Mechanical behavior of two-phase intermetallic compounds based on $L1_2$ $(Al,Cr)_3$Ti was investigated in view of microstructural controls. Al-21Ti-23Cr alloy has two phases consisting of mainly $L1_2$ phase and 20% $Cr_2Al$ phase. In this research, relationship between mechanical properties and microstructure was mainly studied by controlling the second phase, precipitation of third phase and grain size.
The composition of Al-21Ti-23Cr alloy is located in two-phase region at temperature of 1150℃ but in three-phase region at 1000℃ in Al-Ti-Cr phase diagram. From this point, the microstructure was modified by aging treatment below 1000℃ and its influence on mechanical properties was examined. Aging at 800℃ leads to precipitate third phase refinedly in matrix and increases compressive yield strength drastically but decreases compressive strain to failure a little. These phenomena are understood by increasing amount of precipitation phase and it is expected that aging after decrease in second phase will increase strength and ductility of the alloy simultaneously.
The Al-21Ti-23Cr alloy was recrystallized dynamically during isothermal forging at 1150℃, which refined the grain over ten times. Grain refinement increases tensile strength but don't have effect on ductility improvement at room temperature. However, this improves the ductility in 3-4% magnitude and changes fracture mode to intergranular fracture at 800℃.
In order to change the distribution of second phase, boron and zirconium were added in Al-21Ti-23Cr alloy. Most of boron are dissolved in second phase and most of zirconium are dissolved in matrix, which is confirmed by SIMS analysis. The addition of boron and zirconium has different effects on the solidification of second phase. Adding of 0.1 wt.% boron homogenized the distribution of second phase and made second phase more ductile. Adding of 0.25 wt.% zirconium segregated second phase by increasing constitutional super-cooling of $L1_2$ dendrite.
