【作者】 邓英；

【导师】 尹志民；

【作者基本信息】 中南大学 ， 材料物理与化学， 2012， 博士

【摘要】 本课题结合国家总装预研项目,以未添加钪锆和添加0.25Sc+0.10Zr的Al-5.7Zn-2.0Mg-0.35Cu (wt-%)两种合金为研究对象,通过材料制备工艺优化方法及显微分析检测手段,对比研究了复合添加微量钪锆在均匀化、热变形及固溶时效过程中对Al-Zn-Mg合金组织性能影响,在此基础上,详细探讨了微量钪锆对Al-Zn-Mg合金再结晶、各向异性、腐蚀及焊接的作用机理。论文获得了以下主要结论：(1)复合添加0.10Zr+0.25Sc到Al-5.7Zn-2.0Mg-0.35Cu(wt-%)合金后,铸态晶粒显著细化,晶粒尺寸由78μm下降到了58μm。(2)在半连续铸造激冷条件下,铸态合金基体近似为过饱和固溶体,晶界上存在富Zn、Mg低熔点非平衡共晶相和富Fe、Si、Mn难溶杂质相,铸锭必须均匀化处理。随均匀化温度升高,过饱和固溶体析出MgZn2相和T(Mg32(Al, Zn)49)相之后这些相又溶入基体,与此同时晶界上非平衡共晶相溶解。添加微量钪锆的合金均匀化过程中还析出大量细小弥散的Al3(Sc, Zr)粒子,这种合金最佳铸锭均匀化工艺为为350℃/8h+470℃/12h。(3)铸锭热变形条件下,随变形温度的增加,两种合金强度单调下降,伸长率先升后降,断口由穿晶断裂转变为晶间断裂,在375-400。C下变形,合金具有较稳定的热变形抗力和较高的热加工塑性,生产现场试验表明铸锭在该温度区间进行热轧,热轧效果较好。添加微量钪锆形成的Al3(Sc, Zr)粒子在热变形中强烈钉轧位错及亚晶界,阻碍位错运动及晶界迁移,提高了这种合金热变形抗力。(4)复合添加0.10Zr+0.25Sc到Al-5.7Zn-2.0Mg-0.35Cu(wt-%)合金中,在优化的均匀化处理、热变形和470℃/1h固溶后水淬+120℃/24h时效条件下,合金板材拉伸强度和屈服强度分别提高了12%和22%,且延伸率保持在12%的高水平。微量钪锆在Al-Zn-Mg基合金中的强化机制为晶粒细化强化、亚结构强化和Al3(Sc, Zr)粒子析出强化。(5)冷轧板材再结晶退火过程中,复合添加0.10Zr+0.25Sc使Al-5.7Zn-2.0Mg-0.35Cu合金再结晶织构由立方织构转变为轧制织构。考虑具有回复优势(晶界能优势)的形核位置及需要大量的静态回复时间来获取临界尺寸的形核位置,建立了再结晶形核定量模型,计算了钪锆合金不同晶体取向下的储能(驱动力),结果显示受Al3(Sc, Zr)粒子Zener钉扎的影响,钪锆添加使Al-Zn-Mg合金再结晶形核机制由立方形核转变为高储能形核,合金再结晶温度由350℃以下提高到550℃以上。(6)研究合金冷轧板材织构主要由立方织构、p纤维轧制织构和少量的高斯织构构成；固溶时效后,Al-Zn-Mg合金主要以立方织构为主,添加钪锆合金主要以轧制织构为主。钪锆添加使Al-5.7Zn-2.0Mg-0.35Cu合金成品板材屈服强度各向异性指数由2.1%增加到7.2%。基于测量的织构数据,计算了成品板材不同拉伸力轴下的Taylor因子,考虑晶界强化、固溶强化及析出强化对屈服强度的影响,建立了成品板材屈服强度各向异性模型,该模型进一步表明织构是引起板材各向异性的主要原因。(7)微量钪锆添加及在120℃下延长时效时间均可有效提高Al-5.7Zn-2.0Mg-0.35Cu合金的腐蚀抗力。经120℃/36h时效后Al-5.7Zn-2.0Mg-0.35Cu-0.25Sc-0.10Zr合金的最小抗应力腐蚀敏感因子>95%,最大晶间腐蚀深度<34μm,剥落腐蚀等级达到PA级,强度和耐蚀性都能满足航天用户的要求。腐蚀抗力的提高来源于晶粒细化、PFZ窄化、晶界平衡相粗化和晶界平衡相离散度的增加。(8)微量钪锆添加到Al-5.7Zn-2.0Mg-0.35Cu基材中,成品板材焊接接头抗拉强度和屈服强度分别提高了4%和23%,满足了航天对这种合金高焊接性的要求。焊接接头强度的提高归因于微量钪锆形成的弥散Al3(Sc, Zr)粒子抑制了基材焊接接头热影响区的再结晶。更多还原

【Abstract】 Combined the National Assembly Pre-research Project of China, two kinds of Al-5.4Zn-2.0Mg-0.35Cu (wt.%) alloys with and without Sc and Zr additions (0.25Sc+0.10Zr(wt.%)) were investigated comparatively by the methods of process optimizing and microscope observations. Effects of Sc and Zr microalloying additions on the microstructures and properties of Al-Zn-Mg alloys during the processing of homogenization, hot-deformation and solution-aging were studied. Based on this, the mechanisms of the effects of Sc and Zr minor additions on the recrystallization, anisotropy, corrosion and welding of Al-Zn-Mg alloys were studied in detail. The main results of the research are as follows:(1) The as-cast grains of Al-5.7Zn-2.0Mg-0.35Cu (wt.%) ingot were refined from78μm into58μm by adding0.10Zr+0.25Sc (wt.%).(2) Under the condition of semi-continuous casting, the matrix is closest to supersaturated solid solutions, non-equilibrium eutectic phases containing Zn and Mg and indissoluble impurity phases containing Fe and Si are concentrated on grain boundaries, and thus it is necessary to homogenize. With the increase of homogenization temperatures, MgZn2and T(Mg32(Al, Zn)49) phases firstly precipitated and then dissolved into matrix, and the non-equilibrium phases distributed on the grain boundaries dissolved. Meantime, in the alloy with Sc and Zr additions, lots of fine and disperse Al3(Sc, Zr) particles precipitated during homogenization, and the proper homogenization process for this alloy is350℃/8h+470℃/12h.(3) During the hot deformation of the two studied ingots, with the increase of deformation temperatures, the strength decreased, the plasticity increased firstly and then decreased, and the fractures transformed from trans-granular fractures into intergranular fractures. Deformed between375℃and400℃, the alloys have stable deformation resistance and higher hot working plasticity. The actual production of hot rolling proved that375℃-400℃was the proper hot working temperatures for the studied ingots. Besides, during hot deformation, lots of disperse Al3(Sc, Zr) particles in Al-Zn-Mg-Sc-Zr alloy strongly pinned dislocations and subgrain boundaries, inhibiting the movement of dislocations and subgrain boundaries, improving hot deformation resistance.(4) Added0.25Sc+0.10Zr into Al-5.7Zn-2.0Mg-0.35Cu alloy, under the optimal homogenization treatment, hot deformation process and solution-aging treatment (470℃/1h, followed by water quenching,+120℃/24h), the ultimate tensile strength and the yield strength increased by12%and22%, meantime, the elongation remained above12%. The strengthening mechanisms of minor Sc and Zr are substructure strengthening, grain refining strengthening and precipitation strengthening of Al3(Sc, Zr) particles.(5) During recrystallization annealing,0.10Zr+0.25Sc additions changed the recrystallization textures of Al-5.7Zn-2.0Mg-0.35Cu alloy from cube textures into rolling textures. A recrystallization nucleation model was established successfully where two kinds of nucleation sites were considered:(i) nucleation sites that have a recovery advantage (or have a boundary energy advantage) and (ii) nucleation sites which require a considerable static recovery period in order to reach the critical size. Based on the model, the store energies for different orientations were calculated. Affected by the Zener drag from Al3(Sc, Zr) particles, the mechanisms of recrystallization texture changed from cube nucleation into high stored energy nucleation and the recrystallization temperatures increased from below350℃into above550℃.(6) The textures of the cold rolled sheets consisted of Cube, β-fiber rolling and Goss textures. After solution-aging treatment, Cube textures were the dominant textures for Al-Zn-Mg alloy, whereas the main textures of Al-Zn-Mg-Sc-Zr alloy were β-fiber rolling textures. Sc and Zr additions increased the in-plane anisotropy values of yield strength from2.1%into7.2%. Based on the measured texture data, Taylor factors were calculated under different force axis. Considered the effects from grain boundary strengthening, precipitation strengthening and solution strengthening, the model of yield strength in-plane anisotropy for the sheet products was successfully established, exhibiting that textures were the main reason for strength anisotropy.(7) Sc and Zr microalloying additions and increasing aging time at120℃can both effectively improve the resistance of stress corrosion cracking, intergranular corrosion and exfoliation corrosion of Al-5.7Zn-2.0Mg-0.35Cu alloy.Aged at120℃for36h, the minimum value of stress corrosion cracking susceptibility was larger than95%, the maximum corrosion depths were smaller than34μm, and the rank of the exfoliation corrosion reached PA in Al-5.7Zn-2.0Mg-0.35Cu-0.25Sc-0.10Zr alloy. The improved corrosion resistance is from inhibiting recrystallization, narrowing PFZ, coarsening grain boundary precipitates and increasing the spacing of grain boundary precipitates.(8) Added minor Sc and Zr into Al-5.7Zn-2.0Mg-0.35Cu matrix, ultimate tensile strength and yield strength of the welding for sheet products increased by4%and23%, respectively. The increased welding strength caused by Sc and Zr is from inhibiting recrystallization of the heat affected zones by Al3(Sc, Zr) particles.更多还原