Microstructure evolution and dislocation configurations in nanostructured Al-Mg alloys processed by high pressure torsion

Microstructure evolution and dislocation configurations in nanostructured Al–Mg alloys processed by high pressure torsion （HPT） were analyzed by transmission electron microscopy （TEM） and high-resolution TEM （HRTEM）. The results show that the grains less than 100 nm have sharp grain boundaries （GBs） and are completely free of dislocations. In contrast, a high density of dislocation as high as 1017 m^-2 exists within the grains larger than 200 nm and these larger grains are usually separated into subgrains and dislocation cells. The dislocations are 60° full dislocations with Burgers vectors of 1/2〈110〉and most of them appear as dipoles and loops. The microtwins and stacking faults （SFs） formed by the Shockley partials from the dissociation of both the 60° mixed dislocation and 0° screw dislocation in ultrafine grains were simultaneously observed by HRTEM in the HPT Al–Mg alloys. These results suggest that partial dislocation emissions, as well as the activation of partial dislocations could also become a deformation mechanism in ultrafine-grained aluminum during severe plastic deformation. The grain refinement mechanism associated with the very high local dislocation density, the dislocation cells and the non-equilibrium GBs, as well as the SFs and microtwins in the HPT Al-Mg alloys were proposed.

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

Deformation twins and stacking faults were observed in nanostructure A1-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 A1-Mg alloys processed by high pressure torsion. A possible formation process of these nanostructures is proposed based on molecular-dynamics simulations.

Improving the Corrosion Resistance of ZEK100 Magnesium Alloy by Combining High-Pressure Torsion Technology with Hydroxyapatite Coating

Recently,magnesium and its alloys have attracted more and more attention as promising implant materials due to their excellent properties such as good biocompatibility,biodegradation,non-toxicity and comparable mechanical properties with natural bone.They can be gradually degraded and absorbed so as to avoid the second surgery for implants removal after the tissues are healed completely.In addition,they are also able to prevent the stress shielding effect in human body environment because of the density,elastic modulus and yield strength of magnesium closer to the bone.Unfortunately,the high corrosion rate which causes early mechanical failure of the implants in physiological environment limits the widespread use of magnesium alloys for clinical application in biology.And the high corrosion process usually causes huge hydrogen evolution and alkalinization,resulting in problems against the implants as well as the surrounding tissues.In order to enhance the corrosion resistance of magnesium alloys,in this study,the ZEK100 magnesium alloy was pre-deformed with a highpressure torsion(HPT)process and then fabricated hydroxyapatite(HA)coatings with different contents of Mg(OH)2 nanopowder via hydrothermal method.The specimens were characterized by scanning electron microscope(SEM)and X-ray diffraction(XRD).At the same time,prior and after the HPT procedure,the metallography,microhardness and tensile tests of specimens were characterized.Meanwhile,the corrosion behavior of the specimens was evaluated by electrochemical impedance spectroscopy(EIS)and hydrogen evolution tests.And the interface bonding strength of the HA coating on the magnesium alloy substrate was evaluated by a tape adhesion test/scratch test.Results showed that HPT processing refined the grain size and introduced a great number of twins,resulting in the enhancement of microhardness and Young’s modulus of ZEK100 magnesium alloy,but hardness values at the edge were higher than those at the center due to the uneven shear strain.At the same conditions,the HA coating on HPT-ZEK was denser,thicker than that on ZEK sample and the crystal sizes of HA were smaller on HPT-ZEK.These were attributed to fine,uniform distributed secondary phases and lots of fine grains,twins,grain boundaries in HPT-ZEK substrates which can provide more nucleation sites for the HA crystal.In terms of the amount of Mg(OH)2 nanopowder,Mg(OH)2 nanopowder significantly influenced the microstructure and thickness of the HA coating.And at a 0.3 mg/mL content of Mg(OH)2 nanopowder,there was the densest,thickest HA coating on magnesium alloys,and the crystal size of HA was minimum.Specifically,the HA coating thickness on ZEK-03(0.3 mg/mL Mg(OH)2 nanopowder)was 1.8 times of that on ZEK-00(0 mg/mL Mg(OH)2 nanopowder),while the HA coating thickness on HPT-03 was 2.6 times of that on ZEK-00.And the adhesion strength of HA coating on HPT-03 substrate was better than that on ZEK-03.In addition,HPT technology and surface modification by HA coating simultaneously increased the corrosion resistance of ZEK100 magnesium alloy and the corrosion of HPT-ZEK samples occurred in a more uniform manner,while it was pitting on the surface of ZEK100 magnesium alloy.Therefore,there was the best corrosion resistance on HPT-03 sample,which could promote the application of magnesium alloys in biomedical fields.

Exceptional thermal stability and enhanced hardness in a nanostructured Mg-Gd-Y-Zn-Zr alloy processed by high pressure torsion

A Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr(wt.%) alloy is processed by solution treatment and high pressure torsion(HPT) at room temperature to produce a nanostructured light material with high hardness. The stability of this alloy is subsequently tested through isochronal annealing for 0.5 h at 373 K to 673 K. The results reveal a thermal stability that is vastly superior to that of conventional Mg-based alloys processed by severe plastic deformation: the grain size remains at around 50 nm on heating to 573 K, and as the temperature is increased to 673 K,grain growth is restricted to within 500 nm. The stability of grain refinement of the present alloy/processing combination allowing grain size to be limited to 55 nm after exposure at 573 K, appears to be nearly one order of magnitude better than for the other SPD processed Mg-RE type alloys, and 2 orders of magnitude better than those of SPD processed RE-free Mg alloys. This superior thermal stability is attributed to formation of co-clusters near and segregation at grain boundaries, which cause a thermodynamic stabilization of grain size, as well as formation of β-Mg_(5)RE equilibrium phase at grain boundaries, which impede grain growth by the Zener pinning effect. The hardness of the nanostructured Mg-Gd-Y-Zn-Zr alloy increases with increasing annealing temperature up to 573 K, which is quite different from the other SPD-processed Mg-based alloys. The high hardness of 136 HV after annealing at 573 K is mainly due to solute segregation and solute clustering at or near grain boundaries.

Effect of Revolution on Inhomogeneous Deformation of IF Steel in High Pressure Torsion

The effect of revolution on inhomogeneous plastic deformation of HPT processed IF steel was investigated using experimental and simulation approaches. The results indicate that the degree of inhomogeneous plastic deformation increases as the revolutions increase along the radial direction on the transversal plane of disks. In addition, the hardness and the microstructure distributions verify the trend that the effective strain of the HPT processed disks at the early torsion stage is gradually deformed from the edge to the center with the revolutions increases.

Inhomogeneous Hardness Distribution of High Pressure Torsion Processed IF Steel Disks

The inhomogeneous hardness distribution of high pressure torsion (HPT) processed IF steel disks along different directions is investigated. The results indicated that there exists inhomogeneous distribution in HPT processed IF steel disks, giving lower hardness in the center and higher hardness in the edge regions. However, on the axisymmetrical section testing plane of the disks’ thickness direction, there is a soft zone near the surface of disks. Further results from radius testing plane of different depths from the surface of HPT processed disks show that the inhomogeneity rules of hardness distribution on the radius direction are similar to that on the thickness direction. Compared with the initial state, different stages of HPT (compression and compression + torsion) can both remarkably increase the hardness of IF steel disks. Microstructure investigation results can give a well support to verify the rules of hardness distribution, showing hardly no change of grains in center and sever plastic deformation in edge. The inhomogeneous distribution of stress and strain with the huge friction between anvil and disks in the process of HPT play an important role of hardness and microstructure distribution.

HPT法对SiCp/Al复合材料颗粒断裂的影响

采用高压扭转(HPT)法将SiC颗粒与纯Al粉的混合物固结成金属基复合材料。观察试样的显微组织,并对颗粒的数目、粒径及百分比进行测量,研究HPT法对SiCp/Al复合材料颗粒断裂的影响。结果表明,随着扭转半径增大,颗粒断裂现象严重,颗粒棱角钝化更加明显,颗粒数目增加,粒径减小;随着SiC颗粒体积分数的增加,这种趋势会强化;颗粒断裂现象是高压扭转法制备颗粒增强金属基复合材料的固有属性。

室温HPT制备SiC_p/Al复合材料的硬度分布特征

将纯Al颗粒和SiC颗粒混合,室温下采用高压扭转变形(HPT)制备试样,测定不同工艺参数下试样表面的显微硬度,绘制显微硬度—扭转半径曲线图,分析各试样的硬度分布特征。结果表明:由于高压扭转变形应变量的影响,SiCp/Al复合材料经HPT变形后,各试样表面硬度沿径向呈递增分布,边缘硬度值受飞边的影响较大;单一尺寸SiC颗粒强化的试样通过增大加载压力、增大扭转圈数或提高SiC颗粒体积分数可以提高试样的显微硬度值;采用双尺寸SiC颗粒最佳粒径配比或最佳体积配比更有利于提高试样硬度,通过优化双尺寸SiC颗粒粒径配比或体积配比,可以获得更高的试样硬度,且硬度分布更加均匀。

HPT制备高含量SiC颗粒增强Al基复合材料的性能和致密化行为

采用高压扭转法制备了SiCP/Al基复合材料,分析了不同SiC体积分数复合材料的显微组织、硬度、相对密度及SiC颗粒分布的变化情况,并探讨了SiCP/Al基复合材料在高压扭转变形过程中的致密化机理。结果表明:随着SiC体积分数的增加,复合材料的相对密度不断减小,硬度和SiC颗粒的分布均匀程度均先增大后减小,且硬度沿试样径向呈递增趋势。同时,随着SiC体积分数的增大,SiC颗粒破碎和团聚现象也更为严重。

Influence of processing temperature on microstructure and microhardness of copper subjected to high-pressure torsion

Cu samples were subjected to high-pressure torsion (HPT) with up to 6 turns at room temperature (RT) and liquid nitrogen temperature (LNT),respectively.The effects of temperature on grain refinement and microhardness variation were investigated.For the samples after HPT processing at RT,the grain size reduced from 43 μm to 265 nm,and the Vickers microhardness increased from HV52 to HV140.However,for the samples after HPT processing at LNT,the value of microhardness reached its maximum of HV150 near the center of the sample and it decreased to HV80 at the periphery region.Microstructure observations revealed that HPT straining at LNT induced lamellar structures with thickness less than 100 nm appearing near the central region of the sample,but further deformation induced an inhomogeneous distribution of grain sizes,with submicrometer-sized grains embedded inside micrometer-sized grains.The submicrometer-sized grains with high dislocation density indicated their nonequilibrium nature.On the contrary,the micrometer-sized grains were nearly free of dislocation,without obvious deformation trace remaining in them.These images demonstrated that the appearance of micrometer-sized grains is the result of abnormal grain growth of the deformed fine grains.

Effect of high pressure torsion process on the microhardness,microstructure and tribological property of Ti6Al4V alloy

In the present study,a fully lamellar Ti6Al4V alloy was severely deformed by high pressure torsion(HPT)process under a pressure of 7.5 GPa up to 10 revolutions.Experimental results revealed that the microhardness of Ti6Al4V was increased remarkably by about~41%and saturated at about 432 Hv after the HPT process.A relatively uniform bulk nanostructured Ti6Al4V alloy with an average grain size of about52.7 nm was obtained eventually,and no obvious formation of metastableωphase was detected by XRD analysis.For the first time,the tribological properties of the HPT processed Ti6Al4V alloy were investigated by a ball-on-disc test at room temperature under a dry condition.It was found that HPT process had a great influence on the friction and wear behaviors of Ti6Al4V alloy.With increasing the number of HPT revolutions,both friction coefficient and specific wear rate were obviously decreased due to the reduction of abrasion and adhesion wears.After being deformed by 10 HPT revolutions,the friction coefficient was reduced from about 0.49 to 0.37,and the specific wear rate was reduced by about 48%.The observations in this study indicated that HPT processed Ti6Al4V alloys had good potential in structural applications owing to their greatly improved mechanical and tribological properties.

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.

纳米晶Al-xMg铝合金的元素再分布及其对强度的影响

目的针对Al-Mg合金在变形加工过程中容易出现的溶质元素再分布现象,通过微观表征和定量计算深入研究元素再分布的相关机制及其强度贡献。方法利用扫描透射电镜高角环形暗场技术结合几何相位分析,详细研究了溶质元素再分布机理,对经过高压扭转后不同Mg含量的Al-Mg合金中的元素分布进行了表征,并通过原子探针层析技术对原子的三维空间分布情况进行了团簇分析。结果随着Mg的原子数分数从1.0%增加至8.0%,合金硬度从129.0HV显著上升至223.6HV。合金内部的溶质分布状态可以分为溶质原子贫化、溶质原子富集和溶质原子不均匀分布3种,剧烈塑性变形引入的不均匀微观应变诱导了Mg原子的再分布。结论合金中形成的大量原子团簇为Al-8Mg合金贡献了47.7HV的硬度值,占总硬度的21.3%,溶质团簇对合金强度的强化效果十分显著,表明团簇强化是材料强化手段中不可或缺的新型强化机制。

轴向补料量对扭力梁内高压成形质量的影响

研究了轴向补料量对CP800材料扭力梁内高压成形质量(成形壁厚分布和成形精度)的影响,结果表明:轴向补料能改善扭力梁的壁厚分布和成形精度,但补料过多会导致连接区起皱,影响其成形质量;当轴向补料量为12 mm时,最大减薄率为8.3%,整个扭力梁的尺寸精度都在0.5%以内,满足零件的设计要求。

Numerical approach to the modelling of transient interaction of prospective combustor concepts and conventional high pressure turbines

The Institute of Gas Turbines and Aerospace Propulsion at Technische Universitait Damstaxlt conducts research projects in the field of“combustor turbine interaction”(CTI).This paper presents numerical studies on the interaction between novel combustion concepts and conventional“high pressure turbine”(HPT)stages.In order to obtain higher efficiency and reduce emissions of jet engines,it is necessary to apply innovative and revolutionary technologies.The most promising technical solutions are based on the cycle processes,employing“pressure gain combustion”(PGC)methods PGC methods provide a significant thermal efficiency enhancement and low NO_(x)-emission rates at the same time.The investigations presented in this paper give information on the integrability of revolutionary combustion concepts into conventional engine architecture.This paper aims at providing insight into the numerical modelling of the transient behaviour of prospective combustion outflow and its influence on the operation of HPTs,especially on the first stage.The focus is on the aerodynamic effects and loss mechanisms within the blade passage.The interaction between the two components plays an important role.To study the performance under new conditions,an engine-like HPT geometry is used.This study reveals a decrease in turbine efficiency with transient inflow conditions compared to a steady-state inflow case.The decrease is primarily due to the interation between transient inflow and the loss mechanisms in the turbine.The presented research was done as part of the project“Technologien fir REVolutionire Arbeits Prozesse”(TREVAP)

Al-Zn体系高压扭转过程中的相变机理

本工作对常温不互溶的Al-Zn体系进行了高压扭转下的机械合金化研究。将0.03 mm厚的纯Al和纯Zn交替堆叠,在3 GPa的压力下进行50圈的高压扭转处理。结果表明,高压扭转处理的Al-Zn合金中产生两种类型的相变,即晶体到非晶的相变,以及密排六方到面心立方的相变。局部高密度位错可能是Al-Zn合金从晶态转变到非晶态的主要原因,而HCP→FCC相变则归因为密排面原子沿晶界产生的Shockley不全位错的滑动。此外,本工作为深入研究Al-Zn合金的固态相变提供了一种新的途径,为提高铝锌合金的力学性能提供了新的思路。

广州科技图书馆结构设计要点与分析

广州科技图书馆为复杂超限高层结构,采用钢框架-偏置混凝土筒体结构。由于建筑造型需要,外圈框架柱无法贯通落地,采用“上托下吊”重载悬空结构,对该结构形式受力进行了分析。介绍了新型“抗、放”结合的疏水减压抗浮技术在超深地下室中的应用。由于结构存在扭转偏大、抗扭刚度弱等多项不规则,采用YJK、ETABS两个计算软件对结构进行了中震反应谱分析,介绍了针对扭转偏大采取的结构设计措施,补充了中震弹性时程分析,对悬挑桁架和斜柱进行了专项分析,并采用Perform-3D进行了罕遇地震动力弹塑性时程分析。结果表明采取的抗震加强措施有效,可实现预设的抗震性能目标。

武器系统地面设备Ti-B20合金时效过程组织与性能调控研究

基于武器系统地面设备对钛合金材料高强度使用的需求,开展影响Ti-B20合金力学性能的因素分析,采用金相显微镜(OM)、扫描电子显微镜(SEM)和X射线衍射(XRD)等技术研究了高压扭转变形对Ti-B20合金微观组织演变、时效行为及力学性能的影响。研究结果表明,Ti-B20合金在高压扭转(N=10)变形后没有发生相变,变形方式以位错滑移为主。经过固溶处理的合金在时效过程中析出针状α相,并且其沿着晶界析出。高压扭转变形的合金中产生大量的高密度位错区域,这些区域在时效过程中作为α相的优先形核位点,最终形成分布均匀的超细α相,导致显著的析出强化。

高压扭转铜试样的微观组织与压缩性能

通过高压扭转对铜试样施加不同程度的变形,研究了样品扭转面(ND面)和纵截面(TD面)上微观组织特征.对ND面,在较小的剪应变下,原始晶粒形貌模糊,晶粒内部形成等轴状的位错胞及亚晶结构;随变形量的增大,亚晶间取向差及亚晶内部的位错密度增大,最后形成亚微米尺度的等轴晶粒.对TD面,变形初期原始晶粒被拉长,晶粒内部为位错墙分割成的层状结构,层内为拉长的位错胞;随变形程度的增大,拉长晶粒的宽度减小,与剪切方向的夹角减小,晶内层状组织间距减小,并逐渐演化成拉长的亚晶组织;进一步增大变形,晶粒拉长痕迹消失,变形组织与ND面相似,为等轴状亚微米晶粒.压缩实验表明,经16圈扭转后,整个试样上的压缩性能基本均匀,σ_(0.2)达到385 MPa,应变率敏感性指数增大至0.021.

高压扭转法对SiC_p-Al基复合材料颗粒分布的影响

采用高压扭转(High-Pressure Torsion,HPT)工艺制备SiCp-Al基复合材料。通过显微组织的定性分析及样方法的定量计算,深入研究不同工艺参数对SiC颗粒分布的影响规律,结果表明:采用高压扭转法可以直接将8.75%(体积分数)SiC-Al混合粉末制备成金属基复合材料。通过金相分析得出:SiC颗粒在试样不同扭转半径处分布情况具有差异:工艺参数(温度、压力、圈数)对SiC颗粒分布有重要影响,结合样方法对颗粒分布情况的定量分析得出:随着扭转圈数、压力、扭转半径的增大,剪切作用增强,SiC颗粒分布均匀性提高;变形温度升高,基体流动性提高,颗粒分布均匀性得到改善。