微米尺度铜的力学行为及其尺寸效应的研究

IMR OpenIR

	微米尺度铜的力学行为及其尺寸效应的研究
	戴采云
学位类型	博士
导师	朱世杰 ; 张广平
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料物理与化学
关键词	微米尺度铜尺寸效应拉伸性能疲劳性能 Micrometer Scale Copper Size Effect Tensile Property Fatigue Property
其他摘要	" 当微米尺度金属的几何尺度逐渐接近材料内部的微观结构尺度时，其往往表现出与块体材料不同的力学行为。深入研究微米尺度金属材料的力学行为及其尺寸效应，不仅对微/纳米机械系统及器件的可靠性设计具有实际的指导意义，且对揭示材料在小尺度下的基本变形机制具有重要的科学意义。本论文首先选取了微米尺度单晶铜箔作为模型材料，研究了微米尺度单滑移取向单晶铜箔的循环变形行为，考察了不同厚度的单滑移()和双滑移()取向单晶铜箔的疲劳损伤行为及疲劳强度，探讨了晶体取向及几何尺度对材料疲劳行为的作用规律；随后研究了微米尺度多晶铜的厚度、晶粒尺度及两者的比值对材料拉伸与疲劳性能的影响，探究了微米尺度多晶金属力学行为尺寸效应的物理本质；通过对微米尺度轧制态和退火态多晶铜箔的研究，考察了材料的初始微观结构对微米尺度金属力学行为的影响。不同厚度取向单晶铜箔弯曲循环形变行为的研究表明，在总应力幅控制下，50 μm和175 μm厚的单晶铜箔均能在较小的初始分解切应变幅下发生循环硬化饱和，当初始分解切应变幅超过某一临界值时，发生循环软化饱和。单晶铜箔越薄，初始循环硬化和初始循环软化能力越弱，从初始循环硬化向初始循环软化转变的临界初始分解切应变幅越低。50 μm厚的单晶铜箔发生由循环硬化饱和向循环软化饱和转变的临界初始分解切应变幅约为175 μm厚的铜箔的1/3。不同厚度单晶铜箔横截面位错结构的观察表明，悬臂梁铜箔表面的位错密度较高，而中性面附近区域的位错密度较低，中性面处没有明显的位错塞积。相同初始切应变幅下，单晶铜箔越薄，几何必要位错密度和总位错密度越高，但发生初始循环硬化向初始循环软化转变的临界总位错密度非常相近。较薄单晶铜箔中由于高密度的几何必要位错的参与，促使其更容易在较低的初始分解切应变幅下发生循环软化。不同厚度的单滑移和双滑移取向单晶铜箔的弯曲疲劳实验结果表明，两种取向单晶铜箔均表现出相同趋势的疲劳性能尺寸效应，即在总应变幅控制下，单晶铜箔的弯曲疲劳性能随着铜箔厚度的减小而降低。单晶铜箔表面及横截面的疲劳损伤行为及位错结构的观察表明，在相近的总应变幅下，单晶铜箔越薄，位错密度越高，表面的疲劳损伤越严重。对几何必要位错密度的分析表明，弯曲循环变形的较薄单晶铜箔中较大塑性应变梯度产生的高密度几何必要位错促进了材料中位错结构的快速发展及过早循环应变局部化，从而造成了更严重的疲劳损伤，降低了疲劳寿命。厚度恒定与晶粒尺寸恒定的两组微米尺度退火多晶铜箔的拉伸和疲劳实验结果表明，铜箔厚度与晶粒尺寸的比值(t/d)对退火多晶铜箔的拉伸和疲劳性能有显著影响。随着t/d的减小，材料的几何尺度及表面晶粒对位错行为的影响越来越显著，导致其对微米尺度材料力学性能的贡献越来越大。当t/d>1时，屈服强度由晶粒尺寸决定，随着t/d的增加屈服强度升高；当t/d<1时，屈服强度由厚度控制。厚度恒定的铜箔的屈服强度不随t/d的变化而改变，而晶粒尺寸恒定的铜箔的屈服强度随t/d的减小而降低。厚度恒定的铜箔随着t/d的减小，应变硬化速率单调减小，拉伸塑性呈现先增加后减小的趋势；晶粒尺寸恒定的铜箔，拉伸塑性随着t/d的减小而降低；当t/d>1时，应变速率随t/d的减小而降低，而当t/d<1时，则相反。厚度恒定和晶粒尺寸恒定的两组退火多晶铜箔的弯曲疲劳实验结果表明，在总应变幅控制下，对于厚度恒定的多晶铜箔，当t/d>1时，t/d越小，疲劳性能越好，而t/d<1时，则正相反。对于晶粒尺寸恒定铜箔，疲劳性能随t/d的减小而降低。轧制态和退火态多晶铜箔越薄，应变控制的弯曲疲劳性能越差，退火态多晶铜箔的弯曲疲劳性能高于轧制态多晶铜箔。微尺度材料的塑性控制了应变控制的弯曲疲劳性能的优劣。; "As geometrical dimensions of micrometer-scale metals are gradually close to microstructural dimensions, mechanical behavior of the small-scale materials would become different from that of their counterparts. A study of mechanical behaviors and size effects of micrometer-scale metals is of importance not only for the reliability design of micro/nano-systems, but also for the basic deformation mechanism of the small-scale materials. In this PhD thesis, micrometer-scale copper single crystal foils were selected as a model material firstly. Cyclic deformation behavior of -oriented copper single crystal foils with a thickness ranging from tens of micrometers to hundreds of micrometers were investigated. Fatigue damage behavior and fatigue strength of the -oriented and -oriented copper single crystal foils with different thicknesses were examined. Effects of crystallographic orientation and foil thickness on fatigue behavior of micrometer-scale copper single crystal foils were evaluated. After that, effects of the foil thickness, the grain size and the ratio of the foil thickness to the grain size on tensile and fatigue properties of micrometer-scale polycrystalline copper foils were studied to elucidate physical origins of size effects on mechanical behavior. Effects of initial microstructures on fatigue behavior of the micrometer-scale metals were also checked by using as-rolled and annealed polycrystalline copper foils. The investigation of cyclic deformation behavior of -oriented single crystal copper foils under dynamic bending conditions indicated that under the constant cyclic stress amplitude control, initial cyclic hardening of 50 μm and 175 μm thick foils could occur at the small initial resolved shear strain amplitude. As the initial resolved shear strain amplitude was more than a critical value, initial cyclic softening would happen. The ECC observations on dislocation structures on cross-sections of single crystal copper foil show that the dislocation density on the surface of the cantilever beam was high and that close to the central plane was relatively low. There were not evident dislocation pileups in the central plane region. Under the same initial shear strain amplitude, the thinner the single crystal copper foil, the higher the geometrically necessary dislocation(GND) density and the total dislocation density. The critical total dislocation densities for the transition from initial cyclic hardening to initial cyclic softening were similiar for the foils with different thicknesses. Owing to the involvement of a high density of GNDs, cyclic softening could occur more easily in the thinner Cu foil at the lower initial resolved shear strain amplitude. The bending fatigue tests of and -oriented single crystal Cu foils revealed that the single crystal Cu foils with two different orientations had the same trend in fatigue size effects, i.e. under total strain amplitude control the thinner the single crystal Cu foil, the lower the bending fatigue strength. The tensile tests of two kinds of annealed polycrystalline copper foils, i.e. thickness-constant (t-constant) and the grain size-constant (d-constant) foils, showed that the foil thickness, the grain size and the ratio (t/d) of the foil thickness to the grain size had strong influences on tensile properties. When t/d>1, the grain size dominated the yield strength of the micrometer-scale foils, which decreased with decreasing the t/d ratio for two types of the copper foils. As t/d<1, the foil thickness controlled the yield strength, which resulted in the fact that the yield strength of the t-constant foils did not change with t/d, while the yield strength of the d-constant foils decreased with decreasing t/d. For the t-constant foils, the strain hardening rate decreased monotonically with decreasing t/d, while the tensile plasticity firstly increased and then decreased with decreasing t/d. As geometrical dimensions of micrometer-scale metals are gradually close to microstructural dimensions, mechanical behavior of the small-scale materials would become different from that of their counterparts. A study of mechanical behaviors and size effects of micrometer-scale metals is of importance not only for the reliability design of micro/nano-systems, but also for the basic deformation mechanism of the small-scale materials. In this PhD thesis, micrometer-scale copper single crystal foils were selected as a model material firstly. Cyclic deformation behavior of -oriented copper single crystal foils with a thickness ranging from tens of micrometers to hundreds of micrometers were investigated. Fatigue damage behavior and fatigue strength of the -oriented and -oriented copper single crystal foils with different thicknesses were examined. Effects of crystallographic orientation and foil thickness on fatigue behavior of micrometer-scale copper single crystal foils were evaluated. After that, effects of the foil thickness, the grain size and the ratio of the foil thickness to the grain size on tensile and fatigue properties of micrometer-scale polycrystalline copper foils were studied to elucidate physical origins of size effects on mechanical behavior. Effects of initial microstructures on fatigue behavior of the micrometer-scale metals were also checked by using as-rolled and annealed polycrystalline copper foils. The investigation of cyclic deformation behavior of -oriented single crystal copper foils under dynamic bending conditions indicated that under the constant cyclic stress amplitude control, initial cyclic hardening of 50 μm and 175 μm thick foils could occur at the small initial resolved shear strain amplitude. As the initial resolved shear strain amplitude was more than a critical value, initial cyclic softening would happen. The ECC observations on dislocation structures on cross-sections of single crystal copper foil show that the dislocation density on the surface of the cantilever beam was high and that close to the central plane was relatively low. There were not evident dislocation pileups in the central plane region. Under the same initial shear strain amplitude, the thinner the single crystal copper foil, the higher the geometrically necessary dislocation(GND) density and the total dislocation density. The critical total dislocation densities for the transition from initial cyclic hardening to initial cyclic softening were similiar for the foils with different thicknesses. Owing to the involvement of a high density of GNDs, cyclic softening could occur more easily in the thinner Cu foil at the lower initial resolved shear strain amplitude. The bending fatigue tests of and -oriented single crystal Cu foils revealed that the single crystal Cu foils with two different orientations had the same trend in fatigue size effects, i.e. under total strain amplitude control the thinner the single crystal Cu foil, the lower the bending fatigue strength. The tensile tests of two kinds of annealed polycrystalline copper foils, i.e. thickness-constant (t-constant) and the grain size-constant (d-constant) foils, showed that the foil thickness, the grain size and the ratio (t/d) of the foil thickness to the grain size had strong influences on tensile properties. When t/d>1, the grain size dominated the yield strength of the micrometer-scale foils, which decreased with decreasing the t/d ratio for two types of the copper foils. As t/d<1, the foil thickness controlled the yield strength, which resulted in the fact that the yield strength of the t-constant foils did not change with t/d, while the yield strength of the d-constant foils decreased with decreasing t/d. For the t-constant foils, the strain hardening rate decreased monotonically with decreasing t/d, while the tensile plasticity firstly increased and then decreased with decreasing t/d. It was found that with decreasing the t/d ratio, the effects of the geometrical scale and the surface grain on dislocation activity became more and more evident, which had more contributions to mechanical properties of the micrometer-scale metal. The bending fatigue tests of the thickness-constant (t-constant) and the grain size-constant (d-constant) annealed polycrystalline copper foils revealed that under total strain amplitude control, for the t-constant foils, when t/d>1, bending fatigue resistance increased with decreasing t/d, while as t/d<1, fatigue strength decreased with decreasing t/d. The tensile plasticity of the micrometer-scale metals controlled bending fatigue properties."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64416
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	戴采云. 微米尺度铜的力学行为及其尺寸效应的研究[D]. 北京. 中国科学院金属研究所,2012.