Work Hardening Of Nanocrystalline Metals By Molecular Dynamics Simulation

Nanocrystalline metals are widely applied in microelectronic systems, integratedmicroelectromechanical systems, nanocomponents, mask aligners, surfaceengineering and biomedicane etc., as a result of their extremely high strength andpredominant physics-chemical properties. Precious metal silver is also extensivelyapplied in connection wires of super integrated circuits, due to its high thermal andelectrical conductivity. However, an obvious disadvantage for nanocrystalline metalsemerges as the grain size reduced to nanometer size--low ductiliity, which attributesto their low work hardening rate. Fabrication of nanocrystalline materials with highstrength as well as high ductility through material design will make a big difference inthe engineering application and broadening the application of nanocrystallinematerials. Therefore, this thesis is focused on the work hardening of nanocrystallinemetals.Molecular dynamics simulation is the main method adopted in the study for workhardening of nanocrystalline metals in this thesis.Bicrystalline silver nanowires with asymmetric tilt grain boundary of variousinclination angle, symmetric tilt grain boundary and random high angle grainboundary as well as?3coherent twin grain boundary were simulated under uniaxialtensile deformation to investigate their work hardening. The results indicated that onlybicrystalline silver nanowires with symmetric tilt grain boundary and?3coherenttwin grain boundary undergone work hardening during unixial tensile deformation.The analysis showed nanovoids nucleated along the grain boundaries early duringuniaxial tensile deformation for bicrystalline silver nanowires with asymmetric tiltgrain boundary and random high angle grain boundary, and the subsequent growthand coalescence of nanovoids resulted in the failure of bicrystalline silver nanowires,which was the main reason for the absence of work hardening. Whereas obstacleshindering the movement of dislocations, such as Lomer-Cottrell lock, stair roddislocation and jog etc., were detected during the uniaxial tensile deformation ofbicrystalline silver nanowires with symmetric tilt grain boundary. Furthermore, symmetric tilt grain boundary can inhibit the formation of nanovoids. As forbicrystalline silver nanowires with?3coherent twin grain boundary, the impedimentof?3coherent twin grain boundary for dislocation movement and the interaction ofdislocations with each other, led to the occurrence of work hardening. However, thevacancy formed by interplay of dislocation with?3coherent twin grain boundaryand dislocations and its growth and coalescence with each other led to the breakingdown of grain boundary and grain interior early. Therefore, no further work hardeningwas observed during the uniaxial tensile of bicrystalline silver nanowires with?3coherent twin grain boundary.Polycrystalline silver nanowires with a variety of grain size were simulatedunder uniaxial tensile deformation. The results implied that polycrystalline silvernanowires with grain size above and equivalent to4.958nm deformed by dislocations,and undergone slight work hardening. But the emergence of nanovoids along grainboundary softened polycrystalline silver nanowires, gradually resulting in their failure,which reduced their work hardening ability. Whereas polycrystalline silver nanowireswith grain size below4.958nm deform through grain boundary activity, and no workhardening is observed.Laminated polycrystalline silver nanocrystallines were simulated. The resultsindicated that obstacles, such as Lomer-Cottrell lock, stair rod dislocation and jog etc.were responsible for the work hardening of laminated polycrystalline silver bulk, andstable plastic flow was observed during the uniaxial tensile deformation. As forlaminated polycrystalline silver thin film, fewer Lomer-Cottrell lock and stair roddislocation were observed compared with that in laminated polycrystalline silver bulkas a result of the influence of free surface along Z direction, thus work hardeningperiod was relatively short, while stable plastic flow was also observed. Whereas,laminated polycrystalline silver nanowire was influenced by the free surface along Yand Z direction, therefore no work hardening was observed during uniaxial tensiledeformation.Polycrystalline Molybdenum nanowires with grain size1.769nm,5.562nm and8.566nm were simulated under uniaxial tensile deformation. Phase transformationfrom body-centered-cubic (BCC) structure to face-centered-cubic (FCC) structurewere observed during the uniaxial tensile deformation of these nanoeires, which indicated the starting of plastic deformation, and dislocations were detected as thephase transformation further developed. However, the emergence of nanovoids alonggrain boundaries and their growth as well as coalescence ruined the work hardeningability of polycrystalline Molybdenum nanowires, leading to the failure of them.