the FCGR is higher than that at 520 °C, the effects of loading frequency and hold time remain similar to those reported at the lower temperature. Unlike the results at 520 °C, however, the FCGR at low δK is not influenced by variations in lamellar microstructure. Under all test conditions, the fatigue process is predominantly controlled by one single mechanism associated with transcolony fracture and formation of quasi-cleavage facets. The fatigue crack growth results and the associated fracture behavior as obtained in this study are correlated to the crack-tip shear activity and transmission at the α/β interfaces. A general hypothesis accounting for the role of loading frequency, temperature, and microstructure on the observed cracking mechanisms is presented.     

Thermal and Irradiation Creep Behavior of a Titanium Aluminide in Advanced Nuclear Plant Environments

Titanium aluminides are well-accepted elevated temperature materials. In conventional applications, their poor oxidation resistance limits the maximum operating temperature. Advanced reactors operate in nonoxidizing environments. This could enlarge the applicability of these materials to higher temperatures. The behavior of a cast gamma-alpha-2 TiAl was investigated under thermal and irradiation conditions. Irradiation creep was studied in beam using helium implantation. Dog-bone samples of dimensions 10 × 2 × 0.2 mm³ were investigated in a temperature range of 300 °C to 500 °C under irradiation, and significant creep strains were detected. At temperatures above 500 °C, thermal creep becomes the predominant mechanism. Thermal creep was investigated at temperatures up to 900 °C without irradiation with samples of the same geometry. The results are compared with other materials considered for advanced fission applications. These are a ferritic oxide-dispersion-strengthened material (PM2000) and the nickel-base superalloy IN617. A better thermal creep behavior than IN617 was found in the entire temperature range. Up to 900 °C, the expected 10⁴ hour stress rupture properties exceeded even those of the ODS alloy. The irradiation creep performance of the titanium aluminide was comparable with the ODS steels. For IN617, no irradiation creep experiments were performed due to the expected low irradiation resistance (swelling, helium embrittlement) of nickel-base alloys.