氢含量对氢化锆相结构和残余应力的影响

Phase Structure and Residual Stress of Zirconium Hydride with Different Hydrogen Contents

氢化锆是核反应堆理想的固体中子慢化剂,在制备过程中随着基体氢含量的增加,氢化物依次发生α→β→δ→ε的相转变,从而导致氢化锆基体中产生内应力,进一步导致氢化锆开裂。研究了氢化锆相组成、相结构和应力与氢含量的关系。采用X射线衍射(XRD)表征了样品的物相组成,H/Zr(原子比)为0.8时样品含α,γ,δ相,H/Zr为1.2和1.45时样品含γ和δ相,H/Zr为1.60和1.64时样品均只含δ相,H/Zr为1.85和1.90时样品均只含ε相,结果表明氢化锆中氢含量与物相组成的对应关系基本符合样品Zr-H相图。对XRD结果进行精修分析,得到δ相(面心立方(fcc))和ε相(面心四方(fct))氢化物晶格常数与氢含量的关系为:随着氢含量的增加,δ相氢化锆晶格常数a逐渐变大,晶胞体积也逐渐变大;氢含量越大,ε相氢化锆晶格常数a越大,c则越小,晶胞体积也更大。利用X射线衍射应力仪测量了样品表面残余应力,结果表明Zr-1Nb合金表面应力为压应力,大小为-163.7 MPa;氢化锆表面则为拉应力,结果表明:H/Zr为1.57时,表面应力是12.2 MPa;H/Zr为1.67时,表面应力是18.6 MPa;H/Zr为1.74时,表面应力是30.8 MPa;随着氢含量的增加,残余应力逐渐增大。

Zirconium hydride is an ideal solid neutron moderator for nuclear reactors.During the preparation process,with the increase 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 hydrogen content were prepared by gas phase hydride method.The relationship between phase composition,phase structure,stress and hydrogen 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 samples contained δ phase hydride;when H/Zr(atom fraction) was 1.85 and 1.90,zirconium hydride samples contained ε 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) ε phase hydridehydride,in which(nearly) all eight tetrahedral positions were fully(without loss of generality,now referred as ε-ZrH2) occupied by hydrogen,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 influence 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 became larger,the cell volume of δ phase hydride also gradually increased;as the hydrogen content increased,a of ε phase hydride became larger,the lattice constant c of ε phase hydride became smaller,and the cell volume was larger.Compared with α phase,for zirconium 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.Compared with the existing research results,with the increase of hydrogen content,the surface residual stress of zirconium hydride increased 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 hydrogen 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.