In the as-cast samples, the microstructure was primary lamellae, Lp. It is now well known that this structure formed by precipitation of γ laths on the α matrix with an orientation relationship of {111}γ//(0001)α2, 〈110]γ//〈1120〉α2[17]. Lp lamellae are formed at relatively slow cooling rate, and thus are thermally stable. They could be converted to a fine grain structure usually by thermomechanical processing such as isothermal forging, rolling and/or extrusion. A significant finding of the present experiments is that such an Lp structure with a coarse grain size (d>1000 μm) can be drastically refined solely by cyclic heat treatment. The refining result is affected most importantly by cooling rate. This is essentially due to the fact that the Lp structure was transferred to different intermediate states, massive γ (γm), or feathery γ (γf), or lamellar α2/γ under different cooling conditions.