Nano-scale Reinforcements and Properties of Al-Si-Cu Alloy Processed by High-Pressure Torsion

To improve the comprehensive mechanical properties of Al-Si-Cu alloy,it was treated by a high-pressure torsion process,and the effect of the deformation degree on the microstructure and properties of the Al-Si-Cu alloy was studied.The results show that the reinforcements(β-Si andθ-CuAl_(2)phases)of the Al-Si-Cu alloy are dispersed in theα-Al matrix phase with finer phase size after the treatment.The processed samples exhibit grain sizes in the submicron or even nanometer range,which effectively improves the mechanical properties of the material.The hardness and strength of the deformed alloy are both significantly raised to 268 HV and 390.04 MPa by 10 turns HPT process,and the fracture morphology shows that the material gradually transits from brittle to plastic before and after deformation.The elements interdiffusion at the interface between the phases has also been effectively enhanced.In addition,it is found that the severe plastic deformation at room temperature induces a ternary eutectic reaction,resulting in the formation of ternary Al+Si+CuAl_(2)eutectic.

Microstructure and hardness of W-25Re alloy processed by high-pressure torsion

The evolution of microstructure and microhardness was studied in a commercial tungsten-25%rhenium(mass fraction)(W-25Re)alloy processed by the high pressure torsion(HPT)procedure under a pressure of7.7GPa up to10revolutions at different temperatures.The results show that the samples processed by10revolutions at room temperature could have the smallest grain size at around0.209μm.High saturation hardness(HV^1200)could be achieved after the rapid strengthening stage for samples processed by10revolutions both at room temperature and at573K.Microstructural observation and analysis from Hall-Patch relationship could reveal that grain refinement and grain boundaries strengthening are the main factors of hardening mechanism in W-25Re alloy.It is also demonstrated that sintered W-25Re sample may have brittle phase separation phenomenon after HPT processing.

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.

High-pressure torsion deformation process of bronze/niobium composite

The deformation process in the material volume under high-pressure torsion(HPT)was studied.As a model object for the observation of deformation process,we used a composite comprising a bronze matrix and niobium filaments.The arrangements of the niobium filaments in the bronze matrix and their size have regular geometry.This allows us to monitor and measure the displacement of the niobium filaments in the sample volume,which results from HTP.The bronze/niobium composite samples were subjected to HPT at room temperature and 6 GPa,and the number of revolutions N=1/4,1/2,1,2,3 and 5.It was shown that HPT with revolution number of 1 leads to the 360° rotation of entire sample volume without sample slippage.Similar deformational behavior during HPT can be expected for high-ductility metallic materials.The increase in the number of revolutions more than 2 leads to twisting the niobium filaments in the sample volume and the formation of 'vortex' multilayer structure.The mechanisms for the formation of such structures were discussed.

Effect of Bimodal Quasicrystal Phase on the Dynamic Recrystallization of Mg–Zn–Gd Alloy during High-Pressure Torsion

The Mg–Zn–Gd alloy with quasicrystal icosahedral phase was processed by high-pressure torsion (HPT). The effect of bimodal I-phase on the dynamic recrystallization was analyzed by transmission electron microscopy. The results showed that the block I-phase can stimulate obvious particle-stimulated nucleation and dynamic recrystallization (DRX) grains were preferentially formed after HPT for 5 turns, while the granular I-phase only promoted the generation of sub-grains. The orientation relationship was determined as twofold//[1210] and fivefold//(0002)_(Mg). Moreover, after HPT for 9 turns, the DRX grains induced by block I-phase appeared to grow up and coarsened. Compared with block I-phase, the grains induced by granular I-phase presented much smaller size and distributed more homogeneous due to the strong pinning effect.

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.

Application of high-pressure technology in exploring mechanical properties of high-entropy alloys

In recent years,high-entropy alloys(HEAs) have developed a lot as the new style of materials,which have become a research focus in the materials fields.It breaks the normal procedure of traditional alloy design(based on one/two elements as the main component,and adds some other elements to improve the micro structure and required related performance of the alloy),and consists of five or more equal or nearly equal elements,with multiple principal elements.Therefore,HEAs have many excellent mechanical properties compared with traditional alloys,which have aroused great research interest of researchers.The rapid development of high-pressure technology makes it a powerful tool to modulate the structures of HEAs,and brings new chance for the study and optimization of mechanical properties of HEAs.In this paper,the application of high-pressure technology in exploring and improving the mechanical properties of HEAs is reviewed,which could deepen the understanding of high-pressure technology and provide a new avenue for further exploration of HEAs’ mechanical properties.

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.

Nanostructured Ti29.7Ni50.3Hf20 high temperature shape memory alloy processed by high-pressure torsion

High-pressure torsion(HPT)processing under a pressure of 6.0 GPa was applied to Ti29.7Ni50.3Hf20(at.%)alloy.Two types of structure were observed after HPT with 3 revolutions:first one is the mixture of amorphous phase and retained nanocrystalline;second is the alternating bands of amorphous phase and high defect density crystalline.As a result,post deformation annealing(PDA)at 500-700℃leads to the non-uniform distribution of martensite and parent phase grains.The grains of martensite are twice larger compared to that of parent phase.The nanocrystalline and ultrafine grains form after annealing at 500-600℃and 700℃,respectively.The twinning mechanism does not change with the reduction of martensitic grains up to^35 nm.The relationship between strength and grain size in Ti29.7Ni50.3Hf20 alloy obeys the classical Hall-Petch relationship with a coefficient of 10.80±0.39 GPa nm^1/2.

Wear properties of copper and copper composites powders consolidated by high-pressure torsion

The wear characteristics of Cu and Cu-SiC composite microsize powders consolidated by cold com­paction combined with sintering or high-pressure torsion(HPT)were investigated.The HPT processed(HPTed)samples with bimodal and trimodal microstructures and fine Cu grains and SiC particle sizes have superior hardness,reasonable ductility level,and high wear resistance.The wear mass loss and coefficient of friction of HPTed samples were remarkably lower than that of cold-compacted and sintered samples as well as that of micro and nano Cu and Cu-SiC composites from previous studies.The sample fabrication method has an apparent influence on the wear mechanism.The wear mechanism was converted from adhesive,delamination,three-body mechanism,grooves(take off the SiC particles),and cracks into abrasive wear after HPT.Oxidization can be considered a dominant wear mechanism in all cases.The worn surface morphology and analysis support the relationship between wear mechanism and characteristics.

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。

纯铝粉末烧结体锥形件高压扭转有限元模拟及实验

采用有限元法对锥形件高压扭转成形过程进行了数值模拟,着重分析了成形过程的应变和相对密度的分布及变化趋势。通过实验对纯铝粉末烧结体试样变形前后相同部位的金相组织、显微硬度和相对密度进行了对比分析。研究结果表明:高压扭转工艺对试样有着显著的致密效果,模拟中试样的相对密度超过0.996,实验中试样的密度达0.980;试样不同部位的等效应变和显微硬度不同,边缘处的等效应变最大,达到4.55,该处的显微维氏硬度值也最高,达61.12HV0.1/10。

高压扭转对20CrMnTi组织和力学性能的影响

文章以20CrMnTi为研究对象,研究了压力和扭转圈数对试样子午面的显微组织及显微硬度的影响。结果表明,随着压力和扭转圈数的升高,试样子午面的组织得到细化;距试样中心5 mm处的组织细化效果最为显著,显微硬度的测量结果与显微组织观察结果基本一致。

高压扭转法制备超细晶材料的研究进展

高压扭转(HPT)已成为国内外制备超细晶(UFG)材料有效的方法之一。本文介绍了高压扭转的基本原理,综述了压力、扭转圈数和摩擦因子对高压扭转过程中材料显微组织及力学性能的影响,指出了国内高压扭转应用中存在的问题,并对其研究前景进行了展望。

高压扭转IF钢压缩阶段的不均匀变形

观察并研究了IF钢高压扭转压缩阶段过程中的硬度及显微组织不均匀分布特征,并利用ANSYS有限元分析软件对实验结果进行模拟仿真验证。结果表明IF钢在高压扭转压缩阶段过程中的应力、应变及硬度分布是不均匀的:心部变形程度较小,硬度比较低;中间区域变形比较均匀,硬度也趋于一致;在边缘区域,形变程度很大,从而硬度比较高;对于限定型高压扭转,由于模具凹槽立壁对试样变形的阻滞作用,变形量较小从而形成形变"死区"。模拟结果与硬度及显微组织分布的实验结果极为吻合,由此说明,正是由于压缩阶段试样各部位不均匀变形导致了显微组织、硬度的不均匀分布。

转速对高压扭转Cu试样的组织与性能的影响

通过高压扭转(HPT)技术在不同转速条件下实现了Cu试样的晶粒细化。利用光学显微镜(OM)、透射电镜(TEM)及显微硬度计观察并测试了组织的结构与性能,并基于有限元计算了变形诱导试样的温升,研究了转速对Cu试样的组织细化与性能的影响。结果表明:转速由1/3r.min-1增大至1r.min-1,经1圈扭转变形,试样温度由40.8℃升高到54.1℃,变形组织均为100～600nm的高位错密度位错胞/亚晶组织,显微硬度由初始态的52HV0.05增大至140HV0.05;经16圈扭转变形,试样温度由50.4℃升高到97.4℃,组织细化到200nm。慢速扭转变形试样晶内位错密度高,微观组织处于严重变形状态;而快速扭转试样晶内衬度均匀,位错较少,微观组织经历明显的动态回复,显微硬度较慢速扭转变形试样低6%。

高压扭转圈数对碳化硅颗粒铝基复合材料组织和性能的影响

采用高压扭转(high-pressure torsion,HPT)工艺,成功将8.75%体积分数SiC颗粒与Al的混合粉末制成金属基复合材料,并用金相显微镜、显微维氏硬度计、万能材料试验机和扫描电镜等,分析了扭转圈数对SiCp/Al复合材料的显微组织、显微硬度、拉伸性能和拉伸断口形貌的影响。结果显示,扭转圈数从0.5圈增加到4圈,基体的变形抗力降低,SiC颗粒在剪应力的作用下易于与基体协调变形,SiC颗粒分布均匀化程度显著提高;随着扭转圈数增加,试样所受到的剪切作用增大,极易引起位错的运动和增殖,基体晶粒得到细化,单位体积内的晶界增多,使试样显微硬度得到提高;扭转圈数越多,拉伸断口韧窝数量明显增多,且尺寸变小。

高压扭转对铜粉锥形件温压成形的改善

以薄壁锥形件为研究对象,在300℃下将高压扭转工艺应用于铜粉锥形件成形,研究其致密性、显微组织和显微硬度规律,并与相同条件下成形的温压锥形件进行比较。结果表明:温压扭锥形件的相对密度达到0.9605,比温压锥形件提高了5.06%;高压扭转工艺对锥形件的晶粒细化程度在52%～66%之间,明显高于压制工艺的36%;温压扭锥形件口部、中部和锥部的平均显微硬度较温压锥形件分别提高了223%、9%和6%;温压锥形件显微硬度分布规律为中部最高,口部最低,且同一位置内壁高于外壁;温压扭锥形件的显微硬度分布为从锥部到口部逐渐升高,其锥部的显微硬度分布规律与温压锥形件一致,口部和中部的显微硬度分布规律则与温压锥形件刚好相反。