Development of preferential intergranular oxides in nickel-aluminum alloys at high temperatures

The development of intergranular oxides in dilute Ni-Al alloys containing 0.55–4.10% Al in Ni-NiO packs and in 1 atm oxygen at 800–1100°C has been examined. In the Ni-NiO packs, preferential intergranular oxide penetration as well as internal oxidation occurs in every case, except in the higher aluminum-containing alloys at 1100°C. Several different types of intergranular oxide morphology were observed, depending on alloy aluminum concentration and on temperature. The oxides in the more dilute alloys are thin and relatively continuous and are accompanied by preferential penetration of internal oxide particles in the adjacent grains. Thicker intergranular oxides are precipitated in the more concentrated alloys while, in some situations, numerous fine oxide particles are formed well ahead of the main intergranular oxide. The intergranular oxidation is facilitated by high stress development in the specimens due to increases in volume as internal and intergranular oxides are formed. These stresses create microvoids in the grain boundaries immediately ahead of the advancing internal and intergranular oxides, resulting in preferential nucleation and growth of further intergranular oxides. This is the case particularly at the lower temperatures where other stress-relief processes cannot operate. The resulting relatively continuous, incoherent intergranular oxide-metal interface allows a high flux of oxygen to the advancing intergranular oxide front. Preferential intergranular oxidation is much less extensive in the presence of a thickening external NiO scale, due to accommodation of the volume increases on internal oxide formation by vacancies injected into the alloy from the growing cationdeficient scale.British Nuclear Fuels, Windscale Works, Seascale, Cumbria, U.K.     

Penetration of oxygen along grain boundaries during oxidation of alloys and intermetallics

Fast penetration of oxygen into grain boundaries and intergranular oxidation of -NiAl has been observed. Since the solubility of oxygen in NiAl is virtually nil, special ways of oxygen ingress at grain boundaries have to be presumed. Selective intergranular oxidation of binary alloys and fast penetration of oxygen along grain boundaries were analyzed by computer simulation. Interdiffusion caused by consumption of the less-noble component by oxidation at the metal-oxide interface leads to deviation of the alloy composition from the original value. When the diffusivity of the less-noble component is higher than the diffusivity of the other component, a grain-boundary Kirkendall effect may lead to void-chain formation. Experimental evidence for this phenomenon is presented. The deviation in composition and void formation were considered as processes influencing the effective oxygen diffusivity. Both processes were found to allow penetration of oxygen as fast as grain-boundary interdiffusion occurs. In addition, oxygen penetration during intergranular internal oxidation when oxides form at voids beneath the metal-oxide interface was analyzed and treated as a self-propagating process. In this case, fast oxygen penetration is accompanied by fast internal oxide formation and fast displacement of the metal-oxide interface.     

Intergranular oxidation and internal void formation in Ni-40% Cr alloys

Internal void formation and intergranular oxidation behaviour have been studied during the oxidation of two Ni-40Cr alloys in 1 atm oxygen at 1000° to 1200°C. The development of an external Cr₂O₂ scale causes vacancies to be generated in the alloy at the alloy-scale interface as chromium diffuses into the scale, and others to be generated in the alloy due to the different diffusion rates of chromium towards the interface and of nickel back into the bulk alloy. At 1200°C, internal void formation results from condensation of such vacancies at inclusions in the grains and at the grain boundaries. The intergranular oxidation observed at 1000°C, 1100°C and to a lesser extent. 1200°C results from preferential condensation of vacancies to form voids in the alloy grain boundaries. Significant depletion of chromium in the alloy adjacent to the scale facilitates the supply of oxygen from the scale and its penetration into the alloy grain boundaries to form intergranular oxide. Such intergranular oxide develops deep into the alloy following diffusion of this oxygen through a porous network in the oxide, which arises because of the vacancy condensation, and oxidation of chromium at the tip of the intergranular penetration.     

The high-temperature internal oxidation and intergranular oxidation of nickel-chromium alloys

The development of internal oxides and intergranular oxides in dilute NiCr alloys, containing 1–5% Cr, in NiNiO packs and in 1 atm oxygen at 800–1100°C has been investigated. The internal oxide particles were relatively coarse and widely spaced and were Cr₂O₃, except for a narrow band adjacent to the surface where NiCr₂O₄ particles were also present. Several types of intergranular oxide were developed in the Ni/NiO packs, with preferential penetration being more extensive in the higher chromium-containing alloys at the lower temperatures. Discrete intergranular oxide particles were formed deep in the alloy beneath bands of Cr₂O₃ which developed over intersections of the alloy grain boundaries with the surface, or beneath continuous or discontinuous grain-boundary oxides near the surface, possibly due to the development of a relatively flat oxygen profile and a steep chromium gradient in the subjacent alloy. In the presence of a thickening NiO external scale, preferential intergranular oxidation was much less extensive than in the Ni/NiO packs as the rapid growth of the scale prevented development of Cr₂O₃-rich surface bands.     

Effect of Crystalline Structure on Intergranular Failure During Shock Loading

The effect of crystalline structure on intergranular failure during shock loading has been examined. A suite of dynamic tensile experiments, using plate-impact testing, were conducted on copper (face-centered cubic) and tantalum (body-centered cubic) specimens with different grain sizes (30–200 μm). These experiments were designed to probe void nucleation, growth, and coalescence processes that for these materials are known to lead to failure. For the grain sizes examined in the study, post-impact metallographic analyses show that in copper specimens, during the early stages of deformation, voids were present primarily at general or low-coincidence, high-angle grain boundaries (GBs), irrespective of grain size. In tantalum, while some voids developed along the GBs, an increasing amount of transgranular damage was observed as the grain size increased. A scenario based on the availability of potential nucleation sites and number of slip systems inherent to each crystalline structure is discussed. The role that this availability plays in either promoting or hindering plastic processes leading to damage nucleation and growth is then examined.     

The development of internal and intergranular oxides in nickel-chromium-aluminium alloys at high temperature

The development of internal oxides, intergranular oxides and internal voids in Ni-15.1Cr-1.1Al and Ni-28.8Cr-1.0Al during oxidation in 1 atm oxygen at 1000° to 1200°C has been studied. In both cases, the formation of an external Cr₂O₃-rich scale causes vacancies to be generated in the alloy due to the different diffusion rates of chromium towards the alloy-scale interface and of nickel back into the bulk alloy. At 1000°C, condensation of these vacancies at the alloy grain boundaries facilitates formation of intergranular oxides while, at 1200°C, the vacancies condense to give voids in the grains and grain boundaries. Internal oxides are formed at both temperatures. The internal and intergranular oxides are mainly α-Al₂O₃, although some Cr₂O₃-rich oxides are produced near the alloy-scale interface. Possible mechanisms for the development of the internal and intergranular oxides in these alloys are discussed and related to the observed oxide morphologies and compositions.     

AC susceptibility study of Bi1.66Pb0.34Sr2Ca2−xMgxCu3Oy (x = 0, 0.2 and 0.4) superconductor systems

A systematic study of the intergranular properties of Bi_1.66Pb_0.34Sr₂Ca_2−xMg_xCu₃O_y (x = 0, 0.2 and 0.4) samples has been done, using the AC susceptibility technique. The samples were prepared by conventional solid state reaction method. It was found that Mg substitution in place of Ca reduces the intergranular coupling of Bi-2223 system. Analysis of the temperature dependence of the AC susceptibility near the transition temperature (T_c) has been done employing Bean's Critical State Model. The observed variation of intergranular critical current densities (J_c) with temperature indicates that the critical current density decreases by increasing the amount of Mg. The higher electronegativity of Mg in the unit cell promotes more intake of oxygen in the material, and the grain boundaries are in more over-doped regime. These over-doped regions reduce the intergranular coupling and increases weak link behavior of Mg doped samples.     

LOCAL CHEMISTRY AND THE COHESIVE STRENGTH OF GRAIN BOUNDARIES IN Ni_3Al

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Effect of preheating on the microstructure and properties of fiber laser welded girth joint of thin-walled nanostructured Mo alloy

Fiber laser girth welding of a thin-walled nanostructured Mo (NS-Mo) alloy tube was conducted. The microstructures, properties, and residual stresses of the welded girth joints achieved at different preheating temperatures were compared. Combining finite element simulation with experimental data, it was found that as the preheating temperature increased, the maximum welding residual tensile stress monotonically decreased while the tensile strength of the joints increased at first and then declined. At the preheating temperature of 673 K, the tensile strength reached a maximum, which was approximately 50% that of the base metal. The results showed that oxygen content was segregated at the grain boundary of fusion zone during welding and further produced Mo oxides. As the preheating temperature increased, vaporized Mo oxides might escaped from the molten pool, so the oxygen concentration at the grain boundary first decreased. However, when the preheating temperature increased over 773 K, the oxygen concentration rose due to the increased oxidation tendency of Mo at high temperatures and grains in the fusion zone coarsened greatly, while the total area of grain boundaries reduced.     

Characterization of microstructure,local deformation and microchemistry in Alloy 600 heat-affected zone and stress corrosion cracking in high temperature water

With increasing the distance from the weld fusion line in an Alloy 600 heat-affected zone, kernel average misorientation decreases and the fraction of Σ3 boundaries increases. Cr₇C₃ carbides mainly precipitate at random high angle boundaries. Chromium depletion and segregation of boron and phosphorous at grain boundaries are observed. Alloy 600 heat-affected zone specimens exhibit higher intergranular stress corrosion cracking growth rates than that in the base metal in simulated pressurized water reactor water environments. Crack growth rate is enhanced by strain hardening and by increasing temperature. The role of stress on interface oxidation kinetics is analyzed.     

Scanning AES and chemical investigation of materials failure due to phosphorus and sulphur grain boundary segregation in type AISI 304L stainless steel

Investigations have been performed on the influence of heat treatment on grain boundary segregation in austenitic stainless steel. In the present work segregation behaviour at type AISI 304L austenitic stainless steel containing relatively high amount of P and/or S has been examined after heat treatment at 550°C/1000 hours. In order to make grain boundaries accessible, the preparation conditions for a high intergranular fracture part have been determined. This procedure is based on cathodic hydrogen charging followed by thermal exposure and straining up to the yield stress. The enrichment of P and S at grain boundaries has been detected directly by means of Auger Electron Spectroscopy after in situ fracture in ultra high vacuum and indirectly by the Coriou test. Significant evidence for segregation of P and S at grain boundaries has been found after the heat treatment only. The segregation causes increasing intergranular stress corrosion susceptibility and embrittlement indicated by CERT (Constant Extension Rate Test).     

碳和氮元素对高强度奥氏体焊缝组织和性能的影响

采用扫描电镜(SEM)和透射电镜(TEM)等分析研究了碳、氮元素对奥氏体焊条熔敷金属组织和性能的影响.结果表明,随着碳含量的增加,熔敷金属晶界上M23C6碳化物析出物逐渐增多,析出颗粒增大,虽然熔敷金属的抗拉强度有所提高,但韧性明显降低,碳含量增加到一定程度后,对强度的影响趋于平缓,但对晶界碳化物的数量和尺寸仍然有强烈的促进作用,韧性持续降低,耐晶间腐蚀性能大大降低.随着氮含量的增加,抗拉强度呈持续上升趋势,同时韧性仍能保持在较高水平,晶界上析出碳化物少,抗晶间腐蚀性能良好.     

Intergranular corrosion of iron-phosphorus alloys in nitrate solutions

The corrosion of Fe-P alloys with 0.003–2.5 wt.%P has been studied in hot nitrate solutions. The current-potential curves show an extended active range and increased corrosion currents for high phosphorus alloys. The alloy with 2.5P does not passivate at all. The other alloys have been tested for intergranular corrosion in the passive range at 1,000 mV(SHE). The intergranular attack increases with increasing phosphorus content and can be correlated to its grain boundary concentration. The grain boundary concentrations had been established by equilibrium segregation at temperatures between 400 and 80°C and were determined by Auger electron spectroscopy (AES). The presence of phosphorus in solid solution and segregated to the grain boundaries prevents the formation of a passive layer on iron, if its local concentration is higher than ca. 2–3 at.%.     

Intergranular fracture caused by trace impurities in an Al–5.5 mol% Mg alloy

A coarse-grained Al–5% Mg alloy, which does not show high temperature embrittlement, is successfully prepared using high purity raw materials and a graphite crucible. By preventing the contamination of sodium and hydrogen, it becomes possible to examine separately the effects of various trace elements on hot ductility in the Al–5% Mg alloy. Sodium, calcium, or strontium of 2 mol ppm brings about the high temperature embrittlement based on intergranular fracture, while lithium of 4 mol ppm does not. Sodium is the most highly embrittling among such detrimental elements. The detrimental effect of such impurity is due to its segregation to grain boundaries. Further, the embrittlement caused by sodium or strontium of 2 mol ppm is greatly suppressed by an addition of more than 1000 mol ppm of silicon which scavenges those detrimental elements from grain boundaries.     

The morphological and structural development of internal oxides in nickel-aluminum alloys at high temperatures

The formation and development of internal oxides in Ni-Al alloys containing 1–4 wt.% Al in Ni-NiO packs and in 1 atm oxygen at 800 to 1100°C have been studied. The internal oxide particles were relatively fine, closely spaced, and mainly acicular, although more granular near the surface. They were identified as Al₂O₃ at the advancing front, but NiAl₂O₄ at the surface and at a significant distance from that surface. Growth of internal oxide particles resulted in the development of significant compressive stresses in the internal oxide zone when formed in Ni-NiO packs. These stresses led to grainboundary sliding at the higher temperatures and extrusion of weak, internal oxide-denuded zones adjacent to alloy grain boundaries. At the lower temperatures, these stresses also resulted in significant preferential penetration of oxides down grain boundaries and sub-grain boundaries. Stress development and resulting phenomena were much less significant during oxidation in 1 atm oxygen because vacancies injected from the external NiO scale accommodated the volume increase during growth of internal oxide particles.     

Temperature dependence of microstructure and texture in cold drawn aluminum wire

The effect of strain and drawing temperature on the evolution of microstructure and fiber textures of aluminum wires drawn at room temperature and cryogenic temperature was investigated by TEM and EBSD observations. The results show that low angle boundaries frequency increases and high angle boundaries frequency decreases with strain increasing when the strain is low. At high strain, most of grain and dislocation boundaries are parallel to the drawn direction and low angle boundaries frequency decreases and high angle boundaries frequency increases with strain increasing. The decrease of deformation temperature leads to microstructure finer and low angle boundaries frequency increasing. Texture analysis indicates that volume fraction of complex texture component decreases with strain increasing and a mixture of 〈111〉 and 〈100〉 fiber texture forms at high strain. 〈111〉 is stable at low strains but 〈100〉 becomes stable at high strain. The decrease of temperature can enhance the stability of 〈111〉 orientation at high strain.     

Roles of bulk carbon content on grain size,carbide reactions and intergranular rupture life in 2.25cr martensitic heat resistant steels

The steel of a higher bulk carbon content shows the denser precipitation distribution of carbides after the solution treatment followed by tempering. Such a carbide distribution produces the smaller prior austenite grain size after the welding simulation at a high temperature. Because the equilibrium segregation concentration of phosphorus decreases with decreasing prior austenite grain size, the specimen of the higher bulk carbon content shows therefore the longer intergranular rupture life. The rupture life is also increased by the partitioning of phosphorus pre-segregated at prior austenite grain boundary/carbide interfaces onto the fresh surface of precipitates formed on the surface of pre-formed carbides. The intergranular rupture life is additionally increased by the repulsive segregation between carbon and phosphorus which decreases the overall phosphorus segregation concentration at the prior austenite grain boundaries.     

Grain boundary segregation and intergranular fracture in Ferrite containing Mo,Si or Al

Roles of molybdenum, silicon or aluminum in ferrite on grain boundary segregation and hence on intergranular fracture have been investigated by using Auger electron spectroscopy (AES) and tensile test. Competitive segregation between sulfur and carbon or nitrogen, which caused the decrease below 700°C of sulfur content at the grain boundaries, was observed in the pure iron. The intergranular brittleness of the pure iron was caused by sulfur at the grain boundaries. When molybdenum was added to the pure iron, the sulfur contents at the grain boundaries were lowered in comparison to those in the pure iron. The molybdenum-bearing alloy showed higher fracture strength than that of the pure iron, and fractured mostly in the transgranular mode. This arises from the intrinsic effect of molybdenum on the grain boundaries as well as the decrease in sulfur content. Tn the 3.37 wt.%Si alloy, silicon and carbon or nitrogen competitively segregated to the grain boundaries, and such a competitive segregation was also observed between sulfur and carbon or nitrogen. The sulfur content at the grain boundaries decreased with increasing silicon content. The fracture modes in the 3.37- and 4.26 wt.%Si alloys were transgranular in the rolling direction, but were mostly intergranular in the transverse direction and in the as-rolled condition. The intergranular characteristic in the fracture behavior may be attributed to the detrimental effect of silicon as well as sulfur on the intergranular cohesion. Carbon and aluminum only were found at the grain boundaries of the aluminum-bearing alloy. This suggests that aluminum is a strong repulser of sulfur or nitrogen at the grain boundaries. Additionally, it was found that aluminum has a detrimental effect on grain boundary strength of ferrite.     

Effects of grain size and indentation sensitivity on deformation mechanism of nanocrystalline tantalum

Nanoindentation process is performed by molecular dynamics simulations to investigate the plastic deformation phenomena and mechanical properties of the polycrystalline tantalum (Ta) substrate. The comparison of mechanical behaviors between the single crystal and polycrystalline Ta is analyzed. The effects of grain size, indenter radius, indentation velocity, and temperature are deeply investigated. Further, the important factors of mechanical property such as the plastic deformation, dislocation, amorphization process, indentation force, yield pressure, and von Mises stress are evaluated. The results show that the phase transformations and dislocations almost absent in the grains. Instead, the dislocations densely occur in the grain boundaries, the amorphization regions are formed around the indentation areas and develop along the grain boundaries. The high von Mises stresses are mainly distributed around the indentation areas and in the grain boundaries. The single crystal tantalum shows better mechanical characteristics than the polycrystalline tantalum such as harder to deform, higher yield pressure, and lower dislocation.     

疲劳/蠕变复合作用下GH169合金变形和断裂过程的动态观察

利用高温金相显微镜动态观察了GH169合金在疲劳/蠕变复合作用下的变形和断裂过程。结果表明,疲劳/蠕变复合作用下的变形方式有晶内滑移、孪生和晶界滑动,其失效方式因显微组织而不同。沿晶裂纹源于晶界滑动在三叉点处产生的W型裂纹和晶界局部形变区,其扩展机制为空洞的形核、聚集长大和相互连接;穿晶裂纹源于晶内形变损伤区,其扩展机制为沿滑移面的剪切断裂。