Electrochemical studies on cerium(III) in molten fluoride mixtures

This study aims to determine the principal electrochemical characteristics of the electrodeposition of cerium metal from molten fluoride systems. The cathodic process of Ce³⁺ ions in LiF-NaF and LiF-NaF-CaF₂ molten salts was studied using electrochemical techniques as steady state and cyclic voltammetry methods. The decomposition potential (E_d) and the overvoltage(η) were determined for NaCeF₄ using current-potential curves under galvanostatic conditions. The E_d was found to be 2.025 V in LiF-NaF and 2.045 V in LiF-NaF-CaF₂. It was also found that the ohmic drop potential (EΩ) was not dependent on NaCeF₄ concentration and it rose as the current intensity increased. The overvoltage (η) was determined from the polarization curves and the Tafel coefficients and kinetic parameters were calculated on the assumption that the process constitutes of direct discharge of Ce³⁺, with no solvent-solute interaction. In order to elucidate the cathodic process the investigation by cyclic voltammetry technique was finally used. From the evolution of the voltammograms we concluded that the electrochemical reduction of Ce³⁺ ion was actually a reversible process on the molybdenum electrode and cathodic reduction of Ce³⁺ took place in one single step involving three electron exchanges.     

Combined mode I-mode III fracture toughness of a spherodized 1090 steel

The aim of this investigation was to determine the fracture behavior of a spherodized 1090 steel under combined mode I-mode III loading conditions. Suitably defined formulations of the J integral denoted J_ic and J_iiic were used to characterize the elastic-plastic fracture of this steel. As the mode III component in the system is increased, the resolved mode I J integral at initiation decreases, its mode III counterpart increases and the total J value remains nearly a constant. This implies a constant energy requirement for fracture initiation under mixed mode loading. As the crack plane becomes less inclined to the load line, the slopes of the mode I and total J resistance curves increase from their pure mode I values until a crack inclination angle of about 65° is reached. Somewhere in the region of 65-55°, a maximum in these values is reached and they fall off rapidly for larger mode III components. This drop is accompanied by the breakup of the crack front into mode I and mode III steps, which is shown to be an energetically more favorable process for this steel.     

Further observations of SCC in alpha-beta brass: Considerations regarding the appearance of crack arrest markings during SCC

A series of constant displacement and constant extension rate SCC tests was performed on an alpha-beta brass alloy in 1 N Na₂SO₄ solutions. The chosen mechanical and electrochemical conditions resulted in predominantly transgranular, cleavage-like failure at high (≈8 to ≈50 μm· s^•1) average crack propagation rates. Crack arrest markings were observed on selected transgranular facets which almost exclusively bordered regions of ductile overload failure. It is proposed that the observed crack velocities and the specific nature of the arrest mark appearance are most consistent with a cracking mechanism involving adsorption or some other interaction with a damaging environmental species. Formerly with Michigan Tech.     

Effect of strain wave shape on low-cycle fatigue crack propagation of SUS 304 stainless steel at elevated temperatures

Effect of strain wave shape on strain-controlled low-cycle fatigue crack propagation of SUS 304 stainless steel was investigated at 600 and 700 °C. It was found that the rate of crack propagation in a cycle-dependent region was successfully correlated with the range of cyclicJ-integral, ΔJf, regardless of the strain wave shape, frequency, and test temperature. It was also shown that the rate of crack propagation gradually increased from cycle-dependent curve to time-dependent one with decreasing frequency and slow-fast strain wave shape, and that one of the factors governing the rate of crack propagation in such a region was the ratio of the range of creepJ-integral to that of totalJ-integral, ΔJ _c/ΔJT. Based on the results thus obtained, an interaction damage rule proposed semi-empirically was interpreted, with regard to crack propagation. Furthermore, fatigue crack initiation mechanism in slow-fast strain wave shape was studied, and it was shown that grain boundary sliding took an important role in it.     

A study of crack tip blunting and the influence of blunting behavior on the fracture toughness of ultra high strength steels

Cross-sections of strained but not fractured compact tension J_IC speciments have been examined to investigate crack-tip blunting behavior as a function J level for four different microstructures. The microstructures were the as-quenched microstructures of HP9-4-20 and HO9-4-10 steels and the microstructures obtained by tempering these steels at 565°C. Smooth blunting was observed for the as-quenched microstructures while the fatigue cracks for tempered microstructures blunted to geometries characterized by two or three corners or vertices. The blunting geometries were clearly defined at J levels well below J_IC. For the case of smooth blunting voids tended to form directly ahead of the crack tip and crack extension by fracture occurred when the ligament between the blunting crack tip and the void directly ahead of the crack tip failed by shear fracture at an angle of about 45°C to the plane of the crack. Blunting to vertices was characterized by the growth of large voids very close to the corners or vertices of the blunting rack tip; it appears that the blunting geometry was maintained by the coalescence of these voids with the blunting crack tip. The results further suggest that if two microstructures have the same constrained ductility and identical inclusion distributions and one blunts smoothly and the other to vertices the microstructure which blunts to vertices can have substantially higher toughness.     

Enhancement of the Stress Corrosion Sensitivity of AA5083 by Heat Treatment

In this study, the stress corrosion cracking (SCC) resistance of AA5083 is intentionally degraded by a series of progressively longer annealing treatments at 448 K (175 °C) that create a two-phase microstructure. Precipitation of strongly anodic Mg₂Al₃, known as β-phase, occurs heterogeneously with substantial precipitation along the grain boundaries, as observed by differential interference microscopy. Ultimate tensile strength, yield strength, and strain to failure of AA5083 alloy were found to be independent of the amount of β-phase precipitates, making AA5083 an ideal system to study the relative contributions of anodic dissolution and hydrogen embrittlement. Open circuit dropwise exposure SCC tests with precracked double cantilever beam (DCB) specimens made from the AA5083 alloy with different heat treatment conditions were conducted using 3.5 pct NaCl solution at an initial stress intensity factor (K _I ) of \( 1 5\,{\text{ksi}}\sqrt {\text{in}} .\;\left( { 1 6. 5\,{\text{MPa}}\sqrt {\text{m}} } \right). \) Two SCC characteristics, initial crack growth rate and incubation time, were found to be strongly dependent on the amount of β-phase precipitates. Initial crack growth rate increased sigmoidally as a function of heat treatment time with an inflection point between 120 and 240 hours of sensitization time, while the incubation time decreases monotonically with sensitization time. Additionally, fracture surfaces investigated by scanning electron microscopy demonstrated characteristics of intergranular cracking with multiple crack tips. Discussion centers on the evidence supporting anodic dissolution of β-phase grain boundary precipitates as a primary mechanism of SCC in severely sensitized AA5083 alloy and the potential contribution of hydrogen embrittlement in the failure of grain boundary ligaments between β-phase grain boundary precipitates in less severely sensitized conditions.     

Crack paths,microstructure, and fatigue crack growth in annealed and cold-rolled AISI 304 stainless steels

To assist in the understanding of micromechanisms for corrosion fatigue crack growth in metastable austenitic steels, the relationships between the crack paths and the underlying microstructure were investigated for annealed and cold-rolled (CR) 304 stainless steels that had been tested in a deaerated 3.5 pct NaCl solution, air, and vacuum. Corrosion fatigue in the deleterious environments (3.5 pct NaCl and air) was brittle and occurred primarily by {001}_γ and other unidentified, quasi-cleavage (QC), accompanied by preferential cracking along {111}_γ twin and grain boundaries. In contrast, fatigue cracking in vacuum was ductile, fully transgranular, and noncrystallographic. Transformation to alpha prime (α′-) martensite by fatigue was found to be essentially complete in the CR steel, which contained ε-martensite, and in the annealed steel tested in vacuum, but was substantially less in the annealed steel tested in air and 3.5 pct NaCl solution. These results, taken in conjunction with the crack growth and electrochemical reaction data, support hydrogen embrittlement (HE) as the mechanism for corrosion fatigue crack growth in 304 stainless steels in 3.5 pct NaCl solution. Martensitic transformation appears not to be the only responsible factor for embrittlement. Other microstructural components, such as twin and grain boundaries, slip bands, and cold work-induced lattice defects, may play more important roles in enhancing crack growth rates.     

Observation and Modeling of Stress Corrosion Cracking in High Pressure Gas Pipe Steel

Stress corrosion cracking (SCC) is commonly observed to form a colony of closely spaced multiple cracks. Four stages of SCC colony evolution are discussed. The first is the colony initiation stage (CIS), which is associated with formation of corrosion pits randomly distributed over a certain domain of the surface exposed to an aggressive environment. Electrochemical processes play a leading role in CIS. The individual crack growth (ICG) driven by a combination of mechanical stresses and electrochemical processes constitutes the second stage. At the end of the second stage, the individual cracks reach certain proximity of one another resulting in much crack interaction. This becomes a transition to the third, strong crack interaction and clusters formation, stage. Cluster growth and individual crack or a cluster instability leading to the ultimate failure constitute the final, fourth stage of the SCC evolution process. In this article, we present observations and a general approach to modeling the first two stages of SCC, i.e., CIS and ICG, that together constitute the major part of the total lifetime of an engineering structure serving under SCC conditions. A computer simulation of individual SC crack growth is developed and compared with a large set of SCC observation data.     

Dislocation nucleation at metal-ceramic interfaces

The ductile vs brittle behaviour of metal-ceramic interfaces is discussed within an atomistic framework, in which the mechanical response of an interfacial crack is assumed to be ultimately controlled by the competition between atomic decohesion and dislocation nucleation ahead of the crack tip. As in later versions of the Rice-Thomson model, this competition may be evaluated in terms of the parameters G_cleave, the energy release rate for cleavage of the metal-ceramic interface, and G_disl, the energy release rate associated with the emission of a single dislocation within the metal. The various models of dislocation nucleation are discussed, with emphasis on an approach which makes use of Peierls-like stress vs displacement relations on a slip plane ahead of a crack tip. A recent analytical result by Rice shows that for a mode II or III shear crack, with a slip plane parallel to the ack plane, a dislocation is emitted when G = γ_us (G is the energy release rate corresponding to the “screened” crack tip stress field and γ_us is the “unstable stacking” energy associated with the sliding of atomic planes past one another). This treatment permits the existence of an extended dislocation core, which eliminates the need for the core cutoff radii required by the Rice-Thomson model of emission. Results are presented here for the nucleation of dislocations under more realistic assumptions for metal-ceramic cracks, namely, the emission on inclined slip planes within a mixed-mode crack-tip field. The specific case of a copper crystal bonded on a {221} face to sapphire is analyzed, and the results are used to interpret the recent experimental observations of Beltz and Wang [Acta metall. mater.40, 1675 (1992)] on directional toughness along this type of interface.     

Machining of reproducible flaws corresponding to fatigue cracks for fracture mechanics tests of components and welded joints

In connection with the analysis and the development of failure concepts in fracture mechanics for quasistatic loaded components and elastic-plastic behaviour of the material, tests are also carried out with welded and/or complex shaped specimens or structures. Thereby the difficulty arises of generating reproducible flaws in the form of fatigue precracks in definite positions in the components, respectively in the welded joint. It is reported exemplarily about experiments on different CT25 and CCT specimens and on a pressure vessel which contained a fatigue pre-crack, a 0.2 mm saw cut or notches with notch root radius ≤ ≥ 0.1 mm as flaws. The comparison of the results with regard to J-integral at initiation of stable crack, J_i, and J_R curves shows that under certain conditions the 0.2 mm saw cut (notch root radius ≤ ≤ 0.02 mm) is a useful alternative, if reproducible generation of a fatigue pre-crack will not be successful or too expensive. The tests were carried out on StE 460 and on a welded joint of this steel at 25 ± 2°C.     

The mechanical nature of stress-corrosion cracking in Al-Zn-Mg alloys: II. electrochemical-mechanical model

Slow crack growth during SCC of 7075 aluminum has been shown to comprise both an electrochemical and a mechanical component. These findings prompted a review of several possible mechanical models, and seven possible controlling thermally-activated processes. Since no existing interpretation could satisfy all of the observations, an empirical model was developed. The conclusion is that slight modification of many existing proposed mechanisms could explain the general features of SCC but that any theoretical model must contain some aspect of the mechanical rupture process. Formerly with the Lawrence Radiation Laboratory, Berkeley Formerly with the Lawrence Radiation Laboratory Berkeley     

The effect of matrix strength on the fracture resistance of an alpha-Beta titanium alloy,corona-5

This paper presents the results of a study of the effect of matrix yield strength, at constant Widmanstätten α microstructure, on the fracture resistance of an α-β Ti alloy, CORONA-5. Fracture initiation resistance,J _q, and the stable crack growth resistance,T, were evaluated by the single specimen, unloading compliance method for four different microstructures and three yield strengths. The microstructures involved coarse or fine Widmanstätten α particles in a heat treated β-matrix; the yield strength ranged from 765 to 1018 MPa. It was found thatJ _q/σ_Y, where σ_Y is the effective yield strength, decreased with increasingσ _Y.T/σ _Y also decreased with increasing σ_Y for fine structures. For the coarse α structures, however, T/σ_Y revealed intermediate maxima. Coarser structures, in general, revealed higher values ofJ _q/σ_Y andT/σ _Y. The cause was found primarily to be due to the effect of increased α particle thickness. The effect of grain size was secondary. J_q/σ_Y increased with increasing tensile strain hardening rate, obtained at the onset of void nucleation. T/σ_Y was found to decrease with increasing tensile void growth rate. In general, J_q/σ_Y and T/σ_Y revealed different relationships with microstructure. Fatigue precrack front- and the stable crack length-tortuosities did not yield any general relationship to fracture resistance at different yield strengths.     

A critical evaluation of the stress-corrosion cracking mechanism in high-strength aluminum alloys

Attempts have been made to elucidate the mechanism of stress-corrosion cracking (SCC) in high-strength Al-Zn-Mg and Al-Li-Zr alloys exposed to aqueous environments by considering the temperature dependence of SCC susceptibility based upon the anodic dissolution and hydrogen embrittlement models. A quantitative correlation which involves the change of threshold stress intensity,K _ISCC, with temperature on the basis of anodic dissolution has been developed with the aid of linear elastic fracture mechanics. From the derived correlation, it is concluded that the threshold stress intensity decreases as the test temperature increases. This suggestion is inconsistent with that predicted on the basis of hydrogen embrittlement. It is experimentally observed from the Al-Zn-Mg and Al-Li-Zr alloys that the threshold stress intensity,K,_ISCC, decreases and the crack propagation rate,da/dt, over the stress intensity increases with increasing test temperature. From considering the change in SCC susceptibility with temperature, it is suggested that a gradual transition in the mechanism for the stress-corrosion crack propagation occurs from anodic dissolution in stage I, where the crack propagation rate increases sharply with stress intensity, to hydrogen embrittlement in stage II, where the crack propagation rate is independent of stress intensity.     

Combined mode I-mode III fracture of a high strength low-alloy steel

Special formulations of theJ integral were used to quantify the combined mode I-mode III fracture behavior of ASTM A710 Grade A steel, a tough material which displays elastic-plastic behavior at room temperature. Experimental fracture initiation loci for two heat-treated conditions were linear inJ _i-J_iii space, agreeing with analytical predictions based on linear elasticity. As commonly observed in mode I fracture behavior, large tearing modulus values accompanied largeJ values for mode I and mode III components, individually as well as for totalJ values. There was a pronounced tendency toward antiplane shear flow in the modified compact tension specimens tested. This may result from the concentration of mode III shear strains in the plane of the propagating crack as predicted by slip line theory. Shear fractures with the crack propagating at inclined angles to both load line and plate free surfaces are favored energetically over pure mode I cracks even though the total surface area formed by the latter is much smaller.     

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.     

Fracture and toughening mechanisms in an α2 titanium aluminide alloy

The deformation and fracture behaviors of the Ti-24Al-11Nb alloy with an equiaxed α₂ + β microstructure have been characterized as a function of temperature by performing uniaxial tension andJ _IC fracture toughness tests. The micromechanisms of crack initiation and growth have been studied bypost mortem fractographic and metallographic examinations of fractured specimens, as well as byin situ observation of the fracture events in a scanning electron microscope (SEM) equipped with a high-temperature loading stage. The results indicate that quasistatic crack growth in the Ti-24Al-11Nb alloy occurs by nucleation and linkage of the microcracks with the main crack, with the latter frequently bridged by ductile β ligaments. Three microcrack initiation mechanisms have been identified: (1) decohesion of planar slipbands in the α₂ matrix, (2) formation of voids and microcracks in β, and (3) cracking at or near the α₂ + β interface due to strain incompatibility resulting from impinging planar slip originated in α₂. The sources of fracture toughness in the 25 °C to 450 °C range have been attributed to crack tip blunting, crack deflection, and a bridging mechanism provided by the ductile β phase. At 600 °C, a change of toughening mechanisms leads to a lowering of the initiation toughness (theK _IC value) but a drastic increase in the crack growth toughness and the tearing modulus.     

Effect of mean stress on hydrogen assisted fatique crack propagation in duplex stainless steel

Hydrogen assisted subcritical cleavage of the ferrite matrix occurs during fatigue of a duplex stainless steel in gaseous hydrogen. The ferrite fails by a cyclic cleavage mechanism and fatigue crack growth rates are independent of frequency between 0.1 and 5 Hz. Macroscopic crack growth rates are controlled by the fraction of ferrite grains cleaving along the crack front, which can be related to the maximum stress intensity, K_max. A superposition model is developed to predict simultaneously the effects of stress intensity range (ΔK) and K ratio (K_min/K_max). The effect of K_max is rationalised by a local cleavage criterion which requires a critical tensile stress, normal to the {001} cleavage plane, acting over a critical distance within an embrittled zone at the crack tip.     

On macroscopic and microscopic analyses for crack initiation and crack growth toughness in ductile alloys

Relationships between crack initiation and crack growth toughness are reviewed by examining the crack tip fields and microscopic (local) and macroscopic (continuum) fracture criteria for the onset and continued quasi-static extension of cracks in ductile materials. By comparison of the micromechanisms of crack initiationvia transgranular cleavage and crack initiation and subsequent growthvia microvoid coalescence, expressions are shown for the fracture toughness of materials in terms of microstructural parameters, including those deduced from fractographic measurements. In particular the distinction between the deformation fields directly ahead of stationary and nonstationary cracks are explored and used to explain why microstructure may have a more significant role in influencing the toughness of slowly growing, as opposed to initiating, cracks. Utilizing the exact asymptotic crack tip deformation fields recently presented by Rice and his co-workers for the nonstationary plane strain Mode I crack and evoking various microscopic failure criteria for such stable crack growth, a relationship between the tearing modulusT _R and the nondimensionalized crack initiation fracture toughnessJ _Ic is described and shown to yield a good fit to experimental toughness data for a wide range of steels. An erratum to this article is available at .     

On macroscopic and microscopic analyses for crack initiation and crack growth toughness in ductile alloys

Relationships between crack initiation and crack growth toughness are reviewed by examining the crack tip fields and microscopic (local) and macroscopic (continuum) fracture criteria for the onset and continued quasi-static extension of cracks in ductile materials. By comparison of the micromechanisms of crack initiationvia transgranular cleavage and crack initiation and subsequent growthvia microvoid coalescence, expressions are shown for the fracture toughness of materials in terms of microstructural parameters, including those deduced from fractographic measurements. In particular the distinction between the deformation fields directly ahead of stationary and nonstationary cracks are explored and used to explain why microstructure may have a more significant role in influencing the toughness of slowly growing, as opposed to initiating, cracks. Utilizing the exact asymptotic crack tip deformation fields recently presented by Rice and his co-workers for the nonstationary plane strain Mode I crack and evoking various microscopic failure criteria for such stable crack growth, a relationship between the tearing modulusT _R and the nondimensionalized crack initiation fracture toughnessJ _Ic is described and shown to yield a good fit to experimental toughness data for a wide range of steels.     

Effect of Co content on the stress-corrosion cracking behavior of 7091-type aluminum powder alloys

An investigation of the stress-corrosion cracking (SCC) behavior of three aluminum powder alloys, containing 0.0, 0.4, and 0.8 wt pct Co, using double cantilever beam specimens has shown a significant increase in SCC resistance with increasing Co content. This resistance to cracking takes the form of both a decrease in plateau crack velocity and an increase in the threshold stress intensity factor for cracking (K _ISCC ) as the Co content increases. The SCC fracture is intergranular and the crack path is tortuous because of the oxides and Co₂Al₉ intermetallic particles contained within the powder metallurgy alloys. We propose that the improvements in SCC resistance result from the Co₂Al₉ particles, which catalyze the recombination and evolution of hydrogen, thereby reducing hydrogen absorption and embrittlement. Formerly with Martin Marietta Laboratories