Dynamic deformation of silicon-rich V3Si single crystals at elevated temperatures

The plastic deformation behaviour of the intermetallic phase V₃Si (Cr₃Si type) was investigated under dynamic conditions. The experiments revealed that V₃Si deforms plastically at a strain rate of about 4.7 × 10⁻⁵s⁻¹ above 1200°. Flow stress depends strongly on deformation temperature and strain rate. An alloy hardening by deviation from stoichiometric composition within the range of homogeneity is observed.     

Relation between the Activation Parameters of the Plastic Deformation in MgO Crystals Determined by Macroscopic and by In-situ HVEM Experiments

The results of macroscopic strain rate and temperature cycling tests on MgO single crystals are compared with the results of in-situ deformation tests in a HVEM on the basis of the basis of the thermally activated dislocation glide model. The activation volume estimated by strain rate cycling tests is compatible with the mean obstacle distance of the screw dislocations. However, the local stress estimated by in-situ deformation tests was found to be larger than the macroscopic yield stress.     

Study of the instability boundaries of plastic flow in Cu + 14 at.% Al single crystals at low temperatures

The authors experimentally revealed the existence of the temperature-strain rate region of jumplike deformation in Cu + 14 at.% Al single crystals between 4.2 and 25 K. For each particular temperature in this temperature range, there are a lower and an upper critical strain rates, between which deformation is jumplike. The character of transients occurring during strain rate variation was investigated by increasing the strain rate to the lower critical value. The experimental data are explained in terms of the thermomechanical instability concept and inertial effects of the sample-machine system.     

The effect of dynamic recrystallization on the growth velocity of new phase crystals at polymorphic transformation of tin and thallium

The analysis of dynamic recrystallization influencing the crystal growth of the new phase in high purity tin and thallium (99.9995%) is carried out. It is shown that in tin, in which the β → α transition proceeds on the dislocation or the normal mechanisms (depending upon a supercooling degree, ΔT) a superposition of dynamic recrystallization results in an increase of the growth velocity of α-Sn crystals. The most distinct of acceleration is observed at −21 ÷ − 23 °C, when the growth velocity of recrystallization centers and the maximal growth velocity, V_max, of the crystals are comparable values. The dynamic recrystallization superposition in thallium offers to transform the transformation mechanism. Thereby an erase of the features of the martensite α ⇄ β transition mechanism and an appearance of the features of the normal crystal growth mechanism of the new phase are observed.     

Thermal effects in low-temperature deformation: The response to strain rate changes

The response of a deforming crystal to an abrupt strain rate change has been considered in the framework of the thermal model for low temperature anomalies of plastic deformation. Various modes of the deformation behaviour following strain rate changes have been predicted depending on the location of the point representing the state of the specimen in the plane of control parameters. A quantitive analysis of two characteristic modes of the deformation behaviour experimentally observed by KOMNIK and DEMIRSKI has been carried out with the aid of computer calculations and a good agreement with their data has been found.     

Load relaxation behavior of a Zr41.2Ti13.8Cu12.5Ni10 Be22.5 bulk metallic glass

The load relaxation behavior within the supercooled liquid region of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk metallic glass has been investigated. To explain the relationship between normalized stress and relaxation time, two different stress relaxation modes such as a Kohlrausch-Williams-Watts (KWW) behavior and a simple power law were applied to the short and long relaxation time regimes, respectively. The apparent activation energy for stress relaxation is 126 ± 10 kJ/mol. Flow curves were obtained by converting load-displacement data into a flow stress-strain rate relation, resulting in three different deformation characteristics through a wide strain rate region interpreted in terms of strain rate sensitivity. A prediction of hot workability has also been attempted by constructing a power dissipation map based on a dynamic materials model.     

Rapid recrystallization of amorphous silicon utilizing the plasma jet at atmospheric pressure

The rapid recrystallization of amorphous silicon utilizing the very high frequency (VHF) plasma jet of argon at atmospheric pressure is presented. Highly crystallized polycrystalline Si film was synthesized by optimizing the translating velocity of the substrate stage and flow rate of argon. The temperature of the plasma exposure area reached at 1300 °C and the recrystallization of a-Si proceeded with a time constant of 10–50 ms. The effects of the translating velocity of the substrate stage and flow rate of argon on the rapid recrystallization of a-Si are demonstrated along with its mechanism.     

Effect of structural transformations on the texture of Ni?Fe base alloys

1. Processes of texture formation were studied in three Ni Fe system alloys containing Mo and Ti (alloy No. 1), Cr, Ti and Al (alloy No. 2) and Co, Ti and Al (alloy No. 3). The deformation texture is identical in all alloys which agrees with the proximity of stacking fault energies as estimated from X-ray data. 2. Annealing leads to a pronounced diffusion and alteration of the texture in alloy No. 1, but does not alter the texture type and intensity in alloy No. 2, whereas in alloy No. 3 the typical strain texture components are intensified. 3. Structure processes governing texture formation were identified as recrystallization in alloy No. 1, and oriented γ–γ′ transformation in alloys No. 2 and 3 where the intergranular boundaries were blocked by heterogenous precipitation of phases not possessing fcc structure.     

X-ray investigations of the defect structure of aluminium after plastic deformation

With plastic deformation of polycrystalline metals deformation textures are build up. Our experimental results show that the dislocation density increases, even with small plastic deformation. Simultaneously a decrease in the value of coherence length was revealed. The formation of a distinctly marked sub-structure, which is accompanied by an increase of the dislocation density, is essentially completed when plastic strain reaches 30 per cent. Further increase of the degree of deformation is followed by a change of orientation of the subgrains. When plastic strain is 60 per cent the deformation texture is almost completely accomplished.     

Stress relaxation in the intermetallic phase V3Si at elevated temperatures

The plastic deformation behaviour of the intermetallic phase V₃Si (Cr₃Si-type) was investigated in stress relaxation experiments. The experiments revealed that V₃Si shows stress relaxation after dynamic deformation above 1550 K. The characteristic parameters of stress relaxation depend strongly on temperature, chemical composition (within the range of homogeneity) and deformation rate. The decrease of stress is governed by dislocation climb processes.     

Orientation changes during recovery of compressed Mo crystals

The type of the recovery variety which is accompanied by changes in texture is studied. Reorientation of the surface of compressed Mo crystal was observed during its annealing before recrystallization. As a result, new macroscopic {100} orientation areas appear on the contact crystalline surface of {111} orientation. Turned areas have a more perfect inner structure and correspondingly lesser density of deformation defects. As a result of rotation a high-angle boundary interface occurs as a “broken boundary”. The transition layer considered is highly stressed and contains microcracks. The boundary interface is shown to be the intensive recrystallization source, where the recrystallization grains are nucleated at the expence of interface dislocation bulgings. The driving forces of the rotation observed and a scheme of a residual stress distribution in the compressed crystal are analysed. According to the scheme a deformed crystalline surface is affected by the compressive stresses and the inner layers are affected by the tensile ones. A relaxation of the internal stresses results from plastic rotations. A rotation moment arising between the surfaces and internal layers of the crystal leads to reorientation.     

Structural and Density Changes of Rhenium Single Crystals Upon Rolling and Annealing

The rhenium crystals were rolled in the {1000} 〈112 0〉 orientation. Rolling produces cracks in crystals at low deformation (ϵ = 30%). It has been shown that microcracks in the volume of the crystal arise at a more earlier deformation stage than visible surface cracks. Recrystallization during subsequent annealing is strongly intensified with the start of crack formation. Stepwise facets of recrystallizated grains are often observed, this fact being connected with twinning lamellae recrystallization. Microcracks in the crystal body are completely eliminated during high temperature annealing. This process caused by diffusion over microcracks surfaces.     

Evolution of Structure in Deformed Single Crystals of fcc Metals at Heating

The paper deals with conditions resulting in formation of dislocation structures of different types as well as with the change of these structures at subsequent heating which ends in the recrystallization center formation and growth. The measurements were carried out on single crystals of fcc metals. The principal experimental technique was electron microscopy including the annealing of deformed foils directly in the column of electron microscope with accelerating voltage 150 and 1000 kv. Special attention is given to research of the initial stage of recovery in dislocation structures during annealing — the dislocation recovery as the most universal mechanism occurring at any type of structure even when polygonization and recrystallization are hampered. Studies of initial recrystallization stages were mainly carried out while annealing the deformed single crystals of copper. A high degree of local lattice curvature is shown to be a necessary condition for nucleation, and mechanism of the nucleus formation in regions of a high local curvature differs according to the type of the dislocation structure formed during deformation.     

Investigation on the crystal growth process of spherical Si single crystals by melting

Spherical Si single crystals with a diameter of approximately 1 mm were grown by melting for solar cell applications. The start sources were spherical Si multicrystals fabricated by a dropping method, which had various irregular shapes. Spherical Si multicrystals were melted into droplets and recrystallized on a quartz plate sample holder that was coated with Si₃N₄. It was found that a surface coating of SiO₂ layer on the start sources and oxygen atmosphere during melting and recrystallization were essential to achieve almost perfect spherical shape. Defect-free single crystalline spherical Si could be obtained at recrystallization temperature ranging from 1400 to 1330 °C, corresponding to an undercooling ranging from 14 to 84 °C, with a yield of nearly 100%. At recrystallization temperatures higher than 1380 °C, the recrystallized spherical Si crystals were almost perfect spheres, whereas small protuberances were formed when the recrystallization temperature was lower than 1360 °C. It was also found that that melting at a temperature close to the melting point of Si (at ~1414 °C), a slow cooling rate of ~1 °C/min before recrystallization and relatively fast cooling rate of ~20 °C/min after recrystallization were important for achieving high carrier lifetime. The average carrier lifetime was greatly improved from lower than 2.5 μs of start sources up to ~7.5 μs by melting at optimized conditions. The influences of residual oxygen on the carrier lifetime of recrystallized spherical Si are discussed based on the measurement results with Fourier transform infrared spectrometer.     

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).     

Low-temperature anomalies in the plastic response of crystalline materials

The non-uniform, wavy, temperature-dependence of the yield-stress and of the rate of logarithmic creep frequently observed in metals, alloys and non-metallic crystalline solids at low temperature, is shown to be largely a consequence of the progressive inhibition of dynamic recovery processes as T → 0 K. Plastic deformation becomes increasingly localised, resulting in high stress-concentrations. On allowing for this increase in the usual equations of the deformation kineties, the anomalies are very well accounted for.     

Positron Annihilation Study on the Thermal Fatigue in a Die Steel

Samples of 5Cr2NiMoVSi die steel which underwent thermal fatigue with different fatigue cycles have been studied by using microhardness and positron lifetime measurements. It is found that the microhardness and the positron lifetime parameters increase and decrease alternatively with increasing the number of the fatigue cycles. The experimental results reveal that the fatigue strain, the dynamical recovery, and recrystallization and the precipitation are main causes of variation of the defects in the process of thermal fatigue.     

The influence of magnetic effects on the mechanical properties and real structure of nonmagnetic crystals

This review for the first time systematizes the results of our investigations into the influence of magnetic effects on the mechanical properties and the real structure of nonmagnetic crystals. It is found that the preliminary magnetic treatment of alkali halide crystals leads to a decrease in their solubility and a change in the microhardness and yield stress. The magnetic field strongly affects the macroplasticity of LiF, NaCl, and PbS crystals under deformation in a magnetic field. This is accompanied by a change in the shape of stress-strain curves, a shortening of deformation stages, a change in the hardening coefficients, and a decrease in the yield stress. It is revealed that the magnetic effects exhibit threshold behavior. The yield stress is measured as a function of the magnetic induction and the strain rate. It is established that the magnetic and electric fields have a joint effect on the kinetics of plastic deformation. A kinematic model of the macroscopic magnetoplastic effect is proposed.     

Superplastic flow of a Mg-based bulk metallic glass in the supercooled liquid region

The tensile flow behavior of Mg₆₅Cu₂₅Y₁₀ bulk metallic glass was investigated over a range of strain rates (10⁻³–10⁻¹/s) and deformation temperatures (150–170 °C) in the supercooled liquid region. In this region, the relationship between peak flow stress, strain rate and absolute deformation temperature was described adequately by the classic Sellars–Tegart constitutive relationship. There was also a good correlation between the Zener–Hollomon parameter, Z, and the flow characteristics of the material such as the transition from Newtonian to non-Newtonian flow and maximum achievable tensile elongation; the latter was used for determining the optimum conditions for superplastic flow in the material.