Continuous dynamic recrystallization during severe plastic deformation

Severe plastic deformation (strains > 100%) has been shown to create significant grain refinement in polycrystalline materials, leading to a nanometric equiaxed crystalline structure for such metals as aluminum, copper and nickel alloys. This process, termed continuous dynamic recrystallization, is governed by evolution of the dislocation structure, which creates new grain boundaries from dislocation walls. In the proposed model, plasticity occurs which firstly involves dislocation multiplication, leading to strain hardening limited by dynamic recovery. After a critical dislocation density is reached new grain boundaries are formed by condensation of walls of dislocations, creating a new stable configuration that is favored due to a reduction of the system free energy. This evolution of the microstructure continues to develop, with a consequent progressive decrease in the average grain diameter. The proposed model provides a quantitative prediction of the evolution of the average grain size, as well as the dislocation density, during continued plastic strain. The model can be calibrated by use of results from any experiment that involves large plastic deformation of metals, subject to negligible annealing effects. In this paper, the model has been calibrated, and consequently validated, through experiments on machining of Al 6061-T6.     

Microstructure evolution during dynamic recrystallization of hot deformed superalloy 718

Microstructure evolution during dynamic recrystallization (DRX) of superalloy 718 was studied by optical microscope and electron backscatter diffraction (EBSD) technique. Compression tests were performed at different strains at temperatures from 950 °C to 1120 °C with a strain rate of 10⁻¹ s⁻¹. Microstructure observations show that the recrystallized grain size as well as the fraction of new grains increases with the increasing temperature. A power exponent relationship is obtained between the dynamically recrystallized grain size and the peak stress. It is found that different nucleation mechanisms for DRX are operated in hot deformed superalloy 718, which is closely related to deformation temperatures. DRX nucleation and development are discussed in consideration of subgrain rotation or twinning taking place near the original grain boundaries. Particular attention is also paid to the role of continuous dynamic recrystallization (CDRX) at both higher and lower temperatures.     

Constitutive modeling and prediction of hot deformation flow stress under dynamic recrystallization conditions

Simple modeling approaches based on the Hollomon equation, the Johnson–Cook equation, and the Arrhenius constitutive equation with strain-dependent material’s constants were used for modeling and prediction of flow stress for the single-peak dynamic recrystallization (DRX) flow curves of a stainless steel alloy. It was shown that the representation of a master normalized stress–normalized strain flow curve by simple constitutive analysis is successful in modeling of high temperature flow curves, in which the coupled effect of temperature and strain rate in the form of the Zener–Hollomon parameter is considered through incorporation of the peak stress and the peak strain into the formula. Moreover, the Johnson–Cook equation failed to appropriately predict the hot flow stress, which was ascribed to its inability in representation of both strain hardening and work softening stages and also to its completely uncoupled nature, i.e. dealing separately with the strain, strain rate, and temperature effects. It was also shown that the change in the microstructure of the material at a given strain for different deformation conditions during high-temperature deformation is responsible for the failure of the conventional strain compensation approach that is based on the Arrhenius equation. Subsequently, a simplified approach was proposed, in which by correct implementation of the hyperbolic sine law, significantly better consistency with the experiments were obtained. Moreover, good prediction abilities were achieved by implementation of a proposed physically-based approach for strain compensation, which accounts for the dependence of Young’s modulus and the self-diffusion coefficient on temperature and sets the theoretical values in Garofalo’s type constitutive equation based on the operating deformation mechanism. It was concluded that for flow stress modeling by the strain compensation techniques, the deformation activation energy should not be considered as a function of strain.     

Microstructure evolution and dynamic recrystallization mechanism during thermal deformation of GH4698 superalloy

Journal of Materials Science - The microstructure evolution of GH4698 nickel-based superalloy was studied over a temperature range of 1000–1200 °C, a strain rate range of...     

Effect of dynamic recrystallization on the importance of grain-boundary sliding during creep

During creep of polycrystalline materials at elevated temperatures, a certain amount of the strain is accommodated by grain-boundary sliding (GBS). The relative importance of GBS depends on the stress and grain size and sometimes temperature. During high-strain deformation, dynamic recrystallization often occurs with the resultant grain size only related to the stress. In this situation the importance of GBS is then dependent only upon stress and sometimes temperature. In dynamically recrystallized Magnox Al80 deformed atT>0.8T _m, 16 to 23% of the imposed strain is accommodated by GBS. A comparison has been made between the experimental results and some theoretical models for the importance of GBS during creep, modified to take account of recrystallization. The best fit to the data is obtained with the modified form of Langdons model. Deformation mechanism maps constructed with this model suggest that dynamic recrystallization can cause a switch of mechanism from dislocation creep to dominant GBS at intermediate temperature (T<673 K) and low stress. Deformation mechanism maps have also been constructed for calcite based on the data of Schmidet al. These suggest that GBS is an important mechanism in calcite deformed under geological conditions.     

Transition of dynamic recrystallization mechanism during hot deformation of Incoloy 028 alloy

Dynamic recrystallization(DRX) plays significant roles in manipulating of microstructures during hot deformation and the result mechanical properties;however,the underling mechanism leading to multi scale-microstructures remains poorly understood.Here,the DRX mechanism under wide processing conditions(i.e.950-1200℃,0.001-10 s~(-1)) in Incoloy 028 alloy was investigated,where the relationships among flow stress,Z parameter and grain size,as well as the evolution of characteristic microstructures(grain size,sub-grain boundaries,and high angle grain boundaries),are established.As the values of Z parameters decrease(corresponding to decreased flow stresses),three typical softening mechanisms successively occur,ranging from continuous DRX controlled by dislocation glide,discontinuous DRX dominated by dislocation motion(climb and cross/multiple slip) and grain boundary migration,to dynamic normal/abnormal grain growth resulting from grain boundary migration,with transition regions where two adjacent mechanisms occur simultaneously.Correspondingly,these above three softening mechanisms result in ultrafine,fine and coarse grains,respectively.The present findings demonstrate a comprehensive understanding of DRX mechanism over a wide range of processing conditions,and further provide a new guideline for preparing single crystals.     

An investigation of the substructural changes accompanying high-temperature creep of mono- and polycrystalline nickel samples

Results are presented here of an investigation of the kinetics of high-temperature creep of mono- and polycrystalline nickel samples and of the associated substructural changes. It is shown that at low stresses, not exceeding the so-called linear creep limitp _o creep takes place according to the Nabarro-Herring mechanism. The characteristic particle size allowed for in the Nabarro-Herring theory is similar to the size of the blocks (subgrains), measured by X-ray diffraction. In the first stage of creep at stressesp <p ₀ the average block size increases and the dislocation density decreases. In the second and third stages the creep does not cause any noticeable changes in the substructural characteristics (block size, dislocation density). Creep of the material is accompanied by the onset of porosity, the formation and development of which is due, apparently, to the coalescence of vacancies. Atp >p ₀, when Weertman's mechanism is the main creep mechanism, the dislocation density increases with increase in the amount of creep. Creep in this stress range is accompanied by the appearance of pore-cracks which are formed mainly at the grain (block) boundary junctions; these hinder the movement of dislocations. The dependence of the linear creep limit on temperature is determined by the temperature-dependence of the shear modulus.     

Factors affecting dynamic recrystallization of metals and alloys

Abstract

The high-temperature mechanical behaviour of copper, Cu–Al alloys, and nickel has been examined using torsional testing with hollow testpieces in conjunction with microstructural observations on deformed and quenched specimens using both optical and electron microscopy. Dynamic recrystallization occurred in these materials as the restoration process during high-temperature deformation. The factors influencing dynamic recrystallization have been considered, including materials of high stacking fault energy. It was found that the regime of dynamic recrystallization and the transition in flow stress behaviour could be reasonably represented in terms of the Zener–Hollomon parameter. In Cu–Al solid solution alloys, although the addition of the solute aluminium into copper lowered the stacking fault energy, dynamic recrystallization was retarded to higher strains due to the reduced mobility of the grain boundary. By mechanical and microstructural analysis of the behaviour of various single phase metals and alloys during dynamic recrystallization, the factors influencing the behaviour (i.e. stacking fault energy (solute elements), Zener–Hollomon parameter (deformation condition), and strain) can be summarized on a three dimensional schematic.

MST/587     

Dislocation substructures developed during dynamic recrystallisation in polycrystalline nickel

Abstract

The dislocation substructures developed during dynamic recrystallisation (DRX) were studied by means of tensile tests and metallographic observations on polycrystalline nickel. The average cell size in the homogeneous substructures decreased rapidly with straining to about half the peak strain, whereupon DRX nuclei began to form. The average cell size then approached a constant value for increasing strain. Full DRX substructures were distributed heterogeneously throughout all areas, and were classifiable into three categories: (i) DRX nuclei, (ii) growing DRX grains containing a dislocation density gradient, and (iii) large DRX grains with a fairly homogeneous substructure. The average cell size in region (iii) could be expressed as a function of either the peak flow stress or the DRX grain size. The relationship between these microstructures and flow behaviour under DRX are discussed in detail.

MST/1285     

An observation of permanent strain during recrystallization and growth of steel under externally applied stress

An occurrence of permanent strain during recrystallization and growth of an extra low-carbon steel under externally applied stress much lower than the yield stress of the steel was observed by dilatometric measurement. This suggests that the transformation-plasticity-like deformation can occur even in the reaction which is barely accompanied with the internal stress caused by volume change.     

Influence of strain rate and crystallographic orientation on dynamic recrystallization of pure Zn during room-temperature compression

This work investigates the strain rate dependence of dynamic recrystallization behaviour of high-purity zinc in room temperature compression under strain rates of 10-4 s-1,10-2 s-1 and 0.5 s-1.Results from electron backscatter diffraction provide insight into the deformation and dynamic recrystallization mech-anisms operative.Continuous dynamic recrystallization,twin-induced dynamic recrystallization,and discontinuous dynamic recrystallization are all active under compressive deformation at room temper-ature.Due to the high stacking fault energy of Zn,continuous dynamic recrystallization is the dominant mechanism while discontinuous dynamic recrystallization only operates in the early stages of compres-sion at 10-4 S-1.Dynamic recrystallization kinetics are enhanced at higher strain rates(10-2 s-1 and 0.5 s-1)due to an increased contribution from twin-induced dynamic recrystallization.The present study reveals that the controlling mechanisms for continuous dynamic recrystallization are basalslip and 2nd order pyramidalslip activity.Because the activation of slip systems is mainly deter-mined by crystallographic orientation,continuous dynamic recrystallization behaviour varies with grain orientation according to their propensity for basal and 2nd order pyramidal slip.     

The identification of dynamic recrystallization and constitutive modeling during hot deformation of Ti55511 titanium alloy

In the present paper, an internal-variable identification approach has been proposed to investigate the dynamic recrystallization (DRX) behavior during hot deformation and corresponding constitutive model has been constructed. Isothermal compression experiments of Ti55511 titanium alloy were conducted for verification. Plastic behavior is determined by dislocation evolution in many cases while deforming. The comparison between saturated and DRX critical dislocation density was made to distinguish the occurrence of dynamic recrystallization/recovery (DRV) during hot deformation. The influence of deformation parameters on DRX behavior was illustrated by dislocation evolution map, validated by the power dissipation efficiency distribution. DRX process during hot deformation of Ti55511 alloy tends to occur under moderate temperatures and low strain rates. In addition, a physical-based Arrhenius constitutive formula has been derived for DRX criticality. The strain-rate sensitivity coefficients during hot deformation were fixed as a constant equal to 1/6 and the deformation activation energy was related to the material's self-diffusion activation.     

Influence of Bi addition on dynamic recrystallization and precipitation behaviors during hot extrusion of pure Mg

Low material cost and high extrudability for ensuring price competitiveness with Al alloys, as well as excellent mechanical properties, are essential for expanding the application range of Mg extrudates. Bi is a promising alloying element for developing extruded Mg alloys that satisfy such requirements. Bi is inexpensive, exhibits a high solubility limit, and forms a thermally stable Mg_3Bi_2 phase, which improves the commercial viability and enables the high-speed extrusion of Mg–Bi alloys. In this study, the effects of Bi addition on the dynamic recrystallization(DRX) and dynamic precipitation behaviors during hot extrusion of a pure Mg and the resultant microstructure and mechanical properties of the extruded materials were investigated. The addition of 6 wt% and 9 wt% Bi to a pure Mg yielded numerous fine Mg_3Bi_2 precipitates during the early stage of hot extrusion. Consequently, the area fraction of dynamic recrystallized(DRXed) grains decreased because of DRX-behavior suppression by the Zener pinning effect.However, the DRXed grain size was substantially reduced through the grain-boundary pinning effect.The size and number of undissolved Mg_3Bi_2 particles in the homogenized billets increased when the Bi content was increased, which resulted in increased DRX fractions owing to the enhanced levels of particle stimulated nucleation. Bi addition yielded considerable strength improvement of the extruded pure Mg. However, the extruded Mg–Bi binary materials were less ductile than the extruded pure Mg material. This lower ductility resulted from the cracking at twins formed in the coarse un DRXed grains of the Mg-6Bi material and the cracking at large undissolved Mg_3Bi_2 particles in the Mg-9Bi material.     

Effect of initial texture on dynamic recrystallization of AZ31 Mg alloy during hot rolling

Wedge-shaped AZ31 plates with two kinds of initial textures were rolled at 573 K to investigate the effect of initial texture on dynamic recrystallization (DRX). The results indicated that the initiation and nucleation of DRX were closely related to the initial texture. The initiation and completion of DRX in the TD-plate were significantly retarded compared with that in the ND-plate. Twin related DRX nucleation was mainly observed in the ND-plate samples; while gain boundary related DRX nucleation was mainly observed in the TD-plate samples. The different DRX behavior between the TD- and ND-plates was attributed to the different deformation mechanism occurring before DRX initiation. For the ND-plate, dislocation glide was considered as the main deformation mechanism accompanied with {1 0 −1 1}-{1 0 −1 2} double twin, which led to the increment of a faster increasing stored energy within the grains. And {1 0 −1 1}-{1 0 −1 2} double twin was mainly found to be DRX nucleation site for the ND-plate. For the TD-plate, {1 0 −1 2} extension twin was the dominant deformation mechanism which resulted in a basal texture with the c-axis nearly parallel to ND. The stored energy caused by dislocation motion was relatively small in the TD-plate before a basal texture was formed, which was considered as the main reason of that DRX was retarded in the TD-plate compared with that in the ND-plate. Based on the difference in deformation mechanism and DRX mechanism caused by the different initial texture, the variation in grain size, micro-texture and misorientation angle distribution in the ND and TD plates were discussed.     

Multiscale model of dynamic recrystallization in hot rolling

The paper is focused on development of multi-scale CAFE approach, which combines cellular automata (CA) and finite element (FE) methods. The CA model of dynamic recrystallization (DRX) was implemented into the thermal-mechanical FE code, which simulates rolling process using steady state Eulerian approach. Several CA spaces were created at the cross section of the sample and their state was calculated using changes of the external variables along the flow lines. Current local values of temperature, strain, strain rate and stress are calculated by the macro-scale FE model and then passed to micro-scale CA simulations. Improvements in analysing of average grain size in CA simulation were introduced. The CAFÉ calculations were performed for hot rolling of the carbon-manganese steel. The resulting grain sizes after the process were compared with the experimental data, achieving satisfactory consistency.     

Determination of flow stress and the critical strain for the onset of dynamic recrystallization using a hyperbolic tangent function

A new model has been developed to estimate the flow stress under hot deformation conditions up to the peak of the stress–strain curves. This model is derived from the general form of hyperbolic function by introducing an additional parameter to bring the results to a more acceptable level. Stress–strain curves and the critical strain of a ‘304 austenitic stainless steel’ are determined with an average percentage error of 1.24. The model is also used to obtain an equation which has the ability of predicting the critical strain for the onset of dynamic recrystallization.