Activation Energy For Plastic Deformation Research Articles

High temperature nanoindentation as a tool to investigate plasticity upon phase transformations demonstrated on Cobalt

A new field of application for high temperature nanoindentation as a complimentary method to understand the mechanics of plasticity upon bulk phase transformations in thermodynamic equilibrium is introduced. The feasibility is outlined on polycrystalline Cobalt involving a low-temperature hexagonal closed packed phase, and above 700K, a high-temperature face centered cubic phase, which was conventionally characterized by means of differential scanning calorimetry and high temperature X-ray diffraction. Strain rate sensitivity, activation volume and activation energy of plastic deformation were determined up to 873 K to identify the rate-controlling deformation mechanism. From RT to 473 K plasticity was found to be controlled by lattice friction on the basal plane, where dislocations have to overcome the Peierls barrier and deformation twinning was apparent in the hexagonal phase. The thermal activation of cross slip lead to reduced twinning actions when the phase transition temperature is approached. In the high temperature face centered cubic phase deformation was found to be controlled by cutting of forest dislocations and no deformation twinning was observed.

Hot deformation behaviour of bimodal sized Al2 O3 /Al nanocomposites fabricated by spark plasma sintering.

The deformation behaviour of bimodal sized Al2 O3 /Al nanocomposites were investigated by hot compression tests conducted in the temperature range 350-500°C and strain rates of 0.001, 0.01 and 0.1 s-1 . The dynamic recrystallisation behaviour of the nanocomposites strongly depended on the forming parameters. The bimodal sized Al2 O3 particles played a crucial role in the recrystallised microstructure. The addition of bimodal sized Al2 O3 particles led to a significant increase of activation energy of plastic deformation, corroborating the enhanced resistance of the nanocomposite to hot deformation. This was also reflected by the increased compressive yield strength in the nanocomposite due to both dislocation strengthening caused by n-Al2 O3 and preventing the grain growth due to the presence of μ-Al2 O3 at grain boundaries. It was found that with the decrease of Z values, local strain induced by deformation was released and the grain size of aluminium matrix gradually increased, indicating that the main softening mechanism of the bimodal sized Al2 O3 /Al nanocomposites was dynamic recrystallisation (DRX). The lower the Z value was, the easier the DRX occurred. The highly beneficial role of the bimodal sized Al2 O3 reinforcement in improving the high-temperature performance of aluminium matrix nanocomposite was discussed.

Evaluation of the Hot Workability of Commercially Pure Ti Using Hot Torsion Tests.

Optimum processing conditions were obtained by evaluating the hot working behavior of commercially pure Ti using hot torsion tests. Hot torsion tests were conducted at temperatures ranging from 800 °C-1000 °C and strain rates ranging from 0.1-10 s-1. The flow curves show that the peak stress increases as the temperature decreases and the strain rate increases. The optimum processing conditions were derived by comparing the processing and activation energy maps. The microstructure was characterized based on various regions of the processing map. The activation energy for plastic deformation was obtained using the constitutive equation. The activation energy differs depending on the constituent phases.

High-Temperature Strength of Copper-Based Condensed Materials

The temperature dependences of Vickers microhardness, ultimate strength, and offset yield point in the temperature range between 290 and 1070 K were determined for the transversal direction in copper-based composites Cu–Zr–Y–Mo, Cu–Cr, Cu–C, and Cu–W. The materials produced by electron-beam evaporation/condensation are used for the manufacture of current collectors which, when in operation, are subjected to intensive wear and electrical erosion as well as to mechanical loading at elevated temperatures. This production method provides composite materials with a special layered structure, where copper layers are interspersed with dispersed-particle layers of other metals. The paper describes the procedures of Vickers microhardness and tensile testing at high temperatures. A general thermodynamic activation analysis of the derived relationships of hardness and strength has been performed for various copper-based composite systems in the range of (0.2–0.8)Tmelt of copper. A comparative analysis of the obtained values of the activation energy of plastic deformation of copper and copper-based composites as well as the theoretical and experimental data on deformation, internal friction, creep, and self-diffusion processes in copper has revealed that in a wide range of temperatures the activation energy of plastic deformation undergoes a significant change during a transition from one temperature region to another. This is indicative of a successive changeover of effective (governing) thermally activated mechanisms of plastic deformation. It is proved that a heat-induced change in the strength characteristics (hardness, ultimate strength, offset yield point) of the composites studied is controlled by the same plastic deformation mechanisms whose temperature regions coincide.

Rheological properties of EP742-ID alloy in the context of Integrated Computational Materials Engineering (ICME). Part 1. Results of experimental research

The article covers rheological properties of the EP742-ID alloy in high-temperature compression tests of cylindrical samples with different ratios of similar initial diameters and heights (d0 /h0). The results of experimental research in the temperature range t = 1000÷1150 °C and initial deformation rates e · 0 = 3·10–2÷3·10–4 s–1 have shown that an increase in compression flow stress with the growth of the d0 /h0 ratio is observed at all temperatures and deformation rates with a linear dependence on the e · 0 value and the d0 /h0 ratio. The method is proposed to recalculate deformation resistance indicators to the set ratio of similar sizes. Higher compression flow stress is connected with an increase in the coefficient of rigidity of samples and their specific contact surfaces. The dependence of apparent activation energy of alloy plastic deformation (Qdef), its relationship with the phase structure and conditions of the process of γ solid solution dynamic recrystallization is established. In the temperature conditions of the beginning of γ solid solution dynamic recrystallization process (1000–1050 °C) the Qdef value for d0 /h0 = 0,75 samples is 959 kJ/mol. The highest Qdef values for d0 /h0 = 0,75 samples of 1248 and 1790 kJ/mol are observed in the range of temperatures of intensive grain boundary γ ′-phase dissolution and coagulation (1050–1100 °C). Samples with d0 /h0 = 3,0 in this temperature range have Qdef up to 2277 kJ/mol. The apparent activation energy of plastic deformation decreases to 869 kJ/mol in the range of temperatures of homogeneous γ solid solution with grain-boundary primary and secondary carbides (1100–1150 °C). The paper provides the results of alloy sample compression at single and repeated consecutive loading with various times of pauses between deformations. It is shown that meta dynamic recrystallization under experimental conditions does not occur in the γ + γ ′-range, and occurs inertly in the γ-range.

Quantitative description of the flow-stress dependence of aluminum alloys at the stage of steady flow upon hot deformation on the Zener–Hollomon parameter

The deformation behavior of a 1981 aluminum alloy has been studied using a complex for simulating thermomechanical processes in the temperature range of 200–400°C at a deformation rate in the range of 0.001–10 s–1. The models of the relationships between the flow stress, temperature, and deformation rate have been constructed using a power-law dependence, exponential dependence, and hyperbolic-sine function on the Zener–Hollomon parameter (Z). In the calculations according to the power-law and exponential equations, discrepancies between the calculated and experimental values of the Zener–Hollomon parameter have been revealed at low and high values. These discrepancies are caused by the fact that the experimentally obtained dependences of the flow stress on the Z parameter over the entire range of its change with a single magnitude of the effective activation energy of the plastic deformation consist of two linear parts that correspond to the hot and warm deformation and have different magnitudes of the effective activation energy of plastic deformation with a lower value of the activation energy for hot deformation.

Hot deformation and activation energy of a CNT-reinforced aluminum matrix nanocomposite

Hot deformation behavior of a 2.0wt% CNT/2024Al nanocomposite was studied via high-temperature compression over a temperature and strain rate range of 200–400°C and 0.001–0.1s−1, respectively. Both flow stress and Zener-Hollomon parameter increased with decreasing deformation temperature and increasing strain rate. The addition of CNTs led to a significant increase of activation energy of plastic deformation, corroborating the enhanced resistance of the nanocomposite to hot deformation. This was also reflected by the increased compressive yield strength in the nanocomposite due to both Hall-Petch strengthening and effective load transfer of CNTs dispersed in the matrix with well-bonded interfaces. Detailed microstructural examinations at a high strain rate, within the upper and lower temperature limits, revealed the occurrence of second-phase particle shearing, refinement, re-precipitation, and re-orientation. The highly beneficial role of the CNT reinforcement in improving the high-temperature performance of Al alloys was discussed.

Effect of Aluminium Content on High Temperature Deformation Behavior of TiAl Intermetallic Compound

Fundamental studies of microstructural changes and high temperature deformation of titanium aluminide (TiAl) were conducted from the view point of the effect of Al content in order to develop the manufacturing process of TiAl. Microstructures in an as cast state consisted mainly of lamellar structure irrespective of Al content. By homogenization at 1473 K, the microstructures of Ti-49Al and Ti-51Al were transformed into an equiaxial structure which was composed of γ-TiAl, while the lamellar structure that was observed in Ti-46Al and Ti-47Al was much more stable. We found that the reduction of Al content suppressed the formation of equiaxial grains and resulted in a microstructure of only a lamellar structure. On Ti-49Al and Ti-51Al, dynamic recrystallization occurred during high temperature deformation, and the microstructure was transformed into a fine equiaxial one, while the microstructures of Ti-46Al and Ti-47Al contained few recrystallized grains and consisted mainly of a deformed lamellar structure. We observed that on the low-Al alloys the lamellar structure under hard mode deformation conditions deformed as kink observed B2-NiAl. High temperature deformation characteristics of TiAl were strongly affected by Al content. An increase of Al content resulted in a decrease of peak stress and activation energy for plastic deformation and an increase of the recrystallization ratio in TiAl.

Activation parameters and deformation mechanisms of ultrafine-grained copper under tension at moderate temperatures

Mechanical properties, characteristic features of deformation behavior, the activation energy for plastic deformation, strain rate sensitivity and activation volume of ultrafine-grained (UFG) and conventional coarse-grained (CG) copper have been studied by tension in the temperature interval of 293–573K and in the strain rate interval of 1.3×10−2–3.0×10−5s−1. It is found that both the properties and the activation parameters differ significantly in UFG and CG copper suggesting the different deformation mechanisms. Plastic flow is shown to be controlled by grain boundary diffusion in the case of UFG copper. Considering the thermal activation analysis data and deformation relief appearing on the pre-polished surface of the test samples, the significant contribution of grain boundary sliding to the overall deformation during plastic flow of UFG copper at moderate temperatures is supposed.

Hot working behavior of a WE54 magnesium alloy

The plastic deformation and recrystallization behavior of the commercial magnesium alloys WE54 was analyzed using the strain rates 0.01, 0.1, 1, and 5 s−1 in the temperature range from 400 to 550 °C. The dependence of the flow stress on the temperature and the strain rate was modeled using the Garofalo hyperbolic sine equation. Thereby, the activation energy for plastic deformation of 224 kJ mol−1 was determined considering the flow stress at a strain of 0.5. The analysis revealed a stress exponent of 3.2. Furthermore, processing maps were generated by plotting the efficiency of power dissipation and the instability parameter considering different instability criteria as a function of the temperature and the strain rate. Depending on these parameters the extent of the recrystallization and the localization of the nucleation varied, significantly. At 400 °C, the recrystallization is very limited and was observed at grain boundaries (GB), shear bands (SB), and twin boundaries (TW). Increasing temperatures result in an increased recrystallized fraction, while lower strain rates promote grain boundary nucleation and reduce the amount of SBN and TW. The prediction of the processing map was verified by large scale extrusion trials, which proof that the evaluation of hot compression data can provide an effective tool to establish viable processing parameters.

Physical mesomechanics of grain boundary sliding in a deformable polycrystal

The temperature-rate dependences of strain resistance and the mechanisms of grain boundary sliding in Pb polycrystals and Pb-based alloys under active tension were investigated. The activation energy of plastic deformation and grain boundary sliding was determined. The structural mechanisms of grain boundary sliding were studied in a wide temperature range. The conclusion was made that self-consistency of grain boundary sliding and intragranular plastic flow has its origin in rotational deformation modes, with the grain boundary sliding being a primary process. Theoretical analysis of rotational deformation modes involved in grain boundary sliding was performed. It is shown that the dependence of deforming stress on the polycrystal grain size is impossible to describe by one universal Hall-Petch equation.

Hot working behavior of AZ31 and ME21 magnesium alloys

The plastic deformation and recrystallization behavior of the commercial magnesium alloys AZ31 and ME21 were analyzed in a wide temperature range. Using the conventional hyperbolic sine equation the flow stress dependence on temperature and strain rate was modeled. The activation energy for plastic deformation significantly increased with increasing temperature and delivered values above 180 kJmol−1 for both alloys in the very high-temperature regime (400–550 °C). At lower temperatures (250–400 °C) the activation energy of the AZ31 alloy was approximately 108 kJmol−1 considering the peak stress as well as 120 kJmol−1 considering the flow stress at a strain of 0.5. The stress exponent varied in a range between 4.5 and 6.5. During the high-temperature compression tests a partial recrystallized microstructure was formed, which was distinctly different in AZ31 compared to ME21 due to the different onset of dynamic recrystallization (DRX) mechanisms.

Effects of microwave heating in nanostructured ceramic materials

This paper studies the influence of microwave heating on mass transport phenomena and phase transformations in nanostructured ceramic materials. Faster mass transport that depends significantly on the microwave field intensity is observed during microwave annealing of nanoporous alumina membranes. The effect of the microwave field on phase transformations and pore structure evolution in alumina powder compacts is characterized quantitatively. Preferred orientation of pores in ceramics sintered under linearly polarized microwave radiation is predicted theoretically and demonstrated experimentally. Decrease in the activation energy of plastic deformation is experimentally observed for alumina-based ceramics under microwave heating.

Nanoquasicrystalline Al–Fe–Cr-based alloys. Part II. Mechanical properties

Nanoquasicrystalline Al-based alloys show considerable promise for elevated temperature applications compared with commercial Al-based alloys. In particular, a group of Al–Fe–Cr-based alloys-containing Ti, V, Nb or Ta have outstanding thermal stability. In the present work, the elevated temperature mechanical properties of these nanoquasicrystalline alloys were studied by tensile tests at a constant strain rate. Tests were designed in order to compare the mechanical behaviour at different test temperatures. Fractographic analysis was also carried out. The apparent activation energy for plastic deformation was found to be close to that for lattice self-diffusion for pure Al in the Al–Fe–Cr ternary alloy and in the Ti-containing alloy, and for grain boundaries diffusion for pure Al in the V-containing alloy, whereas the activation energy of the alloy with Ta additions was three times higher. All of the alloys showed similar sensitivity of plastic deformation to the strain rate in the range of 10 −3–5 × 10 −6 s −1 at 350 °C. The apparent true stress exponent was n app ∼ 7, which can be associated with a deformation process controlled by dislocation mechanisms.

Effect of grain size on the activation energy for plastic deformation near room temperature in a Zn–28.7 pct Al–1.9 pct Cu alloy

The Zn–22 wt pct Al eutectoid alloy is a superplastic material which has been used in a number of experimental investigations of superplasticity. Considerable attention has been devoted to this alloy, because this material has numerous potential applications [1]. Zinc-rich aluminum alloys with: Fe [2, 3] or with Copper additions [2–5], has been of engineering interest over recent years, because the addition of some alloying elements enhances the wear resistance, elastic modulus, yield strength and corrosion resistance under the service conditions of stress and temperature without having a major effect on the superplastic behavior during production of components. The main goal of this investigation is to study the effect of grain size on the activation energy for plastic deformation, near room temperature, in a Zn–Al–Cu Alloy. The chemical analysis of the alloy used in this work was Zn– 28.7 wt pct Al–1.9 wt pct Cu. The superplastic material was obtained by extrusion resulting in a grain size 2.7 lm. A number of flat tensile specimens were prepared from this material. A set of specimens were heat treated at 548 K for 3 h, followed by rapid quenching in water at room tempeature. This treatment produce a grain size of 0.6 lm, measured by the line intercept method [6]. The experiments were conducted using two grain size specimens. Flat tensile specimens, having 1.6 cm gage length and 0.2 · 0.32 cm cross-section, were used to study creep behavior. Creep tests were carried out using a constant load machine (SATEC), which was modified to provide constant applied stress, with a variation of no more than 0.7% in the chosen stress. Stresses between 1.6 MPa and 20.5 MPa were applied to the samples producing steady state strain rates between 6.4*10 s to 1.6*10 s. The strain during the creep tests was measured with a Schaevitz linear variable differential transformer (LVDT) accurate to ±1.3*10 cm, and this information was monitored using a data acquisition system. The test temperatures ranging from 294 K to 398 K were achieved by using tungsten lamps around the specimen holder. This heating device was capable to rise to the selected temperature of the specimen within 20 s. The test temperatures were constant at the level of ±0.5 K. The plots of true strain, e, against time, t, which were obtained for specimens with grain sizes of 2.7 lm and 0.6 lm are, respectively illustrated in Figs. 1 and 2. Examination of Fig. 1 corresponding to samples with 2.7 lm has shown that the creep curves exhibit a very short decelerating primary stage followed by a steady state stage; and also exhibit that primary transient strain increases with increasing the applied stress. Unlike our experimental J. D. Munoz-Andrade has written this article for partial fulfillment of the Individualized Doctorate Program at the Facultad de Ingenieria, Universidad Central de Venezuela, Caracas, Venezuela.

Internal stress and activation energy of creep

The stress σ acting on a moving dislocation can be divided into two components: the effective stress σ * that opposes the velocity dependent lattice friction and the internal stress σ i that overcomes long-range stress fields generated by neighbouring dislocations or other obstacles. The activation energy of plastic deformation has to be determined for the same state of structure, i.e. at an identical level of the internal stress, and at the same applied stress Q i * = − k ∂ ln ⁡ ε ˙ ∂ ( 1 / T ) σ i , σ , where ε ˙ is the creep rate, T the absolute temperature and k is the Boltzmann's constant. A new method for determining this quantity is proposed. It is shown that at zero applied stress this energy is identical to the apparent activation energy of creep determined at constant applied stress.

Effect of Low Carbon Contents on the Activation Energy for Plastic Deformation upon Steels

Effect of Low Carbon Contents on the Activation Energy for Plastic Deformation upon Steels

High-temperature dynamic thermomechanical analysis of NBS 711 glassy fiber

The complex modulus in bending of NBS 711 glassy fibers was investigated using dynamic thermomechanical analysis (DTMA) in a wide temperature range from room temperature to the supercooled liquid region through the glass transition region. Temperature dependencies of both real part, E′, and imaginary part, E″, of the dynamic modulus as well as the loss angle, tanδ, were studied as a function of heating rate and frequency. Experimental data were compared with results of the static thermomechanical analysis and one kind of dynamic DSC (StepScan DSC), viscosity data of NBS 711 were also used. Good agreement of DTMA and StepScan DSC results was found. Activation energy of plastic deformation calculated from frequency dependence of E″ and activation energy of viscous flow were found close to each other.

Studies of deformation mechanisms in ultra-fine-grained and nanostructured Zn

The temperature, strain rate, grain size and grain size distribution effects on plastic deformation in ultra-fine-grained (UFG) and nanocrystalline Zn are systematically studied. The decrease of ductility with the decrease of average grain size could be an inherent effect in nanocrystalline materials, that is, not determined by processing artifacts. The superior ductility observed in UFG Zn may originate from both dislocation creep within large grains and grain boundary sliding of small nanograins. The stress exponent for dislocation creep is about 6.6. The activation energy for plastic deformation in UFG Zn is close to the activation energy for grain boundary self diffusion in pure Zn.

Flow stress and ductility of duplex stainless steel during high-temperature torsion deformation

Hot torsion tests were performed on a duplex stainless steel (DSS)-type EN1.4462 steel. The temperature was varied in the range from 950 °C to 1200 °C, while the strain rate was varied from 0.01/s to 2/s. The mean flow stress (MFS) was fitted to the hyperbolic sine function proposed by Sellars and Tegart. An activation energy for plastic deformation of Q HW = 425 kJ/mol was obtained. The high value was explained by the fact that, in addition to the softening of ferrite (α) and austenite (γ), there was a decrease in the volume fraction of the high-strength austenite with increasing temperature. For higher values of the Zener-Holomon parameter (Z), the MFS showed a linear dependence on ln (Z), which was related to the gradual disappearance of a yield-point-elongation-like effect. This yield-point-elongation-like effect was characterized by a nonstrengthening plateau during the initial stages of plastic deformation. The strain to rupture and the dynamic softening were both found to decrease for higher values of ln (Z). Therefore, the ductility was directly related to the amount of dynamic softening, i.e., dynamic recrystallization (DRX) of the austenitic phase. At higher strain rates, significant dynamic softening was only observed for temperatures above 1100 °C. The strain-rate sensitivity (m) was found to vary from 0.13 at 950 °C to 0.22 at 1200 °C.