An investigation of the effects of loading rate on resistance-curve behavior and toughening in cast lamellar gamma-based titanium aluminides

This article presents the results of a combined experimental and theoretical study of the effects of loading rate (1, 10, and 100 MPa√m · s⁻¹) on the resistance-curve behavior and toughening in cast lamellar gamma-based titanium aluminides (Ti-48Al-2Cr-2Nb, Ti-45Al-2Mn-2Nb + 0.8 vol pct TiB₂, and Ti-47Al-2Mn-2Nb + 0.8 vol pct TiB₂). Note that compositions are quoted in at. pct unless stated otherwise. The fracture-initiation toughness and resistance-curve behavior in Ti-48Al-2Cr-2Nb are shown to be similar at the three loading rates examined. In the case of the Mn-containing alloys, stronger resistance-curve behavior is observed as the loading rate increases from 1 to 10 MPa√m · s⁻¹. However, the fracture-initiation toughness and resistance-curve behavior of the Mn-containing alloys are similar at loading rates of 10 and 100 MPa√m · s⁻¹. The observed resistance-curve behavior is attributed largely to the role of ligament bridging and, to a lesser extent, to the effects of cracktip plasticity. Small- and large-scale bridging models are also shown to predict the measured resistance curves when the observed/measured bridging parameters and material properties are used in the micromechanical modeling of crack bridging. The implications of the results are also discussed for the design of damage-tolerant gamma alloys and microstructures.     

Fatigue and fracture toughness of a Nb-Ti-Cr-Al-X single-phase alloy at ambient temperature

The fatigue and fracture resistance of a commercially made, single-phase Nb-base alloy with 35 at. pct Ti, 5 at. pct Cr, 6 at. pct Al, and several elements to increase solid solution strengthening have been investigated. The threshold for fatigue crack growth was determined to be ≈7 MPa√m and fracture toughness ≈35 MPa√m. Crack growth was intermittent and sporadic; the fracture path was tortuous, crystallographic, and appeared to favor the {100} and {112} planes. Fatigue crack closure was measured directly at the crack tip. The fatigue and fracture properties of the commercial alloy are compared against those of Nb-Cr-Ti and Nb-Cr-Ti-Al alloys. The comparison indicated that Ti addition is beneficial for, but Al addition is detrimental to, both fracture toughness and fatigue crack resistance.     

Microstructural aspects of sulfide stress cracking in an API X-80 pipeline steel

Crack growth in an API X-80 exposed to sour gas environments was investigated using modified wedge-opening-loaded (MWOL) specimens. The MWOL specimens were tested in the as-received condition and after annealing followed by water spraying to simulate improperly welded regions. It was found that water-sprayed MWOL specimens were susceptible to stress sulfide cracking in a NaCl-free NACE solution. Crack growth was relatively slow when subjected to an initially appliedK _I of 30 MPa . Under these conditions, crack growth rates continually decreased until crack arrest was exhibited at a thresholdK _I (K _ISSC) of 26 MPa . The exhibited crack growth rates were related to the facility with which nucleated microcracks joined the main crack front. Apparently, preferential nucleation and growth of microcracks within the main crack tip plastic zone accounted for the exhibited embrittlement. In particular, favorable microcrack growth followed a path consisting of fractured (cut) carbide regions, as well as various interfaces, including globular inclusions and grain boundary precipitates.     

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.     

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.     

Ambient- to elevated-temperature fracture and fatigue properties of Mo-Si-B alloys: Role of microstructure

Ambient- to elevated-temperature fracture and fatigue-crack growth results are presented for five Mo-Mo₃Si-Mo₅SiB₂-containing α-Mo matrix (17 to 49 vol pct) alloys, which are compared to results for intermetallic-matrix alloys with similar compositions. By increasing the α-Mo volume fraction, ductility, or microstructural coarseness, or by using a continuous α-Mo matrix, it was found that improved fracture and fatigue properties are achieved by promoting the active toughening mechanisms, specifically crack trapping and crack bridging by the α-Mo phase. Crack-initiation fracture toughness values increased from 5 to 12 MPa√m with increasing α-Mo content from 17 to 49 vol pct, and fracture toughness values rose with crack extension, ranging from 8.5 to 21 MPa√m at ambient temperatures. Fatigue thresholds benefited similarly from more α-Mo phase, and the fracture and fatigue resistance was improved for all alloys tested at 1300 °C, the latter effects being attributed to improved ductility of the α-Mo phase at elevated temperatures.     

Microstructural effects on fatigue crack growth in a low carbon steel

A study of the influence of microstructure on fatigue crack growth in an AISI 1018 steel has been carried out. Two distinctly different duplex microstructures were investigated. In one microstructure ferrite encapsulated islands of martensite; in the other martensite encapsulated islands of ferrite. The latter structure resulted in a significant increase in threshold level (18 MPa√mvs 8 MPa√m) together with an increase in yield strength. Fractographic analysis was used to investigate the influence of microstructure on the mode of fatigue crack growth. Formerly at the University of Connecticut     

Fracture toughness of the lean duplex stainless steel LDX 2101

Fracture toughness testing was performed on the recently developed lean duplex stainless steel LDX 2101 (EN 1.4162, UNS S32101). The results were evaluated by master curve analysis, including deriving a reference temperature. The master curve approach, originally developed for ferritic steels, has been used successfully. The reference temperature corresponds to a fracture toughness of 100 MPa√m, which characterizes the onset of cleavage cracking at elastic or elastic-plastic instabilities. The reference temperature, T ₀, was −112 °C and −92 °C for the base and weld materials, respectively. In addition, the fracture toughness is compared with impact toughness results. Complementary crack tip opening displacements (CTODs) have also been calculated. The toughness properties found in traditional duplex stainless steels (DSS) are generally good. The current study verifies a high fracture toughness for both base and weld materials and for the low alloyed grade LDX 2101. Even though the fracture toughness was somewhat lower than for duplex stainless steel 2205, it is still sufficiently high for most low-temperature applications.     

Near-threshold corrosion fatigue crack growth behavior of type 422 stainless steel at controlled maximum stress intensities

The K_max-controlled near-threshold fatigue crack growth behavior was investigated on 422 stainless steel in a boiling NaCl solution. During the test, there was a transition from corrosion fatigue to stress corrosion cracking. The transition occurred at very high load ratios (R=-0.91) and at very lowAK levels (≤2.1 MPa√m). The characteristics of stress corrosion cracking (SCC) were manifested by time-based crack growth rather than cycle-based crack growth, by crack extension under static loading, and by change in fracture mode. In corrosive environments, the small ripple loading imposed on structural materials should be recognized for engineering designs and failure analyses.     

The debonding and fracture of Si particles during the fatigue of a cast Al-Si alloy

Constant-amplitude high-cycle fatigue tests (σ_max=133 MPa, σmax/σy=0.55, and R=0.1) were conducted on cylindrical samples machined from a cast A356-T6 aluminum plate: The fracture surface of the sample with the smallest fatigue-crack nucleating defect was examined using a scanning electron microscope (SEM). For low crack-tip driving forces (fatigue-crack growth rates of da/dN<1 × 10⁻⁷ m/cycle), we discovered that a small semicircular surface fatigue crack propagated primarily through the Al-1 pct Si dendrite cells. The silicon particles in the eutectic remained intact and served as barriers at low fatigue-crack propagation rates. When the semicircular fatigue crack inevitably crossed the three-dimensional Al-Si eutectic network, it propagated primarily along the interface between the silicon particles and the Al-1 pct Si matrix. Furthermore, nearly all of the silicon particles were progressively debonded by the fatigue cracks propagating at low rates, with the exception of elongated particles with a major axis perpendicular to the crack plane, which were fractured. As the fatigue crack grew with a high crack-tip driving force (fatigue-crack growth rates of da/dN>1 × 10⁻⁶ m/cycle), silicon particles ahead of the crack tip were fractured, and the crack subsequently propagated through the weakest distribution of prefractured particles in the Al-Si eutectic. Only small rounded silicon particles were observed to debond while the fatigue crack grew at high rates. Using fracture-surface markings and fracture mechanics, a macroscopic measure of the maximum critical driving force between particle debonding vs fracture during fatigue-crack growth was calculated to be approximately K _max ^tr ≈6.0 MPa √m for the present cast A356 alloy.     

The effects on fracture toughness of ductile-phase composition and morphology in Nb-Cr-Ti and Nb-Siin situ composites

Niobium-chromium alloys, both single and two phase, were alloyed with titanium in order to enhance fracture toughness and fatigue crack growth resistance. The selection of titanium as an alloying element and the relationship of electronic bonding to toughness are examined. The results indicated that toughness increased with a decreasing number of D +s electrons. Titanium was found to increase the toughness of solid-solution Nb-Cr alloys from ≈8 to 87 MPa√m, while for the twophase “insitu composites,” toughness was increased from ≈5 to 20 MPa√m, although this is less than expected. Fracture toughness of the composites correlated nonlinearly with the volume fraction of the phases. The evidence suggests that the toughness of the composites is decreased due to fracture of the intermetallic particles and constraint on matrix deformation imposed by the intermetallic. Fracture characteristics of the Nb-Cr-Ti materials are compared to those of Nb-Cr and Nb-Si materials.     

Fatigue crack growth rates and fracture toughness of rapidly solidified Al-8.5 pct Fe-1.2 pct V-1.7 pct Si alloys

The room-temperature fatigue crack growth rates (FCGR) and fracture toughness were evaluated for different crack plane orientations of an Al-8.5 Pct Fe-1.2 Pct V-1.7 Pct Si alloy produced by planar flow casting (PFC) and atomized melt deposition (AMD) processes. For the alloy produced by the PFC process, properties were determined in six different orientations, including the short transverse directions S-T and S-L. Diffusion bonding and adhesive bonding methods were used to prepare specimens for determining FCGR and fracture toughness in the short transverse direction. Interparticle boundaries control fracture properties in the alloy produced by PFC. Fracture toughness of the PFC alloy varies from 13.4 MPa√m to 30.8 MPa√m, depending on the orientation of the crack plane relative to the interparticle boundaries. Fatigue crack growth resistance and fracture toughness are greater in the L-T, L-S, and T-S directions than in the T-L, S-T, and S-L orientations. The alloy produced by AMD does not exhibit anisotropy in fracture toughness and fatigue crack growth resistance in the as-deposited condition or in the extruded condition. The fracture toughness varies from 17.2 MPa√m to 18.5 MPa√m for the as-deposited condition and from 19.8 MPa√m to 21.0 MPa√m for the extruded condition. Fracture properties are controlled by intrinsic factors in the alloy produced by AMD. Fatigue crack growth rates of the AMD alloy are comparable to those of the PFC alloy in the L-T orientation. The crack propagation modes were studied by optical metallographic examination of crack-microstructure interactions and scanning electron microscopy of the fracture surfaces.     

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.     

Elevated temperature fracture toughness of Al-Cu-Mg-Ag sheet: Characterization and modeling

The plane-strain initiation fracture toughness (K _JICi ) and plane-stress crack growth resistance of two Al-Cu-Mg-Ag alloy sheets are characterized as a function of temperature by a J-integral method. For AA2519 +Mg+Ag, K _JICi decreases from 32.5 MPa√m at 25 °C to 28.5 MPa√m at 175 °C, while K _JICi for a lower Cu variant increases from 34.2 MPa√m at 25 °C to 36.0 MPa√m at 150 °C. Crack-tip damage in AA2519+Mg+Ag evolves by nucleation and growth of voids from large undissolved Al₂Cu particles, but fracture resistance is controlled by void sheeting coalescence associated with dispersoids. Quantitative fractography, three-dimensional (3-D) reconstruction of fracture surfaces, and metallographic crack profiles indicate that void sheeting is retarded as temperature increases from 25 °C to 150°C, consistent with a rising fracture resistance. Primary microvoids nucleate from smaller constituent particles in the low Cu alloy, and fracture strain increases. A strain-controlled micromechanical model accurately predicts K _JICi as a function of temperature, but includes a critical distance parameter (l*) that is not definable a priori. Nearly constant initiation toughness for AA2519+Mg+Ag is due to rising fracture strain with temperature, which balances the effects of decreasing flow strength, work hardening, and elastic modulus on the crack-tip strain distribution. Ambient temperature toughnesses of the low Cu variant are comparable to those of AA2519+Mg+Ag, despite increased fracture strain, because of reduced constituent spacing and l*.     

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.     

The influence of grain orientation on the mode and rate of fatigue crack growth inα-titanium

The high cycle fatigue crack growth characteristics of coarse grainedα-titanium have been studied in vacuum, air, water and brine. Tests were carried out on single-edge-notch tension test-pieces at anR ratio of 0.35, a frequency of 130 Hz, and a ΔK range of 5 to 25 MPa-√m. The use of channelling patterns in the SEM permitted detailed crystallographic information to be correlated with fatigue fracture morphologies. Three distinct modes of fatigue crack growth were identified. Cleavage-like facets on the basal planes (0002); the formation of which was encouraged by increasing severity of environment and increasing stress normal to (0002), striations on planes normal to (0002) consistent with a mechanism involving intersecting prism slip systems, and furrows in the [0001] direction associated with fine lines parallel to the 1123 direction. It is shown how the relationships between grain orientation, stress geometry and testing environment may be presented in the form of Grain Orientation Control Maps. The rate of fatigue crack growth in individual grains was dominated by the mode of growth; below a ΔK of 10 MPa√m the cleavage-like mode was up to 10 times more rapid than the other modes of growth. Formerly with the Department of Physical Metallurgy and Science of Materials, The University of Birmingham, Birmingham, United Kingdom B152TT.     

Toughening mechanisms in ductile niobium-reinforced niobium aluminide (Nb/Nb3Al) in situ composites

Anin situ study has been performed in the scanning electron microscope (SEM) on a niobium ductilephase-toughened niobium aluminide (Nb/Nb₃Al) intermetallic composite to examine the crack-growth resistance-curve (R-curve) behavior over very small initial crack extensions, in particular over the first ~500 μm of quasi-static crack growth, from a fatigue precrack. The rationale behind this work was to evaluate the role of toughening mechanisms, specifically from crack bridging, in the immediate vicinity of the crack tip and to define the size and nature of bridging zones. Although conventional test methods, where crack advance is monitored typically over dimensions of millimeters using compliance or similar techniques, do not show rising R-curve behavior in this material,in situ microscopic observations reveal that bridging zones resulting from both uncracked Nb₃Al ligaments and intact Nb particles do exist, but primarily within ~300 to 400 μm of the crack tip. Accordingly, rising R-curve behavior in the form of an increase in fracture resistance with crack growth is observed for crack extensions of this magnitude; there is very little increase in toughness for crack extensions beyond these dimensions. Ductile-phase toughening induced by the addition of Nb particles, which enhances the toughness of Nb₃Al from ~1 to 6 MPa√m, can thus be attributed to crack-tip shielding from nonplanar matrix and coplanar particle bridging effects over dimensions of a few hundred microns in the crack wake. formerly Research Student, Department of Materials Science and Mineral Engineering, University of California-Berkeley     

Microstructural examination of

Fatigue tests were performed to examine how microstructural conditioning influences crack initiation and propagation in SA508 class 3 low-carbon steel. A 3-mm-long crack was introduced in compact tension (CT) fatigue test specimens under four different loads in order to obtain crack tip plastic zones at different stress intensity factor ranges, ΔK = 18, 36, 54, and 72 MPa√m. The microstructure of the plastic zones around the crack tip were examined by trans- mission electron microscopy (TEM) and selected area electron diffraction (SAD). Micro- orientation of the dislocation cells in the plastic zones of all of the CT samples increased to 4 deg from the level of an as-received sample. Four-point bending fatigue tests were performed for plate shape samples with a large cyclic strain range. The SAD value of the bending samples was also 4 deg in the damaged area where cracks already initiated at an early stage of the fatigue process. These test results indicate that the microstructural conditioning is a prerequisite for the fatigue crack initiation and propagation in SA508. These observations may lead to better under- standing of how fatigue initiation processes transit to cracks.     

The effects of prolonged thermal

Aluminum-lithium alloys are currently being considered for applications at moderately elevated temperatures; accordingly, a study has been made on the effects of prolonged (100 and 1000 hours overaging) thermal exposure at 149 °C and 260 °C on the mechanical properties of a peakaged Al-Li-Cu-Mg-Zr alloy 8090-T8771. In the as-received T8771 temper, the alloy exhibits an excellent combination of strength (˜500 MPa) and toughness (35 MPa√m) with moderate tensile elongation (4 pct). Overaging at 149 °C results in a ˜50 pct reduction in ductility and toughness, primarily associated with the growth of equilibrium phases along grain/subgrain boundaries, resulting in formation of solute-depleted precipitate-free zones and coarsening of matrix8' andS precipitates; strength levels and fatigue-crack growth rates, however, remain largely unchanged. Thermal exposures at 260 °C, conversely, lead to dramatic reductions in strength (by ˜50 to 80 pct), toughness (by ˜30 pct) and fatigue-crack propagation resistance; crack-growth rates at all ΔK levels above ~5 MPa√m are 2 to 3 orders of magnitude faster. Microstructurally, this was associated with complete dissolution of δ′, severe coarsening ofS andT ₂ precipitates in the matrix, and formation of equilibrium Cu- and Mg-rich intermetallic phases in the matrix and along grain boundaries. The resulting lack of planar-slip deformation and low yield strength of 8090 following overaging exposures at 260 °C increase the cumulative crack-tip damage per cycle and reduce the tendency for crack-path deflection, thereby accelerating fatigue-crack growth rates. Despite this degradation in properties, the 8090-T8771 alloy has better strength retention and generally superior fatigue-crack growth properties compared to similarly overaged Al-Li-Cu-Zr 2090 and Al-Cu-Zn-Mg 7150 alloys. formerly with the University of California, formerly with the University of California,     

Fatigue crack propagation in dual-phase steels: Effects of ferritic-martensitic microstructures on crack path morphology

microstructures with maximum resistance to fatigue crack extension while maintaining high strength levels. A wide range of crack growth rates has been examined, from ~10^-8 to 10^-3 mm per cycle, in a series of duplex microstructures of comparable yield strength and prior austenite grain size where intercritical heat treatments were used to vary the proportion, morphology, and distribution of the ferrite and martensite phases. Results of fatigue crack propagation tests, conducted on “long cracks” in room temperature moist air environments, revealed a very large influence of microstructure over the entire spectrum of growth rates at low load ratios. Similar trends were observed at high load ratio, although the extent of the microstructural effects on crack growth behavior was significantly less marked. Specifically, microstructures containing fine globular or coarse martensite in a coarse-grained ferritic matrix demonstrated exceptionally high resistance to crack growth without loss in strength properties. To our knowledge, these microstructures yielded the highest ambient temperature fatigue threshold stress intensity range ΔK₀ values reported to date, and certainly the highest combination of strength and ΔK₀ for steels (i.e., ΔK₀ values above 19 MPa√m with yield strengths in excess of 600 MPa). Such unusually high crack growth resistance is attributed primarily to a tortuous morphology of crack path which results in a reduction in the crack driving force from crack deflection and roughness-induced crack closure mechanisms. Quantitative metallography and experimental crack closure measurements, applied to currently available analytical models for the deflection and closure processes, are presented to substantiate such interpretations. Formerly Lecturer and Research Engineer in the Department of Materials Science and Mineral Engineering, University of California