No abstract

Effects of segregation of solute atoms and vacancies on migration of antiphase domain boundaries (APDBs) in stoichiometric and Fe 3 Al at 673 K have been studied using a phase-field method in which local vacancy concentration is taken into account [Koizumi Y, Allen SM, MinaminoY. Acta Mater 2008;56:5861, ibid. 2009;57;3039]. Boundary mobility (M) of APBs having different phase-shift vectors of a/4<111> and a/2<100> (hereafter denoted as B2-APB and D0 3 -APD, respectively) were evaluated by measuring the boundary velocity of shrinking circular APDs. Similar effects of the segregations on the migration of B2-APBs were observed in both compositions. Vacancies segregated and Al-atoms were depleted at B2-APBs in both compositions. Vacancy concentration at B2-APBs was up to 80 % higher than those in the bulk. As a result, the migration of B2-APBs was greatly enhanced by the vacancy segregation. In contrast, the segregations to D0 3 APBs greatly depended on the composition. Vacancies are depleted and Al-atoms segregated at D0 3 -APBs in the Al-rich Fe 3 Al, whereas vacancies segregated and Al-atoms were depleted at D0 3 -APB in the stoichiometric Fe 3 Al. The Al segregation in the Al-rich Fe 3 Al decreased the M of D0 3 -APB s much more significantly than the Al-depletion in the stoichiometric Fe 3 Al. As the APDs shrank, D0 3 -APBs broke away from the segregation atmospheres and the Ms increased rapidly in both compositions. A greater increase in the M due to the breakaway was observed in the Al-rich Fe 3 Al than in the stoichiometric one.

Segregation of solute atoms and vacancies to migrating D03–type antiphase boundaries (APBs) in Fe3Al of the stoichirometric and an Al-rich compositions has been investigated in detail by a phase-field method developed in the previous study [Koizumi et al. Acta Mater 2009;57:3039] focusing on the quantitative relationship among the segregation, APB energy, driving-force, drag-force and APB mobility. It has been revealed that the drag-force by solute segregation depends considerably on the alloy composition and is responsible for the significant composition dependence of the migration kinetics of D03-APB whereas the composition dependences of driving-force and mobility are negligibly small.

The effects of solute and vacancy segregation on APB migration in Ti 3 Al, and their dependence on composition, have been investigated by using a phase-field simulation in which vacancy distribution is taken into account. Al-atoms are depleted and vacancies segregate at APB in stoichiometric Ti 3 Al (Ti-25Al), whereas Al-atoms segregate and vacancies are depleted in Alrich one (Ti-28Al). The simulation indicates that APB in Ti 3 Al migrates much faster in Ti-25Al than in Ti-28Al with the effect of vacancy segregation whereas it migrates slightly faster in Ti28Al than in Ti-25Al in the absence of the effect of vacancy segregation.

Determination of the mobilities for ordering transformation in Fe3Al has been attempted by comparing the rates of experimentally observed antiphase boundary (APB) migration with that simulated by a phase filed method (PFM). The simulated boundary mobility of D03-APB was sensitive to the ordering mobility. This allowed us to determine the ordering mobility for D03-LRO at 673 K to be 3 × 10 -11 m 3 ·J -1 ·s -1 . However, the simulated boundary mobility of B2-APB was insensitive to the value of ordering mobility, and therefore the ordering mobility for B2-LRO could not be determined. The difference is ascribed to solutedrag which was quite significant at B2-APB.