Acoustoplastic effect and internal friction of aluminum single crystals in various deformation stages

The dependences of the acoustoplastic effect and the internal friction on the oscillatory strain amplitude are measured in various deformation stages of low-purity aluminum single crystals. It is discovered that the acoustoplastic effect is observed not only in the macroscopic plastic region of the stress-strain diagram, but also for microplastic deformation in the “elastic” loading and unloading stages. The sign of the effect reverses during unloading. An increase in the strain rate leads to enhancement of the acoustoplastic effect and the absorption of the energy of ultrasonic vibrations causing this effect with a frequency of about 100 kHz. It is concluded that the acoustoplastic effect observed during both macro-and microplastic deformation is caused by the irreversible high-speed motion of dislocations through the long-range stress field of the other dislocations after breaking through the Cottrell atmospheres. Fiz. Tverd. Tela (St. Petersburg) 39, 1794–1800 (October 1997)     

Point Defect Production Under High Internal Stress Without Dislocations in Ni and Cu

Kiritani et al. have observed a large number of small vacancy clusters without dislocations at the tip of torn portions of fcc metals such as Au, Ag, Cu and Ni. Small vacancy clusters, rather than dislocation cell structures, have also been observed after high-speed compressive deformation, suggesting the possibility of plastic deformation without dislocations. In this paper, in order to investigate the mechanism of deformation without dislocations, change in formation energy of point defects under high internal stress was estimated by computer simulation. Elastic deformation up to - 20% strain was found to provide a remarkable lowering of formation energy of point defects. For example, when Ni is subjected to elastic strain, the formation energy of an interstitial atom decreases to 40% that without strain and the formation energy of a vacancy decreases to 51% that without strain. The number of point defects formed under thermal equilibrium during deformation was evaluated. The number was judged to be insufficient for explaining the formation of vacancy clusters as observed in experiments.     

铝-银合金在疲劳过程中所表现的一些特点

本工作进行了淬火Al-7.27％Ag合金的扭转疲劳试验,测定了各种扭应变下的△E-N曲线,并且观察了经过各种循环数以后试样的表面金相变化。实验结果指出,当扭应变较小时,△E随着循环数N的增加而逐渐下降,△E-N曲线的变化类似Al-Cu和Al-Mg合金在较低扭应变下的情况。但当扭应变较大时,△E开始略有下降,随后上升到某一较高值后再下降,直至试样断裂。△E-N曲线的形状与Al-Cu和Al-Mg合金完全不同。试样表面的金相变化分为两个明显不同的阶段。在疲劳的起始阶段,滑移痕迹细而均匀,但经过一定循环数后,少数滑移痕迹变得集中而深化。随着循环数的增加,新的滑移带在原有滑移带之间不断地出现,没有纯Al和Al-Mg合金中滑移带变宽的情况。还看到了裂纹沿晶界的形成和发展。根据溶质银原子与位错的电交互作用和位错切割银原子簇的观点,对所得到的结果进行了讨论。 关键词：     

Acoustic emission during thermoelastic martensitic transformations in titanium nickelide under isothermal loading

Acoustic emission during thermoelastic martensitic transformations in titanium nickelide is investigated. Phase transitions are initiated by loading the specimen to 200 MPa under isothermal conditions. It is found out that deformation buildup and acoustic emission in the loading-unloading cycles are observed in the first cycle only, during further cycles the acoustic emission is comparable to the background, while the deformation buildup and recovery are not associated with martensitic transformations. It is shown that recovery of the deformation built up during loading occurs due to heating to 600 °C, with the major part of accumulated deformation undergoing recovery already at 250 °C and recovery of its minor part observed at 400 °C. This behavior of acoustic emission and accumulation and recovery of deformation provide evidence of martensitic phase stabilization during cycling of martensitic transformations under conditions of thermo-mechanical cycling. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 89–94, February, 2008.     

Prediction of dislocation formation in epitaxial multilayers subject to in-plane loading

A theoretical model is described to predict equilibrium distributions of misfit dislocations in one or more anisotropic epitaxial layers of a multilayered system deposited on a thick substrate. Each layer is regarded as having differing elastic and lattice constants, and the system is subject to biaxial in-plane mechanical loading. A stress transfer methodology is developed enabling both the stress and displacement distributions in the system to be estimated for cases where the interacting dislocations are of a pure edge configuration. Energy methods are used to determine equilibrium distributions of the dislocations for given external applied stress states. It is shown that the new model accurately reproduces known exact analytical solutions for the special case of just one isotropic epitaxial layer applied to an isotropic semi-infinite substrate having the elastic constants of the substrate but differing lattice constants. The model is used to consider equilibrium dislocation distributions in capped epitaxial systems with misfit dislocations. It is shown that the simplifying assumptions often made in the literature, regarding the uniformity of elastic properties and the neglect of anisotropy, can lead to critical thicknesses being underestimated by 15–18%. The application of uniaxial tensile stresses increases the value of critical thicknesses. The model can be used to analyse dislocations in various non-neighbouring layers provided the dislocation density has the same value in all layers in which dislocations have formed. This type of analysis enables the prediction of the deformation of metallic multilayers subject to mechanical and thermal loading.     

Destruction of SmS single crystals during the semiconductor-metal phase transition under hydrostatic compression

The behavior of internal microstresses, the size of the X-ray coherent scattering region, and the residual amount of the metal phase during the cyclic loading of SmS single crystals at hydrostatic pressure above the critical pressure of the semiconductor-metal phase transition has been investigated. It has been shown that the samples are destroyed as the microstresses reach the values corresponding to the ultimate stress of SmS single crystals. As the number of loading cycles increases, the coherent scattering region gradually decreases, which is accompanied by a decrease in the amount of the metal phase in the samples.     

Influence of electrolytic hydrogen on the etch pit density of iron (110) surfaces

Etch pit densities on iron (110) surfaces in sulphuric acid grow linearly with the interfacial hydrogen activity in excess of a critical activity. The hydrogen activity is approximately proportional to the square root of the cathodic current density. At constant cathodic current density the etch pit density increases with temperature and decreases with external stress. Dislocations at which the excess etch pits form penetrate into the iron at a rate proportional to the hydrogen activity and the square root of time. Effects of prior hydrogen deposition on the shape of etch pits are seen at depths greater than the penetration depth of hydrogen generated dislocations. Changes of etch pit shape similar to those produced by hydrogen are also found when external stress is applied.The results are compared to Prussin's theory in which the assumption is made that stresses accompanying diffusion of an impurity are fully relieved by plastic deformation and formation of dislocations for stresses exceeding a critical stress. While some of the predictions of the theory are met by the experiments, the dislocations penetrate into the iron much slower than diffusion of hydrogen, since dislocations cannot move fast enough, i.e. stresses are not fully relieved.     

Polycrystal creep in the microplastic stress range

A microcreep theory has been developed for polycrystalline materials, which incorporates the microplastic strain in the aggregate on static loading. Experiments have been performed on the creep laws for polycrystalline metals and alloys, and it is found that the theory fits the experiments. The mobile-dislocation density decreases during microcreep.V. D. Kuznetsov Siberian Technical Physics Institute, Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 25–29, June, 1993.     

Deformation behaviour of body centered cubic iron nanopillars containing coherent twin boundaries

Molecular dynamics simulations were performed to understand the role of twin boundaries on deformation behaviour of body-centred cubic (BCC) iron (Fe) nanopillars. The twin boundaries varying from 1 to 5 providing twin boundary spacing in the range 8.5–2.8 nm were introduced perpendicular to the loading direction. The simulation results indicated that the twin boundaries in BCC Fe play a contrasting role during deformation under tensile and compressive loadings. During tensile deformation, a large reduction in yield stress was observed in twinned nanopillars compared to perfect nanopillar. However, the yield stress exhibited only marginal variation with respect to twin boundary spacing. On the contrary, a decrease in yield stress with increase in twin boundary spacing was obtained during compressive deformation. This contrasting behaviour originates from difference in operating mechanisms during yielding and subsequent plastic deformation. It has been observed that the deformation under tensile loading was dominated mainly by twin growth mechanism. On the other hand, the deformation was dominated by nucleation and slip of full dislocations under compressive loading. The twin boundaries offer a strong repulsive force on full dislocations resulting in the yield stress dependence on twin boundary spacing. The occurrence of twin–twin interaction during tensile deformation and dislocation–twin interaction during compressive deformation has been discussed.     

Elasticity and inelasticity of biomorphic silicon carbide ceramics

The effect of the vibration strain amplitude on the Young modulus and ultrasonic absorption (internal friction) in biomorphic SiC ceramics is investigated in the temperature range 116–296 K. The biomorphic SiC ceramics is prepared through pyrolysis of eucalyptus with subsequent infiltration of silicon. It is demonstrated that the vibration loading of samples in air and under vacuum is accompanied by a number of unexpected effects. The behavior of the studied ceramics is governed by at least two mechanisms, which, to a large extent, are responsible for the elastic and inelastic properties of the material. One mechanism is associated with the adsorption-desorption of environmental molecules (hypothetically, owing to the presence of pores and residual carbon), and the other mechanism involves microplastic deformation due to the motion of dislocations or other (similar) structural units.     

Mechanoluminescence of coloured KCl crystals

The gamma-irradiated KCl crystals exhibit mechanoluminescence during elastic, plastic and fracture deformation. The mechanoluminiscence (ml) intensity varies linearly with the number of newly-created dislocations and decreases with successive application and release of uniaxial pressure. The totalml intensity increases with applied pressure as well as with the temperature of the crystals. On the basis of the movement of the dislocations, the pressure and temperature dependence ofml is discussed.     

Emission of partial dislocations by grain boundaries in nanocrystalline metals

A theoretical model is proposed to describe the emission of partial dislocations by grain boundaries in nanocrystalline materials during plastic deformation. Partial dislocations are assumed to be emitted during the motion of grain-boundary disclinations, which are carriers of rotational plastic deformation. The ranges of the parameters of a defect structure in which the emission of partial dislocations by grain boundaries in nanocrystalline metals are energetically favorable are calculated. It is shown that, as the size of a grain decreases, the emission of partial dislocations by its boundary becomes more favorable as compared to the emission of perfect lattice dislocations.     

Low-temperature plasticity in nanocrystalline titanium and copper

The stress-strain compressive curves, temperature dependences of the yield stress, and small-inelastic-strain rate spectra of coarse-grained and ultrafine-grained (produced by equal-channel angular pressing) titanium and copper are compared in the temperature range 4.2–300 K. As the temperature decreases, copper undergoes mainly strain hardening and titanium undergoes thermal hardening. The temperature dependences of the yield stress of titanium and copper have specific features which correlate with the behavior of their small-inelastic-strain rate spectra. Under the same loading conditions, the rate of microplastic deformation of ultrafine-grained titanium is lower than that of coarse-grained titanium and the rate peaks shift toward high temperatures. The deformation activation volumes of titanium samples differing in terms of their grain size are (10–35)b ³, where b is the Burgers vector magnitude. The dependences of the yield stress on the grain size at various temperatures are satisfactorily described by the Hall-Petch relation.     

Damping caused by fatigue in porous 316L steel

An analysis of the effect of microcracks in porous metals on amplitude-dependent internal friction (ADIF) is carried out. A mathematical model is developed, which takes into account an initial distribution of microcracks lengths defining subsequent crack growth during cycling. Using the Dugdale model, the linear density of microplastic zones at the tips of the microcracks is obtained as a function of applied stress and porosity. Resulting simulations of internal friction are compared with corresponding experimental data.     

温度对超薄铜膜疲劳性能影响的分子动力学模拟

用嵌入原子势的分子动力学方法模拟了温度对超薄铜膜疲劳性能的影响. 通过模拟, 首先给出了超薄铜膜的总能及应力随循环周次的变化曲线; 根据叠加经验式得出的叠加量随循环周次变化曲线, 判断出各种恒定温度下超薄铜膜的疲劳寿命. 由 200–400 K温度范围内超薄铜膜的疲劳寿命-温度变化曲线, 可以发现存在两个温度区域: 在约370 K以下, 超薄铜膜的疲劳寿命随温度升高缓慢增加, 而在约370 K以上增加较快. 建立了模型并用位错演化机制解释了超薄铜膜疲劳寿命的温度依赖关系. 关键词： 分子动力学 疲劳 温度效应 位错     

液氮冲击中锑化铟焦平面探测器形变研究

液氮冲击中锑化铟红外焦平面探测器(InSb IRFPAs)的形变研究对理解探测器结构设计可靠性、预测探测器耐冲击寿命具有重要意义.在系统分析液氮冲击结束时模拟得到的InSb IRFPAs形变分布与方向的基础上,提出了降温过程中累积热应变完全弛豫的设想,升至室温后的模拟结果重现了室温下拍摄的InSb IRFPAs典型形变分布特征.经分析认为室温下拍摄的InSb IRFPAs表面屈曲变形源于底充胶固化中引入的残余应力应变,变形幅度随降温过程逐步减弱,至77 K时完全消失,升温过程则依据弹性变形规律复现典型棋盘格屈曲模式.这为后续InSb IRFPAs结构设计、优化及耐冲击寿命预测提供了理论分析基础.     

Theory of microplastic deformation of polycrystals in the stage of lüders band nucleation

This paper develops a model of microplastic deformation of a polycrystalline aggregate in the stage of Lüders band nucleation. The model is used in constructing a theory of microplastic deformation of polycrystals and in calculating the load diagram in the first stage of microdeformation; the proposed theory of a linear stress-strain relation is in accord with the experimental data. An expression is obtained for the coefficient of hardening in the first stage of microdeformaiton.Translated from Izvestiya Vysshkikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 14–18, October, 1981.     

Emission of Dislocation Loops from Nanovoids in an FCC Crystal Subjected to Shear Deformation under Post-Cascade Shock Waves

The method of molecular dynamics is applied to the study of the effect of post-cascade shock waves generated in a solid irradiated by high-energy particles on the heterogeneous formation of dislocation loops in a simulated gold crystal containing a spherical nanovoid, which is subjected to shear deformation. The interaction between atoms is described with the use of a potential calculated by the embedded atom method. Shock waves are created by assigning a velocity exceeding the speed of sound in the simulated material to the boundary atoms of the computational cell. It is shown that two regions of increased mechanical stress are formed under shear deformation near the surface of a nanovoid, which are the sources of emerging partial dislocations. The main mechanism for the formation of dislocations is the displacement of a group of atoms towards the inner surface of the void, which does not contradict modern ideas about the heterogeneous formation of dislocations. It is shown that, when the values of shear stress are insufficient for the formation of dislocations, loop emission can be initiated by a post-cascade shock wave generated in the computational cell. As temperature increases, the number of nucleated dislocation loops increases, and, in addition, the formation of Lomer–Cottrell dislocations is observed, which is attributed to the additional tangential stresses created by the unloading wave. In this case, the formation of a stable dislocation loop in which the linear tension is balanced by the Peach–Koehler force due to external stress requires that the shock wave front affect the regions of increased stress near the void surface while propagating through the simulated crystal.     

Dislocation-crack interaction

In this paper, recent developments in the understanding of the dislocation-crack interaction and its relationship to the phenomena of crack tip deformation and fracture toughness are reviewed. An enhanced research activity in this area began with successful observations of the behavior of crack tip dislocations by various techniques, namely etch pits technique, X-ray topography and transmision electron microscopy. The advantages and limitations of these techniques are compared and the information obtained from these experiments are discussed. The results show that dislocations are emitted from a crack tip when the applied stress is sufficiently high. During crack propagation, dislocations are also generated from other bulk sources and the number of these dislocations relative to those from the crack tip may be an important parameter. The elastic theory of the interaction between dislocations and a crack is presented by considering the force on the dislocations. The theory is applied to derive a dislocation emission condition, which may be expressed in terms of a critical stress intensity factor. It is concluded that the dislocations emitted from a crack tip are repelled from the crack tip and this repulsive interaction is responsible for the formation of a dislocation-free zone. These dislocations shield the crack tip from the applied stress and hence contribute to an increase in the fracture toughness. The physical origin of the dislocation-free zone lies in the presence of a barrier to dislocation emission from the crack tip. One of the barriers to dislocation emission is the image stress. With the dislocation-free zone, the crack tip can maintain a finite stress intensity factor following crack tip deformation. The lattice theories of dislocation-crack interaction indicate that the results are consistent with those of the continuum theory.     

On the core width and Peierls stress of bubble rafts dislocations within the framework of modified Peierls-Nabarro model

Using Foreman’s method, the core structure and Peierls stress of dislocations in bubble rafts have been investigated within the framework of the modified Peierls-Nabarro (P-N) model in which the discrete lattice effect is taken into account. The core width obtained from the modified P-N model is much wider than that from the P-N model owing to the discrete lattice effect. It is found that the core width of dislocation increases with a decrease of the bubble radius. The elastic strain energy associated with the discrete effect is considered while calculating the Peierls stress. The new expression of the Peierls stress obtained in this paper is not explicitly dependent on the particular form of the restoring force law, which is only related to the core structure parameter and can be used expediently to predict the Peierls stress of dislocations. The Peierls stress decreases rapidly with the decrease of the bubble radius.