Electron microscopic investigation of precipitation hardening in Al-Si based alloys

Abstract

The present work was performed on Al-8.5%Si-1.75%Cu-0.55%Mg-0.2%Ti-0.32%Zr-0.015%Sr alloy (coded as 354 alloy) following T6 (at 160 °C) and T7 (at 240 °C) artificial aging conditions using high resolution transmission electron microscopy (HR-TEM). The main results inferred from the present study are the precipitation of GP zones in samples aged for 20 h at 160 °C along with S-Al2CuMg phase particles. Aging at 240 °C was carried out for different times, ranging between 30 min and 200 h. The quenched structures revealed a series of precipitations mainly GP zones at low aging times to Θ’-Al2Cu after 200 h together with the S-phase. No coherency was observed between the S-phase and the aluminum (Al) matrix. The growth of the S- phase takes place by precipitation of S2 type phase on the advancing S1-type phase. Mismatching was clearly observed at particle/matrix interfaces. High magnification images indicated the dissolution of some of the Θ” and Θ’ particles to enrich the nearby particles (coarsening). The start of incoherency is observed when the alloy is aged at 240 °C for 200 h.

Keywords

• HR-TEM
• Precipitation
• Aging
• Al-Si alloys
• Tensile strength