Boundary Fracture Research Articles

Grain boundary and fracture surface analysis of Fe–C–P steel

The present paper investigates the distribution of grain boundary types and fracture surface crystallography in an Fe–C–P alloy. It is shown that electron backscatter diffraction (EBSD) is an effective technique with which to conduct these investigations. The proportions of both Σ1 and particularly Σ3 (in coincidence site lattice notation)present after various heat treatments were higher than would have been expected for random generation. There was limited evidence that both higher annealing temperatures and longer annealing times promoted generation of Σ3 type boundaries. The standard EBSD technique was modified and extended to encompass both the novel ‘matched fracture’ specimen approach and direct mapping from fracture surfaces to provide crystallographic information. A correlation was noted between higher aging temperatures and proportions of cleavage fracture. Furthermore, there was a strong correlation between cleavage fracture surfaces exhibiting river markings and an {001} surface orientation.

Microscopic study for the behavior of grain boundary using molecular dynamics

In this study, MD simulations have been performed to observe the behavior of a grain boundary in an a-Fe plate under 2-dimensional loading. In MD simulation, the acceleration of every molecule can be achieved from the potential energy and the force interacting between each molecule, and the integration of the motion equation by using the Verlet method gives the displacement of each molecule. Initially, four α-Fe rectangular plates which have different misorientation angles of grain boundary were modeled by using the Johnson potential and Morse potential. We compared the potential energy of the grain boundary system with that of the perfect structure model, which does not have a grain boundary inside. Also, we could obtain the width of the grain boundary by investigating the local potential energy distribution. The tensile loading for each grain boundary model at room temperature was applied and the behavior of the grain boundary was studied. From this study it was clarified that when the Johnson potential is used, the obvious fracture mechanism occurs along the grain boundary. With the Morse potential, the diffusion of the grain boundary appears instead of the grain boundary fracture.

ステンレス鋼ＦＣＡＷ溶接金属における再熱割れに関する研究 ４ ノッチ付き試験片を用いた定荷重引張試験による再熱割れ感受性評価

According to previous reports, columnar grain boundary embrittlement caused by bismuth is responsible for the reheat cracking of Type 308 FCAW weld metal. It is necessary to quantitatively evaluate the reheat cracking sensitivity for comprehension of the reheat cracking mechanism. It is considered that grain boundary cracking at elevated temperatures occurred when the strain due to creep deformation caused by residual stress in the relaxation process reached a critical value in the neighborhood of the grain boundary. Therefore it can be said that this critical strain represents the reheat cracking sensitivity of the weld metal.In this report, a constant load tensile test using a notched specimen is performed and the reheat cracking sensitivity of weld metal is quantitatively evaluated by crack opening displacement (Φc) which corresponds to the critical strain of a grain boundary fracture. The results show that Φc is reduced with increasing bismuth content and increasing test temperature. At temperatures above 1100 K, Φc is remarkably decreased in the bismuth-containing weld metal whose fractured surface shows signs of requation. Φc shows a good relation with the cracking ratio of the y-groove self restraint cracking test. Therefore, Φc quantitatively represents the reheat cracking sensitivity of the weld metal.

Mechanical Characteristics of High Nitrogen Contained Weld Metal Based on 316L Stainless Steel

Austenitic stainless steel of SUS316L was welded by GMA process in high-pressure nitrogen atmospheres up to 6.1MPa(60atm). The fracture toughness of the weld metals were measured by means of the Charpy impact test at test temperature range from 77K to 290K The influence of nitrogen content and microstructure on the toughness of weld metal were investigated. The content of nitrogen of GMA weld metal reached about 6700ppm at maximum welding pressure of 6.1MPa. The Vickers's hardness number increased from 225HV to 350HV with increasing content. As weld condition, the toughness increased with tereasing the nitrogen content up to 4000ppm. When the nitrogen content exceeded about 5000ppm, grain boundary fracture was caused by Cr 2 N precipitation.

Kinetics of grain boundary fracture of irradiated materials

The general peculiarities of grain boundary fracture of irradiated materials are discussed. Such physical phenomena as high temperature helium embrittlement of structural fusion materials and irradiation-assisted stress corrosion cracking of austenitic alloys in nuclear reactor core components under neutron irradiation are characterized by the ductility loss and intergranular fracture. In fission and fusion structural materials due to (n, α) and (n, p) reactions, high concentrations of helium and hydrogen can be generated. The effects of accumulation of gaseous (helium, hydrogen) atoms, segregation of solute impurity atoms at grain boundaries (GBs) and grain boundary sliding process of fracture of irradiated materials are discussed. At high temperatures, in irradiated materials containing helium atoms the system of helium bubbles is formed. In stressed materials, the kinetics of helium bubble growth on GBs is considered. The effect of grain boundary bubbles and bubbles on triple grain junctions (TGJs) on the fracture of irradiated alloys is analyzed elsewhere. The fracture process in these conditions is modeled in terms of interaction of bubble arrays on TGJs with a pile up of dislocations during creep and grain boundary sliding. Fracture criteria for a material are formulated in their dependence upon bubble density and size, gas content in bubbles on TGJs and time scale of crack growth.

Influence of grain boundary composition on the moisture-induced embrittlement of Ni 3(Si,Ti) alloys

Ni 3(Si,Ti) alloys containing different levels of boron were tensile tested in air at room temperature at two different strain rates. The grain boundary compositions and fracture modes of these alloys were determined by Auger spectroscopy and scanning electron microscopy, respectively. Tensile elongation and fracture mode depend upon the fabrication procedure, heat treatment, and strain rate. Widely different boron concentrations were observed at the grain boundaries, depending on the fabrication procedure and heat treatment. In addition, silicon and titanium were depleted while nickel was enriched at the grain boundaries in all specimens examined. Tensile elongations correlated well with the grain boundary concentration of boron and also with an embrittlement parameter defined as (Si+Ti−B)/Ti. A sharp brittle-ductile transition was found to occur with increasing grain boundary concentration of boron and with decreasing values of the embrittlement parameter (Si+Ti−B)/Ti. The critical grain boundary concentrations corresponding to this transition were found to be sensitive to the strain rate. All the results can be explained in terms of the effect of grain-boundary composition on moisture-induced environmental embrittlement.

Delayed fracture of aluminum alloys

1. The susceptibility of aluminum alloys to delayed fracture is connected with the nonuniform decomposition of the solid solution and its low stability. The service life of a part is the shorter the higher the anisotropy of grain boundary fracture, the higher the level of residual stresses, and the lower the threshold stress in the tests for corrosion cracking. 2. The susceptibility of aluminum alloys to corrosion cracking can be eliminated or considerably reduced by using controlled cooling in quenching in combination with artificial aging.

Grain boundary segregation and brittle fracture in a Pt-Rh base alloy

Due to an excellent resistance against high-temperature oxidation and corrosion combined with high ductility, platinum and its alloys are widely used as construction materials for operation in aggressive environments. For example, platinum based thermocouples serve for temperature measurements in oxidizing atmosphere and functional components fabricated from Pt-Rh alloys (furnace containers) are often used in glass industry. In some cases, however, brittle cracking has been observed in these materials. Recently, intergranular brittle cracking was observed in Pt-10mass. %Rh glass containers, however, it occurred before their operation in aggressive environment. The aim of this work is the AES study of chemical composition of grain boundary fracture surfaces of these functional components to disclose the nature of this embrittlement.

Grain boundary bonding state and fracture energy in small amount of oxide-doped fine-grained Al2O3

Grain boundary structure and chemical bonding state were characterized in high-purity Al2O3, 0.1 wt% MgO, 0.1 wt% Y2O3 or 0.1 wt% ZrO2-doped Al2O3. High resolution electron microscopy (HREM) and energy dispersive X-ray spectroscopy (EDS) revealed that all samples examined have single phase structure, and that doped cations segregate along grain boundaries. Electron energy loss spectroscopy (EELS) spectra taken from grain boundaries in doped Al2O3 shows slight chemical shift in comparison with those from grain interior. This result suggests that the chemical bonding state in grain boundaries changes by the segregated ions. The change in chemical bonding state seems to affect the grain boundary fracture energy of Al2O3.

Part I-the effects of pre-dissolved hydrogen on cleavage and grain boundary fracture initiation in metastable beta Ti-3Al-8V-6Cr-4Mo-4Zr

The effects of electrochemically pre-dissolved hydrogen on room-temperature fracture initiation in Beta-C titanium (Ti-3Al-8V-6Cr-4Mo-4Zr wt pct) have been investigated using circumferentially notched tensile specimens. Finite element-based analysis of notch stress fields was used to define relationships between the local threshold stress for crack initiation vs total internal hydrogen concentration. The as-received, solution heat treated (ST, σy.2 pct=865 MPa) and the ST + peak-aged conditions (STA, σy.2% pct=1260 MPa) were compared after defining the relationships between the fracture process zone hydrogen concentration, hydrogen-metal interactions (i.e., hydrostatic stress field occlusion, trapping, hydriding), and the resulting fracture initiation behavior of each. Solutionized + peak-aged (β+α) Beta-C fractured intergranularly above total hydrogen concentrations of ∼1000 wt ppm. (5.1 at. pct). A fracture mode consistent with cleavage occurred at ∼2100 wt ppm. (10.7 at. pct). Solutionized Beta-C resisted hydrogen-assisted cracking (e.g., did not crack intergranularly) but was not immune; cleavage cracking was provoked at ∼4000 wt ppm. (20.4 at. pct). Coldworked ST Beta-C (CW, σy.2 pct=1107 MPa) did not crack intergranularly; fracture initiation behavior was similar to the ST condition regardless of specimen orientation. This suggests that high yield strength alone does not account for the susceptibility to intergranular cracking observed in the STA β+α condition. Stroke-rate studies and X-ray diffraction investigation of H partitioning suggests that equilibrium hydriding and/or irreversible trapping does not singularly control intergranular fracture initiation of the STA condition. Fractographic evidence and finite element results show that a finite plastic zone exists prior to intergranular fracture of the STA condition. This suggests that a criterion for fracture that incorporates plastic strain and stress should be considered.

Void nucleation and cracking at grain boundaries

Dimpled grain boundary fracture occurs in creep tests, stress relief cracking and hydrogen attack of steels. While the growth of these voids by grain boundary diffusion is well established, the mode of void nucleation is uncertain. It is shown that the reduction of surface energy by solute adsorption plays an essential role in giving easy void nucleation. A calculation is given for the stress needed for this mode of nucleation using data for phosphorous in steel. Numerous examples exist of strongly adsorbing solute inducing elevated temperature grain boundary cracking. A row of voids growing by stress driven boundary diffusion is shown to develop a tensile stress maximum which aids void nucleation, giving rise to dimpled grain boundary fracture. Our cracking model involves repeated void nucleation and is thus fundamentally different from the steady-state Hull–Rimmer model. At times and temperatures too low for void formation, adsorption can lead to smooth grain boundary cracking at a rate controlled by solute diffusion (adsorption).

Grain boundary cracking

A chronological summary is given of the various types of grain boundary fracture found in metals. In each case, there is an impurity that adsorbs at the new (fracture) surface being formed. For the case of Fe-P alloys, a quantitative argument can show that adsorption of phosphorous on the free surface greatly reduces the barrier to void nucleation compared to that in the absence of phosphorous. The same or larger reduction would appear for any other element, which adsorbs more strongly than phosphorous and displaces it at the surface. Such an argument is shown to explain a great many cases of dimpled grain boundary fracture in strong alloys undergoing creep or hydrogen attack. The reduction in surface energy can also lead to a smooth grain boundary fracture (no void nucleation), in which diffusion of solute to the new surface limits crack growth. Numerous examples of this are also discussed.

Grain boundary cracking

A chronological summary is given of the various types of grain boundary fracture found in metals. In each case, there is an impurity that adsorbs at the new (fracture) surface being formed. For the case of Fe-P alloys, a quantitative argument can show that adsorption of phosphorous on the free surface greatly reduces the barrier to void nucleation compared to that in the absence of phosphorous. The same or larger reduction would appear for any other element, which adsorbs more strongly than phosphorous and displaces it at the surface. Such an argument is shown to explain a great many cases of dimpled grain boundary fracture in strong alloys undergoing creep or hydrogen attack. The reduction in surface energy can also lead to a smooth grain boundary fracture (no void nucleation), in which diffusion of solute to the new surface limits crack growth. Numerous examples of this are also discussed.

Thin film scratch testing in two dimensions—Experiments and analysis

We have modified the microscratch test to create a near plane strain loading condition. In the Microwedge Scratch Test (MWST), a wedge-shaped diamond indenter tip is drawn along a fine line (i.e., narrow strip of film), while simultaneously being driven into the line. We compare microwedge scratching of zone 1 (voided grain boundaries) and zone T (metallurgical grain boundaries) thin film specimens of sputtered tungsten on thermally grown SiO2. Symptomatic of its weak grain boundaries, the zone 1 film displays three separate crack systems. Because of its superior grain boundary strength, the zone T film displayed only one of these—an interfacial crack system. By correlating fracture phenomena to signature events in the load-displacement curve, we develop governing equations for propagating interfacial cracks, including expressions for strain energy release rate, bending strain, and mode mixity. Grain boundary fracture causes zone 1 films to spall before a stable crack is formed. Zone T films survive the bending strains, and hence adhesions may be inferred from stable crack growth mechanics. We conclude by contrasting and comparing experimental results for plane strain indentation versus plane strain scratching.

Effect of two phase warm rolling on aging behavior and mechanical properties of Ti–15Mo–5Zr–3Al alloy

The isothermal aging behavior and change in tensile and mechanical properties of Ti–15Mo–5Zr–3Al alloy have been studied after three different pre-treatments, i.e. PR (rolling at partial solution temperature), PRP (PR+partial solution treatment) and C (complete solution treatment) by hardness and resistivity measurements, tensile and Charpy impact tests. After the PR and the PRP treatment, almost all of the plate-like primary α particles which were left in as-received specimens became granular, furthermore age-hardening was observed in that α+ β bi-phased specimens. Plate-like α is always formed by the aging. Sub-grain size of β phase in the PR and the PRP specimens is 1–2 μm, and no grain growth is observed during the aging. Tensile properties after 673 K-600 ks or 773 K-60 ks aging are improved by the PR or the PRP than the C pre-treatment. The fracture surface of the aged and tensile tested PR and PRP specimens shows dimples, whereas that of the C specimen shows flat surface, area fraction of 40–50%, due to grain boundary fracture. Deflection–load curve shows practically no change in its shape with variation in the pre-treatment, however maximum load is highest in the PRP specimen and decreases in order of the PR and the C. Total absorbed energy is also larger in the PR and the PRP specimens than in the C specimen in age-hardened state. The best strength–ductility balance is attained by the PRP specimen aged at 773 K for 60 ks.

Atomistic Modeling of Grain Boundary Fracture in Diamond

AbstractMolecular dynamics simulations using a bond-order potential were carried out to investigate the behavior under load of several <001> and <011> symmetrical tilt grain boundaries in diamond. Cohesive energies, work for fracture, maximum stresses and strains as functions of the type of grain boundary were evaluated. It was found that special short-periodic GBs possess higher strength and resistance to a crack propagation than GBs in the nearby misorientation range. Crack behavior in polycrystalline diamond samples under an applied load was also simulated, and found to be predominantly transgranular.

Tensile Properties of NiAl Bicrystals

The intermetallic compound {beta}-NiAl continues to receive considerable attention in spite of its lack of room temperature toughness and high temperature strength. Although the dislocations are mobile at room temperature, the lack of a sufficient number of slip systems precludes significant elongation in single and polycrystalline NiAl except in single crystals under special conditions. In the case of polycrystals of stoichiometric NiAl, the room temperature fracture tends to be mostly intergranular; this has been related to the stresses that build up at the grain boundaries during plastic deformation due to the lack of active independent deformation mechanisms or the possibility that the grain boundaries are intrinsically weak. The present study was designed to establish the condition of grain boundary fracture by performing tensile tests at different temperatures and strain rates on bicrystals of NiAl containing natural boundaries produced by Bridgman growth. This approach was selected based on the previous work on NiAl bicrystals produced by diffusion bonding/brazing and Bridgman growth. In some cases, these boundaries were reportedly enriched in nickel although it is unclear, based on the limited atomistic modeling efforts to date, whether this is a result of sample processing or is a characteristic of this compound. Furthermore, themore » previous studies of slip behavior were performed in compression which is less suitable for examining the relative strength of the grain boundaries.« less

Preparation of High Fracture Toughness Alumina Sintered Bodies from Bayer Aluminum Hydroxide

The mechanical properties of hot-pressed, low-temperature-calcined α-alumina powder made from commercial aluminum hydroxide by seeding of wet grinding abrasion powder were investigated. The microstructure of samples sintered at low temperature is composed of very fine equiaxial grains and its bending strength is higher than 800MPa. As the sintering temperature increases, the grains become large and uniaxially elongated, the bending strength of sintered bodies decreases, and the fracture toughness increases. A sample sintered at 1400°C shows a bending strength of 600MPa and a fracture toughness of 7.9MPa⋅m1/2 (SEPB method). A maximum fracture toughness of 9.0MPa⋅m1/2 is achieved in a sample sintered at 1600°C. This high fracture toughness is caused by a uniform and elongated grain structure, and by increment of grain boundary fracture.

Effect of fabrication and microstructure on the fracture initiation and growth toughness of Al–Li–Cu alloys

Fracture toughness properties of three microstructural variants of aluminum–lithium alloy 2020 (4.6 Cu, 1.1 Li-0.5 Mn-0.2 Cd), were analyzed. The first variant is the base alloy, REX-2020, which is completely recrystallized. The second variant was produced by thermomechanically processing the REX-2020 and the third variant was obtained by replacing Mn in the REX-2020 by Zr, as the grain refining element. These two versions of the alloy labeled as TMP-2020 and Zr-2020, resulted in an unrecrystallized microstructure. Fracture toughness data were analyzed in terms of crack initiation toughness K Jc, and the crack growth toughness parameter, T R (tearing modulus). The three variants of the alloy 2020 exhibit minimum differences in both the fracture initiation toughness, K Jc, and the crack growth toughness, T R, in the peak aged condition. However, a considerable improvement in K Jc is observed in the thermomechanically processed (TMP-2020) and the Zr containing 2020 (Zr-2020) in the underaged condition. The improvement in K Jc in the TMP-2020 and Zr-2020 variants over the REX-2020, is attributed to a higher plasticity attainable at the crack tip and finer grain size. The tearing modulus, T R, is however higher in the REX-2020 due to planar slip and conversely the lower T R in the TMP-2020 and Zr-2020 is due to a lower degree of planar slip than the REX-2020 alloy. The low toughness at the peak age condition in the three alloys is due to grain boundary fracture from the presence of grain boundary precipitates. There is some recovery in this lost toughness in the overaged condition where plasticity is gained via the loss in the yield strength of the alloys.

Effect of Cr Dopant on High-Temperature Embrittlement in Fe Alloys Containing High Concentration of Cu.

To evaluate usage of ferrous scraps containing a high concentration of Cu, the mechanical properties of Fe-Cu alloys with 0.3∼4.0 mass% Cu were examined focusing on the effect of the Cu- rich phase segregated at grain boundaries on high-temperature embrittlement; ductility improvement was attempted by adding a small amount of Cr. Excess Cu atoms in the Fe-Cu alloys segregated and formed Cu-rich phase at the grain boundary in α matrix depending on the grain boundary character. Large Cu-rich phase was susceptible to nucleation of microcracks and induced grain boundary fracture, resulting in a loss of ductility at high temperature. Addition of a small amount of Cr effectively improved ductility of α Fe-Cu alloys at high temperature because the formation of Cu-rich phase was suppressed by Cr addition and/or segregated Cr increased the bonding force at grain boundaries. Ductility decreased in γ Fe-Cu alloys with increasing Cu concentration and this reduction was not improved by Cr addition. High strength Fe-Cu alloys containing a large quantity of Cu was obtained, however, by Cr addition.