STUDY OF THREE-STAGE THERMAL TREATMENT EFFECT ON (AlSi)3(Sc,Zr) AND (Mg5Si6) TYPE STRENGTHENING PARTICLES FORMATION IN Al–Mg–Si SERIES ALLOYS WITH LEAN Sc AND Zr ADDITION

10.25712/ASTU.1811-1416.2024.01.012

Abstract

_0.6Mg₁Si alloy with scandium (0.05 %) and zirconium (0.15 %) additions was studied to understand the special features of its strengthening with the aid of both β''-phase (Mg₅Si₆) and Al₃Sc (and its modifications). _0.6Mg₁Si_0.05Sc_0.15Zr was subjected to multi-stage thermal treatment with the following parameters: 550 °С 8 h + 440 °С 8 h + 500 °С 0.5 h + 180 °С 5 h; for _0.6Mg₁Si alloy: 550 °С 8 h + 180 °С 5 h. Fine strengthening particles were studied using transmission electron microscopy (TEM). In addition, mechanical properties were defined after each stage of thermal treatment. It was found out, that Sc and Zr additions facilitate triple yield strength and double ultimate strength improvement if thermal treatment is not applied. Base alloy demonstrates better strength characteristics after thermal treatment compared to the alloy with zirconium and scandium additions. Lower strength values are mainly associated with lower number of β''-phase (Mg₅Si₆) in the alloy with scandium additions vs base alloy during natural aging. This is related to inability to perform full-scale hardening of the alloy with scandium contents, as it will result in dissolution of formed (AlSi)₃(Sc,Zr) particles. At the same time the number of (AlSi)₃(Sc,Zr) particles, formed during thermal treatment, is fairly small, they have non-equiaxed shape and do not significantly contribute to strengthening.