7075铝合金应力时效强化与机制研究

发布时间：2018-07-26 11:43

【摘要】：Al-Zn-Mg-Cu系铝合金属于可热处理强化型合金,广泛应用于航空航天和汽车结构件。但传统的T6等时效处理很难使得合金同时获得高的强度和优良的抗腐蚀性能。从热力学的角度分析,应力是与温度和组分并列的第三个控制材料组织、结构和性能变化的热力学参量。开展弹性应力下时效铝合金组织与性能的系统研究,可以精确调控合金中的第二相组织,为高综合性能铝合金的制备提供新的实验基础和理论指导。本文采用扫描电镜(SEM)、X射线衍射(XRD)和透射电镜(TEM)分析技术,结合力学性能测试,系统研究了外加应力对7075合金时效组织与性能的影响。7075合金在160 oC应力时效1 h后硬度呈现双峰现象,在25 MPa、100 MPa拉应力和25 MPa、112.5 MPa压应力处合金硬度达到最大值,其屈服强度和抗拉强度也明显提高,延伸率略有下降。与无应力时效状态相比,四种应力时效条件下时效析出相的弥散度更高,平均尺寸更小。拉、压应力促进了合金中较大尺寸MgZn2相的长大和η′亚稳相的析出,压应力同时促进了η稳定相的形成,而拉应力则抑制了η相的析出。拉应力使合金中晶界析出相呈现不连续分布状态。研究了温度对7075合金25 MPa应力时效1 h的组织与性能的影响。在120-180oC经压应力时效处理后合金的硬度、屈服强度和抗拉强度均高于同条件下的无应力时效状态,且合金硬度在150oC时达到最高值178 HV;在较低温度时(120oC)拉应力时效使合金的硬度、屈服强度和抗拉强度低于同条件下的无应力时效状态,在较高温度时(160 oC)二者情况刚好相反,拉应力时效的合金硬度在165 o C时达到最高值180 HV。合金在120oC应力时效1 h后,25 MPa拉、压应力抑制了较大尺寸MgZn2相的长大;许多板状GPII区在无应力时效试样中析出,大量似η′片状体出现在拉应力时效试样中,许多η′亚稳相被发现在压应力时效试样中;各时效处理试样中时效析出相的弥散度依次为:压应力时效状态拉应力时效状态无应力时效状态,析出相的平均尺寸分别为3.1 nm、6.3 nm和12.5 nm。研究了时效时间对7075合金在120 oC和160 oC经25 MPa应力时效的组织与性能的影响。与无应力时效处理相比,120oC压应力时效处理1-32 h内和160oC压应力时效处理1-10 h内合金硬度、屈服强度和抗拉强度得到明显提高。120 oC拉应力时效8-24 h内合金硬度升高较快,并在24 h时达到最高值191 HV,其屈服强度和抗拉强度则变化不大;160 oC拉应力时效1-10 h内合金硬度、屈服强度和抗拉强度得到明显提高;在120oC经25 MPa拉应力时效处理24 h后,合金的抗晶间腐蚀和抗剥落腐蚀性能得到显著增强。研究了7075合金的应力时效机制。与无应力时效状态相比,25 MPa拉、压应力时效使合金中的蜷线位错和位错圈转变为直线段位错,应力增加了时效析出相的形核率,析出相的弥散度增大,导致合金的机械性能升高;50 MPa拉应力和75 MPa压应力时效使合金中的位错发生滑移,位错密度变低,并且位错运动破坏了小尺寸的析出相晶核,使得析出相的弥散度降为最低,合金的机械性能变化不大;100 MPa拉应力和112.5 MPa压应力时效使合金中的位错滑移并产生大量增值,析出相的弥散度再次增大,合金的机械性能得到提高。相比于无应力时效,25 MPa拉应力时效降低了合金中较大尺寸MgZn2相的Zn/Mg值,而25 MPa压应力时效增大了MgZn2相中的Zn/Mg值。
[Abstract]:Al-Zn-Mg-Cu aluminum alloy is a heat-treated reinforced alloy, which is widely used in aerospace and automotive structural parts. However, the traditional T6 isoaging treatment is difficult to make the alloy obtain high strength and excellent corrosion resistance. From the thermodynamic point of view, the stress is the third control materials which are parallel to the temperature and components. The systematic study of the microstructure and properties of aging aluminum alloy under elastic stress can accurately regulate the secondary phase in the alloy and provide a new experimental basis and theoretical guidance for the preparation of high comprehensive properties of aluminum alloy. This paper uses scanning electron microscopy (SEM), X ray diffraction (XRD) and transmission electron microscopy (TEM). The effect of applied stress on the aging structure and properties of 7075 alloy 7075 alloy is studied systematically. The hardness of.7075 alloy in 160 oC stress aging is 1 h, and the hardness of the alloy reaches the maximum value at 25 MPa, 100 MPa tensile stress and 25 MPa, 112.5 MPa compressive stress, and its yield strength and tensile strength are also improved obviously. The elongation rate decreases slightly. Compared with the state without stress aging, the dispersion degree of the precipitated phase in the four stress aging conditions is higher and the average size is smaller. The tension stress promotes the growth of the larger size MgZn2 phase in the alloy and the precipitation of the ETA metastable phase, and the compressive stress also promotes the formation of the ETA stable phase, while the tensile stress inhibits the ETA phase. The tensile stress causes discontinuous distribution of the precipitates in the alloy MICROTEK boundary. The effect of temperature on the microstructure and properties of 7075 alloy 25 MPa stress aging is studied. The hardness, yield strength and tensile strength of the alloy after stress aging treatment in 120-180oC are higher than those under the same condition, and the hardness of the alloy is 150oC. At a relatively low temperature (120oC), the hardness, yield strength and tensile strength of the alloy are lower than the non stress aging state under the same condition at a lower temperature (120oC). At a higher temperature (160 oC) two, the hardness of the tensile stress aging alloy reaches the highest value of 180 HV. alloy at the 1 h of 120oC stress aging at 165 o C. After 25 MPa pulling, the compressive stress inhibits the growth of the larger size MgZn2 phase; many plate like GPII regions are precipitated in the non stress aging specimen, and a large number of eta 'flakes appear in the tensile stress aging specimen, and many of the ETA metastable phases are found in the pressure stress aging specimen; the dispersion degree of the aging precipitates in the physical specimens at each aging place is in turn the compressive stress. The aging state has no stress aging state. The average size of the precipitated phase is 3.1 nm, 6.3 nm and 12.5 nm., respectively, to study the effect of aging time on the microstructure and properties of 7075 alloy at 120 oC and 160 oC through 25 MPa stress aging. Compared with the non stress aging treatment, the 120oC pressure stress aging treatment is 1-32 h within and 160oC compressive stress. The hardness of the 1-10 h internal alloy, yield strength and tensile strength are obviously increased in.120 oC tensile stress aging 8-24 h, and the hardness of the alloy increases rapidly, and the maximum value is 191 HV at 24 h, and the yield strength and tensile strength change little; 160 oC tensile stress aging is 1-10 H internal gold hardness, yield strength and tensile strength are obviously raised. After 25 MPa tensile stress aging treatment with 25 MPa, the resistance to intercrystalline corrosion and exfoliation corrosion of the alloy was significantly enhanced. The stress aging mechanism of 7075 alloy was studied. Compared with the state of non stress aging, 25 MPa pull and pressure stress aging made the curled line dislocation and dislocation in the alloy into a straight line dislocation, and the stress increased the aging time. The nucleation rate of the precipitated phase and the dispersion degree of the precipitated phase increase and the mechanical properties of the alloy increase. 50 MPa tensile stress and 75 MPa pressure stress aging make the dislocation slip and the dislocation density lower, and the dislocation motion destroys the small size of the precipitated phase nucleation, which makes the dispersion degree of the precipitated phase to be the lowest, and the mechanical properties of the alloy changes. 100 MPa tensile stress and 112.5 MPa stress stress aging make the dislocation slip and increase a lot of increment in the alloy, the dispersion degree of the precipitated phase increases again, and the mechanical properties of the alloy are improved. Compared to the non stress aging, the 25 MPa tensile stress aging reduces the Zn/Mg value of the larger size MgZn2 phase in the alloy, and the 25 MPa pressure stress aging increases MgZ. The Zn/Mg value in the N2 phase.
【学位授予单位】：燕山大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG146.21

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