Low-Temperature Differential Scanning Calorimetry of an Al-Mg-Si Alloy
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after solutionizing and quenching, many aluminum alloys of the 6000 series (Al-Mg-Si) undergo structural changes even at room temperature that, already after a few minutes, give rise to notable changes, e.g., in the electrical resistivity and hardness. Although these changes at room temperature are technologically not relevant since such alloys are usually artificially age hardened to maximum strength at around 433 to 453 K (160 to 180 C), there is considerable interest in this natural aging (NA) since it influences the subsequent artificial aging (AA) step. In alloys containing typically 0.5 to 1 wt pct of both Mg and Si, this response is negative; i.e., AA is much slower and leads to lower peak strengths after NA,[1,2] expressed by the decrease of the number density and the increase in length of the strengthening phase b¢¢ in the peak-aged condition.[3,4] Although a lot of research work based on different experimental methods was performed to identify the processes during NA and the reasons for negative strength response, the exact mechanisms are not yet known. Using differential scanning calorimetry (DSC), two clustering reactions were shown to take place at temperatures up to 373 K (100 C).[5] It was assumed that the first reaction, which at room temperature is completed after 1 hour, corresponds to Si-rich clusters, whereas the second stems from Mg slowly diffusing to these clusters for at least 1 week and eventually forms Mg-Si coclusters.[5] There is actually not much experimental proof that this is true. The claim of a direct observation of Si clustering by transmission electron microscopy (TEM)[6]
C.S.T. CHANG, Postdoctoral Fellow, is with the Institute of Applied Materials, Helmholtz-Centre Berlin for Materials and Energy, Hahn-Meitner Platz, 14109 Berlin, Germany. Contact e-mail: cynthia. [email protected] J. BANHART, Chair of Structure and Properties of Materials, is with the Institute of Materials Science and Technology, Technische Universita¨t Berlin, 10623 Berlin, Germany. Manuscript submitted March 18, 2010. Article published online January 5, 2011 1960—VOLUME 42A, JULY 2011
could not be verified in later work,[7,8] and it is mainly atom probe (AP) work that provides some evidence that initially Si and Mg clusters are formed, whereas after 70 days of NA, clusters containing both Mg and Si are found.[7] A similar observation was made for an alloy lower in both Si and Mg.[9] Another AP study on alloys high in Mg claimed to have shown the increase in relative Mg content during clustering,[10] but the NA times applied were very long compared to the time in which the first reaction is completed (1 hour). Moreover, there is still some controversy about the reliability of cluster identification from 3-Dimensional(3-D)AP data using different kinds of algorithms.[11] It was postulated that the first clusters formed at lower temperatures, e.g., during NA,[12,13] are not suitable nucleation sites for the strengthening b¢¢ phase during ensuing AA, which could be the reason for the negative effect of NA. These clus
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