The effects of Ti addition (0.1 wt%) on the microstructure evolution and mechanical properties of an Al–7.6Zn–2.6Mg–2.0Cu–0.1Zr alloy were investigated during solidification, extrusion, solution treatment, and aging. The addition of Ti reduces the grain size and degree of phase agglomeration during solidification, thereby improving the strength and ductility of the as-cast alloy. The extrusion forms a bimodal structure consisting of fine dynamically recrystallized (DRXed) grains and coarse elongated unDRXed grains. Ti induces the refinement of the η-phases that enhances dynamic recrystallization (DRX) by particle-stimulated nucleation, resulting in reduced strength of the as-extruded alloy. The dissolution of the η-phase occurs in the initial stages of the solution treatment, followed by coarsening of the η-phase. The solution treatment causes static recrystallization and grain growth, increasing the grain size. The addition of Ti decreases the size and increases the number density of the L12 precipitate by modifying the chemical composition of (Al,Zn)3Zr into (Al,Zn)3(Zr,Ti), and causes the formation of an Al18Mg3Ti2 phase during the solution treatment. The addition of Ti also enhances heterogeneous nucleation of η-Mg(Zn,Cu,Al)2 at the Al18Mg3Ti2 interface and grain boundary but has a negligible effect on the formation of nanoprecipitates (GPII zone, η') in the matrix. The improved strength and ductility of the solution-treated and aged alloys are attributed to the higher number density of the fine L12-(Al,Zn)3(Zr,Ti) precipitates and grain refinement, as well as the finer η-phases acting as cracking sites.
Bibliographical noteFunding Information:
This work was supported by the Industrial Strategic Technology Development Program ( 10062304 ) funded by the Ministry of Trade, Industry & Energy (MOTIE, Republic of Korea) . Electrical conductivity work made use of the MatCI supported by the MRSEC program of the National Science Foundation ( DMR-1720139 ), and atom probe tomography was performed at the Northwestern University Center for Atom-Probe Tomography (NUCAPT). The LEAP tomograph at NUCAPT was purchased and upgraded with grants from the NSF-MRI ( DMR-0420532 ) and ONRDURIP ( N00014-0400798 , N00014-0610539 , N00014-0910781 , N00014-1712870 ) programs. NUCAPT received support from the MRSEC program ( NSF DMR-1720139 ) at the Materials Research Center, the SHyNE Resource ( NSF ECCS-1542205 ), and the Initiative for Sustainability and Energy (ISEN) at Northwestern University . We thank Dr. Se-Hun Kim (KATECH) for performing the extrusion process and Profs. David Seidman and Dieter Isheim ( Northwestern University ) for supporting the APT experiments.
All Science Journal Classification (ASJC) codes
- Materials Science(all)
- Condensed Matter Physics
- Mechanics of Materials
- Mechanical Engineering