搅拌摩擦加工Mg-Y-Nd合金的组织演变、高温力学性能及体外腐蚀性能研究

发布时间：2018-03-08 14:28

本文选题：搅拌摩擦加工　切入点：Mg-Y-Nd合金　出处：《华南理工大学》2016年博士论文　论文类型：学位论文

【摘要】：作为析出强化型镁合金,Mg-Y-Nd合金具有优异的室温强度,高温耐热性和良好的生物相容性,在航空航天、陆上交通工具、生物医用材料等领域都得到了一定程度的应用。然而,Mg-Y-Nd合金为密排六方结构,导致其在室温的塑性变形能力较差,所以Mg-Y-Nd合金的加工成形和工业应用均受到了一定程度的限制。搅拌摩擦加工(Friction stir processing,FSP)是一种新型的剧塑性变形加工技术,在制备具有较好塑性变形能力的细晶金属材料方面具有较大潜力。尤其是在循环水的辅助冷却作用下,再结晶晶粒的长大受到限制,从而能获得更为细小的晶粒组织。因此,本研究选取了铸造Mg-Y-Nd合金作为研究对象,空气中/水下搅拌摩擦加工(NFSP/SFSP)为制备手段,研究了材料在FSP过程中的微观组织演变、后续时效热处理对加工试样微观组织变化及室温力学性能的影响、加工试样的高温超塑性及在模拟体液(Simulated body fluid,SBF)中的腐蚀行为,为铸造Mg-Y-Nd合金力学性能的提升和加工成形性能的改善提供技术参考,为FSP技术在生物医用材料制备领域的应用提供实验依据。首先,选取了不同的加工参数,分别对铸造Mg-Y-Nd合金进行NFSP和SFSP。研究了不同加工条件下,搅拌区的组织演变和力学性能对比。两种不同加工条件下的FSP后,材料的微观组织均得到了显著细化,力学性能也均得到了大幅提高。相比较而言,由于NFSP过程中产生更大的热积累,NFSP试样搅拌区的中心宽度和平均晶粒尺寸均大于SFSP试样,而且NFSP试样中具有部分再结晶晶粒长大而形成的粗晶带状结构。得益于循环水的冷却作用,SFSP可以制备出晶粒尺寸更为细小的Mg-Y-Nd合金,综合力学性能也更为优异,其中相对最优的加工参数为600 rpm的旋转速度和60 mm/min的行进速度。其次,结合NFSP过程中的热历史变化,对铸造Mg-Y-Nd合金在NFSP过程中,第二相的固溶与析出行为进行了研究与探讨。后续时效热处理后,NFSP试样的伸长率略微降低,强度则得到了进一步提高。在150℃的峰值时效条件下,增强相主要为β′′,而在180℃的峰值时效条件下,增强相主要为β1。时效处理后,大量的析出相为裂纹的萌生提供了更多的形核位置,同时也为裂纹的延伸提供了更为复杂的扩展路径。断口形貌表明,组织内部的第二相颗粒带状结构是造成NFSP及峰值时效试样的伸长率较低的主要原因。再次,选取了晶粒组织更为细小的SFSP试样,在不同的测试温度和应变速率下,进行了高温拉伸测试。得益于细小的α-Mg晶粒和良好的组织稳定性,SFSP制备的细晶Mg-Y-Nd合金在733 K和3×10~(-3) s~(-1)的测试条件下获得了最大的伸长率(967%);在758 K和2×10-2 s~(-1)的测试条件下获得了最优的高应变速率超塑性(900%);在应变速率为1×10~(-1) s~(-1)和708-758 K的温度范围内,伸长率都超过了500%。由于与基体之间存在良好的变形协调性,第二相颗粒并未导致变形过程中明显的应力集中,裂纹也主要形成于晶界位置。超塑性变形过程中的主要变形机制为晶格扩散协调的晶界滑移,断裂机制为微孔聚集。最后,研究了微观组织的细化及均匀化对Mg-Y-Nd合金腐蚀行为的影响。电化学测试结果和体外浸泡试验结果共同表明,NFSP和SFSP试样的耐腐蚀性能明显优于铸态试样。三者的主要腐蚀机理均为电偶腐蚀。不同的是,由于微观组织中含有粗大的α-Mg晶粒和第二相,电偶腐蚀导致的粗大第二相脱落或整个晶粒的剥离,让铸态试样的内部基体组织在模拟体液中大面积的裸露,出现严重的局部腐蚀行为。得益于晶粒的细化和第二相的破碎,在同样的腐蚀情况下,NFSP和SFSP试样裸露的内部基体组织相对较少,二者均呈现出了较为均匀的腐蚀形貌。此外,力学性能测试表明,铸态试样在SBF中浸泡5天后,最大拉伸载荷急剧下降为原始值的20%。通过FSP制备的细晶Mg-Y-Nd合金,在模拟体液中浸泡12天后仍能保持一定的机械完整性,仍能具备一定的承载能力。
[Abstract]:As the precipitation hardened magnesium alloy, Mg-Y-Nd alloy has excellent room temperature strength, high temperature resistance and good compatibility in biological, aerospace, land transport, the field of biomedical materials have been applied to a certain extent. However, Mg-Y-Nd alloy is close packed six party structure, resulting in plastic deformation poor at room temperature, so the processing and application of Mg-Y-Nd alloy are subject to certain restrictions. The friction stir processing (Friction stir, processing, FSP) is a new kind of severe plastic deformation processing technology has great potential in fine grain metal materials good plastic deformation ability in the preparation of the particular. In the auxiliary cooling water circulation under the grain is restricted, so as to obtain more fine grain structure. Therefore, this study selected the casting Mg-Y-Nd alloy as the research object Friction stir processing, water / air (NFSP/SFSP) for the preparation method, microstructure of materials in FSP in the process of evolution, affected the subsequent aging heat treatment on the mechanical properties at room temperature change and the microstructure of the specimen processing, processing and superplastic specimens in simulated body fluid (Simulated body, fluid, SBF) corrosion behavior the provided technical reference for the mechanical properties of Mg-Y-Nd alloy casting lifting and shaping performance improvement, to provide the experimental basis for the application of FSP technology in biomedical material preparation field. First, select the different processing parameters, respectively for casting Mg-Y-Nd alloy NFSP and SFSP. were studied under different processing conditions, contrast group evolution and mechanical properties of the stir zone. Two kinds of different processing conditions FSP, microstructure of materials have been significantly refined, the mechanical properties are greatly improved compared. By contrast, due to greater heat generated in the process of NFSP accumulation, NFSP sample mixing area of the center width and the average grain size is greater than SFSP samples, and NFSP samples with coarse grained banded structure part of recrystallized grains formed. Due to the cooling effect of circulating water, SFSP can produce more grain size Mg-Y-Nd alloy fine, comprehensive mechanical properties are more excellent, the rotation speed of the relative optimal processing parameters for 600 rpm and 60 mm/min speed. Secondly, combined with the thermal history changes of NFSP in the process of casting Mg-Y-Nd alloy in the NFSP process, the second phase of the solid solution and precipitation behavior with the subsequent discussion. After aging heat treatment, NFSP specimen elongation decreased slightly, the strength is improved further. In the peak aging condition of 150 DEG C, mainly for the reinforcement of B '', and in the peak of 180 DEG C when The effect under the condition of reinforcement is mainly beta 1. after aging treatment, a large number of precipitates provided more nucleation sites of crack initiation, crack extension also provides a more complex propagation path. The fracture morphology showed that the particle banded structure of the organization is a major cause of NFSP and peak aging specimens of lower elongation. Again, select the grain more fine SFSP specimen, in the test of different temperature and strain rate, the high temperature tensile test. A -Mg due to the small grain and good stability, SFSP preparation of fine grain Mg-Y-Nd alloy in 733 K and 3 * 10~ (-3) s~ (-1) under the conditions of the test to get the maximum elongation (967%); 758 in K and 2 * 10-2 s~ (-1) test conditions to obtain optimal high strain rate superplasticity (900%); the strain rate of 1 * 10~ (-1) s~ (-1 708-758) and K The temperature range of elongation is more than the existence of good distortion coordination between 500%. and the matrix, the second phase particles did not result in the process of deformation obvious stress concentration, cracks are mainly formed in the grain boundaries. For lattice diffusion of grain boundary sliding deformation of the main coordination mechanism in the process of superplastic deformation, the fracture mechanism is micropore aggregation. Finally, the refinement of microstructure and the effect of homogenization on the corrosion behavior of Mg-Y-Nd alloy. The results of electrochemical test and in vitro immersion test results show that NFSP and SFSP, the corrosion resistance of the sample was superior to the cast specimen. The main corrosion mechanism of the three are different. The galvanic corrosion is due to a -Mg grain the microstructure contains thick and the second phase, stripping the galvanic corrosion caused by the coarse second phase shedding or whole grain, make the internal microstructure of as cast samples in simulated body A large area of liquid in bare, local corrosion behavior serious. Due to the grain refinement and the second phase of the crushing, corrosion in the same conditions the NFSP and SFSP specimens exposed internal microstructure is relatively small, the two are showing a more uniform corrosion morphology. In addition, the mechanical properties tests show that the cast samples after soaked in SBF for 5 days, the maximum tensile load decreased sharply in fine grained Mg-Y-Nd alloy as the original value of the 20%. prepared by FSP, after 12 days of immersion in SBF can still maintain a certain mechanical integrity, can still have a certain carrying capacity.

【学位授予单位】：华南理工大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG146.22

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