Discontinuous Precipitation Reaction Front of Cellular Recrystallization for a Single-Crystal Superalloy Studied by Electron Microscopy*Supported by the Scientific Research Foundation of Xi'an University of Technology under Grant No 101-451115007, the National Natural Science Foundation of China under Grant No 51174161, and the Pivot Innovation Team of Shaanxi Electric Materials and Infiltration Technique under Grant No 2012KCT-25. (July 2015)