Morphology of Dispersoids in an Annealed Al-Mg Alloys

Abstract

Morphology of dispersoids in an annealed Al-Mg alloy were investigated using TEM. Five kinds of dispersoids with different structures and morphologies were observed in an annealed Al-Mg alloy. The 1st, or spherical-like one is monoclinic structured θ-Al45(Mn,Cr)7 phase with twin and orientation domain. The 2nd or plate-shaped one is η-Al5(Mn,Cr) phase with monoclinic or pesuo-tetragonal structure. The 3rd or prismatic-like one is a new hexagonal structured Al6.4Mn phase with a unit cell of a=1.72nm, c=1.27nm, and γ=120°, and the 4th or big rod-shaped one is orthorhombic structured Al6(Mn,Fe) phase which is often reported. The 5th one is E-Al18Mg3(Mn–Cr)2 phase with twin or triple twin observed occasionally in Al-Mg annealed alloy. The first two of dispersoids are in majority, followed by the middle two and a small number of the fifth. Formation mechanisms of these particles in Al-Mg alloy are discussed according to phase diagram and possible formation of the twins in the particles are described based on minimum energy.