Aging of an Al-Mg-Si Alloy with a Silicon Excess and Reinforced with Ceramic Particles
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INTRODUCTION
IN
a recent paper,[1] the overaging of an Al-Mg-Si alloy with a high silicon excess was investigated by combining both global and local techniques. This type of alloy is particularly interesting from an industrial point of view with respect to pseudo-binary age-hardenable Al-Mg2Si alloys. Indeed, numerous studies have reported that the presence of a silicon excess tends to increase the maximum mechanical strength reached by the alloy during a T6 treatment (typically 8 hours at 443 K (170 C) and highlighted that the greater the silicon excess, the greater the mechanical strength.[2,3] However, there are two main drawbacks associated with silicon excess: (i) the acceleration of the overaging phenomena leading to a sharp drop in mechanical strength after having reached the peak strength and (ii) a complexification of the precipitation sequence. This is particularly notable in the temperature domain of the semi-coherent precipitation where the b¢ phase (HCP, a = 0.705 nm, c = 0.405 nm) typical of the quasi-binary alloys and its precursor, denoted here b¢0, (HCP, a = 0.407 nm, c = 0.405 nm), can be replaced or
coexist with phases specific to Al-Mg-Si alloys with silicon excess such as the phases of Type-A (HCP, a = 0.705 nm, c = 0.67 nm), Type-B (orthorhombic, a = 0.68 nm, b = 0.79 nm, c = 0.405 nm), and Type-C (HCP, a = 1.04 nm and c = 0.405 nm).[4–12] In the study presented in paper,[1] the precipitation kinetics of the investigated alloy (initially homogenized and water-quenched) were first followed indirectly by ThermoElectric Power (TEP) and Vickers hardness measurements during isothermal treatments performed at temperatures between 373 K and 623 K (100 C and 350 C). The complex TEP variations detected during aging were found to be perfectly correlated with the hardness variations, indicating that TEP is sensitive to overaging. Secondly, the treatment conditions leading to important TEP and hardness variations were characterized by Conventional and High-Resolution Transmission Electron Microscopy (C-TEM and HR-TEM) and by Atom Probe Tomography (APT). This led us to define the following precipitation sequence between 373 K and 573 K (100 C and 300 C)[1]: b00 ! b00 þ Type - B ! Type - B=Type - A=Type - C =b0 þ Si particles ! Type - A þ Type - C þ Si particles:
G. MEYRUEY, V. MASSARDIER, and M. PEREZ are with the University of Lyon, INSA Lyon, MATEIS – UMR CNRS 5510, Baˆt. Saint-Exupe´ry, 25 Avenue J. Capelle, 69621 Villeurbanne Cedex, France. Contact email: [email protected] Manuscript submitted September 24, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
The precipitation of the b¢¢ phase occurs in the early stages of aging. It was found to be responsible for the increase in strength of the alloy to its maximum value
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b¢
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