The grains in the as-cast specimen exhibit a columnar morphology with a width of ≈1000 μm and a length:width ratio of ≈3. They contain parallel γ and α2 plates, showing a lamellar microstructure ( Fig. 1a). In the specimen that had been solution treated and oil quenched, a deeply etched, mottled microstructure prevailed. Detailed examination revealed that this microstructure is made up of γm ( Fig. 1b). After several times of CHT, γm was evolved to lamellar colonies gradually.