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.

Superplastic behavior of friction-stir welded Al-Mg-Sc-Zr alloy in ultrafine-grained condition

The present work was undertaken to improve superplastic ductility of friction-stir welded joints of ultrafine-grained(UFG)Al-Mg-Sc-Zr alloy.In order to suppress the undesirable abnormal grain growth,which typically occurs in the heavily deformed base material,the UFG material was produced at elevated temperature.It was suggested that the new processing route could reduce dislocation density in the UFG structure and thus enhance its thermal stability.It was found,however,that the new approach resulted in a relatively high fraction of low-angle boundaries which,in turn,retarded grain-boundary sliding during subsequent superplastic tests.Therefore,despite the successful inhibition of the abnormal grain growth in the base-material zone,the superplastic deformation was still preferentially concentrated in the fully-recrystallized stir zone of the material.As a result,the maximal elongation-to-failure did not exceed 700%.

超细晶材料制备新工艺——挤扭

近年对块体超细晶材料的研究已成为材料科学领域一大热点。挤扭工艺作为一种新兴的大塑性变形细晶材料制备技术,在细化材料显微组织、改善材料力学性能、提高材料成形性等方面发挥了重要作用,具有广阔的工业应用前景。文章重点介绍了挤扭工艺的基本原理及其变形特点,综述了国内外挤扭工艺的研究现状和研究进展,指出了该工艺目前存在的问题,并对其应用前景进行了展望,具有较好的实用参考价值。

Uniaxial compressive behavior of equal channel angular pressing Al at wide temperature and strain rate range

Uniaxial compressive experiments of ultrafine-grained Al fabricated by equal channel angular pressing(ECAP) method were performed at wide temperature and strain rate range. The influence of temperature on flow stress, strain hardening rate and strain rate sensitivity was investigated experimentally. The results show that both the effect of temperature on flow stress and its strain rate sensitivity of ECAPed Al is much larger than those of the coarse-grained Al. The temperature sensitivity of ultrafine-grained Al is comparatively weaker than that of the coarse-grained Al. Based on the experimental results, the apparent activation volume was estimated at different temperatures and strain rates. The forest dislocation interactions is the dominant thermally activated mechanism for ECAPed Al compressed at quasi-static strain rates, while the viscous drag plays an important role at high strain rates.

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。

连续等径角挤压及其成形过程的三维数值模拟

连续等径角挤压是一种制备大尺寸超细晶材料的新技术,它结合了等径角挤压和连续挤压技术的特点,解决了等径角挤压不能制备大尺寸超细晶材料的问题,该技术对超细晶材料的推广应用具有重要意义。利用DE-FORM3D软件对纯铜连续等径角挤压变形行为进行了数值模拟,分析了变形过程中材料的流动、应变和温度变化情况,并对不同变形速度、摩擦条件和模具结构下的变形过程进行了比较,为连续等径角挤压工艺提供了理论指导。

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.

Structural transformation and energy analysis for pile-up dislocations at triple junction of grain boundary

An energy model for the structure transformation of pile-ups of grain boundary dislocations(GBD)at the triple-junction of the grain boundary of ultrafine-grain materials was proposed.The energy of the pile-up of the GBD in the system was calculated by the energy model,the critical geometric and mechanical conditions for the structure transformation of head dislocation of the pile-up were analyzed,and the influence of the number density of the dislocations and the angle between Burgers vectors of two decomposed dislocations on the transformation mode of head dislocation was discussed.The results show when the GBD is accumulated at triple junction,the head dislocation of the GBD is decomposed into two Burgers vectors of these dislocations unless the angle between the two vectors is less than 90°,and the increase of applied external stress can reduce the energy barrier of the dislocation decomposition.The mechanism that the ultrafine-grained metal material has both high strength and plasticity owing to the structure transformation of the pile-up of the GBD at the triple junction of the grain boundary is revealed.

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.

超细晶纯钛疲劳裂纹的扩展行为

对纯钛进行2道次室温等径弯曲通道变形(ECAP)、等径弯曲通道变形加旋锻复合变形(ECAP+RS)并在旋锻后在300℃和400℃退火1 h,制备出4种具有不同组织的超细晶纯钛。对这4种超细晶纯钛进行疲劳裂纹扩展实验并观察分析超细晶纯钛的显微组织和疲劳断口的形貌,研究了裂纹的扩展行为。结果表明:显微组织对超细晶纯钛的疲劳裂纹扩展门槛值和近门槛区有显著的影响;超细晶纯钛的疲劳裂纹扩展门槛值随着塑性变形量的增大而增大,随着旋锻后退火温度的提高而降低;疲劳裂纹扩展速率曲线因超细晶纯钛晶粒尺寸和强度的影响出现转折,转折前ECAP+RS复合变形纯钛的抗疲劳裂纹扩展能力比ECAP变形强,且随着退火温度的提高而降低;转折后4种超细晶纯钛的疲劳裂纹扩展速率相差较小,呈现出相反的结果。疲劳裂纹扩展寿命中转折前近门槛区裂纹扩展寿命占绝大部分,因而转折前的门槛值与近门槛区的扩展速率对抗裂纹扩展能力更为重要。

Fatigue behavior of CoCrFeMnNi high-entropy alloy under fully reversed cyclic deformation

Bulk ultrafine-grained(UFG) CoCrFeMnNi high-entropy alloy(HEA) with fully recrystallized microstructure was processed by cold rolling and annealing treatment. The high-cycle fatigue behaviors of the UFG HEA and a coarse-grained(CG) counterpart were investigated under fully reversed cyclic deformation.The fatigue strength of the UFG HEA can be significantly enhanced by refining the grain size. However,no grain coarsening was observed in the UFG HEA during fatigue tests. Mechanisms for the superior mechanical properties of the UFG HEA were explored.

Impact of severe plastic deformation on kinetics and thermodynamics of hydrogen storage in magnesium and its alloys

Magnesium and its alloys are the most investigated materials for solid-state hydrogen storage in the form of metal hydrides,but there are still unresolved problems with the kinetics and thermodynamics of hydrogenation and dehydrogenation of this group of materials.Severe plastic deformation(SPD)methods,such as equal-channel angular pressing(ECAP),high-pressure torsion(HPT),intensive rolling,and fast forging,have been widely used to enhance the activation,air resistance,and hydrogenation/dehydrogenation kinetics of Mg-based hydrogen storage materials by introducing ultrafine/nanoscale grains and crystal lattice defects.These severely deformed materials,particularly in the presence of alloying additives or second-phase nanoparticles,can show not only fast hydrogen absorption/desorption kinetics but also good cycling stability.It was shown that some materials that are apparently inert to hydrogen can absorb hydrogen after SPD processing.Moreover,the SPD methods were effectively used for hydrogen binding-energy engineering and synthesizing new magnesium alloys with low thermodynamic stability for reversible low/room-temperature hydrogen storage,such as nanoglasses,high-entropy alloys,and metastable phases including the high-pressureγ-MgH2 polymorph.This work reviews recent advances in the development of Mg-based hydrogen storage materials by SPD processing and discusses their potential in future applications.

Resolving the sintering conundrum of high-rhenium tungsten alloys

Tungsten-rhenium(W-Re)alloys with high-Re contents are the preferred refractory metal materials in many applications because of the improved ductility and processability over pure W and low-Re tung-sten alloys.However,the sintering concurrently becomes increasingly more difficult with increasing Re contents.Here we proposed that the sintering conundrum is caused by the lowered crystal symmetry and the wider dihedral angle distribution when body-center-cubic(BCC)W is alloyed with more hexagonal-close-packed(HCP)Re,which results in inefficient pore removal in the final stage sintering.We showed that the conundrum can be resolved by pressureless two-step sintering(TSS)which suppresses acceler-ating final-stage grain growth,and our proposal is supported by the data of the critical densityρc that is required to start the second step for successful TSS at different W-Re compositions.Dense ultrafine-grained W-Re alloys with∼300 nm average grain size and up to 25 wt%Re were successfully produced.Our work demonstrates the unique opportunities offered by two-step sintering to advance the scientific understanding and technological practices in powder metallurgy and related fields.

Superfunctional high-entropy alloys and ceramics by severe plastic deformation

High-entropy alloys and ceramics containing at least five principal elements have recently received high attention for various mechanical and functional applications.The application of severe plastic deformation(SPD),particularly the high-pressure torsion method,combined with the CALPHAD(calculation of phase diagram) and first-principles calculations resulted in the development of numerous superfunctional high-entropy materials with superior properties compared to the normal functions of engineering materials.This article reviews the recent advances in the application of SPD to developing superfunctional high-entropy materials.These superfunctional properties include(ⅰ) ultrahigh hardness levels comparable to the hardness of ceramics in high-entropy alloys,(ⅱ) high yield strength and good hydrogen embrittlement resistance in high-entropy alloys;(ⅲ) high strength,low elastic modulus,and high biocompatibility in high-entropy alloys,(ⅳ) fast and reversible hydrogen storage in high-entropy hydrides,(ⅴ) photovoltaic performance and photocurrent generation on high-entropy semiconductors,(ⅵ) photocatalytic oxygen and hydrogen production from water splitting on high-entropy oxides and oxynitrides,and(ⅶ)CO_(2) photoreduction on high-entropy ceramics.These findings introduce SPD as not only a processing tool to improve the properties of existing high-entropy materials but also as a synthesis tool to produce novel high-entropy materials with superior properties compared with conventional engineering materials.

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.