A Study on the Nucleation Mechanism of Nano-Scale Precipitates of 7055 Aluminum Alloy

Abstract

7055 aluminum alloy is a typical age-hardening alloy whose properties change with microstructural evolution as a result of heat treatment. In this paper, the microstructural evolution of the alloy after solid solution treatment and manual aging is examined using SEM, TEM and HADDF. The precipitation, nucleation and growth mechanisms of [Formula: see text] and [Formula: see text] phases are also analyzed. Results show that, as aging proceeds, the Al–Cu phase gradually enlarges and thickens the following precipitation sequence of supersaturated solid solution (SSSS) [Formula: see text] GPI zone [Formula: see text] GPII zone [Formula: see text] phase [Formula: see text] phase; some [Formula: see text] phases formed during aging, which have two different nucleation mechanisms: homogeneous nucleation and heterogeneous nucleation, the [Formula: see text] phases of the latter being marginally larger in size than those of the former. There is a transition in the transformation of (Al3Cu) in GPII zone into [Formula: see text]-phase (Al2Cu). The orientation relation between [Formula: see text] phase and Al matrix is (100)Al//(100)[Formula: see text], (010)Al//(010)[Formula: see text], (001)Al//(001)[Formula: see text]. According to this relationship, [Formula: see text] phase has three variants in the matrix: Variants 1, 2, and 3, which represent the [Formula: see text] phase along {100}Al, {010}Al and {001}Al. As [Formula: see text] phase has a special orientation relation with Al matrix (100)Al//(100)[Formula: see text], (010)Al//(010)[Formula: see text], (001)Al//(001)[Formula: see text], [Formula: see text] phase is distributed in disc form on {001}Al. Significantly larger [Formula: see text] phase can be observed. The orientation relation between S phase and Al matrix is [100]Al//[100]S, [021]Al//[010]S, [012]Al//[001]S. [Formula: see text] phase grows in step form for a period equaling to 1.75-fold unit cell of [Formula: see text]. As [Formula: see text] phase grows, the Cu atoms segregated on the edge of [Formula: see text] phase are gradually consumed.