STRESS RELAXATION IN MULTIPHASE SYSTEMS : MICROMECHANICAL STUDY OF THE EFFECTS OF DIFFUSION AND VISCOUS SLIDING AT PHASE INTERFACES

本项目所做的理论和实验工作的主要目的是为了解陶瓷增强金属基复合材料(MMC)的高温变形行为(蠕变)提供一条新的途径。陶瓷增强金属基复合材料是一种轻质、高刚度的先进材料，目前在高温环境中的应用优于传统的金属材料。重点研究了金属-陶瓷相界面的粘性，这种粘性会引起应力松弛，从而对金属基复合材料的蠕变起到重要作用。首先，基于二维和三维连续介质模型进行了一系列分析，建立了两相(或多相)固体体系蠕变的细观力学描述，其中沿相界面同时发生扩散质量流动和粘性滑移。这种描述适用于多晶体中的扩散蠕变，使我们对现有的理论进行了评论，其中…更多的在边界滑移中的作用被完全忽视。研究发现，只有当边界粘性很小，可以瞬间发生滑动时，这种理论才能被证明是正确的。进一步的努力完全致力于我们的主要目标，特别关注通过粉末冶金工艺路线生产的颗粒碳化硅增强铝基复合材料。首先，讨论了陶瓷颗粒周围的扩散蠕变引起的滞弹性松弛。提出了一种松弛的细观力学模型，并通过对循环单轴载荷低幅低频条件下的动态杨氏模量和内耗随温度变化的测量，验证了该模型的有效性。然后将该模型扩展到基质的高应力“位错蠕变”区，即基质本身不保持弹性而是根据“幂定律”蠕变的情况。将注意力集中在以特定应力指数n=3为特征的I类蠕变行为上，以完全解析的方式描述了金属基复合材料的非线性非稳定蠕变行为。这一公式使我们能够对金属基复合材料“最小蠕变速率”数据中异常高的表观应力指数这一“谜题”有了新的认识。问题的症结在于，损伤过程(颗粒破裂和界面脱粘)很可能在高应力下发生在蠕变中，因此并不是所有的最小蠕变速率数据都应该被解释为稳态的代表。在颗粒碳化硅/6061 A1 MMC上进行的精心设计的实验证实了这一点。较少

Theoretical and experimental work has been done in this project with a primary objective to indicate a new route to the understanding of high-temperature deformation behavior (creep) of metal-matrix composites (MMCs) with ceramic reinforcements--a class of light-weight high-stiffness 'advanced materials' currently ranked above conventional metallic materials especially for use in elevated temperature environment. Emphasis is placed on the viscous nature of metal-ceramic phase interfaces which causes 'stress relaxation' and hence plays a substantial role in creep of MMCs. First, as basic preliminaries to our intention, a series of analysis based on 2-D and 3-D continuum models has been made to formulate a micromechanical description of creep in two-phase (or multi-phase) solid systems wherein both diffusional mass flow and viscous sliding occur along phase interfaces. This description is applied to diffusional creep in polycrystals, leading us to make comments on existing theories where … More in the role of boundary sliding is entirely ignored. It is found that such theories can be justified only if boundary viscosity is so small that sliding can occur instantaneously. Further effort is devoted exclusively to our primary objective, giving special attention to particulate SiC-reinforced aluminum-based MMCs produced by powder-metallurgy processing routes. To begin with, 'anelastic relaxation' caused by diffusional creep around ceramic particulates is addressed. A micromechanical model of the relaxation has been presented and its validity confirmed from measurements of dynamic Young's modulus and internal friction, each as a function of temperature, in low-amplitude low-frequency conditions of cyclic uniaxial loading. The model is then extended into the high-stress 'dislocation creep' regime for the matrix, i.e. the case where the matrix itself does not remain elastic but creeps according to the 'power law.' With attention confined to the 'class I creep behavior' characterized by the particular stress exponent n = 3, nonlinear unsteady creep behavior of MMCs has been formulated in an entirely analytical manner. This formulation allows us to gain new insight into a 'puzzle' as to the anomalously high apparent stress exponents in 'minimum creep rate' data for MMCs. The crux of the matter consists in that damage processes (particle cracking and interface debonding) very likely occur in creep at high stresses and hence not all the minimum creep rate data should be interpreted as representatives of the steady state. This has been substantiated by carefully designed experiments on a particulate SIC/6061 A1 MMC. Less