The Structure and Kinetics of the Nanoscale Precipitation Processes in Al-1.0 wt pct Mg 2 Si-0.4 wt pct Mg-0.5 wt pct Ag
- PDF / 981,983 Bytes
- 7 Pages / 593.972 x 792 pts Page_size
- 95 Downloads / 243 Views
INTRODUCTION
THE demand of Al-alloys for automotive applications is increasing as the manufacturers look for high strength and light weight materials.[1–3] In Al-Mg-Si (6000 series), the developed nanoscale precipitates as a result of aging at an intermediate temperature, which are responsible for strengthening the alloys are classified into three successive types. Fully coherent b¢¢-precipitates, semi-coherent or partially coherent b¢-precipitates and incoherent equilibrium b-precipitates are successively precipitated.[4–6] The b¢¢-precipitates are needleshaped zones along h100i direction, typically 1 to 2 nm diameter and about 10 nm long. The b¢-precipitates are rod-shaped (circular cross sections) along h100i, having a hexagonal crystal structure with a = 0.705 nm and c = 0.405 nm.[4] The b precipitates are platelets on {100} of Al having an FCC CaF2 structure with a = 0.639 nm. Although the effect of Cu addition on the precipitation sequence in Al-Mg-Si alloys has been studied extensively, a few publications on the effect of silver (Ag) addition has been published.[7–10] Polmear[7] had earlier reported that addition of small amount of silver (about 0.1 at. pct) to Al-Mg-Si alloys exerts a ATIF MOSSAD ALI, Associate Professor, formerly with the Department of Physics, Faculty of Science, Assiut University, Assiut 71516, Egypt, is now with the Department of Physics, Faculty of Science, King Khalid University, Abha, Saudi Arabia. Contact e-mail: [email protected], [email protected] ABDEL-FATTAH GABER, Professor, is with the Department of Physics, Faculty of Science, Assiut University. KENJI MATSUDA and SUSUMU IKENO, Professors, are with Department of Materials Science and Technology, Faculty of Engineering, Toyama University, Toyama, Japan. Manuscript submitted March 8, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
marked effect on their structure and aging characteristics. It was found that Ag stimulates nucleation of intermediate precipitates which form when the alloy was aged in the medium temperature range [393 K to 493 K (120 C to 220 C)]. The mechanism for this effect was uncertain although it was suggested that it originates in a preferred affinity between Mg and Ag atoms.[7] Nakamura et al.[8] and Matsuda et al.[9] investigated the effect of silver addition on crystal structure of metastable phase (b¢-phase) in Al-Mg-Si alloy. They observed a rod-shaped precipitate in Ag containing AlMg-Si alloy similar to those of b¢-phase in Al-Mg-Si alloy without Ag, however, it has different lattice spacing.[8,9] More recently, Nakamura et al.[10] investigated the distribution of silver using energy filtered mapping and high-angular dark field scanning transmission electron microscopy image. They reported a strong silver enrichment area observed at the precipitates interface with the aluminum matrix. Most of these Ag replace Al in FCC sites.[10] The properties of heat-treatable automotive alloys depend on their microstructure, which is controlled by alloy chemistry and processing. However, the material is characterize
Data Loading...
