In-Situ observation on microfracture behavior ahead of the crack tip in 63Sn37Pb solder alloy

in-situ transmission electron microscopy (TEM) tensile tests on as-cast and aged 63Sn37Pb solder alloys were conducted, and the fracture behavior in nanometer scale ahead of the crack tip was inspected and discussed. Results show that the fracture was completed by connecting the discontinuous cracks or voids. Dislocation behavior was concentrated along the grain boundaries for as-cast samples, and displayed mainly as dislocation climb. The crack was intergranular dominated under the lower strain rate. While remarkable mutual dislocation emission was detected in the aged solder. Transgranular cracks were dominant in the fractured area, and they propagated by linking up with the nanometer scale cracks ahead of the crack tips under the effective promotion of the inverse dislocation emission. At the same time, the partial interphase or intergranular cracks in the thinned area were also found. Under this condition, a new critical stress intensity factorK _c ^′ to define the mutual dislocation emission was proposed.     

In-situ observation on microfracture behavior ahead of the crack tip in 63Sn37Pb solder alloy

In-situ transmission electron microscopy (TEM) tensile tests on as-cast and aged 63Sn37Pb solder alloys were conducted, and the fracture behavior in nanometer scale ahead of the crack tip was inspected and discussed. Results show that the fracture was completed by connecting the discontinuous cracks or voids. Dislocation behavior was concentrated along the grain boundaries for as-cast samples, and displayed mainly as dislocation climb. The crack was intergranular dominated under the lower strain rate. While remarkable mutual dislocation emission was detected in the aged solder. Transgranular cracks were dominant in the fractured area, and they propagated by linking up with the nanometer scale cracks ahead of the crack tips under the effective promotion of the inverse dislocation emission. At the same time, the partial interphase or intergranular cracks in the thinned area were also found. Under this condition, a new critical stress intensity factor K′_C to define the mutual dislocation emission was proposed.     

The effect of hydrogen on the fracture toughness of alloy X-750

The effect of hydrogen on the fracture toughness behavior of a nickel-base superalloy, Alloy X-750, in the solutionized and aged condition was investigated. Notched bend specimens were tested to determine if the fracture process was stress or strain controlled. The fracture was observed to initiate at a distance between the location of maximum stress and maximum strain, suggesting that fracture required both a critical stress and strain. The effect of hydrogen was further investigated and modeled using fracture toughness testing and fractographic examination. The fracture toughness of the non-charged specimen was 147 MPa√m. Charging with hydrogen decreased the fracture toughness, K _Ic , to 52 MPa√m at a rapid loading rate and further decreased the toughness to 42 MPa√m for a slow loading rate. This is consistent with the rate-limiting step for the embrittlement process being hydrogen diffusion. The fracture morphology for the hydrogen-charged specimens was intergranular ductile dimple, while the fracture morphology of noncharged specimens was a mixture of large transgranular dimples and fine intergranular dimples. The intergranular failure mechanism in Alloy X-750 was a microvoid initiation process at grain boundary carbides followed by void growth and coalescence. One role of hydrogen was to reduce the void initiation strain for the fine intergranular carbides. Hydrogen may have also increased the rate of void growth. The conditions ahead of a crack satisfy the critical stress criterion at a much lower applied stress intensity factor than for the critical fracture strain criterion. A model based on a critical fracture strain criterion is shown to predict the fracture behavior.     

Effect of aging on fatigue crack growth at sn-pb/cu interfaces

The effect of isothermal aging on fatigue crack growth behavior at the Sn-Pb solder/Cu interface was examined, with emphasis on the role of interfacial microstructure. Flexural peel interface-crack specimens were made from the joints of eutectic Sn-Pb solder and Cu and were further aged at 443 K for 7 and 30 days. Kinetics of fatigue crack growth along the solder/Cu interfaces were measured from flexural peel specimens as a function of strain energy release rate. Aging was found to induce not only microstructural changes in the solder and at the interface, but also degradation in fatigue crack growth resistance of the interface from the fatigue threshold to the fast fracture. The fatigue threshold decreased from 25 to 20 J/m² after aging for 7 days and to 10 J/m² following aging for 30 days. The degradation in the fatigue crack growth resistance is related to the formation of a Pbrich layer at the interface.     

Correlation of microstructure and tempered martensite embrittlement in two 4340 steels

This study is concerned with a correlation between the microstructure and fracture behavior of two AISI 4340 steels which were vacuum induction melted and then deoxidized with aluminum and titanium additions. This allowed a comparison between microstructures that underwent large increases in grain size and those that did not. When the steels were tempered at 350°C,K _Ic and Charpy impact energy plots showed troughs which indicated tempered martensite embrittlement (TME). The TME results of plane strain fracture toughness are interpreted using a simple ductile fracture initiation model based on large strain deformation fields ahead of cracks, suggesting thatK _Icscales roughly with the square root of the spacing of cementite particles precipitated during the tempering treatment. The trough in Charpy impact energy is found to coincide well with the amount of intergranular fracture and the effect of segregation of phosphorus on the austenite grain boundaries. In addition, cementite particles are of primary importance in initiating the intergranular cracks and, consequently, reducing the Charpy energy. These findings suggest that TME in the two 4340 steels studied can be explained quantitatively using different fracture models.     

Effects of hydrogen on deformation and fracture processes in high-ourity aluminium

The effect of hydrogen on the deformation and fracture processes in high-purity aluminum has been investigated by an in-situ environmental-cell TEM technique. The response of dislocations and cracks to high-fugacity hydrogen atmospheres was monitored by video recording of dynamic events. Hydrogen was found to enhance dislocation mobility and decrease the flow stress. The fracture mode was not changed by the presence of hydrogen, but crack growth occurred at lower stresses because of the enhanced dislocation mobility. Highly localized ductile fracture resulted from the concentration of hydrogen in the stress field ahead of the crack tip.     

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

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

Clean steels for steam turbine rotors—their stress corrosion cracking resistance

This work presents the results of a comprehensive study concerning stress corrosion crack growth rates in steam turbine rotor steels exposed to hot water. The effects of stress intensity, temperature, and dissolved gases in the water have been investigated. Special attention has been given to the influence of impurities and alloying elements in the steel such as P, S, Mn, Si, Mo, and Ni, and to the effect of yield strength and fracture toughness on the growth rates of stress corrosion cracks. The results of this study clearly show that there exists a threshold stress intensity of about 20 MNm^−3/2 above which the invariably intergranular stress corrosion cracks grow at a constant, stress-independent velocity. This plateau stress corrosion crack growth rate isnot affected by the oxygen and carbon dioxide concentration in the water. The temperature and the yield strength of the steel have a strong influence on the growth rate of stress corrosion cracks. In contrast, there isno effect of the steel composition within the range investigated, neither of the impurity elements such as P and S, nor of the major alloying elements such as Mn, Si, Mo, and Ni. Steels with low fracture toughness due to temper embrittlement do not exhibit faster stress corrosion crack growth rates in water than nonembrittled steels. No direct relationship between intergranular temper embrittlement and intergranular stress corrosion crack growth in water can be demonstrated.     

Influence of Grain Size and Age-Hardening on Dislocation Pile-Ups and Tensile Fracture for a Ti-AI Alloy

In an aged Ti-8.6 wt pct Al alloy macroscopic embrittlement occurs with increasing grain size and degree of age hardening. The influence of the grain sizeL on the true fracture strain can be described by εF ∼L-¹ Tensile crack nucleation is caused microscopically by strong dislocation pile-ups which crack the grain boundaries. Using transmission electron microscopy and equations from the dislocation theory, an experimental method was developed to determine quantitatively the shear stress concentrations at the grain boundaries which are produced by the dislocation pile-ups and cause crack nucleation. The experimental results show that for all investigated grain sizes and degrees of age hardening a critical local stress t* _C ≈ 38 GPa leads to crack nucleation. Based on this result equations were derived which describe the combined influence of grain size and age hardening on the true fracture strain and on the true fracture stress. These equations show a good agreement with the tensile test results.     

Experimental assessment of crack-tip dislocation emission models for an Al67Cr8Ti25 intermetallic alloy

A potential explanation for the cleavage fracture of intermetallic alloys with low or moderate critical resolved shear stress (CRSS) is the existence of an energy barrier for crack-tip dislocation emission, as described by models that analyze the energetics of dislocation emission from crack tips. In the present study, an intermetallic alloy with the Ll₂ crystal structure, Al₆₇Cr₈Ti₂₅, has been used to experimentally assess the predictions of the Rice-Thomson dislocation-emission model. The assessment is performed in two ways. First, model predictions of a fracture mode transition at elevated temperature are compared with experimental results. Bend tests performed at temperatures in the range of 293 to 1061 K reveal that the fracture mode of Al₆₇Cr₈Ti₂₅ changes from predominately cleavage fracture at room temperature to a mixed mode of cleavage and intergranular fracture at intermediate temperatures and then to predominately intergranular fracture at high temperatures. The observed cleavage-to-intergranular fracture transition tem-perature is approximately 800 K, in good agreement with the model prediction. Second, model predictions of the effect of grain orientation on the fracture mode are compared with experi-mental results. Electron backscatter patterns and fractographic techniques were used to analyze the grain orientations and fracture modes of grains on the fracture surfaces of specimens frac-tured at four temperatures in the range 439 to 1061 K. Experimental results reveal a correlation between fracture mode and slip system orientation relative to the crack, in good agreement with dislocation emission model predictions. Formerly Graduate Student with the Department of Materials Science and Engineering, University of Virginia.     

Crack initiation and near-threshold surface fatigue crack propagation behavior of the iron-base superalloy A-286

The fatigue behavior of the iron-base superalloy A-286 was studied at room temperature in air for three aging conditions: underaged, peak aged, and overaged. A fatigue strength at 10⁷ cycles of about 200 MPa, independent of aging condition, was measured for an applied load ratio ofR =0.1. Surface crack initiation and propagation were measured using hourglass specimens. Surface cracks were invariably initiated in slip bands orientated between 45 and 55 deg to the load axis, and an average ratio of crack depth to crack length of about 0.45 for these semi-elliptical cracks was measured. These earliest observable short surface cracks grew at an accelerated propagation rate in the near-threshold regime but were retarded in a transition stage, resulting in a minimum in crack growth rate. This behavior was correlated to the interaction of the crack with specific microstructure features. Following this minimum, the crack growth accelerated again with increasing ΔK and appeared to converge with the crack growth behavior expected for long through cracks. The crack propagation rate at fixed ΔK was lowest in underaged, compared to peak aged and overaged microstructures. The minimum and trends in crack growth rate appeared to depend on the development of roughness-induced closure. M. A. DAEUBLER, formerly with Carnegie Mellon University     

Sulphur segregation and high-temperature brittle intergranular fracture in alloy steels

High temperature brittle intergranular fracture has recently been identified as a mode of failure in alloy steels. It is associated with the dynamic segregation of sulphur to cracks in hard microstructures stressed at elevated temperatures in a manner analogous to hydrogen embrittlement at ambient temperature. Several models have been proposed to describe the action of sulphur, but insufficient experimental data have been available for their evaluation. The present study characterises sulphur enrichment at cracks and on free surfaces at high temperature in detail using scanning Auger spectroscopy. Both intergranular and transgranular surfaces were studied at pressures of air from 10⁻⁹ to 10⁻³ torr. Two types of sulphur enrichment at cracks were identified; general segregation to crack faces and local enrichment close to crack tips. The source of sulphur was largely that dissolved in the ferrite matrix. Large sulphides, intersecting grain boundaries, made a minor contribution, while small “overheated” intergranular sulphides were inoperative as sulphur sources. The role of stress in encouraging sulphur segregation was confirmed. In addition, an intermediate pressure of air was found to enhance sulphur enrichment, but only at surface oxygen coverages of 15–25 at.%. These observations were generally consistent with the influence of the crack tip stress field on migration of the sulphur solute, described by the “pure drift” model of high temperature brittle intergranular fracture. Refinement of the model, using finite element stress analysis, is included.     

Further study on the mechanism of the ductile-to-brittle fracture transition in C-Mn base and weld steel

In the present study, the crack opening displacement (COD) tests of specimens of C-Mn base and weld steel were carried out in the ductile-brittle transition temperature region. The majority of the specimens were fractured and others were unloaded prior to fracture after ductile fracture initiated and extended. The cavities and cleavage microcracks located in the vicinities of tips of fibrous cracks of the unloaded specimens were observed in detail. The finite element method (FEM) calculations of the stress and strain distribution ahead of the tip of an extending fibrous crack were completed. The mechanism of the ductile-to-brittle fracture transition was further investigated. It was revealed that in the ductile-brittle transition temperature region, the ductile fracture process was independent of temperature. The ductile-to-brittle fracture transition was triggered by initiating a catastrophic extension of a cleavage crack ahead of the fibrous crack tip, which occurred in a condition satisfying a combined criterion composed of three items, i.e., ε _p ≥ ε _pc for initiating a crack nucleus; σ _m √σ ≥ T _c for preventing the crack nucleus from blunting; and σ _yy ≥ σ _f for propagating the crack nucleus. For a specimen in which a fibrous crack occurred and propagated, the critical event for initiating a brittle cleavage fracture was the propagation of a ferrite grain-sized crack into neighboring grains. With extension of a fibrous crack, the behavior of the ductile-to-brittle fracture transition could be analyzed by the effect of the size of an “active zone” on the initiation of the brittle cleavage fracture.     

Effect of homogenization on microstructure and properties of WE91 alloys

The microstructure and properties of the Mg-9Y-1MM-0.6Zr alloy were studied by scanning electron microscopy, optical microscopy, transmission electron microscopy, hardness and tensile testing. Homogeni...     

Nanometer-scale crack initiation and propagation behavior of Fe3Al-based intermetallic alloy

The initiation and propagation of nanometer-scale cracks have been investigated in detail byin situ transmission electron microscope (TEM) observations for the intermetallic compound Fe₃Al under mode I loading. No dislocation was detected and no dislocation emission was found when cracks propagated directly from the thin edge of a double-jet hole where the thickness of the foil was below a critical thinness. Thinning took place in the thicker region of the foils because a great number of dislocations were emitted from the crack tip, and then an electron semitransparent region was formed in front of the crack tip. Following this process, a dislocation-free zone (DFZ) was formed. The maximum normal stress occurs in the zone. Nanometer-scale cracks initiated discontinuously ahead of the main crack tip in the highly stressed zone. The size of the smallest nanocrack observed was about 3 nm, and the tip radius of the nanocracks was less than 1 nm when the applied loading was low. The radius of the main crack tip was about 2.5 nm. The distances between discontinuous nanocracks and the main crack tip were about 5 to 60 nm, depending on the applied tensile loading. A relationship was found between the tensile loading and the nanocrack distance from the crack tip. The distance increases with the tensile loading, which is consistent with an “elastic-plastic” theoretical model.     

Study on notch fracture of TiAl alloys at room temperature

In-situ observations of fracture processes combined with one-to-one observations of fracture surfaces and finite-element method (FEM) calculations are carried out on notched tensile specimens of two-phase polycrystalline TiAl alloys. The results reveal that most cracks are initiated and propagated along the interfaces between lamellae before plastic deformation. The driving force for the fracture process is the tensile stress, which is consistent with a previous study.^[1] In specimens with a slit notch, most cracks are initiated directly from the notch root and extended along lamellar interfaces. The main crack can be stopped or deflected into a delamination mode by a barrier grain with a lamellar interface orientation deviated from the direction of crack propagation. In this case, new cracks are nucleated along lamellar interfaces of grains with favorable orientation ahead of the barrier grain. The main crack and a new crack are then linked by the translamellar cleavage fracture of the barrier grain with increasing applied load. In order to extend the main crack, further increases of the applied load are needed to move the high stress region into the ligament until catastrophic fracture. The FEM calculations reveal that the strength along lamellar interfaces (interlamellar fracture) is as low as 50 MPa and appreciably lower than the strength perpendicular to the lamellae (translamellar fracture), which shows a value higher than 120 MPa. This explains the reason why cracks nucleate and preferably extend along the lamellar interfaces.     

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. This research was supportd by U.S. Air Force Office of Scientific Research, Office of Aerospace REsearch, Grant No. AF-AFOSR-73-2431.     

HVEM studies of the effects of hydrogen on the deformation and fracture of AISI type 316 austenitic stainless steel

The mechanisms of hydrogen embrittlement in AISI type 316 austenitic stainless steel have been investigated by in situ straining in a high-voltage electron microscope (HVEM) equipped with an environmental cell. Hydrogen effects on strain-induced phase transformations, the generation rate and velocity of dislocation, and crack propagation rates were studied. The salient features of the fracture were similar for cracks propagating in vacuum and in hydrogen gas. In each case, ε and α′ martensite formed at the crack; the ε phase extended ahead of the crack while the α′ phase was restricted to high stress regions near the crack tip. The principal effect of hydrogen was to decrease the stress required for dislocation motion, for phase transformation of the austenite, and for crack propagation.     

Microstructural effects and crack closure during near

Near threshold fatigue crack growth behavior of a high strength steel under different tempered conditions was investigated. The important aspect of the study is to compare the crack growth behavior in terms of the closure-free component of the threshold stress intensity range, ΔK _th,eff While a systematic variation in the absolute threshold stress intensity range with yield strength was observed, the trend in the intrinsic ΔK _th or ΔK _th,eff exhibited a contrasting behavior. This has been explained as due to the difference in fracture modes during near threshold crack growth at different temper levels. It is shown that in a high strength and high strain hardening microstructure, yielding along crystallographic slip planes is difficult and hence it exhibited a flat transgranular fracture. In a steel with low strain hardening characteristics and relatively low strength, a tendency to crystallographic planar slip is observed consequently resulting in high ΔK _th. Occurrence of a predominantly intergranular fracture is shown to reduce intrinsic ΔK _th drastically and increase crack growth rates. Also shown is that crack closure can occur in high strength steels under certain fracture morphologies. A ‘transgranular planar slip’ during the inception of a ‘microstructure sensitive’ crack growth is essential to promote intergranular and faceted fracture. The occurrence of a maximum in the fraction of intergranular fracture during threshold crack growth corresponds to the ΔK value at which the cyclic plastic zone size becomes equal to the prior austenitic grain size.     

The effect of hydrogen on the fracture toughness of alloy X-750

The effect of hydrogen on the fracture toughness behavior of a nickel-base superalloy, Alloy X-750, in the solutionized and aged condition was investigated. Notched bend specimens were tested to determine if the fracture process was stress or strain controlled. The fracture was observed to initiate at a distance between the location of maximum stress and maximum strain, suggesting that fracture required both a critical stress and strain. The effect of hydrogen was further investigated and modeled using fracture toughness testing and fractographic examination. The fracture toughness of the non-charged specimen was 147 . Charging with hydrogen decreased the fracture toughness, K _lc, to 52 at a rapid loading rate and further decreased the toughness to 42 for a slow loading rate. This is consistent with the rate-limiting step forthe embrittlement process being hydrogen diffusion. The fracture morphology for the hydrogen-charged specimens was intergranular ductile dimple, while the fracture morphology of noncharged specimens was a mixture of large transgranular dimples and fine intergranular dimples. The intergranular failure mechanism in Alloy X-750 was a microvoid initiation process at grain boundary carbides followed by void growth and coalescence. One role of hydrogen was to reduce the void initiation strain for the fine intergranular carbides. Hydrogen may have also increased the rate of void growth. The conditions ahead of a crack satisfy the critical stress criterion at a much lower applied stress intensity factor than for the critical fracture strain criterion. A model based on a critical fracture strain criterion is shown to predict the fracture behavior.