استخراج پارامترهاي دانه و مرز دانه در پلي كريستال ها به روش ديناميك مولكولي

چكيده انگليسي :

Extraction of Grain and Grain Boundary Parameters in Polycrystals by Molecular Dynamics Mohammad Saeed Talaei Email ms talaeipashiri@me iut ac ir 17th september 2012 Department of Mechanical Engineering Isfahan University of Technology Isfahan 84156 83111 IranDegree M Sc Language FarsiProf Saeed Ziaei Rad Email szrad@cc iut ac irAbstractNanocrystalline materials are single or multi phase polycrystalline solids with a grain size of a fewnanometers 1nm 10 9 m typically less than 100 nm These materials have properties vastly different fromthe conventional coarse grained material which makes them a perfect candidate as raw materials used invehicle and aero industries In such materials a grain boundary is the interface between two grains Theseboundaries are the cause of decrease in the electrical and thermal conductivity and elastic constant of thematerial In nanocrystalline materials the existence of very fine grain sizes and consequent high density ofinterfaces are the reason to a variety of properties that are different and often considerably improved incomparison with those of conventional coarse grained materials These may include increasedstrength hardness enhanced diffusivity improved ductility toughness reduced density reduced elasticmodulus higher electrical resistivity increased specific heat higher coefficient of thermal expansion lowerthermal conductivity and superior soft magnetic properties In this study the mechanical properties of ironsingle crystal with body centered cubic structure and nanocrystalline iron were investigated by usingmolecular dynamics method In order to obtain results in a reasonable period of time with a large number ofatoms a molecular dynamic package has been used which is executable on the Graphical Processing Units Embedded Atom Method EAM interatomic potential models were utilized as a typical force field for densematerials to simulate the properties of iron Elastic constants of iron single crystal at different temperaturesare obtained employing Nose Hoover integration method The elastic constants in non zero temperature werecalculated using fluctuation formulas which are functions of potential energy as well as the momentum ofthe particles in the system The results have shown good agreement with the existing experimental results Next the Young s modulus of polycrystalline iron was predicted based on results obtained from iron singlecrystal model In addition the coefficients of linear thermal expansion in different temperatures were alsocalculated for iron single crystal Afterward computational investigations on nanoindentation of bcc ironusing a spherical indenter were performed Also additional indentation simulations were performed on thegrain boundaries and the calculated Young s modulus of both grain and grain boundaries were in reasonableagreement with the experimental data The model of nanocrystalline was generated using the Voronoitessellation method The model consists of three grains and about 100 000 atoms In this model the grainshaving a random crystallographic orientation are arbitrarily placed within a cube With Voronoi tesselation one can make a random nanocrystalline sample with a grain boundary structure similar to what is expected inpolycrystalline materials Obtained results implied that the strength of grain boundary is weaker than that ofgrain regarding the results of the indentation process The effect of grain size on the elastic properties ofnanocrystalline alpha iron is also reported Softening of the elastic properties is observed for grain sizesranging from 8 nm down to 6 nm The decrease in the Young s modulus with decreasing grain size is inagreement with experimental data and matches an analytical model based on the rule of mixtures forcomposite materials Keywords Nanocrystalline material Molecular Dynamics Iron GPGPU ElasticModulus Nanoindentation

طلايي پاشيري، محمد سعيد

استخراج پارامترهاي دانه و مرز دانه در پلي كريستال ها به روش ديناميك مولكولي

نانو كريستال , آهن , ثوابت الاستيك , نانو فروبرش