【作者】 许福；

【导师】 张平； 龙志林；

【作者基本信息】 湘潭大学 ， 一般力学与力学基础， 2012， 博士

【摘要】 块体非晶合金具有优异的物理、化学和力学性质及广阔的应用前景，受到科学界和材料工程界的关注与重视，是科技前沿的热点之一，非晶态物质的结构、形成机理、变形机理等基础问题是当前研究的重要内容。块体非晶合金材料的流变行为为流变力学提供了新的研究课题。本文以多种体系的块体非晶合金为研究对象，基于材料的微观结构及物理本质，应用理论与实验相结合的方法，研究了块体非晶合金室温微纳流变力学行为及机理，建立了相应的流变力学模型。本文得到的主要结果如下：1、提出了非晶合金中的微观非均匀结构在中程序到¹0nm尺度或更大尺度（可能达到^μm）范围的分形分布模型，通过综合分析、分形维计算证明了该模型的合理性。2、基于非晶合金微观非均匀结构的分形分布特征，引入能够比拟树状分形网络结构的分数阶微分流变模型研究材料的室温流变行为。根据分数阶微分的Riemann-Liouville定义讨论了分数阶微分流变模型的性质，提出了蠕变柔量和松弛模量的实用简化表达式，并给出了模型参数在热力学限制条件下的取值范围。根据分数阶微分流变模型的本构关系推导了球形压头压入分数阶微分粘弹性半空间问题的解，给出了分数阶微分粘弹性体在阶跃加载和常加载速率模式下的纳米压痕粘弹性响应。3、对Pd₄₀Cu₃₀Ni₁₀P₂₀，Zr₄₈Cu₃₄Pd₂Al₈Ag₈和(Fe_0.432Co_0.288B_0.192Si_0.048Nb_0.04)₉₆Cr₄块体非晶合金进行了室温粘弹性纳米压痕实验，结果表明，分数阶微分流变模型能细致刻画材料的室温延迟弹性行为，恰当、有效地反映材料结构整体信息，流变参数与Tg T的关系明晰，物理意义明确。4、考虑屈服产生的非均匀结构分形生长，提出了表征块体非晶合金室温纳米压痕蠕变行为的新的分数阶微分流变模型，证明了该模型包含常规流变模型，且经适当简化后可得到常用的表征纳米压痕蠕变行为的经验公式。通过精细设计的纳米压痕蠕变实验，分别得到了(La_0.5Ce_0.5)₆₅Al₁₀Co₂₅块体非晶合金室温蠕变中可回复的粘弹性及不可回复的粘性流动变形，结合其他几种不同体系块体的纳米压痕蠕变实验结果考察了蠕变载荷Pm ax、加载速率P对蠕变行为的影响及模型参数的变化趋势。结果表明，流变模型参数少，拟合精度高，物理意义明确。Pmax变化时，粘弹性变形呈现一定程度的线性性质，粘性流变变形表现出明显的压痕蠕变尺寸效应；P对蠕变行为影响显著，并由流变模型参数的变化趋势反映；着重分析了模型参数中粘度指标η、分数阶次α0的变化规律及α0与蠕变应力指数n的关系，结果表明，α0与n随P的变化规律相似，η是加载历史相关的、表征蠕变开始时体系瞬态粘度的指标，α0是反映蠕变过程中材料内部结构信息和流动趋势的参数。由流变模型参数的物理意义合理地解释了文献中报道的n随P变化趋势的争议；以Mg₆₅Cu₂₅Y₁₀和Mg₈₅Cu₅Y₁₀块体非晶合金为例，讨论了弛豫对非晶合金蠕变行为的影响，并进一步验证了流变模型的物理意义。5、基于自由体积理论推导出非晶合金纳米压痕硬度H与应变速率ε （t）、自由体积浓度cf （t）及加载时间t的关系式，提出非晶合金中纳米压痕蠕变、硬度尺寸效应的微观机理是材料在外加载荷作用下发生了应变软化，指出这种软化效应受初始自由体积浓度c f、 ε （t）和t等因素影响，合理地解释了文献中报道的关于块体非晶合金硬度尺寸效应机理的争议，最后由原子力纳米压痕实验验证了应变软化效应。更多还原

【Abstract】 Bulk metallic glass （BMG） offers novel physical, chemical and mechanicalproperties and has perspective potential applications. As one of the hot topics offrontier science and technology, BMG has received extensive attention both fromscientific community and material engineering. Fundamental problems in amorphousmaterials, e.g. structure, glass forming and deformation mechanism etc. are significantissues of current research. Furthermore, rheological behavior of BMGs provides newresearch topic for rheological mechanics. In this thesis, several BMGs were chosen asobjects of the research. Based on the micro-structure and physical essence of thematerials, micro-nano rheological mechanical behavior and related mechanism ofBMGs were studied both by theoretical and experimental methods. Meanwhile, thecorresponding rheological model was established. The main results are summarized asfollows:1. A fractal model was proposed for describing the distribution features of thestructural heterogeneities of metallic glasses （MGs） ranging from the scale of mediumrange order to¹0nm or even larger scale （may reach^μm）. The validity of themodel was proved by comprehensive analysis and fractal dimension calculation.2. Based on the fractal distribution features of the structural heterogeneities ofMGs, fractional-order differential rheological model which can match fractal treenetwork structure was introduced to study the room-temperature rheologicalbehaviors of BMGs. The properties of this model were discussed according to theRiemann-Liouville definition of the fractional derivatives. Practical simplifiedexpressions for the creep compliance and relaxation modulus were given as well asthe range of parameters of the model under the thermodynamic constraints. Theproblem of spherical indenter into fractional viscoelastic half-space was solved byutilizing the constitutive equations of the fractional rheological model. The responsesof the fractional viscoelastic body by step-loading and constant-loading-ratenanoindentation were derived.3. Room-temperature viscoelastic nanoindentation tests were performed onPd₄₀Cu₃₀Ni₁₀P₂₀，Zr₄₈Cu₃₄Pd₂Al₈Ag₈and(Fe_0.432Co_0.288B_0.192Si_0.048Nb_0.04)₉₆Cr₄BMGs.The results show that the fractional rheological model can characterize the delayedelastic behavior of these BMGs accurately and reflect the overall structureinformation of the materials properly and effectively. The rheological parametershave clear relationship withTg T and own clear physical significance. 4. A new fractional-differential rheological model was presented forcharacterizing the room-temperature nanoindentation creep behavior of BMGs byconsidering the fractal growth of the structural heterogeneities caused by yielding.This model contains the traditional rheological model. By appropriate simplification,the empirical equation used to characterize the nanoindentation creep behavior can bederived from the proposed rheological model. Well-designed nanoindentation creepexperiments were carefully performed in (La_0.5Ce_0.5)₆₅Al₁₀Co₂₅BMG to charactize therecoverable viscoelastic deformation and unrecoverable viscous flow respectivelyfrom the room-temperature creep deformation. Combing the nanoindentation creepexperimental results of several BMGs with different constituents, effects of the creepload （Pm ax） and loading rate （P） on the creep behavior as well as the trends of themodel parameters were investigated. It shows that the proposed rheological modelwith few parameters possesses high fitting accuracy and clear physical significance.AsPm axchanging, the viscoelastic deformation shows some of linear nature, whileapparent creep indentation size effect is clearly observed in viscous flow deformation.The effect of P on creep behavior is pronounced and reflected by the trends of therheological model parameters. Trends of parameters e.g. viscosity index （η）,fractional order （α0） and relation betweenα0and creep stress exponent （n） wereanalyzed with great emphasis. The results show thatα0exhibits similar trend as nchanging with P. η is a loading history dependant index which characterizes theinstant viscosity of the materials when creep is starting.α0reflects the internalstructural information and flowing trend during the creep period. The controversialtrends of n changing with P reported in the literatures is explained reasonably onthe base of the physical significance of the model parameters. Taking Mg₆₅Cu₂₅Y₁₀and Mg₈₅Cu₅Y₁₀BMGs as examples, effects of relaxations on the creep behaviorwere discussed, which further verified the physical meanings of the proposedrheological model.5. An expression correlating Nanoindentation hardness （H） with strain rate（ε （t））, free volume concentration （c f（t）） and loading time （t） was derived based onfree volume theory. The micro-mechanism of indentation size effect （ISE） in creepand hardness was proposed as strain softening caused by applied load. The strainsoftening is affected by the initial free volume concentrationcf, ε （t）and t. Withthese in mind, the controversy in literatures about the mechanism of ISE in BMGswas explained in reason. Finally, the strain softening effect was confirmed by nanoindentation performed with atomic force microscopy.更多还原