钇在Fe-Cr-Al合金1200℃氧化中的作用

将含有0.51％Y和不含Y的两种Fe-Cr-Al合金,在1200℃空气中进行了500小时的氧化实验。用X-射线衍射仪、扫描电镜、电子探针及离子探针对不同氧化时间的试样进行了分析,结果表明,除了初期(约20小时)稍提高Fe-Cr-Al合金氧化速度外,总的说来,钇的添加降低了合金的氧化速度。同时,钇的氧化物分布在以α-Al_2O_3为主的氧化膜中,使得α-Al_2O_3氧化物成为柱状晶,这种构造的氧化膜可经受较大压应力作用,因此在冷热交变过程中氧化膜不易剥落。钇与扩散到基体中的氧优先反应生成Y_2O_3,对阻止合金基体的内氧化及力学性能变坏也起到一定的抑制作用。     

Microstructure of Ag-Sn-Cu-Bi-Ni alloy after oxidation

Ag-Sn-Cu-Bi-Ni alloy was internally oxidized in air. The phase constitution, surface morphology and microstructure evolution of the alloy after internal oxidation were analyzed by X-ray diffractometry, optical microscopy and scanning electron microscopy, respectively. The results show that the surface color of samples after internal oxidation is different from the different oxidation time and temperatures. The oxidation reaction firstly takes place on the grain boundaries. The microstructure developed on the initial stage of internal oxidation is fir-tree crystal texture. However, this texture structure disappears accompanied by grain growth and oxides forming during the prolonged oxidation. Finally, the oxide particles are uniformly dispersed in the silver matrix.     

Intergranular environmentally assisted cracking of Alloy 182 weld metal in simulated normal water chemistry of boiling water reactor

Following characterization of grain boundaries in Alloy 182 weld metal, intergranular environmentally assisted cracking (IGEAC) growth behavior of the alloy in simulated normal water chemistry (NWC) of boiling water reactor (BWR) was tested by employing a crack growth rate test. The grain boundary was found consist of about 72% random boundaries, 21% low-angle boundaries and a few coincident boundaries.The correlation between the grain boundary characters and IGEAC was discussed. Based on the IGEAC growth rates obtained in the test and evaluations of the crack tip strain rate, the parameter of the slip-dissolution/oxidation model that embodies the kinetics of oxidation/repassivation in the crack-tip environment of Alloy 182 in BWR-NWC was quantitatively determined.     

Ni-Cr-W合金在1100 ℃的氧化膜演变

对新型Ni-Cr-W合金在1100 ℃下不同保温时间下的恒温氧化行为进行了研究。采用扫描电镜（SEM）以及能谱（EDS）对合金热暴露后的表面氧化膜形貌、元素含量以及合金基体的恶化情况进行了分析,表面氧化膜的相组成通过XRD进行确定。结果表明：在氧化初期（<3 h）,合金表面生成的单层氧化膜主要由Cr2O3组成,随着氧化时间的延长（>7 h）,在Cr2O3外逐渐形成了一层具有尖晶石结构的NiCr2O4。一旦外表面被均匀致密的尖晶石膜所覆盖,双层氧化膜NiCr2O4·Cr2O3便能有效的减慢合金基体被进一步氧化。合金亚表层的恶化形式包括晶界的内氧化、空洞以及无碳化物区的形成。合金中高的W含量并没有明显恶化合金的抗氧化性能。     

Oxidation of ruthenium aluminide-based alloys: The role of microstructure and platinum additions

The cyclic oxidation behavior of six RuAl-based alloys was studied in air over the temperature range of 1000–1300 °C. Oxidation kinetics were influenced by microstructure as well as the addition of platinum. Single phase RuAl-based alloys formed a layered oxide structure during oxidation. The presence of the δ-Ru-rich eutectic along grain boundaries strongly accelerated the oxidation process. A single phase Pt-containing RuAl-based alloy exhibited the highest oxidation resistance.     

Effect of reactive element oxide inclusions on the growth kinetics of protective oxide scales

A model is presented for describing the growth kinetics of a protective oxide scale containing reactive element (RE) oxide inclusions (pegs). The formation of RE oxide inclusions due to dissolution and diffusion of the RE from intermetallic precipitates along grain or phase boundaries in the alloy is considered. The average oxide scale growth kinetics depend on the RE content, the parabolic rate constant of the protective oxide scale, the alloy grain/phase size and the size of the RE containing precipitates. The specimen thickness determines the amount of RE available for oxidation. If the RE in the alloy has been consumed completely, then the RE oxide inclusions attain a maximum size. After this point, a decrease in the average oxidation kinetics occurs. Very good agreement between experiments and calculations was obtained for the oxidation of a free-standing NiCoCrAlY coating at 1373 K.     

A Microstructural Study of Preferential Oxidation at the Grain Boundaries of Ni–Cr Alloys

The oxidation behavior of alloy grain boundaries in model Ni–Cr alloys was investigated. Two binary alloys with nominal wt.% compositions of Ni–10Cr and Ni–20Cr were used. Oxidation was performed in air for 50 hr at 1000°C. The grain boundaries intersecting the alloy surface in Ni–10Cr did not exhibit oxidation, whereas the alloy formed a thick (60 m) oxide layer which formed inwardly. The grain boundaries in this alloy showed a passivating influence at the adjacent regions and retarded oxide formation. An examination of the Ni–20Cr cross section revealed preferential oxidation to a depth of 65 m at the alloy grain boundaries intersecting its surface, while the oxide at the surface was a few micrometers thick. It is noted that the extent to which the grain-boundary oxidation differs from the alloy surface oxidation depends on the Cr content of the alloy. It is also considered that the grain-boundary oxidation behavior in different Ni–Cr alloys changes as a function of Cr content.     

Ti60高温钛合金氧化行为研究

研究Ti60高温钛合金在600～750 ℃范围内的氧化行为.氧化增重试验及XRD、SEM分析结果表明,Ti60合金在600～750 ℃范围内氧化0～100 h条件下,由Wagner的氧化经验公式计算得氧化指数n在1～2之间,氧化激活能为256 kJ/mol,氧化符合线性-抛物线混合规律.在600 ℃氧化100 h及750 ℃氧化10 h,氧化产物为TiO2,经750 ℃、100 h长时间氧化后,表面有少量Al2O3生成,氧化物优先沿原始β晶界形核.氧除了会在试样表面形成氧化层外,还会向基体中扩散形成脆性富氧层,从而影响合金力学性能.随着氧化温度的升高和时间的延长,富氧层厚度增厚.     

Influence of grain size on the oxidation behaviour of MA 956

The influence of grain size of alloy MA 956 on the oxidation kinetics and scale morphology is investigated at 900 and 1100°C for up to 1000 h exposure. Specimens with grain sizes of about 3, 150, and 300 μm mean diameter were studied. Grain growth during oxidation was not observed. The results of this work at 1100°C show that grain size of the alloy does not significantly influence the oxidation kinetics. At 900°C, however, mass gain values for the small‐ and medium‐grain size material are somewhat lower than those for the large‐grain size material, especially for exposures below 100 h. This feature indicates that grain boundaries could have played a role for the supply of Al to the scale.     

Influence of the alloy grain size on the oxidation behaviour of PM2000 alloy

The influence of the alloy grain size on the oxidation behaviour in air between 900 and 1400 °C of oxide dispersion strengthened PM2000 alloy was studied. The grain size of the alloy affects the morphology, the composition of the alumina scale as well as the size, composition and volume fraction of nodules formed on the alumina scale, but has only a small influence in the kinetics. In the fine-grained PM2000, grain boundaries act as rapid pathways for aluminium diffusion, leading to the establishment of a purer alumina scale than that formed on coarse-grained material. On the coarse-grained alloy, there is enough time for the nucleation of oxides other than alumina. These oxides, impeding alumina grain growth, can lead to the formation of a fine-grained alumina layer. The outward cation and inward oxygen diffusion in this fine-grained alumina layer may contribute to both the slight increase in mass-gain and the increased tendency to form nodules at the outer surface.     

Oxidation Behavior of the Directionally Solidified Ni-base Superalloy DS951 in Air

The isothermal oxidation behavior at 1000, 1050 and 1100°C and the cyclic oxidation behavior at 1000°C of the directionally solidified Ni-base superalloy DS951 were investigated. The oxidized samples were characterized by SEM, EDAX, XRD. The alloy DS951 obeyed a two-stage parabolic rate law during isothermal oxidation at 1000–1100°C. In cyclic conditions, the alloy showed no weight loss even after 1100 cycles. Cross-sectional examination revealed the development of faceted and needle-shaped AlN precipitates in the alloy subsurface region during cyclic oxidation, while no internal corrosion products were found in isothermal tests. Furthermore, a selective oxidation of MC carbides (M was predominately niobium with some chromium and tungsten) was observed along alloy grain boundaries and in interdendritic areas.     

Thermal stability and creep behavior of Ti–V–Cr burn-resistant alloys

The thermal stability and creep behavior of Ti–35V–15Cr (35V alloy) and Ti–25V–15Cr (25V alloy) burn-resistant titanium alloys are researched. The results show that post-exposure tensile properties deteriorated with the increase in exposure temperature (450–600 °C). The decrease in tensile properties of the 35V alloy results from the combination of surface oxidation and microstructural changes and the decrease in tensile properties of the 25V alloy results from surface oxidation. The main change of the microstructure during thermal exposure is the heterogeneous precipitation of α phase on β grain boundaries. Increased vanadium content in the alloy shows an adverse effect on alloys’ thermal stability. The creep resistance of the 35V alloy is little better that that of the 25V alloy. During creep exposure at 540 °C for 100 h, the heterogeneous precipitation of α phase on β grain boundaries in 35V alloy strengthens the grain boundary, leading to increases in the creep resistance, while the heterogeneous precipitation of α phase in grains and grain boundaries in the 25V alloy is rod-like, leading to decreases in the creep resistance.     

Laser surface treatment and its influence on the development of healing Cr2O3 scales on nickel-chromium alloys

The influence of laser surface treatment on the isothermal oxidation of Ni-10%Cr and Ni-15%Cr at 1025°C in oxygen at 1 atm pressure has been studied. Particular emphasis has been placed on the progressive establishment of a Cr₂O₃ healing layer, which is facilitated by rapid-diffusion paths for chromium to the surface from the bulk alloy. For nonlaser treated alloys, such paths are alloy grain boundaries. A partial Cr₂O₃ layer forms initially in localized sites at, and immediately adjacent to, these boundaries and progresses into the alloy grains in a stepwise manner following lateral diffusion of chromium from the grain boundaries, thereby developing a contoured configuration. For Ni-15%Cr, there is sufficient chromium in the bulk alloy grains to sustain the eventual development of a self-healing layer parallel to the surface. For Ni-10%Cr, this is not the case and complete development of the healing layer results entirely from the stepwise progression from the grain boundaries. Establishment of the healing layer on laser-glazed surfaces is facilitated by additional rapid-diffusion paths, particularly retained alloy grain boundaries, retained alloy twins, a laser-induced microstructure and solidification artifacts (such as ripples). The relative importance of these features is discussed in relation to the oxidation behavior.     

In situ oxidation of Ni30 wt%Cr and TDNiCr in the high voltage electron microscope

In situ oxidation of Ni-30Cr and TDNiCr (Ni20wt %Cr2 vol. %ThO₂) in the high voltage electron microscope has been used to examine the early stages of oxidation over the temperature range ～ 450–700°C in 15 Torr O₂. Companion studies on thin foils and bulk specimens oxidized out of the microscope gave results similar to the in situ studies, indicating that the in situ results reveal true oxidation behavior. A notable feature was the observation that oxide nucleation and growth occurred on a variety of interfaces, such as grain boundaries, large inclusions, and slip bands. The growth of grain boundary oxides was studied, and a mechanism is proposed to account for the kinetics.     

Development of {110}?001? annealing texture in Al-free and 0.1 %Mn-added 3 %Si-Fe alloy strips containing 95 ppm sulfur

The solubility of sulfur is calculated in 0.1 %Mn-added 3 %Si-Fe alloys. The segregation kinetics of sulfur is compared in the alloy containing 95 ppm sulfur, depending on the annealing atmosphere. The effects of pre-annealing and annealing atmosphere on final annealing texture are investigated. Segregation behaviors of sulfur at free surfaces and grain boundaries are compared and, during the selective growth, the importance of the grain boundary concentration of sulfur is emphasized. Finally, a correlation between the development of the annealing texture and segregation kinetics of sulfur in the alloy strip is discussed.     

Study of localized corrosion in AA2024 aluminium alloy using electron tomography

SEM based tomography of localized corrosion has been achieved using selective detection of backscattered electrons. The high resolution tomography provides direct evidence that links the surface appearance of corroded alloy, the alloy microstructure and the corrosion propagation path. Stable localized corrosion of AA2024-T351 aluminium alloy was initiated at locations where large clusters of S phase particles were buried beneath the surface. Propagating away from the initiation sites, corrosion developed preferentially along the grain boundary network. The grain boundary attack started beneath the alloy surface, proceeded along preferred grain boundaries and may emerge at the alloy surface.     

Mechanisms of damage initiation in a titanium alloy subjected to water droplet impact during ultra-high pressure plain waterjet erosion

Impingement of ultra-high pressure plain waterjets (PWJ) has been considered as a promising surface treatment technology (e.g. peening, milling, cleaning) for some difficult-to-machine engineering materials (such as titanium alloys). However, a comprehensive understanding of the mechanisms of damage initiation on such materials under PWJ impingement has not been established. This paper presents an in-depth investigation into the initial material damage mechanisms which are observed during impingement of a PWJ onto polished samples of an α?β titanium alloy—Ti–6Al–4V, and the influence of the microstructure of the alloy on the initial damage modes. A series of single-track erosion trials were carried out and scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to examine the resulting damage. It was found that the microstructural features of Ti–6Al–4V influence the development of damage, with grain boundaries exhibiting lower resistance to damage than grains themselves. Under waterjet impingement, the initial stage of damage is associated with deformation resulting in grain tilting; a combination of impact damage and lateral outflow jetting then results in preferential damage to the grain boundary regions. Hydraulic penetration into the damaged grain boundaries results in the formation and expansion of microvoids and pits, firstly along the grain boundaries and then across the grains. The subsequent damage is linked with the effects of both hydraulic penetration and lateral outflow jetting on the increasingly roughened surface.     

Corrosion of ultra-fine grained copper fabricated by equal-channel angular pressing

Corrosion of ultra-fine grain (UFG) copper fabricated by equal-channel angular pressing (ECAP) has been investigated in comparison with that in recrystallized coarse grain (CG) copper. Corrosion current was estimated by a Tafel extrapolation method to examine the kinetics of corrosion in a modified Livingstone etchant, which is sensitive to dislocations and grain boundaries. UFG copper exhibited a lower corrosion current in comparison with that in its recrystallized coarse grain (CG) counterpart despite the fact that the dislocation density and total fraction of grain boundaries are much greater in UFG copper than in CG copper. Corrosion damage on the surface of UFG copper is macroscopically rather uniform whereas obvious attack at grain boundaries and selective corrosion of some grain interiors were observed in CG copper.     

预氧化温度对GH3128合金抗高温循环氧化性能的影响

采用热重法并结合SEM、EDAX等分析手段,研究了预氧化温度对GH3128合金抗高温(1100℃)循环氧化性能的影响。结果表明:在适当温度下进行预氧化处理可有效提高合金的抗高温循环氧化性能。GH3128合金经预氧化处理后抗高温循环氧化性能的优劣主要取决于两种因素的综合作用,其一是合金表面元素Cr的选择性氧化,形成致密均匀的保护性预氧化膜,提高合金抗高温循环氧化性能;其二是合金中元素Ti、Cr的晶界偏析加剧,加速晶界氧化并导致显微裂纹等缺陷产生,降低合金的抗高温循环氧化性能。900℃预氧化处理的GH3128合金试样具有最佳的抗高温循环氧化性能,该温度下预氧化膜形成较好且合金元素Ti、Cr的晶界偏析作用相对较弱。