Fabrication of ultrafine-grained AA1060 sheets via accumulative roll bonding with subsequent cryorolling

Ultrafine-grained(UFG)AA1060 sheets were fabricated via five-cycle accumulative roll bonding(ARB)and subsequent three-pass cold rolling(298 K),or cryorolling(83 K and 173 K).Microstructures of the aluminum samples were examined via transmission electron microscopy,and their mechanical properties were measured via tensile and microhardness testing.Results indicate that ultrafine grains in ARB-processed sheets were further refined by subsequent rolling,and the grain size became finer with reducing rolling temperature.The mean grain size of 666 nm in the sheets subjected to ARB was refined to 346 or 266 nm,respectively,via subsequent cold rolling or cryorolling(83 K).Subsequent cryorolling resulted in ultrafine-grained sheets of higher strength and ductility than those of the sheets subjected to cold rolling.

Variation of the uniaxial tensile behavior of ultrafine-grained pure aluminum after cyclic pre-deformation

To explore the influence of cyclic pre-deformation on the mechanical behavior of ultrafine-grained（UFG）materials with a high stacking fault energy（SFE）,UFG Al processed by equal-channel angular pressing（ECAP）was selected as a target material and its tensile behavior at different pre-cyclic levels D（D=N_i/N_f,where N_i and N_f are the applied cycles and fatigue life at a constant stress amplitude of 50 MPa,respectively）along with the corresponding microstructures and deformation features were systematically studied.The cyclic pre-deformation treatment on the ECAPed UFG Al led to a decrease in flow stress,and a stress quasi-plateau stage was observed after yielding for all of the different-state UFG Al samples.The yield strengths_（YS）,ultimate tensile strengths_（UTS）,and uniform straineexhibited a strong dependence on D when D≤20%;however,when D was in the range from 20%to 50%,no obvious change in mechanical properties was observed.The micro-mechanism for the effect of cyclic pre-deformation on the tensile properties of the ECAPed UFG Al was revealed and compared with that of ECAPed UFG Cu through the observations of deformation features and microstructures.

Enhanced mechanical properties of a carbon and nitrogen co-doped interstitial high-entropy alloy via tuning ultrafine-grained microstructures

C-N co-doped interstitial high entropy alloy(iHEA)was reported to have high strength and ductility.However,iHEA with fully recrystallized ultrafine grains(UFGs)and underlying thermally activated pro-cesses associated with dislocation slip,twinning,and solute drag have not been reported yet.In this work,a C-N co-doped iHEA with nominal composition Fe_(48.5)Mn_(30)Co_(10)Cr_(10)C_(0.5)N_(1.0)(at.%)was prepared,and the microstructures were tuned by cold-rolling and annealing treatments to improve mechanical properties.Upon cold-rolling with a strain of 1.74,the main microstructures in the iHEA are composed of nano-grains,nano-twins,HCP laminates,and high density of dislocations,leading to ultrahigh hardness of 466.7 HV and tensile strength of 1730 MPa at the expense of ductility(2.44%).Both the nanostructures and the high hardness of the iHEA can be maintained up to an annealing temperature of 600℃(462.5 HV).After annealing at 650℃ for 1 h,the UFG microstructures are obtained in the iHEA,containing re-crystallized grains with an average grain size of 0.91μm and nanoprecipitates with an average diameter of 90.8 nm.The combined strengthening and hardening effects of UFGs,nanoprecipitates,twinning,and solutes contribute to high strain hardening(n=0.81),gigapascal yield strength(984 MPa),and good duc-tility(20%).The C-N co-doping leads to a strong drag effect on dislocation slip,resulting in a nano-scale mean free path of dislocation slip λ(1.44 nm)and much small apparent activation volume V^(∗)(15.8 b^(3))of the UFG iHEA.

Bulk ultrafine grained microstructures with high thermal stability via intragranular precipitation of coherent particles

Producing ultrafine-grained(UFG)microstructures with enhanced thermal stability is an important yet challenging route to further improve mechanical properties of structural materials.Here,a high-performance bulk UFG copper that can stabilize even at temperatures up to 750℃(∼0.75 T m,T m is the melting point)was fabricated by manipulating its recrystallization behavior via low alloying of Co.Addition of 1 wt.%–1.5 wt.%of Co can trigger quick and copious intragranular clustering of Co atoms,which offers high Zener pinning pressure and pins the grain boundaries(GBs)of freshly recrystallized ul-trafine grains.Due to the fact that the subsequent growth of the coherent Co-enriched nanoclusters was slow,sufficient particles adjacent to GBs remained to inhibit the migration of GBs,giving rise to the UFG microstructure with prominently high thermal stability.This work manifests that the strategy for pro-ducing UFGs with coherent precipitates can be applied in many alloy systems such as Fe-and Cu-based,which paves the pathway for designing advanced strain-hardenable UFGs with plain compositions.

Mechanical properties and microstructural evolution of ultrafine grained zircaloy-4 processed through multiaxial forging at cryogenic temperature

The mechanical properties and microstructural evolution of zircaloy-4 subjected to cumulative strains of 1.48,2.96,4.44 and 5.91 through multiaxial forging（MAF） at cryogenic temperature（77 K） were investigated.The mechanical properties of the MAF treated alloy were measured through universal tensile testing and Vickers hardness testing equipment.The zircaloy-4 deformed up to a cumulative strain of 5.91 showed improvement in both ultimate tensile strength and hardness from 474 MPa to 717 MPa and from HV 190 to HV 238,respectively,as compared with the as-received alloy.However,there was a noticeable decrement in ductility（from 18%to 3.5%） due to the low strain hardening ability of deformed zircaloy-4.The improvement in strength and hardness of the deformed alloy is attributed to the grain size effect and higher dislocation density generated during multiaxial forging.The microstructural evolutions of deformed samples were characterized by optical microscopy and transmission electron microscopy（TEM）.The evolved microstructure at a cumulative strain of 5.91 obtained after MAF up to 12 cycles depicted the formation of ultrafine grains with an average size of 150-250 nm.

Effect of Annealing on Microstructure and Mechanical Properties of Ultrafine-Grained Low-Carbon Medium-Manganese Steel Produced by Heavy Warm Rolling

An ultrafine-grained(UFG) low-carbon medium-manganese steel was fabricated by the heavily warm rolling(HWR) and subsequent quenching, and the effects of annealing temperatures on microstructure and mechanical properties of the UFG HWRed steel were investigated. The results show that the HWRed steel exhibits simultaneous improvements in strength,uniform elongation and work hardening, which is mainly attributed to the refinement of martensitic microstructures. The HWRed steels comprise only a-phase when annealing at lower temperatures below to 550 °C and at higher temperatures above to 700 °C. Whereas, UFG c-austenite is formed by reverse transformation when the HWRed steel was annealed at intermediate temperatures from 550 to 700 °C and the volume fraction increases with increasing annealing temperatures,consequently resulting in a dramatic increase in ductility of the annealed HWRed steels. It was found that the transformed UFG austenite and ferrite remained ～500 nm and ～800 nm in size when the HWRed steel was annealed at 650 and700 °C for 1 h, respectively, showing an excellent thermal stability. Moreover, the HWRed steel annealed at 650 °C exhibits high strength-ductility combinations with a yield strength of 906 MPa, ultimate tensile strength(UTS) of1011 MPa, total elongation(TEL) of 51% and product of strength and elongation(PSE: UTS 9 TEL) of 52 GPa%. It is believed that these excellent comprehensive mechanical properties are closely associated with the UFG austenite formation by reverse transformation and principally attributed to the transformation-induced plasticity(TRIP) effect.

Microstructure Evolution and Microhardness of Ultrafine-grained High Carbon Steel during Multiple Laser Shock Processing

Surface microstructure and microhardness of （ferrite＋ cementite） microduplex structure of the ultrafine- grained high carbon steel after laser shock processing （LSP） with different impact times were investigated by means of scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, X-ray diffraction （XRD） and microhardness measurements. Equiaxed ferrite grains were refined from 400 to 150 nm, and the cementite lamellae were fully spheroidized, with a decrease of the particle diameter from 150 to 100 nm as the impact times increased. The cementite dissolution was enhanced significantly. Correspondingly, the lattice parameter of α-Fe and microhard- hess increased with the impact times.

Microstructure Characteristics in Ultrafine-grained Commercially Pure Aluminium after Laser Shock Processing

The aim of this paper was to address the effect of laser shock processing (LSP) on the microstructure of ultrafine-grained commercially pure aluminium which was produced through severe cold rolling and annealing. The microstructure characteristics of ultrafine-grained commercially pure aluminium were experimentally investigated by TEM during ultra-high strain rate loading. The results show that microstructure was obviously refined due to ultra-high plastic strain induced by a single pass LSP impacts. The grain sizes decrease from 0.6 μm after severe cold rolling and annealing to 0.3 μm at the center of the laser shock wave after a single pass LSP. There is a distinct increase in the dislocation density at the edge of the laser shock wave. These experiments have guide meaning to the practical engineering applications of LSP technique.

超细晶铜在退火与高温变形条件下微观结构的不稳定性研究

通过差示扫描量热仪(DSC)和显微硬度测试研究了等通道转角挤压(ECAP)制备的超细晶铜在退火条件下的热稳定性和硬度变化,同时利用扫描电镜电子通道衬度(SEM ECC)技术和透射电镜(TEM)研究了超细晶铜在室温到300℃的温度范围内分别在单向压缩和循环变形后的微观结构变化.结果表明:超细晶铜即使在低于再结晶温度退火条件下也会以缓慢渐进的方式发生逐步的再结晶和晶粒粗化,该结构软化过程通过DSC随退火时间的响应曲线探测不到.高温压缩下晶粒的粗化行为与应变速率有关,应变速率越大,粗化的局部化越明显;应变速率越小,更多的晶粒发生整体粗化.高温循环加载促使晶粒粗化发生得更为显著、均匀,在粗化的晶粒内可观察到一些典型的位错组态,如墙结构和胞结构等.另外,利用最大晶粒尺度(D_(max))与平均晶粒尺度(D_(aver))的比值V定量讨论了不同高温变形情况下晶粒粗化的不均匀性.

采用等径角挤扭工艺制备块体超细晶铝

采用数值模拟和实验分析方法，对200℃条件下纯铝粉末材料1~4道次A路径等径角挤扭（ECAPT）变形致密行为、晶粒细化机制以及组织和性能演变规律进行研究，制备出组织和性能优良的块体超细晶铝。结果表明：随变形道次的增加，材料内部所累积的应变量不断增大，出现了加工硬化现象，挤压载荷峰值不断上升。ECAPT工艺有效提高了变形材料内部的静水压力，使坯料整体致密程度和变形均匀性得到明显改善。纯铝组织发生了连续动态再结晶，晶粒在不断被细化的同时，其取向差进一步增大，最终在材料内部形成了细小、均匀且被大角度晶界包围的等轴再结晶组织。4道次ECAPT变形后，组织平均晶粒尺寸约为600 nm，抗拉强度达到123.3 MPa。

大变形异步叠轧制备超细晶铜材叠轧过程组织演变研究

采用大变形异步叠轧的方法制备超细晶铜材,从宏观上研究异步叠轧组织变化过程。分析研究表明轧制过程原始等轴晶粒在板面方向上被压扁并延展,当变形量大到一定程度,部分压扁的晶粒碎化;原始等轴晶纵向在轧制过程中晶粒由等轴晶逐渐被拉长成纤维组织,随着应变量的增加,纤维组织逐渐变细长,部分纤维组织在进一步叠轧过程中断掉;横向上晶粒也是逐渐压扁拉长,同时晶粒内部以及晶界多处发生滑移(或兼有孪生)。异步叠轧过程中宏观的组织变化过程类似与同步叠轧过程。均匀的轧制组织再结晶退火后可以获得均匀超细晶,通过再结晶处理,可以获得晶粒尺寸在2μm以下的超细晶铜材。

复合细化制备超细晶纯钛热稳定性研究

用复合细化方法(ECAP+冷轧+旋锻)制备出超细晶纯钛,利用透射电镜(TEM)、万能拉伸机、显微硬度计和扫描电镜(SEM)研究了不同退火温度对超细晶纯钛组织性能的影响.结果表明:复合细化制备超细晶纯钛的晶粒尺寸约为180nm,其抗拉强度达到870 MPa,低温退火时(200~350℃),组织无明显变化,位错密度逐渐降低,晶界逐渐平直清晰,强度和硬度下降较为缓慢,超细晶纯钛的再结晶温度为350~400℃;退火温度高于400℃时,晶粒逐渐长大,超细晶纯钛的硬度显著下降.观察断口形貌可知,超细晶纯钛的断裂形式为韧性断裂,且随着退火温度的升高,等轴韧窝尺寸增大,深度变深.

超细晶粒钢的焊接方法及接头组织特征

作为新一代高性能钢铁结构材料代表的超细晶粒钢,因其晶粒的超细化从而实现了性能的强韧化,但在焊接时,面临的最大障碍是热影响区晶粒过度长大。系统分析了超细晶粒钢焊接接头的组织特征和焊接性,对超细晶粒钢焊接方法进行了新的分类组织保存型焊接工艺与同性能焊接工艺。介绍并讨论了各种焊接方法与焊接接头组织的特征,以期为新型焊接工艺的开发、热影响区组织控制及焊接接头性能预测提供借鉴。

Three-Dimensional Microstructures and Tensile Properties of Pure Iron During Equal Channel Angular Pressing

Commercial pure iron billets having diameter of 60 mm and length of 180 mm were subjected to equal channel angular pressing（ECAP） at 350 ℃ for 1 to 4 passes via route Bc.Microstructural evolutions on three planes（X,Y,Z planes） were characterized by optical microscopy and transmission electron microscopy（TEM）.It was found that after four passes an ultrafine microstructure could be formed on the X plane,but a band structure remained on the Z plane.Accordingly,the mechanical properties exhibited apparent dependence on the orientations.The strength in the x and y directions was higher than that in the z direction.The microstructural refinement and mechanical properties were discussed in terms of experimental results.

Bimodal microstructure – A feasible strategy for high-strength and ductile metallic materials

Introducing a bimodal grain-size distribution has been demonstrated an efficient strategy for fabricating high-strength and ductile metallic materials, where fine grains provide strength, while coarse grains enable strain hardening and hence decent ductility. Over the last decades, research activities in this area have grown enormously, including interesting results onfcc Cu, Ni and Al-Mg alloys as well as steel and Fe alloys via various thermo-mechanical processing approaches. However, investigations on bimodal Mg and other hcp metals are relatively few. A brief overview of the available approaches based on thermo- mechanical processing technology in producing bimodal microstructure for various metallic materials is given, along with a summary of unusual mechanical properties achievable by bimodality, where focus is placed on the microstructure-mechanical properties and relevant mechanisms. In addition, key factors that influencing bimodal strategies, such as compositions of starting materials and processing parameters, together with the challenges this research area facing, are identified and discussed briefly.

Effect of high pressure torsion process on the microhardness,microstructure and tribological property of Ti6Al4V alloy

In the present study,a fully lamellar Ti6Al4V alloy was severely deformed by high pressure torsion(HPT)process under a pressure of 7.5 GPa up to 10 revolutions.Experimental results revealed that the microhardness of Ti6Al4V was increased remarkably by about~41%and saturated at about 432 Hv after the HPT process.A relatively uniform bulk nanostructured Ti6Al4V alloy with an average grain size of about52.7 nm was obtained eventually,and no obvious formation of metastableωphase was detected by XRD analysis.For the first time,the tribological properties of the HPT processed Ti6Al4V alloy were investigated by a ball-on-disc test at room temperature under a dry condition.It was found that HPT process had a great influence on the friction and wear behaviors of Ti6Al4V alloy.With increasing the number of HPT revolutions,both friction coefficient and specific wear rate were obviously decreased due to the reduction of abrasion and adhesion wears.After being deformed by 10 HPT revolutions,the friction coefficient was reduced from about 0.49 to 0.37,and the specific wear rate was reduced by about 48%.The observations in this study indicated that HPT processed Ti6Al4V alloys had good potential in structural applications owing to their greatly improved mechanical and tribological properties.

A crystal plasticity based approach to establish role of grain size and crystallographic texture in the Tension–Compression yield asymmetry and strain hardening behavior of a Magnesium–Silver–Rare Earth alloy

Existence of tension–compression yield asymmetry is a serious limitation to the load bearing capablities of Magnesium alloys in a number of light weight structural applications.The present work is aimed at nullifying the tension to compression asymmetry problem and strain hardening anomalies in a Magnesium–Silver–Rare Earth alloy by engineering different levels of microstructural conditions via friction stir processing and post process annealing.The existence and extent of yield asymmetry ratio in the range of microstructural conditions was experimentally obtained through quasistatic tensile and compression tests.The yield asymmetry problem was profoundly present in specimens of coarse grained microstructures when compared to their fine grained and ultra fine grained counterparts.The impact of the microstructure and associated mechanisms of plasticity on the macroscopic strain hardening behavior was established by Kock–Mecking’s analysis.Crystal plasticity simulations using Viscoplastic Self Consistency approach revealed the consequential role of extension twinning mechanism for the existence of yield asymmetry and anomalies in strain hardening behavior.This was especially dominant with coarsening of grain size.Electron Microscopy and characterization were conducted thoroughly in partially deformed specimens to confirm the predictions of the above simulations.The role of crystallographic texture for inducing the polarity to Tension–Compression yield asymmetry was corroborated.A critical grain size in Magnesium–Silver–Rare earth alloy was hereby established which could nullify influences of extension twinning in yield asymmetry ratio.

Towards ultrastrong and ductile medium-entropy alloy through dual-phase ultrafine-grained architecture

Advanced materials with superior comprehensive mechanical properties are strongly desired,but it has long been a challenge to achieve high ductility in high-strength materials.Here,we proposed a new V 0.5 Cr 0.5 CoNi medium-entropy alloy(MEA)with a face-centered cubic/hexagonal close-packed(FCC/HCP)dual-phase ultrafine-grained(UFG)architecture containing stacking faults(SFs)and local chemical order(LCO)in HCP solid solution,to obtain an ultrahigh yield strength of 1476 MPa and uniform elongation of 13.2%at ambient temperature.The ultrahigh yield strength originates mainly from fine grain strength-ening of the UFG FCC matrix and HCP second-phase strengthening assisted by the SFs and LCO inside,whereas the large ductility correlates to the superior ability of the UFG FCC matrix to storage disloca-tions and the function of deformation-induced SFs in the vicinity of the FCC/HCP boundary to eliminate the stress concentration.This work provides new guidance by engineering novel composition and stable UFG structure for upgrading the mechanical properties of metallic materials.

Deformation temperature and postdeformation annealing effects on severely deformed TiNi alloy by equal channel angular extrusion

Micron TiNi alloy blocks were fabricated at high temperature by equal channel angular extrusion （ECAE） using hotforged Ti-50.3at%Ni alloy as the raw material and the effects of deformation temperature and postdeformation annealing on the severely deformed TiNi alloy by ECAE were investigated. The results show that the TiNi alloy processed by ECAE undergoes severe plastic deformation, and lowering the deformation temperature and increasing the number of extrusions contribute to grain refinement. When the annealing temperature is below 873 K, static recovery is the main restoration process; when the temperature rises to 973 K, static recrystallization occurs. It is found that fine particles are precipitated when the TiNi alloy processed by ECAE is annealed at 773 K.