【作者】 周书才；

【导师】 李华基；

【作者基本信息】 重庆大学 ， 材料学， 2005， 硕士

【摘要】 本文从合金的热力学、动力学及结构形成条件方面系统的讨论了Mg 基大块非晶的形成机制及影响因素,并从非晶形成的热力学观点出发,综合应用Miedema理论和Toop 模型,并考虑纯金属在不同温度下的自由能,在此基础上提出了一种定量预测三元非晶合金形成范围的方法,用该方法计算Mg-Cu-La 三元合金系的非晶形成范围,应用多项技术(三经验原则、二元合金相图及Miedema 理论和Toop模型)设计了Mg65Cu25La10 、Mg68Cu17La15 和Mg57Cu23La20非晶合金。利用纯度较低的原材料通过石墨模具铸造成型了直径为1mm 的Mg-Cu-La 的Mg 基非晶合金棒,用X 射线(XRD)和差热扫描分析仪(DSC)进行检测,同时也与多组元镁基Mg-Cu-Ln 大块非晶合金进行比较,本文的研究结果表明: ①Mg-Cu-La 合金具有较高的非晶形成能力及热稳定性,但较Mg65Cu25Y10、Mg65Cu25Tb10 和Mg65Cu25Gd10 的非晶形成能力及热稳定性弱。Mg-Cu-La 合金的玻璃转变温度Tg、晶化开始温度Tx、过冷液相区ΔTx 如下表: ②稀土(RE)的添加增加了该合金系的抗氧化能力,减轻O、H、C等夹杂物对非晶形成能力的不利影响,且RE氧化物在微观结构上与拉弗斯(Laves)相有很大的不同,因而提高了合金的非晶形成能力。③多元合金化(原子半径相差大,电负性相差大)使合金具有更紧密的无序堆积结构,有利于合金非晶形成能力的提高。主要是增加了合金熔融状态的黏度及固液界面能,从而抑制晶体的形核和生长。④非晶由液相转变时,同时受动力学和热力学因素的控制。本文认为动力学在非晶转变过程中起到关键作用。⑤RE 元素与Mg、Cu 具有大的负的混合焓,能形成更多的原子对,因此增加了过冷液体相的粘度及堆垛密度,降低了液相线温度,原子在过冷状态扩散困难,提高了非晶形成能力。更多还原

【Abstract】 In this paper,the forming mechanism and affecting favtor of bulk metallic glasses is systematically discussed from the aspects of thermodynamic, kinetic and microstructure . Thermodynamic factors which favor the amorphization of alloys analyzed, the glass forming range of ternary alloys system was proposed by using Miedema theory, Toop model and considering the influence of free energy of pure metals at different temperature. The approach was applied to predict the composition range of amorphous Mg-Cu-La alloys. Several methods(such as three empirical criterions ,binary alloys phase diagram , Miedema theory and Toop mode) were used to design a series of Mg65Cu25La10 、Mg68Cu17La15 and Mg57Cu23La20 alloys, These bulk amorphous alloys with maximum diameter thickness reaching to 1mm were prepared by means of graphite-molding casting though the raw materials are of a low purity by conventional casting processes at a low cooling rate. The amorphous nature of the samples was identified by X-ray diffraction (XRD) ,and confirmed by differential scanning calorimeter (DSC) performing at a heating rate of 20K/min,comparing with multicomponent Mg-based Mg-Ln-TM bulk metallic glasses.The following results have been obtained: ①Mg-Cu-La bulk amorphous alloys have a higher glass forming ability (GFA). Thermal stability and glass-forming ability of Mg-Cu-La metallic glasses than these of Mg65Cu25Y10、Mg65Cu25Tb10 and Mg65Cu25Gd10 metallic glasses The glass transition temperature (Tg),the onset temperature (Tx) of crystallization and the supercooled liquid region (△Tx=Tx-Tg)are listed as below: ②Rare earth(RE) elements can increased ignition and oxygen resistances of Mg-based BMGs and have high affinity with other impurities such as H, S, and C which also have harmful effect on the GFA and the RE oxide nucleus with markedly different structure compared with that of Laves phase, and then the GFA is improved. ③Mul-ticomponent alloys element which have the appropriate large atomic size and electronegativity difference amomg the three constituent elements may induce highly dense packed microstructure can significantly improve the glass forming ability of bulk glass forming alloys. Consequently, the supercooled liquid can have a higher viscosity and solid/liquid interfacial energies leading to the suppression of nucleation and growth of crystalline phase during the solidification, and thus the GFA is improved. ④Glass transition from melt state to glassy state was controlled by thermodynamic and kinetic factor .But kinetic factor (such as viscosity)play a key role in the formation of the BMGs. ⑤The RE also has larger negative heat of mixing with Mg and Cu components in the alloy which can form more atomic pairs. Consequently, the packing density and viscous of the supercooled liquid are increased and the liquidus temperature is decreased with adequate amount of RE addition. Thus, the stability of supercooled liquid state of the alloys with low atomic diffusivity is enhanced, and then the GFA is improved.更多还原