Application of aberration-corrected scanning transmission electron microscopy in conjunction with valence electron energy loss spectroscopy for the nanoscale mapping of the elastic properties of Al–Li–Cu alloys

Abstract

The stress and strain play an important role in strengthening of the precipitation-hardened Aluminum (Al) alloys. Despite the determination of relationship between the mechanical properties and the precipitation existing in the microstructure of these alloys, a quantitative analysis of the local stress and the strain fields at the hardening-precipitates level has been seldom reported. In this paper, the microstructure of a T8 temper AA2195 Al alloy is investigated using aberration corrected scanning transmission electron microscopy (AC-STEM). The strain fields in Al matrix in the vicinity of observed precipitates, namely T1 and β′, are determined using geometric phase analysis (GPA). Young's modulus (Ym) mapping of the corresponding areas is determined from the valence electron energy loss spectroscopy (VEELS) measured bulk Plasmon energy (Ep) of the alloys. The GPA-determined strains were then combined with VEELS-determined Ym under the linear theory of elasticity to give rise the local stresses in the alloy. The obtained results show that the local stresses in Al matrix having no precipitates were in the range of 138 ± 2 MPa. Whereas, in the vicinity of thin and thick T1 platelet shape precipitates, the stresses were found to be about 202 ± 3 MPa and 195 ±3 MPa, respectively. The stresses measured in the vicinity of β′ spherical shape precipitates found out to be 140 ± 3 MPa which was near to the local stress value in Al matrix. Our findings suggest that the precipitate hardening in T8 temper AA2195 Al alloy predominantly stems from thin T1 precipitates.