On the role of the solute partitioning and chemistry in initial precipitation of Ω plates

Abstract

The control of the precipitation of Ω phase is the critical factor that determines the mechanical properties and performance of Al-Cu-Mg-Ag alloys at elevated temperatures. As the most important components for Ω nucleation, the Mg and Ag additions usually plays a key role in affecting the precipitation kinetics of Ω phase. In order to clarify the intrinsic association between the bulk Mg (and Ag) content on the initial Ω precipitation, quantitative atom probe tomography (APT) technique is conducted to obtain the evolution of Ω chemistry in different underaged Al-Cu-Mg-Ag alloys with various bulk Mg and Ag contents. Distinct solute partitioning behavior, which strongly depends on the Mg or Ag variations, is clearly identified by APT analysis, and well explains the different Ω precipitation kinetics and strength properties of underaged Al-Cu-Mg-Ag alloys. The Mg or Ag concentration of Ω phase exhibits a linear increasing trend with the increasing bulk Mg or Ag content. This progressive enrichment of Mg or Ag solutes in Ω interior is believed to reduce its elastic strain energy, finally facilitating the enhanced precipitation of Ω phase. Analysis of Ω chemistries in different alloys also indicates that the Mg/Ag atomic ratios of Ω phase are close to that of Al-Cu-Mg-Ag alloys with high Cu/Mg weight ratios. The unchanged Cu concentration of Ω phase show its independence on the bulk Mg or Ag variations. The strength calculation further proves that the formation of high density Ω plates with increasing Mg or Ag content is an effective method to harden the materials by precipitation strengthening mechanism. The present results help to obtain the desirable strength properties of high Cu/Mg weight ratio Al-Cu-Mg-Ag alloys by controlling the Ω chemistry.