A single crystal growth apparatus using modified Bridgman technique has been designed and assembled. A temperature gradient up to 20℃/cm$ at solid-liquid interface could be obtained. The growth rate could be varied from 1mm/hr to 90mm/hr. Single crystals of Al-Li and Al-Li-Mg alloys have been successfully grown by this apparatus. Optimum growth rate for alloy single crystal was determined to be ∼2.5mm/hr. A recovery rate of Li largely increases by using an appropriate reactor and a BN crusible. Growth orientations were observed to be random and composition profile in alloy single crystals well obey the Scheil's theory.
In the case of Li addition into Al, the improvement of elastic property is primarily due to the increase of elastic constants C and C'. The elastic constans $C_1$, C and C' of the δ' phase have been estimated to be 104.81±4.53, 35.25±4.04, and 37.12±2.84GPa respectively. The Young's modulus is thus claculated to be 88.57±5.59GPa. This value is smaller than that previous reported. It is related with the effect of the matrix/δ' interface on the elastic constants. The contribution of δ' phase on the elastic constant have been estimated to be below ∼20%. Anisotropy factor has been estimated to be ∼1.10 for the case of Al-Li single crystal and to be ∼0.95 in the case of δ' phase.
A homogeneous dispersion of primary slip line was observed in the case of single crystals within a single-phase region. Cross slips have been occasionally observed. The slips during the initial stage of aging of the single crystals within a two-phase region are localized to certain slip planes resulting in the formation of intense slip bands. With the progress of aging, the primary slip lines tend to be finer and shorter, which results in the appearance of slip free regions. In the case of peak aging condition, slip lines form discontinuous slip bands which consist of finely distributed short slip lines.
Solid solution strengthening effect was determined to be unusually small ∼0.81MPa/at%Li. It is due to very small atomic size diffrence (-0.2%).
The character of dislocations responsible for the plastic flow was screw type in Al-2.0wt.%Li alloy single crystals. Dislocations present in the as-quenched condition were usually long straight single dislocations. Pair dislocations were occasionally observed. At the initial stage of aging, one observed the presence of long nearly straight pair dislocations. Pair dislocation spacings at various regions were largely different. It is probably due to complex interaction of dislocations. As the aging progresses, one frequently observed the presence of numerous cusps along the pair dislocations. This reflects a pinning process of moving dislocations by the δ' particles. In a further aged condition, the pair dislocations tend to get more pronounced cusps and to be more severely curved δ' particles. At this stage of aging, one observed many small dislocation loops together with some single dislocations. The looping already begins to occur at this stage of aging. In the peak aged condition, many dislocation loops were observed together with the zig-zag shaped dislocations. The pair dislocations mostly break into single dislocations. This indicates a transition from particle shearing to bypassing by the Orowan looping because of the δ' particle strength strong enough at this stage for resisting the shearing force. The overaged structure mostly consists of dislocation loops indicating a predominance of looping over the shearing process.
δ' volume fraction was measured using transmission electron microscopy. It tends to increase with the increase of the foil thickness. This is related with the preferential dissolution of δ' particles near foil surface during specimen preparation. The correct δ' volume fraction could be obtained in the case of foil thickness above ∼$2L_s$(square lattice spacing). Measured δ' volume fraction was similar to that expected from Al-Li binary phase diagram.
The variation of CRSS (critical resolved shear stress) increment as a function of aging time in Al-2.0wt.%Li single crystals appears to be consistent with the prediction of the theory of the order strengthening mechanism. The contribution of the coherency and modulus strengthening was very small. The analysis of the measured CRSS increment data in terms of the order strengthening mechanism indicates that the APB energy is 0.120 and 0.120∼0.135J/㎡ for under aging condition and peak aging condition respectively. The configuration of the trailing dislocation is such that it pulls completely off from the encountering particles.
The loop size measurements using TEM suggested that the critical loop size is close to the average planar particle size at the onset of peak aging condition. The antiphase boundary energy is determined to be∼0.097 J/㎡ from the analysis of this transition behavior of particle shearing to looping. This is smaller than that obtained from the order strengthening mechanism. The similiar analysis of aging curves of polycrystals containing Zr tends to show higher antiphase boundary energy. This is because the peak aging condition appear relatively rapidly due to a bimodal size distribution of δ' particles in these crystals.
In the case of Al-2.0wt.%Li-0.16wt.%Mg alloy single crystals, the variation of CRSS and deformation behavior as a function of aging time at 200℃ was similiar to the case of Al-2.0wt.%Li single crystal. The character of dislocation responsible for the plastic flow in Al-2.0wt.%Li-0.16wt.%Mg alloys was screw type as in the case of Al-2.0wt.%Li alloy. The δ' volume fraction has been significantly increased as a result of Mg addition. It is expected that the increase of the strength above the solid solution strengthening could be obtained by Mg addition. The CRSS increment by the δ' particles in Al-2.0wt.%Li-0.16wt.%Mg alloy well obey the order strengthening mechanism as in the case of Al-Li binary alloy singrystal. The analysis of the measured CRSS increment data in terms of the order strengthening mechanism indicate that the APB energy is∼0.117 J/㎡ for underaging condition. This value is quite similar to that obtained for Al-2.0wt.%Li alloy single crystals. In the peak aged condition, the APB energy of the δ' phase have been determined to be 0.131∼0.142 J/㎡. It is slightly larger than that obtained for under aging condition.
