Defect evolution during annealing of deformed FeSi alloys studied by positron annihilation spectroscopy

Abstract

High silicon steel is widely used in electrical appliances. Alloying iron with silicon improves its magnetic performance. A silicon content up to 6.5 wt. % gives excellent magnetic properties such as high saturation magnetization, near zero magnetostriction and low iron loss in high frequencies. Their workability is greatly reduced by the appearance of ordered structures, namely B2 and D0 3 , as soon as the Si content becomes higher than 3.5 wt. %. This limits the mass production by conventional rolling to this maximum percentage of Si. In this work a series of FeSi (7.5 wt. % Si) samples with different degrees of deformation are investigated with positron annihilation spectroscopy and optical microscopy (OM). The influence of annealing on the concentration of defects of different deformed FeSi alloys has been investigated by positron annihilation lifetime spectroscopy and Doppler broadening of the annihilation radiation. OM is used to investigate the microstructure of deformed samples before and after annealing. The values of the S parameter present a decrease for all studied FeSi alloys with the increase of the annealing temperature, being attributed to a decrease of the concentration of defects. A sudden increase of the S-parameter value at 600C was observed for all samples, which could be related to the change of the ordering of the FeSi alloys at that temperature. At 700 C, the values of the S parameter decreased drastically and starting from 900 C, they became constant. The microstructures of the alloys, investigated by OM, show that recrystallization is completed at 900 C and the samples are mainly free of defects, which is in agreement with the positron annihilation lifetime data. The development of advanced techniques for materials processing requires a corresponding improvement of the materials characterization techniques and development of new ones. Positron annihilation spectroscopy (PAS) is considered to be one of the very important and promising techniques in characterizing materials.1 The method relies on the propensity of positrons to become localized at open-volume regions of a solid and the emission of annihilation gamma rays that escape the system without any interaction. These gamma quanta hold information about the electronic environment around the annihilation site. The binary FeSi phase diagram 2 is shown in Fig. 1. According to the diagram, annealing of the FeSi (7.5 wt. % Si) could lead to the change of ordering from (B2 þ D0 3 ) at room temperature to D0 3 at temperatures above T c ($650 C), which is known as order-disorder temperature. Further increase of the annealing temperature changes the order from D0 3 to B2 and above 900 C the alloy is with fully disordered structure type A2. Therefore in the temperature interval between room temperature and 730 C the size of D0 3 ordered zones increases with the increase in the annealing temperature.

Positron Annihilation Studies on Chemically Synthesized FeCo Alloy

“…Therefore, the first life time in annealed FeCo can be attributed to the average lifetimes of the bulk and the associated vacancies. Such associated vacancies have been reported in other Fe based alloys 18 , 19 . The lifetime component τ 1 for FC-800 is same as that of FC-700 but the former has greater intensity.…”

Hydrogen permeation through deformed and heat-treated Armco pure iron

“…Deformation phenomena in polycrystalline materials are relatively complex due to various interactions between dislocations and grain boundaries [1][2][3]. The deformation causes changes in the density and distribution of mainly vacancies and dislocations.…”