Evolution of antiphase domain (APD)/nanolamellar complex structure in Ti-Al single crystals

We studied the growth of antiphase domain (APD) and the formation of lamellar structure during isothermal annealing of Ti-39at%Al and 40at%Al single crystals quenched from α single phase state, intending to obtain nano-scaled APD/lamellar complex structure. In the Ti-39at%Al crystal, lamellar structure with a lamellar spacing of several tens nm was not obtained by annealing at relatively low temperature such as 973 K even for 5×10 ⁵ s since the precipitation of γ-plate was significantly slow. On the other hand, lamellar spacing of 70 nm was obtained by annealing at 1073 K only for 2×10⁵ s. However, the APD size was larger than 400 nm when the lamellar spacing smaller than 100 nm was obtained. An APD/nanolamellar complex structure in which average APD size (l̄) was smaller than 200 nm and average lamellar spacing (L̄) was smaller than 100 nm was obtained by annealing at intermediate temperatures of 998-1048 K. Finer APD/nanolamellar complex structure was obtained in the Ti-40at%Al crystal even at 973 K and 1073 K since the γ-plate precipitation was faster than in the Ti-39at%Al crystal. The finest structure in the present study with l̄ of 125 nm and L̄ of 25nm, in which lamellar spacing was partly smaller than 10 nm, were obtained by annealing the Ti-40at%Al crystal at 973 K for 5×10⁵ s. Hardness was dependent on both of APD size and lamellar spacing, and tended to increase with decreasing APD size and decreasing lamellar spacing in the range studied. The Ti-39at%Al crystal with l̄ of 250 nm and L̄ of 45 nm exhibited the hardness of HV 386 on its (1120) plane. It is approximately three times higher than that of completely ordered Ti₃Al without APD or lamellar structure.