Deformation and fracture behavior of two Al-Mg-Si alloys

Abstract

Deformation and fracture behavior of two Al-Mg-Si alloys in different aging conditions has been studied by tensile testing, transmission electron microscope (TEM), and scanning electron microscope (SEM) observation. Tensile test results show that the strain hardening exponents (n values) of the two alloys decrease sharply at the early stage of artificial aging and are only 0.045 and 0.06, respectively, in the overaged condition. The sharp decrease of work hardening rate is believed to be one major reason that results in the rapid decrease of elongation to failure at the early stage of artificial aging. In fully aged conditions, dislocations are concentrated in narrow bands during plastic deformation of these alloys, which is responsible for the very low n values of the Al-Mg-Si alloys in peak aged and overaged conditions. The Si particles formed in the interior of grains of the higher Si containing alloy reduce the inhomogeneous deformation behavior. The TEM results show that large precipitates and precipitate-free zones (PFZs) along grain boundaries are formed in peak aged and overaged conditions, and SEM observations demonstrate that the tensile fracture modes of the two alloys in these aging conditions are completely intergranular with many small cusps decorated on facets of the fractured grain boundaries. Thus, the fracture process of both alloys is suggested to be that in which the high local stresses, built up where the slip band impinges on the grain boundaries, nucleate voids at the grain boundary precipitates by decohesion of the particle/PFZ interface, and then coalescence of these voids within the PFZ leads to the final fracture of these alloys.

Impact properties of the aircraft cast aluminium alloy Al-7Si0.6Mg (A357)

“…In the A357 alloy, high quantities of Si may be observed only in the Si particles; they are preferably located at the grain boundaries of the Al-phase. This observation is indicative for an intercrystalline fracture mode and is consistent to previous investigations [14,15]. …”

Effect of Pre-Deformation on Mechanical Response of an Artificially Aged Al-Mg-Si Alloy

“…Precipitation and aging behaviour in AlMg-Si alloys have been extensively studied and characterized in detail by many groups over the last 10-15 years. [1][2][3][4][5] However, the complex interplay between deformation, precipitation and aging response is less characterized and understood. [6][7][8] To get a better understanding and quantitative description of the aging behaviour in deformed materials as a function of alloy chemistry and processing conditions a careful and detailed study on this topic is in progress by the present authors.…”

The Effect of Artificial Aging on Tensile Work Hardening Characteristics of a Cast Al-7 Pct Si-0.55 Pct Mg (A357) Alloy

“…The hardening precipitates in the underaged condition of A357 alloy are b¢¢ rods, which are initially sheared by dislocations due to their coherency with the aluminum matrix and their small size. [34] Even at the condition of peak strength of the material, b¢¢ precipitates are shearable. [12,35] With increasing artificial aging time beyond peak strength, the b¢¢ rods coarsen and reach a critical radius (~2.7 nm [36] ) beyond which the dislocations can only bypass them.…”