不同加热制度对哈氏合金C276耐晶间腐蚀性能影响

采用ASTM G28A法进行硫酸-硫酸铁晶间腐蚀试验,研究了550—1 100℃范围内不同加热温度、加热时间等参数对哈氏合金C276耐晶间腐蚀性能的影响,结果表明,在700—1 050℃区间,哈氏合金C276具有较强的晶间腐蚀敏感性。     

哈氏合金电化学抛光工艺的研究

采用环保型抛光液对离子束辅助沉积技术路线用哈氏合金HastelloyC-276基带进行了电化学抛光,获得了典型的阳极极化曲线,并阐述了电化学抛光的机理.研究了影响电化学抛光的因素(电解液温度、抛光时间、抛光极距)对基带表面粗糙度的影响,优化工艺参数获得了表面粗糙度Rn＜5 nm(5μm×5μm)的高质量表面,满足后续生长过渡层对哈氏合金基带的要求.抛光液中的柠檬酸在50℃左右分解成络合剂,可迅速沉淀金属离子,提高表面的活性.     

哈氏合金电化学抛光工艺的研究

采用环保型抛光液对离子束辅助沉积技术路线用哈氏合金HastelloyC-276基带进行了电化学抛光,获得了典型的阳极极化曲线,并阐述了电化学抛光的机理。研究了影响电化学抛光的因素(电解液温度、抛光时间、抛光极距)对基带表面粗糙度的影响,优化工艺参数获得了表面粗糙度Ra5nm(5μm×5μm)的高质量表面,满足后续生长过渡层对哈氏合金基带的要求。抛光液中的柠檬酸在50℃左右分解成络合剂,可迅速沉淀金属离子,提高表面的活性。     

Pulsed Laser Deposition of YBa2Cu3O7 Superconducting Film on MgO Templates Spray Pyrolyzed on Hastelloy C276

Pulsed laser deposition (PLD) was used to deposit YBCO on MgO-buffered C276 substrates in order to evaluate the quality of the deposited MgO films which were deposited by spray pyrolysis. The characterization of the thin films was done using scanning electron microscopy, atomic force microscopy, electron backscattered diffraction, X-ray diffraction 2??-scans, rocking curve (??-scans), phi scan, pole-figure measurements, and AC susceptibility. It was found that c-axis oriented YBCO films were grown on c-axis oriented MgO films which confirm that the deposited YBCO films copied the out-of-plane texture of the spray pyrolyzed MgO buffer. However, MgO and YBCO films have a very weak in-plane texture. The AC susceptibility measurements show that the YBCO films have a broad superconducting transition temperature which may be attributed to the weak in-plane texture.     

不同焊接方法对哈氏合金C-276焊缝组织和耐晶间腐蚀性能的影响

采用ASTM-G28B法进行晶间腐蚀试验,分别研究了手工GTAW和手工GMAW对哈氏合金C-276焊缝组织和耐晶间腐蚀性能的影响。结果表明,手工GMAW焊接的C-276合金,焊缝区析出碳化物第二相较多,晶间腐蚀倾向更大。     

Investigations on the microstructure and mechanical properties of multi-pass pulsed current gas tungsten arc weldments of Monel 400 and Hastelloy C276

This research article reported the weldability, microstructure and mechanical properties of the dissimilar combinations of nickel alloys such as Monel 400 and Hastelloy C276. Multi-pass pulsed current gas tungsten arc (PCGTA) welding was employed for joining these dissimilar metals using ERNiCrMo-3 filler. Interface microstructures showed the absence of unmixed zone at the HAZ of both the sides. It was evident from the studies that all the tensile failures occurred at Monel 400 side. The average impact toughness portrayed by these dissimilar weldments was found to be 41 J. Bend test results showed that these dissimilar combinations offer augmented ductility. The outcomes of the study substantiated the use of current pulsing for the successful joints of Monel 400 and Hastelloy C276 by correlating the mechanical and metallurgical properties.     

Effect of multiaxial stress state on creep damage in ASME T92 welded joint

This study reports the type IV fracture process and the influence of multiaxial stress state in ASME T92 welded joints during creep. The type IV fracture occurs at the fine‐grained heat‐affected zone (ie, FGHAZ), involving void initiation, growth, and coalescence, microcrack occurrence, propagation and extension, and eventual macrocrack with consequent joint failure. The creep damage is not uniformly distributed along the thickness direction in the FGHAZ, and the central part of the welded joint is the most seriously damaged region. The equivalent creep strain is higher at the external surface, but the stress triaxiality is larger in the centre section. Large equivalent creep strain could promote creep void initiation, whereas high hydrostatic pressure and stress triaxiality factor accelerate void growth in the FGHAZ of T92 joints. Besides, reducing groove angle and HAZ width of the joints is recommended to delay the occurrence of type IV cracking because of lower equivalent creep strain and stress triaxiality factor.     

A damage model for creep crack growth

A model is developed for damage produced by the growth of isolated grain boundary cavities under power law creep. This damage model is combined with the small scale yielding stress and strain fields to predict the damage ahead of a stationary and a steadily propagating crack tip in an elastic-power law creeping material. A failure criterion, based upon the damage ahead of the crack tip attaining a critical value, is invoked. This criterion leads to predictions for the incubation time prior to initiation of crack growth and for the relationship between the remote stress intensity factor and the steady state crack speed. Results are presented for both elastic-primary and -secondary creep crack growth. In either case, there exists a minimum stress intensity factor below which steady state crack growth is not possible. Comparisons of the predictions of this model with others for steady state crack propagation in elastic-secondary creeping materials are also made.

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Résumé On développe un modèle décrivant le dommage produit par la croissance de cavités isolées aux frontières des grains sous l'effet d'un fluage selon une loi parabolique. Ce modèle d'endommagement est combiné aux champs de contrainte et de déformation conduisant à une plastification à petite échelle, en vue de prédire le dommage qui se produit en avant de l'extrémité d'une fissure stationaire et en propagation lente, dans un matériau soumis à fluage selon une loi élastique. On invoque un critère de rupture basé sur la valeur critique atteinte par le dommage en avant de l'extrémité de la fisure. Ce critère conduit à prédire le temps d'incubation avant amorçage de la croissance d'une fissure, ainsi que la relation entre le facteur d'intensité des contraintes appliquées à une certaine distance et la vitesse de croissance stable de la fissure.On présente les résultats dans les cas de croissance d'une fissure de fluage au stade secondaire. Dans les deux cas, il existe un facteur d'intensité de contrainte minimum en dessous duquel une croissance stable de la fissure n'est pas possible. On procède également à des comparaisons des prédictions données par ce modèle avec d'autres, pour la propagation en état stable d'une fissure dans des matériaux soumis à fluage secondaire élastique.

     

Study on the microstructure and mechanical properties of diffusion brazing joint of C17200 Copper Beryllium alloy

In order to study the microstructure and mechanical properties of Copper Beryllium alloy, spreadability test was carried out at two temperatures under Argon atmosphere for different filler metals of Ag content. The results show that BAg2a (Ag–26Cu–21Zn–19Cd) and BAg1a (Ag–18.5Cu–17Zn–14.5Cd) are the best choice for brazing of Copper Beryllium. Zn affects the wetting of interlayer because it spreads preferentially. The bonding process was carried out at a temperature ranging of 650–800 °C for various times under Argon atmosphere using of BAg2a (Ag–26Cu–21Zn–19Cd) film with 100 μm thickness as interlayer.Interfacial microstructures were examined by Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The eutectic and intermetallic compounds such as CuZn, AgZn3 and AgCd3 were formed at the interfaces between the interlayer and substrate. Microhardness and tensile tests were used for evaluating the mechanical properties. Average of hardness at the center of brazed seam decreased with increasing time and temperature that associated with diffusion of main elements to substrate and intermetallic formation at the interface. Maximum tensile strength of 170 MPa was obtained at 750 °C for 20 min for filler metal BAg2a without heat treatment and 227 MPa with heat treatment.     

Modeling creep damage based on real microstructure

This paper outlines a method to model creep failure of polycrystalline materials based on a real microstructure taken from an optical microscope. The creep failure is simulated in 304 stainless steel and the simulation is based on Norton’s creep law. By treating the grain boundaries and the grains differently and adopting the void nucleation process proposed by Shewmon, the creep strain energy density can be used as a failure criterion. The result of the simulation confirmed the results of conventional methods used in a high-temperature remnant life assessment. The intermediate results of the simulation process allow calculation/monitoring of stiffnesses degradation as the material undergoes creep failure.     

A process model for active brazing of ceramics: Part II Optimization of brazing conditions and joint properties

In the present investigation, the process model developed in Part I has been applied to evaluate the microstructure and strength evolution during active brazing of ceramics. As a starting point, reaction-layer growth is assumed to occur isothermally with no restrictions in the supply of reactive element. Different kinds of diagrams are then constructed to show how specific process variables (e.g. the heating and cooling period, the limiting layer thickness, and the diffusion mechanism) affect the growth kinetics. It is concluded that the key to improved joint properties lies in control of the reaction-layer thickness through optimization of the brazing conditions, and an illustration of this is given. This revised version was published online in November 2006 with corrections to the Cover Date.     

Numerical investigation of factors affecting creep damage accumulation in ASME P92 steel welded joint

The present study mainly investigated Type IV cracking occurring in the fine grained heat affected zone (FGHAZ) in the welded joint of ASME P92 steel at high temperature and low applied stress by numerical simulation method. Based on the modified Karchanov–Rabotnov constitutive equation, the user defined material subroutine (UMAT) was complied and the creep damage accumulation was carried out by finite element method using ABAQUS codes for the welded joint at 650 °C and 70 MPa. Calculated results revealed that the most severe creep damage and the highest equivalent creep strain occurred in the FGHAZ because of high maximum principle stress and high maximum principle stress. Furthermore, the effect of groove angle and HAZ width on the creep damage accumulation was investigated. It indicated that a small groove angle and a narrow FGHAZ width could deteriorate the creep damage accumulation because of the degradation of maximum principle stress and stress triaxiality in the FGHAZ.     

Nonlinear creep damage constitutive model for soft rocks

In some existing nonlinear creep damage models, it may be less rigorous to directly introduce a damage variable into the creep equation when the damage variable of the viscous component is a function of time or strain. In this paper, we adopt the Kachanov creep damage rate and introduce a damage variable into a rheological differential constitutive equation to derive an analytical integral solution for the creep damage equation of the Bingham model. We also propose a new nonlinear viscous component which reflects nonlinear properties related to the axial stress of soft rock in the steady-state creep stage. Furthermore, we build an improved Nishihara model by using this new component in series with the correctional Nishihara damage model that describes the accelerating creep, and deduce the rheological constitutive relation of the improved model. Based on superposition principle, we obtain the damage creep equation for conditions of both uniaxial and triaxial compression stress, and study the method for determining the model parameters. Finally, this paper presents the laboratory test results performed on mica-quartz schist in parallel with, or vertical to the schistosity direction, and applies the improved Nishihara model to the parameter identification of mica-quartz schist. Using a comparative analysis with test data, results show that the improved model has a superior ability to reflect the creep properties of soft rock in the decelerating creep stage, the steady-state creep stage, and particularly within the accelerating creep stage, in comparison with the traditional Nishihara model.     

A new model for characterizing primary creep damage

Primary creep damage may occur at a crack tip in steel at room temperature and below. The effect of this time-dependent damage is generally neglected. Recently developed experimental data clearly show that, for some materials, neglecting time-dependent deformation and damage may be quite nonconservative and dangerous in certain practical applications.In this paper, a new theory is developed which incorporates the effects of time-dependent damage into the crack growth and failure process. The predictive ability of the model is verified first by finite element analyses and then by comparison to experimental data. It is clearly shown that, for the material considered here, time-dependent damage effects must be considered or the crack growth process is not adequately accounted for.     

镍基含硼钎料钎焊IC-6合金接头近缝区硼化物研究

研究了采用镍基含硼钎料钎焊IC -6合金时 ,接头近缝区出现的硼化物相的形貌、成分及结构 ,结果表明主要存在两种成分的硼化物相 ,r(Ni) :r(Mo)分别近似为 1 :1和 1 :2 .硼化物取向有一定规律 .对接头近缝区与合金中硼化物的形貌、成分进行了对比 .     

Nonlinear damage creep model based on fractional theory for rock materials

Mechanics of Time-Dependent Materials - In this study, based on the first application of Caputo fractional derivative with respect to the Mittag–Leffler and power functions, two improved...     

The influence of brazing conditions on joint strength in Al2O3/Al2O3 bonding

The brazing of alumina ceramic to itself was performed using Ag₅₇Cu₃₈Ti₅ filler alloy. The bonding was carried out in a vacuum of 7 × 10^?3 Pa, and the joining conditions were at 1073, 1123, 1173, 1223, 1273 and 1323 K for 1.8ks under a pressure of 0.01 MPa, at 1123 K with a pressure of 0.01 MPa for 0, 0.3, 0.9, 1.8, 2.7 and 3.6 ks, and at 1123 K for 1.8 ks with pressures of 0, 0.01, 0.05, 0.10, 0.15, 0.20 and 0.30 MPa, to determine the effects of joining temperature, pressure and holding time on the joint strength. The joint strength was measured by shear tests. The interface microstructures and fractured surfaces after testing were observed by scanning electron microscopy (SEM). It was shown that the shear strength of Al₂O₃/Al₂O₃ joints was largely affected by the joining conditions; it first increased and then decreased with increasing joining temperature, pressure and holding time and depended mainly on the strength of interfacial reaction layer itself and the interface bonding strength between the reaction layer and the ceramic. The maximum joint strength was obtained when the reaction occurred under a suitable temperature, pressure and time, and the reaction layer thickness was about 2 μm. SEM observations revealed that there were four types of fracture and each kind corresponded to a different strength.