Macroscopic dislocation model of a wedge twin with allowance for the shape of its boundaries

Stress fields for wedge twins with various shapes of boundaries are calculated on the basis of a macroscopic dislocation model. It is shown that the deviation of a wedge twin from a symmetric shape leads also to the loss of symmetry in the stress-field distribution near the twin. Cases of a uniform and an equilibrium distribution of twinning dislocations at twin boundaries are considered.     

Plasticity of bcc micropillars controlled by competition between dislocation multiplication and depletion

Recent micropillar experiments have shown strong size effects at small pillar diameters. This “smaller is stronger” phenomenon is widely believed to involve dislocation motion, which can be studied using dislocation dynamics (DD) simulations. In the present paper, we use a three-dimensional DD model to study the collective dislocation behavior in body-centered cubic micropillars under compression. Following the molecular dynamics (MD) simulations of Weinberger and Cai, we consider a surface-controlled cross-slip process, involving image forces and non-planar core structures, that leads to multiplication without the presence of artificial dislocation sources or pinning points. The simulations exhibit size effects and effects of initial dislocation density and strain rate on strength, which appear to be in good agreement with recent experimental results and with a simple dislocation kinetics model described here. In addition, at the high strain rates considered, plasticity is governed mainly by the kinetics of dislocation motion, not their elastic interactions.     

Study on dislocation multiplication in the head of Germanium single crystal

The high strength, radiation hardness and cost-effectiveness make Germanium the substrate of choice for high-efficiency multi-junction solar cells for space applications.Numerical modeling and large-scale simulation are important and indispensable tools in the analysis and development of crystal growth process.In this study, germanium single crystals with low dislocation density were produced by Czochralski method by applying the necking technique.Chemical etching pits method was used to measure the dislocation density, and a professional modeling software CrysVUn was used to obtain the thermal-stress distribution.The results show that the thermal-stress of the sample with diameter of 15 mm is nearly equal to that of other samples, so the thermal-stress does not influence the dislocation multiplication.Based on the result, the dislocation density must be strangely increased caused by gravity.     

On the multiplication of dislocations during martensitic transformations in NiTi shape memory alloys

In situ and post-mortem diffraction contrast transmission electron microscopy (TEM) was used to study the multiplication of dislocations during a thermal martensitic forward and reverse transformation in a NiTi shape memory alloy single crystal. An analysis of the elongated dislocation loops which formed during the transformation was performed. It is proposed that the stress field of an approaching martensite needle activates an in-grown dislocation segment and generates characteristic narrow and elongated dislocation loops which expand on {1 1 0}_B2 planes parallel to {0 0 1}_B19′ compound twin planes. The findings are compared with TEM results reported in the literature for NiTi and other shape memory alloys. It is suggested that the type of dislocation multiplication mechanism documented in the present study is generic and that it can account for the increase in dislocation densities during thermal and stress-induced martensitic transformations in other shape memory alloys.     

Characteristics of dislocation structure in creep deformed lamellar tial alloy within primary regime

In this investigation, dislocations of a lamellar TiAl alloy are analyzed after creeping in the primary range at 800°C/200MPa in order to interpret their mobility It was found that the dislocation density in γ-laths decreased as the creep deformation proceeds within primary creep regime Schmid factor analysis suggests that the creep deformation in the early stage of the primary creep regime is controlled by the gliding of some of the initial dislocations which have a high enough Schmid factor As the creep deformation progressed, those dislocations with high Schmid factors slip preferentially to be annihilated at the α-γ interface For further continuous deformation, dislocation generation is required, and for this, α-phase is transformed to γ-phase in order to generate new dislocations A slow dislocation generation process by phase transformation of α-phase compared with the absorbing rate to sinks is responsible for the decreasing dislocation density as the creep strain increases     

High temperature deformation mechanism and evolution of dislocation structure during creep of Ni-Base superalloy 713LC

Creep deformation of cast nickel base superalloy 713LC has been investigated in a temperature range of 723 to 982°C. The values of the stress exponent and activation energy for creep of the alloy vary with a combination of temperature and stress. Introduction of threshold stress for creep of the alloy provided an explanation of the high values of the stress exponent and the apparent activation energy. Microstructural evolution of the alloy with creep deformation has also been studied. The analysis of the creep mechanism has been supplemented by microstructural observations after deformation under various test conditions. The dislocation structure of the alloy at high temperature and low stress was different from that at low temperature and high stress. Shearing of γ′ particles by dislocation pairs was the dominant creep mechanism at low temperature and high stress whereas dislocation climb over γ′ particles was the rate controlling process of creep at high temperature and low stress.     

Model of grain-boundary diffusion with allowance for near-boundary layers of equilibrium composition

Fisher model of intercrystalline diffusion is refined based on an analysis of previously obtained results of emission Mössbauer studies of grain boundaries. It is shown that under conditions of dominating intercrystalline diffusion, the existence of monatomic near-boundary layers of equilibrium composition should be taken into account. It has been found that the allowance for these layers does not change the expressions for the distribution of the concentration of the diffusing impurity in a polycrystal in the case of a type-B kinetic regime, whereas in the type-C regime the corresponding expressions change somewhat. The technique that was used previously for the determination of parameters of grain-boundary diffusion and segregation, which is based on the results of emission Mössbauer investigations of grain boundaries, has been modified; the application of the new approach has been demonstrated on the example of processing data of Mössbauer investigations of grain boundaries in polycrystalline Nb at ^119mSn nuclei.     

GH4169合金小余量小尺寸静子叶片锻造工艺及显微组织控制

针对GH4169合金叶片锻件粗晶、混晶等晶粒组织不合格问题,以某压气机小余量小尺寸静子叶片锻件为研究对象,首先分析锻件结构,确定锻造工艺路线,然后通过选取2种规格坯料、5个变形量、4个锻造温度来组成12组试验,探究变形量、锻造温度对组织的影响以及演化规律,同时对选取的保温时间、锻造火次的稳定性进行分析,得到合理锻造工艺参数。研究表明,变形量在35%~45%内可获得均匀的组织,但对晶粒度大小无明显影响;温度是影响晶粒度的主要因素,大于1000℃时叶身晶粒粗化现象明显,在990~1000℃之间叶身可获得细腻、均匀的组织;预锻时组织已成形稳定,终锻无明显变化,在选取的保温时间内组织稳定;在变形量40%及锻造温度990℃下获得的叶片锻件具有优良的综合力学性能及细晶组织,均高于标准要求。     

Predicting creep strengths and lifetimes of creep resistant engineering alloys

The physical basis for predicting the long-term creep strengths and lifetimes at application temperatures using creep parameters determined from short-term creep tests is investigated for complex creep resistant engineering alloys. It is shown that the seemingly unpredictable stress and temperature dependence of minimum creep rate of such alloys can be rationalised using an approach based on the new power law creep equation that incorporate the tensile strength. This is demonstrated using the tensile and creep data measured for two completely different types of alloys: steel 11Cr-2W-0.4Mo-1Cu-Nb-V and Ni base superalloy 15Cr-28Co-4Mo-2.5Ti-3Al. For both alloys, the stress exponent n determined does not depend on temperature and activation energy of creep does not depend on stress. Consequently, it becomes possible to use the new power law creep equation in combination with the Monkman-Grant relationship to predict the long term creep rupture strengths and lifetimes and microstructure stability of the two alloys from short term creep test data. The implications of the results for creep mechanism identification and future microstructure analysis are discussed.     

低倍数泡沫灭火系统设计探讨

介绍了中国铝业股份有限公司河南分公司扩建700kt/a氧化铝工程中的储油罐消防设计过程.     

On the effect of grain boundary segregation on creep and creep rupture

The present work investigates the effect of grain boundary chemistry and crystallography on creep and on creep damage accumulation in Cu–0.008 wt.% Bi and Cu–0.92 wt.% Sb at stresses ranging from 10 to 20 MPa and temperatures between 773 and 873 K. Small additions of Bi and Sb significantly reduce the rupture strain and rupture time during creep of Cu. High stress exponents (Cu–Bi) and high apparent activation energies for creep (Cu–Bi and Cu–Sb) are obtained. Sb promotes creep cavitation on random high-angle grain boundaries. Bi, on the other hand, causes brittle failure when small crack-like cavities cause decohesion. Both elements suppress dynamic recrystallization, which occurs during creep of Cu at high stresses and temperatures.     

Apparatus for investigating phase transformations and properties of metals and alloys with programmed electromechanical treatment

Conclusions An apparatus was developed for investigations of phase transformations and mechanical properties under programmed thermal and mechanical conditions, which makes it possible to study electrothermal treatment processes under any desired conditions, high-temperature thermomechanical treatment, lowtemperature thermomechanical treatment, machining, and treatment to obtain superplasticity.This paper will be presented at the All-Union Symposium on New in Metal Science and Heat Treatment of Metals and Alloys, September 10–12, 1975, in Minsk.Peoples Republic of Bulgaria. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 8, pp. 74–76, August, 1975.     

Ferromagnetic resonance in a plate with a stripe domain structure with allowance for the inhomogeneity of the demagnetizing field

A numerical simulation of ferromagnetic resonance (FMR) in the absence of an external magnetic field has been performed for a plate with a high quality factor. A domain structure with Bloch domain walls (DWs) has been studied. It has been shown that the allowance for the inhomogeneity of the intrinsic demagnetizing field leads to the appearance of additional peaks in the FMR spectrum. With decreasing plate thickness, there occurs a displacement of the resonance-curve peaks into the region the low frequencies. An increase in the size of one of the domains at the expense of another in the periodic domain structure leads to an increase in the amplitude of the susceptibility of the low-frequency FMR mode and its shift into the region of low frequencies. Accordingly, the amplitude of the susceptibility of the high-frequency mode decreases and is shifted into the region of high frequencies. Furthermore, in the presence of resonance, in the plate with unequal domains the peak of the resonance curve that corresponds to the high-frequency mode is observed also upon the excitation of oscillations by an external ac field perpendicular to the DW, and the peak corresponding to the low-frequency mode, upon the excitation by an ac field parallel to the DW. These features of the FMR are explained by the corresponding changes in the inhomogeneous demagnetizing field.     

Growth kinetics of nanoshells of the intermediate phase with allowance for finite reaction rates at interphase boundaries

Growth kinetics of the intermediate phase upon reaction diffusion in core-shell systems with simultaneous allowance for barriers at interphase boundaries, curvature, and concentration dependence of the interdiffusion coefficient inside the phase has been analyzed. It has been shown that the throughput capacity of interphase boundaries can change in the process of growth of the phase (both monotonically and nonmonotonically) and the concentration-range width of the phase likewise changes in a nonmonotonic manner.     

Basic model for primary and secondary creep in copper

A model for primary and secondary creep is established. The starting point is a combination of basic models for tensile stress-strain curves and for secondary creep. The derived model can describe experimental creep strain curves for Cu 50 p.p.m. P in the temperature interval 75-250 °C with the same precision as the variation in the experimental creep strain curves for identical conditions. No fitting parameters are involved in this representation. The model has also been applied successfully to creep specimens with round notches, where multiaxial stress states are present. One main aim of deriving a basic creep model is to improve the accuracy of extrapolation. That this is possible has been demonstrated for creep tests with estimated secondary creep rates down to 5 × 10⁻²² s⁻¹. In conventional creep rupture testing strain rates down to 1 × 10⁻¹² s⁻¹ can be recorded. As a background, a summary of empirical methods for the extrapolation of creep strain data is given.     

Structure and creep of Russian reactor steels with a BCC structure

The structural phase transformations have been revealed and the characteristics of the creep and long-term strength at 650, 670, and 700°C and 60–140 MPa have been determined in six Russian reactor steels with a bcc structure after quenching and high-temperature tempering. Creep tests were carried out using specially designed longitudinal and transverse microsamples, which were fabricated from the shells of the fuel elements used in the BN-600 fast neutron reactor. It has been found that the creep rate of the reactor bcc steels is determined by the stability of the lath martensitic and ferritic structures in relation to the diffusion processes of recovery and recrystallization. The highest-temperature oxide-free steel contains the maximum amount of the refractory elements and carbides. The steel strengthened by the thermally stable Y–Ti nanooxides has a record high-temperature strength. The creep rate at 700°C and 100 MPa in the samples of this steel is lower by an order of magnitude and the time to fracture is 100 times greater than that in the oxide-free reactor steels.