The effect of trace additions of sn on precipitation in Al-Cu alloys: An atom probe field ion microscopy study
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I.
INTRODUCTION
TRACEelement or microalloying additions of Sn, Cd, or In are well known to increase the hardening response of A1-Cu alloys during elevated temperature (130 ~ and 190 ~ aging.t 1-4] Figure 1 shows that the rate of hardening of the Sn-containing alloy is faster and that a significantly higher level of hardness is possible. It is also seen that hardening of the Sn-containing alloy proceeds through a single stage at both temperatures and this is associated with the preferred nucleation of the 0' phase at the expense of Guinier-Preston (GP) zones and 0".[4] Additions of Sn and Cd as slight as 0.0! at. pct are effective, and these have been used in commercial Al-alloy 2021 (A1-2.5Cu-(0.1Mn, 0.1Fe, 0.2Si, 0.02Mg, 0.01Cd, 0.01Sn)) (at. pct) which, as rolled plate, has a tensile yield strength of 435 MPa, ultimate tensile strength of 505 MPa, and elongation of 9 pct.[51 This alloy meets requirements for higher tensile properties with no loss in weldability or toughness at cryogenic temperatures and has application in aircraft parts and structures as well as fuel tanks in aerospace and space vehicles. Hence, it is important to understand the strengthening mechanism behind these microalloying additions. The electron micrographs in Figures 2(a) and (b) compare the peak hardness microstructure in the Sn-free and Sn-containing AI-I.7Cu (at. pct) alloy following aging at 190 ~ These micrographs illustrate that ternary additions of Sn result in an extremely fine and uniform dispersion of S.P. RINGER, formerly Japan Society for the Promotion of Science Postdoctoral Research Fellow, Institute for Materials Research, Tohoku University, is Lecturer, Department of Materials Engineering, Monash University, Clayton, Victoria 3168, Australia. K. HONO, formerly Research Associate, Institute for Materials Research, Tohoku University, is Senior Researcher, Materials Design Division, National Research Institute for Metals, Tsakuba 305, Japan. T. SAKURAI, Professor, is with the Institute for Materials Research, Tohoku University, Sendai 980-77, Japan. Manuscript submitted December 2, 1994. METALLURGICALAND MATERIALS TRANSACTIONS A
0' precipitates, which effectively strengthen the alloy. Similar results have been found for the In- and Cd-containing alloys. This general effect on microstructure was first reported by Polmear and Hardy,[61and its origin has been the subject of considerable investigation for the last 4 decades.tU6, 9=26]An important question raised in these studies concerns the mechanism by which the Sn (Cd, In) additions promote the nucleation of the 0' phase, largely at the expense of GP zones and 0". The ~ phase possesses a body-centered tetragonal (bct) structure, with lattice parameters a = 0.404 nm and c = 0.580 nm [271 and orientation relationship {001 }a//{001 }0., (001)cd/(001)0,.[28~ This structure exhibits 4/mmm point group symmetry, allowing three crystallographically equivalent variants, which occur as octagonal-shaped platelets on matrix {001 }~ planes. Even after extended aging, the broad faces of
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