IN-SITU ELECTRON MICROSCOPY STUDY ON PRECIPITATION OF ZIRCONIUM HYDRIDES INDUCED BY STRESS AND STRAIN IN ZIRCALOY-2

The precipitation process of zirconium hydrides induced by stress and strain was investigated by means of electron microscopy in-situ.The precipitating hydrides induced by stress were found to be γ phase with orientation relationship of (110)_γ‖(110)_(αZr),(001)_γ‖ (0001)_(αZr) between γ-hydrides and surrounding matrix.The growth rate of γ-hydrides which was much faster along [110] direction brought them in taper shape.After fracture of y-hydrides,a new one will precipitate at the tip of cracks.This is the essential process of hydrogen-induced delayed cracking in Zircaloy.The precipitating hydrides induced by strain were found to be δ phase with both orientation relationships of(111)_δ‖(0001)_(αZr),(110)_δ‖ (110)_(αZr) or (010)_δ‖(0001)_(αZr),(001)_δ‖(110)_(αZr)between δ-hydride and surrounding matrix.The δ-hydrides become much finer as the strain rate increased.

辐照后N18锆合金氢致延迟开裂临界温度预测

基于辐照后锆合金中H的固溶与析出浓度方程及应力诱导H扩散方程,建立了辐照后锆合金氢致延迟开裂(DHC)临界温度的预测模型.采用此模型对辐照后N18锆合金(Zr-Sn-Nb)的DHC临界温度进行预测,并与未辐照N18锆合金临界温度实验数据进行对比分析.结果表明,中子辐照后N18锆合金屈服强度和H的固溶度增加,N18合金发生DHC的敏感性增加;相同H含量条件下,辐照后N18合金的临界温度最高上升幅度接近20℃;辐照后N18锆合金临界温度变化规律与未辐照实验结果相似,其临界温度介于相同H含量下完全固溶的温度与氢化物析出时的温度之间,且最高开裂温度T_c低于最低止裂温度T_h.

Zr-2.5Nb合金压力管材料微观组织研究

采用电解渗氢法制备出氢含量为60mg/g的压力管材料Zr-2.5Nb合金。利用X射线衍射仪(XRD)、金相显微镜(OM)、透射电子显微镜(TEM)研究了Zr-2.5Nb合金微观结构,采用扫描电镜(SEM)观察了Zr-2.5Nb合金氢致延迟开裂(DHC)断口形貌。结果表明:Zr-2.5Nb合金基体为密排六方结构的a-Zr相,远离裂纹尖端区域晶粒较大(大于5mm),裂纹尖端对应区域的基体晶粒较小(1mm左右);面心立方结构δ-ZrH_(1.66)呈条片状平行于轧制方向分布;体心立方结构的第二相Nb颗粒尺寸为50~500 nm,在晶粒内部均匀分布,晶界附近成串状分布;试验温度为250℃时,预制疲劳裂纹平行于轧制方向的试样,裂纹呈现步进式生长,每步生长的距离大致相当于氢化物的尺寸,约为20mm。

锆合金包壳管氢致延迟开裂应力强度因子阈值研究

针对受对称双集中力作用的含双边缘裂纹的锆合金包壳管,采用数值模拟方法,获得裂纹尖端应力场,建立了应力强度因子与裂纹长度、对称双集中力大小之间的关系。结合氢致延迟开裂(DHC)应力强度因子阈值(KIH)的理论求解方法,获得受对称双集中力作用的包壳管的KIH和临界参数的解析表达式,据此考察实验研究了温度对其产生的影响。研究结果表明,受对称双集中力作用的含双边缘裂纹包壳管的KIH、临界载荷及临界裂纹长度均随温度的升高而增大,与实验所得到的变化趋势相吻合,这种随温度变化的趋势与预开裂区域的应力状态接近于平面应力状态这一事实密切相关。