Dynamic Embrittlement Research Articles

Characteristics of intermediate temperature dynamic embrittlement of age hardenable copper-chromium alloys

Copper-chromium alloys exhibit a severe intermediate temperature loss in ductility during uniaxial tensile loading that has the characteristics of dynamic embrittlement. The dynamic embrittlement is characterised by intergranular fracture caused by stress induced ingress of sulphur to the grain boundaries followed by decohesion.

Oxygen-induced intergranular fracture of the nickel-base alloy IN718 during mechanical loading at high temperatures

There is a transition in the mechanical-failure behavior of nickel-base superalloys from ductile transgranular crack propagation to time-dependent intergranular fracture when the temperature exceeds about 600 °C. This transition is due to oxygen diffusion into the stress field ahead of the crack tip sufficient to cause brittle decohesion of the grain boundaries. Since very high cracking rates were observed during fixed-displacement loading of IN718, it is not very likely that grain boundary oxidation governs the grain-boundary-separation process, as has been proposed in several studies on the fatigue-damage behavior of the nickel-base superalloy IN718. Further studies on bicrystal and thermomechanically processed specimens of IN718 have shown that this kind of brittle fracture, which has been termed dynamic embrittlement, depends strongly on the structure of the grain boundaries.

Oxygen-Induced Dynamic Embrittlement in Nickel-Base Superalloys

AbstractThe phenomenon of dynamic embrittlement involves the stress-induced diffusion of a surface- adsorbed embrittling element into grain boundaries, leading to time-dependent decohesion along these boundaries. Here, the state of our understanding of this generic type of brittle fracture is reviewed, with the focus on cracking of nickel-base superalloys caused by oxygen, including recent and new results on cracking in bicrystals, thermo-mechanical processing to reduce the susceptibility to dynamic embrittlement, and quench cracking.

On the suppression of dynamic embrittlement in Cu–8wt%Sn by an addition of zirconium

The alloy Cu–8wt%Sn alloy is subject to dynamic embrittlement caused by the stress-induced diffusion of surface-segregated tin into grain boundaries that intersect the surface. It is shown here that the cracking caused by this phenomenon can be suppressed by the addition of 0.04wt% zirconium, which can also eliminate the tendency for this alloy to crack during hot rolling (hot shortness).

Lead induced intergranular fracture in aluminum alloy AA6262

The influence of lead on the fracture behavior of aluminum alloy AA6262 is investigated. Under certain conditions, the mode of fracture changes from transgranular microvoid coalescence to an intergranular mechanism. Three different intergranular fracture mechanisms are observed: liquid metal embrittlement, dynamic embrittlement at temperatures below the melting temperature of lead, and intergranular microvoid coalescence. An attempt is made to examine the dependence of these three mechanisms on temperature, strain rate, and stress state using in situ scanning electron microscopy (SEM). Liquid metal embrittlement occurs when the alloy is fractured at temperatures above the melting temperature of lead and at low strain rates. At lower temperatures, the occurrence of dynamic embrittlement depends largely on strain rate, stress state, and temperature. Intergranular microvoid coalescence is not often observed.

High temperature intergranular fracture of 304H austenitic stainless steel: contribution of tests on UHP Fe-Ni-Cr alloys selectively doped with C, P and S

304H stainless steel and a series of Ultra High Purity alloys selectively doped with C, C+P, C+S, C+P+S were used to investigate the conditions of intergranular cracking during simulated stress-relief heat treatment at 610°C. It was demonstrated that this cracking is caused by a synergy between sensitisation (intergranular carbide precipitation and formation of chromium depleted zone) and sulphur segregation, both enhancing grain boundary sliding. Auger spectroscopy measurements indicated (i) intergranular phosphorus segregation after heat treatment as opposed to (ii) sulphur segregation at internal cavity surfaces after high temperature slow strain-rate interrupted tests. Sulphur segregation to free surfaces was confirmed by “in situ” measurement at 600°C within the main Auger chamber. These results support the McMahon’s “dynamic embrittlement model” of intergranular cracking, where dynamic sulphur segregation plays a major role. Healing heat treatment at 800°C is proposed to decrease the stress-relief heat treatment susceptibility of high carbon austenitic stainless steels.

The effect of grain-boundary-engineering-type processing on oxygen-induced cracking of IN718

Polycrystalline Ni-base superalloys are prone to time-dependent intergranular failure when the loading temperatures exceeds about 873 K. The fracture process is then controlled by oxygen grain-boundary diffusion followed by decohesion of the respective boundaries. It is shown that this kind of cracking for IN718 at 923 K in air can be reduced significantly by successive steps of deformation and annealing, which is known as grain-boundary-engineering processing. This illustrates the importance of the grain-boundary-character distribution with regard to this mode of failure, which is known as ‘dynamic embrittlement.’

Thermodynamic and kinetic aspects of interfacial decohesion

Thermodynamic and kinetic aspects of interfacial decohesion are analyzed for a uniformly stressed interface in the presence of an embrittling impurity. During the interfacial separation, the impurity penetrates into the interface and reduces it cohesion. Rice (1976) and Hirth and Rice (1980) analyzed the thermodynamics of this process in great detail with emphasis on the limiting cases of infinitely fast and infinitely slow separation. The ‘dynamic’ model of decohesion proposed in this work extends their analysis to an arbitrary relation between the rates of separation and impurity transport to the interface, including transient kinetics in which the two rates are comparable to one another. Such kinetics play an important role in the phenomenon of dynamic embrittlement observed in many materials. Calculations of dynamic decohesion have been performed for two mechanisms of impurity supply to the interface: the reaction kinetics and bulk diffusion. In both cases a kinetic parameter R has been identified such that R≪1 for slow separation, R≫1 for fast separation, and R∼1 for the transient regime. In the transient regime, the work of decohesion, cohesive strength, the impurity concentration at the new surfaces, and other properties depend sensitively on the strain rate in agreement with experimental observations of dynamic embrittlement.

Dynamic embrittlement of Cu–Sn bicrystals grown from the melt

Bicrystals of Cu–8%Sn grown from the melt to make a Σ=5 (0 3 1)[0 0 1] symmetrical tilt boundary were found to be very difficult to crack by dynamic embrittlement, even along the fast-diffusion direction, in contrast with earlier results on bicrystals made by diffusion bonding that contained particles in the boundary. This has implications for the role of particles and the tendency for cracking in low- Σ boundaries.

Oxygen-induced intergranular cracking of a Ni-base alloy at elevated temperatures—an example of dynamic embrittlement

The brittle intergranular cracking of Alloy 718 at temperatures between 550 and 650°C and at oxygen pressures from atmospheric to 10 −5 torr has been studied by the use of single-edge-notched specimens loaded in pure bending at fixed displacement. The cracking rate was calculated from the load relaxation and a compliance–calibration curve. Rates in the range 10 −9 to 10 −5 m/s were observed, depending on temperature and oxygen pressure. The phenomenon appears similar to the oxygen effects observed by others in cyclic loading and also to the dynamic embrittlement observed in other materials and environments. The mechanism recently modeled for dynamic embrittlement can be applied here directly, and the special characteristics of polycrystalline (as opposed to bicrystalline) behavior, as well as the effects of varying the oxygen pressure, can be explained.

On the dynamic embrittlement of copper-chromium alloys by sulphur

The high temperature ductility of copper-chromium alloys has been examined in the temperature range of 25–700°C. Copper-chromium alloys exhibit severe intermediate temperature loss in ductility. The loss in ductility is associated with the ingress of sulphur along the grain boundaries under the influence of tensile stress, followed by decohesion of grain boundaries. Integranular facets of the fracture surface exhibit striations, indicative of quasi-static, step-wise crack growth process.

The effect of boron on stress-relief cracking of alloy steels

High-purity vacuum-melted laboratory heats of the MnMoNiCr forging steel designated by the ASTM as A508-class 2 were studied to determine the effects of additions of boron and of aluminum on the susceptibility to stress-relief cracking (SRC). The highly deleterious effect of boron was reproduced, and the aluminum addition was found to be at least as deleterious as the boron. High-resolution examination of the fracture surfaces showed that post-fracture faceting occurred on the intergranular fracture surface of the pure steel, but not on the boron-doped steel, indicating a difference in surface composition. From previous AES studies on other steels, it was deduced that the effect of both boron and aluminum is to scavenge nitrogen and to prevent surface segregation of nitrogen with chromium at the test temperature of 550°C. This would allow more sulfur segregation to the surface and thus provide a higher surface concentration of this element, which is known to be the cause of the diffusion-controlled intergranular decohesion that occurs during SRC at high stresses, a process known as dynamic embrittlement. It is concluded that the cosegregation of nitrogen and chromium retards SRC by suppressing sulfur segregation, but this protective effect can be eliminated by scavenging of nitrogen by either boron or aluminum. It is suggested that the susceptibility of a heat of steel to SRC could be assessed by examining the surface-segregation behavior by means of AES.

Tin-induced dynamic embrittlement of Cu–7% Sn bicrystals with a Σ5 boundary

The kinetic model for dynamic embrittlement predicts that the cracking rate is proportional to the diffusivity of the embrittling species along the grain boundary. Diffusion-bonded bicrystals of Cu–7% Sn with a Σ5 (031)/[100] symmetrical tilt boundary were used to test the model and to examine the mechanism of the cracking process. The bicrystals, in which surface-segregated tin was the embrittling species, were tested at 265°C in vacuum parallel and perpendicular to the tilt axis. Cracking occurred parallel to the tilt axis, the fast-diffusion direction, by the propagation of a sharp crack at a maximum rate of ∼2 μm/s and at a stress intensity of less than 3.5 MPa √m. Cracking appeared to be continuous, suggesting that the tin diffusion occurs in the core of the crack tip. No cracking occurred perpendicular to the tilt axis, i.e. the slow-diffusion direction; here, plastic creep occurred with the formation of cavities at the grain boundaries. The hypothesis of a grain-boundary-diffusion process leading to cracking is supported, and the susceptibility to this type of cracking appears to be extremely sensitive to grain boundary structure.

Electron microscopy: probing the atomic structure and chemistry of grain boundaries, interfaces and defects

At the heart of ‘ dynamic embrittlement’ phenomena is the stress-induced segregation of microscopic quantities of embrittling impurities to fracture surfaces. Grain boundaries and interfaces are often the natural weak links in a material. The structural and chemical information of such internal interfaces can be probed on an atomic scale using transmission electron microscopy. High-resolution electron microscopy can be used to determine the atomic coordinates of crystalline interfaces. Electron-energy-loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy can be performed on any boundary or defect and can provide chemical information such as composition and bonding. The spatial resolution of EDS is limited by low collection efficiency to around 10 Å. The more efficient signal collection for EELS allows almost atomic resolution for light elements. EELS fine structure offers a fingerprint of the local boundary arrangements, and also insight into the bonding (and possible reactivity) of boundaries and other defects. Overall, electron microscopy can be used to identify the atomistic characteristics of those interfaces susceptible to dynamic embrittlement. The structural and electronic information obtained can be used as a starting point for semi-empirical and ab initio simulations.

Oxygen-Induced Intergranular Decohesion in IN718

The cracking of IN718 was studied under static loads in air and oxygen and found to undergo oxygen-induced dynamic embrittlement in the same way as in steels with surface-adsorbed sulfur and in bronze with surface-adsorbed tin. When the oxygen supply at the surface is plentiful, the intergranular fracture surface exhibits decohesion with no apparent plasticity, which is consistent with stress-induced oxygen penetration on the nanometer scale ahead of a sharp crack. In one-atmosphere oxygen, cracking was observed to occur at rates on the order of 10 -5 m/sec, An advancing crack could be abruptly arrested by switching the atmosphere from oxygen to vacuum of less than 10 -4 torr.

Quasi-Static Brittle Fracture

AbstractRecent work on the phenomenon of diffusion-controlled quasi-static brittle fracture, known as dynamic embrittlement, is reviewed here with reference to sulfur-induced embrittlement of steel, tin-induced embrittlement of Cu-Sn alloys, and oxygen-induced embrittlement of copper-based and nickel-based alloys. The mechanisms of this generic form of intergranular brittle fracture are discussed.

A study of dynamic embrittlement in bicrystals of Cu-7%Sn

In a kinetic model [1] for the phenomenon of dynamic embrittlement, the cracking rate is predicted to be proportional to the diffusivity of the embrittling species along the grain boundary. To test this model, bicrystals of Cu-Sn with Σ5 symmetrical tilt boundaries in which tin is the embrittling element are being used. The diffusivities parallel and perpendicular to the tilt axis are expected to be different, therefore, the crack growth rates in these two directions should vary in the same ratio as the diffusivities. Measurements of the crack-growth rate along the [100] direction in the Cu-Sn alloy bicrystal at 265 °C showed that the cracking occurred by decohesion along the grain boundary with almost no observable plasticity at a maximum crack growth rate of approximately 10 −6 m s −1. A model for steady-state crack growth has recently been developed and is discussed here.

The effect of pre-segregation of phosphorus on the dynamic embrittlement of iron-vanadium alloys at 550 °C in air

In this paper the author presents preliminary investigations of another example of dynamic embrittlement, together with the study aimed at examining the possible effect of grain boundary pre-segregation of phosphorus on the dynamic embrittlement of iron-vanadium alloys. The preliminary studies suggests that Fe-V containing alloys are susceptible to dynamic embrittlement by oxygen-containing environment. However, intergranular pre-segregation of phosphorus can suppress the dynamic embrittlement process. Efforts are currently underway to further investigate the phenomenon at different temperatures and partial pressures of oxygen.

An Example of Dynamic Embrittlement: Oxygen-Induced Cracking of a Cu-Be Alloy at 200°C

An Example of Dynamic Embrittlement: Oxygen-Induced Cracking of a Cu-Be Alloy at 200°C

Study of Dynamic Embrittlement in Alloy Bicrystals

ABSTRACTIn a kinetic model [1] for the phenomenon of dynamic embrittlement, the cracking rate is predicted to be proportional to the diffusivity of the embrittling species along the grain boundary. To test this model, bicrystals of Cu-Sn and Fe-Si with Σ5 symmetrical tilt boundaries are used in which tin and sulfur, respectively, are the embrittling elements. The diffusivities parallel and perpendicular to the tilt axis are expected to be different, therefore the crack growth rates in these two directions should vary in the same ratio as the diffusivities.Preliminary measurements of crack growth rate along the [100] direction in the Cu-Sn alloy bicrystal are presented. The cracking occurred by decohesion along the grain boundary with almost no observable plasticity. The steady state crack growth was found to be approximately 10∼6 m/sec.