Dynamic Grain Research Articles

Microstructure evolution of ZK40 magnesium alloy during high strain rate compression deformation at elevated temperatures

Microstructure evolution of the homogenised ZK40 magnesium alloy was investigated during compression in the temperature range of 250–400°C and at the strain rate range of 0·01–50 s−1. At a higher strain rate (⩾10 s−1), dynamic recrystallisation developed extensively at grain boundaries and twins, resulting in a more homogeneous microstructure than the other conditions. The hot deformation characteristics of ZK40 exhibited an abnormal relationship with the strain rate, i.e., the hot workability increased with increasing the strain rate. However, the dynamic recrystallisation grain size was almost the same with increasing the temperature at the strain rate of 10 s−1, while it increased obviously at the strain rates of 20 and 50 s−1. Therefore, hot deformation at the strain rate of 10 s−1 and temperature range of 250–400°C was desirable and feasible for the ZK40 alloy.

Deformation resistance in the transition from coarse-grained to ultrafine-grained Cu by severe plastic deformation up to 24 passes of ECAP

Abstract Pure Cu was subjected to severe plastic predeformation by p = 1, 2, 4, 8, 16 and 24 passes of equal channel angular pressing (ECAP) on route B C at ambient temperature and subsequently tested in uniaxial compression parallel to the extrusion direction at constant rate or constant stress and temperatures from ambient temperature up to 418 K. The maximum compressive strength of the ECAPed Cu varies in a systematic fashion with p , until a steady state is finally reached between p = 8 and 16 where the rate sensitivity of flow stress is maximal. The results are quantitatively interpreted in terms of the boundary structure, considering the superposition of hardening due to refinement of low-angle boundaries and softening due to enhanced thermal recovery at high-angle boundaries. Beyond the maximum the compressive strength declines with strain for relatively low rate and/or elevated temperature of compression. This is explained by dynamic grain coarsening towards the new steady state developing in compression.

Numerical Simulation of Microstructure Evolution of TC6 Alloy Blade during Finish Forging

In order to predict the evolution of microstructure and grain size in TC6 alloy blade finish forging process and optimize the parameters, a series of constitutive equations for dynamic recrystallization and grain growth were developed and implemented into a 3D FE simulator. The grain size has been illustrated for the upsetting of TC6 alloy and the calculated grain size is in a good agreement with the experimental results, which shows that the microstructure prediction tool was validated and reliable. Then the distribution of stress, strain, temperature and microstructure, grain size and deformation has been predicted in TC6 alloy blade forging successfully using the 3D FEM at 930°C deformation temperature, 250°C die temperature and 50mm/s deformation velocity. The results show that the temperature of blade edge is higher than that of center because heat transfer, friction work and deformation work. The equivalent stress and strain have the same distribution as temperature after forging and the refined grain is obtained because of dynamic rescrystallization occurred with larger deformation. The uniform distribution of grain size are obtained in the blade and the shape after finish forging under the condition of technology parameters meet the user’s requirement comparing the calculated deformation with experimental results.

Solid structures in a highly agitated bed of granular materials

A series of experiments are described in which bubbles and solid structures are produced in a highly agitated bed of vertically shaken granular materials. To identify the physical mechanisms behind bubbling, three-dimensional simulations of the aforementioned systems are performed on a graphics processing unit (GPU). The gas dynamics above and within shaken granular materials is solved using large-eddy simulations (LES) while the dynamics of grains is described through molecular dynamics. Here, the interaction between the grain surfaces is modeled using the generalized form of contact theory developed by Hertz. In addition, the coefficient of kinetic friction is assumed to depend on the relative velocity of slipping. The results show both a qualitative and a quantitative agreement between simulations and experiments. They imply that the instantaneous formation and failure of granular aggregates could play an important role in the nucleation, growth, departure and collapse of bubbles in shaken granular materials. This promising effort in GPU computing may position the GPU as a compelling future alternative to traditional simulation techniques.

Experimental Study of Squeeze Casting of ZK60 Magnesium Alloy

Tensile behaviors of extruded and rolled AZ80 Mg alloy were investigated with elongation-to-failure tensile tests at constant temperatures of 300 °C, 350 °C, 400 °C, and 450 °C, and constant strain rates of 10-2 s-1 and 10-3 s-1. Experimental data show that the material exhibits tensile ductilities of over 100% at 400 °C and 450 °C, featured by long steady state deformation. Microstructure studies show that annealed coarse grains were remained in the gauge region during the tensile tests, and the enhanced tensile ductilities resulted from dislocation creep, other than dynamic recrystallization or grain boundary sliding. Cavity evolution and recrystallized coarse grains near fracture end caused premature failure of the material.

Enhanced Tensile Ductility of AZ80 Magnesium Alloy

Tensile behaviors of extruded and rolled AZ80 Mg alloy were investigated with elongation-to-failure tensile tests at constant temperatures of 300 °C, 350 °C, 400 °C, and 450 °C, and constant strain rates of 10-2 s-1 and 10-3 s-1. Experimental data show that the material exhibits tensile ductilities of over 100% at 400 °C and 450 °C, featured by long steady state deformation. Microstructure studies show that annealed coarse grains were remained in the gauge region during the tensile tests, and the enhanced tensile ductilities resulted from dislocation creep, other than dynamic recrystallization or grain boundary sliding. Cavity evolution and recrystallized coarse grains near fracture end caused premature failure of the material.

Hot deformation behavior of an 8% Cr cold roller steel

An 8% Cr cold roller steel was compressed in the temperature range 900–1200 °C and strain rate range 0.01–10 s−1. The mechanical behavior has been characterized using stress–strain curve analysis, kinetic analysis, processing maps, etc. Metallographic investigation was performed to evaluate the microstructure evolution and the mechanism of flow instability. It was found that the work hardening rate and flow stress decreased with increasing deformation temperature and decreasing strain rate in 8% Cr steel; the efficiency of power dissipation decreased with increasing Z value; flow instability was observed at higher Z-value conditions and manifested as flow localization near the grain boundary. The hot deformation equation and the dependences of critical stress for dynamic recrystallization and dynamic recrystallization grain size on Z value were obtained. The suggested processing window is in the temperature range 1050–1200 °C and strain rate range 0.1–1 s−1 in the hot processing of 8% Cr steel.

Divided Flow Extrusion of AZ31 Magnesium Alloy with Cerium and Antimony Addition

Divided flow extrusion deformation behaviors and as-extruded tube properties of AZ31-Ce-Sb alloy were investigated. Addition of Sb and RE in AZ31 could accelerate the nucleation of dynamic recrystallization grains and refine tube microstructure. Compared to AZ31 Mg alloy, higher deformation temperature, extrusion speed and ration were allowed. AZ31-1%Ce-1%Sb alloy tubes had much better comprehensive performance.

High-strain-rate superplasticity in oxide ceramics: a trial of microstructural design based on creep-cavitation mechanisms

From existing knowledge about high-temperature cavitation mechanisms, necessary conditions were discussed for the suppression of cavitation failure during superplastic deformation in ceramic materials. The discussion, where special attention was placed on the relaxation of stress concentrations during grain-boundary sliding and cavity nucleation and growth, leaded to a conclusion that cavitation failure could be retarded by the simultaneous controlling of the initial grain size, the number of residual defects, diffusivity, dynamic grain growth and the homogeneity of microstructure. On the basis of this conclusion, high-strain-rate superplasticity (defined as superplasticity at a strain rate higher than 0.01 s−1) could be intentionally attained in some oxide ceramic materials. This was shown in tetragonal zirconia and composites consisting of zirconia, α-alumina and a spinel phase.

Microstructural Evolution of AZ91 Magnesium Alloy during Deformation and Heat Treatment

Microstructural evolutions of AZ91 magnesium alloy, which applied to homogenizing annealing treatment, hot extrusion and ageing treatment respectively, are investigated in this paper. Results exhibit that the massive β-Mg17Al12 phase appearing in the initial structure dissolves to eliminate in α-Mg matrix mostly due to the homogenizing annealing treatment; dynamic recrystallization and consequent grain refinement occur during extrusion; banded structure, in which the β-Mg17Al12 phase precipitates parallel to the extrusion direction in α-Mg matrix, are observed in the ageing treated specimen. Furthermore, the effects of temperature, keeping time of homogenizing annealing treatment and ageing treatment, and the extrusion processing parameter on the microstructural evolution of this alloy are also discussed according to the experimental results in details. In addition, the various mechanisms of the morphology and quantity of α-Mg and β-Mg17Al12 phase are analyzed corresponded to the various states. As for the banded structure appearing in the ageing treated specimen, it can be attributed to the banded segregation which remained in the extruded one and resulted from the casting dendrite and subsequent extrusion.

Research on the Forging Penetration Efficiency of 40Cr Axial Forging by the Simulation of Dynamic Recrystallization

The property of Heavy axial forgings can be modified by the reasonable drawing process with the advantage of dynamic recrystallization. In view of formation, heat transfer, heating generation, dynamic recrystallization a macro-micro coupled process was modeled, and the drawing process of φ300 mm×4000 mm axial forging is simulated .It is found that when the cumulative reduction ratio reaches 9% in the second round, the dynamic recrystallization occurred in the material as it reached a critical strain condition .With the reduction ratio increases, the volume fraction of dynamic recrystallization increases. When the cumulative reduction ratio reaches 60%, about 67% of the inner region of the axial forging is modified fully. After the drawing process, the dynamic recrystallization grain size in 60% of the cross-section area is less than 100 μm , and the grains get refined.

Hot deformation behavior and microstructure evolution of spray formed GH738 superalloy

In order to evaluate the deformation characteristics of spray formed superalloy GH738, the hot compression test was conducted on Gleeble-3500 thermal mechanical simulator at a temperature range of 960–1120 °C, and a strain rates range of 0.1, 1.0 s−1 with engineering strain 50%. The flow stress data at various hot working conditions was obtained, and the influence of deformation temperature and strain rate on the microstructure evolution of spray formed GH738 was analyzed. The result shows that, the flow stress improves with the increasing of strain rate and the decreasing of temperature, the yield drop is observed on true stress-strain curves at strain rate of 1.0 s−1. The temperature and strain rate have a strongly effect on dynamic recrystallization. Higher temperature could efficiently promote the process of dynamic recrystallization, as the increment of temperature, the dynamic recrystallization grain size shows the tendency of growth. Higher strain rate is better for grain refinement.

Influences of Strain Rate and Deformation Temperature on Flow Stress and Dynamic Recrystalliazation of Heat Resistant Steel P91

In power station big caliber thick wall seamless tube typical steel P91for example, The hot deformation behavior and heat flow stress-strain curve of the heat resistant steel P91was investigated with a compression test on Gleeble3500 simulator at a temperature range of 1050°C～1200°C and strain rate range of 10-4～5s-1. The test curve shows that P91 materials have the dynamic recrystallization and recovery behavior in high temperature and deformation. With the rise of temperature and strain rate of decreased, flow stress is lower and the dynamic recrystallization phenomenon is easier to occur. By metalloscope observed, in the high temperature and low strain rate conditions, the dynamic recrystallization area and new nucleation grain size are larger, and microstructure is more uniformly distributed.

Microstructual evolution of AZ80 magnesium alloy during multi-directional compression deformation at elevated temperature

Abstract Microstructural evolution of AZ80 magnesium alloy during multi-directional compression deformation at elevated temperature was systematically investigated. The effects of deformation variables on the as-compressed microstructures and the deformation behavior were analyzed. Grain splitting that developed in various directions due to the formation of microbands was the main characteristic of microstructural evolution during hot multi-directional compression, which was different from that of the continuous uniaxial compression. Such microbands intersected each other, resulting in continuous subdivision of the coarse grains into misoriented fine domains, thus contributing to grain refinement. Further deformation led to increases in the number and misorientation of these boundaries and finally almost full development of fine equiaxed grains at high strain. A more homogeneous microstructure with fine dynamic recrystallization grains could be attained with the applied strain up to a critical strain, and after that, it was difficult to achieve further grain refinement. However, the higher reduction in each pass and the lower deformation temperature in the certain range aided grain refinement. The second phase Mg17Al12 with large particle size that remained after solution treatment experienced a series of changes with the process of breaking up, dissolving, precipitating and then re-dissolving. Fine precipitates were located at the grain boundaries, suppressing dislocation movement and preventing grain growth.

Superplasticity-Like Behavior, Microstructures and Deformation Mechanism Map in Superlight Mg-6Li-3Zn Alloy

Mg-6Li-3Zn alloy sheets were prepared by melting and casting, and heavy rolling with a total reduction of 94%. The high-temperature mechanical behavior, microstructures and deformation mechanisms were investigated. The maximum elongation to failure of 300% was demonstrated at 623K and an initial strain rate of 1.67×10-3s-1. Observations by optical microscope, transmission electron microscope reveal that significant dynamic recrystallization and grain refinement occurred in banded grains at 573K and an initial strain rate of 1.67×10-3s-1, under which the subgrain contour was ambiguous and dislocation distribution was relatively uniform. It is shown by newly constructed deformation mechanism map that the high-temperature deformation mechanism in Mg-6Li-3Zn alloy sheet with banded grains at 573K and an initial strain rate of 1.67×10-3 s-1 is dislocation viscous glide controlled by lattice diffusion, the stress exponent is 3 (strain rate sensitivity exponent 0.33) and deformation activation energy is 134.8 kJ mol-1, which is the same as the lattice diffusion activation energy of magnesium.

A dynamic grain flow model for a mass flow yield sensor on a combine

A model is developed to describe the flow of grain through a clean grain elevator system on a combine in order to facilitate accurate mass flow rate estimation. The relationship between mass flow rate and impact force described by the model depends upon machine operational characteristics, mechanical interactions of the grain and the machine geometry, and material properties of the grain. The model was designed to be adaptable to varying grain conditions, such as those influenced by moisture content, by allowing free parameters of the model to be estimated through a nonlinear regression algorithm. Simulations were performed using discrete element modeling software and data was obtained from experiments conducted on a clean grain elevator system at the University of Kentucky Combine Yield Monitor Test Facility to determine the ability of the model to accurately estimate mass flow rate. The model estimated mass flow rate with a normalized root mean squared residual (NRMSR) less than 2% for discrete element modeling simulations. For experiments involving machine components, NRMSR values were less than 3% for corn at 14% moisture, less than 3% for corn at 21% moisture, and less than 5% for corn at 26% moisture.

Effect of warm forming conditions on AZ31B flow behaviour and microstructural characteristics

The paper presents the mechanical and microstructural characterization of the magnesium alloy AZ31B through tests conducted at elevated temperature on a dedicated uni-axial tensile set-up. The samples are heated in-situ through induction heating and their actual deformation is recorded in-line through the Aramis™ system. Results from the tests in terms of flow stress, plastic properties and anisotropic coefficients are presented and commented. Additionally, the microstructural analysis on deformed samples in terms of grain size and recrystallized fraction is shown, in order to identify the fundamental mechanisms of dynamic recrystallization and grain growth and to correlate them with the testing parameters. The study provides the best combination of process parameters that can assure, at the same time, good formability and sound microstructure in the warm forming temperature range.

Stability of ultrafine-grained structure of copper under fatigue loading

Stability of microstructure of ultrafine-grained materials under cyclic loading is a crucial condition for their good fatigue performance. Changes of microstructure due to fatigue in bulk, localization of cyclic plasticity into cyclic slip bands and related development of microstructure were experimentally studied on ultrafine-grained copper prepared by equal channel angular pressing. Different reaction of ultrafine-grained structure to plastic strain-controlled and load-controlled tests was found. The different susceptibility to dynamic grain coarsening under load and plastic strain-controlled tests reported in literature cannot be explained by differences in purity or details of equal channel angular pressing. The localization of the cyclic plasticity and the development of cyclic slip bands resulting in fatigue crack initiation take place in material volumes which can be characterized as “near by oriented” regions. They correspond to the shear bands, which are characteristic for ultrafine-grained structure after equal channel angular pressing.

Silicon nitride nanoceramics densified by dynamic grain sliding

The densification behaviors of two silicon nitride nanopowder mixtures based respectively on α-Si3N4and β-Si3N4as the major phase constituent were studied by spark plasma sintering. Sintering conditions were established where a low viscous liquid not in equilibrium with the main crystalline constituent(s) stimulated the grain sliding yet did not activate the reprecipitation mechanism that unavoidably yields grain growth. By this way of dynamic grain sliding full densification of silicon nitride nanoceramics was achieved with no noticeable involvement of α- to β-Si3N4phase transformation and grain growth. This processing principle opens the way toward flexible and precise tailoring of the microstructures and properties of Si3N4ceramics. The obtained silicon nitride nanoceramics showed improved wear resistance, particularly under higher Hertzian stresses.

High temperature deformation and microstructural evolution of Mg–Gd–Y alloys

The high temperature deformation behaviours of Mg–10Gd–3Y (GW103) alloys processed by extrusion and extrusion followed by equal channel angular extrusion (ECAE) were investigated and compared, and the maximum tensile elongations of 390 and 490% were obtained in the two specimens respectively. The different processing routes led to different microstructural features, and the extrusion followed by ECAE (EX-ECAE) specimens usually showed higher elongations than as extruded ones under the same deformation conditions. The deformation mechanisms of GW103 alloys were analysed in terms of the strain rate sensitivity exponent, microstructural evolution and activation energy. The high elongations of as extruded specimens were attributed to the combined effects of dynamic recrystallisation and grain boundary sliding, which was accompanied by lattice diffusion. However, grain boundary sliding controlled by the combination of grain boundary and lattice diffusions was dominant in the EX-ECAE specimens.