Superplasticity of fine-grained Mg-10Li alloy prepared by severe plastic deformation and understanding its deformation mechanisms

The superplastic behavior and associated deformation mechanisms of a fine-grained Mg-10.1 Li-0.8Al-0.6Zn alloy(LAZ1011)with a grain size of 3.2μm,primarily composed of the BCCβphase and a small amount of the HCPαphase,were examined in a temperature range of 473 K to 623 K.The microstructural refinement of this alloy was achieved by employing high-ratio differential speed rolling.The best superplasticity was achieved at 523 K and at strain rates of 10^(-4)-5×10^(-4)s^(-1),where tensile elongations of 550±600%were obtained.During the heating and holding stage of the tensile samples prior to tensile loading,a significant increase in grain size was observed at temperatures above 573 K.Therefore,it was important to consider this effect when analyzing and understanding the superplastic deformation behavior and mechanisms.In the investigated strain rate range,the superplastic flow at low strain rates was governed by lattice diffusion-controlled grain boundary sliding,while at high strain rates,lattice diffusion-controlled dislocation climb creep was the rate-controlling deformation mechanism.It was concluded that solute drag creep is unlikely to occur.During the late stages of deformation at 523 K,it was observed that grain boundary sliding led to the agglomeration of theαphase,resulting in significant strain hardening.Deformation mechanism maps were constructed forβ-Mg-Li alloys in the form of 2D and 3D formats as a function of strain rate,stress,temperature,and grain size,using the constitutive equations for various deformation mechanisms derived based on the data of the current tests.

Structure,strength and superplasticity of ultrafine-grained 1570C aluminum alloy subjected to different thermomechanical processing routes based on severe plastic deformation

A comparative study of the structure and mechanical behavior of an Al-5 Mg-0.18 Mn-0.2 Sc-0.08 Zr-0.01 Fe-0.01 Si(wt.%)alloy ingot subjected to multidirectional isothermal forging(MIF)to a strain of 12 or equal-channel angular pressing(ECAP)to a strain of 10 at 325℃,and subsequent warm and cold rolling(WR and CR)at 325 and 20℃,was performed.The results showed that the MIF process of ultrafine-grained structure with a(sub)grain size dUFG=2μm resulted in enhanced room-temperature ductility and superplastic elongation up to 2800%.Further grain refinement under WR as well as development of a heavily-deformed microstructure with high dislocation density by subsequent CR resulted in a yield/ultimate tensile strength increase from 235/360 MPa after MIF to 315/460 and 400/515 MPa after WR and CR,respectively.Simultaneously,WR led to improved superplastic elongation up to 4000%,while after CR the elongation remained sufficiently high(up to 1500%).Compared with MIF,ECAP resulted in more profound grain refinement(dUFG=1μm),which promoted higher strength and superplastic properties.However,this effect smoothed down upon WR,ensuring equal properties of the processed sheets.CR of the ECAPed alloy,in contrast,led to higher strengthening and slightly better superplastic behavior than those after CR following MIF.

Atomistic simulations of the surface severe plastic deformation-induced grain refinement in polycrystalline magnesium:The effect of processing parameters

Magnesium(Mg)based alloys are promising candidates for many applications,but their untreated surfaces usually have low strength and hardness.In this study,a single point diamond turning(SPDT)technique was applied to refine the grain size and improve the mechanical properties of the surface layers of Mg-Li alloys.By refining grains in the topmost layer to the nanometer scale(~60 nm),the surface hardness was found to be enhanced by approximately 60%.The atomic plastic deformation process during the SPDT was then studied by the real-time atomistic molecular dynamics(MD)simulations.A series of MD simulations with different combinations of parameters,including rake angle,cutting speed and cutting depth,were conducted to understand their influences on the microstructural evolution and associated plastic deformation mechanisms on the surface layer of the workpieces.The MD simulation results suggest that using increased rake angle,cutting speed and cutting depth can help to achieve better grain refinement.These simulation results,which provide atomic-level details of the deformation mechanism,can assist the parameter design for the SPDT techniques to achieve the high-performance heterogeneous nanostructured materials.

大塑性变形(severe plastic deformation,SPD)的研究现状

本文概述了近10年来大塑性变形(severe plastic deformation,SPD)研究成果,文章首先简述了各种SPD工艺的特点,其次对SPD强化机理研究现状做了分析,最后基于目前SPD技术的应用现状,对其发展和应用前景展开了必要的讨论和展望.

SEVERE PLASTIC DEFORMATION TECHNIQUES

There is a considerable interest in developing methods for processing of materials with ultrafine grain si- zes. Widely used methods in refining microstructure are severe/intense plastic deformation techniques, such as torsion straining, equal channel angular (ECA) pressing/extrusion,and accumulative roll bonding (ARB) technique.While the torsion technique has been available for decades,a new torsion system has been developed at DSI for simulation of complex stress states such as combination of shear and compression or shear and tension stress states. The equal channel angular pressing technique is ma- inly used for production of ultrafine grain aluminum alloys,which is aimed at develoment of high strain rate superplasticity.The accumulative roll bonding technique has been applied for the rolling of both aluminum alloy and steels to produce ultrafine gained materials. Three different types of hot compression deformation methods, each with a different number of deforma- tion axis, are introduced in this paper. They are single axis deformation, two axis deformation and three axis deformation.The single axis deformation has us or little restraint such as plane strain type testing and axisymmetric compression testing, the two - axis deformation can be fully restrained or un- restrained, and the three - axis deformation has no restraint. The two - axis restrainsd compression deformation techopue is recommended for loboratory side after comparing the single - axis and the three - axis deformation techniques since the bulk volume of the two - axis restraint compression speci- men can be easily machined into mechanical testing speciments for mechanical property measurement and other studies,and the technique can be aplied to studies of any metallic materials.

Severe plastic deformation of commercially pure aluminum using novel equal channel angular expansion extrusion with spherical cavity

Equal channel angular expansion extrusion with spherical cavity(ECAEE-SC)was introduced as a novel severe plastic deformation(SPD)technique,which is capable of imposing large plastic strain and intrinsic back-pressure on the processed billet.The plastic deformation behaviors of commercially pure aluminum during ECAEE-SC process were investigated using finite element analysis DEFORM-3D simulation software.The material flow,the load history,the distribution of effective strain and mean stress in the billet were analyzed in comparison with conventional equal channel angular extrusion(ECAE)process.In addition,single-pass ECAEE-SC was experimentally conducted on commercially pure aluminum at room temperature for validation,and the evolution of microstructure and microhardness of as-processed material was discussed.It was shown that during the process,the material is in the ideal hydrostatic stress state and the load requirement for ECAEE-SC is much more than that for ECAE.After a single-pass ECAEE-SC,an average strain of 3.51 was accumulated in the billet with homogeneous distribution.Moreover,the microstructure was significantly refined and composed of equiaxed ultrafine grains with sub-micron size.Considerable improvement in the average microhardness of aluminum was also found,which was homogenized and increased from HV 36.61 to HV 70.20,denoting 91.75%improvement compared with that of the as-cast billet.

Grain size and texture changes of magnesium alloy AZ31 during multi-directional forging

Grain size and texture changes of magnesium alloy AZ31 were studied in multidirectional forging(MDF) under decreasing temperature conditions.MDF was carried out up to large cumulative strains of 4.8 with changing the loading direction during decrease in temperature from pass to pass.MDF can accelerate the uniform development of fine-grained structures and increase the plastic workability at low temperatures.As a result,the MDFed alloy shows excellent higher strength as well as moderate ductility at room temperature even at the grain size below 1μm.Superplastic flow takes place at 423 K and depends on the anisotropy of MDFed samples.The mechanisms of strain-induced fine-grained structure development and of the plastic deformation were discussed in detail.

Microstructure and texture evolution in titanium subjected to friction roll surface processing and subsequent annealing

Commercial purity and high purity titanium sheets were initially strained by a new technique, named as friction roll surface processing （FRSP）. Severe strain was imposed into the surface layer and strain gradient was formed through the thickness of the sheet. The microstructure and texture in as-strained state were investigated by optical microscopy and X-ray diffraction technique On the surface of the sheets, ultra-fine grains were found to have a sharp texture with a preferred orientation strongly related to the FRSP direction. The evolution of microstructure and crystallographic texture of FRSPed samples during recrystallization were also studied by electron back-scattered diffraction （EBSD） technique after being annealed at selected temperatures and time. The results indicated that the preferred orientations resulting from FRSP and annealing in the surface layer were formed during rolling and its recrystallization textures were reduced by FRSP. In addition, the texture evolved stably without change in main components during the annealing.

Grain refinement in low SFE and particle-containing nickel aluminium bronze during severe plastic deformation at elevated temperatures

The influence of particle size and morphology on grain refinement in low stacking fault energy(SFE)alloys was studied by comparing the grain structures in single-and multi-phase Al-bronze(AB)alloys following equal channel angular pressing(ECAP)between 350 and 500℃.In particular,nickel aluminium bronze(NAB)was chosen as it contained both coarse and fine rounded particles,as well as a lamellar phase which evolved during ECAP.Grain refinement in the single-phase alloy was achieved through dynamic recrystallisation initiated at deformed twin boundaries.By contrast,different mechanisms were observed in the particle-containing NAB.Recrystallisation around the coarseκⅡparticles(~5μm)was promoted through particle stimulated nucleation(PSN),whereas recrystallisation in the region of the fineκⅣ(~0.4μm)was delayed due to the activation of secondary slip.Grain refinement in areas of the lamellarκⅢshowed significant variation,depending on the lamellar orientation relative to the shear plane of ECAP.As the lamellae deformed,numerous high angle grain boundaries were generated between fragments and served as nucleation sites for recrystallisation,while PSN occurred around spheroidised lamellae.The spreading of theκⅢparticles by ECAP then enhanced the total area of recrystallised grains.

Mechanical properties and in vivo biodegradability of Mg-Zr-Y-Nd-La magnesium alloy produced by a combined severe plastic deformation

Permanent implants are going to be replaced by the implementation of biodegradable magnesium-based implants in fixation of internal bone fractures because of many concerns associated with conventional implants.However,biodegradable magnesium-based biomaterials exhibit higher biodegradation rate and low mechanical properties which are the main challenges.This work aims to almost overcome both disadvantageous by grain refining of a WE43 magnesium alloy containing 93.04 wt% Mg,4.12 wt% Y,2.15 wt% Nd,0.43 wt% Zr,and 0.26 wt%La.In this study,the consequences of combined severe plastic deformation(SPD) on the mechanical properties,microstructure,and in vivo degradation behavior of WE43 magnesium alloy were investigated.To do so,WE43 magnesium alloy was chosen and processed through multipass equal channel angular pressing(ECAP) at 330℃ for up to four passes followed by an extrusion process.The results showed that higher strength and hardness with minimum ductility less was obtained in the sample processed via two-pass ECAP followed by extrusion.In vivo biodegradation experiments showed higher degradation rate for the unprocessed coarse-grained(CG) WE43 sample.The two-pass ECAP and extruded sample with ultrafine-grained(UFG) structure exhibited the lowest in vivo biodegradation rate besides appropriate mechanical properties.It may be concluded that the WE43 magnesium alloy processed via two-pass ECAP and extrusion could be a very promising candidate for biodegradable implants from both mechanical and biocorrosion viewpoints.

Equal-Channel Angular Pressing as a New Processing to Control the Microstructure and Texture of Metallic Sheets

The extended band structures of as-cold-rolled high Cr steel sheets are recrystallisation-resistant, and tend to become aggregates of the so-called grain colonies as a partially recovered state after final annealing. Such band structures diminish formability and become origin of the so-called ridging. A novel processing will be shown here, which involves strain-path change by introducing one-pass ECAP prior to cold-rolling, and facilitates recrystallisation. Indeed, the recrystallisation temperature was reduced by 100℃, compared with cold-rolling alone imposing an equivalent strain. Grain- scale microshear bands introduced during one-pass ECAP perturbed the banded structures in post-ECAP cold-rolling and enhanced the recrystallisation at the final annealing.

Molecular Dynamics Simulation of Grain Refinement in a Polycrystalline Material under Severe Compressive Deformation

Grain refinement in a polycrystalline material resulting from severe compressive deformation was simulated using molecular dynamics. A simplified model with four square grains surrounded by periodic boundaries was prepared, and compressive deformation was imposed by shortening the length in the y direction. The model first deformed elastically, and the compressive stress increased monotonically. Inelastic deformation was then initiated, and the stress decreased drastically. At that moment, dislocation or slip was initiated at the grain boundaries or triple junction and then spread within the grains. New grain boundaries were then generated in some of the grains, and sub-grains appeared. Finally, a microstructure with refined grains was obtained. This process was simulated using two types of grain arrangements and three different combinations of crystal orientations. Grain refinement generally proceeded in a similar fashion in each scenario, whereas the detailed inelastic deformation and grain refinement behavior depended on the initial microstructure.

Mechanical and Microstructural Properties of Ultra-fine Grained AZ91Magnesium Alloy Tubes Processed via Multi Pass Tubular Channel Angular Pressing(TCAP)

Ultra-fine grained （UFG） cylindrical tubes were produced via recently developed tubular channel angular pressing （TCAP） process through different passes from as-cast AZ91 magnesium alloy. The microstructure and mechanical properties of processed tube through one to four passes of TCAP process at 200℃ were investigated. Microhardness of the processed tube was increased to 98.5 HV after one pass from an initial value of 67 Hr. An increase in the number of passes from one to higher number of passes has no more effect on the microhardness. Yield and ultimate strengths were increased by 4.3 and 1.4 times compared to those in as-cast condition. Notable increase in the strength was achieved after one pass of TCAP while higher number of passes has no more effect. Microstructural investigation shows notable decrease in the grain size to around 500 nm from the primary value of - 150 μm. Dissolution and distribution of hard MglTAI12 phase in the grain boundaries of dynamically recrystallized UFG AZ91 with a mean grain size of - 500 nm was an interesting issue of TCAP processing at 200 ℃ compared to other severe plastic deformation processes.

Corrosion behavior of severely plastically deformed Mg and Mg alloys

Magnesium(Mg)alloys have several advantages,such as low density,high specific strength and biocompatibility.However,they also suffer weak points,such as high corrosion,low formability and easy ignition,which makes their applications limited.Many studies have been conducted to overcome these disadvantages and further improve the advantages of Mg alloys.Severe plastic deformation(SPD)is one of the most important techniques and has great effects on the microstructure refinement of Mg alloys and improvements in their strength and formability.Several researchers have studied the corrosion behavior of SPD-processed Mg alloys in recent decades.However,these studies have reported some controversial effects of SPD on the corrosion of Mg alloys,which makes the research roadmap ambiguous.Therefore,it is important to review the literature related to the corrosion properties of Mg alloys prepared by SPD and understand the mechanisms controlling their corrosion behavior.Effective grain refinement by SPD improves the corrosion properties of pure Mg and Mg alloys,but control of the processing conditions is a key factor for achieving this goal because texture,dislocation density,size and morphology of secondary phase also importantly affects the corrosion properties of Mg alloys.Reduced grain size in the fine grain-size range can decrease the corrosion rate due to the increased barrier effect of grain boundaries against corrosion and the formation of a stable passivation layer on the surface of fine grains.Basal texture reduces the corrosion rate because basal planes with the highest atomic planar density are more corrosion resistant than other planes.Increased dislocation density after SPD deteriorates the corrosion resistance of the interior grains and thus proper annealing after SPD is important.The fine and uniform distribution of secondary phase particles during SPD is important to minimize the micro-galvanic corrosion effect and retain small grains during annealing treatment for removing dislocations.

Texture evolution and mechanical anisotropy of an ultrafine/nano-grained pure copper tube processed via hydrostatic tube cyclic expansion extrusion

Texture evolution and mechanical anisotropic behavior of an ultrafine-grained(UFG)pure copper tube processed by recently introduced method of hydrostatic tube cyclic expansion extrusion(HTCEE)was investigated.For the UFG tube,different deformation behavior and a significant anisotropy in tensile properties were recorded along the longitudinal and peripheral directions.The HTCEE process increased the yield strength and the ultimate strength in the axial direction by 3.6 and 1.67 times,respectively.Also,this process increased the yield strength and the ultimate strength in the peripheral direction by 1.15 and 1.12 times,respectively.The ratio of ultimate tensile strength in the peripheral direction to that in the axial direction,as a criterion for mechanical anisotropy,are 1.7 and 1.16 for the as-annealed coarse-grained and the HTCEE processed UFG tube,respectively.The results are indicative of a reducing effect of the HTCEE process on the mechanical anisotropy.Besides,after HTCEE process,a low loss of ductility was observed in both directions,which is another advantage of HTCEE process.Hardness measurements revealed a slight increment of hardness values in the peripheral direction,which is in agreement with the trend of tensile tests.Texture analysis was conducted in order to determine the oriental distribution of the grains.The obtained{111}pole figures demonstrate the texture evolution and reaffirm the anisotropy observed in mechanical properties.Scanning electron microscopy micrographs showed that different modes of fracture occurred after tensile testing in the two orthogonal directions.

Revealing the grain size dependent hot workability and deformation mechanisms in a Mg-Zn-Y alloy

Despite the industrial significance of grain size for enhancing mechanical properties and formability,the in-depth deformation mechanisms at elevated temperature are still unclear.To investigate the functions of grain size on hot workability and deformation mechanisms,three groups of Mg-1.2Zn-0.2Y alloy specimens with different grain sizes were hot compressed and then studied by combining constitutive model,processing map and microstructural observations.The results showed that the enhanced hot workability accompanying low deformation activation energy and small instability regime was obtained with refined grain size.During hot deformation,the decreased grain size in Mg1.2Zn-0.2Y alloy mainly improved the plastic deformation homogeneity,especially for the weakened local straining around grain boundaries.As a result,the dynamic recrystallization nucleation and texture development at lower strain level were influenced by the initial grain size.At higher strain magnitude,the growth and coarsening of dynamic recrystallized grains would further release strain localization and improve hot workability,while the texture was less impacted.Further,unlike the primary basal slip and deformation twinning in the specimen with coarse grain at low temperature,non-basal slips of dislocations were initiated with less deformation twins in the specimens with refined grain size.

Enhancing plastic deformability of Mg and its alloys—A review of traditional and nascent developments

Mg and its alloys have continued to attract interest for several structural and super-sensitive applications because of their light weight and good combination of engineering properties.However for some of these applications,high plastic deformability is required to achieve desired component shapes and configurations;unfortunately,Mg and its alloys have low formability.Scientifically,the plastic behaviour of Mg and its alloys ranks among the most complex and difficult to reconcile in metallic material systems.But basically,the HCP crystal structure coupled with low stacking fault energies(SFE)are largely linked to the poor ductility exhibited by Mg alloys.These innate material characteristics have regrettably limited wide spread applicability of Mg and its alloys.Several research efforts aimed at exploring processing strategies to make these alloys more amenable for high formability–mediated engineering use have been reported and still ongoing.This paper reviews the structural metallurgy of Mg alloys and its influence on mechanical behaviour,specifically,plasticity characteristics.It also concisely presents various processing routes(Alloying,Traditional Forming and Severe Plastic Deformation(SPD))which have been explored to enhance plastic deformability in Mg and its alloys.Grain refinement and homogenising of phases,reducing CRSS between slip modes,twinning suppression to activate non-basal slip,and weakening and randomisation of the basal texture were observed as the formability enhancing strategies explored in the reviewed processes.While identifying the limitations of these strategies,further areas to be explored for enhancing plasticity of Mg alloys are highlighted.

Evolution of microstructure and texture in copper during repetitive extrusion-upsetting and subsequent annealing

The evolution of the microstructure and texture in copper has been studied during repetitive extrusionupsetting（REU） to a total von Mises strain of 4.7 and during subsequent annealing at different temperatures. It is found that the texture is significantly altered by each deformation pass. A duplex 001 ＋ 111 fiber texture with an increased 111 component is observed after each extrusion pass,whereas the 110 fiber component dominates the texture after each upsetting pass. During REU, the microstructure is refined by deformation-induced boundaries. The average cell size after a total strain of 4.7 is measured to be ~0.3 μm. This refined microstructure is unstable at room temperature as is evident from the presence of a small number of recrystallized grains in the deformed matrix. Pronounced recrystallization took place during annealing at 200？C for 1 h with recrystallized grains developing predominantly in high misorientation regions. At 350？C the microstructure is fully recrystallized with an average grain size of only 2.3 μm and a very weak crystallographic texture. This REU-processed and subsequently annealed material is considered to be potentially suitable for using as a material for sputtering targets.

Evolution of dislocation cells during plastic deformation

In recent years, materials with ultrafine grain size(UFG) have attracted much attention. By using severe plastic deformation(SPD) techniques, materials with fine grain size as small as 200250 nm have been obtained. However, the nature of the grain boundaries has not been theoretically understood. It is still an unsolved question whether or not finer grain sizes down to 100 nm could be reached. A semi-quantitative model for the evolution of dislocation cells in plastic deformation was proposed. The linear stability analysis of this model leads to some interesting results, which facilitate the understanding of the formation of cell structures and of the factors determining the lower limit of the cell size of SPD materials.

Texture evolution during semicontinuous equal-channel angular extrusion process of interstitial-free steel

Semicontinuous equal-channel angular extrusion（ SC-ECAE） is a novel severe plastic deformation technique that has been developed to produce ultrafine-grain steels. Instead of external forces being exerted on specimens in the conventional ECAE,driving forces are applied to dies in SC-EACE. The deformation of interstitial-free（ IF） steel w as performed at room temperature,and individual specimens w ere repeatedly processed at various passes. An overall grain size of 0. 55 μm w as achieved after 10 passes. During SC-ECAE,the main textures of IF steel included { 111} ,{ 110} ,{ 112} ,{ 110} ,and { 110} At an early stage,increasing dislocations induce new textures and increase intensity. When the deformation continues,low-angle boundaries are formed betw een dislocation cell bands,w hich cause some dislocation cell bands to change their orientation,and therefore,the intensity of the textures begins to decrease. After more passes,the intensity of textures continues to decrease w ith high-angle boundaries,and the sub-grains in dislocation cell bands continuously increase. The present study reports the evolution of textures during deformation; these w ere examined and characterized using high-resolution electron backscattered diffraction（ EBSD） in a field emission scanning electron microscope. The mechanisms of texture evolution are discussed.