Boundary Embrittlement Research Articles

インバー合金の溶接に関する検討 １ インバー合金の溶接金属割れ発生機構に関する検討

The weldability of Fe-36%Ni alloy (Invar alloy) was investigated to clarify the main factor influential for weld cracking in multi-pass weld metal. The tests were carried out using the Fe-36%Ni plates containing various contents of sulfur. The weld cracking susceptibility was evaluated by double bead longitudinal Varestraint test and restraint weld cracking test using gas tungsten arc welding (GTAW). The ductility of weld metal at an elevated temperature was evaluated by hot ductility test. The crack in multi-pass weld metal occurred in the grain boundary away from the fusion boundary of the subsequent weld pass. From the result of hot ductility test of weld metal, the reduction of area was lower at temperature range from 1073 to 1173K. This range was consistent within the temperature range where cracking portion in the multi-pass weld metal was heated by subsequent weld thermal cycles. Consequently, the crack in the multi-pass weld metal was specified as ductility dip cracking. The ductility dip cracking susceptibility was increased in the higher sulfur content weld metals and sulfur concentration was detected on that crack surface. Further, the oxides and sulfides, such as Mn2SiO4 and α-MnS, were very slightly observed on the grain boundary in weld metal. From these results, the ductility dip cracking in weld metal was concluded to be mainly caused by grain boundary embrittlement by segregation of sulfur.

Hydrogen Assisted Intergranular Crack Propagation during Environmental Embrittlement in an Al-Zn-Mg-Cu Alloy

Hydrogen absorbed from the test environment was detected experimentally from an Al-4.9%Zn-2.2%Mg-1.3%Cu alloy deformed in laboratory air, which is a direct evidence of hydrogen embrittlement. The alloy exhibited grain boundary embrittlement when deformed at a strain rate of 10 -7 s -1 in laboratory air and hydrogen behavior was studied using deuterium as a tracer of hydrogen. The direct detection of hydrogen supports the idea that gradual propagation of intergranular crack at sufficiently low strain rate may enable continuous absorption and accumulation of hydrogen in the stress field near the crack tip and eventually embrittle the material.

Synergistic effect of hydrogen and impurity segregations on the grain boundary embrittlement in Nb

Niobium and its alloys are the candidate materials for fusion reactors and can be used at high-temperatures. This paper was intended to study embrittlement of niobium by high-concentrations of hydrogen and impurity segregation at grain boundaries. Specimens of commercial Nb were subjected to heat treatment at 1100°C and 500°C and subsequently charged with deuterium in an electrolytic cell. The charged specimens were placed into the high-vacuum chamber of a special self-made Auger electron spectrometer. They were then fractured under high-vacuum conditions and the chemistry of grain boundaries was analysed. Carbon and oxygen were found as the main impurities on the grain boundaries and effective energies for hydrogen–impurity-grain boundary interaction have been estimated. It was found that there is a noticeable reduction of fracture strength corresponding to the grain boundary oxygen and carbon segregation levels.

The effect of Li, He and Ca on grain boundary cohesive strength in Ni

Boron is added to nickel-base superalloys such as Alloy X-750 in order to enhance high temperature strength and ductility so that the alloy may be more easily hot worked[1]. Boron additions also have been shown to ameliorate intergranular hydrogen embrittlement in nickel[2], and to improve the high temperature resistance of Alloy X-750 to aqueous stress corrosion cracking (SCC) in the absence of irradiation[3]. Recent quantum mechanical calculations demonstrate that boron strengthens grain boundaries in pure nickel[4], and may contribute to the observed benefits of boron on workability and fracture resistance of nickel alloys. Alloy X-750 exhibits greater susceptibility to intergranular stress corrosion cracking (IGSCC) when irradiated[5], and it has been proposed that the presence of grain boundary helium and/or lithium is responsible. Arguments have been advanced that helium embrittlement of the grain boundaries is primarily responsible for the greater observed susceptibility to IGSCC in irradiated X-750[1]. Alternatively, it has been proposed that lithium promotes IGSCC either by entering the water at the crack tip and lowering the local pH, or by inducing a restructuring of the grain boundary itself[1]. Direct embrittlement of grain boundaries by lithium also has been investigated by ion bombardment in Nimonic PE16, illustrating that under certain conditions lithium can produce degrees of embrittlement in nickel comparable to that produced by helium[6]. It is important to understand the relative roles of these species in grain boundary embrittlement in nickel alloys so that better predictive abilities and mitigation strategies can be developed. Toward that end, quantum mechanical calculations have been performed to investigate the influence of isolated lithium and helium atoms on the cohesive strength of an ideal grain boundary in pure nickel.

A study of grain boundary embrittlement in an Fe–8%Mn alloy

A time–temperature ‘C’ curve for intergranular fracture in an Fe–8wt.%Mn alloy has been established by tensile ductility tests at −78°C. In the water quenched condition Auger electron spectroscopy measurements showed N on the intergranular facets with no enhancement of Mn. Two peaks were evident on age hardening curves. On ageing at 450°C a rapid grain boundary segregation of Mn, P, and N was noted with the Mn concentration approaching its equilibrium value. An Arrhenius plot of the embrittlement data suggested that diffusion of N in ferrite was the rate controlling step in the embrittlement of the alloy on ageing. However it is argued, but not proved, that Mn per se will cause embrittlement in the absence of N and P. A kinetic analysis of the segregation rate of Mn and P at 450°C was consistent with the bulk diffusion of these elements in ferrite. Methods of improving impact toughness for possible cryogenic applications are summarised.

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.

Al Grain Boundary Embrittlement Promoted by Na Impurity: An ab initio Study

AbstractWe calculate the electronic structure of AlΣ9 tilt grain boundary with substitutional Na impurity atoms by first principles pseudopotential method. Results show that by Na segregation Al grain boundary expands and the valence charge density decreases significantly along the boundary. There is no stronger bond than metallic bond in the boundary even with Na impurity. We therefore conclude that the mechanism of Na-promoted Al grain boundary embrittlement should be one kind of ‘decohesion model’.

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 ３ 定速度引張試験による溶接金属の高温延性の評価

According to previous reports, the main factor causing reheat cracking in stainless steel FCAW weld metal is bismuth added in the flux to improve slag detachability. The cracks, whose surfaces have a dendritic appearance, propagate along the columnar grain boundary. In this report, the constant speed tensile test at elevated temperature in an inert gas atmosphere was performed and the ductility of the FCAW weld metal was evaluated to clarify the temperature range of weld metal embrittlement, and characterization of the fractured surfaces was performed to categorize the fracture type. Also, the effect of initial ferrite content and precipitates such as the σ phase on the ductility was evaluated.As for the bismuth-free weld metal, a significant ductility drop was not observed at any test temperature and the fractured surface showed a dimple pattern which suggests a ductile and transgranular fracture. On the other hand, in the case of the bismuth-containing weld metal, ductility is remarkably depressed at temperatures above 873 K and the fractured surfaces showed a dendritic appearance. Moreover, at temperatures above 1173 K, the fractured surface showed sighs of Equation at higher magnification. The ferrite content in as-welded condition and aging time at 973 K had no effect on the ductility at elevated temperature. Therefore bismuth is the main factor that causes the grain boundary embrittlement at elevated temperature. However, in the case of the high ferrite content under as-welded conditions, the aged weld metal showed slightly depressed ductility compared with the low ferrite content material, therefore, the precipitates in ferrite are considered to be a secondary factor for reheat cracking.

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 ２ クリープ破断試験による溶接金属の高温延性の評価

To evaluate the high temperature ductility, which is considered to affect the reheat cracking sensitivity, creep rupture tests with Type 308 stainless steel weld metals made from various commercial welding materials at 823 K-1023 K have been performed. To clarify the effect of bismuth on creep embrittlement, creep rupture test with FCAW weld metal including Type 308, Type 316 and Type 347 containing various bismuth contents have been carried out at 823 K and 973 K.The experimental results indicate that with an increasing Larson-Miller parameter value, the rupture strength of the Type 308 weld metal decreased linearly with decreasing rupture ductility. Among the welding processes using commercial welding materials, the FCAW weld metal showed the poorest rupture ductility. However the bismuth-free FCAW weld metal showed a better ductility compared to the bismuth containing FCAW weld metal. It is concluded that the main factor which causes creep embrittlement in the FCAW weld metal is bismuth which is usually added to the flux for improvement of slag detachability. Also, the Type 316 and Type 347 weld metals were embrittled by bismuth as in the case of the Type 308 weld metal.The characterization of cracks in the creep rupture test specimen indicated that regardless of the bismuth content in the weld metal, the fracture mechanism can be categorized into two types, the transgranular type and the intergranular type. At the higher test temperature and/or the prolonged rupture time, the intergranular type fracture becomes dominant and the rupture ductility decreases. Bismuth is considered to severely affect the grain boundary embrittlement and enhances the degradation of rupture ductility.

Materials Issues In Modern Reactor Safety

ABSTRACTNuclear power currently supplies about 20% of the nation's electrical power needs. The nuclear power industry is going through evolutions that will lead to greater efficiency and competitiveness. Many changes in the nuclear power industry are limited by materials issues. This paper discusses a representative range of the materials issues that are arising in the industry. Materials issues are associated with prolonged use of reactor fuel, extended operation of power plants and realistic analyses of power plant safety. The issues include questions of fuel stability at high burnups, metallurgical issues of fuel cladding, embrittlement of pressure boundaries, detecting and sizing flaws in steam generators and issues of environmental qualification of reactor control and instrumentation systems. The paper concludes with a discussion of the future of nuclear power and the continuing need for better materials for power plants.

Al Grain Boundary Embrittlement Promoted by Na Impurity: An ab initio Study

Al Grain Boundary Embrittlement Promoted by Na Impurity: An ab initio Study

A fine γ′+α cellular structure in Fe-37.3 wt pct Ni-3.6 wt pct Al-3.3 wt pct Ti-0.2 wt pct C and its influence on high-temperature tensile properties

Fe-37.3 wt pct Ni-3.6 wt pct Al-3.3 wt pct Ti-0.2 wt pct C alloy, which reveals an excellent combination of high strength and good elongation endowed by formation of homogeneously dispersed fine γ′ precipitates in the matrix during aging at 823 K, has been investigated by means of transmission electron and optical microscopies, electron diffractions, and tensile tests. The influence of unique γ′+α cellular products on the mechanical properties has also been studied. Because of low elastic mismatch between the austenitic γ matrix and isomorphic γ′ precipitate phases, the homogeneously distributed precipitate particles, which formed at the early stage of aging, were observed to persist even after long-term aging. After very lengthy aging, the fine γ′ phase particles were changed to coarser γ′ lamellae at the grain boundary reaction front, which were alternately arranged with fine α lamellae that were estimated to have been transformed from the austenite-stabilizing-solute(Ni, C)-depleted γ lamellae. The fine duplex γ′+α cellular product did not affect deleteriously the room-temperature tensile properties of the alloy. However, the cellular structure was observed to cause the grain boundary embrittlement of the aged alloy at elevated temperatures higher than 681 K.

Effect of nitrogen on the mechanical properties and microstructure of hot isostatically pressed nanograined Fe

The effect of nitrogen content in Fe powders on the mechanical properties and microstructure of cold isostatic pressed (CIPed) and sintered or hot isostatic pressed (HIPed) Fe powders which was produced by attrition ball milling was investigated. Microhardness and compression tests were used to determine the mechanical properties. Optical, scanning, and transmission electron microscopy were used to investigate the microstructural changes that occurred during consolidation. Sintering of the CIPed bodies at temperatures below 850°C caused no change in density, with only minor mechanical properties improvement, and the bodies remained friable. HIPing was found to be essential for effective consolidation of ball-milled powders. Increasing HIPing temperatures increased the density, the compression yield stress, the maximum compressive strength, and the hardness to a maximum value and then decreased. The maximum in density preceded the maximum in the compression yield stress, and its value depends on the nitrogen content in the powder. At low HIPing temperatures (<580°C) the nitrogen content has no influence on the mechanical properties. However, above 580°C, the nitrogen content reduces the mechanical properties as well as density, and increases the oxide precipitation. There are two microstructural effects of nitrogen on the microstructure: void formation in the Fe bodies HIPed above 580°C for the higher nitrogen concentrations, and severe grain boundary embrittlement above 965°C. Iron bodies processed in argon have smaller grain size than iron bodies processed in nitrogen throughout the entire consolidation temperature range. However, due to the smaller grain size, the optimum in mechanical properties (maximum strength with reasonable elongation) is reached only above 965°C. Processing in argon improves machinability.

Hydrogen embrittlement of B-doped Ni3Al-based alloy

Environmental and internal-hydrogen embrittlements in stress-relieved and recrystallized Ni−16.5Al−8Cr−0.8Hf−0.1B−0.03Y (at. %) have been studied. The stress-relieved Ni3Al-based alloy showed environmental embrittlement when tested in air or in hydrogen gas. The embrittlement is more severe in hydrogen gas than that in air. The recrystallized Ni3Al-based alloy was not susceptible to air, but it was embrittled severely by hydrogen gas and exhibited not only a grain interior but a severer grain boundary embrittlement. When tested in air, the stress-relieved Ni3Al was insensitive to internal hydrogen; however, the recrystallized Ni3Al showed a certain susceptibility to internal hydrogen. With increasing the internal hydrogen content in the alloy, the ductility of the recrystallized Ni3Al decreased steadily and a certain degree of grain boundary embrittlement was observed.

Experimental Characterization of Crack Tip Deformation Fields in Alloy 718 at High Temperatures

A series of fracture mechanics tests were conducted at temperatures of 650°C and 704°C in air, using Inconel® 718. A noncontacting measurement technique, based on computer vision and digital image correlation, was applied to directly measure surface displacements and strains prior to and during creep crack growth. For the first time, quantitative comparisons at elevated temperatures are presented between experimentally measured near-crack-tip deformation fields and theoretical linear elastic and viscoelastic fracture mechanics solutions. The results establish that linear elastic conditions dominate the near-crack-tip displacements and strains at 650°C during crack growth, and confirm that KI is a viable continuum-based fracture parameter for creep growth characterization. Postmortem fractographic analyses indicate that grain boundary embrittlement leads to crack extension before a significant amount of creep occurs at this temperature. At higher temperatures, however, no crack growth was observed due to crack tip blunting and concurrent stress reduction after load application.

Premature failure of a leaf spring due to improper materials processing

Failure analysis of a leaf spring which failed prematurely during service was carried out using optical and scanning electron microscopy, hardness and tensile testing, and residual stress evaluation by X-ray diffraction. Quench cracks, which are aligned normal to the length of the leaf, have been identified in shot peened and polished surfaces. These cracks have been attributed to an improper quenching process. It is established by theoretical analysis that the leaf thickness is smaller than the critical plate thickness required for this composition of steel, and that leads to an increase in quench severity. It appears that some of these quench cracks have propagated by a fatigue mode which is confirmed by the presence of beach marks on the fractures surface. Observation of intergranular cracking and the presence of FeS inclusions at the prior austenite rain boundaries implies that some sort of grain boundary embrittlement might have facilitated crack growth and led to failure. It has been suggested that quenching should be carried out by recommended procedures guided by the thickness of the component and chemical composition of the steel. Careful inspection of the surface after quenching must also be carried out to maintain quality assurance in order to avoid premature failure.

Grain boundary segregation and fracture behavior of boron containing Fe-Mn-Ni-(Mo,w) age hardening alloys

The effects of boron addition on the grain boundary segregation and fracture behavior of tempered Fe-Mn-Ni-Mo and Fe-Ni-Mn-W steels were investigated. High segregation of Mn to prior austenitie grain boundaries resulted in severe grain boundary embrittlement in W-bearing alloys. Boron addition did not significantly affect the grain boundary segregation of other alloying elements. Nevertheless, improvement of tensile properties is observed in 16 ppm boron doped W-bearing steel. Segregation of boron itself to grain boundaries is believed to affect the grain boundary strength of this alloy. Lower Mn segregation in Mo containing steels resulted in the ductile fracture when tempered at 480°C.

Serrated Flow and Dynamic Precipitation in Elevated Temperature Tensile Deformation of Fe&amp;ndash;Mn&amp;ndash;Al&amp;ndash;C Alloys

To investigate the characteristics and effects of dynamic strain aging (DSA) and dynamic precipitation of austenitic Fe-Mn-Al-C alloys, a hot-rolled alloy (Fe-26Mn-9Al-0.9C, in mass%) and two cast alloys (Fe-35Mn-9Al-1C, Fe-32Mn-9Al-lC) were selected to perform tensile test at various temperatures from 298 to 1073 K. The strain rates were chosen in the range from 9.4 x 10 -5 to 9.4 x 10 -3 s -1 . As indicated from the appearance of serrated flow, DSA occurs in two temperature regimes. Judging from the apparent activation energy obtained at the onset temperatures of serration, carbon atmosphere drag plays a role in the serration and the temperature independent flow stress in the low temperature regime. In the high temperature regime, the serration cannot be solely accounted for by the diffusion of manganese or aluminum solute atoms, or both. During high temperature deformation, the precipitation of Fe 3 AlC x occurs on active slip planes and along grain boundaries, giving rise to a substantial strengthening effect as compared with that normally associated with DSA. Grain boundary precipitation also leads to grain boundary embrittlement at the temperatures higher than about 773 K. All the above features and effects are common in the three test alloys although the hot-rolled alloy contains about 4 vol% ferrite.

Impurity-induced embrittlement of heat-affected zone in welded Mo-based alloys

Characteristics of strength and plasticity, fracture mode and grain boundary segregation for two Mo-based alloys with different bulk compositions, recrystallized by either furnace annealing or rapid heating followed by quenching, are studied as a function of heating temperature by mechanical test, scanning electron microscopy, Auger electron spectroscopy and computer simulation. There exists an essential difference in both segregation behaviour and mechanical properties between as-annealed and as-quenched structural states. The rapid quenching causes strong oversaturation of the grain boundaries. In this case, intergranular enrichment is approximately twice as high as that in as-annealed alloys, and spontaneous nucleation of brittle microcracks is observed at certain embrittled boundaries. The proposed high-speed heat treatments are considered as a promising method for modelling of the structural states of the heat-affected zone of weldments. The results obtained are discussed from the viewpoint of possible reasons of impurity-induced embrittlement of Mo-based alloys.

Irradiation performance of stainless steels for ITER application

The proposed normal operating temperature range for the ITER first wall/shield structure (100–250°C) is below the temperature regimes for void swelling (400–600°C) and for grain boundary embrittlement (500–700°C). However, the neutron doses for the basic performance phase (3–4 dpa) and the extended performance phase (20–30 dpa) are such that large changes in yield strength, deformation mode, and strain hardening capacity will be encountered which could significantly affect fracture properties. Yield strength increases rapidly with dose in the 60–300°C regime with the increase tending to saturate at 1–3 dpa. Under certain conditions, radiation hardening is accompanied by changes in the stress-strain relationship with the appearance of an initial yield drop and a significant reduction in strain hardening capacity. This paper reviews the low temperature ( < 400°C) tensile and fracture toughness properties changes seen in 300-series stainless steels after neutron irradiation and discusses the implications for the design of the ITER first wall/shield.