【作者】 朱宝宏；

【导师】 熊柏青；

【作者基本信息】 北京有色金属研究总院 ， 材料学， 2011， 博士

【摘要】 2139合金是在Al-Cu-Mg合金基础上,通过添加微量Ag元素而开发出来的一种新型2000系铝合金,该合金具有良好的损伤容限性能、耐热性能和抗弹性能,有可能被用于下一代超音速飞机蒙皮和战车装甲等领域。本文针对2139合金的均匀化处理、固溶处理、时效处理等热处理工艺展开研究,分析了合金在时效过程中的组织和性能变化规律,探讨了微量Mn元素在合金中的作用,并初步探索了合金的耐热性能和疲劳性能。铸态2139合金晶粒尺寸较大,晶间存在着严重的成分偏析,利用扫描电镜对合金表面进行成分面扫描,结果表明,Cu元素主要在晶界上偏析,Mg和Ag元素均匀分布在基体上。根据差热分析结果,并结合对合金微观组织的分析及性能测试,确定了铸态2139合金的均匀化处理制度为520℃/24h,挤压变形态合金的固溶热处理制度为(520～525)℃/(1.5-2)h,合金时效温度在160～180℃之间选择。固溶态2139合金的差热分析曲线存在三个放热峰,结合对各放热峰的微观组织分析,确定了三个放热峰分别对应着合金中GP区、Q相和θ’相的形核与长大。2139合金的时效过程分为三个阶段,即欠时效、峰时效和过时效,合金在160℃下达到峰时效的时间为10～12h,峰值强度为473MPa；在180℃下达到峰时效的时间为4～6h,峰值强度为479MPa。在过时效阶段,180℃下的性能衰减速度快于160℃。微量Mn元素能够起到细化合金晶粒的作用,但依然存在晶间偏析。Mn元素对2139合金的相变行为无明显影响,可以采用与2139合金相同的均火制度、固溶制度和时效热处理制度。加Mn的2139合金T6状态极限抗拉强度为453MPa,延伸率大于15%。进入过时效阶段,2139合金强度及延伸率均衰减较快,而加Mn的2139合金则能在较长时间内保持合金强度及延伸率变化不大,表明微量Mn元素有利于提高2139合金的延伸率,但合金强度降低。从热力学和动力学角度分析了微量Ag元素对Ω相形核和长大的影响。微量Ag元素能够抑制高Cu:Mg比Al-Cu-Mg合金中θ’相的形核和长大,并促进Ω相在{111}Al面上析出。研究了2139合金在不同时效阶段的微观组织特征,发现当2139合金在较低温度下时效处理时,时效初期θ’相易先发生析出,随着时效时间的延长,Ω相才逐渐成为合金中的主要强化相；而当合金在较高温度下时效时,时效初期θ’相的析出受到抑制,Ω相在(111}Al面上优先析出并成为合金的主要强化相。高分辨电镜研究表明,Q相与θ’相均与基体呈共格关系。在时效过程中,Ω相以台阶机制长大,Mg、Ag原子在长大过程中偏析到Ω/a界面处,阻碍了Cu原子的自由扩散,有利于阻止Ω相在厚度方向的长大。利用STEM原子像观察,进一步证明了高原子序数的Ag原子分布在Ω/α界面处。在透射电镜下,加Mn的2139合金的主要强化相仍为Q相,同时含有少量的θ’相,以及少量的粗棒状相。能谱分析及衍射花样表明粗棒状相为Al11Cu5Mn3相,不利于提高合金的常规力学性能。分别对固溶态2139合金施加2.5%和5%的预变形,并迅速在180℃下进行人工时效处理,结果表明,2.5%的预变形将2139合金T6态极限抗拉强度提高了11MPa；5%的预变形则将合金的T6态极限抗拉强度提高了34MPa,但预变形降低了合金的延伸率,预变形量越大,延伸率损失越大；预变形不影响合金达到峰时效的时间。将T6态2139和加Mn的2139合金分别在150℃、175℃、200℃和225℃下热暴露100h后,测试其室温强度,结果表明加Mn的2139合金具有良好的耐热性能,特别是合金在150℃下热暴露100h后,其室温强度几乎没有损失。只有当热暴露温度提高到200℃以上时,加Mn的2139合金性能才出现较大幅度下降。峰时效状态2139合金的疲劳极限为141MPa,而加Mn的2139合金的疲劳极限达到了275MPa,表现出了良好的疲劳性能,这主要是因为加Mn的2139合金中形成的AlCuMn三元相能够改变Fe杂质在合金中的存在形式,有利于提高合金的疲劳性能。更多还原

【Abstract】 2139 alloy is a new aluminum alloy developed by addition a small amount of Ag element to the Al-Cu-Mg alloy with high Cu:Mg ratio. The high damage tolerant, high heat resistance and high ballistic properties of 2139 alloy make it possible to be applied in the next generation ultrasonic transport airplane and some armor tanks. The study of this thesis focused on the homogenization, solution and ageing treatments of 2139 alloy. The microstructure and properties including heat resistance and fatigue properties of the alloy, as well the effect of Mn on the alloy, were also analized.The microstructure of both as-cast alloys has been studied. It is found that the grain size of 2139 alloy is large, and there is heavy segregation along the grain boundaries. Surface scanning of the as-cast 2139 alloy shows that Cu element is rich along the grain boundary, while both elements Mg and Ag are evenly distributed across the matrix. An optimized homogenizing treatment is performed at 520℃for 24h according to the thermal analysis by Differential Scanning Calorimeter (DSC) method and the microstructure characterization of as-cast 2139 alloy. The optimized solution treatment for 2139 alloy is also fixed as 520-525℃for 1.5-2h. The fit aging temperature can be chosen from160℃to 180℃.There are three exothermal peaks in the DSC curves of as-quenched 2139 alloy, each of which is corresponding to the nucleation and growth of the GP zone,Ωphase andθ’ phase, respectively. Furthermore, the mechanical properties and microstructures after artificial ageing treatment at both 160℃and 180℃were compared. The research results show that 2139 alloy experiences three stages during aging treatment, namely underage stage, peak-age stage and overage stage. It cost 10-12h for 2139 alloy to reach peak ageing status and the peak strength is about 473MPa. When the alloy was aged at 180℃, only 4-6h is needed to reach the peak-aged status and the peak strength is about 479MPa. The strength of 2139 alloy declines faster at 180℃than at 160℃when the ageing treatment is extended to over-aged stage.The grain size of 2139 alloy with trace Mn element is smaller than that of 2139 alloy due to the refining effects of Mn element to the alloy, although there still is obvious segregation along the grain boundary. The homogenization treatment, solution treatment and aging treatment optimized for 2139 alloy are also applicable to alloy 2139 with Mn element, based on the fact that Mn element has no substantial effect on phase transformation of the latter alloy, either as-cast or as-extruded. The UTS of peak-aged 2139 alloy with Mn element decreased to 453MPa, compared with 474MPa of 2139 alloy, while the elongation gained to more than 15%, from about 10% of 2139 alloy. Both strength and elongation of 2139 alloy decline rapidly after going into over-aged stage, but neither changes much for 2139 alloy with Mn, even if hold for a relative long time after peak-aged stage, indicating that Mn element is helpful to improve elongation but detrimental to strength.The addition of trace element Ag to an Al-Cu-Mg alloy with high Cu:Mg ratio results in the Mg-Ag clustering along{111}Alplane due to the extremely high binding energy between Mg and Ag atoms, promotes the nucleation and growth ofΩphase there and, as a result of that, suppresses the nucleation and growth ofθ’ phase.The microstructure of 2139 alloy during age treatment is also studied in this thesis. And it is found that theθ’phase would precipitate prior toΩphase at the early stage when the ageing temperature is relatively lower, and with time going on theΩphase becomes the main strengthening phase gradually. While the 2139 alloy was aged at a higher temperature, the precipitation ofθ’phase is suppressed at early stage, andΩphase precipitates preferentially along{111} Al habit plane to become the main strengthening phase. The images from HREM show that bothΩphase andθ’phase are coherent with the matrix. During aging treatment process, theΩphase grows by a step-control mechanism. At the same time, the alloying elements are redistributed, causing both Mg and Ag atoms to segregate toΩ/αinterface. The Mg-Ag atom-pair wall, therefore, forms to prevent the free diffusion of Cu atoms, helping restrain the growth ofΩphase along thickness direction.The STEM images further confirm the Ag atoms, with high atomic number, distributed at boundaries betweenΩ/αinterface. TEM images of 2139 alloy with Mn show that theΩphase is the main strengthening phase, with a small amount ofθ’phase and a thick-bar-like phase, too. The thick-bar-like phase is identified, with the help of EDS and diffraction pattern, as Al11Cu5Mn3, which is detrimental to the mechanical properties of the alloy.Different pre-stretch deformation by 2.5% and 5% were performed on as-quenched 2139 alloy, and then were artificial aged at 180℃immediately. It is found that a 2.5% pre-deformation increased the ultimate tensile strength (UTS) at peak-aged status by 11MPa, and a 5% pre-deformation contributed to an increase of 34MPa. However, the pre-deformation reduced the elongation of the alloy, and the more the pre-deformation was, the more it reduced the elongation. Pre-deformation doesn’t affect the time to reach peak-aging status.Both peak-aged 2139 and 2139 with Mn alloys were held for 100h at 150℃、175℃, 200℃and 225℃, respectively, before the tensile test at room temperature. Then 2139 alloy with Mn shows a less loss of the strength after the exposure at high temperature than 2139 alloy, indicating the former one is more heat-resistant. Especially, the tensile strength of peak-aged 2139M alloy after being held at 150℃for 100h is almost the same as before heat exposure. Substantial strength loss is observed only when the exposure temperature is over 200℃.The endurance limit of peak-aged 2139 alloy is about 141MPa, lower than that of 2139 alloy with Mn element, which is 275MPa, indicating the latter one presenting better fatigue resistance. It is believed that the ternary AlCuMn phase in alloy 2139M can change the existence form of Fe impurity, which is helpful to improve fatigue resistance.更多还原