【作者】 宗瑞磊；

【导师】 潘峰；

【作者基本信息】 清华大学 ， 材料科学与工程， 2009， 博士

【摘要】 人们在关注非晶态合金强度的同时也致力于提高其室温塑性。研究发现,非晶态合金的强度和塑性变形行为与合金成分、第二相的加入及变形速率密切相关。非晶薄膜中插入晶体层或非晶层形成多层结构,能影响多层膜的强度和塑性变形行为,值得进行深入的研究。本文使用磁控溅射制备了CuW非晶薄膜和CoZrNb/S(S=Ag, Cu, CuTa)多层膜,使用多种技术表征了其结构,采用纳米压入技术测量了其力学性能(硬度、弹性模量、塑性变形等),研究了多层膜周期结构对硬度的影响,讨论了非晶基多层膜塑性变形的表征、特征及机制。结果表明,非晶和非晶多层膜发生局域剪切变形,变形特征与应变率、成分、多层结构有关。对于CuW非晶合金,当应变速率和铜含量较低时,载荷-位移曲线表现为锯齿流变特征;随应变率和铜含量的增大,锯齿流变特征减弱,纳米压痕周围的剪切带数量增多,塑性变形能力提高。对于CoZrNb非晶合金,晶体层的加入形成非晶/晶体多层结构能抑制剪切带的扩展,促进剪切带的萌生,提高塑性变形能力;随调制周期的减小,塑性增强效果更加明显。纳米划痕实验表明,非晶薄膜从剪切带萌生、扩展阶段发展到剪切断裂阶段存在一临界载荷,我们可用这个临界载荷定量表征非晶和非晶/晶体多层膜的塑性。CoZrNb薄膜对应的临界载荷是46mN,而周期为8nm的CoZrNb/Cu多层膜的临界载荷大于100mN。非晶CuTa层加入到CoZrNb非晶合金中对塑性变形行为影响不明显。非晶/晶体多层膜随周期减小表现出不同程度的硬度增强。硬度增强效应与多层膜层间强度错配相关:强度错配变小,硬度增强效应增大,说明针对晶体/晶体多层膜提出的强度错配理论也适用于非晶/晶体多层膜。在强度错配最大的CoZrNb/Ag体系中,在周期较大时(Λ≥16nm)出现软化现象,这是由于大的强度错配导致塑性变形主要局域于硬度较低的Ag层中。非晶-CoZrNb/非晶-CuTa多层膜由于缺乏阻碍位错运动的增强机制,硬度不随周期变化。实验证实多层膜退火后致密度的提高是由原子平均距离变小(Co-Co平均键长从0.2496nm变为0.2397nm)和缺陷率降低共同引起的。致密度的提高导致CoZrNb/CuTa多层膜的弹性模量和硬度随退火温度的升高而增大。更多还原

【Abstract】 People concern about the strength of amorphous alloys, but also commit to improving their plasticity at room temperature. It has been found that the strength and plastic deformation behaviors of amorphous alloys are related with deformation rates, compositions and the addition of a second phase. The multilayer structure formed by inserting amorphous or crystalline layer into amorphous films may affect the strength and plastic deformation, which is worthy of an in-depth study. In this dissertation, amorphous CuW films and CoZrNb/S(S=Ag, Cu, CuTa) multilayers were prepared by magnetron sputtering. Their structure was characterized with various methods, and their mechanical properties including hardness, elastic modulus and plastic deformation behavior were tested by nanoindentation technique. After studying the mechanical behaviors of amorphous alloy films, we analyzed the relationship between hardness and periodical structure, and explored the characterizations, characteristics and mechanisms of plastic deformation of amorphous-based multilayers.The results show that both amorphous alloy films and amorphous-based multilayers deform by localized shear bands. The deformation characteristics vary with strain rate, composition and multilayer structure. For amorphous CuW films, lower strain rate and lower Cu content promote more obvious serrated flow on load-displacement curves. With the increase of strain rate and Cu content, the serrated flow characteristics weaken. The increasing strain rate and Cu content also result in increasing number of shear bands around nanoindents, and then increasing the plasticity. For amorphous CoZrNb films, the inserting crystalline layers, forming amorphous/crystalline multilayer structure, could both inhibit the propagation of shear bands and promote the nucleation of shear bands, which increases the plasticity. What’s more, the effect is more pronounced when the periodicity is smaller. Nanoscratch results show that there exists a critical load, above which the samples fracture along the scratch tracks. The critical load can be used to characterize quantitatively the plasticity of CoZrNb and CoZrNb/Cu films. For amorphous CoZrNb film, the critical load is only 46mN. For the multilayer withΛ=8nm, the critical load is larger than 100mN. The addition of CuTa amorphous layers into the CoZrNb amorphous alloy has little effect on the plastic deformation.For amorphous/crystalline multilayers, there exists hardness enhancement with the decrease of the periodicity. The extent of hardness enhancement is closely related to the strength mismatch of component elements. With the decrease of the strength mismatch, the hardness enhancement increases. The strength mismatch theory, which is originally put forward to the crystalline/crystalline multilayers, is also suited to amorphous/crystalline multilayers. CoZrNb/Ag system with the largest strength mismatch shows a softening effect, whenΛ≥16nm, which can be ascribed to the localization of plastic deformation in softer Ag layers.For CoZrNb/CuTa multilayers, there is no hardness enhancement with decreasing periodicity, which can be attributed to the absence of the dislocation movement. After thermal annealing, the density increases due to the decrease of the bond distance (the Co-Co average bond distance decreases from 0.2496nm to 0.2397nm) and the annihilation of defects with increasing annealing temperature. The densification results in the enhancement of both elastic modulus and hardness.更多还原