Nickel base superalloy K5 w as directionally solidified to study the developm ent of m i crostructure under super high tem perture gradient and rapided cooling. The results of the experim ent showthat the surper fine directionalsolidified structures are obtained w ith super tem perature gradientand surper cooling rate w hen solidification rate arrives 600μm /s: The prim ary arm spacing λ1 is less that 23 8μm ; Thesecondary arm spacing λ2 is less than 10μm . In addition,the interface stability theory...