Microstructural development and mechanical properties of interrupted aged Al-Mg-Si-Cu alloy
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heat-treatable aluminum alloys are commonly subjected to a single stage, T6 aging treatment following solution heat treatment and quenching. Two types of modified heat treatments have been developed, termed ‘‘T6IX’’ wherein artificial aging at a typical T6 aging temperature is interrupted (I) by holding the alloy at a reduced temperature for a prolonged period of time.[1–5] In the first of these heat treatments, aging is allowed to continue at the reduced temperature, hence the designation T6I4 (cf. T4) and typically produces tensile properties close to, or sometimes greater than, those for the T6 temper.[3,4] If artificial aging is resumed after the interrupt, a wide range of aluminum alloys show a simultaneous improvement in both the tensile and fracture properties, compared to the T6 condition.[1,2,3] This heat treatment has been designated a T6I6 temper. The process of secondary precipitation during the interrupted aging stage is responsible for the improvements observed.[2,3] For many years, it has been widely assumed that once an aluminum alloy is artificially aged at an elevated temperature (e.g., a T6 temper), its microstructure and mechanical properties remain stable for an indefinite time if the alloy is then exposed to a significantly lower temperature. However, Lo¨ffler et al.,[6,7] reported that highly saturated Al-Zn alloys, aged initially at 180 °C, were found to undergo what has been termed ‘‘secondary precipitation’’ if the alloy was then held at ambient temperature. Secondary precipitation is observed when underaged, and sometimes even fully aged alloys, are held at a reduced temperature for an
extended period of time. As a result, the mechanical properties of the material are altered. For example, in the Licontaining aluminum alloy 2090, secondary aging of peak-aged material has been found to reduce ductility and fracture toughness, and this has been ascribed to secondary precipitation of a fine dispersion of the Al3Li (d9) hardening phase throughout the matrix.[8] Secondary precipitation was also found to reduce the positive creep performance in the underaged condition of an experimental Al-Cu-Mg-Ag alloy.[9] Because secondary precipitation in aluminum alloys occurred generally in an uncontrolled manner, and in such cases had mostly an adverse effect on the mechanical properties, this phenomenon was initially considered to be problematic and undesirable. However, it has been shown recently that through the T6I6 aging procedure, secondary precipitation can also be manipulated and exploited to enhance the mechanical properties of a wide range of age-hardenable aluminum alloys.[1–5,10] Alloys based on the Al-Mg-Si-Cu system are widely used as medium strength alloys with major applications in extruded products and automotive body sheet. AA6061 is one of the most widely used alloys from this group and optimal mechanical properties are achieved by aging in the temperature range from 175 °C to 180 °C for 10 to 20 hours after solution treatment and quenching (T6 temper). These alloys undergo a complex decompositi
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