Ａｌ‐Ｌｉ合金のδ′相析出に及ぼすＣｕおよびＭｇ微量添加の影響の計算機シミュレーション

Abstract

The Monte Carlo computer simulation has been performed to elucidate the effects of the trace-additional elements of Cu and Mg on the δ′ phase precipitation in an Al–Li alloy. The assumed alloy composition in this work was Al–8 mol%Li with additional 0.25 mol%Cu or Mg. The aging temperature employed for the simulation was 463 K. With increasing Monte Carlo step the precipitation of the δ′ phase with the L12-type ordered structure was clearly confirmed in the simulated atom configurations. Based on the simulated atom configurations the solute atom concentration of the matrix, size and number density of the δ′ phase were determined as a function of the Monte Carlo step. While Cu addition to an Al–Li alloy exerts almost no effects on the precipitation of the δ′ phase, Mg addition has the significant effects to accelerate the precipitation of the δ′ phase and also to produce increased number density of the refined δ′ precipitates. These characteristic effects of the additional elements on the δ′ precipitation are closely related with the ordering parameters among quenched-in excess vacancies, Li atoms and trace-additional elements. The computer simulation demonstrates that Mg atoms preferentially exist at the second nearest neighbor positions of the Li atoms and attracts vacancies to form Mg/Li/Vacancy complexes in the alloy containing Mg. These Mg/Li/Vacancy complexes further attract Li atoms and act as the effective nucleation sites for the δ′ phase. This mechanism explains the accelerated precipitation of the δ′ phase in the Al–Li alloys containing Mg.