【作者】 张永青；

【导师】 张金山；

【作者基本信息】 太原理工大学 ， 材料加工工程， 2010， 硕士

【摘要】 准晶的发现,不仅打破了传统晶体学对于晶体的定义,而且也带来了材料学科的一场革命。随着人们对准晶研究的深入,准晶的理论不断的完善,而且准晶的应用中也取得了积极的进展。由于准晶材料硬而脆的特性,极大地限制了其在工程领域的应用。但是准晶所独有的准周期晶体结构,决定了准晶具有高硬度、高耐热性和低表面能等性质。从而,使其可以用作韧性材料中的增强相。采用锌基准晶增强锌基合金为实现准晶在工业领域中的应用提供了一条途径。本文利用常规铸造金属型冷却的方式制备Zn-Mg-Ti准晶中间合金,并通过外加的方式应用于ZA27中,以提高ZA27的耐磨性,并细化其组织。主要利用了X-ray衍射分析仪,金相显微镜,扫描电子显微镜,能谱分析仪等手段,研究了Zn-Mg-Ti准晶中间合金的显微组织变化规律以及Zn-Mg-Ti准晶相形成的成分范围。并且对Zn-Mg-Ti准晶相及其近似相的生长规律和形态特征进行了研究。在利用Zn-Mg-Ti准晶增强ZA27的过程中,主要研究了Zn-Mg-Ti准晶中间合金的加入量对ZA27的组织细化程度和硬度的影响。本文主要的研究结果表明(1)通过调整铸造工艺,在常规铸造金属型冷却的条件下,可以在成分为Zn67Mg28Ti5的合金中制备含有Zn-Mg-Ti准晶相及其近似相的中间合金。通过X-ray衍射分析确定所制备的Zn67Mg28Ti5合金中包含有两种结构:基体MgZn2和二十面体结构。研究发现Zn-Mg-Ti准晶相的形态是六边形相和六瓣花瓣状相,其成分为Zn71Mg21Ti8;Zn-Mg-Ti准晶近似相的形态是四边形相和四瓣花瓣状相,其成分为Zn84Ti16。六瓣状准晶相的核心是六边形相,四瓣状的近似相的核心是四边形相,核心的形态决定了后期所形成相的形态。(2)在常规铸造金属型冷却的条件下制备Zn-Mg-Ti准晶中间合金,Ti的比例应该是5at.%,当超过这个值时凝固后的合金中会形成一种块状相,其成分为Zn68Mg13Ti19。同时由于块状相的形成,组织中准晶相及其近似相的数量急剧的减少。在Ti含量不变的情况下,锌的比例应该为65-70at.%,相应的镁的比例为30-25at.%,在这个范围内均会有Zn-Mg-Ti二十面体准晶的形成。当成分为Zn67Mg28Ti5的时候,所形成的Zn-Mg-Ti二十面体准晶相及其近似相的量达到最多。(3)在Zn-Mg-Ti准晶中间合金熔体中,Zn-Mg-Ti准晶相及其近似相的形核需要克服一定的形核势垒。Zn-Mg-Ti准晶相在形核后的长大过程所需的动力来源于Zn-Mg-Ti准晶相变驱动力。这个Zn-Mg-Ti准晶的相变驱动力的大小取决于熔体的过冷度,而且驱动力越大,形成的Zn-Mg-Ti准晶相及其近似相的体积也越大。Zn-Mg-Ti准晶及其近似相的的形态取决于各个方向上的生长速率,而生长速率的差异性主要的影响因素是界面曲率变化引起的过冷。在Zn-Mg-Ti准晶及其近似相的相界面曲率变化较大的尖角部位的生长速率最快,所以形成了花瓣状的形态。Zn-Mg-Ti准晶及其近似相的生长是层状生长,其界面为光滑界面。Zn-Mg-Ti准晶相及其近似相的生长行为与晶体的差别不大。(4)在利用Zn-Mg-Ti准晶中间合金增强锌铝合金ZA27时,Zn-Mg-Ti准晶中间合金的加入温度应该是600℃。在此温度加入到ZA27中,并且保温一定的时间,能够使Zn-Mg-Ti准晶中间合金基体MgZn2熔解,而Zn-Mg-Ti准晶相得以在ZA27熔体中保留下来。Zn-Mg-Ti准晶中间合金的加入,能够增加ZA27形核的质点,同时对其枝晶生长起到一定的抑制作用。当Zn-Mg-Ti准晶中间合金加入量为3wt.%时,ZA27的组织细化最为明显。而且此时的布氏硬度的值也达到最大为140HB。这是由于Zn-Mg-Ti准晶增加了基体的局部抗变形能力。当加入量继续增加并超过3wt.%的时候,ZA27的组织开始有一定的粗化,而且布氏硬度也开始下降。这主要是由于Zn-Mg-Ti准晶相在熔体中发生了团聚,并且沉降到熔体的底部,失去了Zn-Mg-Ti准晶增强的作用。更多还原

【Abstract】 Discovery of quasicrystal breaks the conventional definition of crystal but also brings about a revolution in the material field. The in-depth studies and perfection of quasicrystal theory cause positive progress in the field of quasicrystal application. However, quasicrystal can not be directly used as structural materials due to its innate high brittleness. Whereas, some unique properties of quasicrystal, 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. Zinc based quasicrystal strengthening Zinc alloy provides a new way for application of quasicrystal in industry. Zn-Mg-Ti quasicrystal master alloy, prepared by conventional metal casting method, was added into the ZA27 alloy to improve its wear-resistance and refine its grains. The microstructure evolvement law and phase composition range, in which Zn-Mg-Ti quasicrystal can be formed, has been detailedly discussed by using X-ray diffraction, optical microscope, scanning electron microscope equipped with energy dispersive spectrum. Besides, the growth law as well as morphological characteristics of Zn-Mg-Ti quasicrystal phase and its approximate phase were also investigated. The study on Zinc based quasicrystal strengthening ZA27 alloy is focused on the effect of Zn-Mg-Ti quasicrystal master alloy on the grain refinement and macro-hardness of ZA27 alloy. Results show that:(1) Zn-Mg-Ti quasicrystal phase and its approximate phase were obtained by conventional metal casting method in alloys with composition of Zn67Mg28Ti5. The microstructure of prepared Zn67Mg28Ti5 alloy was indentified as MgZn2 matrix and icosahedral structure by XRD analysis. Results show that Zn-Mg-Ti quasicrystal phase, with stoichiometric composition of Zn71Mg21Ti8, appear to be hexagonal and six-petaled petal-like. Cores of the six-petaled quasicrystal phase and four-petaled approximate phase are hexagonal and quadrilateral. It is indicated that the morphology of phase subsequentially formed is determined by morphology of the core.(2) Atomic ratio of Ti element in Zn-Mg-Ti quasicrystal master prepared by conventional metal casting method should be 5at.%. A new blocky phase with stoichiometric composition of Zn68Mg13Ti19 appears in the solidification when Atomic ratio of Ti exceeds 5at.%. Besides, formation of the blocky phase results in sharp reduction of quasicrystal phase and its approximate phase. With Ti content remains constant,atomic ratios of Zn and corresponding Mg are in the range of 65-70at.% and 30-25at.%, in which Zn-Mg-Ti icosahedral quasicrystal will be formed. The amount of Zn-Mg-Ti icosahedral quasicrystal and its approximate phase reach the maximum when the alloy composition is Zn67Mg28Ti5.(3) The nucleation of Zn-Mg-Ti quasicrystal phase and its approximate phase have to overcome certain nucleating potential barrier in the Zn-Mg-Ti quasicrystal master alloy melt. The power of Zn-Mg-Ti quasicrystal growth after nucleation is derived from phase transformation driving force of Zn-Mg-Ti quasicrystal. The value of phase transformation driving force is depended on the undercooling of surrounding melt. The volume of Zn-Mg-Ti quasicrystal phase and its approximate phase increase with the increasing driving force. The morphologies of Zn-Mg-Ti quasicrystal phase and its approximate phase depend on growth rate in different directions. And the main factor that affects the growth rate is undercooling caused by curvature variation on the interface. The growth rate of sharp corner of Zn-Mg-Ti quasicrystal phase and its approximate phase is fastest, which leads to the petal-like morphology of quasicrystal phase. Growth patterns of Zn-Mg-Ti quasicrystal phase and its approximate phase are laminar growth and its growth interface is smooth interface. Growth behavior of Zn-Mg-Ti quasicrystal phase is similar to that of its approximate phase.(4) The adding temperature should be 600℃by using Zn-Mg-Ti quasicrystal master alloy strengthening ZA27 alloy. This adding temperature could make sure that MgZn2 matrix of Zn-Mg-Ti quasicrystal master alloy dissolve and Zn-Mg-Ti quasicrystal phase can be inherited to ZA27 alloy. Zn-Mg-Ti quasicrystal master alloy addition can increase the nucleation sites for ZA27 alloy and prevent dendrite growth. The microstructure is best refined when Zn-Mg-Ti quasicrystal master alloy addition is 3wt.%. Meanwhile, its hardness reaches the peak value (140HB). This could be attributed to the improvement of local deformation resistance caused by Zn-Mg-Ti quasicrystal. The microstructure of ZA27 alloy begins to coarse and its hardness tends to decrease when Zn-Mg-Ti quasicrystal master alloy addition is more than 3wt.%. This phenomenon can mainly ascribed to the aggregation of quasicrystal phase at the bottom of melt and lose of its strengthening effect.更多还原