Characteristic temperature of deformation of crystalline materials

Using the ceoncept of a characteristic deformation temperature, t^* (knee temperature), below which the PEIERLS -NABARRO stress becomes significant, some general regularities have been determined on the influence of temperature upon structural state and mechanical properties of crystalline materials. It is demonstrated that t^* represents a natural boundary between the intervals of warm and cold deformation. At the temperature t^*, the mechanisms of deformation, fracture, and recovery of properties in annealing vary significantly in the same manner for different crystals.     

The Influence of High Temperature Predeformation on Ductility of LiF and BaF2 Single Crystals

The effect of high temperature predeformation on deformation characteristics of ductile LiF and brittle BaF₂ crystals was investigated. The same predeformation, as has been shown previously, leads to the increasing of the limited plastic deformation of MgO crystals by one order of magnitude. It was found that in both investigated crystals, as in the case of MgO crystals, at the test temperature T₂ ≅ 0.1 T_m limited plastic deformation before fracture obviously increases if the specimen was prestrained up to ε_p ≦ 1% at T₁ = 0.5 T_m. This increase is as higher as the plasticity of crystal is lower. In BaF₂ crystals it reaches a factor of 20 and more. The effect is connected with the creation of mobile dislocations during the high temperature predeformation and dislocaton sources for subsequent deformation at lower temperatures and with the homogeneity of the process of plastic deformation. These conditions prevent the appearance of dangerous places in which the fracture can begin.     

Morphological diagram of Savart-Masson bands of macrolocalized deformation

The dynamics and morphology of Savart-Masson bands of macrolocalized deformation during a tension test of flat samples of aluminum-magnesium alloy Al-6wt.%Mg with a constant stress rate ([(s)\dot] ₀ = const)(\dot \sigma _0 = const) have been investigated in situ using high-speed video recording. A phenomenological classification of the Savart-Masson bands according to the morphological and kinetic peculiarities is presented. This classification includes six types of deformation bands which replace each other with an increase in stress. A morphological diagram of Savart-Masson deformation bands is obtained for the first time. These bands differ significantly from the A, B, and C deformation bands during a tension test with a constant strain rate ([(e)\dot] ₀ = const)(\dot \varepsilon _0 = const) under the conditions of the Portevin-Le Chatelier effect.     

X-ray trajectories in crystals with x-depending deformation gradients

Based on the Eikonal theory analytical solutions of the trajectory-equation were calculated for some special x-depending deformation gradients, where x shows in the direction of the diffraction vector. For the discussion deformation gradients are chosen which are similar to those in crystals with varying concentrations of impurity atoms. The calculated trajectories allow to obtain properties of section- and traverse-topographs.     

Qualitative and quantitative determination of X-ray trajectories

X-Ray trajectories are qualitatively determined only from the behaviour of the potential (of the lattice distance) in crystals with x-dependent deformation gradients. the quantitative calculations show the varying degrees of sensitivity of X-ray trajectories to deformation for different radiations, different orders of reflection and different maximum values of relative change of lattice distance.     

Connection between electrical and deformation characteristics of PbS crystals

The influence of plastic deformation on temperature dependence of carrier concentration n and mobility μ in PbS crystals has been studied. Comparison of the results obtained with selective etching patterns from original and deformed crystals makes it possible to conclude that during the plastic deformation there takes place a dissolution of precipitates by passing of dislocations through them. As a results, rows of point defects appear along the path of dislocation movement which may be the cause of a change in n and μ due to deformation. A change in the electron scattering mechanism below 110 K has been discovered.     

Influence of chemical composition on dislocation structure and its change by plastic deformation of V3Si single crystals

The dislocation structure and its change by plastic deformation of V₃Si single crystals has been studied by an etch technique. The solution hardening effect established elsewhere is interpreted in terms of the observed dependence of the dislocation density ϱ(r) and its increment with the plastic deformation on the chemical composition within the range of homogeneity.     

On the change of low-temperature thermal conductivity of gap after bending

The thermal conduction of a sulphur-doped single crystal plastically deformed by bending at 963 K was measured between 2 and 50 K. The thermal resistivity W at temperatures > 14 K is practically independent on deformation. At lower temperatures W is found to be proportional to AT⁻³ before and to AT⁻³ + BT⁻² after deformation. A is due to boundary scattering, B due to dislocations.     

Defect structure in nickel single crystals in dependence on the cyclic deformation prehistory

The development of the dislocation structure in nickel single crystals push-pull fatigued at room temperature is investigated for different deformation types using electrical resistivity measurements and results of TEM observations. The dislocation density in the bundles of the matrix is higher after changing the strain amplitude from region A to region B (CD_AB-type) of the cyclic stress-strain curve (N_dB ≈︁ 1.6 ≈︁ 10¹⁵ m⁻²) than after virginal deformation in region B (VD_B-type) especially at higher strain amplitudes (N_dB ≈︁ 0.75 ≈︁ 10¹⁵ m⁻²). Increasing the strain amplitude within region B (CD_BB-type) does not change N_dB. The farther development of the matrix structure after reaching the v_H-minimum during the VD_B-type of deformation suggests the action of the „formation and dissolution”︁ – mechanism, whereas the CD_BB-type causes the „cord”︁-mechanism of the PSB nucleation.     

The plastic deformation of ultrafine grained aluminum at 0.52 K

The plastic deformation of ultra-fine-grained aluminum subjected to equal-channel angular pressing has been studied at a temperature below the superconducting transition point. The stress-strain curves σ(ɛ) for polycrystals and the effect of the superconducting transition on the jump of flow stress Δσ _NS have been investigated. It is shown that the grain refinement, along with the increase in the flow stress, leads to a correlated change in the shapes of the dependences σ(ɛ) and Δσ _NS (σ). The results obtained are explained by the features of dislocation accumulation in ultra-fine-grained polycrystals and by the manifestation of the inertial properties of dislocations under low-temperature plastic deformation.     

X-ray investigation of the mechanism of phase transformation in single crystals of ZnS,ZnxCd1−xS and ZnxMn1−xS (I). Calculation of diffraction effects by a three parameter model

Hexagonal close-packed (2H) single crystals of ZnS, Zn_xCd_1−xS and Zn_xMn_1−xS are known to undergo solid state transformation to the cubic close-packed (3C) and 6H-structures on annealing at elevated temperatures. The transformations occur by the non-random nucleation of stacking faults on individual close-packed layers parallel to (0001). The nature of the faults involved and their probability distribution during transformation determine the diffraction effects produced along the 10. L reciprocal lattice row by a crystal quenched in an intermediate state of transformation. We have investigated the mechanism of the transformation by comparing the diffraction effects recorded from such crystals on a single crystal diffractometer, with those calculated for an assumed model of the transformation. It is known that in these materials the faults involved in the transformation are deformation faults. To explain the observed diffraction effects we develop a three parameter theoretical model employing a fult probability α for the radom insertion of a deformation fault in the 2H structure, a fault probability β for the deformation faults to occur at three layer separations and a fault probability γ of their occcurrence at 2-layer separations. The probability α corresponds to the development of a fresh nucleus, the probability β to the growth of the 6H nucleus and the probability γ to the growth of a 3C nucleus. This paper develops the necessary theory of X-ray scattering for such a model of the transformation and predicts the diffraction effects for different values of α, β, and γ. The next paper compares these results with experimental observations.     

Dislocation energy levels in deformed silicon

Abstract-Energy levels in deformed silicon have been directly measured by transient junction capacitance techniques. If complex slip band formation is not suppressed, a large variety of states are observed after deformation with a prominent state at E_v − 0.68 ev. After annealing at 900°C the complex defect spectra simplify to two dominant states at E_v + 0.35 ev and E_c − 0.38 ev. In more homogeneously deformed silicon where slip band formation is suppressed, the dominant state at E_c − 0.68 ev is not observed after deformation and the spectra resembles that obtained after annealing the complex spectra at 900°C.     

High-temperature Localization of Plastic Deformation in Lithium Fluoride Single Crystals

Experimental results on the superlocalization of plastic deformation at high strains and high temperatures in LiF single crystals are described. The physical conditions of transition to a localized plastic flow in the temperature range of 603 K to 1073 K T (0.53–0.94T_mp) using constant strain rate compression tests under strain rates from 10⁻³s⁻¹ to 10⁻²s⁻¹ are found. The results indicate that the deformation mechanism involves dislocation climb, controlled by diffusion. The connection of high temperature flow instability with an excessive concentration of point defects (strain vacancies) in zone of shear has been confirmed.     

Plastic Deformation of Cesium Iodide Single Crystals at High Temperatures

The plastic deformation of CsI crystals of three crystallographic orientations 〈100〉. 〈110〉. and 〈111〉 at temperatures from 423 to 773 K (0.5 to 0.86T_m) and strain rates from 2 × 10⁻³ to 10⁻⁵ s⁻¹ is studied. Four-stage stres-strain curves were found, three stages being more and more distinct with an increase in temperature up to 623 K above which stage III prevails. For all the temperatures, strain rates and oreintations studied the superplasticity features (jerky flow, deformation localization, active dynamical recovery etc.) were observed. The thermal activatoin analysis has shown that the rate of high temperature deformation of CsI is controlled by dislocation climb limited in its turn by mobility of cation vacancies (quasiviscous creep).     

Plastische Verformung von Siliziumeinkristallen unterschiedlicher Ausgangsversetzungsdichte im Streckgrenzenbereich

Floating-zoned Silicon crystals orientated for single slip both dislocation-free and with grown-in dislocations (EPD₀ = 6,5 · 10⁴ cm⁻²) were dynamically deformed in compression in the temperature range 1120 … 1300 K up to the lower yield point. The dislocation structure of plastically deformed crystals (T = 1138 K; a₀ = 1,45 · 10⁻⁴ s⁻¹) was examined by etching and high-voltage electron microscopy. The crystals exhibit essential differences in deformation behaviour and in the resulting dislocation structure. Typical for the dislocation-free basic material are (i) an inhomogeneous deformation in the range of the upper yield point (ii) a greater portion of sessile lomer dislocations at the lower yield point in relation to the crystal with grown-in dislocations. The measured stresses were related to those values calculated from structure data by employing plasticity theory of diamond-like semiconductors.     

TEM-Untersuchungen von Gitterfehlern in ZnSiP2-Einkristallen

Results are dealt with concerning TEM investigations of lattice defects in ZnSiP₂ single crystals. After the crystal growth dislocations or stacking faults were found in a few cases only. More frequently twins were present in the microstructure. The crystallographic elements of twinning are {112} 〈111〉. After plastic deformation by bending at 900 °C local dislocation arrangements with high defect density (N_v ≈ 10⁶…︁ 10⁷ cm⁻²) were observed. By means of the diffraction contrast one Burgers vector b = 1/2 〈111〉 could be identified. In some cases the crystals also contained wide deformation stacking faults, which were limited by partial dislocations. The density of twins was not increased under the conditions of deformation reported here. As it can be concluded from investigations of Oettel et al. and from the results of the twin analysis, slip and generation of stacking faults take place on {112}-planes in ZnSiP₂ crystals. Crystallographic considerations on both processes are dealt with.     

Deformation behaviour of single crystals of GaP in uniaxial compression

LEC n-GaP:S single crystals were deformed in uniaxial compression parallel [321] and [110] direction. Plastic deformation by (111) slip has been observed between 511 °C and 717 °C. Indications for (111) twinning were obtained. Yield stress exhibits activation enthalpies U = Q(m + 2) and stress exponents m of U₃₂₁ = (0.54 ± 0.05) eV, U₁₁₀ = = (0.46 ± 0.05) eV, m = 1.1 ± 0.2 and m = 3 ± 1, respectively.     

Dislocation arrangements in cyclically deformed polycrystalline molybdenum

The present paper deals with the cyclic stress-strain behaviour of polycrystalline molybdenum at room temperature and the dislocation structures built up within this material during the fatigue process. A cyclic stress-strain curve of molybdenum deformed in a strain-controlled and symmetrical push-pull test is shown. At strain amplitudes e_a < 3 × 10⁻³ arrangements of relatively homogeneously distributed dislocations are observed in the stage of the stabilization of mechanical properties. The characteristics of these dislocation arrangements are similar to those of dislocation structures of unidirectionally deformed molybdenum single crystals. At strain amplitudes e_a > 3 × 10⁻³ dislocation structures are developed with an inhomogeneous dislocation distribution (bundles structures). The dislocation density in the surface layers of fatigued specimens shows larger values than within the material. The cyclic deformation after a change from a small deformation amplitude to a larger one, or vice versa, is connected with characteristic changes of dislocation density.     

Erholung und Rekristallisation des stabilen Austenits bei der Verformung durch Walzen

For the explanation of the processes which take place during a high-temperature thermomechanical treatment (HTTT) in the stable austenite hardness, grain structure and dislocation structure of a Fe 24Ni 0,5C alloy are examined in dependence of the degree and temperature of rolling deformation (ϵ = 13 to 52%; T_v = 830 and 1150°C). The observed structures can be represented by a recovery-recrystallization-diagram, that describes the influence of technological parameters on the recovery and the recrystallization of stable austenite.