【作者】 聂鑫；

【导师】 赵东山； 王建波；

【作者基本信息】 武汉大学 ， 凝聚态物理， 2015， 博士

【摘要】 在时效强化镁合金中,镁锡系(Mg-Sn)合金和镁锌稀土系(如Mg-Gd-Zn)合金以其优良的可铸造性、高温下的机械强度和抗蠕变性能得到了广泛关注,特别是关注并研究其时效过程中析出的沉淀相的晶体学特征以及对合金力学性能的影响机理。本论文采用当前较先进的透射电子显微术包括高分辨透射电子显微术(HRTEM)以及球差矫正的高角环形暗场扫描透射电子显微术(HAADF-STEM)结合X射线衍射(XRD)与扫描电子显微术(SEM)等常规微观分析手段,研究了平衡凝固下铸态镁锡(Mg-9.76wt.%Sn)合金和镁钆锌(Mg-9.05wt.%Gd-2.85wt.%Zn)合金经过热处理后晶粒内沉淀相的析出规律、晶体学特征及时效强化机理；同时研究了快速凝固技术制备的相同成分Mg-9.76wt.%Sn合金的微观组织特征,并将非平衡快速凝固同铸态的结果进行了对比。对固溶处理后(823K保温12小时后水淬)的铸态Mg-9.76wt.%Sn合金分别进行高温(573K)和低温(453K)的时效处理,测得硬度曲线。结果表明,该合金高温(573K)时效5小时达到峰值硬度63.5Hv,低温(453K)时效425小时达到峰值硬度67.0Hv。采用透射电子显微术(TEM)的选区电子衍射方法,对固溶处理后(823K保温12小时后水淬)的铸态Mg-9.76wt.%Sn合金在高温(573K)和低温时效(453K)硬度峰值时样品中析出相β-Mg2Sn颗粒同基体a-Mg的取向关系进行了观察和统计分析。结果发现：高温时效时(573K)析出相同基体的取向关系为OR-3((110)β//(0001)α,[111]β／／(1互10)α)的颗粒数目占75.1%；析出相同基体的取向关系为OR-4((110)β//(0001)α,[001]β／／(2110)α)的颗粒数目占24.3%；析出相同基体的取向关系为OR-1((111)β//(0001)。,[110k//(2110)。)的颗粒数目占0.6%。低温时效(453K)峰值硬度时析出相β-Mg2Sn同基体的取向关系为OR-4的颗粒数目占77.4%,析出相同基体的取向关系为OR-3的颗粒数目占22.6%。本文根据固态相变的晶体学理论(三维不变线应变模型),在倒易空间建立了由基体α-Mg母相(HCP)到析出相β-Mg2Sn (FCC)的转变矩阵,解释了镁锡系(Mg-Sn)合金在高温时效过程中析出相β-Mg2Sn更容易以OR-3的取向从基体中析出和生长,丰富了HCP到FCC固态相变的晶体学理论。本文根据在高温时效过程中析出相β-Mg2Sn同基体α-Mg界面的高分辨透射电子显微相位衬度像和高角环形暗场原子序数(Z)衬度像,提出了OR-3和OR-4取向下析出相β-Mg2Sn颗粒同基体α-Mg间惯习面的界面结构模型,与实验结果符合较好。对快速凝固技术制备的非平衡凝固Mg-9.76wt.%Sn条带进行了力学性能(显微硬度)测量与微观组织分析,测得平均显微硬度为69.4Hv,α-Mg晶粒平均粒径3.824μm,相比铸态的一百微米级的平均粒径,大幅降低。根据霍尔佩奇关系式σs=σ0+kd-1/2,由于镁合金的k值较大,因此当粒径d大幅降低时镁合金的强韧性可获较大提高。在快速凝固Mg-9.76wt.%Sn条带样品中发现具有D019结构的亚稳相β"-Mg3Sn相,它属于密排六方结构(HCP),晶格常数aβ"=2αα, cβ"=cα,与基体α-Mg的取向关系为(0110)β"//(0110)α,(0001)β"//(0001)α,(21 10)β"//(2110)α。利用高分辨透射电子显微术(HRTEM)以及球差矫正的高角环形暗场扫描透射电子显微术(HAADF-STEM),观察和表征了经773K保温16小时后铸态Mg-9.05wt.%Gd-2.85wt.%Zn合金中14H型长周期结构相,验证了前人提出了此14H长周期结构相的原子结构模型,且与实验结果符合较好。更多还原

【Abstract】 Among age-hardening magnesium-based alloys, the magnesium-tin (Mg-Sn) based alloys and magnesium-gadolinium-zinc (Mg-Gd-Zn) based alloys have been attracting increasing attentions of researchers due to their excellent castability, mechanical strength and creep resistance at elevated temperatures. Now, reseachers especially discuss and study the precipitates’crystallography and the affection mechanism of mechanical properties during aging treatment.In this thesis, the precipitation process, the precipitates’crystallography and the age-hardening mechanism for heat-treated as-cast Mg-9.76wt.%Sn alloy and as-cast Mg-9.05wt.%Gd-2.85wt.%Zn alloy have been investigated using X-ray diffraction (XRD) technology, scanning electron microscopy (SEM) and advanced transmission electron microscopy (TEM) including high-resolution TEM (HRTEM) and Cs-corrected atomic resolution high-angle annular dark-field scanning TEM (HAADF-STEM). The referential Mg-9.76wt.%Sn alloys prepared by rapidly solidification technology have been studied and compared with those as-cast alloys at the same time.The as-cast Mg-9.76wt.%Sn alloys, which were dissolved at 823K for 12 hours and quenched in water, have been heat-treated at 573K and 453K respectively using traditional aging method. The aging hardness curves for the two aging processes had been measured. The experiments revealed that, the peak harness is 63.5Hv for the as-cast Mg-9.76wt.%Sn alloy which was heat-treated at 573K for 5h; and the peak harness is 67.0Hv for the as-cast Mg-9.76wt.%Sn alloy which was heat-treated at 453K for 425h.The orientation relationships between the precipitated β-Mg2Sn particles and the matrix a-Mg had been analyzed and counted statistically in peak hardness samples for as-cast Mg-9.76wt.%Sn alloy aging at 573K and 453K using the selected-area electron diffraction method in TEM. The results aging at 573K showed that, the proportion of the precipitated β-Mg2Sn particles which had the orientation relationship of OR-3 ((110)β//(0001)α, [111]β//(1210)α) was 75.1%; the proportion of the precipitated β-Mg2Sn particles which had the orientation relationship of OR-4 ((110)β/(0001)α, [001]β//(2110)α) was 24.3%; the proportion of the precipitated β-Mg2Sn particles which had the orientation relationship of OR-1 ((111),//(0001)α, [110]β//(2110)α) was 0.6%. The results aging at 453K showed that, the proportion of the precipitated β-Mg2Sn particles which had the orientation relationship of OR-4 was 77.4%; the proportion of the precipitated P-Mg2Sn particles which had the orientation relationship of OR-3 was 22.6%.According to the crystallographic theory in solid-state phase transformation, the crystallographic factor which determines the predominance of OR-3 in precipitated P-Mg2Sn particles aging in 573K was explained by a three-dimensional invariant line model constructed using a transformation matrix from a-Mg (HCP) to β-Mg2Sn (FCC) in reciprocal space.The interface structure models between the precipitated β-Mg2Sn particles with OR-3 and the matrix a-Mg, the interface structure between the precipitated P-Mg2Sn particles with OR-4 and the matrix a-Mg, had been constructed using HRTEM and HAADF-STEM images, and had a good greement with the experimentals.The microstructure investigation and mechanical strength (micro-hardness) measurement were carried out on the rapidly solidified Mg-9.76wt.%Sn ribbons. The result showed that, the average micro-hardness was 69.4 Hv, and the grain size of a-Mg was 3.824 μm and decreased sharply compared with as-cast samples. According to the Hall-Petch relationship σs=σ0+kd-1/2 and owing to the large value of k of the magnesium alloys, the strength of the magnesium alloys would be greatly improved by the deep drop in the grain size d. A non-equilibrium phase β"-Mg3Sn with the structure of DO19 had been found in this rapidly solidified Mg-9.76wt.%Sn ribbons. It has a close-packed hexagonal structure (HCP) with αβ"=2αα, cβ= cα and the orientation relationship between the DO19 phase and the matrix a-Mg is (0110)β"//(0110)α, (0001)β"//(0001)α, (2110)β"//(2110)α.HRTEM and HAADF-STEM method were carried out to observe and characterize these 14H long period stacking ordered (LPSO) phase in as-cast Mg-9.05wt.%Gd-2.85wt.%Zn that was heat-treated at 773K for 16h, and the atomic structure model was improved and had a good greement with the experimental images by summarizing the results of previous studies.更多还原