【作者】 曹庆平；

【导师】 周尧和； 李金富；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2007， 博士

【摘要】 非晶合金因其独特的机械、物理和化学性能成为极具应用潜力的先进材料。然而,合金玻璃形成能力的表征仍然是非晶研究领域未解决的关键问题,现有的各种表征方法都无法解释一些例外的情况。传统快淬薄片非晶至少有一维方向的尺寸很小,室温变形时获得的应变极其有限,限制了在不同应变量下微观结构及性能变化的研究,而块体非晶合金的出现使得室温下的大塑性变形成为可能,为该研究创造了条件。本文基于非晶合金形成过程的热力学和动力学,对非晶形成能力的本质进行了探讨。综合利用X射线衍射仪（XRD）、差示扫描量热仪（DSC）和高分辨透射电子显微镜（HRTEM）分析了Cu₆₀Zr₂₀Ti₂₀块体非晶合金在等温和连续加热退火过程中的晶化机制,考察了Cu₆₀Zr₂₀Ti₂₀和Cu_47.5Zr_47.5Al₅块体非晶合金在轧制变形到不同应变量时的自由体积和微观结构,并测量了其显微硬度,由此阐明了它们随轧制温度、应变速率和应变量的变化规律,以及它们之间的关联特性,为块体非晶合金的成分设计和非晶基复合材料的开发提供新思路。在综合考虑非晶形成热力学和动力学的前提下,提出表征玻璃形成能力的新判据,即T_k /T_l （ T_k -Kauzmann温度, T_l -合金液相线温度）和D（合金液体的脆性参数）。研究发现单独在强液体特性的合金或脆液体特性的合金中, T_k /T_l可以很好地表征合金的玻璃形成能力, T_k /T_l越大,玻璃形成能力越好,并且D值的改变并不影响T_k /T_l的表征作用。同时,在对这两类合金的玻璃形成能力进行比较时,发现当T_k /T_l相等时,D值越大,玻璃形成能力越高。Cu₆₀Zr₂₀Ti₂₀块体非晶合金的晶化过程为两阶段晶化,第一个晶化反应为初生型晶化,析出纳米尺寸的Cu₅₁Zr₁₄相,第二个晶化反应为共晶型反应,析出晶化相为Cu₅₁Zr₁₄和Cu2ZrTi。时间关联形核过程存在于Cu₆₀Zr₂₀Ti₂₀非晶合金的初生晶化和剩余非晶的晶化过程中。Cu₆₀Zr₂₀Ti₂₀非晶合金在过冷液相区等温晶化时初生Cu₅₁Zr₁₄相的形核激活能不仅与等温退火前的加热速度有关,加热速度越快,形核激活能越小,而且还与孕育期起始点算法有关。但是有效晶化激活能与加热速度和孕育期起始点算法无关。Cu₆₀Zr₂₀Ti₂₀块体非晶合金在室温和150 K低温轧制过程中获得的最大应变量均高达97%,且大变形后的样品仍具有很好的韧性。对应于每一种变形工艺都存在着一个临界应变量,低于这个临界应变量,样品中只有剪切带产生,高于这个应变量,则有相变发生。无论是提高轧制温度还是提高应变速率,发生相变的临界应变量都下降。相变类型都从单纯的相分离向同时发生相分离和纳米晶化的方向转变,并且相分离后的非晶成分更加偏离合金原始成分。Cu₆₀Zr₂₀Ti₂₀块体非晶合金在150 K低温进行轧制时,如果应变速率ε&= 1.0×10^-4 s^-1,试样在轧制到最大应变量97%时仍保持单一非晶状态。当ε& = 5.0×10^-4 s^-1时,试样在应变量ε大于93%时发生相分离。提高ε&至5.0×10^-3 s^-1时,发生相分离的临界应变量降低到89%。在更大的应变速率5.0×10^-1 s^-1下,在ε> 67%时就同时发生相分离和纳米晶化。固定应变速率ε&= 5.0×10^-3 s^-1,提高轧制温度到室温,试样在ε> 87%也同时发生相分离和晶化。轧制后的试样只有剪切带而没有相变发生时,非晶合金的热稳定性没有明显变化。相分离一旦出现则会降低非晶合金的热稳定性和退火时的晶化激活能。如果相分离和晶化同时发生,合金的热稳定性和晶化激活能会进一步降低。在Cu₆₀Zr₂₀Ti₂₀块体非晶合金低温、低应变速率的轧制过程中观察到了单一非晶状态下自由体积在高应变量下的饱和现象。提高应变速率,自由体积在变形初期的增加速度、单一非晶合金试样中自由体积达到饱和的临界应变量以及饱和自由体积含量也随之上升。虽然自由体积饱和的临界应变量随应变速率的上升而上升,但是相变发生的临界应变量却随之下降,这两个临界应变量之间的竞争决定了塑性变形过程中自由体积含量能否在样品还是单一非晶相时达到饱和。剪切带中自由体积崩溃成纳米空洞是导致自由体积饱和的根本原因。相分离的出现并不明显改变自由体积含量,但是晶化的发生使自由体积含量降低。应变速率为5.0×10^-3 s^-1的298 K室温和150 K低温轧制变形都没有导致Cu_47.5Zr_47.5Al₅非晶样品发生相分离或晶化,只是在靠近剪切带的非晶基体上,出现了原子尺度的成分结构不均匀性。应变量在从0到最大值97%的变化过程中,自由体积含量持续升高。由于塑性变形过程中绝热加热使剪切带中的材料被加热到均匀变形温度区间,强化了变形过程中自由体积的湮灭,导致低温轧制样品中的自由体积含量高于室温轧制样品,并且相同应变量下低温和室温轧制样品之间自由体积含量的差别也随应变速率的增加而减小,使不均匀变形过程中自由体积演化呈现与已有理论完全不一致的变化。Cu₆₀Zr₂₀Ti₂₀块体非晶合金在轧制变形过程中发生相分离或晶化时材料的显微硬度急剧上升。相分离析出的富Cu非晶相的硬度高于原始非晶相,相分离是导致强化的主要原因。在塑性变形过程中形成的纳米晶体含有明显的晶体缺陷,使得其强化作用下降。在变形非晶合金中相分离可能是比晶化更加有效的强化手段。更多还原

【Abstract】 Amorphous alloys have become one of the most active fields in the research of advanced materials due to their unique mechanical, physical and chemical properties. However, the criterion of glass forming ability （GFA） of alloys is a critical issue that has not been unsolved yet, and exceptional cases always exist for the known criterions. Traditional quenched amorphous alloys possess very small sizes in at least one dimension, and the obtained strain during deformation at room temperature （RT） is quite limited. The discovery of bulk amorphous alloys （BMA） makes it possible to achieve severe plastic deformation at RT, and provides an opportunity to investigate the microstructural change with changing strains and its influence on various properties of amorphous alloys.In this dissertation, based on thermodynamics and kinetics of glass-forming process, the essence of GFA of alloys is revisited. By using X-ray diffractometry （XRD）, differential scanning calorimetry （DSC） and high resolution transmission electron microscopy （HRTEM）, the crystallization mechanism during the isothermal and isochronal annealing of the Cu₆₀Zr₂₀Ti₂₀ BMA, the evolution of microstructure and free volume during severe plastic deformation of Cu₆₀Zr₂₀Ti₂₀ and Cu_47.5Zr_47.5Al₅ BMAs are systematically investigated. The microhardness of as-rolled specimens with different amounts of deformation is measured. The present work reveals the evolutions of microstructure, free volume and mechanical property of amorphous alloys with deformation temperature, strain rate and strain during plastic deformation. It provides a new approach to designing the compositions of BMGs and developing amorphous matrix composites.On the basis of the preconditions of the thermodynamics and kinetics of glass forming, a new criterion evaluating the GFA is proposed, i.e. T_k /T_l （ T_k - Kauzmann temperature, T_l - liquidus temperature of an alloy）, and D （fragile parameter of alloy liquid）. It is found that T_k /T_l can reflect the GFA well for either the alloys with strong liquid behavior or the alloys with fragile liquid behavior. The larger the T_k /T_l , the better the GFA, and the change in D does not change the role of T_k /T_l . Meanwhile, when the GFAs of these two different alloys are compared, and if T_k /T_l is kept to be the same, the larger the D , the higher the GFA.The crystallization of Cu₆₀Zr₂₀Ti₂₀ BMA proceeds through two reactions. The first is a primary crystallization with precipitation of nano-sized Cu₅₁Zr₁₄, and the second is a eutectic one with simultaneous formation of Cu₅₁Zr₁₄ and Cu₂ZrTi. Time-dependent nucleation process exists in both crystallization reactions of Cu₆₀Zr₂₀Ti₂₀ BMA. The nucleation activation energy of primay Cu₅₁Zr₁₄ phase during isothermally annealing the Cu₆₀Zr₂₀Ti₂₀ BMA in the supercooled liquid region is not only dependent on the heating rate before isothermal annealing, but also correlated with the definition of incubation time. A higher heating rate leads to a smaller value of nucleation activation energy. However, the effective activation energy for crystallization is not related to the heating rate or the definition of incubation time.The strain as high as 97% has been achieved during rolling the Cu₆₀Zr₂₀Ti₂₀ BMA at both RT and cryogenic temperature （CT, about 150 K）, and the as-rolled specimens with large strains remain ductile. Corresponding to each deformation condition, there is a critical strain, below which only shear bands form in the as-rolled specimens, and above which phase transformation occurs. When the rolling temperature or strain rate is increased, the critical strain for occurrence of phase transformation decreases. Meanwhile, the type of phase transformation varies from phase separation to phase separation plus nanocrystallization, and the deviation of the average chemical composition of phase-separated amorphous phases from that of the original alloy is enhanced. When the Cu₆₀Zr₂₀Ti₂₀ BMA is rolled at CT at the strain rateε& = 1.0×10^-4 s^-1, the specimen remains in a monolithic amorphous state up to the highest strain of 97%. Atε& = 5.0×10-4 s-1, phase separation occurs when the strainεexceeds 93%. With further increasingε& to 5.0×10^-3 s^-1, the critical strain for the occurrence of phase separation drops to 89%. At the higherε& of 5.0×10^-1 s^-1, phase separation plus nanocrystallization occur atε> 67%. At a givenε& of 5.0×10^-3 s^-1, and with increasing the rolling temperature to RT, phase separation plus nanocrystallization also occur atε> 87%. The thermal stability of as-rolled specimens does not obviously change as compared with the as-cast specimen when only shear bands form in the as-rolled and no phase transformation occurs. As phase separation occurs, the thermal stability and crystallization activation energy decrease. If phase separation and nanocrystallization simultaneously take place, the thermal stability and crystallization activation energy will be further reduced.The saturation of the free-volume content in the monolithic metallic glass without phase transformation at high strains are observed in the Cu₆₀Zr₂₀Ti₂₀ BMA during CT-rolling at low strain rates. With increasingε& , the increase rate of the free-volume content in the initial stage of deformation, the critical εabove which the free-volume content in the monolithic metallic glass begins to saturate and the saturated amount of the free-volume content increases. Although the criticalεfor the saturation of the free-volume content in the monolithic metallic glass rises with the strain rate, the criticalεfor phase transformation tends to decrease. The competition of these two critical strains determines whether the saturation of the free-volume content can be obtained when the specimen is still in the monolithic amorphous phase. It is revealed that formation of nano-voids in shear bands through the coalescence of free volume leads to the saturation of free volume. The occurrence of phase separation does not considerably change the free-volume content, while partial nanocrystallization decreases it.Neither phase separation nor crystallization is induced when the Cu_47.5Zr_47.5Al₅ BMA is rolled at RT and CT withε& = 5.0×10^-2 and 5.0×10^-3 s^-1, and only compositional and structural heterogeneity in atomic level exists in the amorphous matrix near the shear bands. The free-volume content continuously increases with increasing strain during the whole rolling process. Since the material in the shear bands is adiabatically heated up to the temperature region of homogeneous flow during plastic deformation, enhancing the annihilation of free volume, results in the fact that more free volume is stored in the CT-rolled specimen as compared with the RT-rolled specimen with the same strain, and the difference in the free-volume content between the CT-rolled and RT-rolled specimens with the same strain decreases with the strain rate. The free-volume evolution with temperature and strain rate during the inhomogeneous deformation is totally different from that predicted by the free volume theory.During rolling the Cu₆₀Zr₂₀Ti₂₀ BMA, microhardness dramatically increases when phase separation or phase separation plus nanocrystallization occurs. Since Cu-rich separated amorphous phases from the matrix possess higher strengths than the original amorphous phase, phase separation is the main reason for strengthening. As the deformation-induced nanocrystallites contain lots of crystal defects, their resistance to yielding is deteriorated. In the deformed amorphous alloy, phase separation may be a more effective way to strengthen amorphous alloys than crystallization.更多还原