研究了Sn (0.2%,质量分数)的添加对一种富Mg的Al-Mg-Si合金经历不同时间的自然时效后在250 ℃下人工时效过程中的时效硬化行为的影响,并利用TEM观察揭示了其微观机理。结果表明,在Al-Mg-Si合金中添加微量Sn (0.2%)可以改变合金250 ℃人工时效的强化相析出路径：当直接进行人工时效时,析出相主要为β"相;当进行自然时效+人工时效处理时,随着自然时效时间的延长,β"相的比例不增反降,而β'相的比例不降反增,但最终β"相的比例仍高于β'相的比例。Sn对强化相析出路径的改变,可以明显提高合金高温人工时效的硬化能力。Sn的添加提高了基体中有效的Si浓度,从而改变了不含Sn合金的强化相析出路径。

The 6xxx series aluminum alloys (Al-Mg-Si(-Cu) alloys) are widely used for the industrial applications in the lightweight construction, automotive and architecture because of their light weight, medium to high strength, excellent formability and good corrosion resistance. It has been reported that trace Sn addition can accelerate ageing kinetics and increase peak hardness of Al-Mg-Si alloys when ageing at high temperatures (>210 ℃). However, the mechanism about it has not been investigated comprehensively yet. For Mg-excess Al-Mg-Si alloys, when aged at 250 ℃, the alloys are hardened by the β'-precipitates. While after applying natural ageing prior to artificial ageing, the β"-precipitates will be formed, with the percentage of which increasing with natural ageing time, and eventually become the main hardening precipitates. In this work, the effect of Sn on natural ageing and subsequent artificial ageing at 250 ℃ in a Mg-rich Al-Mg-Si alloy was investigated by Vickers microhardness measurements and TEM. The results show that adding a small amount (0.2%, mass fraction) of Sn in the Mg-rich Al-Mg-Si alloy can modify the precipitation pathways upon 250 ℃-ageing: when the alloy is directly artificially aged, the β"-precipitates are dominant, whereas when the alloy is subjected to "natural ageing+artificial ageing" treatment, upon prolonged natural ageing time, the percentage of β"-precipitates would not increase but decrease and that of β'-precipitates would not decrease but increase, but ultimately the β"-precipitates are still dominant over the β'-precipitates. The Sn-induced modification of the precipitation pathways can significantly enhance the age-hardening potential of the alloy upon high-temperature artificial ageing. The addition of Sn increases the effective Si-concentration in the matrix, and consequently changes the precipitation pathways in the Sn-free alloy, which is different from the explanation proposed in literatures.