Creep crack growth in the absence of grain boundary precipitates in UDIMET 520

The effects of grain size and environment on creep crack growth (CCG) in Ni-base superalloy, UDIMET 520, were studied through experiments at 540 °C. Specially designed solution and aging treatments were used to produce γ′ strengthened microstructures with different grain sizes but without any M₂₃C₆ grain boundary precipitates. Five grain sizes, which fall into three groups (i.e., small, medium, and large), were employed. The creep crack growth rates (CCGRs) in specimens with small grain sizes were approximately 2.5 times lower than those with medium and large grain sizes, as a result of crack branching and the presence of some undissolved primary MC carbides at the grain boundaries. Otherwise, the CCGRs were insensitive to the grain size. Fractographic observations on the fracture surfaces and metallographic examinations on the cross sections of the interrupted CCG specimen revealed intergranular microcracks and a faceted intergranular mode of fracture in both air and argon environments. The test results suggest that the formation and propagation of intergranular cracks by grain boundary sliding (GBS) is the main micromechanism responsible for CCG in both air and argon environments at the relatively low test temperature employed. Grain boundary oxidation attack in the air environment simply accelerates the crack growth process. The present results are in agreement with the theoretical predictions of the GBS-controlled CCG model previously developed by the authors. S. XU, formerly Graduate Student, Department of Engineering Physics and Materials Engineering, Ecole Polytechnique de Montreal A. K. KOUL, formerly Senior Research Officer, NRC, Ottawa     

Near- threshold fatigue crack growth in copper and alpha- brass: Grain- size and environmental effects

The fatigue propagation rates and fatigue threshold ( ΔK _th) values were studied (R = 0.1 and frequency = 20 Hz) on copper and 70-30 α-brass of two different grain sizes in laboratory air and dry argon. With decreasing grain size, the threshold increased in copper, while it decreased in α-brass. These results suggest that in copper, crack tip plasticity considerations were more important in determining the threshold values than crack closure effects. Dry argon increased ΔK _th slightly in copper and more significantly in α-brass. A transition from completely transgranular to partially intergranular and back to completely transgranular cracking was observed with decreasing crack growth rates in both materials and environments. The growth rates for which intergranular cracking was obtained were found to be consistent with a hydrogen embrittlement mechanism, associated with adsorption of water molecules and dislocation transport of hydrogen.     

Effect of Environment on Fatigue and Creep Crack Growth in Inconel X-750 at Elevated Temperature

The fatigue crack growth rates (FCGR) of Inconel X-750 were measured in air and in vacuum at 25 °C and 650 °C as a function of test frequency. The wave shape was triangular and the frequency varied from 10 Hz to 0.01 Hz. The creep crack growth rates (CCGR) were also measured on single edge notch specimens at 650 °C in air and in purified argon. For a givenAK, the FCGR increases when temperature increases and frequency decreases. At low frequency the FCGR approach the creep crack growth rates. The mode of fracture changes from transgranular at 10 Hz to intergranular at 0.01 Hz. The effect of air environment is to accelerate the transition from transgranular to intergranular fracture modes with decreasing frequency. The role of oxidation in accelerating crack growth rate in fatigue and in creep is discussed in detail. F. GABRIELLI, was formerly a visiting scientist at Massachusetts Institute of Technology.     

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.     

An intergranular creep crack growth model based on grain boundary sliding

An intergranular crack growth model is developed to describe the effect of microstructural features such as grain size, grain boundary precipitates, and serrated grain boundaries on creep crack growth under grain boundary sliding (GBS) conditions. The model considers quantitatively that several deformation mechanisms contribute to the stress redistribution ahead of the crack tip through a stress relaxation process. The crack tip region is divided into three zones: (a) the intragranular-deformation-controlled stress relaxation zone, (b) the GBS-controlled stress relaxation zone, and (c) the elastic region. Intergranular creep crack growth is considered to occur as a result of the GBS-controlled process in all cases. The derived creep crack growth model shows a complex dependence of the creep crack growth rate (CCGR) on fracture mechanics quantities, such as C(t) (the path-independent energy integral with its steady-state value as C*) and K (the stress intensity factor). For creep-brittle materials, the model predicts that the CCGR depends on K to the power of 2 and this is verified experimentally; however, when environmental effects contribute to the crack growth process, the power exponent will increase. A semiempirical factor is introduced to account for the effects of oxidation on CCGR.     

Influence of gaseous environments on rates of near-threshold fatigue crack propagation in nicrmov steel

The influence of hydrogen environment (448 kPa) on near-threshold fatigue crack propagation rates was examined in a 779 MPa yield strength NiCrMoV steel at 93 °C. An automatically decreasing and increasing stress intensity technique was employed to generate crack growth rates at three load ratios(R = 0.1, 0.5, and 0.8). Results show that the crack propagation rates in hydrogen are slower than those in air for levels of stress intensity range, ΔK, below about 12 MPa√m. The crack closure concept does not explain the slower crack growth rates in hydrogen than in air. Near-threshold growth rates appear to be controlled by the levels of residual moisture in the environments. In argon and air, the fracture morphology is transgranular, while in H₂ the amount of intergranularity varies with ΔK and achieves a maximum when the cyclic plastic zone is approximately equal to the prior austenite grain size.     

The creep fracture of wrought nickel-base alloys by a fracture mechanics approach

Creep fracture in the 500 to 750°C temperature range was by an intergranular crack growth process involving the formation of microcracks in grain boundaries slightly ahead of the main crack. The crack growth was proportional to an exponential power of the stress intensity. Wide differences in cracking behavior were seen between different alloys, but their differences were due primarily to material processing history and not to compositionper se. Transverse sample orientation and coarser grain sizes significantly improved the resistance to cracking. Both slow crack growth and the final fast fracture toughness changed appreciably with test history. A good correlation was found between the notch properties and the creep behavior of an unnotched sample loaded to the yield strength.     

Grain boundary sliding and stress concentration during creep

Importance of grain boundary sliding to creep intergranular fracture is focussed. Previous metallographic and fractographic studies of creep intergranular fracture on metal bicrystals and polycrystals are briefly reviewed in order to show the close relationship between grain boundary sliding and fracture. Deformation ledge and migration irregularity are shown to be potential sites of stress concentration and crack nucleation on sliding grain boundaries without particles. The effect of grain boundary structure on creep intergranular fracture is discussed on the basis of the effect of grain boundary structure on sliding, the contribution of sliding to the overall creep deformation, and a sliding-fracture diagram. Recent observations of the effect of grain boundary structure on creep intergranular fracture on alpha iron-tin alloy polycrystals are shown. This paper is based on a presentation made at the symposium “The Role of Trace Elements and Interfaces in Creep Failure” held at the annual meeting of The Metallurgical Society of AIME, Dallas, Texas, February 14-18, 1982, under the sponsorship of The Mechanical Metallurgy Committee of TMS-AIME.     

The influence of microstructure and environment on the crack growth behavior of powder metallurgy nickel superalloy RR1000

A study of crack growth in vacuum and air at 725 °C (T/T _m=0.6) highlights the relative importance of creep and environmental crack-tip damage mechanisms in Powder Metallurgy (P/M) disc alloy RR1000. Both of these mechanisms are associated with a transition to intergranular fracture during fatigue crack growth at 0.25 Hz. Crack growth under sustained loads reveals the precise nature of these mechanisms in RR1000. The severity of creep and environmental mechanisms is controlled by the grain-boundary microstructure and the crack-tip stress. Near-tip cavitation leads to fracture in vacuum. Sigma-phase precipitation causes an increase in crack growth rate through increased crack-tip cavity nucleation. Rapid near-tip stress relaxation induced by γ′ coarsening has a beneficial effect on the severity of this type of damage. In air, increases in crack growth rates are associated with near-tip intergranular oxidation. It is proposed that the extent of this damage and subsequent growth rates are increased by sigma-phase precipitation through enhanced oxidation due to chromium depletion and subsequent decreased passivation. Again, a beneficial effect of rapid near-tip stress relaxation due to selective γ′ coarsening is apparent and environmental damage is reduced under these conditions.     

A theoretical model for creep crack growth

A theory of creep crack growth has been developed with the presumption that the crack growth occurs by the diffusion of vacancies along the grain boundaries. This is consistent with many experimental results that show that creep fracture is generally of intergranular type and the activation energies for crack growth rates fall within the range of grain boundary diffusion energies. The theory is based on the concept that creep crack growth results from a balance of two competing processes-the diffusion of point defects that contributes to the growth and the creep deformation process that retards the growth and causes even its arrest. The present analysis shows that crack growth via grain boundary diffusion occurs within some temperature range. The upper limiting temperature is determined by the bulk diffusion process which disperses the vacancies, that are diffusing to the crack tip, to the plastic zone ahead of the crack front. The lower temperature limit is set by the fact that the grain boundary diffusion rates decrease with the decrease in temperature and thus large stress intensities approaching the fracture toughness value are required to accomplish crack growth by the grain boundary diffusion. Outside these limits creep crack growth occurs via deformation which is significantly slower than growth by the grain boundary diffusion process. The importance of the present analysis rests on the fact that service conditions for many high temperature structural materials fall within the regime wherein creep crack growth occurs via grain boundary diffusion.     

Effect of carbon content and helium gas environment on creep crack growth properties of Ni-26 pct Cr-17 pct W-0.5 pct Mo alloy at 1273 K

Creep crack growth tests were conducted on Ni-26 pct Cr-17 pct W-0.5 pct Mo alloys with different carbon contents in air and in helium gas environment at 1273 K using the compact-type (CT) specimen, and the effects of carbon content and environment on creep crack growth rate are discussed. Creep crack growth rateda/dt is evaluated by theC* parameter. Theda/dt is faster in higher-carbon alloys than in lower-carbon alloys in each environment. This effect of carbon content is attributed to the lower creep ductility due to the increase of fine trans-granular carbides in higher-carbon alloys. The environmental effect on theda/dt vs C* relations is scarcely observed in higher-carbon alloys. In the 0.003 pct C alloy, however,da/dt is much lower in the He gas environment than in air. Carburization is observed ahead of the crack tip in the He gas environment at 1273 K. The intergranular carbides precipitated due to carburi-zation have a granular configuration and are considered to prevent the grain boundary sliding in lower-carbon alloys.     

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     

Role of Localized Deformation in Irradiation-Assisted Stress Corrosion Cracking Initiation

Intergranular cracking of irradiated austenitic alloys depended on localized grain boundary stress and deformation in both high-temperature aqueous and argon environments. Tensile specimens were irradiated with protons to doses of 1 to 7 dpa and then strained in high-temperature argon, simulated boiling water reactor normal water chemistry, and supercritical water environments. Quantitative measurements confirmed that the initiation of intergranular cracks was promoted by (1) the formation of coarse dislocation channels, (2) discontinuous slip across grain boundaries, (3) a high inclination of the grain boundary to the tensile axis, and (4) low-deformation propensity of grains as characterized by their Schmid and Taylor factors. The first two correlations, as well as the formation of intergranular cracks at the precise locations of dislocation channel–grain boundary intersections are evidence that localized deformation drives crack initiation. The latter two correlations are evidence that intergranular cracking is promoted at grain boundaries experiencing elevated levels of normal stress.     

Effects of grain boundary chemistry on the intergranular cracking behavior of Ni-16Cr-9Fe in high-temperature water

An experimental program is conducted to determine the role of carbon, chromium, and phosphorus on the intergranular (IG) cracking behavior of Ni-16Cr-9Fe in 360 °C argon and water. Both constant extension rate tensile (CERT) tests and constant load tensile (CLT) tests are used to determine the susceptibility to IG cracking. Results show that carbon in solution strongly suppresses IG cracking behavior through an increased resistance to power-law creep, which promotes failure by the formation and linkup of grain boundary voids. The mechanical deformation at 360 °C is very time dependent, with slower extension rates resulting in greater IG cracking and lower elongation due to the longer time afforded the creep process. Although creep-induced grain boundary fracture is dominant in both water and argon, there is a substantial environmental enhancement in water. Grain boundary carbides do not appear to play a primary role in the grain boundary deformation process. In both environments, addition of P to Ni-16Cr-9Fe improves the IG cracking resistance, but chromium depletion has no effect. Results imply that carbon in solution plays a critical role in strengthening and increasing resistance to creepinduced grain boundary void formation and fracture.     

On the mechanisms of high-temperature intergranular embrittlements of Ni3Al-Zr Alloys

Recent studies on the room-temperature fracture behavior of Ni₃Al-Zr alloys after preexposure at elevated temperatures show various types of intergranular failure. In the presently studied Ni₇₈Al₂₁Zr₁B_0.2 alloy, a strong intergranular fracture tendency at room temperature has been found after preexposure at 750 °C, which is caused by the grain boundary precipitation in this alloy. After short-term exposure above 1200 °C and bending fracture at room temperature, the alloy also suffers intergranular embrittlement due to grain boundary melting. The intergranular fracture appearance is quite different from that observed in a previous study for a Ni_77.4Al₂₂Zr_0.6B_0.2 alloy after air exposure for 100 hours at 1200 °C. In that case, the intergranular fracture was accompanied by grain boundary diffusion (invasion) and segregation of oxygen. The mechanisms of these types of grain boundary failure are discussed. Formerly Doctoral Candidate, Institute of Materials Science and Engineering, National Taiwan University.     

Effects of grain boundary chemistry on the Intergranular Cracking Behavior of Ni-16Cr-9Fe in High-Temperature Water

An experimental program is conducted to determine the role of carbon, chromium, and phosphorus on the intergranular (IG) cracking behavior of Ni-16Cr-9Fe in 360 °C argon and water. Both constant extension rate tensile (CERT) tests and constant load tensile (CLT) tests are used to determine the susceptibility to IG cracking. Results show that carbon in solution strongly suppresses IG cracking behavior through an increased resistance to power-law creep, which promotes failure by the formation and linkup of grain boundary voids. The mechanical deformation at 360 °C is very time dependent, with slower extension rates resulting in greater IG cracking and lower elongation due to the longer time afforded the creep process. Although creep-induced grain boundary fracture is dominant in both water and argon, there is a substantial environmental enhancement in water. Grain boundary carbides do not appear to play a primary role in the grain boundary deformation process. In both environments, addition of P to Ni-16Cr- 9Fe improves the IG cracking resistance, but chromium depletion has no effect. Results imply that carbon in solution plays a critical role in strengthening and increasing resistance to creep- induced grain boundary void formation and fracture.     

Hydrogen-Induced subcritical crack growth of a 12 Cr-1 Mo ferritic stainless steel

Subcritical crack growth and tensile ductility measurements have been made on a 12 Cr-1 Mo ferritic stainless steel at cathodic potentials in a 1 N H₂SO₄ solution at 25 °C. The tensile ductility was found to be a minimum at −600 mV (SCE) and both the subcritical crack growth behavior and tensile ductility were similar for material in the tempered (760 °C/2.5 h) or tempered-plus-segregated (540 °C/240 h) condition. A rising-load crack growth threshold of 20 MPa √m was measured and a rising-load fracture toughness of 110 MPa √m was determined from extrapolation of the stage III crack growth curve. A K-independent stage II was observed and a stage II crack growth rate of about 1 × 10⁻⁵ mm/s was measured. The fracture mode was a mixture of intergranular and quasi-cleavage for both heat treatments and for subcritical and tensile fracture tests. Impact fracture properties were independent of heat treatment and grain boundary composition with the fracture mode predominantly transgranular. The difference in the fracture mode for hydrogen-induced crack growth and dynamic crack growth was explained by a difference in the relationship between their stress profiles and the maximum grain boundary segregation distribution.     

Effects of temper level on the dependence of fatigue crack growth threshold and crack closure on the prior austenitic grain size

An attempt has been made to systematically investigate the effects of microstructural parameters, such as the prior austenite grain size (PAGS), in influencing the resistance to fatigue crack growth (FCG) in the near-threshold region under three different temper levels in a quenched and tempered high-strength steel. By austenitizing at various temperatures, the PAGS was varied from about 0.7 to 96 μm. The microstructures with these grain sizes were tempered at 200 °C, 400 °C, and 530 °C and tested for fatigue thresholds and crack closure. It has been found that, in general, three different trends in the dependence of both the total threshold stress intensity range, ΔK _th , and the intrinsic threshold stress intensity range, ΔK _{eff, th} , on the PAGS are observable. By considering in detail the factors such as cyclic stress-strain behavior, environmental effects on FCG, and embrittlement during tempering, the present observations could be rationalized. The strong dependence of ΔK _th and ΔK _{eff, th} on PAGS in microstructures tempered at 530 °C has been primarily attributed to cyclic softening and thereby the strong interaction of the crack tip deformation field with the grain boundary. On the other hand, a less strong dependence of ΔK _th and ΔK _{eff, th} on PAGS is suggested to be caused by the cyclic hardening behavior of lightly tempered microstructures occurring in 200 °C temper. In both microstructures, crack closure influenced near-threshold FCG (NTFCG) to a significant extent, and its magnitude was large at large grain sizes. Microstructures tempered at the intermediate temperatures failed to show a systematic variation of ΔK_th and ΔK_{eff, th} with PAGS. The mechanisms of intergranular fracture vary between grain sizes in this temper. A transition from “microstructure-sensitive” to “microstructure-insensitive” crack growth has been found to occur when the zone of cyclic deformation at the crack tip becomes more or less equal to PAGS. Detailed observations on fracture morphology and crack paths corroborate the grain size effects on fatigue thresholds and crack closure. K.S. RAVICHANDRAN, formerly Research Scholar, Department of Metallurgy, Indian Institute of Science