Some aspects of the static and dynamic compressive behaviour of β-brass single crystals

The compressive behaviour of β-brass single crystals has been investigated in uninterrupted static and dynamic tests and in interrupted tests using static-static, static-dynamic, dynamic-dynamic and dynamic-static loading sequences. Static and dynamic strain-rates were 1.5×10^?4 and 3.2×10³ sec^?1 respectively. Slip traces on the statically deformed crystals were wavy and deformation occurred by single slip on either (ī01) [111] or (¯211) [111] or by a transition mode involving both (ī01) [111] and (¯211 [111]. Except for anomalous behaviour in the dynamic reload following static preload the dynamic slip traces were straight with deformation occurring by multiple slip on four {110} planes involving two 〈111〉 directions. It is shown that there is no direct causal relationship between the lower work-hardening rate and level of flow stress and the crystallography of slip in dynamic deformation. The work-hardening rate and flow stress in static and dynamic loading are rather determined by the dynamics of the deformation. The differences in the substructural features as observed by transmission electron microscopy arise principally from the differences in the slip modes and cannot be interpreted as controlling the stress-strain behaviour. The low work-hardening rate and flow stress in dynamic deformation is believed to be due to the production of short-lived disorder. The absence of a/2〈111〉 dislocations in thin foils is explained in terms of the fast reordering reaction in β-brass.     

FEM-simulation of the hardening behavior of FCC single crystals

Summary Although the deformation behaviour of single crystals has been investigated experimentally and theoretically for several decades, a generally accepted theory for the underlying hardening processes has not been found yet. In the last years, computer simulations offered additional investigation possibilities, whereby especially the use of constitutive equations in combination with the finite element method has delivered important results. The publication reports on an FEM crystal plasticity model-which was previously used for multicrystals-and its application to single crystals. The model is designed for the low temperature behaviour of pure fcc metal crystals. The rate dependent equations include kinematic and isotropic hardening, with formulations founded on the responsible slip system processes. The obtained simulation results coincide very well with typical single crystal experiments like tensile or latent hardening tests, which confirms the chosen mathematical approaches. It is shown that both kinematic and isotropic processes determine hardening, whereby the underlying slip system interactions play an important role. Moreover, experimentally not visible processes can be studied in detail and are discussed concerning the metal physics theories, which finally contributes to a better understanding of metal deformation behaviour.     

Deformation of single crystals of Cd3Mg

The deformation behaviour of single crystals of Cd₃Mg and Cd-28.5 at.% Mg has been studied over a range of temperature from –196 to 200° C, and the critical resolved shear stresses associated with both basal and {1¯100} 11¯20 prismatic slip have been measured. The CRSS for prismatic slip has been found to show a marked increase with increasing temperature prior to the onset of appreciable disordering while that for basal slip remains athermal over the same range. Both slip systems, however, showed a CRSS peak within the two-phase, ordered plus disordered, region. The athermal CRSS for basal slip in the stoichiometric alloy is shown to be too low to be consistent with a previous explanation advanced to account for the CRSS for basal slip in Mg₃Cd.     

A study in the work-hardening behaviour of austenitic manganese steels

The work-hardening behaviour of Hadfield's steel and the effect of vanadium additions have been examined. At room temperature, deformation twinning plays a significant role. The rate of work-hardening was found to be sensitive to the presence of vanadium above 1 wt%. Transmission electron microscopy studies indicate that the work-hardening is due to a combination of structural defects. The mechanical behaviour of the alloys in compression can be described by a mathematical expression derived from the familiar Ludwik expression.     

Prolonged plastic deformation related to the microindentation of MgO single crystals

The plastic deformation regions near the indentations on the (0 0 1) plane of MgO single crystals were investigated for an indentor under load and after unloading. It was established that the completion of slip-line structures arising during indentor penetration occurred during indentor removal. The presence of prolonged plastic deformation has been explained by considering the impulse character of the microindentation process of MgO at room temperature.     

Athermal dislocational anomalies in microcontact spectra of zinc single crystals

Regularities in the behavior of athermal dislocational anomalies of microcontact spectra of zinc single crystals are established. It is shown that they arise due to emission of terahertz waves by dislocations. In this case hexagons of basis dislocations surrounding the microcontact generate in most cases monochromatic high-frequency waves, which promote the appearance of a nondecaying current. A model is proposed according to which properties of dislocations are determined by regularities in interactions of atoms that keep moving at an arbitrarily low temperature. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 70, No. 1, pp. 23–26, January–February, 1997.     

The plastic deformation of copper-2 at. % cobalt alloy single crystals

Copper-2 at. % cobalt alloy single crystals containing coherent and/or incoherent spherical cobalt precipitates (60 Å相似文献   

Effect of frictional forces on the behaviour of dislocation loops

A detailed analysis of the effect of frictional forces on the stability of dislocation loops lying in their glide planes and the effect of these forces on the cross-slip behaviour of these loops under both applied stresses as well as internal stresses arising from other dislocation loops, has been made. In addition, the passing as well as cross-slip behaviour of non-coaxial dislocation loops has been analyzed and it is shown that these loops could give rise to Frank-Read sources by way of cross-slip; and the conditions for generations of such Frank-Read sources have been determined. The results show that depending upon strain, the cross-slip of dislocation loops could lead to regeneration of dislocation and subsequent work-hardening, or to dynamic recovery or stage III deformation. The significance of the above results in relation to the behaviour of dislocation loops observed in real crystals is discussed in detail.     

Influence of second-phase particles on plastic deformation and lattice reorientation of single crystals during uniaxial compression

Changes in the orientation of crystal axes of Al-0.8 wt% Si single crystals in the presence of two different particle sizes and inter-particle spacings were investigated during uniaxial compression. The orientation of crystal axes before and after deformation was precisely determined by the diffractometer method in the unit triangle of the stereographic projection. The reorientation of the compression axes after deformation was found to be influenced by particle size and initial orientation of the undeformed crystals. It was found that particle size also influenced the yield strength and work-hardening characteristics which was explained on the basis of Orowan stresses and solid-solution strengthening.     

Plastic behaviour of CuAl2

The plastic behaviour of CuAl₂ was studied by compression testing of single crystals and polycrystals in the temperature range 300–575 °C. While single crystals were grown from the melt by the Bridgeman technique, ingot and powder metallurgy routes were adopted for polycrystalline specimens. In addition to exploring their flow behaviour, the deformation mechanism was assessed through thermal activation analysis. It was observed that CuAl₂ failed in a brittle manner in compression below 375 °C and its ductility improved progressively with temperature. The brittle-ductile transition (BDT) temperature was influenced by the initial dislocation density but not by the grain size. The strong temperature dependence of flow stress and grain size strengthening effect as per the Hall-Petch relation, were dominant up to nearly the melting temperature of CuAl₂. The measured activation parameters for deformation suggest that the Peierls mechanism is rate controlling over the investigated temperature range.     

New method for measuring the non-elastic work-hardening rate of solid polymers

Probing the non-elastic deformation of solid polymers in the pre-yield range has proved to be a very sensitive test of the structural response to plasticity nucleation. A work-hardening rate parameter,K, deduced from repeated stress relaxations was thus introduced to quantify this behaviour. It is shown that considerable improvement and more extensive information may be gained by measuring the work-hardening rate by the so-called direct method from a simple stress-non-elastic strain plot. A comparison is given of the two methods in the case of unsaturated polyester resins, together with a careful evaluation of potential sources of discrepancy.     

Dislocations produced in magnesium oxide crystals due to contact pressures developed by softer cones

An experimental investigation shows that dislocations, encompassing a predictable dislocated volume, are produced in MgO single crystals subjected to circular contact pressures due to cones of other solids which may be an order of magnitude softer. The mean contact pressure necessary to produce localized plastic flow, without fracture, is shown to be directly related to the critical resolved shear stress of the MgO. Dislocation etching is used to investigate the influence of cone hardness and normal loading on the nature of the deformed zone in the harder crystal and the results are discussed in terms of the deformation characteristics of the softer cones and the crystal.     

In situ nuclear magnetic resonance study of defect dynamics during deformation of materials

Nuclear magnetic resonance techniques can be used to monitor in situ the dynamical behaviour of point and line defects in materials during deformation. These techniques are non-destructive and non-invasive. We report here the atomic transport, in particular the enhanced diffusion during deformation by evaluating the spin lattice relaxation time in the rotating frame, T ₁,in pure NaCl single crystals as a function of temperature (from ambient to about 900K) and strain-rate (to 1.0s^–1) in situ during deformation. The strain-induced excess vacancy concentration increased with the strain-rate while in situ annealing of these excess defects is noted at high temperatures. Contributions due to phonons or paramagnetic impurities dominated at lower temperatures in the undeformed material. During deformation, however, the dislocation contribution became predominant at these low temperatures. The dislocation jump distances were noted to decrease with increase in temperature leading to a reduced contribution to the overall spin relaxation as temperature is increased. Similar tests with an improved pulse sequence (CUT-sequence), performed on ultra-pure NaCl and NaF single crystals revealed slightly different results; however, strain-enhanced vacancy concentrations were observed. The applicability of these techniques to metallic systems will be outlined taking thin aluminium foils as an example.     

Cyclic hardening/softening behaviour in AZ31B magnesium alloy based on infrared thermography

The work-hardening/softening behaviour of AZ31B magnesium alloy during high cycle fatigue was investigated. The superficial temperature evolution during fatigue tests was used as a criterion for the different levels of work-hardening/softening. The microstructures under different cycles were observed by transmission electron microscope. Tensile test (with post-fatigue) was conducted to quantify the work-hardening/softening behaviour which showed that high dislocation density after cyclic loading lead to high tensile strength. The temperature evolution of the specimens with different levels of work-hardening/softening during tensile tests is related to the microstructures; the results indicated that the temperature rise of the specimen with high density dislocation was lower. Microstructures after tensile tests showed that high dislocation density after cyclic loading would lead to high twinning density.     

The yield strength and dynamic behaviour of dislocations in MgO crystals at high temperatures

The mechanical behaviour and mobility of dislocations in MgO crystals in the temperature range 20 to 800° C are shown to be dependent on the concentration and the valence state of impurities. Impurities of trivalent state are found to interrupt the motion of screw dislocations and lead to an increase of the yield strength throughout the temperature range investigated. Impurities of divalent state, of which Ca²⁺ ions are the most prevalent, are shown to retard the motion of edge dislocations by forming an atmosphere at elevated temperatures and are responsible for a peak of the yield stress appearing around 700° C.     

铜单晶体循环应力应变曲线的平台行为的取向效应

系统讨论和总结了单滑移,双滑移和多滑移取向铜单昌体循环形变中呈现的不同平台行为。分析结果表明,晶体取强烈影响双滑移和多滑移铜晶体的循环变行为,平台区的出现与否及平台应力的高低,不仅与闰错反应模式和强度有关。     

Work-hardening rate of glassy polymers: evolution with curing of thermo-set resins

A new parameter, the work-hardening rate,K, is introduced to characterize the non-elastic deformation behaviour of glassy polymers. Related to the defects nucleation in the preyield stage, this parameter is shown to be a very sensitive probe of the structural evolution of materials. The variation ofK with cross-linking in the curing of a polyimide resin is reported.     

Studies on the stress-strain relations of dual-phase steels

The correlations of the work-hardening exponent,n, with quenching temperature, martensite volume-fraction (MVF) and solute concentration in ferrite are discussed and derived for dual-phase steel. The flow stress of dual-phase steel at low strain is suggested to be expressed by the combination of the terms due to plastic deformation in ferrite and elastic deformation of martensite. Previous experimental results are compared with the behaviour suggested by this theoretical work. In addition, an expression for the work hardening exponents at moderate strains and at the onset of necking are also theoretically suggested.     

TEM study of the recovery process of cyclically deformed MgO single crystals

Single crystals of MgO were subjected to plastic strain-controlled push-pull cyclic deformation at elevated temperatures. Below 400° C the crystals were very brittle and failed with a few fatigue cycles. At 470° C a large number of cycles could be obtained before failure, and the cyclic stress-strain response showed a period of rapid hardening followed by a period of decreasing hardening rate. TEM investigations of the lower temperature samples show structures of isolated dislocation dipoles, multipoles and debris. At 470° C dense bundles of dislocations were observed aligned perpendicular to the Burgers vector direction. The regions between the bundles were relatively dislocation free, but they contained a high density of debris. Bowed out screw dislocations are observed between the edge dislocation bundles, suggesting that screw dislocations were largely mobile. Comparisons are made with the cyclic deformation and structure of fcc metals and other NaCl structure single crystals.