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.     

Intrinsic fatigue crack growth

The influences of microstructure and deformation mode on inert environment intrinsic fatigue crack propagation were investigated for Al-Li-Cu-Mg alloys AA2090, AA8090, and X2095 compared to AA2024. The amount of coherent shearable δ (Al₃Li) precipitates and extent of localized planar slip deformation were reduced by composition (increased Cu/Li in X2095) and heat treatment (double aging of AA8090). Intrinsic growth rates, obtained at high constantK _max to minimize crack closure and in vacuum to eliminate any environmental effect, were alloy dependent;da/dN varied up to tenfold based on applied ΔK or ΔK/E. When compared based on a crack tip cyclic strain or opening displacement parameter (ΔK/(σ_ys E)^1/2), growth rates were equivalent for all alloys except X2095-T8 which exhibited unique fatigue crack growth resistance. Tortuous fatigue crack profiles and large fracture surface facets were observed for each Al-Li alloy independent of the precipitates present, particularly δ, and the localized slip deformation structure. Reduced fatigue crack propagation rates for X2095 in vacuum are not explained by either residual crack closure or slip reversibility arguments; the origin of apparent slip band facets in a homogeneous slip alloy is unclear. Better understanding of crack tip damage accumulation and fracture surface facet crystallography is required for Al-Li alloys with varying slip localization.     

The effect of intermediate thermomechanical treatments on the fatigue properties of a 7050 aluminum alloy

A study was undertaken to determine the effect of microstructures produced by different ingot processing techniques on the fatigue properties of a 7050 aluminum alloy. The different microstructures investigated were produced by hot-rolling to simulate commercial processing (CP) methods or intermediate thermomechanical treatments (ITMT). Characterization of the microstructures revealed that the CP 7050 material was partially recrystallized (<50 pct) due to the use of hot-rolling as the final deformation step. The ITMT materials were examined in the as-recrystallized (AR) condition or in AR + hot rolled condition (AR + HR). Results of the investigation showed thattotal fatigue life, both low and high cycle, were not greatly affected by the grain structures of the experimental materials. However, metallographic studies indicated that crack initiation is probably more difficult in the fine-grained AR material. The results of fatigue crack growth tests showed that higher crack growth rates observed at low ΔK values for ITMT {dy7050} were most likely due to the detrimental effects of undissolved Al₂CuMg particles. These particles, which also contribute to low fracture toughness and higher crack growth rates at high ΔK levels, are formed during a furnace-cooling step in the ITMT processing schedule.     

Elevated temperature fatigue crack growth in incoloy alloy 800 in sulfidizing environments

Rates of fatigue crack growth in INCOLOY alloy 800 were measured in He-SO₂ and He-H₂S environments at a frequency of 0.1 Hertz over a temperature range of 316 to 650 °C. Increasing concentrations of sulfur and increasing temperatures increased the growth rates at higher △K values. H₂S environments were more aggressive than SO₂-containing gases, probably because of the inhibiting effects of oxygen when SO₂ reacted with the alloy. At lower crack growth rates, sulfide particle formation at the crack tips caused cracks to decelerate and stop due to crack tip blunting and inhibition of crack closure. Sulfide particle formation also promoted the formation of periodic crack opening jumps. Under static loading, cracking could be produced only by a combination of prior cyclic deformation and the presence of a sulfidizing environment. The results suggest that sulfidizing environments enhanced fatigue crack growth through enrichment of sulfur ahead of the crack tip, producing a time-dependent crack growth process.     

Effects of Processing Residual Stresses on Fatigue Crack Growth Behavior of Structural Materials: Experimental Approaches and Microstructural Mechanisms

Fatigue crack growth mechanisms of long cracks through fields with low and high residual stresses were investigated for a common structural aluminum alloy, 6061-T61. Bulk processing residual stresses were introduced in the material by quenching during heat treatment. Compact tension (CT) specimens were fatigue crack growth (FCG) tested at varying stress ratios to capture the closure and K _max effects. The changes in fatigue crack growth mechanisms at the microstructural scale are correlated to closure, stress ratio, and plasticity, which are all dependent on residual stress. A dual-parameter ΔK–K _max approach, which includes corrections for crack closure and residual stresses, is used uniquely to connect fatigue crack growth mechanisms at the microstructural scale with changes in crack growth rates at various stress ratios for low- and high-residual-stress conditions. The methods and tools proposed in this study can be used to optimize existing materials and processes as well as to develop new materials and processes for FCG limited structural applications.     

Influence of heat treatment on the fatigue crack growth rates of a secondary hardening steel

The relationships between microstructure and fatigue crack propagation behavior were studied in a 5Mo-0.3C steel. Microstructural differences were achieved by varying the tempering treatment. The amounts, distribution, and types of carbides present were influenced by the tempering temperature. Optical metallography and transmission electron microscopy were used to characterize the microstructures. Fatigue fracture surfaces were studied by scanning electron microscopy. For each heat treatment the fatigue crack growth properties were measured under plane strain conditions using a compact tension fracture toughness specimen. The properties were reported using the empirical relation of Paris [da/dN = C_oΔK^m]. It was found that secondary hardening did influence the fatigue crack growth rates. In particular, intergranular modes of fracture during fatigue led to exaggerated fatigue crack growth rates for the tempering treatment producing peak hardness. Limited testing in a dry argon atmosphere showed that the sensitivity of fatigue crack growth rates to environment changed with heat treatment.     

Fatigue crack growth through alloyed niobium, Nb-Cr2Nb, and Nb-Nb5Si3 in situ composites

Fatigue cracks were grown through several niobium-based materials. For Nb-Cr-Ti composition materials, the single-phase alloy represented the matrix of two in situ composites with about 22 and 38 vol pct Cr₂Nb. Grain boundaries were coated with intermetallic in the lower-volume fraction material, while the 38 vol pct Cr₂Nb composite consisted of mainly spherical, dispersed intermetallic. The Nb-10Si composite was composed of about 28 vol pct primary Nb₅Si₃, with most of the matrix alloy in “fiberlike” shapes due to extrusion. Crack growth rates through the composites were generally faster than for unalloyed Nb, roughly in proportion to the volume fraction of intermetallic, although differences in microstructure make this comparison difficult. The presence of intermetallic greatly alters deformation of material near the crack tip. Particles of Cr₂Nb were broken during the crack growth process, leading to increased crack growth rates. These results suggest microstructural modifications that could be expected to enhance fatigue crack growth resistance.     

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.     

The effects of test temperature,temper, and alloyed copper on the hydrogen-controlled crack growth rate of an Al-Zn-Mg-(Cu) alloy

The hydrogen-environment embrittlement (HEE)-controlled stage II crack growth rate of AA 7050 (6.09 wt pct Zn, 2.14 wt pct Mg, and 2.19 wt pct Cu) was investigated as a function of temper and alloyed copper level in a humid air environment at various temperatures. Three tempers representing the underaged (UA), peak-aged (PA), and overaged (OA) conditions were tested in 90 pct relative humidity (RH) air at temperatures between 25 °C and 90 °C. At all test temperatures, an increased degree of aging (from UA to OA) produced slower stage II crack growth rates. The stage II crack growth rate of each alloy and temper displayed an Arrhenius-type temperature dependence, with activation energies between 58 and 99 kJ/mol. For both the normal-copper and low-copper alloys, the fracture path was predominately intergranular at all test temperatures (25 °C to 90 °C) in each temper investigated. Comparison of the stage II HEE crack growth rates for normal- (2.19 wt pct) and low- (0.06 wt pct) copper alloys in the peak PA aged and OA tempers showed a beneficial effect of copper additions on the stage II crack growth rate in humid air. In the 2.19 wt pct copper alloy, the significant decrease (∼10 times at 25 °C) in the stage II crack growth rate upon overaging is attributed to an increase in the apparent activation energy for crack growth. In the 0.06 wt pct copper alloy, overaging did not increase the activation energy for crack growth but did lower the pre-exponential factor (v ₀), resulting in a modest (∼2.5 times at 25 °C) decrease in the crack growth rate. These results indicate that alloyed copper and thermal aging affect the kinetic factors that govern stage II HEE crack growth rates. The OA, copper-bearing alloys are not intrinsically immune to hydrogen-environment-assisted cracking, but are more resistant due to an increased apparent activation energy for stage II crack growth.     

The role of heat treating on the sour gas resistance of an X-80 steel for oil and gas transport

In this work, the role of the microstructure in the stress sulfide cracking (SSC) resistance of an API X-80 steel was investigated by exposure of as-received and heat-treated specimens to a H₂S-saturated aqueous National Association of Corrosion Engineers (NACE) solution. It was found that for similar corrosive environments and applied stress intensity factors of 30 to 46 MPa√m, crack growth in LEFM (linear elastic fracture mechanics) compact specimens is strongly influenced by heat treating. In the as-received alloy, crack growth in the direction normal to rolling was controlled by metal dissolution of the crack tip region in contact with the corrosive environment, with crack growth rates of the order of 1/W(da/dt)∼8.3×10⁻⁴ h⁻¹. Alternatively, crack growth in the direction parallel to the rolling direction did not show metal dissolution, but instead hydrogen embrittlement along segregation bands. In this case, crack growth rates of the order of 1.2×10⁻³ h⁻¹ were exhibited. In the martensitic condition, the rate of crack propagation was relatively fast (1/W(da/dt)∼4.5×10⁻² h⁻¹), indicating severe hydrogen embrittlement. Crack arrest events were found to occur in water-sprayed and quenched and tempered specimens, with threshold stress intensity values (K _ISSC) of 26 and 32 MPa√m, respectively. Apparently, in the water-sprayed condition, numerous microcracks developed in the crack tip plastic zone. Crack growth occurred by linking of microcracks, which were able to reach the main crack tip. In particular, preferential microcrack growth occurred across carbide regions, but their growth was severely limited in the ferritic matrix. Quenching and tempering (Q&T) resulted in a tempered martensite microstructure characterized by fine distribution carbides, most of which were cementite. In this case, the crack path continually shifted to follow the ferrite interlath boundaries, which contained mostly fine cementite precipitates. As a result, the crack was tortuous with numerous bifurcations along ferrite grain boundaries. Most of the tests were carried out in NaCl-free NACE solutions; the only exception was the as-received condition where 5 wt pct NaCl was added to the sour environment. In this case, crack growth did not occur after exposing the specimen to the salt-free NACE solution for 30 days, but addition of 5 pct NaCl promoted crack propagation.     

The effects of test temperature,temper, and alloyed copper on the hydrogen-controlled crack growth rate of an Al-Zn-Mg-(Cu) alloy

The hydrogen-environment embrittlement (HEE)-controlled stage II crack growth rate of AA 7050 (6.09 wt pct Zn, 2.14 wt pct Mg, and 2.19 wt pct Cu) was investigated as a function of temper and alloyed copper level in a humid air environment at various temperatures. Three tempers representing the underaged (UA), peak-aged (PA), and overaged (OA) conditions were tested in 90 pct relative humidity (RH) air at temperatures between 25 °C and 90 °C. At all test temperatures, an increased degree of aging (from UA to OA) produced slower stage II crack growth rates. The stage II crack growth rate of each alloy and temper displayed an Arrhenius-type temperature dependence, with activation energies between 58 and 99 kJ/mol. For both the normal-copper and low-copper alloys, the fracture path was predominately intergranular at all test temperatures (25 °C to 90 °C) in each temper investigated. Comparison of the stage II HEE crack growth rates for normal- (2.19 wt pct) and low- (0.06 wt pct) copper alloys in the peak PA aged and OA tempers showed a beneficial effect of copper additions on the stage II crack growth rate in humid air. In the 2.19 wt pct copper alloy, the significant decrease (∼10 times at 25 °C) in the stage II crack growth rate upon overaging is attributed to an increase in the apparent activation energy for crack growth. In the 0.06 wt pct copper alloy, overaging did not increase the activation energy for crack growth but did lower the pre-exponential factor (v ₀), resulting in a modest (∼2.5 times at 25 °C) decrease in the crack growth rate. These results indicate that alloyed copper and thermal aging affect the kinetic factors that govern stage II HEE crack growth rates. The OA, copper-bearing alloys are not intrinsically immune to hydrogen-environment-assisted cracking, but are more resistant due to an increased apparent activation energy for stage II crack growth. An erratum to this article is available at .     

Near-threshold fatigue crack growth in 8090 Al-Li alloy

Near-threshold fatigue crack growth was studied in 8090-T8771 Al-Li alloy tested in moist laboratory air. The testing was conducted using (1) the ASTM E-647 load-shedding procedure, (2) a power-law load-shedding procedure, and (3) a constant-amplitude (CA) loading procedure. Crack closure in the three procedures was analyzed. In reconciling fatigue crack growth rates (FCGRs) with different crack closure levels under identical testing parameters, the conventional ΔK _eff (=K _max —K _op) fails to correlate the test data and the modified ΔK eff (=K _max - _χK_op, where χ is the shielding factor, defined by an energy approach) is proven to be the true crack driving force. A parallel slip-rupture model is proposed to describe the mechanism of near-threshold fatigue crack growth in this alloy. The model explains the mode transition from crystallographic slip band cracking (SBC) to subgrain boundary cracking (SGC)/brittle fracture (BF) in terms of a microstructure-environment synergy. The transition is related to the material’s short-transverse grain size.     

The Effect of Grain Size on Fatigue Growth of Short Cracks

The influence of alloy grain size on growth rates of surface cracks 20 to 500 μm in length was studied in Al 7075-T6 specimens prepared in 12 and 130 μn grain sizes. Grain boundaries temporarily interrupt the propagation of cracks shorter than several grain diameters in length. Linear elastic fracture mechanics is inadequate to describe resulting average growth rates which must instead be characterized as a function of cyclic stress amplitude, σ_a, and alloy grain size as well as stress intensity range, σK. These observations are rationalized using two models, one that relates crack closure stress to alloy grain size, and a second that relates the development of microplasticity in a new grain in the crack path to grain size. In addition, growth rates were found to be faster in fully reversed loading than in tension-tension loading, especially in the large grained material. Evidence is presented to demonstrate that this is a consequence of the fatigue induced development of a compressive residual surface stress during tension-tension loading. These complex effects, and the role of grain size in determining short crack growth, are discussed.     

High cycle fatigue and fatigue crack growth of the oxide dispersion strengthened alloy MA 754

The high cycle fatigue (HCF) behavior of the oxide dispersion strengthened (ODS) MA 754 alloy has been determined as a function of specimen orientation. The fatigue life showed anisotropic behavior with the longest and shortest lives in the longitudinal and short transverse directions, respectively. Surface porosity, due to oxidation, was found to affect fatigue life in the long transverse orientation more than in the longitudinal orientation. The fatigue crack growth behavior in MA 754 exhibited a directional dependence. In general, the crack growth rates in the longitudinal direction were lower than those in the long transverse direction. The ΔK _th was ∼11 MN ·^-3/2 and 9 MN · m^-3/2 for the longitudinal and the long transverse orientation, respectively. This behavior was explained on the basis of the unusual grain structure and the texture exhibited by this alloy as well as different crack closure effects. It was found that a consideration based on the crack growth rates results, obtained from fracture mechanics specimens, could not explain the anisotropic behavior of the HCF properties of MA 754. However, the anisotropic HCF properties could be rationalized on the basis of the differences in the modes of crack initiation.     

Mechanisms of Slow Fatigue Crack Growth in High Strength Aluminum Alloys: Role of Microstructure and Environment

The role of microstructure and environment in influencing ultra-low fatigue crack propagation rates has been investigated in 7075 aluminum alloy heat-treated to underaged, peak-aged, and overaged conditions and tested over a range of load ratios. Threshold stress intensity range, ΔK₀, values were found to decrease monotonically with increasing load ratio for all three heat treatments fatigue tested in 95 pct relative humidity air, with ΔK ₀ decreasing at all load ratios with increased extent of aging. Comparison of the near-threshold fatigue behavior obtained in humid air with the data forvacuo, however, showed that the presence of moisture leads to a larger reduction in ΔK₀ for the underaged microstructure than the overaged condition, at all load ratios. An examination of the nature of crack morphology and scanning Auger/SIMS analyses of near-threshold fracture surfaces revealed that although the crack path in the underaged structure was highly serrated and nonlinear, crack face oxidation products were much thicker in the overaged condition. The apparent differences in slow fatigue crack growth resistance of the three aging conditions are ascribed to a complex interaction among three mechanisms: the embrittling effect of moisture resulting in conventional corrosion fatigue processes, the role of microstructure and slip mode in inducing crack deflection, and crack closure arising from a combination of environmental and microstructural contributions.     

Dispersoid growth in Ni-2ThO2 due to self-diffusion sintering of particle clusters

The growth rates and activation energy for growth of thoria particles in thoria dispersion-strengthened nickel were measured after elevated temperature exposure. Growth was determined by direct measurement of thoria particle size on transmission electron micrographs. The thoria particles were distributed spatially in clusters in the hot consolitated alloy. Presence of particle clusters was found to provide the proper environment for a self-diffusion sintering-type particle coarsening. The rapid particle growth observed in this alloy form had too high a rate constant and too low an activation energy to constitute an Ostwald ripening mechanism, and was considered to reflect the grain growth stage of sintering. Further studies showed that thermomechanical treatments of hot consolidated material retarded this type of particle growth, by mechanically dispersing the ThO₂ particle clusters. W. SCHEITHAUER, JR. is currently on leave of absence at Lehigh University, Department of Industrial Engineering.     

The Small Fatigue Crack Growth Behavior of an AM60 Magnesium Alloy

The effects of thermomechanical processing and subsequent heat treatment on the small fatigue crack growth (FCG) behavior of an AM60 (Mg-6.29Al-0.28Mn wt pct) alloy were evaluated. The effects of mechanical loading parameters, such as maximum stress and load-ratio, on the small FCG behavior were also determined. Maximum stress did not appear to affect the crack propagation rate of small cracks in the stress and crack size ranges considered. Materials with different microstructures and yield stresses, introduced by different processing conditions, showed similar crack growth rates at equivalent stress intensity factor ranges. The effect of load ratio on small crack growth rates was recorded. Fracture surface characterization suggested that the fatigue crack propagation mechanism was a mixture of transgranular and intergranular cracking. Porosity and other material defects played respective important roles in determining the fatigue crack initiation and propagation behavior.