【作者】 赵燕春；

【导师】 寇生中；

【作者基本信息】 兰州理工大学 ， 材料加工工程， 2010， 博士

【摘要】 CuZr基块体金属玻璃在很宽的成分范围内有较强的玻璃形成能力,特别是Cu-Zr-Al体系,具有良好的综合性能,包括高热稳定性、优良的力学性能,且低成本。本文采用悬浮熔炼-水冷铜模吸铸方法制备了具有较高热稳定性的Cu₅₀Zr₄₂Al₈三元BMG,研究了凝固中的不同冷却速度对微观组织的影响,其非晶的临界尺寸为Ф4.8mm。微观组织的变化表现出不同的力学行为。Ф4mm全非晶结构铸态试样的σcf达到2260MPa,弹性应变和塑性应变分别为2.0%和0.4%。而Ф5mm原位自生非晶/晶体复合棒的屈服强度、断裂强度和杨氏模量分别为1670MPa、1849MPa和104.4GPa,塑性应变为1.9%,其最终断裂方式取决于增强增韧的CuZr马氏体相和脆性相力学行为的竞争。研究了Cu₅₀Zr₄₂Al₈ BMG的变温晶化行为,运用Kissinger法和FWO法分别计算出各特征激活能和阶段激活能,考察了不同加热速度下晶化体积分数和晶化温度、晶化激活能的关系。退火态非晶试样受不同冷却介质的影响,其有效退火时间不同。本文考察了Ф5mm铸态Cu₅₀Zr₄₂Al₈试样在过冷液相区内退火,分别采用炉冷和液氮两种方式冷却后的晶化和力学行为。Zr-Cu体系中具有最好玻璃形成能力的四元BMG Cu₃₆Zr₄₈Al₈ ,由于其熔体脆性参数小,具有高的断裂强度以及一定的塑性。由合金的熔化曲线,分析Cu₃₆Zr₄₈Al₈合金熔化时发生四元共晶转变,其过冷熔体具有良好的热稳定性。分析了铜模吸铸前反复熔炼对母合金的微观组织的影响,并在相同的冷却速率下,通过改变铸造条件,考察熔体过热度和保温时间对BMG热稳定性和力学性能的影响,以确定出最佳的铸造环境,并研究了弛豫退火对不同制备态的BMG力学性能的影响。结果表明,高的熔体过热水平制备非晶合金的比热容大,热稳定性高。过热度越小,压缩塑性越小;抗压强度和维氏硬度随着过热度的减小先增加而后下降。残余应力在高过热度下起主要作用,导致抗压强度、维氏硬度和压缩塑性同时下降;低过热度下自由体积影响较大,抗压强度和维氏硬度降低,塑性增加。金属玻璃的力学行为在一定程度上可由熔体过热水平控制。而在同样的铸造温度下,随着过热保温时间的延长,合金的热稳定性降低,且由于优先形成的Zr2Cu+AlCu2Zr共晶相为脆性相,随着原子排列的有序化甚至晶化,其力学性能下降。将不同制备态的BMG分别进行低温和高温弛豫退火,试样的力学行为发生显著改变,文中对其变化机制进行了分析。选择Cu₃₆Zr₄₈Al₈ BMG作为基体合金,研究了Fe元素合金化对其热稳定性和力学行为的影响,并讨论其作用机理。Fe原子半径较小,提高了体系的混乱度,但与Ag和Cu的原子对混合热为正值。由相图液态Fe与Cu的溶混间隙小,和Ag几乎完全不混溶,快速冷却导致液固界面推进速度增大,原子的长程扩散被抑制,Ag“选择性溶解”于Cu中,从而形成了富Cu、Ag相和富Fe相的分离,并通过XRD和TEM测试得到证实。其中富Fe相为硬相,结构的原子结合力较大、结构密堆性高,而富Cu、Ag相为软相,相分离程度主要受Fe/Cu,Fe/Ag的原子比的控制。结果表明,随着Fe的添加,非晶合金的热稳定性降低,ΔT_x和T_rg减小。Fe含量为3at.%时,DSC曲线只存在一个尖锐的吸热峰,熔化过程中可能只发生一级相变,其成份靠近共晶点,其ΔTx为103,Trg和γ分别为0.566和0.424,具有较高的热稳定性和玻璃形成能力,且受具有不同的模量和临界剪切应力的相分离两相对剪切带的影响,力学性能较基体提高。Fe含量为5at.%时,相分离程度增加且有纳米晶析出,对基体弥散强化,合金的强度和塑性分别提高至2249MPa和4.9%。随着Fe含量增加,试样逐渐晶化并先后析出了脆性相Cu10Zr₇和FeZr₃,力学性能下降,合金的脆化敏感性增大。文中采用维氏硬度计考察了Fe的掺杂对合金各相的相对硬度、塑性和压痕形貌的影响规律。显微硬度在压头压入时由于多轴应力的束缚,能够依据各组成相上压痕边缘出现滑移线或裂纹的情况来评定各相之间的相对塑性,本文对各试样组织结构与非晶相和晶体析出相的关系也进行了分析。更多还原

【Abstract】 BMGs are considered to have many potential applications as the structural materials, because of their unique physical and mechanical properties. In particular, the Al enhanced the GFA of Cu-Zr alloy system have a better combination of thermal stability, strength, ductility and lower production cost. In this dissertation, tapered sample of Cu₅₀Zr₄₂Al₈ was fabricated by suspend melting under an argon atmosphere using a water-cooled Cu mold. And the microstructure under different cooling rate was investigated. The critical thickness of Cu₅₀Zr₄₂Al₈ bulk glassy alloy is up to 4.8 mm. The mechanical behavior changes distinctly due to the presence of different microstructure. TheФ4mm BMG exhibits high compressive fracture strength of 2260MPa with elastic strain of 2.0% and plastic strain of 0.4%. TheФ5mm BMG composites containing CuZr martensite phase and Cu10Zr7 crystallizing phase with orthorhombic structure, exhibits a combination of high compressive fracture strength （1849MPa） and yielding strength （1670MPa） with elastic strain of 1.6% and plastic strain of 1.9%. The final fracture behavior is controlled by competition of the two crystallizing phases. Thermal crystallization of Cu₅₀Zr₄₂Al₈ BMG was studied. The characteristic and local activation energy were calculated by Kissinger and FWO methods respectively, thus the relationship between crystalline volume fraction and crystallization temperature, crystallization activation energy were obtained. The as annealed BMGs treated by different cooling medium exhibits different microstructure and mechanical behavior, because of their different effective annealing time. And the effects of liquid nitrogen quenching and furnace cooling on crystallization and compression fracture behavior of as annealedФ5mm BMG composites in the supercooled liquid region, were investigated respectively.The best quaternary glass former in Zr-Cu based alloy system, Cu₃₆Zr₄₈Al₈ BMG is a strong liquid with a fragility parameter m of 33, resulting in high glass forming ability and high fracture strength with a distinct plastic strain. Moreover, the Cu₃₆Zr₄₈Al₈ alloy shows a single-event feature on its melting curve, indicating that the alloy is located at a quaternary Zr-Cu-Al-Ag eutectic point. And the high GFA has been considered to result from the high stability of the undercooled melt. In this dissertation, the effect of repetitive melting on the master alloy microstructure before copper mold suction was analyzed. And the effects of overheated level and overheated time on the thermal stability and mechanical properties of Cu₃₆Zr₄₈Al₈ BMG were investigated under the same cooling rate, thus the optimal cast conditions were determined. Furthermore, the effect of structural relaxation on mechanical properties of the different as-deposited states BMGs was studied. The results show that higher overheated temperature enhances the thermal stability of bulk amorphous alloys, corresponding to higher specific-heat capacity and the smaller initial defect concentration. Bulk amorphous alloys exihibit good compressive plasticity at small overheat levels, whereas the compressive fracture strength and microhardness exihibit a significant increase first and then a slightly decrease. The mechanical properties of bulk metallic glasses fabricated by different casting process are determined by the interaction of the free volume and residual stresses. Under the same casting temperature, the combination of thermal stability and mechanical properties decrease with the overheated time prolonged. As the Cu₃₆Zr₄₈Al₈ BMG is located at a Zr2Cu+AlCu2Zr eutectic point, the eutectic with the brittle structure nucleate preferentially in the melt. Thus as the atomic arrangement is gradually ordered and even crystallized, the combination properties of BMG are destroyed. BMGs at different as-deposited states were annealed in the Sub-Sub-T_g and Sub-T_g region respectively. And mechanism of the pronounced mechanical properties changes was also discussed.The effect and mechanism of addition of Fe element in Cu₃₆Zr₄₈Al₈ BMG on the thermal stability and mechanical behavior were investigated. Fe element with smaller atomic size has the positive heat of mixing with Cu and Ag elements. From the phase diagram, the binary Fe-Cu alloy exhibits a small miscibility gap, as well as Fe-Ag is almost immiscible. Rapid cooling accelerates the fltting speed of the liquid-solid interface, and inhibits the long range diffusion of atoms. Thus Ag selectively dissolves in Cu, and BMG displays a distinct phase separation with a Fe-rich phase homogenously distributed in the Cu, Ag-rich phase matrix. And the phase separation was confirmed by XRD and TEM tests. Fe-rich phase has higher atomic packing density than Cu, Ag-rich phase. Therefore, the Fe-rich phase should be the hard phase, and Cu, Ag-rich phase be the soft one in BMG. The formation of a distinct phase-separating glassy structure depends on the ratio of Fe/Cu and Fe/Ag contents in the quinary Cu–Zr–Al–Ag-Fe alloys. The results show that, the addition of Fe delays the glass transition of BMG, and accelerates the crystallization reaction, thus decreasesΔTx and T_rg. When xFe=3, the bulk amorphous alloy shows a single-event feature on its melting curve, indicating that the alloy is almost located at the eutectic point. AndΔT_x, T_rg andγof this alloy are 103, 0.566 and 0.424 respectively. As the two amorphous separating phases possess different modulus and critical shear stresses in the BMG, and take effect on the shear band during compression, thus mechnical properties increase evidently compared with the matrix alloy. When xFe=5, the phase separation increases as well as the nanocrystalline form and dispersion strengthen the amorphous matrix. The compressive fracture strength and plastic strain increase to 2249MPa and 4.9% respectively. With the increasing Fe content, the alloys start to crystallize and precipitate the Cu10Zr₇ and FeZr₃ brittle phases, and exhibit the increase of sensitivity of embrittlement as well as the decrease of mechanical properties. Vickers hardness tests were used to study the effect of addition of Fe on the hardness and shear band features of the BMG.更多还原