Orginal Article

# Sn-Induced Modification of the Precipitation Pathways upon High-Temperature Ageing in an Al-Mg-Si Alloy

XIANG Xuemei, LAI Yuxiang, LIU Chunhui, CHEN Jianghua

College of Materials Science and Engineering, Hunan University, Changsha 410082, China

Abstract

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.

Keywords： Al-Mg-Si alloy ; ageing ; precipitation ; trace element ; transmission electron microscopy

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XIANG Xuemei, LAI Yuxiang, LIU Chunhui, CHEN Jianghua. Sn-Induced Modification of the Precipitation Pathways upon High-Temperature Ageing in an Al-Mg-Si Alloy[J]. Acta Metallurgica Sinica, 2018, 54(9): 1273-1280 https://doi.org/10.11900/0412.1961.2018.00125

6xxx系(Al-Mg-Si(-Cu))合金具有较高的强重比、良好的抗腐蚀性能及优异的成型性能等,广泛应用于汽车行业及建筑领域[1,2,3,4,5]。6xxx系铝合金的强度主要是通过在560 ℃左右进行固溶处理并快速淬火到室温后进行后续的人工时效获得。在人工时效过程中,合金中会析出大量的纳米尺寸的析出相。在Al-Mg-Si三元合金中,被广泛认同的析出相的析出序列为：过饱和固溶体(SSSS)→团簇/GP区→pre-β"/β"β'β,其中β"相被认为是该系列合金中强化效果最好的析出相,而β'相的析出发生在β"相析出之后,且对合金的时效强化效果不利[6,7,8,9,10]。但最近有研究[11]表明,低含量的Al-Mg-Si合金在180 ℃下进行人工时效时能够直接形成β'相。因此,β"相与β'相的形核生长是相互独立的,不存在先后顺序。

## 2 实验结果

### 2.1 自然时效对人工时效硬化行为的影响

Fig.1   Evolution of hardness during natural ageing for the Al-Mg-Si alloy (a) and Al-Mg-Si-Sn alloy (b)

Fig.2   Evolution of hardness during artificial ageing at 250 ℃ after different natural ageing delay times for the Al-Mg-Si alloy (a, b)[20] and Al-Mg-Si-Sn alloy (c, d). Figs.2b[20] and d show the corresponding peak-hardness for artificial ageing vs natural ageing time

### 2.2 自然时效对人工时效析出行为的影响

Fig.3   Bright-field TEM images of the precipitate morphologies (a, c, e) and the corresponding precipitate length distributions (b, d, f) of the Al-Mg-Si-Sn alloys peak-aged at 250 ℃ for 5 min after different natural ageing time
(a, b) without natural ageing (c, d) natural ageing for 4 d (e, f) natural ageing for 2 weeks

Fig.4   HRTEM images and the corresponding FFT patterns (insets) of the main precipitates in the Al-Mg-Si-Sn alloys peak-aged at 250 ℃ for 5 min after different natural ageing times
(a) without natural ageing (b) natural ageing for 4 d (c, d) natural ageing for 2 weeks

Fig.5   The relative frequencies of β"-precipitate and β’-precipitate in the Al-Mg-Si alloy (a)[20] and Al-Mg-Si-Sn alloy (b) upon peak ageing at 250 ℃ following different natural ageing durations (NA—natural ageing)

## 3 分析讨论

Fig.6   An illustration of the effect of matrix Si-concentration (CSi(+Sn)) on nucleation energy barriers (∆Gβ’ and ∆Gβ”) of β’ and β” ($C0Si(+Sn)$—the value at which ∆Gβ’=∆Gβ”; $Ceqβ"Si(+Sn)$ and $Ceqβ’Si(+Sn)$—the critical CSi(+Sn) needed for the formation of β” and β’, respectively; ΔG—nucleation energy)[11]

## 4 结论

(1) Sn的添加对富Mg的Al-Mg-Si合金250 ℃人工时效硬化特性的影响与Sn改变了合金的析出路径有关。不含Sn的Al-Mg-Si合金的析出路径为：SSSS→β'相(直接人工时效)和SSSS→原子团簇→β"相(长时间自然时效+人工时效);而含Sn的Al-Mg-Si合金的析出路径为：SSSS→β"相(直接人工时效)和SSSS→原子团簇→β"相+β'相(长时间自然时效+人工时效)。

(2) Sn的添加能显著提高富Mg的Al-Mg-Si合金在250 ℃下人工时效的时效硬化能力。推测在不同热处理状态下,Sn的加入增加了基体中Si的有效浓度。

The authors have declared that no competing interests exist.

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