【作者】 刘新宝；

【导师】 杨根仓；

【作者基本信息】 西北工业大学 ， 材料加工工程， 2003， 博士

【摘要】 采用惰性形核涂层技术与循环过热相结合的方法，使大体积Al₇₂Ni₁₂Co₁₆准晶合金获得深过冷，并系统研究了深过冷Al₇₂Ni₁₂Co₁₆合金熔体中十面体准晶的形核与生长行为。借助经典形核理论与瞬态形核理论讨论了过冷熔体中十面体准晶形核的热力学和动力学条件；研究了宽过冷度范围内Al₇₂Ni₁₂Co₁₆准晶合金的凝固组织演化规律；通过分析十面体准晶在不同过冷度下的生长形貌，界定了十面体准晶生长方式随过冷度发生转变的临界条件，制备出完全呈单相的块体十面体准晶；采用粉末X-射线衍射、差热分析、扫描电镜和微观硬度等手段，研究了单相十面体准晶的热稳定性及其基本力学性能。本文的主要研究内容及获得的结论如下： （1）采用惰性形核涂层技术和循环过热的方法，使Al₇₂Ni₁₂Co₁₆合金熔体最大获得了180K的高过冷度。大量实验结果表明，惰性形核涂层的非晶化程度、熔体过热度、过热保温时间以及循环过热次数等工艺参数将显著影响熔体的初始过冷度。 （2）在已有实验基础上，通过经典形核理论计算，发现在不考虑异质形核条件下，准晶熔体所能获得的最大过冷度取决于初生相的晶体结构。初生相晶体结构与过冷熔体中存在的二十面体短程有序（ISRO）结构越接近，熔体的过冷能力则越小。通过引入表征晶体结构的参数α，发现初生相结构与熔体相对过冷度ΔT_rel间存在线性关系：ΔT_rel=-0.033+0.44α。 （3）理论计算表明，在过冷度为180K的Al₇₂Ni₁₂Co₁₆合金熔体中，十面体准晶的形核仍为异质形核。采用惰性形核涂层技术和循环过热的方法，仅去除了熔体中润湿角小于92°的异质形核质点。十面体准晶的临界晶核表面能、形核孕育时间均比通常晶体相小得多，并且匀质形核时的最大形核率要比晶体相高出几个数量级。 （4）实验结果和理论分析均表明，当Al（72）Ni₁₂Co₁₆合金熔体的初始过冷度小于65K时，随着凝固过程的进行，液相中出现的溶质贫乏导致了晶体相γ在十面体准晶柱状晶界处析出，单相组织被破坏。当过冷度超过65K时，显著的溶质截留效应使Al₇₂Ni₁₂Co₁₆合金凝固组织演变为完全呈单相的准球状等轴晶。 （5）热力学计算结果表明，深过冷Al（72）Ni₁₂Co₁₆合金凝固组织中的细化晶粒是由再辉过程中初始枝晶骨架的重熔而引起。在大约70K的初始过冷度下凝固时，凝固组织中的初始枝晶骨架具有最大的重熔趋势。过冷度进一步增加时，再辉过程中的重熔作用随之减小而使最终凝固组织相对粗大。西北工业大学工学博卜学位论文 （6）随熔体初始过冷度的增加，十面体准晶呈现出不同的生长形貌。小过冷度下，十面体准品沿二次轴方向以台阶方式生长形成多棱柱组织。当过冷度超过某一临界值时，十面体准晶由小平面台阶生长方式转变为非小平面连续生长，形成等轴晶组织。结合卡恩生长动力学统一理论和过冷熔体中晶体生长的BCT模型，界定了十面体准晶生长方式发生这一转变的临界过冷度为65K。 （7）粉末X一射线衍射、差热分析等结果表明，在深过冷快速凝固条件下，十面体准晶的非小平面连续生长有效抑制了晶体相的形成，所获得的单相十面体准晶在室温和退火温度范围内均保持稳定。 （8）通过压痕法对单相AI7剑i.Zc。，6十面体准晶的显维硬度进行测量，结果发现随载荷的增加，其硬度从9.07GPa降低到8.32GPa，减少幅度只有8%，与陶瓷材料的性质相仿。根据1009载荷下首次出现的裂纹，估算出单相Al，剑i12Co，6十面体准晶的断裂韧性K.。为1 .6MPa了而，脆性为5.63扳而，表现出强烈的脆性。更多还原

【Abstract】 A high undercooling was got in bulk Al72Ni12Co16 alloy melt by means of cyclic superheating and a special coating crucible. On this basis, the solidification behavior of decagonal quasicrystal in the undercooled Al72Ni12Co16 alloy melt, such as nucleation and growth mechanism, was systematically investigated. Based on the classical nucleation theory and the time-dependent nucleation theory, the thermodynamics and the kinetics for the nucleation of decagonal quasicrystal have been discussed in detail. The microstructure evolution of the quasicrystal-forming Al72Ni12Co16 alloy under a wide undercooling range was studied. With increasing undercooling, the growth mode of decagonal quasicrystal was investigated and bulk single-phase decagonal quasicrystal materials were prepared. In addition, the thermal stability and basic mechanical properties of single-phase decagonal quasicrystals were researched by scanning microscopy (SEM), X-ray powder diffraction (XRD), differential thermal analysis (DTA) and microhardness technology. The main results are as follows:(1) A maximum undercooling, 180K, was achieved in the Al72Ni12Co16 alloy melt by means of cyclic superheating and the nucleating inhibition crucible. The undercooling of alloy melts was markedly influenced by some technical parameters, such as content of noncrystal in crucible coating, superheating temperature, superheating time and cyclic superheating times.(2) Based on the experimental results about the undercooling of quasicrystal-forming alloy melts, the structure dependent α- factor of primary phases have been calculated within the framework of classical nucleation theory. It indicated that the maximum undercooling of alloy melts is determined by the primary-phase structure only in consideration of homogeneous nucleation and a linear relation exits between the primary-phase structure and the relative undercooling of melts. The more similar the structure of ISRO in the undercooled melt and the primary phase, the lower is the undercoolability of the melt.(3) According to the relation between the primary-phase structure and the relative undercooling of melts, the maximum undercooling of Al72Ni12Co16 alloy was calculated when decagonal quasicrystal was preferentially nucleated. The results indicated that the heterogeneous nucleation mechanism is adopted by thedecagonal quasicrystal in the undercooling range achieved in the experiments and the heterogeneous sites with witting angles smaller than 92?have been effectively eliminated by cyclic superheating and the nucleating inhibition crucible in the Al72N12Co16 alloy melts undercooled by 180K. The interfacial energy and the incubation time for nucleation of decagonal quasicrystal are smaller than those of crystal phases. Besides, the homogeneous nucleation rate of decagonal quasicrystals is several orders of magnitude larger than that of crystal phase.(4) According to the experimental results and theoretical analysis, it indicated that at the undercoolings smaller than 65K, the crystal-phase y would form near the columnar-grain boundaries of decagonal quasicrystal in the solidification process of Al72Ni12Co16 alloy because of the solute lack in front of the L/S interface, which causes the destruction of single-phase microstructure. When the undercooling is larger than 65K, due to the development of solute trap, an equiaxed microstructure of single-phase decagonal quasicrystal could be formed.(5) Thermodynamic calculation indicated that the fine equiaxed microstructure in the Al72Ni12Co16 alloy originates from the break-up of primary dendrites under the action of remeiting in recalescence. Near the undercooling of 70K, the initial dendrite skeleton has the maximum tendency to be remelted. With increasing undercooling, the remeiting in recalescence decreases to a low level and is not severe enough to make the initial structure disintegrate into fine grains. So the relatively coarse microstructures are formed.(6)更多还原