Effect of Two-Step Aging on Cluster Formation in Al–Mg–Si Alloys

Abstract

The change in the state of a MgSi cluster with pre-aging at 363 K, followed by aging at 303 or 323 K, was studied by means of a tensile test, three-dimensional atom probe (3DAP), and differential scanning calorimetry (DSC) measurements. MgSi clusters formed during isothermal aging (one-step aging) at 363 K after solution heat treatment were different from the ones formed at 303 and 323 K. Furthermore, during aging at 303 and 323 K following pre-aging at 363 K (two-step aging), the clusters that were originally formed at 363 K (high-temperature clusters) grew in size and a new type of clusters (low-temperature cluster) were newly formed at 303 and 323 K. The increase in yield stress with aging time at 303 and 323 K was greater with the pre-aging at 363 K than without the pre-aging. The greater increase in the yield strength was attributed to the growth of the high-temperature clusters formed in the pre-aging and the nucleation and growth of the low-temperature clusters, both of which proceeded during the aging at 303 or 323 K.

Improvement of Strength–Elongation Balance of Al–Mg–Si Sheet Alloy by Utilising Mg–Si Cluster and Its Proposed Mechanism

“…However, no study has yet veri ed this observation. Results of DSC measurements reported in a previous paper 7) reveal that the exothermic peaks at 363, 530, and 585 K for the as-quenched specimen as shown in Fig. 5(a) indicate the formation of low-temperature clusters, β ′′ precipitates, and β ′ precipitates, respectively.…”

“…5(b)) shows that the exothermic peaks at 363, 530, and 585 K disappeared and a new exothermic peak appeared at 567 K; no such new exothermic peak was observed for the as-quenched alloy. We assumed that the endothermic peak observed at around 500 K was similar to the peak observed in an alloy with high-temperature clusters in a previous study 7) , although the origin of this peak remains to be understood. It was reported that when an Al-Mg-Si alloy was aged at 363 K for a long time following solution heat treatment, the exothermic peaks corresponding to the formation of low-temperature clusters and β ′′ precipitates decreased and the peak corresponding to the formation of β ′ precipitates shifted to a lower temperature 7) .…”

“…TEM diffraction analysis indicated that the crystal structure of the clusters was the same as that of Al metal 7) . Since the average number of atoms evaluated in the present study was similar to that reported in a previous study, it could be inferred that the aggregates of Mg and Si atoms observed in Fig.…”

“…The DSC curve of the as-quenched alloy is also shown for comparison. Previous reports on DSC measurements 7) of an as-quenched specimen revealed the appearance of exothermic peaks in the following sequence: low-temperature cluster formation at 363 K, high-temperature cluster or β ′′ precipitate formation at 548 K, and β ′ precipitate formation at 593 K. Endothermic peaks appeared during low-temperature cluster dissolution at temperatures below 500 K. Based on previous reports 6,7,11) , we classi ed the clusters formed during aging at temperatures below 343 K as low-temperature clusters and the clusters formed during aging at temperatures above 343 K as high-temperature clusters. When aging was performed at 448 K, which corresponds to the temperature used for paint baking in automotive manufacture, high-temperature clusters grew into β ′′ precipitates but low-temperature clusters did not 11) .…”

“…This suggests that clusters may be sheared by dislocations and be decomposed into Mg and Si atoms, such as GP zones in an Al-Cu alloy. Clusters grow in size with increasing time of aging at their formation temperature 7) . An increase in the size of clusters can lead to an increase in their resistance force to dislocations.…”