Microstrucure and strengthening of Al–Li–Cu–Mg alloys and MMCs: I. Analysis and Modelling of Microstructural changes

Abstract

A complete and detailed analysis of the microstructural development during ageing in an 8090 (Al-2.3Li-1.2Cu-1Mg-0.1Zr) alloy, an 8090/20wt%SiC p MMC, an Al-1.5Li-Cu-Mg MMC and an Al-Cu-Mg MMC (all with similar Cu and Mg contents) has been performed. Volume fractions of all precipitates relevant for precipitation strengthening of the alloys (δ' phase, S' phase and GPB zones) have been determined using a recently derived method based on differential scanning calorimetry (DSC). The volume fractions have subsequently been successfully fitted using a novel model for transformation kinetics. The sizes of these precipitates have been analysed using newly derived expressions consistent with the latter model. As a result of dislocation generation around misfitting SiC particles the volume fractions of both GPB zones and S' phase depend strongly on the presence of these particles. Also the amount of Li present in the alloys influences the volume fractions of the phases significantly. The sizes of S' are similar for the four alloys. Introduction AimsMonolithic Al-based alloys for structural applications are generally strengthened by 4 basic mechanisms: precipitation strengthening, solution strengthening, grain and subgrain strengthening and strengthening by dislocations. The first two mechanisms depend strongly on composition of the alloy and on heat treatment and are generally employed in a metastable state, whilst the latter two mechanisms depend mostly on thermo-mechanical processing routes. In metal matrix composites (MMCs) additionally load transfer to ceramic inclusions in the matrix can increase the strength of the alloy (see e.g. [1,2]). Apart from this direct effect the presence of ceramic inclusions will also influence the precipitation in the matrix [1,3,4] and thus influence strengthening in an indirect way. The main aim of the present work and the companion paper [5] is to derive and validate a complete microstructure-property model for reinforced and unreinforced Al-Li-Cu-Mg type alloys and link this model with a novel model for the kinetics of microstructure development.

Al–Mg–Cu based alloys and pure Al processed by high pressure torsion: The influence of alloying additions on strengthening

“…In these applications they derive their strength mostly from solution strengthening and strain hardening [13]. The Al-Mg-Cu alloys with higher Cu content (>2.5wt%) that are being studied in this work are applied predominantly in the aeronautical industry due to their in-service strength, which is due primarily to precipitation hardening [14,15,16,17], combined with good damage tolerance properties [18]. Recent development in these types of alloys has seen the introduction alloys with Li at relatively low levels (<2wt.…”

DSC study of precipitation in an Al–Mg–Mn alloy microalloyed with Cu

“…For instance, in commercial Al-Cu-Mg alloys (e.g. AA2024: Al-1.6at%Cu-1at%Mg) the amount of Cu and Mg atoms that precipitate to form S phase is about 2at% [2,3], in an Al-1at%Si alloy the amount of Si that precipitates is about 1at% [4], in commercial Al-Zn-Mg-Cu based alloys the amount of precipitates is in the order of 4 to 7at% [5,6] (but these precipitates will contain some Al), in Al-Li-Cu-Mg alloys the amount of Li that precipitates to form δ' phase (Al 3 Li) is about 2 to 3 at% [7,8] and in Al-16at%Mg the amount of Mg atoms that precipitates to form β'' (Al 3 Mg) phase is about 6at% [9]. The Al alloys with the smallest amount of precipitation which have been studied by calorimetry are probably the Al-4.7at%Mg-0.25at%Cu-0.14at%Si alloy, in which about 0.5at% of Cu and Mg are expected to combine to form S phase (Al 2 CuMg) [10,11] and the Al-0.5at%Mn-0.3at%Fe based AA3003 alloy in which most of the Mn and Fe is thought to form intermetallic Al, Mn and Fe containing precipitates [12].…”

“…(The rod or lath shaped precipitates in Al-Cu-Mg alloys, which have often been indicated by S', are a slightly strained semicoherent version of the (incoherent) S phase. In recent works [7,13,14] several researchers have thus decided to discontinue the use of the indication S'. This is thought to be appropriate, and for the present paper we will not use the term S' phase, and instead indicate all precipitates with the same structure as S phase.)…”

Microstrucure and strengthening of Al–Li–Cu–Mg alloys and MMCs: II. Modelling of yield strength

“…[22,23]). Details on production route, compositions and heat treatments are given in the companion paper [19]. The grain sizes and PFZs were studied by TEM [19].…”

“…For instance, precipitation strengthening in 8090 MMCs contains contributions from modulus hardening, order hardening and dispersion hardening mechanisms, whilst load transfer is influenced by the formation of a precipitate free zone (PFZ) around the reinforcing particles. In the companion paper [19] the development of the microstructures of 4 reinforced and unreinforced Al-Li-Cu-Mg-Zr alloys during ageing was analysed and modelled. In the present paper these results are used to model and predict the strength of the reinforced and unreinforced Al-Li-Cu-Mg-Zr alloys at various stages of the ageing process.…”