Capillarity Effect Controlled Precipitate Growth at the Grain Boundary of Long-Term Aging Al 5083 Alloy
- PDF / 1,857,002 Bytes
- 12 Pages / 593.972 x 792 pts Page_size
- 88 Downloads / 199 Views
tance for automotive and ship structural applications owing to their balance of strength, formability, weldability, and corrosion resistance.[1–3] However, alloys containing more than 3 wt pct Mg can become unstable with time at moderately elevated service temperature for the precipitation of Mg-rich b phase (Al3Mg2) at grain boundaries.[4] This leads to the susceptibility to intergranular corrosion (IGC) and stress corrosion cracking (SCC).[5–7] According to previous research, IGC and SCC behavior of Al 5xxx depends strongly on the size and dispersion of grain boundary precipitates.[8] Modeling of grain boundary precipitation behavior has been carried out by several research groups. Aaron and Aaronson[9] first developed a model for grain boundary precipitation. By realizing the rapid lengthening and thickening rates of allotriomorphs can’t be explained by a volume diffusion mechanism[10] alone, they accounted for the role of grain boundary short circuit diffusion using a collector plate mechanism. In their model, thickening of h phase (CuAl2) was controlled by diffusion of Cu at the interface of a:h, and lengthening was controlled by diffusion from the bulk to the grain boundary and then to the advancing edge of h phase. Their theory was successfully applied to Cu-Al alloys. GAOSONG YI and YAKUN ZHU, Ph.D. Candidates, MICHAEL L. FREE, Professor, and ALEXANDER T. DERRICK, Master Candidate, are with the Department of Metallurgical Engineering, University of Utah, 135 S 1460 E, Rm 412, Salt Lake City, UT 84112. Contact e-mail: [email protected] Manuscript submitted January 25, 2014. Article published online July 29, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
Based on the theory of diffusion along grain boundaries developed by Fisher[11] and Shewmon,[12] and the theory of the surface morphology change resulting from diffusion developed by Mullins[13] and Nichols and Mullins,[14] Brailsford and Aaron[15] put forward a theory to solve simultaneously the lengthening and thickening rate of h phase at the grain boundary of Cu-Al alloys. In their research, an equation was used to describe the shape of h phase, and a corresponding mass balance of Cu was taken into account. The model was solved numerically, and the results agreed well with experimental observation. Faulkner and Caislery[16] combined the approaches of Aaron and Aaronson for grain boundary precipitation in Nimonic PE16. In order to avoid the complicated mathematical approach of solving lengthening and thickening simultaneously, they assumed a constant aspect ratio throughout the early stage of growth, which led to a rough assessment of overall size of precipitates as a function of time and temperature. Carolan and Faulkner[17] modified the previous collector plate mechanism based theory and applied it to the precipitation of M23C6 precipitation at the grain boundary of austenitic steel. They adopted a time-dependent collector plate and took into account different precipitation morphologies such as cap, faceted, disk and conical shapes. The model accurately predict
Data Loading...