Fatigue crack propagation in aluminum-base copper alloys

Fatigue crack propagation ratesda/dN in binary Al alloys with 3.6 wt pct Cu and 6.3 wt pct Cu and commercial 2024 aged at 21°C were compared with 99.95⁺ wt pct aluminum. Omitting an anomalous region at lowΔK, the extrapolated rates for “pure” aluminum are more than 100 times greater than those in the three alloys at the same ΔK. The data for the alloys fit into a single scatter band of a factor of three. It was suggested thatda/dN varies inversely with the square of the strength of the alloy but that another parameter related to the fatigue crack propagation energy per unit area is also important. Theda/dN vs ΔK curves were determined for 3.6 wt pct Cu single crystals aged seven days at 21°C which containGP zones and two and seven days at 160°C which contain mixtures ofθ′ andθ′’. No systematic variation of (da/dN _Δ with crystallographic orientation was discerned, but the naturally aged specimen had a strong orientation dependence on crack initiation. At low ΔK 21°C aged specimens gave the lowestda/dN while at high ΔK the warm aged specimens gave the lower values ofda/dN. Measurement ofda/dN vs ΔK curves were conducted on specimens of 3.6 wt pct Cu with 1 mm equiaxed grains aged for various times at 130°C, 160°C, and 190°C. All warm aged specimens experienced brittle intergranular fracture at sufficiently high ΔK. The transition ΔK where intergranular fracture first appears is inversely proportional to the aging temperature. The change of fracture mode from intra to intergranular occurs gradually over a broad range of ΔK which shifts to lower ΔK with increase in aging temperature.     

Effect of microstructure,strength, and oxygen content on fatigue crack growth rate of Ti-4.5AI-5.0Mo-1.5Cr (CORONA 5)

Fatigue crack growth rate behavior in CORONA 5, an alloy developed for applications requiring high fracture toughness, has been examined for eight material conditions. These conditions were designed to give differences in microstructure, strength level (825 to 1100 MPa [120 to 160 ksi]), and oxygen content (0.100 to 0.174 wt pct), in such a manner that the separate effects of these variables could be defined. For all eight conditions, fatigue crack growth rates (da/dN) are virtually indistinguishable over the full spectrum of stress-intensity range (ΔK) examined,viz., 8 to 40 MPa√m (7 to 36 ksi√in). Concomitantly, it is noted that over the sizable solution annealing range studied (830° to 915 °C [1525° to 1675 °F]), the primary α-phase morphology was substantially invariant. Eachda/dN curve exhibits a bilinear form with a transition point (ΔKT) between 16 and 19 MPa√m (15 and 17 ksi√in). A change in microfractographic appearance occurs at ΔKT, as extensive secondary cracking along α/β interfaces is observed at all hypertransitional levels ofAK, but not for AK < ΔKT. For each material condition, the mean length of primary α platelets is approximately the same as the cyclic plastic zone size at ΔKT. Accordingly, locations ofAKT (and their similarity for the different material conditions) are rationalized in conformance with a cyclic plastic zone model of fatigue crack growth. Finally, the difference in behavior of CORONA 5, as compared to conventional α/β alloys such as Ti-6A1-4V, is rationalized in terms of crack path behavior.     

Fatigue and fracture of porous steels and Cu-infiltrated porous steels

The effects of Cu infiltration on the monotonic fracture resistance and fatigue crack growth behavior of a powder metallurgy (P/M) processed, porous plain carbon steel were examined after systematically changing the matrix strength via heat treatment. After austenitization and quenching, three tempering temperatures were chosen (177 °C, 428 °C, and 704 °C) to vary the strength level and steel microstructure. The reductions in strength which occurred after tempering at the highest temperature were accompanied by the coarsening of carbides in the tempered martensitic steel matrix, as confirmed by optical microscopy and by microhardness measurements of the steel. Each steel-Cu composite, containing approximately 10 vol pct infiltrated Cu, had superior fracture toughness and fatigue properties compared to the porous matrix material given the same heat treatment. Although the heat treatments given did not significantly change the fatigue behavior of the porous steel specimens, the fatigue curves (da/dN vs ΔK) and fracture properties were distinctly different for the steel-Cu composites given the same three heat treatments. The fracture toughness (K _IC and J _IC ), tearing modulus, and ΔK _TH values for the composites were highest after tempering at 704 °C and lowest after tempering at 177 °C. In addition, the fracture morphology of both the fracture and fatigue specimens was affected by changes in strength level, toughness, and ΔK. These fractographic features in fatigue and overload are rationalized by comparing the size of the plastic zone to the microstructural scale in the composite. This article is based on a presentation made in the symposium “Fatigue and Creep of Composite Materials” presented at the TMS Fall Meeting in Indianapolis, Indiana, September 14–18, 1997, under the auspices of the TMS/ASM Composite Materials Committee.     

Low cycle fatigue crack propagation in hastelloy-X at 25 and 760 °C

Fatigue crack propagation tests were conducted on Hastelloy-X in air, at 25 °C and at 760 °C under controlled plastic strain amplitudes in the fully plastic low cycle fatigue regime. The crack growth rate data for different strain levels were correlated with the range of theJ integral ΔJ. The ΔJ values were calculated from finite element numerical solutions. It was found that the assumption thatda/dN =A(Δε _ρ ) ^α a is only an approximation of the more general equationda/dN =B(ΔJ) ^α in a narrow range of crack lengths. It is shown that theoretical models predicting low cycle fatigue lives by integrating the fully plastic crack growth rates will be in error if the (da/dN, ΔJ) relationship is not used.     

Environmental fatigue of an Al-Li-Cu alloy: part I. Intrinsic crack propagation kinetics in hydrogenous environments

Deleterious environmental effects on steady-state, intrinsic fatigue crack propagation (FCP) rates(da/dN) in peak-aged Al-Li-Cu alloy 2090 are established by electrical potential monitoring of short cracks with programmed constant ΔK andK _maxI loading. Such rates are equally unaffected by vacuum, purified helium, and oxygen but are accelerated in order of decreasing effectiveness by aqueous 1 pct NaCl with anodic polarization, pure water’ vapor, moist air, and NaCl with cathodic polarization. Whileda/dN depend on ΔK^4.0 for the inert gases, water vapor and chloride induce multiple power laws and a transition growth rate “plateau.” Environmental effects are strongest at low ΔK. Crack tip damage is ascribed to hydrogen embrittlement because of acceleratedda/dN due to parts-per-million (ppm) levels of H₂O without condensation, impeded molecular flow model predictions of the measured water vapor pressure dependence ofda/dN as affected by mean crack opening, the lack of an effect of film-forming O₂, the likelihood for crack tip hydrogen production in NaCl, and the environmental and ΔK-process zone volume dependencies of the microscopic cracking modes. For NaCl, growth rates decrease with decreasing loading frequency, with the addition of passivating Li₂CO₃ and upon cathodic polarization. These variables increase crack surface film stability to reduce hydrogen entry efficiency. Small crack effects are not observed for 2090; such cracks do not grow at abnormally high rates in single grains or in NaCl and are not arrested at grain boundaries. The hydrogen environmental FCP resistance of 2090 is similar to other 2000 series alloys and is better than 7075. ROBERT S. PIASCIK formerly Graduate Student, Department of Materials Science, University of Virginia.     

The effects of temperature on fatigue crack propagation in linepipe steel

Fatigue crack propagation rates in an A.P.I. 5L Grade steel were investigated by means of constant deflection amplitude bending fatigue tests at 640 c.p.m. on single edge notched specimens at — 50, — 10, 20 and 70°C in argon. The data were evaluated in terms of the crack propagation rate (da/dN) as a function of the stress intensity range (Δ/K), according toda/dN = ΔK ^m . It was found that dynamic strain aging has a major influence on fatigue crack propagation, resulting in a maximum of the crack propagation rate at room temperature. Similarly, the cyclic plastic zone size is a maximum at room temperature. D. H. Andreasen, formerly with the Department of Mining and Metallurgy, University of Alberta, Edmonton, Alberta, Canada.     

Fatigue crack propagation in a cobalt base aligned eutectic

Fatigue crack propagation rates were determined for directionally solidified Co-10Ni-10Cr-14Ta-1.0C (CoTaC) at room temperature in laboratory air. Single edge crack specimens, 0.25 cm thick, tested in tension-tension at a stress ratio of less than 0.1 produced a relationship between crack growth rates,da/dN, and stress intensity range,AK, as follows:da/dN = 8 × HF △K(m and MN/m). A stress ratio ofR = 0.5 increasedda/dN by a factor of six. A prestrain sufficient to break fibers into 5 to 10 μm long segments had no effect upon subsequent crack growth rate. Compact tension specimens, tested with the stress axis normal to the fiber axis, exhibited more rapid cracking for equivalent △K and a steeper slope, obeying the relationshipda/dN = 1.2 × lO △K. Fractographic examination showed Stage I cracking for △K less than 10 MN/m, mixed Stage I and Stage II cracking for 10 MN/m <AK < 20 MN/m and only Stage II cracking for larger △K. The extent of fiber failure was measured and found to be proportional toK _max. The plastic zone size was shown to be three times greater at the surface than at the interior.     

Fatigue-crack growth in Ti-6Al-4V-0.1Ru in air and seawater: Part I. Design of experiments, assessment, and crack-grown-rate curves

Fatigue-crack growth rates for different simulated ocean environments and loading conditions have been investigated for beta-annealed Ti-6Al-4V-0.1Ru (extra-low interstitials, (ELI)), a candidate material for oil production risers; the focus was on uncovering whether certain combinations of conditions could produce unexpectedly high crack growth rates. A two-level, one-quarter-fraction factorial design-of-experiments (DOE) approach was used to ascertain which testing variables and environmental conditions warranted further study. This study used eight different combinations of variables: parent/deformed material, 27 °C/85 °C temperature, 2 Hz/20 Hz loading frequency, 0.1/0.6 load ratio (R=σ _min/σ _max, where σ _min is the minimum and σ _max is the maximum stress during a fatigue cycle), and aerated/deaerated seawater. Comparisons were based on crack growth rates at ΔK=17 MPa , roughly the middle of the Paris portion of the da/dN vs ΔK curves. The da/dN vs Δ_K curves were also examined, and conclusions based upon these data were compared with those from the DOE. Consideration of the microstructure’s influence on the crack path is postponed until Part II of this article. Samples tested at the higher load ratio showed a statistically significant increase in the crack propagation rate compared to those tested at R=0.1; the same was true of specimens tested at 20 Hz vs those tested at 2 Hz, but the level of significance was lower. The parent material had somewhat higher crack growth rates than the deformed samples. Changes in environmental conditions other than frequency produced little effect on the crack growth rate. Comparison of crack growth rates over the ΔK range measured revealed details that would have not been uncovered in comparisons at a single ΔK value. The Paris exponent ranged between 3.7 and 6.7, and the only systematic variation observed was an increase in the exponent with increasing test frequency. In seawater, cold work (a 5 pct reduction in thickness by rolling) reduced fatigue-crack growth rates by a factor of 2 (compared to the parent material) at intermediate and high ΔK values. There was a crossover of crack growth rates for low ΔK values: below 10 MPa , growth rates were lower for the parent material than for the cold-rolled material, suggesting a higher ΔK _th for the parent material, while above this value, fatigue cracks grew more rapidly in the parent material than in the cold-rolled material. Crack growth rates were slightly higher in seawater than in air, but only slightly more than the sample-to-sample variation of crack growth rates, and cold work reduced fatigue-crack growth rates in air by about the same amount as in seawater. Somewhat more scatter was observed for the R=0.1 tests than for the R=0.6 tests. Differences in temperature (27 °C, 53 °C, and 85 °C) do not appear to affect fatigue-crack growth rates. For ΔK<20 MPa , crack growth rates were similar for 0.2 and 2 Hz but were higher for 20 Hz; above 20 MPa , the crack growth rates were similar for all three frequencies. One explanation for the unusual frequency dependence relies on the possibility that the environment produces different amounts of closure for different test frequencies. According to this view, closure is effective in air and in seawater at 0.2 and 2 Hz but not at 20 Hz: perhaps the higher loading rate breeches the passive layer at a rate more rapid than it can reform. Because the crack growth rate appeared independent of temperature, it is unlikely that there is a significant influence of thermally activated corrosion-fatigue mechanisms for the conditions tested. The results demonstrate that beta-annealed Ti-6V-4Al-0.1Ru (ELI) possesses a robust response to the combinations of environment and loading expected in oil production riser service. The value of the DOE approach was clear, and supplementary tests verified the main effects predicted by the DOE results. Comparison of the single-value DOE results with the da/dN curves reveals a limitation in the former: different slopes of the Paris curves and crossover effects are or would be missed for DOE comparing crack growth rates derived from constant ΔK tests. The use of constant Δσ data and a second level of interrogation, following DOE analysis and based on the da/dN curves, addressed this limitation effectively. A DOE comparison based, for example, on three ΔK values (the lower, middle, and upper portions of the Paris regime) might be another way of proceeding.     

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 effect of cold work on the precipitation of Ω and θ′ in a ternary Al-Cu-Mg alloy

The effect of plastic deformation on the microstructural evolution of an Al-5.0Cu-0.5Mg (wt pct) ternary alloy was investigated. Hardness measurements and quantitative precipitate analysis were performed on specimens that were water quenched from a solution heat treatment, stretched either 0 or 6 pct and immediately aged at ambient temperature or artificially aged at 200 °C or 250 °C for times up to 3000 hours. Quantitative transmission electron microscopy (TEM) was used to characterize Ω and θ′ number density, diameter, and thickness as a function of preage mechanical stretch and artificial aging condition. Age hardening curves for naturally and artificially aged specimens revealed an increase in hardness corresponding with a preage stretch. Quantitative TEM verified an increase in the number density and a refinement of precipitates for both Ω and θ′ between the 0 and 6 pct stretch condition for those samples artificially aged. When aged at 200 °C, θ′ exhibited superior coarsening resistance relative to the Ω phase. The quantified Ω coarsening kinetics were greater than similar Ag-containing alloys. To investigate the effects of trace Si additions on subsequent microstructural evolution, a series of Al-Cu-Mg-Si quaternary alloys were produced. The addition of 0.1Si (wt pct) was found to suppress Ω precipitation in most Al-4.0Cu-xMg alloys investigated. These initial results indicate that Ω precipitation may be related to the Mg/Si ratio.     

The effects of heat treatment on fracture toughness and fatigue crack growth Rates in 440C and BG42 steels

The fatigue crack growth rates,da/dN, and the fracture toughness, K_Ic have been measured in two high-carbon martensitic stainless steels, 440C and BG42. Variations in the retained austenite contents were achieved by using combinations of austenitizing temperatures, refrigeration cycles, and tempering temperatures. In nonrefrigerated 440C tempered at 150 °C, about 10 vol pct retained austenite was transformed to martensite at the fracture surfaces duringK _Ic testing, and this strain-induced transformation contributed significantly to the fracture toughness. The strain-induced transformation was progressively less as the tempering temperature was raised to 450 °C, and at the secondary hardening peak, 500 °C, strain-induced transformation was not observed. In nonrefrigerated 440C austenitized at 1065 °C,K _Ic had a peak value of 30 MPa m^1/2 on tempering at 150 °C and a minimum of 18 MPa m^1/2 on tempering at 500 °C. Refrigerated 440C retained about 5 pct austenite, and did not exhibit strain-induced transformation at the fracture surfaces for any tempering temperature. TheK _Ic values for corresponding tempering temperatures up to the secondary peak in refrigerated steels were consistently lower than in nonrefrigerated steels. All of the BG42 specimens were refrigerated and double or quadruple tempered in the secondary hardening region; theK _Ic values were 16 to 18 MPa m^1/2 at the secondary peak. Tempered martensite embrittlement (TME) was observed in both refrigerated and nonrefrigerated 440C, and it was shown that austenite transformation does not play a role in the TME mechanism in this steel. Fatigue crack propagation rates in 440C in the power law regime were the same for refrigerated and nonrefrigerated steels and were relatively insensitive to tempering temperatures up to 500 °C. Above the secondary peak, however, the fatigue crack growth rates exhibited consistently lower values, and this was a consequence of the tempering of the martensite and the lower hardness. Nonrefrigerated steels showed slightly higher threshold values, ΔK_th, and this was ascribed to the development of compressive residual stresses and increased surface roughening in steels which exhibit a strain-induced martensitic transformation.     

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 .     

The effect of microstructure on fatigue crack propagation in iron-carbon alloys

The dependence of fatigue crack growth rate on the cyclic stress intensity factor was determined for six iron-carbon alloys ranging in carbon content from 0.23 to 1.08 wt pct carbon. Both ferrite/pearlite and ferrite/free iron carbide microstructures were studied. Scanning electron microscope fractography studies correlated the fatigue mechanism with microstructure. It was found that when the predominant mode of crack growth was ductile, the crack growth rateda/dN could be related to the cyclic stress intensity factor ΔK by an equation of the formda/dN = (ΔK)_m where andm are constants. The constantm was approximately equal to four when the crack growth mechanism presumably was the blunting and resharpening of the crack tip by slip processes. The constantm was greater than four when the crack growth mechanism was void coalescence in the interlamella ferrite of pearlite colonies. The preferred fatigue crack path through the pearlitic alloys was through the free ferrite phase. formerly Research Assistant at Materials Science and Engineering Department and Materials Research Center, Northwestern University.     

Fatigue crack initiation and microcrack growth in 4140 steel

Initiation and growth of fatigue microcracks were studied in vacuum degassed 4140 steel in three conditions: as-quenched, tempered at 400°C, and tempered at 650°C. Micro-scopic examinations were made of specimens with metallographically polished notches using a 400 times long working distance microscope with an x-y micrometer base mounted directly on an MTS machine. Following Barsom and McNicol, the cycles to fatigue crack initiationN _i were plottedvs ΔK/√p and threshhold values of gDK√p were determined. The data of logN _i vs log [ΔK/√p-ΔK/√p|_th] fit on a straight line. Microcracks grew most rapidly in as-quenched specimens and least rapidly in 650°C tempered specimens at the same ΔK/√p. In as-quenched specimens, fatigue cracks initiated at grain boundaries but in the 400 and 650°C tempered specimens they initiated at intrusions-extrusions. They are also associated with Northwestern’s Materials Research Center.     

Environmental fatigue of an Al-Li-Cu alloy: Part II. Microscopic hydrogen cracking processes

Microscopic fatigue crack propagation (FCP) paths in peak-aged unrecrystallized alloy 2090 are identified as functions of intrinsicda/dN- ΔK kinetics and environment. The FCP rates in longitudinal-transverse (LT)-oriented 2090 are accelerated by hydrogen-producing environments (pure water vapor, moist air, and aqueous NaCl), as defined in Part I. Subgrain boundary cracking (SGC) dominates for ΔK values where the cyclic plastic zone is sufficient to envelop subgrains. At low ΔK, when this crack tip process zone is smaller than the subgrain size, environmental FCP progresses on or near (100) or (110) planes, based on etch-pit shape. For inert environments (vacuum and He) and pure O₂ with crack surface oxidation, FCP produces large facets along 111 oriented slip bands. This mode does not change with ΔK, andT ₁ decorated subgrain boundaries do not affect an expectedda/dN- ΔK transition for the inert environments. Rather, the complex dependence ofda/dN on ΔK is controlled by the environmental contribution to process zone microstructure-plastic strain interactions. A hydrogen embrittlement mechanism for FCP in 2090 is supported by similar brittle crack paths for low pressure water vapor and the electrolyte, the SGC and 100/110 crystallographic cracking modes, the influence of cyclic plastic zone volume ( ΔK), and the benignancy of O₂. The SGC may be due to hydrogen production and trapping atT ₁ bearing sub-boundaries after process zone dislocation transport, while crystallographic cracking may be due to lattice decohesion or hydride cracking. Robert S. Piascik, formerly Graduate Student, Department of Materials Science, University of Virginia.     

Structure and properties of a β solution treated,quenched, and aged si-bearing near-α titanium alloy

The microstructure, tensile properties, and fractographic features of a near-α titanium alloy, IMI 829(Ti-6.1 wt pct Al-3.2 wt pct Zr-3.3 wt pct Sn-1 wt pct Nb-05 wt pct Mo-0.32 wt pct Si) have been studied after aging over a temperature range of 550°C to 950°C for 24 hours following solution treatment in the β phase field at 1050°C and water quenching. Transmission electron microscopy studies revealed that aging at 625°C and above produced discrete silicides at α′ interplatelet boundaries. However, aging at 900°C and above has also resulted in the precipitation of β phase along the lath boundaries of martensite. The silicides have been found to have a hexagonal structure withc=0.36 nm anda=0.70 nm (designated as S₂ by earlier workers). There is a significant improvement in yield and ultimate tensile strength after aging at 625°C, but there is less improvement at higher aging temperatures. The tensile ductility is found to be drastically reduced. While the fracture surface of the unaged specimen shows elongated dimples, the aged samples show a mixed mode of fracture, consisting of facets, featureless parallel bands, and extremely fine dimples.     

Fatigue crack propagation in 4140 steel

The fatigue crack propagation rates, da/dN, of 4140 steel were measured in dry argonvs tempering temperature. In specimens 3.2 mm thick at a given ΔK between 15 and 30 MN/ m^3/2, da/dN decreases with increasing tempering temperature, reaches a shallow minimum for tempering at 400°C. The rate for as-quenched specimens increases withR ratio; this is not the case for the 400, 550 and 650°C tempers. Reducing the specimen thickness to 1.3 mm has little effect on the 650°C temper but causes a large decrease in da/dN for the asquenched condition and 200°C temper. Edge notch specimens tempered at 550 and 650°C are subject to crack arrest from cycling prior to crack initiation. The results are discussed in terms of the metallurgical structures and various fatigue crack propagation equations which have been proposed. The results cannot be explained on the basis of da/dN being determined only by Young’s modulus andK _c.     

The effect of molybdenum on γ′ coarsening and on elevated-temperature hardness in some experimental nickel-base superalloys

The coarsening of γ′ and the elevated-temperature hardness have been studied as a function of molybdenum content, time, and temperature in experimental wrought nickel-base superalloys. The alloys were selected from a systematic series containing 3, 4 1/2, and 6 wt pct Al and 1 wt pct Al plus 3 1/2 wt pct Ti. Each of the aluminum (plus titanium) series consisted of four alloys containing 0, 2, 5, and 8 wt pct Mo. The alloys were solution-treated plus aged up to 112 h at 1700°F (925°C) and up to 1000 h at 1400°F (760°C). Molybdenum retards the coarsening of γ′ on aging; this retarding effect is most pronounced in alloys containing 6 wt pct Al. The coarsening of γ′ particles follows Ostwald ripening kinetics. Hardness testingin vacuo at temperatures up to 1750°F (955°C) shows that molybdenum also increases the elevated-temperature hardness significantly. The relation of elevated-temperature hardness to the volume fraction of γ′ is considered, and the influence of aluminum and titanium contents is discussed.     

Fatigue-crack growth in Ti-6Al-4V-0.1Ru in air and seawater: Part II. crack path and microstructure

In Part I of this article, the influence of various testing parameters and environments on the fatigue-crack growth rates in samples of beta-annealed Ti-6Al-4V-0.1Ru (extra-low interstitials) ELI was reported.^[1] A design-of-experiments (DOE) approach was used to survey different combinations of variables, all expected to be important for dynamically loaded offshore oil and gas production risers, and to identify significant effects on the fatigue-crack propagation rate at a stress-intensity range of ΔK=17 MPa . The da/dN vs ΔK curves also were examined for the DOE and supplementary tests, and the results of the two approaches were compared. In this part of the study, the microstructural basis for the robust fatigue-crack growth resistance of beta-annealed Ti-6Al-4V-0.1Ru (ELI) samples was investigated with optical metallography and scanning electron microscopy (SEM). A gradual transition from structure-sensitive (microfacet formation) to structure-insensitive (striation formation) crack propagation centered at ΔK _trans ≅24 MPa , regardless of the combination of testing/environmental conditions examined; the absence of a sharp transition in the slope of the da/dN vs ΔK curves was, therefore, entirely consistent with the fracture-surface morphology. The size of the reversed cyclic plastic zone at the transition ΔK value correlated with the size of the lamella packets, but the number of cycles required to generate a striation ranged between one and ten, suggesting that the crack actively grew over only a portion of its front at any one instant. It is interesting to note that ΔK _trans was the same as the stress range where the da/dN curves at 0.2, 2, and 20 Hz (in seawater) converged to a single curve (refer to Part I of this article): at lower stress ranges, crack growth rates at 20 Hz were significantly higher than those at 0.2 and 2 Hz. The quantitative data showed that fatigue cracks propagated parallel to lamellae interfaces when the long axes of the lamellae made a relatively small angle (<30 deg) to the nominal crack-propagation direction. The crack cut directly across lamellae (i.e., perpendicular to their surfaces) when the long axes of the lamellae were nearly perpendicular to the nominal crack-propagation direction. If the lamellae long axes lay 45 deg to the crack-propagation direction, the crack deflected to run parallel or perpendicular to the lamellae. This behavior occurred regardless of the environment and loading conditions investigated. There was considerable variation in the amount that the cracks deviated from their nominal plane (i.e., the plane normal to the load axis and through the notch tip), with much greater deflections in the cold-rolled than in the parent material, but the angle of the macroscopic crack plane did not exceed 11 deg. Crack branching was observed both at the center and outer surfaces of the samples, regardless of ΔK or other parameters. The relationship between micro- and macrobranches was examined, and branching was more prominent below ΔK _trans, which separated the structure-sensitive and continuum-mode crack-propagation regimes. The relative amounts of micro- and microbranches are reported, and this branching may explain the large scatter in the measurements of the fatigue-crack growth rate often encountered in Ti-6Al-4V and its variants and points to the need for thorough characterization of crack paths, both midplane and surface, as part of the interpretation of da/dN vs ΔK data.     

Cu precipitation in a prestrained Fe-1.5 wt pct Cu alloy during isothermal aging

The control of Cu precipitation at low temperatures, e.g., bake hardening of Cu bearing steels, has recently attracted considerable attention due to the potential of achieving good formability and high strength. An Fe-1.5 wt pct Cu alloy, solution treated and 10 pct prestrained, exhibits a two-step age-hardening behavior, i.e., a smaller, but substantial hardening around 200 °C to 300 °C and a major hardening around 500 °C, while only the latter hardening occurs in undeformed specimens. The precipitation behavior of nanoscale Cu particles or bcc Cu clusters that plays a major role in age hardening was simulated by Cahn-Hilliard nonclassical nucleation theory and the Langer-Schwartz model. Simulation results are compared with the distribution of Cu particles observed under three-dimensional atom probe field ion microscope (3-D APFIM) and transmission electron microscope (TEM), and age hardening behavior as well. The increase in hardness in prestrained specimens at low temperatures (≤400 °C) can be ascribed to Cu particles nucleated preferentially at dislocations or to Cu particles that were formed in the matrix as early as at dislocations presumably due to excess vacancies introduced by prestraining.