The physical interpretation of the parameters measured during the tensile testing of materials at elevated temperatures

Hot tensile (or compression) testing, where the stress developed in a material is measured under an imposed strain rate, is often used as an alternative to conventional creep testing. The advantages of the hot tensile test are that its duration can be more closely controlled by the experimenter and also that the technique is more convenient, since high precision testing machines are available. These factors can be particularly important when extensive testing programmes of radioactive samples are involved. The main disadvantage is that the interpretation of results is more complex. Confusion can easily arise when attempts are made to extend the use of parameters which satisfactorily categorize behaviour at lower temperatures, into regimes where concurrent thermal recovery can occur. The present paper relates the parameters which are measured in hot tensile tests, to physical processes which occur in materials deforming by a variety of mechanisms. For cases where no significant structural changes occur, as in viscous or superplastic flow, analytical expressions are derived which relate the stresses measured in these tests to material constants. When deformation is controlled by recovery processes, account has to be taken of the structural changes which occur concurrently. A wide variety of behaviour may then be exhibited which depends on the initial dislocation density, the presence of second-phase particles and the relative values of the recovery rate parameters and the velocity imposed by the testing machine. Numerical examples are provided for simple recovery models.     

Negative creep and recovery during high-temperature creep of MgO single crystals at low stresses

High-temperature creep equipment with very high precision has been used to measure the creep of MgO single crystals above 1948 K and stresses lower than 4 MPa. A transition in exponent,n, from 3 at stresses higher than 2 MPa to almost unity at lower stress region was observed. Since in a single crystal deformation can only occur by the generation and movement of dislocations, the transition in stress exponent from high to low stress region cannot be interpreted in terms of a change from dislocation to diffusional creep processes. Decreasing the stress by a small amount during steady-state creep resulted in an incubation period of zero creep rate before creep commenced at lower stress. However, large stress reduction led to a period of negative creep during which the dislocation substructure coarsens and the subgrain cell boundaries straighten. On the basis of dislocation substructure studies, it is proposed that the kinetics of backflow are thought to be based on the local network refinement caused by the reverse movement of dislocations and that recovery is necessary before further movement of dislocation can occur. It is shown that the network theory proposed by Davis and Wilshire can satisfactorily account for all stress reduction observed during forward creep.     

The incorporation of ambipolar diffusion in deformation mechanism maps for ceramics

Ambipolar diffusion gives rise to four distinct types of diffusion creep in ceramic materials, depending on whether the processes of lattice and grain-boundary diffusion creep are controlled by the anions or cations, respectively. These four processes are incorporated in a deformation mechanism map for diffusion creep in pure Al₂O₃, using a new form of map which is independent of the selected stress level. This map may be used to determine the rate-controlling mechanism for diffusion creep under any selected experimental conditions. By superimposing dislocation creep on to the map, it is possible to estimate the highest permissible stress and the lowest feasible temperature for experimental observation of any of the diffusion creep processes.     

A high-temperature deformation model based on dislocation movement for wrought aluminium alloys

This paper presents a high-temperature deformation model based on dislocation movement for wrought aluminium alloys, which can exhibit the dynamic recovery and dynamic recrystallisation processes of a wrought aluminium alloy at the same time. In the model, work hardening corresponds to the increase of dislocation density. Dynamic recovery occurs in two ways, namely by the condensation of dislocations into new low-angle boundaries, and by the absorption of dislocations into pre-existing boundaries. High- and low-angle boundaries disappear by the sweeping of high-angle boundary migration. The prediction of the model is presented for the high-temperature deformation of the 7050 aluminium alloy. Predicted true stress–strain curves and microstructure evolution from the model are consistent with experimental data.     

Effects of cyclic stress on the creep behaviour and dislocation microstructure of pure copper in the temperature range 0.4 to 0.5T m

The creep deformation behaviour of polycrystalline pure copper under static and cyclic stress was studied in the temperature range 0.4 to 0.5T _m. Both cyclic creep acceleration and retardation occurred depending on the condition of peak stress and temperature combination. The comparison of dislocation microstructures, developed during steady state static and cyclic creep deformation, has also been performed to determine the effect of cyclic stress on the dislocation microstructure and evidence for the enhanced recovery of the cell wall under cyclic stress was found. These effects of cyclic stress on the creep rate and dislocation microstructure were interpreted on the basis of diffusion-controlled recovery creep theory and the cyclic creep acceleration mechanism is suggested as the enhanced recovery of the cell wall with the help of athermally generated excess vacancies.     

Internal stress and unloading experiments in creep

Internal stresses are developed during deformation and have an important role in determining the mechanical properties and, in particular, the creep properties of crystalline materials. The strain transient dip test is the generally accepted method for the determination of internal stresses developed during creep. The strain transient dip test has been analysed using a number of very general creep models and it is concluded that, for glide-controlled creep, the dip test can only be interpreted if the relation between dislocation velocity and the force on the dislocation is linear. When this is the case it measures not an average internal stress but an average back stress for all the dislocations, mobile and immobile, where the back stress is the resolved component of the internal stress plus the glide component of the line tension force divided by the Burgers vector. The dip test does not allow separation of the back stress into internal stress and line tension components. For recovery models the results of the dip test cannot be simply interpreted because expressions for the creep rate do not define a unique average internal stress or back stress. However, for the recovery model in which strain occurs by athermal or jerky glide there will be a reverse yield stress, i.e. there will be a stress reduction below which there will be instantaneous reverse strain followed by reverse creep. By averaging the instability condition for all the dislocations participating in jerky glide it is shown, subject to assumptions, that the sum of the average internal stress experienced by dislocations involved in both forward and reverse creep can be obtained from the reverse yield stress. Separate values for these internal stresses cannot be obtained, however. Determination of the reverse yield stress for recovery creep is the experiment equivalent to the strain transient dip test for glide-controlled creep.Research visitor from University of Aix-Marseille 11, France.     

Ti65合金的初级蠕变和稳态蠕变

使用透射电镜(TEM)研究了Ti65合金在600~650℃、120~160 MPa条件下的蠕变变形行为及其微观变形机制。结果表明：初级蠕变变形机制主要由受攀移控制的位错越过α₂相的过程主导;稳态蠕变阶段蠕变机制主要由受界面处扩散控制的位错攀移的过程主导,且应力指数为5~7。在初级蠕变阶段α₂相与位错的相互作用是α₂相对合金高温强化的主要方式,在稳态蠕变阶段沿α/β相界分布的硅化物阻碍位错运动与限制晶界滑移是硅化物对合金强化的主要方式。     

High-Temperature Physicochemical Mechanics of Materials

We propose to regard investigations in the field of high-temperature strength of structural metals and alloys interacting with corrosive media as high-temperature physicochemical mechanics of materials (HTPCMM). The most important feature of HTPCMM is the principle of correlation between deformation processes and physicochemical phenomena, which allows one to describe the features and regularities of changes in the properties of materials under service conditions most completely and correctly. We emphasize the most important role of diffusion as a controlling factor in a metal–medium system at high temperatures. Results of analytical investigations aimed at the development and investigation of physicomathematical models of elastic and elastoviscous multicomponent solid solutions with inherent degradation processes (accumulation of damage) are presented. The fact that, as a rule, these models are constructed within the framework of continuum mechanics on the basis of principles of nonequilibrium mechanics is noted Experimental data obtained, in particular, on refractory metals and titanium interacting actively with components of a vacuum or an inert atmosphere testify to the intensification of saturation of metals by interstitial impurities under conditions of long-term loading and to significant changes in the character of their creep, namely, under the influence of oxygen diffusion the creep rate decreases as stresses increase.     

Creep behaviour of a heat-resistant ferritic chromium steel in terms of stress exponents

In many publications the high-temperature deformation behaviour of materials is described by the stress sensitivity of steady-state creep rate, the creep exponent, n. In order to investigate the mechanisms of dislocation motion, it is more promising to evaluate the constant structure creep properties. This leads to the constant structure creep exponent, m, which is not influenced by the stress dependence of the substructure. Therefore, the investigation of deformation mechanisms is less difficult. Additionally, m is the basis for the calculation of the effective stress exponent, m, of dislocation velocity, which permits the investigation of the strength of interactions between alloying atoms and moving dislocations. It is shown that the creep exponent, n, is between 5 and 10 in the power-law creep region (where diffusion-controlled glide processes of dislocations cause deformation). However, it increases to about 50, if exponential creep is working (in this region the glide processes are thermally activated but diffusion is not the rate-controlling mechanism). The constant structure creep exponent, m, is relatively small and independent of stress in the power-law creep region. It increases almost linearly with the applied stress, if thermally activated glide dominates creep. The evaluation of the stress exponent, m, which can be calculated from m and the effective stresses, showed that dislocation motion is influenced by alloying atoms as long as power-law creep works. There is experimental evidence that power-law breakdown is due to a breakdown of the alloying effect, because dislocations can escape from their dragging Cottrell clouds at high applied stresses.     

Procedure of Determining Creep and Creep-Rupture Strength Parameters for Isotropic Materials under Nonisothermal Loading

A procedure has been developed for determining the parameters of creep and creep-rupture strength that appear in constitutive equations of thermoviscoplasticity for describing nonisothermal processes of deformation and damage accumulation in isotropic materials due to creep. The procedure is tried out using a high-temperature chromium-nickel alloy ÉI437.     

Microstructure evolution and deformation features of AZ31 Mg-alloy during creep

By means of the measurement of the creep curve and the observation of SEM and transmission electron microscope (TEM), an investigation has been made into the microstructure evolution and deformation features of AZ31 Mg-alloy during high temperature creep. Results show that the deformation features of the alloy in the primary stage of creep are that significant amount of dislocation slips are activated on basal and non-basal planes, then these ones are concentrated into the dislocation cells or walls as creep goes on. At the same time, twinning occurs as an additional deformation mechanism in the role of the compatibility stress. During steady state creep, the dislocation cells are transformed into the subgrains, then, the protrusion and coalition of the sub-boundaries results in the occurrence of dynamic recovery (DRV). After the dynamic recrystallization (DRX), the multiple slips in the grain interiors are considered to be the main deformed mechanism in the later stage of the steady state creep. An obvious feature of creep entering the tertiary stage is that the cracks appear on the locations of the triple junction. As creep continues, the cracks are viscous expanded along the grain boundaries; this is taken for being the fracture mechanism of the alloy crept to failure. The multiple slips in the grain interiors and the cracks expanded viscous along the grain boundary occur in whole of specimens, that, together with the twins and dynamic recrystallization, is responsible for the rapid increase of the strain rate in the later stage during creep.     

Power-law and exponential creep in class M materials: discrepancies in experimental observations and implications for creep modeling

This paper discusses our current understanding of the processes thought to be dominant in the exponential creep regime as well as the implications for creep modeling relating to both power-law and exponential creep regions. The significance and implications of creep controlled by vacancy diffusion along dislocation cores are discussed. It is pointed out that creep substructures, other than subgrains, have been reported in the literature, and a bifurcation diagram is presented to demonstrate how this evolution can occur from an initially homogeneous dislocation substructure. The use of nonlinear dislocation dynamics in creep modeling is advocated to rationalize the observed diversity in the creep substructures. It is demonstrated that the dislocation substructure evolution models can be coupled with a viscoplastic model through the volume fractions of the ‘hard’ and ‘soft’ phases. This coupling is shown to lead to the stress-subgrain size relationship in a simple and a natural way.     

Creep and recovery of nonlinear viscoelastic materials of the differential type

In this work, the creep and recovery properties of rubberlike viscoelastic materials in simple shear are studied by two special constitutive equations for isotropic, nonlinear incompressible viscoelastic material of the differential type. The creep and recovery processes are of significant importance to both the mechanics analysis and engineering applications. The constitutive equations introduced in this work generalize the Voigt-Kelvin solid and the 3-parameter model of classical linear viscoelasticity. They describe the uncoupled non-Newtonian viscous and nonlinear elastic response of an isotropic, incompressible material. The creep and recovery processes are treated for simple shear deformation superimposed on a longitudinal static stretch. Closed form solutions are provided and both processes are described effectively by the exponential function.     

Atomistic simulation and modeling of localized shear deformation in metallic glasses

Since the end of 1980s, bulk metallic glasses became available for various multi-component alloys. Because bulk metallic glasses are applicable to structural materials, their mechanical properties have become a matter of great interest in these decades. A characteristic feature of plastic deformation of metallic glasses at the ambient temperature is the localized shear deformation. Since we have no appropriate experimental technique, unlike crystalline matter, to approach microscopic deformation process in amorphous materials, we have to rely on computer simulation studies by use of atomistic models to reveal the microscopic deformation processes. In this article, we review atomistic simulation studies of deformation processes in metallic glasses, i.e., local shear transformation (LST), structural characterization of the local shear transformation zones (STZs), deformation-induced softening, shear band formation and its development, by use of elemental and metal-metal alloy models. We also review representative microscopic models so far proposed for the deformation mechanism: early dislocation model, Spaepen’s free-volume model, Argons’s STZ model and recent two-state STZ models by Langer et al.     

基于分数阶黏弹性模型的木塑复合材料蠕变/回复性能分析

木塑复合材料(WPC)是一种木质纤维增强聚合物的新型环保复合材料,为分析WPC在非恒定荷载下的变形行为,进行结构的长期变形设计,对WPC的蠕变/回复变形进行计算分析。采用叠加原理对比分析既有蠕变计算模型对WPC蠕变/回复的整体预测效果。结果表明,现有模型均不能良好预测其蠕变/回复行为。采用基于分数阶微积分的黏弹性模型对其蠕变/回复行为进行预测,提出一种双参数法的修正分数阶黏弹性模型。通过与已有实测数据对比表明,该模型能够准确反映WPC的静态黏弹性行为。结合实验数据,给出了不同WPC蠕变/回复模型的参数取值。      

Powder compaction interpretation using the power law

The effects of strain hardening and axial strain rate, over a wide range of rates (10^–3 to 10⁵ s^–1), on the compaction properties of a variety of pharmaceutical powders have been investigated. The powders tested are: Di Pac sugar, paracetamol d.c., Avicel and lactose. These materials have been assessed using the constants derived from the power law as a criterion to describe their behaviour. All the materials tested show, with varying degrees, a non-linear increase in the yield pressure (flow stress), the constantG, the strain rate exponentm and the strain hardening exponentn as the strain rate increases. These variations are more clear in the materials known to deform plastically, such as Avicel. This is attributable to a change either from ductile to brittle behaviour or a reduction in the amount of plastic deformation due to the time-dependent nature of the plastic flow. This, however, is explained in terms of dislocation and diffusion processes involved in the plastic deformation mechanisms during the compaction process. As the speed of compaction increases the characteristics of deformation, including the value of the strain rate exponent, the shape of the creep curve and the nature of creep rate, suggest that the creep behaviour is therefore controlled by some form of diffusion process. Meanwhile, the creep characteristics of the low and medium rate tests appear to be consistent with dislocation climb and viscous glide. For the materials tested, Avicel is found to be the most strain-rate sensitive material, while paracetamol d.c. is found to be the least strain-rate sensitive material.     

Quasistatisches Verformungsverhalten reiner Sintereisen im Temperaturbereich von –184 bis 600°C

Quasistatic deformation behaviour of pure sintered iron in the temperature range between –184 and 600°C The deformation behaviour of pure sintered iron materials with densities between 6,88 und 7,57 g/cm³ was investigated in tension tests in the temperature range of –184 and 600°C. Supplementary compression tests were carried out at 20°C. Increasing density leads to increasing material resistances and ductility properties due to the increase of the bearing specimen cross sections as well as due to smaller numbers of pores, more spherical pores with smaller notch effects and smaller numbers of mircocracks, which are initiated at pores. After equal deformations, due to pore closing effects and the impediment of crack initation, the flow stresses of compressively deformed specimens are larger than those of tensily deformed. The deformation behaviour is dominated at low temperatures by thermal activated glide processes of dislocations and their interactions with short range obstacles, at middle temperatures by dynamic strain ageing due to elastic interactions of glide dislocations and diffusing carbon atoms and at high temperatures by recovery controlled dislocation creep processes.     

Grain boundary sliding in nanomaterials at elevated temperatures

The unique deformation behavior of nanocrystalline materials is considered to be caused by suppression of conventional lattice dislocation slip (which dominates in coarse-grained materials) and effective action of alternative deformation mechanisms occurring through motion of grain boundary defects. A significant role of grain boundary sliding in deformation processes in nanocrystalline materials was shown in models and was revealed experimentally.     

Dislocation network in additive manufactured steel breaks strength–ductility trade-off

Most mechanisms used for strengthening crystalline materials, e.g. introducing crystalline interfaces, lead to the reduction of ductility. An additive manufacturing process – selective laser melting breaks this trade-off by introducing dislocation network, which produces a stainless steel with both significantly enhanced strength and ductility. Systematic electron microscopy characterization reveals that the pre-existing dislocation network, which maintains its configuration during the entire plastic deformation, is an ideal “modulator” that is able to slow down but not entirely block the dislocation motion. It also promotes the formation of a high density of nano-twins during plastic deformation. This finding paves the way for developing high performance metals by tailoring the microstructure through additive manufacturing processes.