The strengthening behavior of δ' and $T_1$ particles have been investigated in the Al-Li-Ag and Al-Li-Cu ternary single crystals using transmission electron microscopy and mechanical testing. Single crystals of Al-Li-Ag and Al-Li-Cu alloys have been successfully grown by modified Bridgman method (using an appropriate designed reactor and BN crucible). The compositions of grown ternary single crystals was determined to be Al-~2.0%Li-~0.45%Ag and a Al-~1.2%Li-~1.9%Cu. The result of Laue X-ray experiment, growth directions were observed to be random both ternary single crystals.
Part Ⅰ : Study on the strengthening mechanism of δ' particle in Al-Li-Ag ternary single crystal
The CRSS(Critical Resolved Shear Stress)drastically increased with aging time due to the precipitation of δ' particles and appear to be peak aging condition at near 20hr. As a result of microstructure observation, it consists of only matrix and spherical δ' particles. At the initial stage of aging, one observed the presence of nearly long straight pair-dislocations. As the aging progress,one frequently observed the presence of numerous cusps along the pair-dislocation. This reflects that dislocations moving impeded by the δ' particles. At the near peak aging condition, the pair-dislocation separate a single one and one observed many small dislocation loops. This indicates transition of deformation mode 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 analysis of order hardening theory indicates that the APB(Anti-Phase Boundary) energy is $γ_apb=0.129J/㎡$ for under aging and $γ_apb=0.114~0.135J/㎡$ for peak aging condition. The configuration of the trailing dislocation is such that it pulls completely off from the encountering particles.
As a results of direct measurements of loop size, critical planar loop diameter is close to the average planar particle size at the onset of peak aging condition. The average loop sizes increase with aging time, but the minimum loop sizes converge into the constant value. The critical planar loop diameter, dloop is determined to be ~17nm from an average minimum loop sizes for the various aging condition. The APB energy is estimated to be $γ_apb=~0.123J/㎡$ from the analysis of this transition behavior of particle shearing to looping. This is well consistent with that obtained from the order hardening mechanisms. This value was higher than one calculated from binary Al-Li single crystals. As a result of the addition of Ag, critical planar loop diameter decreased and the APB energy increased. This fact well consistents with the reducing of pair-dislocation spacings.
Part Ⅱ : Study on the strengthening mechanism of $T_1$ particle in Al-Li-Cu ternary single crystal ; The CRSS(Critical Resolved Shear Stress) increased with aging time at 180℃ and 200℃ aged, but the CRSS values were high in the case of low temperature aging(180℃). The CRSS was closed to the peak values in the near 20hr aged at 180℃. However, in case of higher aging temperature, the CRSS appears to be constant values until the long aging time.
As a results of TEM observation, the microstructures consist of mainly matrix and T1 particles but small amount of δ' particles coexisted. However, the strengthening effects of δ' particles could be negligible because the sizes are large and the distributions are inhomogeneous. The diameter, thickness and volume fraction of $T_1$ particles continuously increased with aging time at both aging temperatures. As the aging temperature decreasing, the diameter and thickness appear to be low but volume fraction of $T_1$ particles increased. This is consistent with the results of CRSS measurements. This reflects that the distribution of fine and large amount of precipitation play a important role increament of CRSS values. The dislocation moving effectively impeded by $T_1$ particles of the plate type due to the strong particle strength.
The square lattice spacing, $L_s$ for plate type $T_1$ particles calculated from considering of orientation relationship and shape effects. The results of calculation, the square lattice spacing $L_s$ was independent of particle diameter and only dependent on the volume fraction and thickness.
From the analysis of modified chemical strengthening(using calculated $L_s$),one could calculate the interfacial energy between matrix and $T_1$ particles. As a result of analysis, experimental measured increment of CRSS well consistent with theoretical predictions by the modified chemical strengthening. If we assume that $T_1$ particles are weak obstacle, interfacial energy was estimated to be $γ_s=6.199 J/㎡$ and are strong obstacle, was determined to be $γ_s=7.184J/㎡$. From the calculation of particle strength c, it is considered that $T_1$ particles belong to strong obstacle.
Application : Effect of Ag Addition on the Precipitation and Mechanical Properties in Al-Li-Cu-Mg polycrystals
The effect of Ag on the precipitation and Mechanical properties have been investigated in Al-2Li-2.5Cu-0.5Mg(-Zr) alloys using transmission electron microscopy and mechanical testing.
The δ' PFZ(Precipitate Free Zone) width increased with square root of aging time because of growing of $T_2$ and particles. The PFZ width tends to decrease with Ag addition, it is considered the diffusion of Li atom limitted by the Ag addition. The precipitation of $T_1$ particles drastically increased by the stretching, but the precipitation of δ' and S' particles decreased. It was considered the supersaturations of Li and Cu reduce by the active precipitation of $T_1$ particles. The toughness appears to be decreased by prolonged thermal exposure treatment both alloy but the amount of decreament was very different, i.e.,60%(No Ag)and 30%(Ag added)respectively. The reason for this is that the growth rate of S' particles increase and the δ' particles appear to be bimodal distributions by the Ag addition.