DEFORMATION AND FRACTURE-BEHAVIOR OF 2 AL-MG-SI ALLOYS

Deformation and fracture behavior of two Al-Mg-Si alloys in different aging conditions has been studied by tensile testing, transmission ele ctron microscope (TEM), and scanning electron microscope (SEM) observa tion. Tensile test results show that the strain hardening exponents (n values) of the two alloys decrease sharply at the early stage of arti ficial aging and are only 0.045 and 0.06, respectively, in the overage d condition. The sharp decrease of work hardening rate is believed to be one major reason that results in the rapid decrease of elongation t o failure at the early stage of artificial aging. In fully aged condit ions, dislocations are concentrated in narrow bands during plastic def ormation of these alloys, which is responsible for the very low n valu es 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 containin g 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 tw o alloys in these aging conditions are completely intergranular with m any 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 o n the grain boundaries, nucleate voids at the grain boundary precipita tes by decohesion of the particle/PFZ interface, and then coalescence of these voids within the PFZ leads to the final fracture of these all oys.