Boundary Embrittlement Research Articles

Effects of neutron irradiation on mechanical properties and microstructures of dispersion and precipitation hardened copper alloys

Tensile specimens of CuAl2O3, CuCrZr and CuNiBe alloys were irradiated with fission neutrons to fluences of 5 × 1022, 5 × 1023and1 × 1024n/m2 (E > 1MeV) at 47°C. Tensile properties and Vickers hardness were determined at 22°C. Microstructures of irradiated as well as unirradiated specimens were examined using a transmission electron microscope and the fractured surfaces were investigated in a scanning electron microscope. The most significant effect of irradiation is a drastic decrease in the ductility of copper alloys at a dose level as low as 0.2 dpa. The loss of ductility appears to be related to the intrinsic hardness of the grain interior and not to the grain boundary embrittlement. It is suggested that the irradiation-induced hardening and the lack of uniform elongation may be understood in terms of difficulty in dislocation generation due to pinning of grown-in dislocation by defect clusters (loops) and/or impurity atoms.

STEM investigation of the chemistry and bonding changes associated with the grain boundary embrittlement of Cu by Bi

It has long been known that trace amounts of Bi can embrittle Cu after appropriate heat treatments. The Bi segregates to the grain boundaries and weakens them such that failure occurs through intergranular fracture without plastic deformation. This behavior is demonstrated in the scanning electron micrograph of a typical Cu-Bi fracture surface in Figure 1. It is known that the Bi extends for only a few atomic layers into the grains on either side of the grain boundary. This narrow segregation width was been confirmed using Energy Dispersive X-ray Spectroscopy (EDS) on a VG HB603 STEM. Figure 2 shows the ratio of Bi to Cu as the probe is stepped across the grain boundary.The segregation behavior is well understood, however it is not yet properly understood how the Bi causes embrittlement once it is at the grain boundaries. The Bi must change the bonding at the boundaries so that the boundaries become weak and hence the most likely fracture path. The Electron Energy Loss Near Edge Structure (ELNES) coupled with the small probes and high current density available in a field emission STEM can provide information about the localized electronic structure and hence bonding at grain boundaries. Previous investigations indicated that the near edge structure of Cu was altered at the grain boundaries due to the presence of Bi.

Role of grain boundary segregation in corrosion fatigue process of high strength aluminium alloy

Effects of grain boundary segregation on fatigue crack growth in 7050 aluminium alloy have been investigated under various aging conditions in a humid air environment. The results show that with increased aging, the concentration of Mg segregation on the grain boundary decreases, resulting in a reduction in the fatigue crack growth rate for the same humidity and value of Δ K. The effect of grain boundary Mg segregation on the fracture stress has been calculated using the quasi-chemical method. The result indicates that Mg segregation induces fracture reduction and grain boundary embrittlement.

Investigations of the Bonding Changes Associated with Grain Boundary Embrittlement

ABSTRACTThe embrittlement of materials through the segregation of impurities to the grain boundaries is a common and industrially important problem. Despite considerable investigation, the mechanism by which the impurity elements cause embrittlement is not well understood. A change in the electron energy loss near edge structure (ELNES) has been observed at Cu grain boundaries containing Bi. This result provides experimental evidence that a change in the electronic structure at the grain boundary is responsible for embritdement.

Relationship between misorientation and bismuth induced embrittlement of [001] tilt boundary in copper bicrystal

The grain boundary embrittlement (GBE) of metal copper caused by bismuth segregation has been extensively studied both experimentally and theoretically. In this paper, the GBE was further studied for [001] symmetrical tilt Cu bicrystals with the misorientation between 0{degree} and 90{degree}. Attention was paid to the influence of the GB structure and the Bi doping time (the Bi bulk concentration) on the fracture behaviors of the bicrystals. The fracture stresses and SEM micrographs are presented, discussed and compared with the density of the CSL and the dependence of the GB energy on the misorientation.

Microstructure and mechanical behaviour of TZM and Mo-5% Re alloys irradiated with fission neutrons

The response of microstructural and mechanical properties of TZM and Mo-5% Re alloys to neutron irradiation are reported. Irradiations were carried out at five temperatures between 323 and 723 K to a dose level of ∼ 0.16 dpa. The resulting microstructures consisted of a high density of small loops in both alloys with the addition of characteristic loop rafting in TZM. There was a marked sensitivity of microstructure to temperature. Void formation was found in both TZM and Mo-5% Re after the 623 and 723 K irradiations. The irradiations induced a significant increase in Vickers hardness in all cases but with no obvious dependence on irradiation temperature. Even at the present low neutron dose, large increases in tensile strength and a drastic decrease in ductility were observed. These results are discussed in terms of source hardening of the matrix within grains rather than obstacle hardening. Although the fracture processes are concentrated at the grain boundaries, these are not due to grain boundary embrittlement.

Effect of calcium on grain boundary embrittlement of structural steel strengthened with carbonitrides

Economically alloyed structural steels with carbonitride strengthening are widely used in the pipe and ship-building industries. These steels are liable to cold brittleness, one of whose causes is low-energy intergrain fracture due to intercrystal lite, internal adsorption (IIA). The authors investigate the effect of calcium on the cold brittleness of structural steels.

The deuterium-impurity interaction and the grain boundary embrittlement in nickel

Sulphur and carbon grain boundary segregations are studied by Auger spectroscopy. Differently heat-treated samples are electrolytically deuterium doped and fractured by tension in the chamber of a special Auger spectrometer. The deuterium content is calculated on grain boundaries in the frame of Guttmann's formal approach taking into account the interatomic interaction in 2HSC complex segregation. The effective energy of deuterium segregation and its grain boundary concentration increase with growing S and C segregation level.

A Creep Damage Estimation Method for In-Service Fossil Fuel Boiler Superheater Tubes

Abstract Because mechanical properties of structural materials for high-temperature use, such as boiler tubing, degrade during long-term service, it is essential to detect toughness degradation by means of a nondestructive and simple field test technique. A grain boundary etching technique is developed to detect material degradation, and assess creep strength and notch toughness. An etching test using a picric acid solution with a wetting agent or using 20 percent HNO3 with alcoholic solution was found to have great potential for the nondestructive estimation of grain boundary embrittlement caused by carbide and sigma precipitation in SUS stainless steel. The feasibility of this estimation procedure was determined showing the relationships between Charpy impact energy (CVN) and grooving width (WGS), and creep damage ratio (Φ) and WGS. Superheater tubes of fossil fuel boiler were tested on site to demonstrate the validity of this technique.

Effect of the non-equilibrium phosphorus segregation on the brittle fracture of iron

The kinetics of segregation embrittlement is controlled by phosphorus diffusion to the grain boundary, and this process can be described by the MacLean equation. The equilibrium state, corresponding to an infinite time of isothermal holding, has been the object of intensive study. On the other hand, only a few papers have presented a non-equilibrium analysis of segregation. In the initial states of the segregation process, a difference in the chemical potential of the impurity in the grain boundary and in the bulk produces a change of the free energy of the system and, correspondingly may cause a change in the work of fracture. Thus, if non-equilibrium segregation occurs, one can observe a decrease of the true surface energy of cracked surfaces and attendant grain boundary embrittlement. On the other hand, if the diffusion is a random process, the influence of the instantaneous level of grain boundary composition may play a major role in this kind of brittleness. The aim of the present paper is to analyze the influence of non-equilibrium phosphorus segregation at grain boundaries on the embrittlement of iron.

Grain boundary embrittlement due to reactor pressure vessel annealing

The tendency of the Cr-Ni-Mo low-alloyed steel to brittle fracture as a function of the sizes of austenitic grains and the phosphorus concentration at the grain boundary has been studied. A simple analytical dependence connecting the temperature of ductile-to-brittle transition of steel with the boundary phosphorus concentration and the austenitic grain size has been found. In estimating the kinetics of the development of intergranular embrittlement the decrease in the diffusion coefficient of phosphorus in α-Fe in the presence of molybdenum was taken into account. The effect of the mutually increasing grain boundary adsorption of phosphorus and nickel was considered as well. The possibility to predict the tendency of the Cr-Ni-Mo low-alloyed steel to temper embrittlement is shown. The technique proposed was successfully used to estimate the degree of recovery of the Ni-containing materials of the nuclear reactor vessels after annealing radiation defects.

Effect of environment on the rupture behavior of alloys 909 and 718

The effect of moisture on the high temperature rupture behavior of an Ni-base superalloy and an FeNiCo superalloy was investigated. The high temperature life and ductility of Inconel alloy 718 and Incoloy alloy 909 is severely reduced when specimens are exposed to moist air. The major role of the moist air is to accelerate the kinetics of the grain boundary embrittlement by oxygen. For alloy 718 it was found that heat treatments which produce fully recrystallized microstructures also lead to lifetime and ductility reduction in laboratory air environments. In both alloys the reduction in lifetime and ductility is accompanied by a transition from a transgranular ductile fracture where failure occurs in the smooth gage section to a brittle intergranular fracture which occurs in the notch of the specimens. The transition in fracture mode is attributed to the diffusion of atomic oxygen along the grain boundary resulting in a reduction in the grain boundary surface energy. This allows for intergranular fracture to be favored over transgranular fracture within the present rupture stress-temperature regime. The brittle intergranular fracture is described quantitatively using the principal facet stress approach as opposed to the SAGBO mechanism previously thought responsible. In accordance with the principal facet stress approach the presence of atomic oxygen reduces the frictional forces on the grain boundaries inclined to the applied stress direction. The reduction in frictional force is suggested to occur via the oxygen increasing the grain boundary diffusivity. This then allows for extensive grain boundary sliding on the inclined boundaries which redistributes the stresses to the grain boundary triple junctions. The high stresses acting at the triple junctions give rise to accelerated cavity nucleation and growth causing eventual failure of the specimen. Values for the principal facet stress are calculated and it is shown that this is a viable approach for the present rupture conditions. The unrecrystallized microstructure of alloy 718 is found to exhibit an increased resistance to grain boundary embrittlement by oxygen in moist air. Microstructural variables which may contribute to the increased resistance of uncrystallized microstructures to grain boundary oxygen embrittlement are discussed.

Microstructure and tensile properties of the bismuth-containing 2091 AlLi alloy

Studies show that the ductility and toughness of Al-Li alloys are lower than those of 2024 and 7075 commercial alloys, which are widely used in the aerospace industry. Therefore, it is very important to improve the ductility and toughness of Al-Li alloys for industrial applications. Among the many reported reasons for the low ductility and toughness of Al-Li alloys, grain boundary embrittlement resulting from the grain boundary segregation of trace impurity elements is a important reason. There are two approaches to reducing or eliminating the embrittlement, which resulted from the low melting point metals. One is to use high pure materials, containing lower amounts of Na, K and Ca impurities. The other way is to add some alloying elements to change the distribution of these impurities or to form compounds with them in order to reduce the grain boundary segregation of these harmful impurities. Previous work showed that a small addition of bismuth could neutralize the harmful effect of sodium, decrease the trend of cracking during hot rolling, and increase the ductility of Al-Mg alloys. The purpose of this paper is to study the influence of a small addition of bismuth on the microstructure and tensile behavior of a 2091more » Al-Li alloy, containing higher sodium content, and to investigate the efficacy of bismuth in reducing the harmful influence of sodium.« less

Effects of P and B adsorbates on the Fe(111) surface.

To gain insights into the segregation energy differences between grain boundaries and free surfaces which drive impurity embrittlement of boundaries, the effects of P and B adsorbates on the Fe(111) surface have been determined using the all-electron full-potential linearized augmented-plane-wave total-energy method. The calculated nearest P(B)-Fe distance in P(B)/Fe(111) is 2. 14(1.82) A. Both P and B affect the surface relaxation of the Fe(111) substrate strongly. In contrast to the P-Fe bonding, there is no evident charge transfer from B to Fe

Review of nondestructive techniques designed to evaluate elevated temperature toughness degradation

This paper represents a brief review of a selection of semi-nondestructive, nondestructive techniques designed to assess a materials in-service toughness degradation in the elevated temperature regime. Such an assessment in one prerequisite in arriving at realistic life evaluations of high temperature components. Both the auger electron spectroscopy (AES) and analytical electron microscopy (AEM) have shown some potential, but, to-date, AEM is not really a practical method, for measuring, from both a quantitative and reproducible viewpoint, monolayer type elemental segregation. The Electrochemical Polarisation Kinetic Reactivation (EPR) test and the chemical etching techniques have been continuously developed over the past decade and represent attractive, totally nondestructive, toughness degradation assessment techniques. Discussed are some of the limitations of the EPR test. Chemical etching techniques have demonstrated unique relationships between FATT; the shift in FATT, and such parameters as grain boundary groove width, M, replica surface roughness, R max , and grain boundary ratio factor D o /D a where D o is austenite grain size revealed by a nital etch and D a is grain size revealed by a picric acid etch. They exhibited excellent potential in the field of the practical nondestructive assessment of grain boundary embrittlement whether as a result of phosphorus (Ferritic steels) or carbide or sigma phase precipitation (Stainless steels). Finally, it has been shown that the recent development of a miniature sample sampling technique now makes the small punch test an attractive technique for assessing the material toughness of critical plant locations. Indeed, the small punch test exhibited the least data scatter in the prediction of FATT than do other indirect methods utilising composition and grain boundary evaluations.

Neutron damage in beryllium

The effects of neutron irradiation on the mechanical properties and helium retention of beryllium are being studied using postirradiation tensile testing, fractography, and microstructural examinations on arc cast beryllium that was irradiated in the FFTF/MOTA to support the International Thermonuclear Experimental Reactor (ITER) design effort. Miniature sheet tensile specimens were embrittled by irradiation at 400°C to 8 displacements per atom (dpa). Failure with zero ductility occurs with both transgranular brittle cleavage and grain boundary embrittlement. Based on microstructural examination, embrittlement is ascribed both to helium bubble formation at grain boundaries and to c-type loop formation. Loop formation of this type has rarely been reported in the literature. Companion microstructural examination specimens of identical material were irradiated at 500°C to 1 dpa. The major effect of irradiation at 500°C is the development of helium bubbles on matrix and grain boundary dislocations, but evidence of the onset of void swelling was also found.

Effects of elastic anisotropy on CTEM dislocation images in L10 alloys: An assessment of the sensitivity computed images to input parameters

Ordered alloys have important applications as high temperature structural materials. Grain boundary embrittlement, however, is a major obstacle in their practical applications. The details of grain boundary structure have been suggested to be important in determining the mechanical properties, and it is therefore important to have a better understanding of grain boundary structure in these alloys.Conventional g.b and g.b×U techniques for the determination of Burgers vectors of lattice and grain boundary dislocations are limited by the magnitude of the Burgers vectors and spacing of the dislocations. The dislocations may not be detected if the magnitude of the Burgers vectors is less than 0.05nm or the spacings less than one third of the extinction distance. The strengths of grain boundary dislocations are usually very small, therefore in determining the Burgers vectors of grain boundary dislocations, computer image matching techniques are generally used in conjunction with the usual g.b and g.b×U techniques.

Aging embrittlement and grain boundary Aluminum Alloys Reinforced with SiC Particles

“Clean” 3.5NiCrMoV steels with limited contents in trace elements (P, Sn, As, Sb) are commonly provided for manufacturing big rotor shafts. The possible increase in temperature in future steam turbines has promoted the development of “superclean” steels characterized by an extra drastic decrease of manganese and silicon contents. Their higher cost in comparison to “clean” steels leads to concern above which temperature they must be considered as mandatory for resisting aging embrittlement in operation. 3.5NiCrMoV “clean” steel samples (Mn = 0.30 pct; Si = 0.10 pct) were aged at 300 °C, 350 °C, and 400 °C for 10,000 hours up to 30,000 hours. No embrittlement results from aging at 300 °C and 350 °C, but holding at 400 °C is highly detrimental. Auger spectroscopy confirms that, when aging at 400 °C, phosphorus is the main embrittling trace element. It is suggested that grain boundary embrittlement is associated with the building of a layer that contains, on the one hand, Ni and P and, on the other hand, Mo and Cr.

“dual phase” thin films prepared by deposition of aluminum on liquid nucleant gallium layers

Aluminum thin films have been universally employed as interconnections in integrated circuits for the last quarter of a century. However, during this time Al metallization has never been totally immune from assorted reliability problems. This present research involves preparation and characterization of “dual phase” thin films, comprised of Al (or a transition metal) vacuum evaporated onto a liquid Ga (or other low melting temperature metal) nucleant layer. Ga is well known for causing grain boundary embrittlement in structural Al alloys. However, the Ga may well enhance the mechanical properties in Al thin films used as interconnect metallization, where this “dual phase” microstructure will prevent the buildup of stress that has historically resulted in electromigration failures in semiconductor devices. In addition, the presence of a liquid nucleant layer results in thin films (up to 2 μm thick) having surface roughness of the order of the film thickness, which in turn would enhance the bondability of such films.

Morphology and atomic structure of segregated grain boundaries in Cu-Sb

The properties of polycrystalline materials are strongly dependant upon the strength of internal boundaries. Segregation of solute to the grain boundaries can adversely affect this strength. In copper alloys, segregation of either bismuth or antimony to the grain boundary will embrittle the alloy by facilitating intergranular fracture. Very small quantities of bismuth in copper have long been known to cause severe grain boundary embrittlement of the alloy. The effect of antimony is much less pronounced and is observed primarily at lower temperatures. Even though moderate amounts of antimony are fully soluble in copper, concentrations down to 0.14% can cause grain boundary embrittlement.