Dislocation density profile of mechanically polished iron single crystals determined by positron annihilation

Abstract

Positron annihilation techniques have become important nondestructive evaluation methods. Dislocation densities can be determined nondestructively [1-4]. In comparison, the conventional technique of microstructure characterization, transmission electron microscopy, requires special specimen preparation of a destructive nature because very thin specimens are required. Only a pair of relatively flat specimens treated in the same way are required for the positron annihilation measurement. The thickness can be several centimeters and is restricted only by the stopping power of the metal for 7 rays. This method has been applied to studies of vacancy production [5-7] and fatigue damage [8, 9]. It is known that there are deformed layers at the surface of mechanically polished metal specimens [10]. In ductile materials, the wear debris is formed by the microplowing and microcutting mechanisms and the worn surface is plastically deformed by these processes [11]. Positron annihilation affords a new method of determining the depth of damage and of measuring the profile of the density of dislocations or other defects. Plastically deformed surface layers have been studied by several methods. Dance and Norris [12] measured the microhardness of the subsurface of the abraded specimens. Samuels and co-workers [10, 13, 14] used metallographic examination combined with electropolishing to measure the deformation depth. Moore and Douthwaite [15] measured the strain profile of abraded specimens directly by forming a composite structure of copper and the silver solder leaves. But these techniques do not show the defect concentration below the abraded surface. A transmission electron microscopic study [16] as well as some models [17-19] of the surface layer of the worn specimen, show that dislocations are the major defect produced. The pure iron single crystal strain was prepared by the strain-annealing method. Specimens of 0.7 cm x 0.8cm and 0.1 cm in thickness were cut with a lowspeed diamond saw and polished mechanically by SiC paper and diamond paste down to 1/~m. Polished specimens were purified and annealed in the clean hydrogen atmosphere generated in a circulating ZrH2 furnace system at 1113 K for 72 h. Positron annihilation lifetimes were measured with annealed specimens first. The measurement was done by the fast coincidence system described elsewhere [20]. The same specimens were then polished with new SiC paper lubricated with running water. The specimens were rubbed in one direction with a fixed weight which corresponded to a pressure of 0.045 M N m 2.