细观和纳观力学的若干进展讲稿杨卫.ppt

杨卫 清华大学工程力学系,细观和纳观力学的若干进展,2001. 9. 1-5,毫米尺度疲劳断裂量测,Biaxial Fatigue Testing,细观显微加载,纳米尺度 疲劳断裂量测,2001. 9. 1-5,CM/MD 联合模拟,Green function method for lattice statics, Zhou et al. (1992) Combined CM/MD simulation by an overlapping layer, Tan & Yang (1994),2001. 9. 1-5,CM/MD 联合模拟,Adaptive quasi-continuum method, Phillips & Ortiz (96-99) Material Point Method Lattice-Material Point Method, Tan (2000),Hierarchical Adaptive Dynamic Seamless Multi-scales Triple identities,裂尖位错发射,Xing, Dai & Yang Science in China（2000）,initial crack tip,current crack tip,Dislocation Emission from a Crack Tip,多向喷丸引起表面纳米化,吕坚与卢柯的喷丸引起表面纳米化实验,初始晶粒为50m的铁材料经过喷丸撞击450秒后，表层形成尺寸为10nm的纳米晶粒。,2001. 9. 1-5,喷丸引起位错增殖,喷丸撞击的二维简化模型,实验参数 试件表面尺寸：3035 mm2 喷丸直径： 2 mm 喷丸数目： 300 个 喷丸速度： 520 m/s 喷丸流量： 990 次/(smm2),斜入射比垂直入射可引入更高的表层位错密度,Data input, time step selection,Pressure & friction force of shot-peens,FEM computes velocity and stress fields,Density evolution by dislocation migration,Density evolution by dislocation interaction,Total change in dislocation density,Density evolution by dislocation generation,Plastic strain rate by dislocation motion,Change in sample surface profile,Acceleration, velocity, spin & location of shots,Output dislocation density,Flow Chart,2001. 9. 1-5,Numerical Simulation,Parameters: Iron sample: E = 2.001011 Pa，=0.3，=7.8 103 Kg/m3 Shots: = 7.8 103 Kg/m3 Dislocations: b = 2.48 10-10 m Hardening 0 = 200 MPa，0 = 0.1 Mobility VS = 2.98 103 m/s, B=4.00 1010 Pa Source CR = Y, mS = 1.00 106 mm-2 Initial density ALL = 1.00 108 mm-2, = 1.00 106 mm-2,Normal Impact,Dislocation density across the depth V/R=20000 s-1，0, x = 0,Oblique Impact,Dislocation density across the depth V/R=20000 s-1， 20, x = 0,2001. 9. 1-5,Forming Nano-grains,Low angle grain boundary,Grain boundary of larger angle is energetically favored, = 10， = 2.17 1011 mm-2，D = 6.5 nm, = 20，D = 13 nm,Low angle GB forms if qD 2b,Forming Nano-grains,初始构型,6000弛豫步后,Superplastic Extensibility of Nano-grained Metals,Critical dislocation spacing is 1520nm in metals Dislocations are depleted in nano-grained metals Room temperature superplasticiy was predicted (Karch et al., Nature, 1987) for nanograined metals, but seldom realized.,Superplastic Extensibility of Nano-grained Copper,Experiment by Lu et al. (Science, 2000) for nanograined bulk electro-deposition copper,Superplastic Extensibility of Nano-grained Copper,No texture was formed under rolling, and 5100% extensibility in RT was reported.,Deformation of Nanocrystalline Metals,Dislocations are seldom found within the grains. Gao and Gleiter (1987), Thomas and Siegel (1990), Milligan et al. (1993),Mass diffusion and grain boundary sliding dominate. Lu et al. (2000), Science,MD Simulation,2-Dimensional, periodic BC 18-grain cluster + repetition Random (111) plane orientations, relaxed for 1000 MD steps (5fs) NVT ensemble, T=313K 0.1% Strain increment is imposed on atom centers + fluctuation of 0.005a, then relax for 800 MD steps; 700 loading increments Effective medium theory on copper (Jacobsen et al, 1996),2001. 9. 1-5,Energetics,Energy Minimization,Principle of Minimum Plastic Dissipation,2001. 9. 1-5,Energetics,Principle of Virtual Work,Simulation,Low Fluctuation (1/40),Simulation,High Fluctuation (1/5),Stress-Strain Curve,Ashby-Verrall Model,4-grain clusters Predict a creep rate 10 times as the Coble creep The process is incomplete,2001. 9. 1-5,9-Grain Cluster,Complete and repeatable,2001. 9. 1-5,Fully-Relaxed Creep,Constitutive Law,Creep Rate,Threshold,Creep Data for NC Copper,Cai et al., Scripta Materialia (1999),2001. 9. 1-5,Data of Copper,Elasticity: E = 115GPa，=0.31 Atomic volume: W =8.78 10-30 m3 Diffusion: D0B=3 10-9 m2/s; DQB=0.72eV; d = 0.5nm Cai et al (1999) D0V=4.4 10-6 m2/s; DQV=1.98eV Wang & Han (1990) Specific GB energy: 0.53Jm-2 Handros (1969) Grain Size: 30nm Cai et al (1999), Lu et al (2000) Order/Disorder Transition: r0=0.75