摘要
镁基复合材料由于高的比强度和比刚度、低的热膨胀系数等优点。而成为提高镁合金高温性能的主要途径之一。原位内生镁基复合材料除了可以克服外加增强体所造成的污染外,还具有组织细小、界面结合好等优点,被认为是镁基复合材料研究和发展方向之一。
本文利用Mg-Si的结晶特点,采用熔融铸造法在镁基体上原位生成Mg_2Si增强颗粒,成功制备出不同Mg_2Si含量的Mg_2Si/Mg基复合材料。研究表明,不同Si的加入量会影响生成的Mg_2Si颗粒的数量、形态和分布。当Si的含量超过Mg-Si共晶成分(1.34wt.%)后,随着Si的加入量增加,虽然不能改变共晶Mg_2Si的数量、形态和分布,但是初生Mg_2Si的量会逐渐增加,形态会呈“细小的颗粒-粗大的颗粒-花瓣状-单支树枝晶-多支树枝晶”的规律演变,当Si含量达到3wt.%时,初生Mg_2Si颗粒局部有团聚现象。
随后考察了不同温度、不同外加应力载荷条件下,原位自生Mg_2Si/Mg基复合材料的高温蠕变行为。结果表明:温度、载荷、增强相含量是影响Mg_2Si/Mg基复合材料蠕变性能的三个主要因素。在较高的蠕变温度和应力下,复合材料表现出较差的蠕变性能,随着Mg_2Si含量的增加,复合材料的蠕变性能逐渐提高。通过材料的蠕变表观应力指数和蠕变表观激活能分析,发现原位自生Mg_2Si/Mg基复合材料的蠕变变形机制为蠕变由位错攀移和第二相增强控制。当Mg_2Si增强相含量较低,蠕变温度较高时,蠕变过程主要由位错攀移控制;而当增强相含量较高,蠕变温度较低时,主要由第二相增强控制;当蠕变条件处在两者之间时可能由位错攀移和第二相增强共同控制。通过对蠕变拉伸断口的扫描分析可知,Mg_2Si/Mg基复合材料的蠕变断裂过程是由韧窝和准解理断裂混合控制的蠕变断裂。
Due to the high specific strength and specific stiffness, low thermal expansion coefficient, high young’s modulus, Magnesium matrix composites are considered as one of the main choice to improve the performance of magnesium alloys. In-situ magnesium matrix composite are considered as one of magnesium matrix composites research and development directions, not only it can overcome the pollution caused by external enhanced,but also it have small organizations,well interface bonding, etc.
In this thesis, make use of the crystallization characteristics of Mg-Si alloy, in-situ high-performance Mg_2Si particle can be formed in the process of melt casting, therefore it successfully prepared Mg_2Si/Mg composites with different content of Mg_2Si. Besides, adding different Si content will affected the amount,shape and distribution of Mg_2Si particle. When the Si content is more than Mg-Si eutectic composition (1.34wt.%), with the increase of Si content, the amount of paimary Mg_2Si will increase gradually, and its morphology will evolve, and take on“small particles-coarse particles-petal-single dendrite-multivessel dendrite”. When the Si content is up to 3wt.%, the paimary Mg_2Si will agglomerate obviously.
The high temperature creep behavior of in-situ Mg_2Si/Mg composites is researched by a series of creep experiment. The results indicate that temperature, stress and the content of Mg_2Si are the main effect factors of creep properties. The Mg_2Si/Mg composites show more poor creep properties with the increase of temperature and stress, and composits will shown higher creep properties with the increase of Mg_2Si content. It is belived that the creep mechanism of Mg_2Si/Mg composites is dislocation climb and enhanced control of the second phase through analysis of apparent stress exponent and apparent activation energy. Through analysising of the tensile creep fracture, it can conclude that the creep fracture process of Mg_2Si/Mg is a kind of hybrid control of dimples and quasi-cleavage fracture.
引文
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