Identification of Metastable Rod-Like Particles in an Isothermally Aged Al–10Mg–0.5Ag (mass%) Alloy

Abstract

The structure and morphology of a precipitation product in an Al-10Mg-0.5Ag (mass%) alloy aged isothermally at 240 C have been characterised using transmission electron microscopy and microbeam electron diffraction. A metastable T phase (Mg 32 (Al, Ag) 49 , b.c.c., a ¼ 1:41 nm) which is identified by the electron microdiffraction patterns, has been found in the Al-10Mg-0.5Ag (mass%) alloy after solution treatment, water quenched and then aged for 2 h at 240 C. The orientation relationship between the T phase and matrix -Al phase was of the form ð010Þ T k ð11 1 1Þ and ½001 T k ½1 1 10 . Qualitative microanalysis suggested that the metastable crystalline T phase was a ternary compound that contained all three elements Al, Mg and Ag. The morphology of the metastable T phase is the h110i rod-like in shape and those precipitate particles are homogeneously nucleated, and finely and uniformly dispersed in the Al matrix. The icosahedral quasicrystalline phase was observed in the early stages of ageing (0.5 h at 240 C), and it was to be replaced by the metastable crystalline T phase Mg 32 (Al, Ag) 49 after the alloy is aged for 2 h at 240 C. The T phase formed as faceted rods parallel to h110i directions of the -Al matrix appeared to be the primary strengthening constitute exhibiting maximum hardness in the Al-10Mg-0.5Ag (mass%) alloy aged at 240 C.

Identification of β Phase Particles in an Isothermally Aged Al–10 mass%Mg–0.5 mass%Ag Alloy

“…22) are not the same compared with those of the equilibrium phase (f.c.c. ), 13) there is evidence of structural and chemical similarities between the T and phases identified in the aged samples in the Al-10Mg-0.5Ag (mass%) alloy.…”

“…Previous reports have indicated that the equilibrium phase is generally observed only in binary Al-Mg alloys and there has been no prior evidence of equilibrium phase forming in Ag-containing Al-Mg alloys. [18][19][20][21] Although based on the present TEM observations the formation mechanism for two forms of the equilibrium phase is not clear, from the distribution of the rod-like T phase particles reported in the previous paper, 22) it is presumed that the coarse globular particles may be derived from the metastable rod-like T phase particles, 22) while the finer spheroidal particles may be nucleated independently and homogeneously.…”

“…Firstly, lattice parameter of the phase (a ¼ 2:824 nm) 13) is approximately twice larger than that of the T phase (a ¼ 1:41 nm). 22) Secondly, the ratio of constitute elements between Al:Mg in the compounds of the phase (Al 3 Mg 2 ) and T phase (Mg 32 (Al,Ag) 49 ) is approximately the same of 1 : $1:50. These similarities would imply a possibility for the formation of the coarse globular phase transformed from the T phase during the isothermal ageing process in the Al-10Mg-0.5Ag (mass%) alloy.…”

“…At the peak-aged stages of ageing (e.g. 2 h at 240 C), 22,23) fine-scale, uniformly distributed rod-like particles were observed. When the ageing time was increased to 72 h at 240 C, the microstructure was dramatically changed as shown in Fig.…”

“…It thus remains that precipitate microstructures and precipitate phases in Al-Mg-Ag alloys during isothermal ageing are not thoroughly and convincingly understood. The purpose of this paper is to report results of a detailed microstructures examination of the equilibrium precipitate phase transformed from the metastable rod-like precipitate T phase 22) during isothermal ageing process in an Al-10Mg-0.5Ag (mass%) alloy. Identification of the equilibrium precipitate phase was carried out using the transmission electron microscopy with the electron diffraction techniques.…”