Effect of grain size distribution on the strength and strain properties of Zr-Nb alloys under tension at high strain rates

Two-level computer simulation is used to study the effect of bimodal grain size distribution on the plastic flow, damage evolution, and fracture of Zr-Nb alloys with a hexagonal close-packed crystal lattice under tension at strain rates of 100 and 1000 s$^{-1}$. The developed computational model allows one to describe the strain and fracture of the Zr-1 % Nb alloy with unimodal and bimodal grain structures under tension at high macroscopic strain rates. It is shown that the damages that cause the fracture of the Zr-1 % Nb alloy arise at the boundaries between coarse grains and volumes with an ultrafine-grained structure at high tensile strain rates.
A sharp increase in the strain to fracture and a smooth decrease of the yield strength and tensile strength of the Zr-1 % Nb alloy are observed at increasing volume concentration of large grains from 0 to 30 %. A rational combination of the increased yield strength and tensile strength with satisfactory ductility for strain rates ranging from 100 to 1000 s$^{-1}$ can be achieved in the Zr-1 % Nb alloy when the ratio of the volume of submicron grains to the volume of coarse grains is about 3:7. Numerical simulation results show an insignificant impact of the concentration of dispersed particles of zirconium hydrides with sizes varying from 25 to 40 nm segregated in a grain boundary phase on the tensile strength of the Zr-1 % Nb alloys and on the strain to failure in the studied range of strain rates and temperature.