【作者】 马毅；

【导师】 蒋建中； 曹庆平；

【作者基本信息】 浙江大学 ， 非晶合金， 2014， 博士

【摘要】 非晶合金又称金属玻璃,是非晶固体材料家族中的新成员。非晶合金作为具有玻璃、金属、固体和液体特性的新型金属材料,表现出非常独特的性能,同时还是人们探索材料科学与凝聚态物理中一些重要问题的模型体系。半个多世纪以来,非晶合金已经从最初的被人们嘲笑为“愚蠢的合金”到如今成为科研工作者的研究热点。从工业应用的角度出发,非晶合金研究领域目前存在两大基本科学问题：非晶合金的变形机制是什么?非晶合金内部结构与性能、变形行为之间存在什么关系?本论文的主要研究工作围绕着这两个关键问题展开,通过对非晶合金薄膜变形行为的研究,探索了非晶合金变形模式转变的尺寸效应；通过纳米压痕实验测试了非晶合金的剪切转变区体积,从基本流变单元的角度分析了非晶合金薄膜的变形行为。得到的主要结果如下：(1)本文在精密抛光的钛基底上沉积了Ni-Nb非晶合金薄膜,通过基底的弯曲带动薄膜的变形。通过扫描电镜对Ni-Nb薄膜的5%拉伸应变区进行了变形行为的研究,发现随着薄膜厚度的降低,拉伸变形会从剪切变形转变为非局域变形。并且变形模式的转变存在过渡区间,即在一定厚度范围内,薄膜可能发生剪切变形也可能发生非局域变形。通过对Ni-Nb薄膜退火处理,发现结构弛豫可以使Ni-Nb非晶合金薄膜发生非局域变形的临界厚度变小,同时两种变形模式转变的厚度区间也被压缩。基于Griffith裂纹扩展准则,从剪切带扩展的驱动能角度理论上预测了发生非局域变形的临界厚度。考虑到基底的束缚作用,引入了薄膜弹性能转移比例Ps这个参数,使理论计算与实验结果基本符合。而退火处理会使Ni-Nb薄膜的杨氏模量变大,同时薄膜与基底结合力降低,导致临界厚度的变小。(2)研究了Ni-Nb非晶合金薄膜中应力状态对变形模式转变的影响,发现在非晶合金薄膜拉伸区变形模式转变的临界厚度要明显低于压缩区的临界厚度。基于非晶合金样品在拉伸和压缩时的屈服强度差异和Griffith裂纹扩展准则,提出了正是非晶合金样品在拉伸时屈服强度低于压缩时的屈服强度20%左右,导致了拉伸应力区的临界厚度值低于压缩应力区的临界厚度值。(3)进一步开展了Ni-Nb、Cu-Zr-Al、Zr-Cu-Ni-Al、Pd-P和Pd-Ni-P非晶合金样品变形模式转变的温度和应变速率相关性研究。发现非晶合金的变形模式是受实验温度和应变速率调控的,变形模式转变的临界厚度值会随着温度的升高和应变速率的降低而增加。基于Griffith裂纹扩展准则和非晶合金弹性参数与温度和应变速率的关系,温度升高或应变速率降低会减小非晶合金样品的杨氏模量,导致了变形模式转变临界厚度值的下降。(4)通过纳米压入技术,研究了八种不同体系的非晶合金薄膜的蠕变性能。发现具有高玻璃转变温度的非晶合金样品抵抗蠕变的能力更强,同时具有更低的应变速率敏感系数。而从稳态蠕变阶段计算得到的各个体系剪切转变区相关信息发现,非局域变形过程中材料的流动性越好(蠕变越明显),那么剪切转变区中包含原子数越少,激活能越低。这说明剪切转区尺寸大小不仅与非晶合金的塑性相关,还和非晶合金的变形模式密切相关。无论是在本课题的研究工作,还是前人对非晶纳米柱状样品的力学研究中,非晶合金的玻璃转变温度越低,发生非局域变形的临界尺寸越大。表明剪切转变区尺寸及激活能越小,非晶合金越容易发生非局域变形。(5)同样通过纳米压痕设备,在Ni-Nb非晶合金薄膜中研究了不同结构状态下剪切转变区的体积和不同测试手段对剪切转变区体积的影响。发现结构弛豫可以使剪切转变区体积显著变大。而薄膜变形过程中应变速率越大,对应的剪切转变区体积也越大。这也从内部流变单元角度解释了为什么结构弛豫和应变速率会影响非晶合金发生非局域变形时的临界尺寸。更多还原

【Abstract】 Amorphous alloy also often referred to as metallic glass, is a relative new member of amorphous solid materials family. As a new-structure metal, amorphous alloy integrates characters of glass, metal, solid and liquid. Amorhpous alloy exhibits very unique properities, meanwhile this new material could be an important part of model system in the material science and condensed matter physics reaserches. As emerged more than half a century, it has been at the cutting edge of material researches though it was treated as a kind of "stupid alloy" at the very beginning. From the sight of industrial application, amorphous alloy researchers are faced with two basic scienctific issues:what is the mechanism of deformation; what is the relation between inernal structure and external performance such as mechanical property and deformation behavior. In this thesis, the main research contents are carried out around these two key questions. We focus on the size effect of deformation mode by the defomation behavior in amorphous alloy film. On the other side, we measure the size of "shear transformation zone" in amorphous alloy by experimental methods. Thus we could analysis the deformation behavior at the point of the basic rheology unit. The main results are summarized as follows.(1) By depositing on the polished Ti substrate, Ni-Nb amorphous allopy thin film could be deformed with the bending of substrate. We study the deformation behavior of5%tensile area of thin film by the SEM. As the thickness decreases, defomation mode could be changed from localized shearing to non-localized deformation. In addition, there exists a critical transition zone for the two deformation modes, i.e within a thickness range, the deformation mode is neither complete shearing or non-localized deformation. By annealing of the Ni-Nb thin film, we find the structure relaxation could reduce the critical thickness for non-localized deformation, the "thickness window" is also shorten. Based on the Griffith crack theoy, we theoretically calculate the critical thickness for non-localized deformation at the angle of driving energy for shear banding. Considering the substrate constraint effect,we introduce a paremeter to describe the transfer ratio of thin film elastic energy. Annealing would enlarge the Young’s modulus and weaken the adhension between film and substrate,thus results in the reduction of critical thickness for non-localized deformation. (2) The transition in deformation mode from highly localized to non-localized deformation was investigated in Ni6oNb4o glassy film bymonitoring the reduction in thickness during film/substrate co-bending. It is revealed that in addition to the film thickness, the mode of plastic deformation depends on the stress state. With the reduction in thickness of thin film, tensile stress can efficiently suppress the change in deformation mode from highly localized to non-localized deformation in comparison with compressive stress. A mechanism for the stress-state-dependent deformation mode change in glassy alloys is discussed on the basis of the pressure/stress effect of plastic deformation and Griffith’s crack-propagation criterion. This study provides distinct evidence of the deformation mode change in metallic glassy film via the variation in stress state, and also sheds light on the deformation mechanism of glassy alloys.(3) Both the effects of temperature and strain rate on the deformation behavior of metallic glass thin films (Ni60Nb40, Gu44Zr44Al12Zr55Cu15Ni13Al17, Pd79P21, and Pd41Ni39P20) were systematically investigated. The evolution of surface morphology of magnetron sputtered thin films co-bent with the Ti substrate was monitored. A transition from highly localized deformation to non-localized deformation was observed at various temperatures and strain rates by reducing the film thickness to a critical thickness, which can be understood on the basis of Young’s modulus, Poisson’s ratio, Griffith’s crack-propagation criterion and elastic energy transferring efficiency of the material. Temperature and strain rate dependence of the critical thickness for transition are discussed to shed light on the deformation mechanism of glassy thin films.(4) By nanoindentation technique, we study the creep behaviors of eight kinds of amorphous alloy films. A phenomenon was observed that the higher Tg or Young’s modulu an amorphous alloy film has, the stronger resistance to creep could be detected, and also a lower strain rate sensitivity. The size of "shear transformation zone" could be calculated from the steady-state creep. We found that the material has better fluidity during the non-localized deformation (creep), its STZ size is smaller. This result indicates the STZ size not only influences the plasticity of amorphous alloy but also plays an important part on the transformation of deformation modes. Weather in our work or other groups’researches on the mechanical properties of amorphous alloy nanopillars, there exists a tendency that if an amorphous alloy owns higher Tg, its critical size for non-localized deformation would be larger. That is to say, smaller STZ size could promote the non-localized deformation.(5) Using nanoindentation technique, we study the STZ siz of Ni-Nb amorphous alloy film in different structure state and experiment methods. The results show that structure relaxation could remarkably enlarge the STZ size in all the tesing. Meanwhile larger STZ size would be detected if the strain rate during the deformation of thin film is faster. This could explain the reason why structure relaxation and strain rate would affect the critical size for non-localized deformation in amorphous alloy in theaspect of internal rheology unit.更多还原