Recovery during stress relaxation in Cd-Ag alloy single crystals

Stress relaxations in a series of cadmium-silver alloy single crystals containing up to 0·25 at% Ag were carried out at temperatures of 77 K, 145 K and 200 K. The samples were deformed in tension at a strain rate of 1×10^–4 s^–1. When the specimens deformed at 145 K and 200 K were reloaded after stress relaxation, the deformation started with the flow stress which was lower than that before stress relaxation. The observed differences in the stresses could be due to a variation in the internal stress. The activation volume should be estimated from the part of stress relaxation in which the internal stress remains practically constant.     

Influence of molecular relaxation processes on mechanical properties of polyvinyl chloride in tensile experiments

The relation between the molecular relaxation processes and the results of tensile experiments has been studied for hard and toughened polyvinyl chloride (PVC). The relaxation spectra have been determined between 10^?4 and 10⁺⁴ sec by means of stress relaxation, flexural vibration experiments, and tensile tests. Yield stress, yield strain, rupture stress, rupture strain, and rupture energy have been determined at 23° and 50° C as functions of yield time or rupture time, respectively, over a time-range from 10^?2 to 10⁺⁴ sec approximately. It is shown that the molecular relaxation processes observed in the range of small deformations also influence the behavior in the nonlinear range, especially the elongation at rupture in tensile experiments. In addition, effects of the molecular processes on the yield stress, the rupture stress, and, consequently, the rupture energy have been observed. These effects are due to the time- and temperature-dependent decrease of the macroscopic stress and of the microscopic notch stresses by certain relaxation mechanisms. The most suitable characteristic parameters in tensile experiments are the yield stress and the rupture strain. It is pointed out that the usefulness of a single value of a mechanical property as determined in a standard “one-point” test is very restricted.     

The role of internal stresses on the plastic deformation of the Al–Mg–Si–Cu alloy AA6111

In this work, we have investigated the internal stress contribution to the flow stress for a commercial 6xxx aluminium alloy (AA6111). In contrast to stresses from forest and precipitation hardening, the internal stress cannot be assessed properly with a uniaxial tensile test. Instead, tension–compression tests have been used to measure the Bauschinger stress and produce a comprehensive study which examines its evolution with (i) the precipitation structure, and (ii) a wide range of applied strain. A large set of ageing conditions was investigated to explore the effect of the precipitation state on the development of internal stress within the material. It is shown that the Bauschinger stress generally increases with the applied strain and critically depends on the average radius of the precipitate and is thus linked to the shearable/non-shearable transition. Further work in the case of non-shearable particles shows that higher strain eventually leads to particle fracture and the Bauschinger stress then rapidly decreases. Following the seminal work of Brown et al. a physically based approach including plastic relaxation and particle fracture is developed to predict the evolution of the internal stress as a function of the applied strain. Knowing the main characteristics of the precipitation structure–such as the average precipitate radius, length and volume fraction–allows one to estimate accurately the internal stress contribution to the flow stress with this model.     

Photostimulated relaxation of internal mechanical stresses in epitaxial SOS structures

The nature of internal mechanical stresses in the thin silicon-on-sapphire (SOS) epitaxial films is studied, and their value is estimated. They are ～10¹⁹ Pa and have a compressive character. The effects of pulsed laser and lamp annealings on stress relaxation are analyzed, and stress relaxation is shown to reach 90% under certain annealing conditions. An electron mechanism, which is based on a change in the quantum state of the electron subsystem of a crystal during pulsed photon annealing, is proposed for annealing of structural defects.     

冲击加载下Zr51Ti5Ni10Cu25Al9金属玻璃的塑性行为

进行了10—27 GPa应力范围内Zr₅₁Ti₅Ni₁₀Cu₂₅Al₉金属玻璃的平面冲击实验以研究其高压-高应变率加载下的塑性行为.由样品自由面粒子速度剖面的分析获得了冲击加载过程的轴向应力,并通过轴向应力与静水压线的比较获得剪应力.实验结果表明,尽管存在明显的松弛效应,但Zr基金属玻璃的Hugoniot弹性极限随着冲击应力的增加而增加.然而,塑性波阵面上的剪应力则显示先硬化而后软化现象,而且软化的幅度随冲击应力的增加而增加.冲击加载下Zr基金属玻璃的上述剪应力变化特征与分子动力学模拟结果比较一致,但与压剪实验结果和一维应力冲击实验结果明显不同.     

On the homogeneous nucleation and propagation of dislocations under shock compression

The dynamic response of crystalline materials subjected to extreme shock compression is not well understood. The interaction between the propagating shock wave and the material’s defect occurs at the sub-nanosecond timescale which makes in situ experimental measurements very challenging. Therefore, computer simulation coupled with theoretical modelling and available experimental data is useful to determine the underlying physics behind shock-induced plasticity. In this work, multiscale dislocation dynamics plasticity (MDDP) calculations are carried out to simulate the mechanical response of copper reported at ultra-high strain rates shock loading. We compare the value of threshold stress for homogeneous nucleation obtained from elastodynamic solution and standard nucleation theory with MDDP predictions for copper single crystals oriented in the [0 0 1]. MDDP homogeneous nucleation simulations are then carried out to investigate several aspects of shock-induced deformation such as; stress profile characteristics, plastic relaxation, dislocation microstructure evolution and temperature rise behind the wave front. The computation results show that the stresses exhibit an elastic overshoot followed by rapid relaxation such that the 1D state of strain is transformed into a 3D state of strain due to plastic flow. We demonstrate that MDDP computations of the dislocation density, peak pressure, dynamics yielding and flow stress are in good agreement with recent experimental findings and compare well with the predictions of several dislocation-based continuum models. MDDP-based models for dislocation density evolution, saturation dislocation density, temperature rise due to plastic work and strain rate hardening are proposed. Additionally, we demonstrated using MDDP computations along with recent experimental reports the breakdown of the fourth power law of Swegle and Grady in the homogeneous nucleation regime.     

Grain boundary segregation and relaxation in nano-grained polycrystalline alloys

The present study carries out systematic thermodynamics analysis of Grain Boundary(GB)segregation and relaxation in NanoGrained(NG)polycrystalline alloys.GB segregation and relaxation is an internal process towards thermodynamic equilibrium,which occurs naturally in NG alloys without any applied loads,causes deformation and generates internal stresses.The analysis comprehensively investigates the multiple coupling effects among chemical concentrations and mechanical stresses in GBs and grains.A hybrid approach of eigenstress and eigenstrain is developed herein to solve the multiple coupling problem.The analysis results indicate that the GB stress and grain stress induced by GB segregation and relaxation can be extremely high in NG alloys,reaching the GPa level,which play an important role in the thermal stability of NG alloys,especially via the coupling terms between stress and concentration.The present theoretic analysis proposes a novel criterion of thermal stability for NG alloys,which is determined by the difference in molar free energy between a NG alloy and its reference single crystal with the same nominal chemical composition.If the difference at a temperature is negative or zero,the NG alloy is thermal stable at that temperature,otherwise unstable.     

The synergetic theory of the glass transition in liquids

The glass transition is treated as a spontaneous emergence of the shear components of strain and stress elastic fields upon cooling a liquid at a rate exceeding the critical value. The stationary elastic strains and stresses and the effective relaxation time are determined within the adiabatic approximation. It is shown that the glass transition process occurs through the mechanism of a first-order kinetic transition with allowance made for the strain dependence of the shear modulus. The critical cooling rate turns out to be proportional to the thermal diffusivity and unrelaxed shear modulus and inversely proportional to the temperature derivative of the relaxed shear modulus and the square of the heat conductivity length of the sample.     

低频内耗测量时标准滞弹性固体的内耗行为

用三参量力学模型的恒应力和恒应变弛豫时间描述了标准滞弹性固体的内耗行为.在低频内耗测量时,振动系统的惯量不可避免地影响测量的内耗值,当材料的内耗较大和测量频率与振动系统的共振频率可以比较时,惯量对内耗测量的影响较大,这时惯量的影响不可忽略. 关键词：     

Defect formation processes and their influence on mechanical stresses in green glow gallium phosphide structures

Defect formation and their influence on internal mechanical stresses in epitaxial gallium phosphide layers grown from a gallium melt with the addition of finely-dispersed gallium nitride particles in an atmosphere of hydrogen with ammonia are investigated in this paper. It is established that an increase from 0.04 to 0.1 vol. % in the ammonia content in the gas mixture will result in growth in the quantity of defects, in particular, inclusions of the second phase, as well as internal mechanical stresses, while for an NH₃ content greater than 0.1 vol. % – in the formation of shallow cracks and stress relaxation. The dependence between the internal mechanical stresses, the volume fraction, and the dispersion of the GaN inclusions in gallium phosphide is shown. The results obtained are discussed within the framework of the proposed model.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 23–27, January, 1991.     

Origins of growth stresses in amorphous semiconductor thin films

Stress evolution during deposition of amorphous Si and Ge thin films is remarkably similar to that observed for polycrystalline films. Amorphous semiconductors were used as model materials to study the origins of deposition stresses in continuous films, where suppression of both strain relaxation and epitaxial strain inheritance provides considerable simplification. Our data show that bulk compression is established by surface stress, while a subsequent return to tensile stress arises from elastic coalescence processes occurring on the kinetically roughened surface.     

The relaxation-element method in models of the plastic strain of structurally inhomogeneous materials

It is shown that it is possible to represent a continuous field of plastic strain with high gradients in local regions of a material by methods of the continual theory of defects. A practical method of constructing highly inhomogeneous fields of plastic strain in local regions of a solid under load is proposed, and the stress fields connected with them are determined. The method is used to propose a principle of a unique connection between the process of inelastic deformation and stress relaxation in local volumes of the material. This enables one to simulate the inhomogeneous fields of plastic strain and stresses connected with the external stress without discontinuities and singularities at the interfaces between the phases and crystallites.Institute of Physics of the Strength and Study of Materials, Siberian Branch, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 16–22, February, 1994.     

Interferometric measurement of displacements and displacement velocities for nondestructive quality control

It is shown that the interferometric measurement of small displacements and small-displacement velocities can be used to determine internal stresses or the stresses induced by an applied load in solids and to control structural changes in them. The interferometric method based on the measurement of the reaction of a solid to a small perturbation in its state of stress is applied to determine stresses from the deviation of the reaction to perturbations from that in the standard stress-free case. For structural control, this method is employed to study the specific features of the characteristics of microplastic deformation that appear after material treatment or operation and manifest themselves in the temperature and force dependences of the rate of a small inelastic strain.     

Interfacial diffusion relaxation of internal stresses in solids

The relaxation of internal stresses due to interfacial diffusion in a two-phase solid is studied theoretically with the help of the onsageristic approach of irreversible thermodynamics. In this note we derive an expression for the rate at which internal stresses associated with misfit caused by bonding a flat surface of one material to a rough surface of another. The two phases are treated as isotropic clastic substances. It is assumed that the components of only of the solids are capable of leaving their positions of migrating along the interface. The driving force for this process is minimization of total energy-clastic plus interfacial energy. We show that the time constant for relaxing these stresses is proportional to the cube of the wavelength of the roughness.     

Surface deformation due to non-homogeneous distribution of solute atoms in crystals

Deformation caused by a modulated distribution of solute atoms, periodic in one dimension, in a plate-like crystal is studied within the framework of the isotropic theory of elasticity. The exact formulae for the strain and displacement field, which changes near the surface due to relaxation of the internal concentration stresses, can be used for determination of the distribution of the solute atoms by X-ray diffraction techniques.     

Stress relaxation through ageing heat treatment – a comparison between in situ and ex situ neutron diffraction techniques

The effect of ageing heat treatment on the relaxation of residual stress in a water quenched polycrystalline nickel-base superalloy has been measured using neutron diffraction. Two separate experiments have been conducted; the first experiment was an ex situ study in which samples were individually processed with varying degrees of age time before measurement. The second experiment was an in situ heat treatment, which required heating and then holding the sample at ageing temperature while measuring strain using neutron diffraction. The in situ experiment was carried out twice using the same setup to assess the repeatability of the technique and found to be repeatable within experimental error. The agreement between in situ and ex situ experiments was found to be reasonable, particularly the manner in which the stresses relaxed with time. In both studies it was found that initial stress relaxation was rapid, approximately 200 MPa in 15–30 min, after this a slower linear relaxation remained for the rest of the ageing heat treatment. This behaviour suggests creep may be the means by which stress relaxation takes place in this material during ageing.     

Crystal-amorphous and crystal-crystal phase transformations via virtual melting

A new mechanism of crystal (c)-amorphous (a) and c-c phase transformations (PTs) and internal stress relaxation via virtual melting (VM) induced by internal stresses was justified thermodynamically and kinetically. VM removes interface friction, reduces kinetic barrier, increases atomic mobility, and can reduce thermodynamic melting temperature. We combine VM and nonequilibrium PT diagrams to develop new scenarios of c-a and c-c PTs. Results are applied for a new interpretation of c-c and c-a PT mechanisms in ice Ih and are also applicable for other materials.     

Comparison of electron beam and pulsed laser irradiation effects in Fe80B20

The effects of high-energy electron beam and pulsed excimer laser irradiation on the magnetic properties of Fe₈₀B₂₀ amorphous alloy are investigated by means of transmission and conversion electron Mössbauer spectroscopy (CEMS) and scanning electron microscopy (SEM). Both types of irradiation were found to induce changes in the magnetic anisotropy of Fe₈₀B₂₀, but the influence of high-energy electron beam irradiation was weaker. Depending on the number of applied laser pulses and repetition rate, controlled magnetic anisotropy and/or relaxation of internal stresses could be obtained by excimer laser irradiation. The presence of molten zones subsequently solidified and of laser-induced internal stresses supports the model of closúre domain structure.     

Thermal evolution of residual stress in IN718 alloy subjected to laser peening

The thermal relaxation behaviors of residual stresses induced by laser peening (LP) in IN718 alloy were investigated using an integrated numerical simulation and experimental approach. LP and heat treatments (HT) were carried out after which the X-ray diffraction (XRD) technique was employed in measuring the residual stresses. Micro-structures were observed using an optic microscope (OM) and transmission electron microscope (TEM). Dislocations induced by LP were also observed by TEM and characterized using the XRD technique. The effects of the applied temperature and the exposure time on residual stress and micro-structures were investigated. The results show that the extent of the residual stresses relaxation increased accordingly with the increase in the applied temperature. The relaxation rate was initially high and tended to stabilize for a longer exposure time. Grain size evolution during the process was subsequently discussed. Furthermore, a conceivable mechanism of residual stresses thermal relaxation behavior was obtained.     

Radiation response of strained Si---SiO2 layered system

In the presented paper radiation effects in silicon-silicon dioxide systems in the presence of additional tensile and compressive stresses were investigated. These stresses were produced by placing additional layers of SiO₂ or Si₃N₄ on the back side of oxidized silicon wafers.The results obtained indicate that relaxation takes place in the oxide during irradiation and leads to the decrease of the compressive stress in the oxide film. Relaxation of strained bonds causes defect generation and the increase of the oxide charge density. This increase is larger for oxides with higher level of compressive stress. No significant changes of the density of surface states were observed since the additional coatings did not affect the strain gradient in the SiO₂.