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CONCLUSIONS AND RECOMMENDATIONS Ordered alloys have a number of properties that make them extremely attractive for structural uses, particularly at elevated temperatures. Ordered alloys have other very important uses, for example, as superconductors; however, only structural uses will be discussed in this report. Since many of them show both a high yield strength and high modulus along with a low density at elevated temperatures, their specific modulus and specific strengths are very attractive when compared to those of other materials. The work hardening rate of ordered alloys also is quite high compared to that of disordered alloys. The self-diffusion rate of these alloys is low compared to that of disordered alloys; therefore, properties that are determined by self-diffusion rate also can be quite different. For example, since the steady state creep rate is directly proportional to the self-diffusion rate, the creep rate of ordered alloys is slower than that of disordered materials. Thus, ordered alloys are stronger (deform more slowly) at elevated temperatures than do disordered alloys. Ordered alloys also tend to have longer high cycle fatigue lives at room temperature than do disordered alloys; however, this improvement has not been documented at elevated temperatures, except under crack growth conditions. Finally, the oxidation resistance of many ordered alloys, particularly the aluminides, is very high. Ordered alloys, however, have rather low ductilities and, as a result, they traditionally have been used in structural applications only as second-phase particles added to strengthen the disordered matrix, as, for example, in nickel-base superalloys. The reasons for the low ductility of ordered alloys are quite diverse; they include an insufficient number of slip systems (primarily in noncubic alloys), a limited cross slip, impurity locking of dislocations, and intrinsically brittle grain boundaries. Recent work at a number of different laboratories, however, has shown that the ductilities of ordered alloys can be dramatically improved by the
intelligent application of physical metallurgical principles. Ductilities have been improved by the removal of impurities and second-phase particles; by alloying to transform a noncubic to a cubic structure and thereby increase the number of available slip systems; by grain refinement through thermal-mechanical treatments; by rapid solidification through the melt spinning technique to produce ribbons, or the rapid solidification rate (RSR) technique to produce powders (which are subsequently processed by powder metallurgical techniques); or by microalloying with boron to improve grain boundary cohesion. As a result of these efforts, a new class of single-phase ductile ordered alloys (or possibly multiphase alloys with an ordered matrix) is now possible. These alloys show promise of offering extremely good properties at elevated temperatures while not suffering from the ductility problems previously encountered in these alloys. Although it is clear that the ductilities of ordered alloys can be dramatically improved, no one method is a panacea. Physical metallurgists have learned that a variety of treatments can improve the ductility of ordered alloys but, without detailed experimentation on a given alloy, they do not yet know enough to decide which treatment will be effective for that particular alloy. Furthermore, they do not yet know why some of these ductilizing treatments succeed. For example, small additions of boron to Ni^Al result in dramatic improvements of the grain boundary strength, but this effect is found only if the alloy is slightly aluminum-deficient. The mechanism by which boron improves the grain boundary strength and, furthermore, why it does not operate in many other grain boundary-brittle ordered alloys is simply not understood. The number of research programs in the United States that concentrate on ordered alloys is rather small, particularly when compared to 15 years ago when there were many more programs in both industrial and university laboratories. An extensive, long-term program at the Air Force Wright Aeronautical Laboratories under the direction of Harry A. Lipsitt has produced impressive results leading to possible practical applications of several aluminides. Another successful program is underway at Oak Ridge National Laboratory (ORNL) under the direction of C. T. Liu; Ni3Al and 0037 are being studied, and Ni3Al-based alloys with remarkably high ductility and strength have been developed. Additional Department of Energy (DOE) supported work on Ni3A1 is also being performed at Dartmouth University. The National Aeronautics and Space Administration, Lewis Research Center for the Conservation of Strategic Aerospace Materials (COSAM) program also supports research on several aluminides at NASA-Lewis and at Texas A & M, Stanford, and Dartmouth Universities. Also the Office of Naval Research (ONR) supports programs at Ohio State University, General Electric Company, and ORNL. Smaller programs are being conducted at the University of Pennsylvania (Pope and Vitek) with National Science Foundation (NSF) funding, at Rensselaer Polytechnic Institute (Stoloff) with DOE and ONR funding, and at Michigan Technological University. Thus, we can conclude that the U.S. effort in this area is rather small, but possibly growing. The level of effort in Japan is modest but appears to be increasing quite rapidly. Some information to support this conclusion appears in the literature, but it is based primarily on conversations between members of this committee and their Japanese colleagues who have stated that there is
now a concerted effort being made in Japan to develop a new class of high-temperature materials based on ordered alloys. A large number of Soviet researchers are working on ordered alloys, but one can only speculate as to whether any work is being performed to produce ductile ordered alloys since this kind of work may not be reported in the open literature. Only a few ordered alloy research programs are being conducted in Europe. There is a program on titanium aluminides at the National Gas Turbine Establishment in Farnborough in the United Kingdom, and there are three programs at universties at Groningen, Holland, under de Hosson; at Poitier, France, under Rabier; and at Lille, France, under Escaig. The university programs are highly scientific in nature and are aimed at developing an understanding of the basic physical phenomena rather than at developing particular alloy systems. The committee believes, based on its survey of research in progress, that, while the U.S. research effort is much smaller than it was 15 to 20 years ago, it is highly productive and appears to be ahead of other programs being carried out in other parts of the world. For early applications, in systems whose preliminary design and performance requirements have already been established, it must be recognized that a new alloy with better properties than an existing alloy will be used only if it allows the designer to meet performance specifications at lower cost or if use of the new alloy is necessary to meet the specified performance. For future designs, a new alloy may permit establishment of higher system performance requirements and thus become the necessary material of choice. However, even on currently designed systems there are windows of opportunity for new alloys wherein these may be substituted for others in the design stage (to reduce unanticipated weight gains, for example). Opportunities may also be presented during prototype testing or to correct a service problem. Before any alloy is incorporated into a system it will normally have gone through the stages of laboratory demonstration, development (including processing development where large costs are incurred), and finally, qualification. An adequate body of data must be available when qualification testing is required in order to support a decision to undertake such testing and the high costs involved. The titanium aluminides developed under Air Force sponsorship are well along the path towards usage, but development of the iron aluminides and the ductile nickel aluminides has proceeded only to the laboratory stage with some processing work only recently begun. The committee proposes that these new materials be considered for application in rotating parts of gas turbine engines because of their low density, high strength, and high oxidation resistance at elevated temperatures. Also, if these new materials show improved thermal and high-cycle fatigue resistance at elevated temperatures, they could be used as a turbine blade material in gas turbine engines in rocket propulsion systems. The combination of high strength at elevated temperatures and low density also suggests a number of applications to space power systems.
Before the designer can consider using a new material he must have considerable information on the engineering properties of the material, including data on its physical properties, time-independent and time-dependent properties, environmental compatibility, and fabricability. The data base for titanium aluminides is rather large but it is much more limited for iron aluminides and even more limited for nickel aluminides. In fact, virtually no data of the type required by a designer are available on the ductile iron and nickel aluminides. Since ductile ordered alloys show such unusual promise, the committee recommends that a three-phased program be initiated in which some of the engineering properties of ductile iron and nickel aluminides are measured, cost trade-off studies then are performed, and, if the potential cost and performance advantages of these alloys are clear, additional properties are measured. In the first phase, physical properties, tensile properties, creep and creep-fatigue properties, toughness, crack growth properties, and corrosion properties should be measured on heats of sufficient size that several laboratories can perform experiments on the same material. This phase should be relatively inexpensive compared with the second and third phases. If the results of Phase I show that these alloys have sufficient promise, Phase II should be started in which both experimental and production lots of material are tested, materials processing optimization is undertaken, the effects of multiaxial stress states and product anisotropy are studied, joining techniques are investigated, and compatibilities with various environments are studied. Phase III should involve testing of multiple production runs of material, development of statistical distributions of mechanical property data, and the development of specifications. To date there has been very limited work done on the processing of ductile ordered alloys. Based on the limited information available in the literature, a wide range of primary processing methods are available for the production of ordered alloys, including various melting and casting techniques, several deformation processing methods, and powder metallurgy (PM) methods. The best primary processing route for ordered alloys has not yet been identified and this must be done in Phase II of the program recommended above. Primary processing is followed by secondary processing when the product is made into its final shape (bar, sheet, rod, etc.), and no work has yet been performed on aluminides to determine the optimum secondary processing steps to be followed to provide the best properties at the lowest cost. Although work is under way to provide the kinds of information required by producers and designers, the development of ductile ordered alloys is very much based on the science of these materials, and a great deal of scientific information about these materials is still needed. Needed are additional phase diagrams; data on the effects of order and ordering kinetics on properties; an understanding of the deformation behavior of binary, ternary, and higher order systems including the effects of fault energies, dislocation cores, and domain structures; an understanding of grain boundary properties including the effects of impurity segregation; diffusion data; and an understanding of the effects of point defects.
Finally, a number of other ordered alloys have not yet been considered for structural usage. For example, ordered alloys having long period superlattices in which periodic faults exist have not yet been examined for such usage, but since the period of the fault spacing can be changed by alloying and since the mechanical properties are expected to depend strongly on that spacing, these alloys are good candidates for future consideration. Over the longer term, the committee expects that quantum mechanical calculations of equilibrium properties of ordered alloys will have a major impact on the development of additional alloys. Since only some of the trends in properties with changes in composition, state of long-range order, and defect structure, are understood, the development of new alloys is necessarily Edisonian in nature. The additional predictive capabilities of such calculations are expected to be invaluable in future development programs. SUMMARY 1. In the past, interest in ordered alloys has been limited by problems of brittleness and inadequate creep resistance, but recent work on a number of alloys has suggested that these problems can be overcome. 2. Ordered alloys potentially provide a combination of properties which are of great interest for DOD applications, particularly for high temperature load-bearing structures (i.e., in gas turbine engines, rocket propulsion systems, and space power systems). 3. The fact that high-strength ductile aluminides have been developed after only limited research on a few compositions suggests that there is potential for the development of even better systems with the application of a coordinated research effort. 4. A development program is recommended in which engineering property and processing data are gathered in a phased manner, and after each phase, cost and performance analyses are performed to see if further phases in the development are justified. 5. Areas of further scientific investigation of ordered alloys are suggested which can form the basis for the development of future ordered alloys of practical interest.
