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

Abstract

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.

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

“…5, the microstructures contained coarse globular particles. Based on the previous investigation of precipitate phases, 7) these coarse-scale globular particles were identified to be metastable 0 and/or stable phases. 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.…”

“…6) In the recent work, the other orientation relationship between the phase and -Al matrix in the aged Al-10 mass%Mg alloy has been reported as the cube-on-cube orientation, such as ð1 1 11Þ k ð1 1 11Þ and ½011 k ½1 1 1 1 1 , or ð100Þ k ð100Þ and ½001 k ½001 , depending upon morphologies of the precipitate particles. 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.…”

“…In order to improve fundamental understanding of the ageing behaviour in Al-Mg alloys with and without Ag, the present work has thus paid attention to the 0 and phases [1][2][3][4][5][6][7] using TEM. The purpose of this paper is to report further results of a microstructural examination of the metastable 0 and equilibrium precipitate phases formed during isothermal ageing process in an Al-10Mg alloy with and without 0.5Ag (mass%).…”

Effect of Trace Additions of Ag on Precipitation in Al–Mg Alloys

“…10,22) With prolonged ageing time, the hardness decreased gradually 24) and during overageing the T phase particles were replaced by a bimodal distribution of the equilibrium phase (Al 3 Mg 2 ). 9,23) This distribution corresponded coarse-scale ($500 nm), globular particles together with fine-scale ($100 nm), sparsely distributed spheroidal particles. According to early work on Al5Mg-0.5Ag alloy using the bulk X-ray diffraction technique, 6) the early stages of the precipitation hardening response was attributed to the formation of GP zones, while the maximum hardness was associated with formation of what was described as equilibrium T phase.…”

“…Detailed characterisations of those particles using TEM will be discussed elsewhere. 23) 4. Discussion 4.1 Precipitation hardening response and precipitation sequence It has been reported that the age-hardening response of the Al-10Mg alloy is dramatically enhanced by trace additions of Ag.…”