V.G. Shadrov - Ph.D. (Phys.-Math.), Leading Research Scientist, Scientific-Practical Materials Research Centre (Minsk) A.E. Dmitrieva - Junior Research Scientist, Scientific-Practical Materials Research Centre (Minsk) A.V. Boltushkin - Ph.D. (Phys.-Math.), Senior Research Scientist, Scientific-Practical Materials Research Centre (Minsk). E-mail: nemtsevich@ifttp.bas-net.by