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

理论和实验工作已经完成，在这个项目的主要目标是指出一个新的路线，以了解高温变形行为（蠕变）的金属基复合材料（MMCs）与陶瓷增强体-一类轻质高刚度的“先进材料”目前排名高于传统的金属材料，特别是在高温环境中使用。重点放在金属-陶瓷相界面的粘性性质，导致“应力松弛”，因此在金属基复合材料的蠕变中起着重要的作用。首先，作为我们的意图，一系列的分析的基础上的2-D和3-D连续模型已制定的两相（或多相）固体系统中的蠕变的微观力学描述，其中扩散质量流和粘性滑动发生沿着相界面。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作出评论， ...更多信息 在边界滑移中的作用被完全忽略。据发现，只有当边界粘性是如此之小，滑动可以瞬间发生，这样的理论是合理的。进一步的努力专门致力于我们的主要目标，特别注意颗粒SiC增强铝基金属基复合材料生产的粉末冶金工艺路线。开始，“滞弹性松弛”所造成的扩散蠕变陶瓷颗粒周围解决。的松弛的微观力学模型已被提出，其有效性确认从测量的动态杨氏模量和内摩擦，每个作为温度的函数，在低振幅低频率条件下的循环单轴加载。然后将该模型扩展到高应力的“位错蠕变”制度的矩阵，即的情况下，矩阵本身不保持弹性，但蠕变根据“幂律”。考虑到以特殊应力指数n = 3为特征的“I类蠕变行为”，MMCs的非线性非稳态蠕变行为已完全解析地表达出来。这一提法使我们能够获得新的洞察到一个“难题”，因为在“最小蠕变速率”的数据金属基复合材料的异常高的表观应力指数。问题的关键在于，损伤过程（颗粒开裂和界面脱粘）很可能发生在高应力下的蠕变中，因此并非所有的最小蠕变速率数据都应被解释为稳态的代表。这已被证实的颗粒SIC/6061 A1 MMC的精心设计的实验。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