Study of grain boundary segregation using the Auger electron emission technique. Annual technical progress report, VII, January 1, 1977--December 31, 1977

Abstract

An analysis of plastic flow at the tip of a propagating crack during brittle fracture has been developed. The plastic flow analysis takes into account the anisotropy of slip by a mixed mode opening scheme. Values of the true fracture energy for different crack systems are obtained; the effects of an embrittling agent can be taken into account. The fracture stresses of Cu--Bi alloys having varied grain sizes and Bi concentrations were measured. Dilute alloys exhibited a decreasing fracture stress with decreasing grain size. Auger analysis indicated a large variation of Bi segregated to boundaries in coarse grained materials with smaller variations in fine grained material. The grain boundary diffusion of nickel was studied in the Cu--Bi alloy system by means of Auger spectroscopy. Bi retards boundary diffusion of Ni; however, the results are quite complex since the Ni presence alters the Bi concentration at the boundary. The Bi concentration tends to decrease to a value approximately equal to that of Ni. Possible effects of solute segregation in determining SCC susceptibility of copper alloys in acid sulfate solution were examined with high purity binary alloys. Grain boundary segregation of Sb and Bi in Cu promoted intergranular SCC in sulfuric acid. Cracking was macroscopically brittle, but significant dissolution occurred along the intergranular crack path. Electrochemical measurements revealed an active path for SCC in segregated Cu--Sb and Cu--Bi alloys. Grain boundary segregation has no effect on the bulk electrochemical properties of unstressed Cu--Sb and Cu--Bi.