Structure evolution of pure iron upon low-temperature deformation under high pressure

Structure evolution of iron (99.97% purity) deformed by shear under pressure at 80 K in a medium of liquid nitrogen has been investigated. It has been found that, along with dislocation slip, twinning and development of deformation microbands become operative mechanisms of low-temperature deformation. This led to specific type of inhomogeneity of the structure in which, up to ultimately attained degrees of deformation, low-angle misorientations are retained and, unlike room-temperature deformation, no homogeneous submicrocrystalline (SMC) structure is formed. Twinning contributes to the refinement of structure elements that are more than 1 μm in size; the further refinement occurs by the dislocation-disclination mechanism and goes to the steady-state stage.     

7A60铝合金高压扭转变形微观组织及力学性能分析

对不同高径比H/D的7A60铝合金坯料进行高压扭转实验,借助于EBSD技术分析了变形7A60铝合金微观组织(晶界取向差、晶粒取向)演变规律,同时对变形7A60铝合金进行拉伸实验和断口扫描,分析7A60铝合金的抗拉强度、断裂方式和断裂机理。研究结果表明:高压扭转变形后7A60铝合金晶粒发生了明显破碎细化,当高径比H/D为0.25时,晶粒取向沿001、101方向;当进一步减小高径比H/D时,在三叉晶界处出现了细小的动态再结晶晶粒,取向角在2°≤θ≤5°范围内的小角度晶界比例较大,同一晶粒内部出现了颜色渐变区,大部分晶粒取向沿111方向;高压扭转变形后7A60铝合金抗拉强度明显提高,最高抗拉强度达到了751 MPa,提升幅度达到了20.74%,在强度得到大幅提高的同时伸长率也得到了较好的保持,最高伸长率为14%;高压扭转变形后7A60铝合金断裂方式为塑性断裂,断裂机理属于微孔连接断裂。     

Evolution of the amorphous structure in melt-spun Ti50Ni25Cu25 alloy subjected to high pressure torsion deformation

Peculiarities of structure and mechanical behaviour of amorphous Ti₅₀Ni₂₅Cu₂₅ alloy were the focus of this research. The melt-spun ribbons of amorphous Ti₅₀Ni₂₅Cu₂₅ were subjected to high pressure torsion (HPT) at temperatures of 20–150 °C in order to modify their structure and mechanical behaviour. Some features of obtained HPT-processed samples were compared with initial state with the help of x-ray diffraction (XRD), transmission electron microscopy (TEM) and nanohardness testing. Analysis of structural data and mechanical behaviour allowed us to assume that severe plastic deformation (SPD) processing of melt-spun Ti₅₀Ni₂₅Cu₂₅ alloy might lead to the formation of the structure similar to the new kind of noncrystalline state – “nanoglass” state.     

Special nanostructures in Al-Mg alloys subjected to high pressure torsion

Deformation twins and stacking faults were observed in nanostructure Al-Mg alloys subjected to high pressure torsion. These observations are surprising because deformation twinnings have never been observed in their coarse-grained counterparts under normal conditions. Experimental evidences are introduced on non-equilibrium grain boundaries, deformation twinnings and partial dislocation emissions from grain boundaries. Some of these features can be explained by the results reported from molecular-dynamics simulations of pure FCC metals. Special emphasis is laid on the recent observations of high density hexagonal and rhombic shaped nanostructures with an average size of 3 nm in the Al-Mg alloys processed by high pressure torsion. A possible formation process of these nanostructures is proposed based on molecular-dynamics simulations.     

Cold-consolidation of ball-milled Fe-based amorphous ribbons by high pressure torsion

Cold-consolidation using high pressure torsion (HPT) has been applied to ball-milled Fe-based amorphous ribbons to process bulk disks of approximately 9 mm in diameter and 0.3 mm in thickness. The mainly amorphous bulk samples demonstrated soft magnetic behavior, higher Curie temperature and enhanced microhardness with respect to the ribbons.     

限定型高压扭转变形分析

利用量纲分析研究限定型高压扭转(HPT)变形的相关因素,在此基础上用有限元分析不同侧边摩擦条件和不同径厚比和不同侧面摩擦约束大小对纯铜试样角位移场的特点,讨论理想高压扭转公式适用的范围。量纲分析揭示,试样上变形与几何位置、径厚比、施加压力、材料弹性参数以及模具侧面的摩擦状况相关。有限元分析结果表明,可用幂函数形式的角位移约束来简化侧面摩擦,当幂指数不小于8时,试样上非HPT变形区域大小趋于稳定;当径厚比不小于5时,试样中心存在一个可用纯扭转变形描述的区域,非理想HPT区域大小不超过试样厚度尺寸;当径厚比不大于2时,试样上不存在理想HPT区域。     

Thermal effects in mixtures of aluminum with polyvinyl chloride after plastic deformation under high pressure

PVC and mixtures of PVC with aluminum of different compositions were subjected to plastic deformation under a pressure of 0.5–3.0 GPa using setups of Bridgman anvil type. DSC data showed that ΔC _p in the polymer was doubled and T _c increased by 20°C. Chemical reactions occurred on the interfaces of freshly opened metal surface/polymer phase under pressure treatment in the mixtures. They were accompanied by formation of volatile and soluble products. When metal-polymer mixtures were heated under pressure after deformation, chemical reactions occurred in them starting at 40°C and reaching the maximum intensity in the temperature range of polymer transition from the vitreous to highly elastic state. Thermogravimetric studies of mixture samples of different compositions were carried out.     

纯镁在高压扭转处理中的结构及硬度演变

研究了纯镁在室温高压扭转处理过程中的结构及硬度演变,系统探索了硬度达到稳态后时晶粒尺寸的演变。结果表明,在变形的初始阶段硬度HV随着应变的增加而增大,并达到一个最大值(~530 MPa),随后硬度HV随着应变的增加将降低到一个稳定值。然而,当硬度值处于稳态时,晶粒尺寸并不是处于稳态。在高压扭转的过程中,硬度值随应变的演变和晶粒尺寸随硬度值的演变是不一致的。当硬度值处于稳态时,晶粒尺寸进一步减小是由于动态回复和动态再结晶造成的位错湮灭。而动态回复和动态再结晶的发生来源于高压扭转过程中的温度上升以及纯镁较低的熔点。     

Finite element analysis of the effect of friction in high pressure torsion

High pressure torsion (HPT) is one of the most important techniques among various methods that create severe plastic deformation in the production of bulk materials with nano/ultrafine grained microstructures. Since the driving force in deforming the workpiece in HPT is surface friction, understanding of the friction effect is critical for successful application of HPT. In this study, the friction effect in HPT was analyzed using the finite element method. The distribution of effective strain on the contact surface of the HPT samples under different friction conditions was investigated. The friction force influenced the effective strain more in the middle and edge regions than in the central region. The condition for the minimum friction factor that could achieve a sticking condition between the surfaces of the dies, and the samples in the middle and edge regions, was investigated. There was a critical friction coefficient in which the effective strain varies sharply with an increasing friction coefficient.     

Melting of high- and low-density polyethylene in mixtures with aluminum after plastic deformation under high pressure

Mixtures of low- and high-density polyethylene with aluminum were subjected to plastic deformation at a pressure of 0.5–4.0 GPa. DSC analysis showed that heating of samples at 110 and 140°C results in addition to the melting of polymers in the melting of ultrafine polymer crystallites at 40°C and also in chemical interaction at 80°C at the polymer–metal phase interface; crystallization of the polymer phase in deformed mixtures occurs at 14–18° higher than that of the initial polymers. Enthalpies of thermal processes in baric dependences reached extremum values at the pressures at which restructuring of the electron subsystem occurs in the deformed mixtures.     

Effect of pressure on the evolution of copper microstructure upon large plastic deformation

Copper of 99.99% purity has been deformed by shear under pressure of 2, 6, or 9 GPa at room temperature. The structural changes observed correspond to the temperature and strain-rate conditions of deformation and occur as a result of three processes, namely, dynamic recovery, dynamic recrystallization, and post-dynamic recrystallization, relationship between which depends on the pressure applied. At a pressure of 2 GPa, the dynamic recovery plays a significant role, showing up in the absence of deformation-induced hardening and in a low driving force for grain growth upon the post-dynamic recrystallization. As the pressure rises to 6 GPa, the recovery is hampered and the post-dynamic recrystallization leads to growth of some coarse grains. Grains of more uniform size are retained after deformation at a pressure of 9 GPa.     

高压扭转变形Mg-Zn-Y合金的组织演化和时效行为

采用透射电镜(TEM)、扫描电镜(SEM)、X射线衍射(XRD)仪和显微硬度计研究了高压扭转(HPT)变形Mg-Zn-Y合金的微观组织演变和时效行为.结果表明,在HPT变形前后,合金中的第二相颗粒是W相和Mg24Y5相,且在HPT处理后没有观察到任何新沉淀物的形成,说明HPT变形并未导致合金发生相变.同时,发现HPT变...     

Formation of a W–25%Cu nanocomposite during high pressure torsion

A coarse-grained W–25%Cu composite is subjected to high pressure torsion (HPT) at room temperature, 200 °C, and 400 °C, to different very large strains. The evolution of microstructure with increasing strain is investigated. It is shown that the HPT causes a strong refinement of W particles. No significant influence of the deformation temperature on the microstructure is revealed at small strains (64). A strong effect of the HPT temperature on the microstructure is found at larger strains (>64). It is demonstrated that the HPT can be successfully used to fabricate a W–25%Cu nanocomposite.     

高压扭转对Al-Zn-Mg-Cu-Zr合金抗应力腐蚀性能影响

在400 ℃下对铸态Al-Zn-Mg-Cu-Zr合金进行压强为1GPa的高压扭转和热处理实验,并利用金相显微镜、电化学工作站、慢应变速率拉伸机测试初始及变形试样的显微组织、电化学腐蚀性能及抗应力腐蚀性能。结果表明,高压扭转通过调控晶粒和第二相的尺寸与分布,可以同时提升7系铝合金的抗拉强度和抗应力腐蚀性能。铸态合金由于晶界处团聚的粗大第二相持续腐蚀,导致明显的晶间腐蚀及较差的抗应力腐蚀性能。0.5圈变形后第二相破碎及含量减少导致试样腐蚀敏感性降低,同时晶粒细化能减小面滑移而增大均值滑移模式,使HPT变形后试样的抗应力腐蚀性能显著提升。2圈变形后试样为尺寸更加细小的均匀细晶组织,其均值滑移模式相比于0.5圈变形试样进一步增强,但在剧烈点蚀与应力集中的共同作用下,抗应力腐蚀性能略有降低。     

Fabrication of CuZr(Al) bulk metallic glasses by high pressure torsion

In this paper, pure Cu, Zr and Al metal sheets were alternatively stacked and, then, cold roll-bonded. The overall compositions of the stacks were Cu₇₁Zr_29, Cu₆₂Zr_38, Cu₅₃Zr₄₇ and Cu₅₂Zr₃₈Al₁₀. The bonded materials were subsequently sliced into disks and, then, used for high pressure torsion (HPT) process. After HPT process up to maximum equivalent strain of about 1200 at ambient temperature, bulk materials with 10 mm in diameter and 0.30 mm in thickness were obtained. The phase constitution and microstructural evolutions of the HPTed samples with various rotations ranging from 0.5 to 20 were studied by XRD, FESEM and TEM. Thermal properties of the HPTed samples were analyzed by DSC. It was found that CuZr(Al) BMGs could be fabricated with a wide composition range by the HPT process. Tensile tests were used to measure the mechanical properties of the HPT processed samples. The mechanism of the solid-state amorphization during the HPT process was put forward and discussed.     

高压扭转纳米结构Al-Mg铝合金的微观结构演变和位错组态

利用透射电镜（TEM）和高分辨透射电镜（HRTEM）研究高压扭转大塑性变形纳米结构Al-Mg合金的微观结构演变和位错组态。结果表明：对尺寸小于100 nm的晶粒,晶内无位错,其晶界清晰平直;而尺寸大于200 nm的大晶粒通常由几个亚晶或位错胞结构组成,其局部位错密度高达10^17 m^-2。这些位错是1/2〈110〉型60°位错,且往往以位错偶和位错环的形式出现。在高压扭转Al-Mg合金的超细晶晶粒中,用HRTEM同时观察到分别由0°纯螺型位错和60°混合位错分解产生的Shockley部分位错而形成的微孪晶和层错。这些直接证据证实,通常存在于FCC纳米晶中由晶界发射部分位错而产生孪晶和层错的变形机制,同样可以存在于超细晶FCC金属中。基于实验结果,分析了高压扭转Al-Mg合金中的局部高密度位错、位错胞、非平衡晶界、层错和孪晶等对晶粒细化的作用,提出了相应的晶粒细化机制。