I NFORMATION ON REFRACTORY COMPOSITES FOR USE ABOVE

This report summarizes information on refractory composites for use above 2500 F, as presented at the Seventh Meeting of the Refractory Composites Working Group, held March 12-14, 1963. Reports presented at the meeting were concerned with pro tective coatings, insulating ceramics, materials for rocket thrust chambers, dispersion strength ening of metals, joining of refractory materials, and testing techniques. Emphasis in the field of coatings for refractory metals has shifted from the rudiments of developing oxidation-resistant coatings to improvement and advanced evaluation of coatings to predict their utility for re-entry vehicles. (Author).

Fracture-toughness testing using principles of fracture mechanics has developed to the point where it can be used as a basis for selection of materials, for estimating limiting design stresses assuming the presence of small flaws, and for analyzing failures. Current methods of measuring plane-stress and plane-strain fracturetoughness parameters are presented in this report. The specimens include center-cracked, edge-cracked, single-edge-cracked, surface-cracked, and notched round bars, which are subjected to tensile loading, and notched bars for bend tests. The different types of specimens permit evaluating sheet, plate, bar stock, and forgings as well as material from failed structures. Application of fracture-toughness parameters to design of high-strength structures is reviewed for both static and fatigue loading. Consideration of the fracture-mechanics concepts in design should lead to fewer problems with brittle fracture in high-strength structures. (Author).

Although a considerable amount of information concerning the applications for arc plasmas in the materials sciences is available, it is contained in literally thousands of separate manuals, technical notes, textbooks, and government and industrial reports. Each source generally deals with only one specific application or, at best, a narrow range of utilization. This book was developed to provide a comprehensive and up-to-date compilation of information in the technology of arc plasma utilization. The book is divided into two general categories: flame spraying and materials evaluation. In the flame spraying section a comprehensive review of the plasma spraying process is presented. The design and operation of plasma spraying equipment are described. Included are a description of the nature of a plasma, and the design and operation of plasma generators, powder feed systems and accessory control equip ment. The general process procedures, and associated process variables are de scribed. Particular emphasis is given to the particle heating process and the mechanisms for adherence and cohesion of coatings. Competitive flame spraying equipment is also detailed (combustion process, detonation and electric arc) and compared with the plasma spray process. A discussion and compilation of flame sprayed ceramic and metal materials, their properties and applications are also included.

Because of a special interest in Alloy 718, the Defense Metals Information Center has summarized information on the welding and brazing of the alloy. Two principal resons for the interest in this alloy are its good formability and its good weldability, expecially under restraint. The greater portion of the fusion welding of Alloy 718 has been done by the gas tungsten-arc (TIG) process. A discussion of welding atmosphere, filler metals, heat treatment of welds, welding under restraint, and weld toughness is presented in the section on fusion welding. Information is also presented on resistance welding and electron-beam welding of Alloy 718. The final section of this report deals with the brazing of Alloy 718.

Metallic fibers in the form of whiskers, chopped fine wire, or wool have been incorporated into metallic matrices by two principal methods: classical powder-metallurgical methods, and infiltration of the fibers with molten matrix metal. Ceramic fibers, notably alpha-A1203, have been used as whiskers or wool for reinforcing metallic matrices, the methods of preparation of the composite being the same as with metallic fibers. When metallic fibers are used to reinforce ceramics, the ceramic is either slip cast into the fiber mat and dried, or a mixture of the fiber and ceramic is hot pressed. Plastics and elastomers are combined with fibers by several methods, for example, dipping a sintered skeleton of the fibers into liquid Teflon. Several programs have resulted in combinations which show that reinforcement of metals by either metallic or alpha-A1203 fibers is possible. Both the room-temperature and elevatedtemperature tensile properties of the composite seem to increase linearly with the volume fraction of the fiber phase. (Author).

Studies on the effects of heat-treating and testing environments for the refractory metals have been limited primarily to the unalloyed metals and a few alloys of columbium and molybdenum. Most of this work has relied on compositional changes as a means of assesing the effects of various environments on these materials. It has been shown that all of these materials are subject to contamination or purification in various test environments. The residual gases H2, CO(or N2), and H20, constitute the major sources of contamination when testing columbium and tantalum materials in vacua. Under the same conditions, molybdenum and molybdenum alloys containing carbon and reactive-metal additions are subject to serious decarburization. Nonreactive gaseous atmospheres also cause serious changes in material chemistry, since small quantities of noxious gases are contained in the atmosphere. Several promising methods of circumventing material chemistry changes during various longtime, high-temperature exposures are being used and/or evaluated. (Author).