High strength-superplasticity combination of ultrafine-grained ferritic steel:The significant role of nanoscale carbides

There is currently a gap in our understanding of mechanisms that contribute to high strength and high plasticity in high strength UFG ferritic steel with nano-size Fe3 C carbides in situations that involve combination of various strain rates and high temperature.In this regard,we describe the mechanistic basis of obtaining high strength-high plasticity combination in an ultrafine-grained(UFG)(~500±30 nm)ferritic steel with nano-size carbides,which sustained large plastic deformation,exceeding 100%elongation at a temperature significantly below 0.5 of the absolute melting point(Tm).To address the missing gap in our knowledge,we conducted a series of experiments involving combination of strain rate and temperature effects in conjunction with electron microscopy and atom probe tomography(APT).Strain rate studies were carried out at strain rates in the range of 0.0017-0.17 s^(-1)and at different temperatures from 25℃to 600℃.Dynamic recrystallization occurred at 600℃,resulting in a significant decrease in yield and tensile strength.Nevertheless,the UFG ferritic steels had an advantage in tensile strength(UTS)and elongation-to-failure(εf)at 600℃,especially at strain rate of 0.0017 s^(-1),with high UTSof 510 MPa and excellent low temperature(<0.42 Tm)superplasticity(εf=110%).These mechanical properties are significantly superior compared to similar type of steels at identical temperature.A mechanistic understanding of mechanical behavior of UFG ferritic steels is presented by combining the effect of strain rate,temperature,and nano-size carbides.

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.

Tensile behavior of ultrafine-grained low carbon medium manganese steel by intercritical annealing treatment

The intercritical annealing treatment at 650 and 700 ℃ results in two ultrafine-grained (UFG) dual-phase ferrite-austenitesteels. The two steels exhibit different and special discontinuous yielding and pronounced Lüders-like strain phenomenawith large yielding strain which are related to their retained γ-austenite (RA) volume fractions and RA stabilities. The steelannealed at 650 ℃ shows an absent or very small strain hardening, while the steel annealed at 700 ℃ shows an obviousstrain hardening upward curvature with increasing strain. The results show that before and during straining, the steel annealedat 650 ℃ exhibits a mixture of equiaxed and elongated UFG α-ferrite and austenite phases;however, the steel annealed at700 ℃ exhibits only elongated UFG α and γ phases. It was found that most of the γ-austenite to α′-martensite transformationoccurred at the initial deformation stage and very small or almost no transformation occurred afterward. This demonstratesthat the strain-induced martensite (SIM) transformation (γ-α′) or transformation-induced plasticity (TRIP) effect dominatesonly at the initial deformation stage. RA remained stable, and no TRIP effect was observed at the final deformation stage. Theload-unload-reload test was performed to evaluate the back stress (σb) hardening effect. It is believed that the pronouncedstrain hardening behavior at the later deformation stage is mainly associated with σb enhancement induced by the strainpartitioning between the soft and hard phases due to SIM transformation during tensile deformation.

Effect of martensitic transformation on nano/ultrafine-grained structure in 304 austenitic stainless steel

304 austenitic stainless steel was cold rolled in the range of 20%-80%reductions and then annealed at 700-900°C for 60 sto obtain nano/ultrafine-grained（NG/UFG）structure.Transmission electron microscopy,electron backscatter diffraction and X-ray diffraction were used to characterize the resulting microstructures.The results showed that with the increase of cold reduction,the content of martensite was increased.The steel performed work hardening during cold-working owing to the occurrence of strain induced martensite which nucleated in single shear bands.Further rolling broke up the lath-type martensite into dislocation-cell type martensite because of the formation of slip bands.Samples annealed at 800-960°C for 60 swere of NG/UFG structure with different percentage of nanocrystalline（60-100 nm）and ultrafine（100-500 nm）grains,submicron size（500-1000 nm）grains and micron size（〉1000 nm）grains.The value of the Gibbs free energy exhibited that the reversion mechanism of the reversion process was shear controlled by the annealing temperature.For a certain annealing time during the reversion process,austenite nucleated first on dislocation-cell type martensite and the grains grew up subsequently and eventually to be micrometer/submicrometer grains,while the nucleation of austenite on lath-type martensite occurred later resulting in nanocrystalline/ultrafine grains.The existence of the NG/UFG structure led to a higher strength and toughness during tensile test.

日本关于超细晶粒钢制备与焊接新工艺的研究进展

介绍了日本3家公司的3种不同超细晶粒钢制备工艺,以及两项于2002年获得日本溶接学会奖励的超细晶粒钢焊接新技术。

超细晶粒钢制备工艺及机制与传统控轧控冷(TMCP)钢的异同

总结对比了传统控轧控冷(或热机械控制处理,TMCP)钢与超细钢开发制备所用的新型TMCP工艺的特征及其冶金机制。轧前急冷、低温加工与大应变变形(强加工)是超细钢制备工艺的3个必要条件。TMCP的晶粒细化主要靠加工硬化奥氏体的静态铁素体转变。新工艺晶粒细化主要靠形变诱导动态铁素体相变。

超细晶粒钢制备技术及工程应用

介绍了钢铁材料强化方式的发展历程,对超细晶粒钢的制备工艺和冶金机制进行了详细阐述。比较了传统TMCP与新型TMCP的异同,总结了目前采用的细化晶粒低温大变形方法的两种机制。分别简单介绍了鞍钢、本钢、攀钢和宝钢中超细晶粒钢的生产工艺及应用情况。

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。

大变形异步-同步轧制超细晶TWIP钢的组织及变形性行为

通过大变形异步-同步轧制及随后的退火处理,成功制备了超细晶高锰TWIP钢,并详细研究了不同退火温度对大变形TWIP钢的组织和性能的影响机理,结果表明:大变形轧制材料经700℃退火后,组织发生完全再结晶,获得了稳定的晶粒尺寸小于600nm的超细晶TWIP钢。经大变形轧制TWIP钢的抗拉强度从677MPa提高到了1591MPa,随着退火温度的升高,材料的强度降低,塑性增加;大变形纳米组织TWIP钢在600℃退火后,组织为部分再结晶状态,屈服强度、抗拉强度和延伸率分别达到了980MPa、1126MPa和32.6%。退火处理后的超细晶TWIP钢呈现出较高的应变硬化能力。优异的综合力学性能主要归因于大变形轧制后的纳米组织的部分回复再结晶和高锰TWIP钢相对较低的层错能。

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.

Impacts of multiple laser shock processing on microstructure and mechanical property of high-carbon steel

Multiple laser shock processing （LSP） impacts on microstructures and mechanical properties were investigated through morphological determinations and hardness testing. Microscopic results show that without equal channel angular pressing （ECAP）, the LSP-treated lamellar pearlite was transferred to irregular ferrite matrix and incompletely broken cementite particles. With ECAP, LSP leads to refinements of the equiaxed ferrite grain in ultrafine-grained microduplex structure from 400 to 150 nm, and the completely spheroidized cementite particles from 150 to 100 nm. Consequentially, enhancements of mechanical properties were found in strength, microhardness and elongations of samples consisting of lamellar pearlite and ultrafine-grained microduplex structure. After LSP, a mixture of quasi-cleavage and ductile fracture was formed, different from the typical quasi-cleavage fracture from the original lamellar pearlite and the ductile fracture of the microduplex structure.

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.

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.