Theoretical Investigation of The Defect Interactions in Dilute Copper Alloys Intended for Nuclear Waste Containers
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sion resistance is limited because of the effect of intergranular emnibrittleinent at temperatures above 100-150TC. Dilute copper alloys containing S, P, and Ag inpirities and vacancies are studied theoretically on the basis of total energy calculations. The dissolution energies, volunie misfits, and defect interaction energies are calculated and used to study the microscopic miechanisin behind the effect of these impurities on the eiibrittlenieiit of colpper at interinetemiperatures. A large linding energy of a sulfur-vacancy defect pair (-0.46 eV) is dia found. The sulfur-vacancy and sulftir-sulftir interactions in the copper matrix seeni to favor precipitation of copper sulphide Cu,2 S which is the most probable cause of the einhrittleiieiit. The effect of phosphorus and silver ilmpurities oii the einbrittlement of sulfiir-contaminated copper can he related to their competition with sulfuir to attract to vacancies as well as to other lattice defects. INTRODUCTION The efecut of intergraniilar emibirittleiient of copper at temperatures above 100-150lC' is of particular importance in connection with the potential application of pure, oxygen-free copper as a Iiaterial for nucler waste containers. Intergranular cavitation and fracture were observed l)referentially at randoni grain boundaries [1] and the fracture surface was found to be enriched in sulfur [2]. Thus, there is strong evidence that this eIibrittlerilent is caused by grain boundary segregation of sulfur. The sulfur content in commercially pure topper is very low, but it is nevertheless well above the equilibrium solubility of sulfur in copper. A
drop in ductility has been observed even for ultra-high purity copper [3]. It is also of considerable interest that a certain aniount of phosphorus or silver additions to coiptper has been found to improve its ductility at interniediate temperatures (180-450TC) [2]. The iniiroscti)ic Inechanisin of this p)henomenon is not clear. Two possible explanations have been proposed: The ductility improvement could be either due to an increase ini S solid solubility or due to a segregation of P or Ag and a competition with .5 for the grain boundary sites [2]. Thus, reliable information oil the interactions of these impurities with each other and with other defects in Cu is highly desirable. It is not known what structure the segregated sulfur forIms at grain boundaries. Judging from the equilibrium Cu - S phase diagrami [4], it is natural to expect formation of col)per (I) sulp)hide C'u2 S. It is noteworthy that stoichiometric CU2S miay exist in three crystal structures: low clialcocite ((oCli, unonoclinic), high chalcocite (Chli, hexagonal), and high digenite (Dg, cubic). The temtperatures of the structural transformations in C'o2 S, 103.50C (vtChi10-/K'ih) and 435()C (f3ChievDg), correlate with the euibrittlenientt temperattres of Colpper. Therefore, the structural properties of copper sulphide as well as the thermodynamics and kinetics of the solid state reactions in the C0 - S systemi are imtportant for understanding t
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