Investigations of the influence of grain size on structure and properties of nickel were done by packet hydroextrusion method (one - stage route regime) within the range of relative deformation degree epsilon = 0 - 60 %. For formation the ultrafine grained structure in nickel with the initial grain size d = 0.008 mm the packet hydroextrusion method was also used (many-stage route regime) with the maximum logarithmic deformation e = 12.
The structure and properties of nickel samples were studied by methods of optical and transmission electron and field ion microscopy methods, as well as by using resistometry and durometry. Structure parameters were quantitatively analyzed by the standard secant method. Measurements of the specific electrical resistance rho were made in liquid nitrogen. The accuracy of rho estimation was 1 10^-10 omega cm that corresponded to rho_D value (dislocation density) of the order of 10 8 m -² calculated by used coefficient eta= delta rho_d/rho_D (for Ni - eta= 9.4 10^-19 omega cm m². The degree of strain hardening was found by measuring hardness by Vickers method with a 0.98 N load.
Investigations have been done of the influence of grains size on the value of nickel strain hardening during the deformation by packet hydroextrusion method (one-stage route regime). It is shown that the grain-boundary component of the hardening is conditioned by the increase of dislocation density during the deformation. A relationship between the value of strain hardening and the grain size has been derived. It coincides with the Hall-Petch relationship for low values of the degrees of deformation.
The results of investigations of changes in structure and properties of Ni by packet hydroextrusion method (multistage route regime) show that the ultrafined grained structure is formed at the logarithmic deformation e = 8.
It was for the first time when the direct field ion microscopy method gave evidence of a high density of defects in ultrafine
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