Slip Band Crack Research Articles

Investigation of micro-mechanisms for facet formation during dwell fatigue in Ti6321 alloy with equiaxed microstructure

Dwell fatigue failure has always been the most concerning threat to titanium alloy components, and the dominant mechanism is still not thoroughly understood. In this work, room temperature stress-controlled dwell fatigue tests were performed on a near α titanium, Ti-6Al-3Nb-2Zr-1Mo alloy with equiaxed microstructure. Quantitative characterization was conducted to reveal the surface morphology and the crystallographic orientation of facets on the fracture. The results shows that faceted primary α grains well aligned with (0001) basal plane. Facets at crack-initiation sites were found with orientation distribution ranging from 20° to 60°, while the facets at crack path shows a more scatter orientation distribution. Based on the results, phenomenological models were proposed to explain the formation of facets from the aspect of slip-assisted cracking, including basal slip band cracking, and mixed-deformation induced cracking, which well elucidated the formation of traditional sharp facet and special step-liked facet morphology.

Quantitative investigation on the deformation modes and cracking behavior during cyclic torsional loading of extruded pure Mg and Mg‐3Y alloy

AbstractThis study explores the effect of adding 3 wt.% Y to pure magnesium (Mg) on its mechanical behavior under cyclic torsional loadings at room temperature. The research examines deformation and cracking modes in both pure Mg and Mg‐3Y samples. Deformation modes are monitored using quasi‐in‐situ EBSD observations coupled with slip trace analysis. The findings reveal that basal slip dominates the cyclic deformation throughout the fatigue life of the pure Mg sample, while both basal and pyramidal slip dominate the cyclic deformation in the Mg‐3Y sample. Intergranular cracking is the primary cracking mode for both samples under cyclic torsional loadings. Basal and pyramidal slip persistent slip band (PSB) cracking serves as a primary transgranular cracking mode in the pure Mg and Mg‐3Y samples, respectively. The study also investigates the underlying mechanism governing the activity of various deformation modes, cracking modes, and mechanical behavior.

On the low temperature fatigue crack growth behavior of AA7075-T651 in ultra-high vacuum environments

It is well-established that the fatigue crack growth behavior of aerospace aluminum alloys in high altitude applications is sensitive to both temperature and water vapor pressure. To isolate the role of temperature, fatigue crack growth experiments were performed on AA7075-T651 under ultra-high vacuum conditions (<6x10−6 Pa) at temperatures ranging from 23 °C to −65 °C. All da/dN–ΔK relationships exhibit the following: (1) a temperature-independent Paris regime at high ΔK, (2) a transition from the Paris regime to a sharply reduced da/dN, which occurs at higher ΔK as the temperature decreases, and (3) a coalescence in da/dN for all temperatures in the threshold region. Interestingly, the point of deviation from the Paris regime is found to correlate with a transition from a flat, transgranular fracture morphology to slip band cracking (SBC) across all conditions. The likely mechanisms governing the observed behaviors are discussed, focusing on the role of slip.

Short-range ordering plays a determining role in the low-cycle fatigue life improvement of fcc metals: A conclusive study on low solid-solution hardening Ni-Cr alloys

The effect of short-range ordering (SRO) on the low-cycle fatigue (LCF) behavior of low solid-solution hardening Ni-Cr alloys with high stacking fault energies (SFEs) was systematically studied under cycling at constant total strain amplitude (Δεt/2) in the range of 0.1%–0.7%. The results show that an inducement of SRO structures can notably improve the fatigue life of the alloy regardless of Δεt/2, and several unique fatigue characteristics have been detected, including the transition of fatigue cracking mode from intergranular cracking to slip band cracking, the non-negligible evolution from non-Masing behavior in pure Ni to Masing behavior in the Ni-40Cr alloy, and the secondary cyclic hardening behavior in the Ni-10Cr and Ni-20Cr alloys. All these experimental phenomena are tightly associated with the transformation in cyclic deformation mechanisms that is induced by SRO based on the “glide plane softening” effect. Furthermore, a comprehensive fatigue life prediction model based on total hysteresis energy has been reasonably proposed, focusing on the analyses of the macroscopic model parameters (namely the fatigue cracking resistance exponent β and the crack propagation resistance parameter W0) and microscopic damage mechanisms. In brief, on the premise that the effects of SFE and friction stress can be nearly ignored, as in the case of the present low solid-solution hardening Ni-Cr alloys with high SFEs, an enhancement of SRO in face-centered cubic metals has been convincingly confirmed to be an effective strategy to improve their LCF performance.

Slip transfer accelerates slip band cracking of high cycle fatigue in high-Mn austenitic steel at 77 K

Slip transfer accelerates slip band cracking of high cycle fatigue in high-Mn austenitic steel at 77 K

Microstructure, Fatigue Properties and Stress Concentration Analysis of 6005 Aluminum Alloy MIG Welded Lap Joint.

This paper studies the microstructure and mechanical properties of MIG (Melt Inert Gas) lap welded 6005 aluminum alloy plates. Microstructure analysis (OM) of the joint showed that 15~30 μm small grains were observed at the fusion line. Mechanical analysis shows that the small grains are broken by shielding gas and molten pool flow force. Hardness test shows that there is a softening zone (41~43 HV) in HAZ much lower than BM and WZ. The low cycle fatigue test showed that the performance of lap joint decreased sharply, and the fatigue strength of weld decreased significantly, which was only 27.34% of the base metal. The fatigue fracture (SEM) of the weld observed slip band cracking and a large number of brittle fracture characteristics. Using the stress concentration factor Kt for analysis, it was found that the cause of brittle fracture was mostly stress concentration. Lap joint stress concentration model appears in two ways: firstly, at the weld toe, the weld is subjected to eccentric force, secondly, there is a small gap between the two plates at the weld root, which cracks along the direction of 45° of the maximum shear stress.

Microstructural Evolution, Mechanical Properties, and Preosteoblast Cell Response of a Post-Processing-Treated TNT5Zr β Ti Alloy Manufactured via Selective Laser Melting.

A Ti–34Nb–13Ta–5Zr (TNT5Zr) β Ti alloy with a high strength-to-modulus ratio has been developed, showing its potential to become another candidate material in load-bearing implant applications. This work mainly investigates the microstructural evolution, mechanical properties, and biocompatibility of a post-processing-treated TNT5Zr alloy manufactured via selective laser melting (SLM). Transmission electron microscopy observation shows the existence of the single beta grain matrix and alpha precipitates along the grain boundary in the SLM + HIP manufactured TNT5Zr alloy (TNT5Zr-AF + HIP), and ellipsoidal nano-sized intragranular α″ precipitates (approx. 5–10 nm) were introduced after the subsequent low-temperature aging treatment. The precipitation strengthening enables the SLM + HIP + aging manufactured TNT5Zr (TNT5Zr-AF + HIPA) alloy to show a comparable ultimate tensile strength (853 ± 9 MPa) to that of the reference material (Ti64-AF + HIP, 926 ± 23 MPa). Including the inferior notch-like surface of the test pieces, the slip-band cracking that occurs in this ductile TNT5Zr-AF + HIPA alloy is regarded as the main factor in determining its fatigue strength (170 MPa). In vitro short-term biocompatibility evaluation reveals almost no significant difference in the preosteoblast viability, differentiation, and mineralization between TNT5Zr-AF + HIPA and the reference biomaterial (Ti64-AF + HIP).

Crack growth behavior of 725 in seawater under cathodic polarization

Hydrogen embrittlement behavior of four heats of alloy 725 (H1, H2, H3, and H4) were characterized by measuring the crack growth rate response over a range of cathodic potentials. Significant grain boundary precipitation in two heats (H1 and H3) may have contributed to higher susceptibility, as evidenced by higher values of crack growth rate. The crack growth rate increased with increasing twin fraction. The crack morphology exhibited evidence of intergranular as well as slip band cracking. The crack growth rate response of the two heats that were less susceptible (H2 and H4) were evaluated in more detail over a range of applied potentials and stress intensity factors. Evidence of nano void formation at the intersection of dislocation slip bands, and grain boundaries suggests that hydrogen stabilized vacancies may play a role in the crack growth mechanism. Crack growth rate exhibited a strong response with applied potential varying by as much as 100 times in the potential range of −1050mV SCE to −850mV SCE. Over the range of K values evaluated, the crack growth rate increased by a factor of 3to 30 with applied K depending on the applied potential. The crack growth rate was related to the water adsorption on fresh metal surfaces of alloy 725 in the anticipated crack tip conditions. A crack tip strain rate based approach was applied to model the measured crack growth response.

Improving the stress-controlled fatigue life of low solid-solution hardening Ni-Cr alloys by enhancing short range ordering degree

The tension–tension fatigue behavior of low solid-solution hardening Ni-Cr alloys with an approximately constant stacking fault energy was systematically investigated under different stress amplitudes. With an enhancement of short range ordering (SRO) degree, the fatigue crack initiation mode is evolved from intergranular cracking to slip band cracking, and the cyclic deformation mechanism changes from wavy to planar dislocation slip. Both the fatigue strength coefficient (σ′f) and exponent (b) can be simultaneously increased by inducing SRO structure that greatly enhances the cyclic strain hardening capacity, deformation homogeneity and slip reversibility, and the fatigue life is thus significantly improved.

Influence of microstructure on fatigue crack growth behavior of Ti–6Al–3Nb–2Zr–1Mo alloy: Bimodal vs. lamellar structures

The influence of microstructure (bimodal and lamellar structure) on the fatigue crack growth (FCG) rate of Ti–6Al–3Nb–2Zr–1Mo alloy was investigated using compact-tension (CT) specimens. The da/dN curves showed that the FCG rate of lamellar structure was slower than that of bimodal structure, especially in the early stage of crack growth. The FCG paths in the early stage were observed by optical microscopy and scan electric microscopy. The results revealed that the FCG paths of lamellar structure were more tortuous than that of bimodal structure. The frequency of crack deflection and bifurcation of the lamellar structure is significantly higher than that of bimodal structure. The dominant mode of crack propagation for bimodal structure is crossing βtrans and αp, and for lamellar structure is crossing α colonies. Fatigue crack growth mechanisms at the early stage have been studied by observation of crystalline orientation near the crack propagation path at the crack tip, in combination with the slip trace analyzing. The results show that the fatigue crack growth directions in αp or βtrans for bimodal structure and α colony for lamellar structure are mostly consistent with that of prismatic or basal plane trace. This indicated that the cracks propagation modes of crossing αp or βtrans in bimodal structure and crossing α colonies in lamellar structure are resulting from the slip band cracking of the microstructure at crack tip, as the evidence of {101‾0}112‾0 prismatic dislocation lines were observed in αp or βtrans for bimodal structure and α colony for lamellar structure by transmission electron microscope (TEM) near the fracture surface.

Data rich imaging approaches assessing fatigue crack initiation and early propagation in a DS superalloy at room temperature

Crack initiation and early propagation behavior of the directionally solidified (DS) superalloy CM247LC has been assessed by data rich imaging approaches. These include conventional characterization methods such as replica record analysis, 3D optical surface imaging, optical and scanning electron microscopy (SEM) as well as more recent techniques like digital image correlation (DIC) and synchrotron radiation computed tomography (SRCT). Three modes of secondary crack behaviors were found during evaluation of the fatigue process. The early stages of fatigue damage were controlled by microstructure-induced cracking, mainly consisting of carbide cracking. Fatigue damage was then promoted via slip band cracking and opening mode controlled carbide-cracking. The mechanisms of these different cracking behaviors are associated with the plastic zone of the main crack tip. Even though the early localized strain levels were of the same intensity within slip bands and at the intersection sites with carbides, carbide-induced cracking occurred prior to slip band cracking, characterized by SEM-DIC. This indicated that carbide-induced cracking was more likely to occur in the early stages of the fatigue process. Early crack growth behaviors were further investigated in situ at the microstructural scale via SRCT. The effect of carbides on crack initiation and propagation processes were evaluated in 3D. This revealed the phenomenon around pores, that cracks simultaneously grew on different slip planes in 3D, contrary to previous theories that such cracks tend to grow on a single favourable slip plane (in polycrystalline alloys). The SRCT result demonstrates the importance and necessity of 3D characterization of the crack propagation behavior at sub-surface, which is not fully analyzed by 2D characterization.

Fatigue crack propagation of aerospace aluminum alloy 7075-T651 in high altitude environments

Fracture mechanics fatigue testing of 7075-T651 in environmental conditions pertinent to high altitude airframe operation established the fatigue crack growth behavior over a wide span of stress intensity range (ΔK). Two distinct methods were used to control the environmental severity parameter of water vapor pressure over frequency (PH2O/f): (1) at 23°C the PH2O value was controlled by an ultra-high vacuum system into which purified water was leaked, and (2) the testing temperature was decreased to temperatures as low as −65°C allowing an equilibrium PH2O over ice at a given temperature to be maintained. Decreasing PH2O at 23°C (at a constant f) results in a drastic reduction in the fatigue behavior, strongly suggesting that the reduction in moisture is a primary mechanism controlling the retarded fatigue crack growth behavior observed at low termperatures. Additionally, studies conducted at a corresponding PH2O/f with various testing temperatures down to −15°C show that fatigue crack growth rate (FGCR) behavior is consistent with data generated at 23°C. However, at temperatures below −30°C, low T tests exhibit crack growth rates below the 23°C results despite having the same PH2O/f value. This suggests that the strong role of environmental moisture reduction is augmented by a separate temperature dependent effect on either the inherent dislocation behavior and/or a separate aspect of the Hydrogen Environment Embrittlement (HEE) process. Mechanisms based purely on temperature dependent dislocation behavior fail to fully describe the experimental observations. Temperature dependent impacts on the HEE process (apart from the reduction of the PH2O) via influencing the surface reaction process to generate atomic H at the crack tip or the diffusion of the H within the crack tip process zone do not adequately describe the observed experimental data. Furthermore, a dip in the crack growth rate at −30°C suggests that molecular flow of H2O from the bulk to the crack tip is in part responsible for the temperature dependent behavior. Such behavior is hypothesized to be caused by the preferential onset of a rough crack wake morphology (likely slip band cracking) at low temperatures, that impedes molecular flow from the bulk to the crack tip. This behavior strongly suggests an important temperature dependence of the dislocation motion or H-dislocation interactions on the observed FCGR behavior.

Effects of microstructures on fatigue crack initiation and short crack propagation at room temperature in an advanced disc superalloy

Fatigue crack initiation and early short crack propagation behaviour in two microstructural variants of a recently developed Low Solvus, High Refractory (LSHR) disc superalloy at room temperature has been investigated by three-point bending with replication procedure. The results shows that fine gained (FG) LSHR possesses higher fatigue life due to its better crack initiation resistance, limited crack coalescence and comparable Stage I crack propagation resistance to the coarse grained (CG) LSHR, although its resistance to Stage II crack propagation is inferior. Twin boundary (TB) cracking in the relatively large grains dominates the crack initiation process along with occasional crack initiation due to slip band cracking. Activation of the primary slip systems parallel to the TB at matrix and twin and high resolved shear stress associated with high Schmid factor (SF) are required for TB crack initiation. Cracks preferentially propagate along slip bands associated with high SF slip systems after initiation. But cracks also propagate along slip bands associated with slip systems with lower SF if the inclination angle between the slip band ahead of the crack tip and the crack segment of the crack tip is small enough to enable a steady transition (or non-deflected growth) of cracks across the grain boundary.

Fatigue extrusions, slip band cracking and a novel hybrid concept for fatigue crack closure close to the crack tip

Small fatigue cracks in the aluminium alloys 2024-T351 and 8090-T8771 have been observed to grow by a heterogeneous process of three-dimensional slip band decohesion. Local, finely-divided crack path excursions at slip bands, the pull-out of small sections of decohered slip band and the formation of fatigue extrusions at slip bands produced micro-roughness on the crack surfaces. The hybrid form of plastic deformation induced surface roughness provides a potential source of crack closure. This may be effective in reducing the nominal crack growth driving force at small cracks and near threshold for long cracks when the crack tip opening displacement is small. A higher magnitude of closure at small mode I crack sizes and a spread in the levels of closure for a given crack size are consistent with the dependence of the slip band related micro-roughness on the individual crack path. Fatigue extrusions imply a dynamic form of crack closure that could develop immediately at the crack tip even for fatigue at high mean load. Complex influences of crack size, grain orientation and crack path on crack closure and crack growth rate indicate that a stochastic approach to determining fatigue life is appropriate for the critical small crack regime in these and similarly behaving materials.

Vibration fracture resistance of 5052H112 aluminum alloy processed by cold rolling and friction stirring

Abstract Hot-rolled Al–3Mg alloy processed by various cold-rolled reductions exhibited excellent vibration fracture resistance, however subsequent friction stirring did not endow better vibration fracture resistance than cold-rolled samples in spite of possessing microstructural refinement and homogenization. The D–N curve under a constant initial deflection amplitude (6.5 mm) also showed that the vibration fracture resistance of cold-rolled specimens increased as a result of the slower crack propagation rate. All samples used in this investigation showed a work hardening feature from the initial stage of the D–N curve, and the deflection amplitude tended to increase as the number of vibration cycles increased. During this ascending stage, slip band cracking and strain hardening in the vicinity of the main crack could be recognized. The D–N curve feature of the cold-rolled samples depicted a longer plateau region while the deflection amplitude was at its maximum. It is suggested that the dislocation tangles introduced by cold rolling can strengthen the matrix. As a result, the deflection amplitude drops could be associated with increasing the dislocation tangles due to prior cold rolling while the main crack achieved a critical crack length.

Effects of microstructure on room temperature fatigue crack initiation and short crack propagation in Udimet 720Li Ni-base superalloy

An assessment of the effects of microstructure on room temperature fatigue crack initiation and short crack propagation in a Ni-base superalloy is presented. The assessment was carried out on microstructural variants of U720Li, including as-received U720Li, U720Li-LG (large grain variant) and U720Li-LP (large intragranular coherent γ′ variant). Fatigue tests were carried out at room temperature using a 20 Hz sinusoidal cycling waveform on plain bend bars. Tests were conducted in 3-point bend under load control with an R-ratio of 0.1. A maximum load of 95% σ y was used in all tests. Room temperature fatigue crack initiation was noted to occur due to slip band cracking and from porosity on or just beneath the surface in all materials. Crack propagation was noted to be highly faceted (due to planar slip band cracking) immediately after crack initiation followed by a transition to a flatter Stage II type crack path as crack length increases. U720Li-LP was noted to show the longest fatigue lifetime, followed by U720Li-LG while U720Li shows the shortest life. The longer lifetime of U720Li-LP was linked to a higher resistance to both fatigue crack initiation and short crack propagation. U720Li and U720Li-LG show approximately similar crack initiation resistance although U720Li-LG showed slightly improved short crack growth resistance. The observations have been rationalised in terms of the microstructural characteristics of the materials, and it is believed that larger grain size, larger coherent γ′ precipitate size and higher volume fractions of both coherent and primary γ′ precipitates will improve overall fatigue lifetimes in PM Ni-base alloys which exhibit planar slip characteristics at room temperature.

Crystallography of Fatigue Crack Propagation in Precipitation-Hardened Al-Cu-Mg/Li

A combined electron backscatter diffraction (EBSD)/stereology method successfully quantifies the orientation of fatigue crack surfaces for Al-Li-Cu and Al-Cu-Mg alloys stressed at low ΔK, in which deformation is localized in slip bands and cracking is highly faceted. The method orients features as small as ∼1 μm in complex microstructures. Vacuum fatigue facets align within 15 deg of up to four variants of {111} slip planes, governed by the distribution of crack tip resolved shear stress. The small fraction of precisely oriented {111} facets suggests that cracking involves complex intraband and multiple-band interface paths. Water vapor and NaCl solution affect a similar dramatic change in the crack path; near-{111} facets are never observed, at odds with mechanisms for H-enhanced slip localization and associated slip band cracking. Rather, two environmental crack facet morphologies, broad flat and repeating step, exhibit a wide range of orientations between {001} and {110}, as governed by crack tip resolved normal stresses. The repetitive stepped facets appear to contain areas parallel to {100}/{110} on the ∼1-μm scale, coupled with surface curvature consistent with a mechanism of discontinuous fatigue crack growth involving H-enhanced {100}/{110} cleavage and intermingled crack tip plasticity. Broad-flat faceted regions are parallel to a variety of planes, consistent with a mechanism combining high crack tip tensile stresses and H trapped at the dislocation structure from cyclic deformation, within 1 μm of the crack tip.

Analysis of Biaxial Fatigue Crack Growth in Microstructure Modeled by Voronoi-Polygon

Cracking behavior in low cycle fatigue regime depends on the level and the multiaxiality of the applied stress and also on the microstructure. Such a complex cracking behavior affects failure life significantly. More realistic assessments of failure life and integrity require a new appropriate procedure to analyze the crack growth process in multiaxial fatigue. A model of the fatigue process has been proposed to describe the cracking behavior in biaxial stress state. There is, however, no adequate model to present features of material microstructure. In this work, simulations of crack initiation and propagation based on a previous model were carried out in microstructure modeled by using Voronoi-polygon. In a crack initiation analysis, slip-band crack was modeled for the slip system given randomly in each grain composing the modeled microstructure. In modeling crack growth, a competition model between the coalescence growth and the propagation as a single crack was applied. Simulated cracking morphology and failure life were compared with experimental results observed in biaxial fatigue using circumferentially notched specimens of a pure copper, and the applicability of the proposed model was discussed.

EBSD-AFM Hybrid Analysis of Crack Initiation in Stainless Steel under Fatigue Loading

Both EBSD and AFM methods were used to investigate the active slip systems and fatigue crack initiation behavior in face-centered cubic polycrystalline metal, austenitic stainless steel, SUS316NG, under cyclic torsional loading. Most active slip planes are the primary slip planes having the largest Schmid factor. Grains with slip band cracks or transcrystalline cracks have larger Taylor's factors. On the basis of EBSD and AFM observations, h, the depth of intrusion vertical to the surface, S, and the component of the slip displacement perpendicular to the surface trace, SB, showed a sharp increase at the onset of crack initiation. The critical value of SB at crack initiation was 170 nm.

Low cycle fatigue in rene 88DT at 650 °C: Crack nucleation mechanisms and modeling

Intermediate-temperature low-cycle-fatigue experiments were conducted on three microstructural conditions of the nickel-base superalloy Rene 88DT over several strain ranges. The most significant difference between the microstructural conditions was the grain size; two of the conditions had an average grain diameter of approximately 20 µm, while the third condition had an average grain diameter of 6 µm. Two dominant crack nucleation mechanisms were observed on the specimen fracture surfaces: crack nucleation from surface slip band damage accumulation and crack nucleation around subsurface inclusion clusters. The experimental results indicate that both the grain size and the applied strain range are contributing factors in the prevailing crack nucleation mechanism. The Fatemi-Socie parameter, a multiaxial fatigue damage parameter, was used to examine the ease and probability of surface slip band crack nucleation for these microstructural conditions. The parameter was modified to explicitly include grain size, fatigue R-ratio, and applied strain range, thus giving it a more physically meaningful basis. This modified Fatemi-Socie parameter was found to be a suitable parameter for characterizing the ease of surface slip band cracking for materials with different grain sizes and is able to explain the observed disparity in fatigue lives based on microstructural design.