TEM STUDY OF THE INFLUENCE OF PRE-AGING AT 100ºC ON PRECIPITATION DURING ARTIFICIAL AGING IN Al-Mg-Si

M. V. CASTRO RIGLOS; A. TOLLEY; V.NOSEDA GRAU; A. CUNIBERTI

PRAGA

Congreso; 18TH INTERNATIONAL MICROSCOPY CONGRESS IMC 2014; 2014

SOCIEDAD CHECA DE MICROSCOPIA

Age hardenable Al-Mg-Si alloys are widely used as structural materials in automotive and architectural industries due to their low density, high formability and corrosion resistance (1). In these alloys the principal strengthening process is related to the formation of needle-shaped ? precipitates, that strongly depends on the time and temperature of artificial aging. Pre-aging at room temperature or at higher temperatures prior to artificial aging is known to influence the precipitation process significantly. It has been reported that room temperature pre-aging delays the formation of the strengthening precipitates (2), whereas if pre-aging is carried out at temperatures of 70ºC or 100ºC, this delay is reduced (3). The detailed process by which these effects take place is not yet fully understood. In this work, the influence of pre-aging at 100ºC on the subsequent precipitation process during artificial aging at 180ºC in an Al-Mg-Si alloy is addressed.A commercial AW-6082 alloy with composition Al-0.64Mg?0.50Si?0.60Mn?0.05Cu?0.05Fe (wt%), determined by optical emission spectrometry, and a mean grain size was 0.3 mm was used. Specimens were cut from this alloy, solution-treated for 1 h at 530 ºC and water quenched. Before artificial aging at 180ºC, one batch was aged at room temperature and another at 100ºC. Microstructural characterization was carried out using a Tecnai F20 transmission electron microscope at Centro Atómico Bariloche. Figure 1 shows two-beam bright field images with g = (200) obtained near the [011] matrix zone axis of specimens pre-aged 3 hours at room temperature (a) or at 100ºC (b), and subsequently aged at 180ºC for 30 minutes. In both specimens the observed contrast corresponds to needle-shaped ? precipitates whose projections are oriented parallel to the [0 11] direction. The density of precipitates is higher and the average size is smaller in the specimen with pre-aging at 100ºC that those in the room temperature pre-aged specimens. Figure 2 shows STEM annular bright field (ABF) images along the [001] matrix zone axis (a,b) and High Resolution images (c,d) of specimens with 3 hours pre-aging at room temperature (a,c) or 100ºC (b,d), subsequently aged for 2 hours at 180ºC. Three needle-shaped precipitate variants, oriented along the matrix directions, are indicated. Comparison of the precipitate lengths was carried out using the ABF images, while that of the average diameters of the precipitates were analysed using the high resolution images. The results are shown in Table I. Pre-aging at room temperature leads to fewer precipitates that grow to larger sizes. The results indicate that the pre-aging treatment at 100ºC favours the nucleation of beta? precipitates.