Abstract
In this work, various aspects of age hardening in 6xxx alloys were analyzed with a strong focus on the effect of the early ”clustered” microstructure on the final peak-strength precipitate microstructure. First, the effect of a typical industrial ramp up to artificial aging (AA) temperature was compared to a quick and direct heating to AA temperature. There, it was shown that hardness curves with different shapes and peak-hardnesses can be produced during AA by changing the early clustered state. The resulting differences in strength were explained by the differences in the emerging precipitate microstructures. Confirmed through strength modelling, it was shown that an alloy can reach higher strength values with precipitate microstructures that have a reduced precipitate number density.
The reason behind an observed wider hardness plateau for alloys that were quickly brought to AA temperature was investigated more in depth. It was shown that by quickly heating an alloy to the final AA temperature, more and thicker precipitates form early on during the course of AA. At a later stage, few but significantly longer precipitates form and compensate the strength-loss from Ostwald ripening. Those longer precipitates where found to have building-blocks from mainly post-β′′ phases.
Eventually early conditions that did not show fully developed precipitates yet, were investigated. It was shown that the early Guinier-Preston zones (GP-zones) can be imaged by direct characterization techniques such as scanning transmission electron microscopy (STEM). Imaging with elastically scattered electrons instead of phonon scattered ones by the use of low-angle annular dark-field (LAADF) made this possible. Through simulations it was shown that the acquired images are incoherent in nature for the chosen imaging parameters. Thus, the images acquired through LAADF-STEM can be interpreted directly. Furthermore, it was shown that imaging with LAADF even allows to differentiate between some atom species in 6xxx alloys.
The ”negative effect of natural aging” was analyzed more in depth in dense and lean 6xxx alloys. For this, the natural aging (NA) time and temperature were varied, as well as the alloy composition. It was suggested that the competitive behavior of homogeneous and heterogeneous nucleation, i.e. seeding of strength giving precipitates by GP-zones, results in the observed change in precipitate number density. With this view on precipitation in the 6xxx series other observed effects, e.g. the ”reversal of the negative effect” or the increased number of post-β′′structures found in precipitates after NA, could be explained by a well-established theory.
Through the correlative approach of differential scanning calorimetry (DSC) analysis and (scanning) transmission electron microscopy ((S)TEM) different, precisely defined conditions could be fabricated and characterized. It was possible to analyze among others the effect of NA by characterizing the emerging GP-zones’ microstructure and the resulting precipitate microstructure. Based on the theory of heterogeneous nucleation of precipitates on GP-zones, i.e. seeding, the observed changes in precipitate length, number density and type were explained. The formation of square GP-zones allows post-β′′ precipitates to develop easier, resulting in longer and thicker precipitates with a reduced number density. A structure of the square GP-zone was proposed that explains the different contrasts of square GP-zones typically seen in LAADF-STEM. Finally, the formation of square GP-zones could be correlated the characteristic shape of conductivity curves acquired during NA.