Aspects of intergranular fracture of CuBi studied in the scanning electron microscope

Abstract

IntroductionKossel X-ray diffraction within the SEM can be used to obtain orientation relationships (to 0.2° routinely) and crystal perfection information from microvolumes (5μm)3, this being the principal feature of the technique1. A study of the cracking behaviour of CuBi alloys has been made in an attempt to understand the mechanism by which intergranular fracture occurs due to the addition of down to 0.001at.% Bi2. Orientation relationships (angle/axis pairs) were needed for cracked boundaries but can only be found from Kossel patterns from annealed materials. Thus orientation work was done on annealed material, which was then deformed in a 4-point bending machine. Observations of changes in crystal perfection were made at various strains, as cracks appeared, at positions within grains and at boundaries. Finally, optical study of slip lines at boundaries and fracture surfaces was made.A Cu0.02 wt.% Bi sample was given a maximum embrittlement heat treatment (1073K,10min,air-cooled; 773K, 4h, furnace-cooled) which caused a large percentage of the Bi to segregate to the boundaries.