Direct observation of the 12R transition structure during the C15 -> C14 phase transformation in Zr(Fe, Cr)2 Laves structured nanoparticles in Zircaloy-4

The presence of 12 R long-period stacking-ordered (LPSO) structure and the associated C15 -> 12 R -> C14 phase transformation of Zr(Fe, Cr)2 secondary phase particle (SPP) in Zircaloy-4 (Zr-4) were investigated by using transmission electron microscopy. Results revealed that the 12 R structure, acting as a transition state, played a critical role in the C15 -> C14 phase transformation. The C15 -> 12 R and 12 R -> C14 phase transformations were realized via the gliding of 16 [112] partials on every four (111)C15 planes and the motion of 16 [112] partials on every four (111)12R atomic layers, respectively. The above two transition processes together constitute the C15 -> C14 phase transformation, viz. the gliding of 16 [112] partials on every other (111)C15 planes. The knowledge attained from the present study, shedding lights on the 12 R LPSO transition structure, would also benefit the under-standing of the C15 -> C14 phase transformation.