AlZnMgCu合金单级时效热处理微观组织演变规律研究
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摘要
7xxx系AlZnMgCu铝合金是一种典型的可时效强化铝合金,广泛应用于航空航天、交通运输及其它军工、民用行业。然而,关于时效过程中晶内析出相的演变规律以及析出相的形貌、大小、种类以及分布对强度的贡献尚有争论。另外,关于晶界无析出带(PFZ)的形成机理及其对材料综合性能的影响研究者们也未能形成统一的观点。
本文针对Al-6.47%Zn-1.63%Mg-2.05%Cu铝合金板材,采用固溶淬火以及时效热处理,对合金进行了显微硬度测定、拉伸试验、电导率测量、金相分析(OM)、扫描电子显微镜(SEM)和透射电子显微镜(TEM)分析等,对不同时效条件下合金的微观组织、晶内强化析出相及晶界PFZ的演变规律进行了研究。主要研究内容和创新点如下:
(1)在120℃单级时效过程中发现一种新的亚稳相:η前驱相,具有六角结构,点阵参数c=4d111Al=0.935nm,与η相(c=0.860nm)接近,但比η′相(c=6d111Al =1.406nm)小。点阵参数a= 0.496nm,与η′相相同。η前驱相作为新观测到的一种强化相,在以往的文献中尚未报道过,本实验发现其对材料的强度以及晶界无析出带(PFZ)有重要影响。
(2)采用TEM观察,结合硬度表征对7xxx系铝合金在120~150℃不同温度下单级时效之后的微观结构的演变规律进行跟踪研究。研究发现120℃时效72h合金的硬度最高,可达202HV,对应的抗拉强度为708MPa,屈服强度为691MPa,延伸率为14%。单级峰值时效的主要强化相为GP区,η'相和η前驱相。
(3)采用TEM观察,结合扫描透射-高角环形暗场像(STEM-HAADF)对合金单级时效状态下晶界析出相以及晶界无析出带(PFZ)演变规律进行跟踪研究。研究发现时效温度在150℃左右,晶界周围产生PFZ。时效温度越高,PFZ越宽。150℃恒温时效2h以后,PFZ的宽度不随时效时间的变化而变化。通过EDX能谱分析发现,PFZ区域为贫溶质原子区,尤其是贫Zn。对150℃时效4h合金进行HAADF观察发现,小角度晶界附近PFZ的宽度小于大角度晶界。长时间时效后,小角度晶界上晶界析出相仍为η前驱相,而大角度晶界析出相很快由η前驱相转变为η相。在同一时效温度下,随着时效时间的延长,晶界析出相由连续分布变为不连续分布,而PFZ的宽度对时效时间并不敏感。
7xxx series (AlZnMgCu) aluminum alloys are among the age-hardening aluminum alloy, widely used as structural materials in aviation and aerospace industry, transportation, and other military or civil sectors. Nonetheless, there are still many problems to be solved about their microstructures, such as the structures of the hardening precipitates and their evolution in morphology and in overall distribution upon thermal ageing. Also, it is not clear about the contribution of these factors to the mechanical properties of the alloy. In addition, about the precipitate free zone (PFZ), there are still many ambiguous conclusions that need to be clarified, such as their contribution to the mechanical properties, and their width change during ageing, as well as their formation mechanism.
An Al-6.47%Zn-1.63%Mg-2.05%Cu alloy was used in the present study. All samples were solution-treated and then quenched in water to room temperature. After that, one-step aging treatments were carried out in an oil bath furnace. In order to understand their microstructure evolution of the hardening precipitates formed in grains and at grain boundaries and the evolution of PFZs, micro-Vickers hardness, tensile test, conductivity measurement,optical microscopy (OM) and scanning / transmission electronic microscopy analysis (SEM / TEM) were carried out. The obtained main results are as follows:
(1) An intermediate phase between theη' phase and theηphase, when aged at 120℃. This intermediate phase with a hexagonal structure can be termed theη-precursor, since its a lattice parameter (a= 0.496nm) is the same as that of theη' phase and its c lattice parameter (c=4d111Al=0.935nm) is approximately the same as that of theηphase (c=0.860nm) but smaller than theη' phase (c=6d111Al =1.406nm). Theη-precursor is a newly discovered precipitate phase, which has never been reported proor to the present study. It is found that theη-precursor significant effect on hardness and PFZs.
(2) Microstructure evolution of the targetting alloy aged at 120, 130, 140 and 150℃, respectively, was systematically studied using TEM in combination with hardness test. It was found that the alloy aged at 120℃for 72h has the maximum hardness up to 202HV. Correspondingly, its tensile strength is 708MPa, yield strength 691MPa and elongation 14%. Three types of precipitates, GP zone,η' phase andη-precursor , all are observed in the peak aged samples at different temperatures.
(3) The evolution of PFZs in the samples aged at 150℃has been studied in detail by means of TEM and scanning transmission electron microscopy high-angle annular dark field (STEM-HAADF) imaging. It has been found that PFZs formed in the samples aged at near 150℃. And the higher the ageing temperature, the wider the PFZs. The width of the PFZ does not change with increasing ageing time at the same ageing temperature. However, along with the increasing of ageing time, the successive precipitates lying at grain boundaries evolve into distributional larger precipitates. Combining the Scanning Transmission Electron Microscopy (STEM) with energy dispersive X-ray analysis (EDX), information about distributions of solute elements were detected. The solute elements are poor in the PFZ, and rich in the precipitates at grain boundaries, especially for zinc. With high resolution HAADF observation, it was found that the width of the PFZs in the vicinity of small-angle grain boundaries is smaller than that in the vicinity of high angle grain boundaries. Also, the precipitates formed on small angle grain boundaries are theη-precursor other thanηphase at high angle grain boundaries.
本文针对Al-6.47%Zn-1.63%Mg-2.05%Cu铝合金板材,采用固溶淬火以及时效热处理,对合金进行了显微硬度测定、拉伸试验、电导率测量、金相分析(OM)、扫描电子显微镜(SEM)和透射电子显微镜(TEM)分析等,对不同时效条件下合金的微观组织、晶内强化析出相及晶界PFZ的演变规律进行了研究。主要研究内容和创新点如下:
(1)在120℃单级时效过程中发现一种新的亚稳相:η前驱相,具有六角结构,点阵参数c=4d111Al=0.935nm,与η相(c=0.860nm)接近,但比η′相(c=6d111Al =1.406nm)小。点阵参数a= 0.496nm,与η′相相同。η前驱相作为新观测到的一种强化相,在以往的文献中尚未报道过,本实验发现其对材料的强度以及晶界无析出带(PFZ)有重要影响。
(2)采用TEM观察,结合硬度表征对7xxx系铝合金在120~150℃不同温度下单级时效之后的微观结构的演变规律进行跟踪研究。研究发现120℃时效72h合金的硬度最高,可达202HV,对应的抗拉强度为708MPa,屈服强度为691MPa,延伸率为14%。单级峰值时效的主要强化相为GP区,η'相和η前驱相。
(3)采用TEM观察,结合扫描透射-高角环形暗场像(STEM-HAADF)对合金单级时效状态下晶界析出相以及晶界无析出带(PFZ)演变规律进行跟踪研究。研究发现时效温度在150℃左右,晶界周围产生PFZ。时效温度越高,PFZ越宽。150℃恒温时效2h以后,PFZ的宽度不随时效时间的变化而变化。通过EDX能谱分析发现,PFZ区域为贫溶质原子区,尤其是贫Zn。对150℃时效4h合金进行HAADF观察发现,小角度晶界附近PFZ的宽度小于大角度晶界。长时间时效后,小角度晶界上晶界析出相仍为η前驱相,而大角度晶界析出相很快由η前驱相转变为η相。在同一时效温度下,随着时效时间的延长,晶界析出相由连续分布变为不连续分布,而PFZ的宽度对时效时间并不敏感。
7xxx series (AlZnMgCu) aluminum alloys are among the age-hardening aluminum alloy, widely used as structural materials in aviation and aerospace industry, transportation, and other military or civil sectors. Nonetheless, there are still many problems to be solved about their microstructures, such as the structures of the hardening precipitates and their evolution in morphology and in overall distribution upon thermal ageing. Also, it is not clear about the contribution of these factors to the mechanical properties of the alloy. In addition, about the precipitate free zone (PFZ), there are still many ambiguous conclusions that need to be clarified, such as their contribution to the mechanical properties, and their width change during ageing, as well as their formation mechanism.
An Al-6.47%Zn-1.63%Mg-2.05%Cu alloy was used in the present study. All samples were solution-treated and then quenched in water to room temperature. After that, one-step aging treatments were carried out in an oil bath furnace. In order to understand their microstructure evolution of the hardening precipitates formed in grains and at grain boundaries and the evolution of PFZs, micro-Vickers hardness, tensile test, conductivity measurement,optical microscopy (OM) and scanning / transmission electronic microscopy analysis (SEM / TEM) were carried out. The obtained main results are as follows:
(1) An intermediate phase between theη' phase and theηphase, when aged at 120℃. This intermediate phase with a hexagonal structure can be termed theη-precursor, since its a lattice parameter (a= 0.496nm) is the same as that of theη' phase and its c lattice parameter (c=4d111Al=0.935nm) is approximately the same as that of theηphase (c=0.860nm) but smaller than theη' phase (c=6d111Al =1.406nm). Theη-precursor is a newly discovered precipitate phase, which has never been reported proor to the present study. It is found that theη-precursor significant effect on hardness and PFZs.
(2) Microstructure evolution of the targetting alloy aged at 120, 130, 140 and 150℃, respectively, was systematically studied using TEM in combination with hardness test. It was found that the alloy aged at 120℃for 72h has the maximum hardness up to 202HV. Correspondingly, its tensile strength is 708MPa, yield strength 691MPa and elongation 14%. Three types of precipitates, GP zone,η' phase andη-precursor , all are observed in the peak aged samples at different temperatures.
(3) The evolution of PFZs in the samples aged at 150℃has been studied in detail by means of TEM and scanning transmission electron microscopy high-angle annular dark field (STEM-HAADF) imaging. It has been found that PFZs formed in the samples aged at near 150℃. And the higher the ageing temperature, the wider the PFZs. The width of the PFZ does not change with increasing ageing time at the same ageing temperature. However, along with the increasing of ageing time, the successive precipitates lying at grain boundaries evolve into distributional larger precipitates. Combining the Scanning Transmission Electron Microscopy (STEM) with energy dispersive X-ray analysis (EDX), information about distributions of solute elements were detected. The solute elements are poor in the PFZ, and rich in the precipitates at grain boundaries, especially for zinc. With high resolution HAADF observation, it was found that the width of the PFZs in the vicinity of small-angle grain boundaries is smaller than that in the vicinity of high angle grain boundaries. Also, the precipitates formed on small angle grain boundaries are theη-precursor other thanηphase at high angle grain boundaries.
引文
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[2]陈亚莉.波音777及其侯选动力装置的选材(一).航空制造工程,1995, 25: 32-34.
[3]吴一雷,李永伟.超高强度铝合金的发展和应用.航空材料学报,1994, 2: 49-51.
[4]李红英,董显娟.高强高韧铝合金研究现状及展望.湖南有色金属,2002,18: 33-36.
[5] H. A. Recent development in aluminum alloys for aerospace applications. Mater. Sci. Eng. A, 2000,280: 102-107.
[6] J.T. Staley, J. Liu, W.H. Hunt. Aluminum alloys for aerostructures.Adv Mater Process, 1997 152: 17-20.
[7]陈昌麒.超高强铝合金的发展.中国有色金属学报, 2002,12: 22-26.
[8]王涛,尹志民.高强变形铝合金的研究现状和发展趋势.稀有金属, 2006,30: 197-202.
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