Synthesis of ultra high strength Al–Mg–Si alloy sheets by differential speed rolling

The ultrafine-grained Al–Mg–Si alloy sheets, which were fabricated by severe plastic deformation (SPD) using a high-speed-ratio differential speed rolling (HRDSR) and subsequent low temperature aging, exhibited an ultra high strength (yield stress: 455 MPa, ultimate tensile strength: 489 MPa). The strengthening effect was impressive compared with the results obtained by using other SPD techniques. The achievement could be attributed to formation of very fine grains due to significantly increased dislocation density in solute supersaturated matrix, high Hall-Petch constant and particle strengthening gained by formation nano-scale precipitates during the low temperature aging after the HRDSR process.     

Mechanical,corrosion and magnetic behavior of a CoFeMn1.2NiGa0.8 high entropy alloy

In this study,a magnetic high entropy alloy (HEA) of CoFeMn1.2NiGa0.8 was designed and prepared by arc melting in order to investigate its mechanical,corrosion and magnetic behavior.The results show that the alloy mainly possesses body-centered cubic (BCC) phase and face-centered cubic (FCC) phase.A high compressive strength of 1450 MPa,a strain of 18.5 ％ and a relatively low yield strength of 303 MPa in as-cast condition at room temperature can be achieved in the present alloy.In-situ high-energy X-ray diffraction technique was employed to reveal the deformation mechanism of CoFeMn1.2NiGa0.8 under uniaxial compression and the results show that the competition between BCC phase and FCC phase plays a significant role during the compressive process.The corrosion behavior of CoFeMn1.2NiGa0.8 was investigated in 3.5 wt％ NaCl solution and it turned out that the alloy possessed good corrosion resistance.At last,the magnetic behavior of the CoFeMn1.2NiGa0.8 alloy was studied and it can present a high saturation magnetization of 94.5 emu/g and a coercivity of 26.4Oe at 4 K.This work indicates that the present CoFeMn1.2NiGa0.8 HEA has promising applications as future magnetic functional materials.     

Enhancement of strength-ductility balance of heavy Ti and Al alloyed FeCoNiCr high-entropy alloys via boron doping

As one of the most effective mechanisms,precipitation-hardening is widely used to strengthen high-entropy alloys.Yet,heavy precipitation-hardened high-entropy alloys usually exhibit serious embrittlement.How to effectively achieve ultra-high strength and maintain reliable ductility remains a challenge.Here,we report a study of doping extremely little boron to meet this target.We found that adding of 30 ppm boron into the heavy Ti and Al alloyed FCC FeCoNiCr high-entropy,(FeCoNiCr)88Ti6Al6 HEA(at.％)which is strengthened mainly by both coarse BCC-based(Ni,Co)2TiAl Heusler and fine L12-type FCC-based(Ni,Co)3TiAl precipitates and shows ultrahigh strength but poor ductility,could significantly change the original microstructure and consequently improve mechanical performance,owing to the well-known effect of boron on reducing the energy of grain boundaries.The boron addition can(1)eliminate microcavities formed at Heusler precipitate-matrix interfaces;(2)suppress the formation and segregation of coarse BCC Heusler precipitates;(3)promote the formation of L12 nanoparticles.This changes of microstructure substantially improve the tensile ductility more than by～86％and retain comparable or even better ultimate tensile strength.These findings may provide a simple and cost-less solution to produce heavy precipitation-strengthened HEAs with ultrahigh strength and prevent accidental brittleness.     

NbMoTiVSi_x refractory high entropy alloys strengthened by forming BCC phase and silicide eutectic structure

In order to improve mechanical properties of refractory high entropy alloys,silicide was introduced and NbMoTiVSi_x(x=0,0.1,0.2,0.3,and 0.4,molar ratio) refractory high entropy alloys are prepared by vacuum arc melting.Phase composition,micro structure evolution and mechanical properties were systematically studied.Results show that the silicide phase is formed in the alloys with addition of silicon,and the volume fraction of silicide increases from 0 to 8.3 % with increasing of silicon.Microstructure observation shows that the morphology of dendrite changes from columnar to near equiaxed,eutectic structure is formed at grain boundaries and composed of secondary BCC phase and silicide phase.The average length of the primary and second dendrites decreases with the increasing of silicon.Whereas,the ratio of eutectic structure increases from 0 to 19.8 % with the increment of silicon.The refinement of microstructure is caused by heterogeneous nucleation from the silicide.Compressive tests show that the yield and ultimate strength of the alloys increases from 1141.5 MPa to 2093.1 MPa and from 1700.1 MPa to 2374.7 MPa with increasing silicon content.The fracture strain decreases from 24.7 %-11.0 %.Fracture mechanism is changed from ductile fracture to ductile and brittle mixed fracture.The improvement of the strength is caused by grain bounda ry strengthening,which includes more boundaries around primary BCC phase and eutectic structure in grain boundary,both of them is resulted from the formation of silicide.     

Stability of ultra-fine and nano-grains after severe plastic deformation: a critical review

In this critical note, the thermal stability behavior of ultra-fine grained (UFG) and nano-structured (NS) metals and alloys produced through severe plastic deformation (SPD) techniques is reviewed. For this case, the common engineering metals with body-centered cubic (BCC), face-centered cubic (FCC), and hexagonal close-packed (HCP) crystal structures such as aluminum, copper, nickel, magnesium, steel, titanium, and their relating alloys were assessed. Microstructural evolution in these severely deformed materials following post-processing annealing treatment was investigated for various times and temperatures below the recrystallization point. The microstructure development reported in the literature was studied in terms of the stable grain structures correlated with different levels of plastic straining. The stacking fault energy (SFE) is noted to be a key issue which has a critical influence in predicting the coalescence or coarsening behavior of ultra-fine and nanoscale grains after SPD treatment by controlling the cross-slip phenomenon for screw dislocations.

Graphical Abstract

     

Analysis of strengthening mechanisms in a severely-plastically-deformed Al–Mg–Si alloy with submicron grain size

Methods of severe plastic deformation of ductile metals and alloys offer the possibility of processing engineering materials to very high strength with good ductility. After typical amounts of processing strain, a submicrocrystalline material is obtained, with boundaries of rather low misorientation angles and grains containing a high density of dislocations. In the present study, an Al–Mg–Si alloy was severely plastically deformed by equal channel angular pressing (ECAP) to produce such a material. The material was subsequently annealed for dislocation recovery and grain growth. The strength of materials in various deformed and annealed states is examined and the respective contributions of loosely-arranged dislocations, many grain boundaries, as well as dispersed particles are deduced. It is shown that dislocation strengthening is significant in as-deformed, as well as lightly annealed materials, with grain boundary strengthening providing the major contribution thereafter.     

Influence of twist extrusion process on microstructure and mechanical properties of 6063 aluminum alloy

In this study, aluminum alloy 6063 was severely deformed by twist extrusion (TE) technique and its mechanical properties, before and after TE, was investigated using a die with the twist line slope of β = 30°. It was revealed that large strains imposed on the material by this advanced method of severe plastic deformation (SPD) led to a nano-scale ultra-fine microstructure and to an enhancement of the mechanical properties. The more passes of TE the finer grained microstructure was produced. Also with increasing the number of TE passes, yield strength, ultimate tensile strength and hardness increased, while after relative reduction of uniform elongation and elongation to failure by intermediate passes they remained almost unchanged. Therefore, both the strength and ductility of the material were improved when deformed by twist extrusion.     

On the room-temperature tensile deformation behavior of a cast dual-phase high-entropy alloy CrFeCoNiAl0.7

The microstructure and room-temperature tensile deformation behavior of the cast CrFeCoNiAl0.7 high-entropy alloy (HEA) were studied in details.The cast HEA consisted of a dual-phase structure of 77.3 vol.％ face-centered-cubic (FCC) phase plus 22.7 vol.％ B2 phase,and exhibited excellent room-temperature tensile properties with a high yield strength of 876 MPa,ultimate tensile strength of 1198 MPa and a relatively large elongation to fracture of ～9 ％.Dislocations gliding in the FCC phase governed the plastic deformation at the early stage of room-temperature tensile,and disordered dislocations were to form dislocation walls as the deformation proceeded.With further increase in strain to a high level,the stacking faults were generated through the dissociation of the geometrically necessary dislocations,serving as the potential heterogeneous nucleation sites for the deformation twins.     

Ultrafine grained copper alloy sheets having both high strength and high electric conductivity

The ultrafine grained (UFG) microstructure, mechanical properties and electric conductivity of the Cu alloys severely deformed by accumulative roll bonding (ARB) process were systematically investigated. High density of grain boundaries introduced by the ARB process has significant effect on strengthening but little effect on the electric conductivity. The UFG Cu alloys with submicometer grain sizes can achieve both superior mechanical properties and high electric conductivity.     

Methods and mechanisms for uniformly refining deformed mixed and coarse grains inside a solution-treated Ni-based superalloy by two-stage heat treatment

The uniform refinement mechanisms and methods of deformed mixed and coarse grains inside a solution-treatment Ni-based superalloy during two-stage annealing treatment have been investigated.The two-stage heat treatment experiments include an aging annealing treatment(AT)and a subsequent recrystallization annealing treatment(RT).The object of AT is to precipitate some δ phases and consume part of storage energy to inhibit the grain growth during RT,while the RT is to refine mixed and coarse grains by recrystallization.It can be found that the recrystallization grains will quickly grow up to a large size when the AT time is too low or the RT temperature is too high,while the deformed coarse grains cannot be eliminated when the AT time is too long or the RT temperature is too low.In addition,the mixed microstructure composed of some abnormal coarse recrystallization grains(ACRGs)and a large number of fine grains can be observed in the annealed specimen when the AT time is 3 h and RT tem-perature is 980℃.The phenomenon attributes to the uneven distribution of δ phase resulted from the heterogeneous deformation energy when the AT time is too short.In the regions with a large number of δ phases,the recrystallization nucleation rate is promoted and the growth of grains is limited,which results in fine grains.However,in the regions with few δ phases,the recrystallization grains around grain boundaries can easily grow up,and the new recrystallization nucleus is difficult to form inside grain,which leads to ACRGs.Thus,in order to obtain uniform and fine annealed microstructure,it is a prereq-uisite to precipitate even-distributed δ phase by choosing a suitable AT time,such as 12 h.Moreover,a relative high RT temperature is also needed to promote the recrystallization nucleation around δ phase.The optimal annealing parameters range for uniformly refining mixed crystal can be summarized as:900℃×12 h+990℃×(40-60 min)and 900℃×12 h+1000℃×(10-15 min).     

S32750双相不锈钢相界与晶界特征对其力学性能和耐蚀性能的影响EI北大核心CSCD

通过固溶处理获得不同初始组织状态的S32750双相不锈钢样品,然后进行厚度压下量80%的冷轧变形和1050℃的退火处理,采用SEM-EBSD和XRD技术研究合金相界与晶界特征以及相组成分布情况,并利用拉伸实验、纳米压痕和双环电化学动电位再活化法(DL-EPR)分析不同初始状态样品的组织对力学性能与耐晶间腐蚀性能的影响规律。结果表明:高温固溶处理的合金样品经冷轧退火后晶粒细小均匀,两相分布接近1∶1,且相界占内界面(晶界+相界)比例较高,同相晶粒团簇程度最低,表现出优异的综合力学性能。合金样品经敏化处理后,σ相易沿α相晶界析出,高温固溶并经轧制退火后的样品中,由于α晶界比例较少且满足K-S取向关系的相界比例较高则又表现出良好的晶间腐蚀抗力。因此,通过适当的工艺来调控合金的相界与晶界分布可以实现材料强度和晶间腐蚀抗力的同步改善。     

放电等离子烧结对TiB_2/AlCoCrFeNi复合材料组织与性能的影响

采用放电等离子烧结方法(SPS),制备体积分数5%TiB_2的等摩尔AlCoCrFeNi高熵合金基复合材料。通过密度测试、X射线衍射、扫描电镜及力学性能测试等方法,研究SPS烧结温度及烧结压力对复合材料的微结构演变与力学性能影响。结果表明:随着SPS烧结温度及烧结压力的增加,复合材料的硬度及抗压强度得到明显提高。在1200℃/30MPa进行SPS烧结后,复合材料的致密度达99.6%,抗压强度达2416MPa,屈服强度达1474MPa,硬度超过470HB。烧结过程中,复合材料的基体高熵合金发生相变,1200℃及30~45MPa烧结时,复合材料由BCC,B_2,FCC,σ及TiB_2相组成。     

Tensile deformation of a duplex Fe-20Mn-9Al-0.6C steel having the reduced specific weight

The room temperature deformation characteristics of a duplex Fe-20Mn-9Al-0.6C steel with the reduced specific weight of 6.84 g/cm³ in the fully solutionized state were described in conjunction with the deformation mechanisms of its constituent phases. The phase fraction was insensitive to annealing temperature in the range of 800-1100 °C. The ferrite grain size was also nearly unaltered but the austenite grain size slightly increased with increasing annealing temperature. This revealed that there is little window to control the microstructure of the steel by annealing. The steel exhibited a good combination of strength over 800 MPa and ductility over 45% in the present annealing conditions. Ferrite was harder than austenite in this steel. Strain hardening of both phases was monotonic during tensile deformation, but the strain hardening exponent of austenite was higher than that of ferrite, indicating the better strain hardenability of austenite. In addition, the strain hardening exponent of austenite increased but that of ferrite remained unchanged with increasing annealing temperature. The overall strain hardening of the steel followed that of austenite. Considering element partitioning by annealing, the stacking fault energy of austenite of the steel was estimated as ∼70 mJ/m². Even with the relatively high stacking fault energy, planar glide dominantly occurred in austenite. Neither strain induced martensite nor mechanical twins formed in austenite during tensile deformation. Ferrite exhibited the deformed microstructures typically observed in the wavy glide materials, i.e. dislocation cells. The mechanical properties of the present duplex steel were compared to those of advance high strength automotive steels recently developed.     

Cr含量对CrMnFeNi系高熵合金腐蚀行为的影响EI北大核心CSCD

利用XRD,SEM/EDS,EBSD,电化学测试等表征手段研究Cr含量对Cr_(x)MnFeNi(x=0.8,1.0,1.2,1.5)高熵合金微观组织与耐蚀性能的影响。结果表明:Cr_(0.8)MnFeNi高熵合金为单相FCC结构,Cr_(x)MnFeNi(x=1.0,1.2,1.5)高熵合金为FCC+BCC双相结构,且BCC相比例随着Cr含量升高而增加。在0.5 mol/L H_(2)SO_(4)溶液中,高熵合金的耐蚀性能随着Cr含量降低而增强,其中,Cr_(0.8)MnFeNi单相高熵合金的耐蚀性能最好,这是因为Cr_(0.8)MnFeNi高熵合金的成分更为均匀。此外,Cr_(x)MnFeNi高熵合金在0.5 mol/L H_(2)SO_(4)溶液中均具有宽泛的钝化区域以及明显的伪钝化区域,表明合金在耐蚀性能上具有较大的研究价值和开发潜力。     

Strain path effects on microstructural evolution and mechanical behaviour of constrained groove pressed aluminium sheets

Change in strain path during severe plastic deformation (SPD) of metallic materials has shown significant influence on the microstructural evolution, grain boundary characteristics and mechanical behaviour. In the present work high purity aluminium sheets are severe plastically deformed at room temperature by conventional constrained groove pressing (CGP) technique and cross-CGP technique up to 2,4, and 6 passes thereby imparting total effective plastic strain of 2.32, 4.64 and 6.96 respectively. Change in strain path is imposed during cross-CGP by rotating the sheets by ± 90° along thickness axis between each pass. Microstructural evolution of processed sheets studied by electron back scattered diffraction (EBSD) analysis revealed ultra-fine grains (~ 1 μm) irrespective of change in strain path. Analysis of grain boundary characteristics showed significant influence of strain path change on the evolution of relative fraction of low angle and high angle boundaries. The grain refinement mechanism during deformation processing in both conventional CGP and cross-CGP is corroborated to the evolution of misorientation distribution. Though considerable improvement in room temperature tensile characteristics is observed in both cases, cross-CGP processed Al sheets exhibited superior tensile properties.     

Body-centered-cubic martensite and the role on room-temperature tensile properties in Si-added SiVCrMnFeCo high-entropy alloys

We present a new class of metastable high-entropy alloys(HEAs),triggering deformation-induced martensitic transformation(DIMT)from face-centered-cubic(FCC)to body-centered-cubic(BCC),i.e.,BCC-DIMT.Through the ab-initio calculation based on 1st order axial interaction model and combined with the Gibbs free energy calculation,the addition of Si is considered as a critical element which enables to reduce the intrinsic stacking fault energy(ISFE)in SixV(9-x)Cr10Mn5Fe46Co30(x=2,4,and 7 at.％)alloy system.The ISFE decreases from-30.4 to-35.5 mJ/m2 as the Si content increases from 2 to 7 at.％,which well corresponds to the reduced phase stability of FCC against HCP.The BCC-DIMT occurs in all the alloys via intermediate HCP martensite,and the HCP martensite provides nucleation sites of BCC martensite.Therefore,the transformation rate enhances as the Si content increases in an earlier deformation range.However,the BCC-DIMT is also affected by the phase stability of FCC against BCC,and the stability is the highest at the Si content of 7 at.％.Thus,the 7Si alloy presents the moderate transformation rate in the later deformation range.Due to the well-controlled transformation rate and consequent strain-hardening rate,the 7Si alloy possesses the superior combination of strength and ductility beyond 1 GPa of tensile strength at room temperature.Our results suggest that the Si addition can be a favorable candidate in various metastable HEAs for the further property improvement.     

Dilatometry: a powerful tool for the study of defects in ultrafine-grained metals

Vacancies, dislocations, and interfaces are structural defects that are deliberately introduced into solids during grain refinement processes based on severe plastic deformation (SPD). Specific combinations of these defects determine the improved mechanical properties of the obtained ultrafine-grained materials. High-precision, non-equilibrium dilatometry, i.e., measurement of the irreversible macroscopic length change upon defect annealing, provides a powerful technique for the characterization and the study of the kinetics of these defects. It is applied to determine absolute concentrations of vacancies, to characterize dislocation processes, and to assess grain boundary excess volume in pure, FCC and BCC ultrafine-grained metals processed by SPD.     

Microstructure and mechanical properties of novel Al-Y-Sc alloys with high thermal stability and electrical conductivity

The microstructure and mechanical properties of novel Al-Y-Sc alloys with high thermal stability and electrical conductivity were investigated.Eutectic Al3 Y-phase particles of size 100-200 nm were detected in the as-cast microstructure of the alloys.Al3 Y-phase particles provided a higher hardness to as cast alloys than homogenized alloys in the temperature range of 370-440℃.L12 precipitates of the Al3(ScxYy) phase were nucleated homogenously within the aluminium matrix and heterogeneously on the dislocations during annealing at 400℃.The average size of the L12 precipitates was 11±2 nm after annealing for 1 h,and 25-30 nm after annealing for 5 h,which led to a decrease in the hardness of the Al-0.2 Y-0.2 Sc alloy to15 HV.The recrystallization temperature exceeded 350℃and 450℃for the Al-0.2 Y-0.05 Sc and Al-0.2 Y-0.2 Sc alloys,respectively.The investigated alloys demonstrated good thermal stability of the hardness and tensile properties after annealing the rolled alloys at 200 and 300℃,due to fixing of the dislocations and grain boundaries by L12 precipitates and eutectic Al3 Y-phase particles.The good combination of strength,plasticity,and electrical conductivity of the investigated Al-0.2 Y-0.2 Sc alloys make it a promising candidate for electrical conductors.The alloys exhibited a yield stress of 177-183 MPa,ultimate tensile stress of 199-202 MPa,elongation of 15.2-15.8%,and electrical conductivity of 60.8%-61.5% IACS.     

退火工艺对H62黄铜显微组织与力学性能的影响

采用不同的加热温度和冷却方法对H62黄铜进行退火工艺试验,对退火处理后H62黄铜的显微组织、硬度及抗拉强度进行了分析。结果表明:在600~750℃退火温度范围内,H62黄铜出炉后直接空冷得到的显微组织均为α+β相,且随着退火加热温度的升高,显微组织中的β相逐渐增多,材料的硬度和抗拉强度逐渐升高,且抗拉强度均高于技术要求;H62黄铜经650℃退火保温后先随炉缓冷至500℃再出炉空冷,可得到单一均匀的α相组织,材料的硬度和抗拉强度均最低,且抗拉强度满足技术要求。     

Damascene Light‐Weight Metals

Besides widely investigated severely plastically deformed materials that are available in laboratory scale and size only, there is a high demand for semi‐finished products such as sheets and wires with similar mechanical properties. A damascene‐like technology applying swaging and bundling/swaging allows to deform Ti? Nb? Al composites up to a log. deformation strain of 8.4. Here, Al and Ti are used because of their low density, while Nb acts as diffusion barrier to prevent the formation of hardly deformable intermetallic phases. The obtained wires show an ufg microstructure with grain sizes of Ti and Al between 100 and 200 nm. In the cold‐worked condition the wires with a density of 4.0 g cm^?3 reveal an ultimate tensile strength of 790 MPa.