Electroplasticity Mechanisms in hcp Materials

Herein, the mechanisms of the electroplastic effect (EPE) in different hexagonal close-packed (hcp) metals under varying loading conditions and current densities through the analysis of flow curves and microstructural changes are investigated. The investigations show a significant change in the forming behavior of the hcp materials as a result of superimposed electric current impulses. This behavior could be attributed to two effects. On the one hand, additional dislocation types are activated; on the other hand, new characteristic twin bands are formed. This is shown for all three hcp materials under investigation: Ti, Mg, and Zn. Furthermore, the hypothesis of the existence of a critical value of the current density at which a significant change in the plastic behavior occurs is verified by the experiments. The magnitude of this critical value for the analyzed hcp materials corresponds approximately to the theoretical values reported to be in the range of 1.6 to 2.0 kA mm⁻². In addition to the current density, the duration of the pulses also has an influence on the EPE. Understanding the correlation between the individual activated deformation mechanisms during electric pulse treatment can be crucial for controlling the electroplastic forming processes in a systematic and targeted manner.     

On the additivity of acoustoplastic and electroplastic effects

The influence of high density current pulses (approximately 10³ A mm⁻² for 25–80 μs) and the ultrasonic deformation (2 × 10⁴ Hz) on the flow stress of Cu tested in uniaxial tension at 300 K were investigated with the objective of determining the mechanisms of the joint action of electroplastic (EPE) and acoustoplastic (APE) effects. The experimental procedure used allows synchronization of the current pulse and the ultrasonic deformation and allows the current pulse and the ultrasonic deformation to be shifted in phase. It was found that the combined APE + EPE is smaller than the sum of separate effects and depends on the phase shift between the current pulse and the ultrasonic deformation. An extremum of the combined APE + EPE at a certain phase shift between the current pulse and the ultrasonic deformation was detected. An increase in the EPE after prior action of ultrasonic deformation was detected. Employing the thermally activated plastic flow concept, a differential equation for combined APE + EPE is proposed.     

Determination of the energy of electroplastic deformation of metals

On the basis of the well-known mechanism of electroplastic deformation, we suggest a method for determination of the fraction of energy of pulses of the electric current directly spent on the work of plastic deformation. The experimental data presented in the work confirm the validity of the proposed approach. The present work was partially financially supported by the International Science Foundation and the Ukrainian Government (Grant UBW 200). Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 38–43, July–August, 1997.     

Influence of electric current on superplastic deformation mechanism of 5083 aluminium alloy

5083 aluminium alloy superplastic forming adopted resistance heating can not only improve efficiency and cut energy but also generate electroplastic/electrosuperplastic effect to make the material deformation possess lower flow stress and higher plasticity. By analysing the influence of current on dislocation slipping, grain boundary migration and dynamic recrystallisation, it is found that the electron wind force can enhance the mobility of dislocations; meanwhile, the current also can reduce the activation heat of dislocation motion by joule heating effect. What is more, the grain size of resistance heating forming sample is significantly smaller than furnace heating, and the cavities in the sample become small and dispersive, so the resistance heating forming specimen possesses better performance.     

Uniform texture in Al-Zn-Mg alloys using a coupled force field of electron wind and external load

Cylindrical Al-Zn-Mg alloys were processed by electroplastic compression with forced air cooling.Compared to a simple compression process,an unequal intensity of {110} <111> was obtained,and other textures were eliminated by electroplastic compression,that is,electroplastic compression can promote a uniform texture.The various textures formed in different regions along the radial direction under a simple compression process were illuminated by analyzing the relationship between the crystal rotation and stress state.Furthermore,the interaction between the electrons and dislocations was studied in electroplastic compression.The electrons enhanced {110} <111> by promoting slipping of the dislocations when the Burgers vectors of the dislocations were parallel to the drift direction of the electrons.However,the electrons also inhibited crystal rotation by pinning the dislocations with the Burgers vectors perpendicular to the drift direction of the electrons.Therefore,textures other than {110} <111>have difficulty forming under electroplastic compression.The effect of the current energy on the texture(enhancement or attenuation) was in accordance with the law of conservation.The results provided reasonable explanations for the test phenomena.     

ELEVATED TEMPERATURE FATIGUE CRACK GROWTH IN ALLOY 718—PART II: EFFECTS OF ENVIRONMENTAL AND MATERIAL VARIABLES

Observations concerning the effects of the environment and material variables on the crack growth process in alloy 718 are reviewed and analyzed on the basis of deformation characteristics in the crack tip region. The review of the role of material variables has focused on the effects of chemical composition and microstructure parameters including precipitate size and morphology as well as grain size and morphology. These analyses have suggested that the governing mechanism at the crack tip is the degree of homogeneity of plastic deformation and associated slip density. For conditions promoting homogeneous plastic deformation, with a high degree of slip density, the environmental damage contribution is shown to be limited, thus permitting the dominance of cyclic damage effects which are characterized by a transgranular crack growth mode and a lower crack growth rate. Under conditions leading to inhomogeneous plastic deformation and lower slip density the crack tip damage is described in terms of grain boundary oxidation and related intergranular fracture mode. Considering that the crack growth damage mechanism in alloy 718 ranges from fully cycle dependent to fully environment dependent, conflicting experimental observations under different operating conditions are examined and a sensitizing approach is suggested to increase the alloy resistance to environmental effects.     

On the nature of plastic deformation generated by hydrostatic pressure in silicon single crystals

Macroscopic plastic deformation of silicon single crystals, caused by annealing at hydrostatic pressure and high temperature, was studied by X-ray topography and transmission electron microscopy. The analysis is given of elastic and thermal properties of material around surface cracks and scratches from which deformation process is propagated. The idea of elastic misfit between damaged self-strained material at cracks and scratches and defect-free silicon matrix, is introduced. On the basis of theoretical and experimental data it is concluded that the plastic deformation of silicon at high pressure consists of two processes. The first is a loss of coherency of cracks and scratches by the emission of dislocations at misfitting second phase precipitates present in silicon. The second is the macroscopic yielding from incoherent cracks and scratches at lower elastic strain energies.The presented mechanism explains also the deformation behaviour of silicon crystals subjected to tensile stress at high temperatures; the generation and propagation of dislocations at oxide precipitates before the macroscopic yielding [3].     

Creep and Superplasticity of Gadolinium-Doped Ceria Ceramics under AC Electric Current

Shaping of dense ceramics is difficult due to their inherent brittleness. Nanograined ceramics like tetragonal zirconia (TZP) can be superplastically deformed and shaped at high temperatures owing to grain boundary sliding (GBS). Herein, the enhanced plasticity of gadolinium-doped ceria (GDC) ceramics under mild and strong AC electric current in terms of steady state creep rate under both compressive and tensile loading is demonstrated. A current density of 25 and 200 mA mm⁻² is used for the creep deformation. The creep rate increases by up to two orders of magnitude under electric current. The stress exponent remains unchanged for creep experiments at 1200 °C with and without electric current, suggesting a GBS mechanism of plastic deformation in both cases. The field-enhanced creep rate is attributed to the interaction of space–charge layer and the electric field resulting in enhanced GBS. A higher current density results in enhanced ductility of GDC even when the Joule heating effect is compensated by reducing the furnace temperature.     

AZ31 镁合金板材自阻加热超塑成形及电流宏微观作用分析

目的为提高镁合金板材超塑成形加热过程的效率,将自阻加热引入到超塑成形过程中。方法通过自阻加热实验,分析了电流密度对坯料温度及升温速率的影响;设计了自阻加热胀形装置并确定了工艺参数,实现了AZ31镁合金的自阻加热超塑胀形。结果采用该加热方式可将坯料加热时间从数十分钟缩短至几十秒,极大地提高了加热速率,降低了能耗。结论分析了在电流作用下镁合金的变形机制,发现电流可以通过促进材料的再结晶形核、位错滑移来提高材料塑性,并具有钝化和阻碍空洞扩展的作用。     

一维钛纳米材料的SEM和TEM形貌研究

在10mol／L氢氧化钠溶液中，以溶胶状纳米TiO2为钛源，分别于150℃和180℃采用水热法合成了钛纳米晶须和钛纳米线；以金红石型纳米TiO2为原料在150℃水热合成了钛纳米管。用扫描隧道显微镜（SEM）、电子透射电镜（TEM）对产物进行了表征，结果表明钛酸钠纳米晶须扫描形貌为球形颗粒状，透射形貌为直径l～3nm针尖...     

Inhibition of Electromigration in Eutectic SnBi Solder Interconnect by Plastic Prestraining

Plastic prestraining was applied to a solder interconnect to introduce internal defects such as dislocations in order to investigate the interaction of dislocations with electromigration damage. Above a critical prestrain, Bi interfacial segregation to the anode, a clear indication of electromigration damage in SnBi solder interconnect, was effectively prevented. Such an inhibiting effect is apparently contrary to the common notion that dislocations often act as fast diffusion paths. It is suggested that the dislocations introduced by plastic prestraining acted as sinks for vacancies in the early stage of the electromigration process, but as the vacancies accumulated at the dislocations, climb of those dislocations prompted recovery of the deformed samples under current stressing, greatly decreasing the density of dislocation and vacancy in the solder, leading to slower diffusion of Bi atoms.     

Dynamic observation of dislocation-free plastic deformation in gold thin foils

Ductile fracture of metals produces a thin foil portion, which is observable by transmission electron microscopy, at the fractured edge. The thin foil portion shows unusual deformation microstructure, which contains no dislocations, but contains vacancy-type point defect clusters at extraordinarily high density. Dynamic observation of the deformation process revealed that these defect clusters are produced in the portion of local heavy deformation; however, no dislocation motion was observed during the course of the heavy plastic deformation, constituting direct evidence that the unusual deformation microstructure is produced by plastic deformation without dislocations. Also, the deformation was found to involve 14% elastic deformation, indicating that the dislocation-free plastic deformation occurs under an extraordinarily high internal stress level of more than 10 GPa, which is comparable to the ideal strength of metals. Furthermore, during the dislocation-free plastic deformation, equal-thickness fringes were found to disappear temporarily, suggesting that instability of crystalline state under extraordinarily high internal stress level is a key factor for the mechanism of dislocation-free plastic deformation.     

Mechanistic model for grain size and temperature dependence of flow stress in 316L austenitic stainless steel

Abstract

During plastic deformation of a polycrystalline material, both the grain interior and the grain boundary regions exhibit distinctly different dislocation behaviours at a given strain and temperature. Studying the variation of experimental flow stress with temperature, it seems that the flow stress of a fine grained polycrystalline material is mainly controlled by dislocation dynamics at and in the vicinity of grain boundaries. At low temperatures in a polycrystalline material, the dislocations are piled up at grain boundaries and the density of dislocations increases significantly in the grain boundary region, while at high temperatures the annihilation of dislocations take place at and in the vicinity of the grain boundaries during deformation. Therefore, the flow stress behaviour of a polycrystalline material can be understood in terms of the process of accumulation and annihilation of dislocations at and in the vicinity of grain boundaries at a given strain and temperature.     

Grain refinement of pure Al with different electric current pulse modes

A comparative study on grain refinement of pure Al by applying electric current pulse (ECP) at different positions in the mould was carried out. The experimental results showed that fine equiaxed grains were generated by exerting ECP at different electrode positions, and the highest proportion of fine equiaxed grain area was obtained when ECP was applied at the upper and lower lateral wall of the mould. The refinement mechanism of exerting ECP and the reasons of different refining efficiency with various ECP employing modes were also discussed.     

Features of the electrodiffusion-plastic deformation of silicon

The effect of a change in the electron subsystem of a single crystal, caused by the electric-field-assisted transfer of impurity atoms, on the features of electroplastic deformation in silicon was studied. The new method of action upon the motion of dislocations revealed a considerable decrease in the yield point and an increase in the general plasticity as compared to those observed in the material deformed by traditional methods. Possible mechanisms explaining the observed effects are considered.     

Generation of vacancies in high-speed plastic deformation

In a recent experiment, crystalline metals were subjected to high-speed plastic deformation, and subsequently a number of vacancy clusters were observed without any trace of dislocations. In an effort to explain this result, in the present study fluid-like behavior of solid in ultra-high-speed deformation is considered, and the possibility of spontaneous generation of vacancies analogous to cavitation in high-speed fluid flow is discussed. Similar to a large velocity gradient that induces turbulence in a high-speed fluid flow, large shear stress induced in a solid material during the course of high-speed deformation may generate vacancies instead of dislocations, if the dislocations cannot follow the deformation speed. In this paper, similarities between dislocation in solid and vortex in a fluid discussed, along with similarities between vacancy in a solid and cavitation in fluid, and a mechanism of vacancy production under high-speed plastic deformation of crystalline materials is proposed.     

Correlation of the microdislocation parameters with the hardness of plastically deformed heat-resistant steels

By the methods of electron-transmission microscopy and nondestructive testing, we study basic regularities of the influence of high-temperature plastic deformation on the kinetics of hardening of 15Kh13MF and 25Kh1M1F heat-resistant steels. The difference between the intensities of exhaustion of the plasticity of steels is discovered. Thus, the process of hardening of 15Kh13MF ferritic-martensitic steel is more intense, which is explained by the increase in nonparallelism and the decrease in the distance between subboundaries of the dislocation microstructure, reduction of sizes, and fragmentation of dislocation martensite. The 25Kh1M1F ferritic-pearlitic steel is characterized by less intense hardening explained by a different type of evolution of the dislocation structure connected with the transformation of dislocation nets and cells into globular and cellular disoriented structures and then into fragmented structures. It is shown that the relationship between the characteristics of substructural hardening, the density of dislocations within the low-angle boundaries, and the hardness (microhardness) of the material in the process of plastic deformation of heat-resistant steels can be described by a linear dependence independently of the level of strains. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 2, pp. 41–45, March–April, 2008.     

Defect hardening modeled in 2D discrete dislocation dynamics

Two-dimensional discrete dislocation dynamics simulations are used to model the plastic deformation of an fcc metallic material containing large densities of defects. An obstacle model is proposed, based on the line tension concept. Increasing yield strength and hardening are obtained when the obstacle density is increased and destroyable junctions are included. A high dislocation source density is used to obtain a good dissemination of dislocations. Over 30% of the total density is stored as junctions. Slip is shown to be localized within a few intense slip bands, whatever the obstacle density. This localization is quantified as a function of the density of obstacles.     

麦秸防护内衬包装材料隔热保温性能的研究

研究了麦秸防护内衬包装材料的工艺和产品特性之间的关系,以及密度对这种材料性能的影响。结果证明麦秸防护内衬包装材料可以作为珍珠棉等高分子发泡材料的替代品,是可以在运输包装中使用的理想的隔热保温材料。     

Research on solidification structure refinement of pure aluminum by electric current pulse with parallel electrodes

The influence of electric current pulse (ECP) with parallel electrodes on the solidification structure of pure aluminum has been investigated. The experimental results indicate that the solidification structure cannot be refined when the ECP is applied before the molten metal starts nucleating. However, significant refinement of the solidification structure can be achieved by applying ECP during the nucleation stage. In addition, under the same cooling conditions, ECP with parallel electrodes has better refinement effect than ECP with up–down electrodes. The refinement mechanism of ECP with parallel electrodes has also been proposed.