Phase Structure and Residual Stress of Zirconium Hydride with Different Hydrogen Contents
Zirconium hydride is an ideal solid neutron moderator for nuclear reactors.During the preparation process,with the in-crease of hydrogen content in the matrix,the phase changes of α→β→δ→ε in turn occur in hydrides.This results in internal stress in zirconium hydride matrix and further leads to zirconium hydride crack.In this paper,zirconium hydride samples with different hydro-gen content were prepared by gas phase hydride method.The relationship between phase composition,phase structure,stress and hy-drogen content of zirconium hydride with different hydrogen content was studied.X-ray diffraction(XRD)was used to characterize the phase composition of the sample.When H/Zr was 0.8,zirconium hydride sample contained α,γ and δ phase hydrides;when H/Zr was 1.2 and 1.45,zirconium hydride samples contained γ and δ phase hydrides;when H/Zr was 1.60 and 1.64,zirconium hydride sam-ples contained δ phase hydride;when H/Zr(atom fraction)was 1.85 and 1.90,zirconium hydride samples contained e phase hydride.The results showed that the corresponding relationship between hydrogen content and phase composition in zirconium hydride basically conforms to the Zr-H phase diagram published in literature.In different zirconium hydrides,the hydrogen tended to occupy tetrahedral interstitial positions in the unit cell.Compared to commonly observed hydrogen rich face centered tetragonal(fct)e phase hydridehy-dride,in which(nearly)all eight tetrahedral positions were fully(without loss of generality,now referred as ε-ZrH2)occupied by hy-drogen,and non-stoichiometricface centered cubic(fcc)δ phase hydride,which had the same sites partially and randomly filled.The lattice constants of zirconium hydride with different hydrogen content were different due to different hydrogen atom occupancy.The in-fluence of hydrogen content on the lattice constant and cell volume of δ phase hydride(fcc)and ε phase hydride(fct)was obtained through refined analysis of XRD results as followed:as the hydrogen content increased,the lattice constant a of δ phase hydride be-came larger,the cell volume of δ phase hydride also gradually increased;as the hydrogen content increased,a of ε phase hydride be-came larger,the lattice constant c of ε phase hydride became smaller,and the cell volume was larger.Compared with α phase,for zir-conium hydride,the higher the hydrogen content was,the larger the cell volume was,so the larger the volume expansion rate was,and the matrix was easier to crack.X-ray diffraction stress meter was used to measure the residual stress on the surface of the sample.The surface stress of Zr-1Nb alloy was compressive stress and its size was-163.7 MPa.The surface of hydride was tensile stress,the results showed that when H/Zr was 1.57,the surface stress was 12.2 MPa;when H/Zr was 1.67,the surface stress was 18.6 MPa;when H/Zr was 1.74,the surface stress was 30.8 MPa.And the residual stress increased with the increase of hydrogen content.Com-pared with the existing research results,with the increase of hydrogen content,the surface residual stress of zirconium hydride in-creased gradually.When the residual stress reached the tensile strength,the matrix would crack.To sum up,the influence of lattice constant on the cracking of zirconium hydride was studied from the micro point of view,and the variation of residual stress with hydro-gen content was studied from the macro mechanical point of view.The reason of hydrogen induced cracking of zirconium hydride was given,which had a certain guiding significance for the preparation of crack free zirconium hydride.
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