【作者】 严杰；

【导师】 张金山；

【作者基本信息】 太原理工大学 ， 材料物理与化学， 2009， 硕士

【摘要】 自二十面体准晶(I-phase)被发现以来,一直是材料科学界所研究的热点。准晶的制备、性能及应用研究也一直受到人们普遍的关注,但是由于准晶自身脆性的限制,使得它不能够作为结构材料而直接应用。尽管如此,准晶由于其独特的准周期性晶体结构,使得准晶具有高硬度、耐热性和低表面能等独特性能,使其特别适合作为韧性基体材料中的强化相。采用镁基准晶相作为镁合金的增强相,能够很大程度地提高镁合金的室温及高温性能,具有重大的应用价值。本文正是基于准晶颗粒增强镁基复合材料的思想,为准晶增强镁基复合材料设计一种新型准晶中间合金。实验采用常规金属型铸造方式制备含球形二十面体准晶相颗粒的Mg-Zn-Nd中间合金。利用X-ray衍射分析仪、金相显微镜、扫描电子显微镜及透射电子显微镜等分析手段,研究了Mg-Zn-Nd中间合金的显微组织变化规律,确定了Mg-Zn-Nd球形二十面体准晶相的结构类型,确定了球形准晶相形成的合金成分范围。本文的主要研究结果如下:1)研究表明,通过常规铸造方法,在本实验所设计的几种Mg-Zn-Nd中间合金中,能够得到具有五重轴对称特征结构的二十面体准晶相,其宏观形态异于其它准晶体,呈现出高圆整度的球形状,准晶相颗粒尺寸在15μm以下。TEM及X-ray衍射分析表明,该球形准晶相是一种典型的Frank-Kasper型简单二十面体准晶,准晶格参数aR = 0.525nm,化学成分为Mg40Zn55Nd5。球形Mg40Zn55Nd5二十面体准晶的价电子浓度比e/a = 2.05,在由Hume-Rothery规则确定的Frank-Kasper型准晶的价电子浓度比值(2.0-2.15)之内。球形准晶相能够在327℃的退火温度下长期稳定存在。此外,在一些实验合金组织中出现的杆状相是一种具有六方晶体结构的三元金属间化合物相,成分近似为Mg68Zn30Nd2,晶格参数为a = 0.3975nm,c = 0.6529nm,c/a = 1.64,晶体结构及c/a值与Mg相近。2)通过研究合金中Nd元素含量的变化对合金铸态显微组织的影响,发现球形准晶相颗粒的数量是随着Nd元素的递增呈正态函数分布的。当合金的Mg/Zn原子比值维持在2.5-2.6之间时,合金的显微组织变化规律如下:①当Nd含量小于1.2at%时,合金的铸态显微组织主要由Mg7Zn3基体+α-Mg相+六方Mg68Zn30Nd2杆状相及少量的球形Mg40Zn55Nd5二十面体准晶相组成;②当Nd含量在1.2at%左右时,合金的铸态组织由柱状Mg7Zn3单相基体+球形Mg40Zn55Nd5准晶相颗粒组成;③而当Nd含量等于和大于1.5at%时,合金组织中球形Mg40Zn55Nd5准晶相颗粒的数量急剧减少和消失,并出现了一种呈六边形形状的Mg28Zn65Nd7金属间化合物相(在此称之为Z相),并且随着Nd元素含量的继续增多其数量也在增多。同时,合金组织中还有枝晶状α-Mg相,少量的Zn2Mg Laves相以及α-Mg +MgZn的细层片状共晶组织。3)实验结果表明,Mg/Zn原子比值能够影响到合金中球形准晶相颗粒的数量,要获得较多球形准晶相,需要控制合金的Mg/Zn原子比值。仅当Nd含量在1.2at%,而Mg/Zn原子比值在2.5-2.6之间时,合金中的球形准晶相颗粒数量最多。超出这个比值范围,球形准晶相颗粒数量会减少,并且合金的组织也会发生改变,当Mg/Zn原子比值在2.0-2.4之间时,合金组织中会出现枝晶状α-Mg相;而当Mg/Zn原子比值在2.7-3.3之间时,合金组织中则出现六方Mg68Zn30Nd2杆状相。4)要获得数量较多的球形准晶相,不仅要严格将Mg-Zn-Nd中间合金的成分控制在合适范围,而且要制定合适的制备工艺,主要是冷却速度和浇铸温度控制。如果合金熔体的冷却速度太慢,球形准晶相就会充分生长,不仅尺寸增大,而且其形态会演化成多面体形状甚至枝晶状。实验结果表明,形成球形准晶相的冷却速度应大于某一个临界冷却速度,依据阶梯型冷却速度实验,形成球形准晶相的临界凝固模数M应小于3.3。更多还原

【Abstract】 Since the first discovery of icosahedral quasicrystal phase (I-phase), it has been a research focus in materials science area all along. Many attentions have been made on the investigation into its preparation, properties and application. However, quasicrystal can not be directly used as structural application materials due to its innate high brittleness. Whereas, quasicrystal possesses some unique properties such as high hardness, heat-resistant and low surface energy due to its unusual quasi-periodic lattice structure, which quite favors for its application as a strengthening phase in toughness matrix materials. The room and elevated temperature properties of magnesium alloy can be remarkably improved by using quasicrystal as reinforced phase, which show great promising application future. This research is based on the idea of quasicrystal strengthening magnesium-based composite materials and provides a new quasicrystal master alloy for strengthening magnesium alloy. The Mg-Zn-Nd master alloy containing spherical I-phase particles has been prepared by conventional metal casting method in the paper. The microstructure evolvement law of Mg-Zn-Nd master alloy has been detailedly discussed by using X-ray diffraction, optical microscope, scanning electron microscope and transmission electron microscope. At the same time, the crystal structure type of Mg-Zn-Nd spherical I-phase and the composition range of alloy forming spherical I-phase were both confirmed in the paper.The main research conclusions can be summarized as follows:1) I-phase with five-fold symmetry can be obtained in the designed Mg-Zn-Nd master alloy by conventional metal casting method. The macroscopical morphology of Mg-Zn-Nd I-phase is different from others, which present high roundness spherical morphology. The average diameter of spherical I-phase is confined to 15μm. X-ray diffraction and TEM analysis results indicate that the spherical quasicrystal phase is a typical Frank-Kasper type simple icosahedral quasicrystal with stoichiometric composition of Mg40Zn55Nd5 and quasi-lattice aR = 0.525nm. The e/a value of I-phase are 2.05, which is at the range of e/a value (2.0-2.15) for Frank-Kasper type icosahedral structure through the Hume-Rothery rule. The spherical I-phase can be at least stable up to annealing temperature of 327℃. Furthermore, the rod-like phase obtained in Mg-Zn-Nd experimental alloy is a hexagonal ternary intermetallic phase with stoichiometric composition of Mg68Zn30Nd2 and crystal lattice a = 0.3975nm,c = 0.6529nm,c/a = 1.64. The crystal structure and crystal lattice ratio c/a of rod-like phase is quite close to magnesium.2) After the investigation into the effect of Nd content on the microstructures of as-cast alloy, it can be clearly seen that the quantity of spherical I-phase particles vary with the increasing content of Nd element. When Mg/Zn atom ratio value limit within 2.5-2.6, the microstructure evolvement law of Mg-Zn-Nd alloy can be concluded as follows:①When the content of Nd less than 1.2at%, the alloy microstructures mainly consist of Mg7Zn3 matrix phase,α-Mg phase, hexagonal Mg68Zn30Nd2 rod-like phase and spherical Mg40Zn55Nd5 I-phase.②When the content of Nd is 1.2at%, the as-cast microstructure of alloy mainly consists of spherical Mg40Zn55Nd5 I-phase and Mg7Zn3 single-phase matrix.③After the content of Nd exceed 1.5at%, the spherical I-phase particles decreased a lot or even disappeared. At the same time, a new Mg28Zn65Nd7 intermetallic phase with hexagon morphology can be obtained in the alloy and its quantity increased with the increasing content of Nd. Furthermore, the microstructures of alloy containα-Mg phase, Zn2Mg Laves phase and eutectic microstructure ofα-Mg + MgZn.3) The research results indicate that the quantity of spherical I-phase particles can be affected by the variation of Mg/Zn atom ratio value. Only when the content of Nd is 1.2at% and atom ratio value of Mg/Zn at the range of 2.5-2.6, the amount of spherical I-phase particles is maximal among all the experimental alloys. If the Mg/Zn atom ratio value besides 2.5-2.6, the spherical I-phase particles decreased a lot with the variation of alloy microstructures. The α-Mg phase can be obtained in the alloy microstructures with Mg/Zn atom ratio value of 2.0-2.4, while hexagonal Mg68Zn30Nd2 rod-like phase can be obtained when the value is 2.7-3.3.4) To obtain more spherical I-phase particles, the cooling rate, casting temperature and nominal composition of Mg-Zn-Nd alloy should be accurately controlled. If the cooling rate is too slow, the spherical I-phase would grow more heavily and the morphology of I-phase transform into polyhedron or even dendritic shape. According to the metal mould cooling rate experiment, the cooling rate of alloy melt should reach a critical rate to form spherical I-phase and the critical solidification modulus (M) should less than 3.3 in this research.更多还原