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

该项目已经完成了理论和实验工作，其主要目标是为理解带有陶瓷增强的金属基复合材料（MMCs）的高温变形行为（蠕变）指明一条新的途径。MMCs是一种轻质高刚度的“先进材料”，目前排名高于传统金属材料，特别是在高温环境中使用。重点放在金属-陶瓷相界面的粘性特性上，它会导致“应力松弛”，因此在mmc的蠕变中起着重要作用。首先，作为我们意图的基本准备，我们进行了一系列基于二维和三维连续体模型的分析，以制定两相（或多相）固体系统中蠕变的微观力学描述，其中扩散质量流动和粘滞滑动都发生在相界面上。这一描述适用于多晶中的扩散蠕变，这使我们对现有的理论做出评论，其中…More在边界滑动的作用完全被忽略了。研究发现，只有当边界黏度小到可以瞬间发生滑动时，这些理论才能成立。进一步的努力专门用于我们的主要目标，特别关注通过粉末冶金加工路线生产的颗粒sic增强铝基mmc。首先，由陶瓷颗粒周围的扩散蠕变引起的“非弹性松弛”被解决。提出了一种微力学松弛模型，并通过测量动态杨氏模量和内摩擦（作为温度的函数）在低振幅低频循环单轴加载条件下的有效性得到了证实。然后将该模型扩展到基体的高应力“位错蠕变”状态，即基体本身不保持弹性，而是根据“幂律”蠕变的情况。本文将重点放在以特定应力指数n = 3为特征的“I类蠕变行为”上，以完全解析的方式阐述了mmc的非线性非定常蠕变行为。该公式使我们对mmc的“最小蠕变率”数据中异常高的表观应力指数的“难题”有了新的认识。问题的关键在于，损伤过程（颗粒开裂和界面剥离）很可能发生在高应力下的蠕变中，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。这已通过精心设计的颗粒SIC/6061 A1 MMC实验得到证实。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