纳米金属材料制备

1、块体金属纳米材料结构性能汇报人：日期：2015.7.8目录块体金属纳米材料的制备方法1SPD法制备块体金属纳米材料2MD模型分析块体金属纳米材料特性3总结41 1、块体金属纳米材料的制备方法、块体金属纳米材料的制备方法01气相法高真空反应室的惰性气体保护下加热金属，升华凝结为纳米尺度的金属粉，在真空中给金属纳米粉加压、烧结成。02液相法它是在尽可能消除异质核心的前提下，使液态金属保持到液相线以下数百度，而后突然形核并获得快速凝固组织 的一种工艺方法。03固相法通过机械研磨过程粉粒进行反复熔结、断裂，使得粉粒不断细化到纳米尺度，得到纳米颗粒。然后在经过压制。04电沉积法在浸入电解液的阴、 阳极之

2、间加以电流，使电解液中的金属离子向阴极表面迁移，并沉积到阴极表面，生成块体金属纳米晶材料。ECAERCASPSEARBMDFSPDsevere plasticdeformation（强塑性变形法）equal channel angular extrusionPure shear extrusionaccumulative roll bondinghigh pressure torsionrepetitive corrugation and straighteningmulti-directional forging2 2、SPDSPD法制备块体金属纳米材料法制备块体金属纳米材料HPTPure

3、shear extrusionPure shear extrusion(PSE)(PSE)Fig 1.(a) Schematic illustration of pure shear extrusion, showing the ram in green color, the sample in red in zone I (the entry channel), the upper (zone II) and lower (zoneIII)deformation zones and the exit channel (zone IV) and (b) Top view of PSE defo

4、rmation illustrating the changes of the cross section of the sample at the half course of PSE deformation.2211=22WL lDW and D are the side and diagonal of the initial square cross section and L and l are the long and short axes of the rhombic.Fig. 2.Shear strain states, (a) imposing shear stress, (b

5、) pure shear and (c) simple shear condition.PSEFig.3.Changes in the geometry of a small two-dimensional element (a) before PSE, squareABCD, (b) during the process, rhombic EFGHand (c) after PSE, rhombic.xx=lnlnOA（）= ROAln()lnyyOBROB zzxzyz=0Normal strain component in x and y directionszzxzyz+=0（R di

6、agonal ratio）PSExy=tan(- )4Shear strain components in PSE(切变应变分量)2212()1xyRRltan( )LEquivalent strain222222eqxxyyxxzzyyzzxyxzyz21=-+-+-+93eq221=ln+13R222R（ R） （）222tot221=ln)()13NRRR（PSEExperimental procedureTable 1 Chemical composition of the AA1050 alloy used in the studyThe nanostructures of the

7、PSEed samples were investigated using electron backscatter diffraction(EBSD电子背散射衍射).Experimental result and discussionPSEFig.1.Microstructure of the sample in the (a) as-rolled and (b) as-annealedconditions.It is clear that the microstructure of the as-rolled sample is composed of elongated grains f

8、ormed during rolling. In order to dispose the effects of previous deformation steps on thefinal results of this investigation, the samples were fully annealed before PSE. Fig 5 Variations of strain components and equivalent strain with R.Fig. 6.Cross section of the sample at the conjunction plane of

9、 PSE dies at different values for R.Experimental result and discussionPSEstrain componentsExperimental result and discussionPSERatio of normal to shear strains at variousRThe ratio of normal to shear strains is presented as a function of R. It should be noted that for calculation of the ratio, equiv

10、alent strains are calculated, once only with considering the normal strains and once only with shear strains. It is clear that the share of simple and pure shear deformations varies by changing the diagonal ratio of the PSE process which provides a broad range of strain states for SPD processing of

11、metals and alloys. The fact that the ratio is always greater than 1 indicates that the governing deformation mode in PSE is pure shear.Experimental result and discussionPSEFig.7.illustrates EBSD maps of the specimen after one pass PSE at room temperature. The low angle grain boundaries (with misorie

12、ntation angles between 11and 151) are shown by the red lines and the high angle grain boundaries (with misorientation larger than 151) by the black lines. The results indicates the formation of cell structures at the size range of around 500 nm in the sample. However, one may notice that most cells

13、are surrounded by low angle grain boundaries. This is quantified in Fig. 7(b) which indicates a histogram for the misorientation angles of the microstructure. It is clear that a large fraction of grains has low angle grain boundaries. (EBSD电子背散射衍射)Fig.7.(a) EBSD map and (b) histogram of the distribu

14、tion of misorientation angles of the microstructure after one pass PSExperimental result and discussionPSEFig.8.Variations in hardness alongside the diagonal of the specimen before and after PSEThe effect of PSE deformation on the hardness of the specimen is shown in Fig. 8. Hardness is measured on

15、the front side of the deformed sample (alongside the diagonal of the square). It is clear that hardness is increased about two times when one pass of PSE deformation is applied to the specimen. Obviously, it is expected to have more significant strain hardening by application of further PSE passes.T

16、he feasibility of SPDing metallic samples by PSE are investigated using a die imposing a strain of 2.12. ABThe PSE process provides the possibility of severely deformed metals and alloys in a combined mode of pure and simple shear.CVerification of grain refinement and increase in hardness of the pro

17、cessed samples is also an indication for the efficiency of the introduced process to serve as a new SPD technique.PSE3.MD3.MD模型分析块体金属纳米材料结构特性模型分析块体金属纳米材料结构特性Molecular-dynamics simulations(MD)Molecular-dynamics simulations are used to elucidate the coupling between grain growth and grain-boundary dif

18、fusion creep in a polycrystal consisting of 25 grains with an average grain size of about 15 nm and a columnar grain shape.The comparison of the grain growth observed in the presence of the applied stress with that solely in response to temperature as the driving force enables us to identify the mec

19、hanisms by which external stress affects grain growth.Computational approach1Columnar polycrystal(invested without stress)22D, Time3With misorientation angle more than 14.9o4Grain size d15nm,(518208atoms)MD5Parameter cohesive energy,elastic constants vacavcy-formation energyFig.9.The 25 grains in th

20、e001textured, initial microstructure with a grain diameter of 15 nm are clearly seen as linesof miscoordinated atoms.6t=2.56fs,Stress 0.4-0.8GPa,T=1200KMDComputational approachFig.10.Grain-growth simulation performed at 1200 K. (a) The microstructure att =5.49 ns (no stress), and (b) att=5.42ns,e=7.

21、74% (with stress). Very little grain growth has taken place in (a); in contrast, in (b) the presence of stress results in a considerable enhancement of grain growth.MDComputational approachFig.11.The strain in the stress directionexx, is characterized by three approximately linear regions; the const

22、ant strain rate in each region is consistent with Coble creep. Also shown (normalized relative to their initial values) are the change in the system volume Vand that in the fraction of miscoordinated atoms c; for clarity both have been scaled up, and the elastic volume change has been subtracted off

23、 the volume curve. Changes in the curve of c, which are reflections of the rate of grain growth, correlate with those in the strain rate. The change in Vatt 3.5 ns, visible only due to the high scaling factor, is related to the topological discontinuity associated with the disappearance of grain 13.

24、MDComputational approachFig.12.The faster decrease in the numbers of miscoordinated atoms (solid symbols) than in the total excess-energy (open symbols; both quantities are normalized relative to their respective t =0 values) indicates a more rapid decrease in the fraction of high-angle than that of

25、 low-angle GBs. The increase in the relative separations of the curves with stress indicates an increase in the rate of grain growth.The total excess-energy and the fraction of miscoordinated atoms represent slightly different quantitative measures for the occurrence of grain growth in the system MD

26、(i) The fraction of miscoordinated atoms decreases more rapidly if stress is applied to the system. Moreover, as the stress is increased froms=0.4 to 0.6 GPa, this decrease becomes evenmore pronounced.(ii) The relative separation of the excess-energy and miscoordinated-atom curves, indicating the co

27、nversion of high-angle into low-angleGBs during growth, also increases withhigher stress.(iii) In the presence of stress, the total excessenergy of the system provides a considerably less transparent measure of growth since it contains contributions from the GBs and the rapidly stored elastic energy

28、 In summary, the comparison of the stress-free grain-growth results with those obtained under applied stress provides three indicators for the existence of stress-enhanced grain growth.MDMechanisms of stress-enhanced graingrowthFig.13.Thefirst topological event in the evolution of the microstructure

29、 is the disappearance of grain 23 (see Fig. 1) by curvature-driven GB migration. The time at which this event takes place, plotted here, is an indicator of the enhancement of GB migration under the influence of stressshows the time of disappearance of grain 23 as a function of the applied stress,s=0

30、, 0.4, 0.6 and 0.8 GPa. This figure clearly demonstrates that GB migration is enhanced by the stress.MDMechanisms of stress-enhanced grain growthFig.14.The influence of stress on the rotations of neighboring grains 8 and 14. In contrast to the simulation at the higher temperatureT=1400 K , without stress grains 8 and 14 rotate only very slowly; the application of stress substantially increases th