The transition phase η′ in AlZnMg alloys

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.

Section: Discussionmentioning

confidence: 51%

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

Kubota¹,

Nie²,

Muddle³

2005

“…It is worthy to note 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 Agcontaining Al-Mg alloys. [8][9][10] However, the previously report 7) and current research revealed to form the equilibrium phase in the ternary Al-10Mg-0.5Ag (mass%) alloy. This phase could be the products of transformation from the metastable rod-like T particles, but based on the present TEM observations the formation mechanism is not clear.…”

Section: Discussionmentioning

confidence: 88%

“…7) There has been little attention paid to the precipitation sequence in ternary Al-Mg-Ag alloys aged above the GP zone solvus. Early studies [8][9][10] suggested that the decomposition process might be summarised as follows: Supersaturated solid solution ( 12) as the equilibrium phase in the aged Al-Mg-Ag alloy. Recent studies of the microstructural observations by transmission electron microscopy (TEM), 13,14) however, revealed the formation of both the quasicrystalline phase rather than T 0 phase in the early stages and the equilibrium phase in over-aged conditions in Al-10Mg-0.5Ag (mass%) alloy.…”

Section: Introductionmentioning

confidence: 99%

Observation of Beta Phase Particles in an Isothermally Aged Al-10 mass%Mg Alloy with and without 0.5 mass%Ag

Kubota

2008

Characterization of the 0 and precipitate phases formed in the aged Al-10 mass%Mg alloy with and without 0.5 mass%Ag at 200 C and 240 C has been carried out using TEM. The plate-like precipitate particles were formed in binary alloys aged from 2 h at 200 C. The plate-like particles had habit planes of f100g , and were identified as the 0 phase. Whereas the very fine scale, uniformly distributed precipitate particles with a spherical shape were formed as the dominant precipitate particle in ternary alloy aged at 200 C for periods of 4 h and 6 h. These small spherical particles were identified to be icosahedral phase while no icosahedral phase was observed in the Ag-free alloys when samples were aged at 200 C. Formation of a precipitate cluster consisting of two distinguishable parts has been observed in binary alloys aged 5 h at 240 C, and these precipitate particles were identified as the equilibrium and metastable 0 phases. Whereas a higher volume fraction of dispersed precipitate phases which comprised a mixture of a coarse-scale, rod-like particles and blocky particles, was observed in ternary alloy aged 5 h at 240 C. With further increase of ageing time up to 24 h at 240 C, the microstructure contained a high volume fraction of coarse-scale, globular precipitate particles, together with a secondary distribution of fine-scale spheroidal precipitates, these coarse globular particles were identified to be the equilibrium phase.

“…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.…”

Section: Discussionmentioning

confidence: 90%

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

Kubota¹

2005

The structure and morphology of coarse-scale globular and smaller spheroidal particles in an isothermally aged Al-10Mg-0.5Ag (mass%) alloy have been charcterised by means of transmission electron microscopy. Coarse-scale globular and smaller spheroidal particles were commonly observed in an over-aged sample aged for 72 h at 240 C. The coarse-scale globular and smaller spheroidal particles were identified by electron microdiffraction to have the face-centered cubic structure of equilibrium phase (Al 3 Mg 2 ) (lattice parameter a ¼ 2:824 nm). The orientation relationship between the phase and -Al matrix was such that: ð100Þ == ð100Þ and ½001 == ½001 . This orientation relationship differs from those reported previously for the phase formed in binary Al-Mg alloys. The coarse-scale globular particles had invariably internal faulted structures.