Ultrastrong and ductile(CoCrNi)_(94)Ti_(3)Al_(3)medium-entropy alloys via introducing multi-scale heterogeneous structures

The coarsening-grained single-phase face-centered cubic(fcc)medium-entropy alloys(MEAs)normally exhibit insufficient strength for some engineering applications.Here,superior mechanical properties with ultimate tensile strength of 1.6 GPa and fracture strain of 13.1%at ambient temperature have been achieved in a(CoCrNi)_(94)Ti_(3)Al_(3)MEA by carefully architecting the multi-scale heterogeneous structures.Electron microscopy characterization indicates that the superior mechanical properties mainly originated from the favorable heterogeneous fcc matrix(1-40μm)and the coherent sphericalγ’precipitates(10-100 nm),together with a high number density of crystalline defects(2-10 nm),including dislocations,small stacking faults,Lomer-Cottrell locks,and ultrafine deformation twins.

Enhancing {1012} twin boundary migration capability in Ti-Al solid solution alloys with increasing Al content

1.Introduction Titanium(Ti)and its alloys have been widely used in the aerospace,biomedical and chemical industries due to their high specific strength,low Young’s modulus,superior biocompatibility and strong corrosion resistance[1–5].While Ti alloys have high strength,they usually present relatively poor ductility and formability at room temperature[5].Therefore,their deformation behaviors and corresponding mechanisms have been the focus of attention in order to obtain better formability and mechanical properties.Forα-Ti and its alloys with hexagonal close-packed(HCP)structure,the most common slip system is prismatic a slip at room temperature[6–9].Pyramidal c+a slip can coordinate c-axis strain,but it is general difficult to activate c+a slip in polycrystalline Ti alloys[6,9]although reducing grain size can increase the propensity of c+a slip[9].Due to the limited slip systems for accommodating c-axis strain,deformation twinning plays an important role to coordinate strain along the c-axis of the HCP structure during plastic deformation.

超低温环境下亚稳态CoCrFeNi高熵合金的优异塑性和锯齿流变行为（英文）

低温环境下,位错的运动受到限制而导致极少数的金属和合金能保持优异的力学性能,尤其是塑性.本文研究了具有面心立方结构的CoCrFeNi高熵合金的超低温服役,发现其在低温环境下具有优异的综合性能. 4.2 K时的拉伸强度达到1260 MPa,同时延伸率达到62%,展现出极强的低温应用潜力;超低温环境下,高熵合金极低的层错促进了变形孪晶的产生,使其表现出高强高韧的优异力学性能.此外,在液氦环境下,该合金中FCC-HCP的相转变和锯齿流变行为使得合金在77 K以下温度的塑性降低;同时,关于锯齿特征的动态模型分析证实由于相变行为的出现导致该合金中锯齿行为的不稳定特点.液氦环境下,大量的变形孪晶和相变行为的共同作用导致了较高的应变硬化率,从而使高熵合金的塑性变形维持在较高的应力水平,并且形成了锯齿特征.

Ultra-strong and thermally stable nanocrystalline CrCoNi alloy

Grain refinement to the nanocrystalline regime is the most effective way to strengthen materials but this often deteriorates the grain-size thermal stability and plasticity. Here we manufactured a nanocrystalline face centred cubic Cr Co Ni medium entropy alloy with columnar grains via magnetron sputtering. Compression of CrCoNi pillars with diameters of ~1 μm revealed a record high yield strength of ~5 GPa for pillars with face centred cubic structures and engineering plastic strain of > 30%. The alloy possessed an outstanding grain-size thermal stability even at 1073 K. Both nanocrystalline grain size and a high density of nanotwins/stacking faults are critical to the exceptional yield strength. Deformation twinning, grains refinement during deformation, grain boundary sliding and random grain orientation all contribute to the large plasticity. The outstanding thermal stability is attributed to the sluggish diffusion effect and the low energy of twin boundaries.

体心立方金属及合金的高力学性能的尺寸效应（英文）

体心立方(BCC)金属在亚微米和纳米尺度表现出不同于块体粗晶尺度的力学性能,包括高屈服强度和大延展性.这些突出性能使其不仅可用于微米、纳米甚至原子尺度的器件,而且可作为结构材料的组成部分发挥重要作用.导致小尺度BCC金属这种不同寻常的力学性能的一个重要原因就是它们的尺寸,降低单晶金属的尺寸和多晶金属的晶粒度可以显著增加强度.在过去十年,小尺度BCC金属塑性变形行为的研究取得了大幅进展.本综述全面、系统地总结了小尺度BCC金属塑性变形尺寸效应的塑性机制理论,并综合介绍了研究小尺寸样品塑性变形的原位方法和技术.

Understanding formation of Mg-depletion zones in Al-Mg alloys under high pressure torsion

Redistribution of elements may take place in alloys during severe plastic deformation, which significantly alters the mechanical properties of the alloys. Therefore, comprehensive knowledge about deformationinduced redistribution of elements has to be established. In the present paper, the distribution of Mg in an Al-Mg alloy processed by high pressure torsion was examined using atom probe tomography(APT).With crystallographic information extracted by APT data analysis, this research reveals that the movement of dislocations plays an important role in the formation of Mg-depletion zones in the deformed microstructure.

Effects of temperature and alloying content on the phase transformation and{10■1}twinning in Zr during rolling

The effects of temperature and Ti content on the deformation mechanisms of pure Zr and Zr-Ti alloys were investigated by transmission electron microscopy.The results indicate the existence of a relation between deformation-induced phase transformation from a hexagonal close-packed structure to a facecentered cubic structure and{10■1}deformation twinning.That is,when one is suppressed,the other will be promoted.The phase transformation was suppressed while the{10■1}compressive twinning was promoted with increasing the rolling temperature and/or Ti content.This can be attributed to the activation of basal<a>dislocations at high temperature and the increased stacking fault energy with Ti content.

双相纳米结构CrCoNi中熵合金变形行为的尺寸效应

本文结合原位扫描电子显微镜微柱压缩与透射电子显微镜技术,研究了具有双相多级纳米结构的CrCoNi中熵合金变形行为的尺寸效应.研究表明,该合金的屈服强度高达~4 GPa,这主要归因于其多级微观结构特征,包括柱状纳米尺寸晶粒、织构、高密度的层错、孪晶界、晶界和相界.在变形过程中,该合金的屈服强度基本与微米尺度样品的尺寸无关,但其变形行为却强烈依赖于样品大小.具体来说,随着微柱直径减小,材料主要的变形模式从突发的局部剪切带变为具有明显塑性的均匀变形.这种转变是由剪切带穿过微柱所需的临界应力与样品尺寸紧密相关所决定的,剪切诱导的软化和随后的硬化机制之间的竞争也起了重要作用.此外,变形引起了六方密排结构到面心立方结构的相变,该相变导致的应变软化对材料变形中的塑性有重要贡献.这些发现揭示了样品尺寸对可用于微纳机电系统的纳米结构金属材料的力学行为有着重要影响.

Phase transformation and structural evolution in a Ti-5at.%Al alloy induced by cold-rolling

The phase transformation,deformation mechanism and their correlation in a cold-rolled Ti-5 at.%Al alloy were investigated.Two types of phase transformations from a hexagonal close-packed(HCP)structure to a face-centered cubic(FCC)structure were observed:the basal-type(B-type)with an orientation relation-ship of<1210HCP//<110FCC and{0001}HCP//{111}FCC,and the prismatic-type(P-type)with an orientation-relationship of<1210HCP//<110FCC and 1010HCP//110FCC.The two types of transformations both accommodate the strain along the<c>axis of the HCP matrix.With the proceeding of deformation,different deformation mechanisms were activated in the FCC and the HCP structures,respectively,which led to a faster grain refinement rate in the FCC structure than in the HCP matrix.Deformation twins with zero macroscopic strain were prevalent in the FCC domains produced by the B-type transformation,while deformation twins with macroscopic strain were normally observed in the FCC domains produced by the P-type transformation.This is in accordance with the lattice mismatches produced during phase transformation.The easy occurrence of deformation twinning in the FCC structure contributed significantly to the grain refinement process.In addition,the interaction between neighboring FCC domains produced by the two types of phase transformations also accelerated the grain refinement process.

Unveiling the grain boundary-related effects on the incipient plasticity and dislocation behavior in nanocrystalline CrCoNi medium-entropy alloy

The incipient plasticity and dislocation behavior in a nanocrystalline(NC)CrCoNi medium-entropy alloy were systematically investigated in terms of pop-in events during instrumental nano-indentation tests.Quantitative statistical analysis and molecular dynamic simulations were performed to reveal the effects of grain boundaries(GBs)on initial stages of plastic deformation.Multiple pop-in events appeared during loading on the NC CrCoNi.The first pop-in that represents the initial yielding was identified to be controlled by dislocation nucleation,which is in sharp contrast to the continuous elastic-plastic transition mediated by GB mechanisms in NC pure metals.This can be attributed to the sluggish kinetics of the chemically complex GBs(CCGBs)in the NC CrCoNi that hinders diffusive GB activities but facilitates dislocation nucleation.Subsequent pop-ins were also found to be closely related to the extra dragging effects imposed by the CCGBs on dislocation propagation in the NC alloy.Moreover,the extremely small grain sizes and the consequent high-volume fraction of GBs in the NC alloy severely restrict the lengths of dislocation source and the radii of dislocation loop,giving rise to a higher critical stress,smaller activation volume and lower pop-in width as compared with its coarse-grained counterpart.These results provide new insights into the onset of nano-plasticity in concentrated multi-principal element alloys.

Composition-dependent dynamic precipitation and grain refinement in Al-Si system under high-pressure torsion

Understanding composition effects is crucial for alloy design and development. To date, there is a lack of research comprehensively addressing the effect of alloy composition on dynamic precipitation, segregation and grain refinement under severe-plastic-deformation processing. This research investigates Al-x Si alloys with x = 0.1, 0.5 and 1.0 at.% Si processed by high pressure torsion(HPT) at room temperature by using transmission electron microscopy, transmission Kikuchi diffraction and atom probe tomography. The alloys exhibit interesting composition-dependent grain refinement and fast dynamic decomposition under HPT processing. Si atoms segregate at dislocations and Si precipitates form at grain boundaries(GBs) depending on the Si content of the alloys. The growth of Si precipitates consumes most Si atoms segregating at GBs, hence the size and distribution of the Si precipitates become predominant factors in controlling the grain size of the decomposed Al-Si alloys after HPT processing. The hardness of the Al-Si alloys is well correlated with a combination of grain-refinement strengthening and the decomposition-induced softening.