Positron annihilation studies of recrystallization in the subsurface zone induced by friction in magnesium—effect of the inhomogeneity on measured positron annihilation characteristics

Abstract

The discussion of the positron annihilation studies of crystal structure defects, like vacancies, dislocations, grain boundaries and the defect depth profile, is presented. The role of the positron implantation depth and positron diffusion in such studies has been considered in detail. For description of the measured annihilation characteristics the proposed theoretical models take into account both effects. The annealing studies of defects created in pure magnesium by compression or dry sliding-wear were used for demonstration of the discussed thesis. The positron lifetime measurements were applied for monitoring open volume defects behavior. It was demonstrated that annealing at the temperature of about 300°C removes the defects created by compression. Application of the proposed model to description of the data obtained allows to determine the activation energy of the grain boundary mobility in pure magnesium equal to Q = 0.56 ± 0.18 eV. However, defects created by the dry sliding are not completely annealed up to the temperature of 500°C. The defect depth profile induced by dry sliding evolves with the annealing temperature in such a way that at the worn surface concentration of defects gradually decreases but at the depth between 60 and 100 µm the generation of new defects takes place at temperature of 150 and 225°C. Above 300°C the defects still are extended up to the depth of about 80 µm.

The Annealing Behavior of the Subsurface Zone Induced by Friction in Bismuth Detected by Positron Lifetime Technique

“…It is worth noticing that similar dependency was obtained also for the samples with lower thickness reduction, i.e., 23 and 34 %. Dryzek [11,12] proposed a theoretical model that allows describing this temperature dependency. It takes into account two processes: the positron diffusion inside the new spherical grains and increase in the grain radius with the temperature and the time of annealing.…”

“…The solid line in Fig. 2 represents the best fit of the equations (12 b) and (14) (see Dryzek [12]) to the experimental points.…”

“…Indeed, our former studies performed for deformed pure copper samples have shown that these processes run with different rates at different depths what can be understood, taking into account that the stored energy deposited during sliding changes with the depth as well [14]. However, the recent studies for pure magnesium indicate that the SZ is completely restored in much higher temperature than it is for a sample exposed to compression [12]. The studies have shown that part of the SZ close to the worn surface is rebuilt at higher temperature than deeper regions.…”

“…As it was pointed out in numerous papers by the author of this paper and his coworkers, positron annihilation methods are suitable for the studies of SZ in ductile metals and alloys exposed to dry sliding (see Dryzek et al [9]). However, these methods were applied also with success to studies of recovery and recrystallization processes in plastically deformed samples [10][11][12][13]. The annihilation of defects and formation of defect-free new grain during recrystallization is well reflected in the measured positron annihilation characteristics.…”

The Positron Probe Microanalyser Studies of Defect Distribution Induced by Machining of Copper, Iron and Titanium

“…It should be noted that such a behaviour is slightly different from that observed in the case of dry sliding, where after annealing at 400°C, a maximum in the profile occurs at the depth of about 80 lm from the worn surface [23]. The maximum indicates the creation of new open-volume defects, like vacancy clusters; presumably they occur due to the dislocation motion release at higher temperature [24]. For Fe, the dependencies of the S-parameter depicted in Fig.…”