【作者】 杨孟锦；

【导师】 赖文生；

【作者基本信息】 清华大学 ， 材料科学与工程， 2007， 硕士

【摘要】 本论文就金属纳米多层膜的力学性能,选取了Cr/Nb和Ag/Nb两个具有鲜明特色的系统。系统的选择是在考虑晶体结构、晶胞参数、弹性模量差等因素下,着重突出生成热这个因素而得出的。多层膜采用电子束蒸发镀膜制备,其力学性能通过纳米压痕实验测得。实验制备的Cr/Nb多层膜呈多晶态,具有显著的协调应变效应。在调制波长Λ> 6nm的范围内,多层膜的硬度随Λ的减小没有产生相应的增加,反而略有下降。这主要是由于生成热为负的Cr/Nb系统没有形成理想的界面结构,故在Λ减小时,晶界滑移的效应超过了Hall-Petch的贡献。在Λ=3nm时,多层膜的硬度有了一些增加,这恰巧和小角掠入射X射线衍射中观察到的新相的形成同时出现。进一步的退火实验表明,Cr/Nb中的亚稳新相对多层膜的硬度强化起了重要作用。另外Cr/Nb体系虽然有较大的协调应变,但相邻层的相反作用(Nb受到压应变和Cr受到拉应变)相互抵消,因而Cr/Nb体系的弹性模量并没有很大的变化。Ag/Nb体系由于小的晶格尺寸差和模量差,突出了生成热为正的因素。X射线反射率实验表明Ag/Nb多层膜具有较好的层状调制结构。Ag/Nb多层膜具有强烈的Ag(111)和Nb(110)织构。多层膜的力学性能测试表明,硬度随着Λ的减小而单调增加,到Λ=4nm处增幅达80%,而薄膜的弹性模量变化在8%之内。通过高分辨电镜观察多层膜的截面,发现多层膜的层间和晶粒间存在着无序的非晶区域。在非平衡制备条件下产生的非晶合金对位错的形成和扩展起了阻碍作用,因而提高了其硬度。至于模量的微小变化,是来自非晶合金的自由体积对模量的负作用和多层膜层间间距减小的正作用相互抵消的结果。更多还原

【Abstract】 The comprehensive study of mechanical properties of metallic nano-multilayers in this thesis is carried out in Cr/Nb and Ag/Nb systems with distinctive characteristics. The choice of systems is based on the factors of enthalpy of formation, crystal structure, lattice constant and modulus difference, with focus on the factor of enthalpy of formation. Multilayers were prepared by e-gun evaporation equipment, and the mechanical properties were measured by nano-indentation method.Cr/Nb multilayers are in polycrystalline state, and have a remarkable reconciling effect. The hardness of multilayers does not increase with decreasing modulation wavelength (Λ), but in contrast, it drops a little whenΛis down to 6nm. This phenomenon can be interpreted by the weak interfaces, formed in Cr/Nb system with a negative enthalpy of formation, that the effect of grain boundary sliding takes place of Hall-Petch effect. WhenΛ= 3nm, the hardness of multilayers has a noticeable climb, in coincide with the formation of a new phase detected in the small angle grazing X-ray diffraction pattern. Through a further annealing experiment, it is demonstrated that the formed meta-stable phase has a great contribution to this hardness enhancement. In regards to modulus, although the reconciling strain is obvious in Cr/Nb multilayers, compressive strain in Nb and tensile strain in Cr between neighboring layers have a cancellation, and therefore the modulus of Cr/Nb multilayer is almost unchanged.The small difference of crystal parameter and modulus makes the factor of positive enthalpy of formation dominant in Ag/Nb system. The result of X-ray reflectivity indicates a well modulated layer structure in Ag/Nb multilayers as designed. Multilayers have a strong Ag(111) and Nb(110) texture. The hardness of multilayers increases monotonically with decreasingΛ, reaching about 80% enhancement whenΛ= 4nm, whereas the fluctuation of modulus is within 8%. Through the exploration of cross-sections of multilayers by a high resolution transmission electron microscope, it is seen that some disordered amorphous locations exist in some parts of interfaces and grain boundaries. These amorphous alloys, formed during the nonequilibrium preparation, make the formation and propagation of dislocations more difficult than in crystalline state, and therefore contribute to hardness enhancement. As to the little change of modulus, it is due to cancellation between the negative effect of the free volume in amorphous alloys and positive effect with decreasingΛof mutilayers.更多还原