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

The precipitation hydrides induced by stress were found to be gamma phase with orientation relationship of (110)_γ//(1120)_(αZr),(001)γ//(0001)_(αZr)between γ- hydride and surrounding matrix. The growth of-hydrides exhibiter a quickest rate along [110] direction, that brings them in taper shape. After cracking at hydride matrix beundary, a new one will precipitate at the tip of cracks. This is the essential process of hydrogen-induced delayed cracking in zircaloy The pre- cipitating hydrides induced by strain were found to be delta phase Phase with both orientation relationships of (111)_δ//(0001)_(αZr)(110)_δ//(1120)_(αZr) and (010)_δ//(0001)_(αZr) (001)_δ//(1120)_(αZr) between δ-hydride and surrouding matrix. The δ-hydride becomes much finer as the strain rate increased.