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Browsing by Author "Rangaswamy, Mukundhan"

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    Computer simulation of high fluence ion beam surface modification processes
    Rangaswamy, Mukundhan (Virginia Polytechnic Institute and State University, 1989)
    Various processes that participate in ion beam surface modification are studied using phenomenological, analytical and first principle models. The processes that are modelled phenomenologically include preferential sputtering, radiation-damage induced migration and second phase precipitation. The models are based on numerical solutions of the transport equation and include the processes of ion collection, sputtering, lattice dilation or accommodation and diffusion as well. The model for preferential sputtering takes into account the depletion of the preferentially sputtered element at the surface and the atomic transport process that results from the concentration gradients caused by the depletion. Results are presented for the case of Ta implantation into Fe. ln the radiation-damage induced migration the flux of the solute atoms is coupled to the concentration gradient of the continuously introduced defects. Examples of implantation of Sn into Fe and N into Fe are modeled to demonstrate the influence of radiation-damage induced migration. The precipitation of second phases during irradiation is modelled using thermodynamic considerations but with solubility values under irradiation obtained from experiment. In the model the solute atoms in excess of the solubility limit are assumed to precipitate out. Calculations are presented for the case of N implantation into Nb. Using first principle calculation for binary collisions in solids a computer simulation code was developed to study the collisional mixing occurring during high fluence ion implantation. It is based on the Monte Carlo code TRIM, and is capable of updating the target composition as the implantation process proceeds to high fluences. The physical basis for the dynamic simulation as well as a detailed analysis on the statistics required for obtaining the profiles with a given accuracy are presented. Vectorized results in a high computational efficiency. The predicted collisional broadening of the implantation profiles is presented for Ar bombardment into a Sn-Fe target as well as Ti implantation into C-Fe. The results are compared to those of the diffusion approximation. A semi-empiricaI model based on an analytical evaluation of ion mixing at low temperatures was developed taking into account collisional mixing and thermal spike effects, as well as the thermal spike shape. The ion beam mixing parameter for the thermal spike is derived as being proportional to different powers of the damage parameter, i.e. the damage energy scaled by the cohesive energy of the matrix, dependent on the thermal spike shape and point defect density in the thermal spike regions. Three different regions of ion beam induced mixing were recognized according to different density levels of the damage parameter. An experiment was conducted to determine the effect of chemical or thermodynamic factors in the migration of C in the presence of Fe and Ti atoms. A marker layer of C in a Fe-Ti matrix was ion beam mixed using Ar. The large mixing effect is tentatively attributed to a favorable heat of mixing values.
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    Modeling of high fluence Ti ion implantation and vacuum carburization in steel
    Rangaswamy, Mukundhan (Virginia Polytechnic Institute and State University, 1985)
    Concentration-versus-depth profiles have been calculated for Ti and C in Ti-implanted 52100 steel. A computer formalism was developed to account for diffusion and mixing processes, as well as sputtering and lattice dilation. A Gaussian distribution of Ti was assumed to be incorporated at each time interval. The effects of sputtering and lattice dilation were then included by means of an appropriate coordinate transformation. C was assumed to be gettered from the vacuum system in a one-to-one ratio with the surface Ti concentration up to a saturation point. Both Ti and C were allowed to diffuse. A series of experimental (Auger) concentration-versus-depth profiles of Ti implanted steel were analyzed using the above-mentioned assumptions. A best fit procedure for these curves yielded information on the values of the sputtering yield, range and straggling, as well as the mixing processes that occur during the implantation. The effective diffusivity of Ti was found to be 6x10⁻¹⁵ cm²/sec, a value that is consistent with the cascade mixing mechanism. The effective diffusivity of C was found 6x10⁻¹⁵ cm²/sec, and the sputtering yield by Ti atoms was best fit by a value of about 2. The observed range and straggling values were in very good agreement with the values predicted by existing theories, so long as the lattice was allowed to dilate.