Abstract:The type, morphology and orientation relationship of the hydride precipitates in a hydrogen absorbed Zr-4 alloy have been studied by means of transmission electron microscopy (TEM). The results show that δ-ZrH1.6 and ε-ZrH1.8 hydrides are found to precipitate successively with the increase of hydrogen absorption. For the δ-ZrH1.6 hydrides, they are mainly in the form of lath and precipitate in the orientation relationship of (0001)α//(001)δ and [2 0]α //[110]δ with the α-Zr matrix, while for the ε-ZrH1.8 hydrides, they are in acicular morphology and precipitate in the orientation relationship of ( 1 )δ //( 1 )ε and [ 12] δ//[011]ε with the δ-ZrH1.6 hydrides. The close-packed planes of these two hydrides are identified to be parallel. High-resolution TEM observation shows that there exists a great deal of distortion inside the crystal lattice of the δ-ZrH1.6 and ε-ZrH1.8 hydrides, indicating a large stress concentration produced during their precipitation.