Modulated structures and GP Zones in Al-Mg Alloys
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INTRODUCTION
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A1-Mg alloys have been widely used for commercial wrought and casting alloys due to their high resistance to corrosion. Wrought alloys contain 1 to 5 wt pct Mg and casting alloys - 5 and - 1 0 wt pct Mg. In general, both wrought and casting alloys are used without precipitation hardening treatment. AI-10 wt pct Mg casting alloys are used after solution treatment, so that the/3 phase (A13Mg2) that crystallizes in the course of solidification is redissolved in the matrix. In this case, a good ductility is achieved by the solution treatment but the alloys later exhibit such unfavorable changes as a decrease in ductility and an increase in the susceptibility to stress corrosion cracking, together with an increase in the mechanical strength, 1,2 due to subsequent natural aging over a period of several years. In a previous paper, 3 it was concluded that these changes in properties during natural aging were caused mainly by the formation of GP zones within grains. The formation of GP zones in A1-Mg alloys, that is, the early stage of the precipitation process during low temperature aging, was investigated by means of electrical resistivity measurement, 4,s X-ray diffraction, and electron microscopy. 3'6-9These results suggested that GP zones (Mgrich clusters) formed in the early stages of low temperature aging as in most aluminum alloys, for example, the conventional A1-Cu and A1-Zn alloys, and had a spherical form and some ordered lattice structure. The structure of GP zones, however, was not completely clarified because the difference in scattering amplitudes between A1 and Mg atoms and the volume fraction of GP zones formed at the beginning of low temperature aging is small. The elucidation of the precipitation behavior during the early stages of low temperature aging in these alloys is important for clarifying the changes in the various properties during natural aging. The purpose of this paper is to confirm the precipitation behavior at the beginning of aging and to clarify the structure of GP zones using direct evidence from high resolution electron microscopy.
A1-5 wt pct Mg and AI-10 wt pct Mg alloy slabs were prepared from 99.99 wt pct pure A1 and 99.9 wt pct pure Mg. The chemical compositions of these slabs are shown in Table I. The slabs were homogenized at 450 ~ for 50 hours, following which A1-5 pct Mg alloy slabs were hotand cold-rolled, with intermediate annealing at 450 ~ to 1.3 mm thick sheets, and the AI-10 pct Mg alloy slabs were cold-rolled, with intermediate annealing at 450 ~ to 1.3 mm thick sheets. The specimens taken from the sheets for micro-Vickers hardness measurements had dimensions 10 mm • 10 mm • 1.3 ram. The 1.3 mm sheet was also drawn to 0.5 mm diameter wire, which was used for electrical resistivity measurements. These were carried out by the usual potentiometric method on specimens with dimensions 0.5 mm diameter • 300 mm long immersed in liquid nitrogen. A1-10 pct Mg alloy specimens for tensile tests were also prepared, using Federal-type sand molds. 2 Thin foils
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