Development of toughened metal/intermetallic layered materials

最近要求材料具有各种功能。具有优异强度和抗氧化性能的金属间化合物有望成为高温结构材料。然而，由于其缺乏延展性和脆性，在整体形式下的应用受到很大限制。具有韧性金属和脆性金属间化合物的层状材料有望具有新的性能，特别是在特定方向上增强抗断裂能力。制备这些金属/金属间化合物层状材料的方法之一是自蔓延高温合成，我们已经研究了Ni/Ni- al， Ti/Ti- al和Nb/Nb- al层状材料。然而，热压加工过程中金属间层中产生的微空洞影响了材料的断裂行为，降低了材料的强度。在本研究中，我们尝试了用金属薄板加工制备具有较少微孔的层状材料，以及在不同的热处理温度和压力条件下金属间相的变化。对层状材料的力学性能和断裂机理进行了研究，并对优质层状材料的加工工艺进行了探讨。采用热压自蔓延高温合成技术，成功制备了不同金属层厚度和不同金属间相的Ni/Ni-Al层状材料，金属间相的变化使材料的抗拉强度和伸长率都得到了提高。通过对裂纹尖端周围应力场的分析，评价了裂纹在这些材料中的扩展行为，并提出了裂纹是直接扩展还是横向扩展的准则。

Materials are recently requested to have various functions. Intermetallics which have excellent strength and oxidation resistance are expected as high temperature structural materials. However, their application in monolithic form is very limited because of their lack of ductility and brittleness. Layered materials with ductile metals and brittle intermetallics are expected to have novel properties, especially enhancement of fracture resistance in specific direction. One of the methods to fabricate these metals/intermetallics layered materials is a self-propagated high temperature synthesis, and we have investigated Ni/Ni-Al, Ti/Ti-Al and Nb/Nb-Al layered materials. However, microvoids which are produced in intermetallic layer during hot-press processing affect the fracture behavior and reduce the strength of materials. In the present research we have tried the fabrication of layered materials with fewer microvoids by thin metal sheets processing and the change of intermetallic phase by different processing conditions of heat temperature and pressure. Also the mechanical properties and fracture mechanism of layered materials were investigated as well as the processing for excellent layered materials. Layered materials of Ni/Ni-Al with different thickness of metal layer and different intermetallic phase were successfully fabricated by self-propagating high temperature synthesis using hot-press from thin metal sheets of Ni and Al. The change of intermetallic phase gave the increase of both tensile strength and elongation. Crack propagation behavior in these materials was evaluated, and the criterion for crack direction whether a crack propagates straightly or deflects laterally was proposed using the analysis of stress field around crack tip.