Correlative STEM-EELS and First-Principles Probing of Nanoscale Elastic Heterogeneity Around T1 and Al3Zr Phases in AA2195 Al-Li-Cu Alloy.

Abstract

The local elastic response of the Al matrix surrounding T₁ precipitates and Al₃Zr dispersoids in a T8-treated AA2195 Al-Li-Cu alloy was investigated using a correlative approach combining advanced scanning transmission electron microscopy, low-loss electron energy-loss spectroscopy, image simulation, and first-principles calculations. Aberration-corrected HAADF-STEM and STEM-EDS confirmed the presence of plate-like T₁ precipitates and spheroidal Al₃Zr dispersoids, with Cu enrichment in the T₁ phase and Zr enrichment in the dispersoids. Low-loss STEM-EELS spectrum imaging was used to probe the local valence-electron response by analyzing the first bulk plasmon peak of the Al-rich matrix. Quantitative fitting of the plasmon peak revealed spatial variations in peak amplitude, full width at half maximum, and energy position near the precipitate-matrix interfaces, indicating local changes in electron density, bonding environment, and interfacial strain. Using an empirical plasmon energy-elastic modulus relationship, local effective Young's modulus values of approximately 70.4 GPa for the Al matrix, 69.3 GPa for the T₁-adjacent Al-rich matrix/interface region, and 72.1 GPa for the Al₃Zr-adjacent Al-rich matrix/interface region were obtained. These values reveal nanoscale elastic heterogeneity associated with the distinct precipitate-matrix interfacial environments. QSTEM-based multislice HAADF-STEM simulations reproduced the experimental T₁ contrast and validated the Cu-rich layered atomic arrangement of the precipitate. Density functional theory calculations were further performed on idealized Al/T₁ and Al/Al₃Zr interface supercells. Strain-energy analysis yielded effective directional Young's moduli of approximately 78 GPa and 73 GPa, respectively, for the selected interface configurations. The combined experimental and computational results show that the local elastic response of AA2195 is governed by precipitate chemistry, interfacial coherency, lattice mismatch, and electronic-structure modification. This work establishes low-loss STEM-EELS plasmon mapping, supported by atomistic simulations and DFT calculations, as a powerful framework for linking nanoscale structure, chemistry, and elasticity in precipitation-strengthened Al-Li-Cu alloys.