Plastic Flow In Single Crystals Research Articles

A brief response to ‘Intermittent plastic flow of single crystals: central problems in plasticity: a review’, by M. Niewczas

Issues raised by Niewczas are addressed.

Intermittent plastic flow of single crystals: central problems in plasticity: a review

The relationships between microstructure and plastic flow properties of copper single crystals deformed at low temperatures are reviewed based on the analysis of experimental results of mechanical, structural and electrical resistivity characterisation. Electrical resistivity measurements suggest that the flow stress correlates with the average density of defects accumulated in the microstructure. The characteristic microstructural length scale that determines the flow stress corresponds closely to the size of dislocation free channels and/or cells produced during plastic flow. The correspondence between micro- and macroscopic approaches to the plastic flow has been analysed based on the behaviour of dislocation mean free path and mean slip distance and their relationship to the microstructure. Production and annihilation of nanoscale debris and point defects and their role in plastic flow and questions pertinent to the influence of microstructural instabilities on the flow stress, workhardening, storage and annihilation of lattice defects have been discussed.

Strain hardening upon twinning of % MathType!MTEF!2!1!+- % feaagaart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaai4waiqaig % dagaqeaiaaigdacaaIXaGaaiyxaiaacYcaieaacaWFGaGaai4waiqa % igdagaqeaiaaisdacaaI0aGaaiyxaaaa!3F5F! $$ [\bar 111], [\bar 144]$$ , and [011] single crystals of hadfield steel

Transmission electron microscopy and X-ray diffraction have been used to experimentally study the mechanism of deformation of Hadfield steel single crystals oriented for tension along \( [\bar 111], [\bar 144]\) , and [011] directions. It has been established that at T = 23 ° C starting from the early degrees of deformation (ɛ > 0.5%) the process of plastic flow in single crystals of these orientations is connected with the development of predominantly mechanical twinning. The influence of the number of operative shear systems on the rate of strain hardening has been shown.

Autowave model of crystal plasticity: Macro- and microdefects

A new approach to the problem of the plastic flow of solid crystals is proposed. This approach is based on studying the macroscopic localization patterns of plastic deformation, which can be considered as different types of autowave processes of defect self-organization. An unambiguous correspondence between the localization patterns and stages of plastic flow in single crystals and polycrystals is established. The propagation velocity of localized plasticity autowaves is inversely proportional to the strain-hardening coefficient, and the dispersion relation is quadratic. A new model is proposed to describe the development of plastic flow localization.

Strain burst phenomena in the necking of a sheet that deforms by non-associated plastic flow

In non-close-packed crystalline lattices, e.g. bcc metals and intermetallics, the stress-state dependence of the Peierls barrier for the motion of a screw dislocation violates Schmid's law and leads to non-associated plastic flow in single crystals and polycrystalline aggregates (Bassani 1994 Adv. Appl. Mech. 30 191–258, Bassani et al 2001 Mater. Sci. Eng. A 319–321 97–101). Plasticity models based upon distinct yield and flow functions are proposed to describe polycrystalline behaviour and specialized to the case of isotropic response. Studies of sheet necking using both a Marciniak–Kuczynski analysis and finite elements predict that non-associated flow has a significant effect on the evolution of inhomogenieties in the sheet. For nearly rate-insensitive response, intermittent bursts of strain arise as a consequence of non-associated flow, particularly for deformations near the plane-strain state. These strain bursts, which are due to instantaneously unstable deformation behaviour, are observed to be sensitive to mesh refinement in the case of a purely local constitutive description. Strain-gradient effects are introduced, which significantly improves convergence.

Localized strain autowaves at the initial stage of plastic flow in single crystals

Plastic strain localization in single crystals of pure metals and alloys is studied on the yield plateau and at the easy glide stage with a zero or small strain hardening coefficient. The difference between localization patterns in the two cases is explained, and strain localization mechanisms are suggested. At these stages of plastic deformation, various types of autowaves are observed.

Crystallographic aspects of macroinhomogeneous plastic flow in single crystals of metals

Various crystallographic aspects of the distribution of strain-localization zones have been studied in single crystals of metals subjected to tensile stresses at different orientations of the tension axes and characterized by different mechanisms of plastic flow (slip of dislocations and martensite transformations). It is shown that the crystallographic orientations of the strain-localization zones (interpreted as the patterns of plastic-flow self-organization) are preserved within the whole deformation process. Some characteristic features of the dynamics of the strain sites are considered.

A self-excited wave model of plastic deformation in solids

A self-excited wave model of plastic flow in crystalline solids is proposed. The experimental results pertaining to plastic flow in single crystals and polycrystals with different underlying mechanisms responsible for plastic deformation are summarized. The salient feature types of strain localization in the materials under consideration are captured and related to the respective steps on the deformation curves. The conditions at which the salient self-excited wave types of plastic deformation reveal themselves are determined. The applicability of the synergic self-organization approach to describing plastic deformation is substantiated. A self-excited wave model of plastic flow in materials with different underlying mechanisms of deformation is suggested.

Plastic deformation modelled as a self-excited wave process at the meso- and macro-level

A self-excited wave model is developed to describe plastic flow phenomena in crystalline solids. Experimental observations suggest that by plastic flow in single crystals and polycrystalline materials, different underlying mechanisms are responsible for key features of strain localisation corresponding to different stages of the deformation curve. The major autowave (self-excited wave) types manifest themselves in plastically deforming materials. The self-excited wave model could explain plastic flow pattern behaviour corresponding to different physical mechanisms.

Plastic deformation of solids viewed as a self-excited wave process

A self-excited wave model of plastic flow in crystalline solids is proposed. Experimental data on plastic flow in single crystals and polycrystalline solids involving different mechanisms have been correlated. The main types of strain localization in the materials investigated have been established and correlated with the respective stages of plastic flow curves. The best observing conditions have been defined for the major types of autowaves emerging by plastic deformation. The synergetic concepts of self-organization are shown to apply to description of plastic deformation. Suggested is a self-excited wave model of plastic flow in materials with different mechanisms of deformation.

Non-associated plastic flow in single crystals

P lastic flow by shears on well-defined crystallographic planes is associated, most often, with the Schmid yield criterion which is expressed in terms of the resolved shear stress on those planes in the direction of the shears. In this case, for time-independent (low-temperature) flow, the yield function for each slip system is also the potential for the shear and. therefore, flow is said to be associated with that function (i.e. normality). Qin and Bassani (1992. J. Mech. Phys. Solids40, 813) propose a yield criterion to accommodate various non-Schmid behaviors. In this paper it is shown that this criterion leads to a nonassociated flow rule (i.e. non-normality). Time-independent constitutive relations are derived for single crystals undergoing non-associative plastic flow. The tension-compression asymmetry predicted for initial yield persists in strain hardening as well. Strain localization in the form of shear bands is investigated for both a single-slip and a symmetric double slip model. It is shown that non-Schmid effects increase the tendency for localization.

The Influence of Substances Reducing the Surface Energy of Naphthalene Single Crystals on Plastic Flow

Abstract The effect of surface active substances on plastic flow of naphthalene single crystals is considered. The role of normal and shear stresses is revealed. Rates of single crystals flow in different glide systems under constant stress are compared. The stress of plastic flow in single crystals can be decreased in active media by as much as 50%.

Critical shear stresses and plastic flow of 3% silicon-iron single crystals

1. Slip in 3% silicon-iron single crystals during extension in the directions 〈151〉 and 〈111〉 takes place on the system {110} 〈111〉, while during extension in the direction 〈331〉 it occurs on the {110} and {112} planes, and in the direction 〈111〉 with one system of the type {110} 〈111〉) predominating. 2. The critical shear stresses τcr for the slip direction 〈111〉 and the planes {110} and {112} vary between 9.5–10 and 11–11.3 kg/mm2, respectively. 3. The development of plastic flow in single crystals takes place by the movement of Luders-Chernov bands along the specimen and their propagation by the transverse slip mechanism.