通过对Al-Li合金的凝固偏析行为的系统研究,首次揭示了Al-Li合金的凝固机制与元素偏析规律。结果表明,元素Cu具有严重的偏析倾向,而Mg的偏析倾向较小,所以2090合金和8090合金具有相同的凝固机制,L → α - Al + L_1是基本凝固过程。由于Cu的严重偏析,导致凝固温度范围显著增大,在最后凝固区形成α - Al + T_2共晶和α - Al + T_1共晶。杂质元素Fe和Si具有强烈的偏析倾向,将形成α - Al + Al_3Fe共晶,在最后凝固区形成AlLiSi和Al_7Cu_2Fe。冷却条件对凝固行有显著影响。冷却速度越小,偏析越大,使得共晶组织以及富Fe、Si的金属间化合物更加粗大。对于一定的含Fe量,有一个临界冷却速度,小于这个冷速,便可形成Al_3Fe或Al_7Cu_2Fe。含Fe量越高,临界冷速越大。对于冷速小于10 ℃/min的凝固过程。为避免粗大α - Al + Al_3Fe共晶析出,其含Fe量应小于0.1wt%。对Al-Li合金定向凝固行为的研究表明,界面形态对溶质偏析有显著影响。合金在以粗大树枝晶界面凝固时,元素偏析最大。通过定向凝固,合金塑性显著提高。Al-Li合金的晶粒组织对力学性能的显著影响。通过热机械处理,尤其是冷变形前过时效,可以在一定程度上调整合金的晶粒组织。在高温过时效,析出粗大的Al_2MgLi相,促进合金再生晶,而在低温过时铲析出细小的δ相,阻碍合金再生晶。完全未再结晶材料具有较高的力学性能。合金完全再结晶仍然能保持相当高的性能。而当合金发生少部分再结晶时,性能显著降低。δ'相是8090合金的主要强化相,随着δ'相析出长大,晶界平衡相也大量析出长大,使塑性降低,但是,通过双级时效,即可获得相当大的δ'相,使合金强化,又可抑制晶界平衡相大量析出。对一个8090合金的研究表明,通过100 ℃,8h + 190 ℃,4h双级时效,合金具有较高的力学性能。δ'相和晶界平衡相的尺寸不同,导致合金具有不同的性能。对8090合金应力腐蚀行为的研究表明,过时效,析出大量的粗大的平衡相,显著提高合金的应力腐蚀抗力。在峰时效状态,合金的应力腐敏感性最大。本文首次将回复再时效工艺应用到Al-Li合金中,取得了良好的效果。通过325 ℃,2.5min或230 ℃,45min回复处理,然后再时效,既能保持峰时效状态的拉伸强度,又能获得过时效状态的应力腐蚀抗力。微观组织研究表明,回复时δ'回溶,再时效时重新析出和长大,导致了合金具有高强度,平衡相在回复和再时效过程中大量析出和长大,导致应力腐蚀抗力显著提高。
其他摘要
In this paper, considerable investigations have been made systematically for understanding the solidification mechanism and segregation behavior of Al-Li alloys. The results show that alloying element Cu has quite large segregation tendeney while Mg comparatively small. Hence, the 2090 alloy and 8090 alloy have the similar solidification behavior. L → α - Al + L_1 is the main solidification reaction. The segregation of Cu results in lowering of solidification temperature and formation of α - Al + T_2 and α - Al + T_2 eutectics in final solidification zone. The impurities Fe and Si are seriously prone to segregation, which leads the formation of insoluble constituents Al_3Fe, Al_7Cu_2Fe and AlLiSi, although the compositions of Fe and Si in the alloy are quite small. The cooling condition has great influence on solidification behavior. The smaller the cooling rate, the large the segregation and the coarser the constituents. The research results on unidirectional solidification of the alloys show that the interface morphologies have remarkable influence on solute segregation. The interface transforming to coarse dendrite from cellular leads to increase of solute segregation. The enrichments of Cu, Fe and Si in interdendrite are very large, while Mg is quite small. The unidirectional solidification improves the ductility remarkably. The grain structure has great influence on mechanical properties. It can be controlled by thermomechanical treatment, especially overaging prior to cold work. The coarse Al_2MgLi precipitated during overaging promotes recrystallization while fine δ phase precipitated during overaging retards recrystallization. The fully unrecrystallized structure possesses quite high mechanical properties, only a little drop of properties takes place when grain is fully recrystallized, while partially recrystallized structure lowers mechanical properties markedly. The precipitation strengthening of 8090 alloy has been studied systematically. With precipitation and growth of δ' phase, grain boundary equilibrium phases precipitate and grow also. The former strengthens the alloy, while the latter lowers the ductility of the alloy. Double-aging can promote the precipitation and growth of δ' phase, while equilibrium phases are retarded. A study on a 8090 alloy shows that double-aging of 100 ℃, 8h + 190 ℃, 4h improves the mechanical properties remarkably. On the other hand, the microstructure influences on corrosion resistance greatly. In the peak aging state, the 8090 alloy is much susceptible to local corrosion and stress corrosion cracking (SCC). Aging alloy from peak aging to overaging condition results in remarkable increase in corrosion resistance. The Retrogression and Reaging (RRA) treatment, used in 7075 aluminum alloy, was studied for 8090 Al-Li alloy. The results show that the RRA is suitable for 8090 Al-Li alloy. Retrogressing the material at 230 ℃ for 45min or at 325 ℃ for 1.5min, and then reaging in peak aging condition can get a better combination of tensile strength and SCC resistance, TEM examination indicates that δ' phase dissolves during retrogression and reprecipitation during reaging, which allows strength not to drop. Equilibrium phases precipitate and grow during both retrogression and reaging, which results in the increase of SCC resistance.
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