TRANSPORT PHENOMENA IN MATERIALS PROCESSING
TRANSPORT PHENOMENA IN MATERIALS PROCESSING
复制标题
DOI：
10.1111/j.1749-6632.1983.tb19473.x
复制
发表时间：
1983-05
影响因子：
5.2
通讯作者：
S. Ostrach
中科院分区：
文献类型：
--
作者：
S. Ostrach
直到最近十年左右，人们才认识到材料加工过程中流体相的传输现象对固相的结构和质量有着深远的影响。在此期间，人们不断加大努力，以了解这些现象，从而改进商业产品。这种活动的很大一部分是针对单晶的生长和有关的凝固领域。在大多数情况下，这方面的早期工作是相当初级和经验性的，基本上包括对当前处理技术中观察到的具体方面的定性描述或解释。合金的晶体生长和凝固本质上是传质过程，在几乎所有常用的技术中，母相都是流体。当存在固体-蒸气-固体转变时，形成晶体的过程称为蒸气生长。当晶体从液体中获得时，该过程称为熔体或溶液生长。粗略地说，熔体生长是比溶液生长温度更高的过程。在大多数涉及流体的非平衡过程中，对流（宏观流体运动）起着主导作用，因为它影响相界面处的流体相组成和温度。反过来，流体运动由外部施加的运动（强制对流）和局部产生的运动（自然对流）的相互作用决定。后一种流动是由于体积力场中的温度和浓度密度梯度的影响或表面张力梯度的影响。因此，一般来说，晶体生长和固化是包括传热和传质、流体流动、相变和化学反应的耦合过程。大多数早期的工作，特别是关于晶体生长的工作，都是基于对流是有害的这一前提。因此，人们认为，随着太空飞行的到来，晶体生长将通过低重力环境来增强，这种环境可以消除或大大减少自然对流。然而，在天空实验室上进行的晶体生长实验并没有产生如此明确的结果。也许迄今为止太空计划中材料加工的最大贡献是它使材料科学家和流体动力学家之间的互动更加密切。然而，这本身并没有立即带来理解的提高，因为许多相关的交通问题超出了相关领域当前知识的前沿。所发生的是一个更密集的基础研究工作，开始对这些问题产生影响。这是本文的目的，以确定的problenis的重要性，并指出一些结果所取得的我的小组迄今为止。这将为本届会议的其他文件奠定基础，这些文件将提出其他重要成果。256
Only in the last decade or so has it been realized that transport phenomena in the fluid phase during the processing of materials have a profound influence on the structure and quality of the solid phase. During this period there has been an ever-increasing effort to gain some understanding of these phenomena so that commercial products could be improved. A great deal of this activity has been directed to the growth of single crystals and to the related field of solidification. For the most part, the earlier work in this regard was rather rudimentary and empirical and consisted essentially of qualitative descriptions or explanations of specific aspects that are observed in current processing techniques. Crystal growth and solidification of alloys are inherently mass transfer processes and the parent phase is a fluid in almost all the commonly used techniques. The process of forming a crystal is called vapor growth when there is a solid-vapor-solid transformation. The process is called melt or solution growth when the crystal is obtained from a liquid. Roughly speaking, melt growth is a higher temperature process than solution growth. As in most nonequilibrium processes involving a fluid, convection (macroscopic fluid motion) plays a dominant role, in that it affects the fluid-phase composition and temperature a t the phase interface. The fluid motion, in turn, is determined by the interplay of externally imposed motion (forced convection) and locally generated motion (natural convection). The latter flows are due either to the effects of temperature and concentration density gradients in a body force field or to the effects of surface tension gradients. Thus, in general, crystal growth and solidification are coupled processes that include heat and mass transfer, fluid flow, phase transformations, and chemical reactions. Most of the earlier work, particularly in regard to crystal growth, was based on the premise that convection is deleterious. Thus, it was thought, with the advent of space flight, that crystal growth would be enhanced by a low-gravity environment that could eliminate or greatly reduce the natural convection. However, crystal growth experiments aboard Skylab did not produce such definitive results. Perhaps the greatest contribution to date of the materials processing in space program is that it brought about closer interactions among materials scientists and fluid dynamicists. However, this by itself did not bring about an immediate improvement in understanding because many of the relevant transport problems were beyond the frontiers of current knowledge in the related fields. What has occurred is a more intensive and basic research effort that is beginning to have an impact on the problems. It is the purpose of this paper to identify the problenis of importance and to indicate some of the results obtained by my group to date. This will set the stage for the other papers of this session, which will present other significant results. 256