Mg-7Gd-3Y-0.4Zr合金显微组织与性能研究

【作者】 官迪凯；

【导师】 梁叔全；

【作者基本信息】 中南大学 ， 材料物理与化学， 2010， 硕士

【摘要】 采用金相显微镜(OM)、透射电镜(TEM)、场发射扫描电镜(FE-SEM)、能谱仪(EDS)、X射线衍射仪(XRD)、失重试验及极化曲线等方法研究Mg-7Gd-3Y-0.4Zr (GW73K)中显微组织与合金力学性能与耐腐蚀性能的关系,同时还对合金表面不同温度形成的氧化膜结构及腐蚀性能进行了初步研究,揭示合金基体显微组织对力学性能和腐蚀性能的影响规律。获得了如下的研究结论：(1) GW73K合金的时效脱溶序列为：(S.S.S.S.)β”(D019)→β’(BCO)→β(Mg5Gd,FCC),不论在200℃,225℃还是250℃进行等温时效,峰值时效的主要析出相都为p’相(BCO)。(2)GW73K在225℃和250℃高温时效时,Gd和Y在基体中固溶度的增加和粗大的p’相的形成导致整个样品的析出相体积密度降低,p’相的体积分数随着时效温度的升高而降低。而p’相的减少直接导致合金的力学性能下降,硬度值大幅度降低。GW73K合金在200℃时效120h后硬度达到最大值的原因是因为合金基体中析出了大量弥散分布的细小β’相。(3)热处理影响GW73K合金腐蚀产物的形貌,尽管腐蚀产物几乎都为Mg(OH)2,但是F态样品主要由蓬松的颗粒构成,T4态样品由紧凑和均匀分布的三角形粒子组成。而T6态样品由于p’在基体中的出现。使得腐蚀产物具有两种典型形貌,少部分区域发生了局部腐蚀。由于腐蚀产物的形貌不同,导致腐蚀性能有显著的差异,耐腐蚀性能的大小顺序为T4>T6>F。(4)GW73K峰值时效的样品随着等温时效温度的增加而增大。高的腐蚀速率可以归因于p’相与基体间的电偶腐蚀。在高温等温时效时,β’相比较大更容易作为电偶腐蚀阴极相加速腐蚀。极化曲线测得200℃峰值时效的腐蚀电流密度要小于225℃和250℃峰值时效样品,结果与峰值时效样品的显微组织变化相一致。(5)时效膜和常温膜的致密度低于固溶膜,且固溶膜的厚度也为最大,使得固溶膜样品在浸泡腐蚀时更容易阻隔镁合金基体与外部腐蚀溶液的接触。而且稀土元素Gd,Y在固溶处理过程中也同时参与了氧化过程。并且与氧的亲和力要大于Mg,因此镁合金耐腐蚀性能的提高与生成的Gd,Y氧化物有着密切的联系。更多还原

【Abstract】 The microstructure, mechanical property and corrosion behavior of Mg-7Gd-3Y-0.4Zr (GW73K) were investigated by optical microscopy(OM), transmission electron microscopy (TEM), field emission scanning electron microscope (FE-SEM), EDS, XRD, immersion test and potentiodynamic polarization. Meanwhile, the microstructure and corrosion resistance of oxide films formed in different temperatures were also discussed. The main results show that:(1) The precipitation sequence for GW73K was super-saturated solid solution (S.S.S.S.)→β"(DOi9)→β’(BCO)→β(Mg5Gd,FCC), and the main precipitate wasβwhen the alloy was peak-aged at 200℃,225℃and 250℃.(2) When GW73K was aged at 225℃and 250℃, the solubility of Gd and Y in Mg and coarseβ’largely reduced the density number ofβ’. The hardness improvement of peak-aged alloy was mainly ascribed to the presence ofβ’phase. The volume fraction ofβ’phase decreased with increasing aging temperature while the average size of the precipitate particles increased. This alloy, when peak-aged at 200℃for 120h, displayed the highest hardness owing to the existence of a great manyβ’ precipitate particles with a quite small size and uniform distribution in the matrix.(3) Heat treatment influenced the morphologies of corrosion products of GW73K alloy. The corrosion product was identified as Mg(OH)2 by XRD. For F sample, the corrosion product was composed of fluffy particles. For T4 sample, the product was compact and uniform with triangle shape particles. Two totally different morphologies appeared in T6 sample because of the presence ofβ’precipitations with a discontinuous distribution. The corrosion rates were T4> T6>F due to different morphologies of corrosion products.(4) The corrosion rates of the peak-aged samples increased as the isothermal aging temperature rise. The higher corrosion rates can be ascribed to the micro-galvanic corrosion acceleration byβ’phases. Theβ’precipitates were larger and appeared to be more active as galvanic cathodes for the alloy peak-aged at the higher aging temperatures. The potentiodynamic polarization curves showed that the corrosion current densities for the samples peak-aged at 225℃and 250℃were higher than for the sample peak-aged at 200℃. The results for the polarization curves were consistent with the microstructures of all the samples.(5) The film formed at solution temperature was more compact than the films formed in room temperature and aging temperature, and the thickness of the solution film was the thickest. Hence, the film can be easier to protect Mg matrix from NaCl solutions. In addition, during solution, Gd and Y were also oxidized. Therefore, the improvement of corrosion resistance of the alloy had strong connection with the formed oxide.更多还原