Effect of the addition of Ag on the strengthening of Al 3Li phase in Al–Li single crystals

Abstract

The variation of the critical resolved shear stress as a function of aging condition has been measured in ternary Al–∼(1.9–2.0) wt% Li–∼0.4 wt% Ag single crystals and has been analyzed using the same version of order-hardening theory as in our previous work in binary single crystals. The result showed that the antiphase boundary energy of the Al 3Li lattice on {111} definitively increases from about 0.118 J/m 2 in binary crystals to 0.128±0.012 J/m 2 in ternary crystals, in good agreement with the analysis of the transition behavior from shearing to looping and of the dislocation pair spacings. A strong segregation tendency of Ag into the Al 3Li lattice is believed to be responsible for this increment. Our previous result of γ apb (0.118±0.006 J/m 2) in binary crystals is, although unlikely from the recent suggestion of Nembach, the best value one can obtain from the recent knowledge of order-hardening theory. This is slightly larger than that (about 0.100 J/m 2) obtained from the analysis of the transition from shearing to looping and somewhat larger than the result (about 0.085 J/m 2) of the first-principles calculation. Three factors are discussed as principal causes of a possibility of an overestimation of γ apb on applying order-hardening theory.