Improving ageing kinetics and precipitation hardening in an Al-Mg-Si alloy by minor Cd addition

Abstract

This study reports significant improvements in both the age hardening kinetics and peak hardness of an Al-Mg-Si alloy by microalloying with 0.06 at.% Cd. A detailed high-angle annular dark-field scanning transmission electron microscopy study shows that most of the needle-shaped hardening precipitates β" (~82%) formed in the Cd-containing alloy are attached to or around Cd-rich precipitates, indicating that the Cd-rich precipitates can act as effective nucleation sites for needle-shaped precipitates, and therefore substantially increase the number density and decrease the size of needle-shaped precipitates. It also shows that most of the β" precipitates in the Cd-containing alloy have a disordered structure, even at the peak-aged state.Additionally, Cu and/or Cd atoms are found to be incorporated into β" and some precipitates with Q'/C-like triangular sub-units are formed, which are very rare in Al-Mg-Si alloys with a trace Cu content of 0.01 wt.%.

Newer Developments in Aluminium Alloys Through Ordered Precipitates and Segregation of Transition Elements

“…The authors reported the enhanced precipitation of the b'' metastable phase due to increased quenched-in vacancies. Though the heat treatment modifications like pre-ageing, intermediate quenching, etc., suppress the determinantal effects of natural ageing, the alloying addition with Ge [52,53], Cu [10,11,54,55], Cd [56], Ag [57][58][59], Zn [60], Li [52], Sc and Zr has recently attracted attention due to the accelerated ageing response and significant improvement in strength. The enhanced ageing response is related to the modification in the precipitation sequence by the alloying additions.…”

“…Due to similar electronic properties, Ge was reported to replace Si atoms in the b'' phase [53], while Li was reported to substitute Mg sites in the b'' structure [52]. Cd forms Cd-rich precipitates in the early stage of artificial ageing and acts as nucleation sites for needle-shaped b'' precipitates that lead to the observed Q'/C type triangular sub-units in the matrix [56]. HAADF-STEM further reveals that Cd-atoms incorporated into b'' precipitates leads to a disordered structure.…”

Modelling the Age-Hardening Precipitation by a Revised Langer and Schwartz Approach with Log-Normal Size Distribution

“…wherein the first term on the right of Eq. [10], vð rÞ is the approximation for the growth rate of particles, which can be described with the classic Zener equation. [35] The second term corresponds to the change of r contributed by dissolution of u(r*)dr* particles with radius r*+dr* >r>r*.…”

“…According to Eq. [10], an increase of the mean particle radius is contributed by the Zener growth equation and dissolution of particles with instantaneous radius smaller than r* at late aging (coarsening) stage, wherein the nucleation rate at this stage is approaching~0, with no contribution of newly nucleated particles to mean particle radius. As a result of the same aging temperature, the diffusion controlled Zener growth equation is not supposed to cause large discrepancy in the growth [49] 1.342 9 10 À19 m 2 /s Diffusion Mobility of Si in Matrix [49] 2. rate of the two precipitates.…”

“…[1][2][3][4][5][6] 6xxx and 7xxx aluminium alloys can generally be precipitation strengthened via artificial aging, wherein complex precipitation of multiple precipitates occurs, contributing to the hardening of the material. [5,[7][8][9][10][11] The extent of precipitation strengthening is largely determined by the precipitate shape, size, and number density. Up to now, various modelling approaches have been developed to predict the precipitation behavior of secondary phases during age-hardening process.…”