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.

Evolution of dispersoids during multistep heat treatments and their effect on rolling performance in an Al-5% Mg-0.8% Mn alloy

“…It can be found that the cube-like dispersoids have a higher Mn content than that of rod-like ones. Combined the results of SADP and their TEM-EDS as well as information from the literature [5,13,17,29,30], the cube-like dispersoids seem mostly to be Al4Mn (Hexagonal, a=b=2.812 nm, c =1.274 nm, γ=120 o [31]), while the rod-like ones are believed to be Al6Mn (Orthorhombic, a=0.6498 nm b=0.7574 nm, c =0.887 nm [5,32]). However, α-Al(Mn,Fe)Si dispersoids were hardly detected in the experimental alloy, which is likely contributed to the higher Mg and Mn contents in the present work [5,14,17].…”

“…Fig. 6a shows a bright-field TEM image after the first-step treatment (275 °C/12 h), where a large number of plate-like precipitates appeared with a length range of 280-430 nm and a width of 24-32 nm, which were identified as β'-MgSi [10,32]. When the samples continued to be treated at 375 °C/48 h, the formation of dispersoids became more obvious with a large fraction of DZ (marked with a red dotted line) in addition to a small fraction of DFZ (marked with a yellow line), as shown in the optical image in Fig.…”