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MATERIALS SCIENTISTS have long known that if two (or more) metal or ceramic slabs of different composition are pressed together at high temperature, atoms from each slab will diffuse into adjoining slabs, forming new phases. This approach has long been used to determine phase diagrams and diffusion coefficients, but it hasn't been used to survey the properties of the new phases formed. Author has now coupled this traditional synthetic technique with newer microscale techniques for measuring compositions and properties, creating what he calls "a powerful combinatorial approach" for structural (load-bearing) materials. Author describes this approach for studying a complex system involving four components: molybdenum; nickel; iron; and Inconel 706 (IN706), a commercial superalloy consisting of (by weight) about 42% nickel, 38% iron, 16% chromium, and smaller amounts of several other elements. The goal of the experiment was to study how adding various amounts of molybdenum, nickel, and iron to the alloy would affect its properties. To promote the interdiffusion of these four materials, author assembled machined components made of these materials into a metallic "birthday cake." The entire outside of the cake--the "frosting"--was pure nickel. Buried inside was a smaller cake, consisting of four quarter slices--one each of the four starting metals. In this arrangement, every metal slice was in contact with all the other metal slices. Author ensured good contact by welding the assembly together and subjecting it to high temperature and high pressure. Then the entire cake--with frosting--was baked at 1,100 ºC for more than two months to give the constituents plenty of time to intermix by diffusion. After heating, the cake was cut in half horizontally, and the cut surfaces were polished flat. Author was then able to chemically analyze the various phases that had formed (using electron probe microanalysis) and determine their mechanical properties. From these data, for example, he mapped out the Ni-Mo-Fe phase diagram and found that it agrees very well with that measured earlier from equilibrated alloys. And by performing nanoindentation tests, he found that iron has little effect on the hardness and elastic modulus (stiffness) of a major phase known as the g-phase, whereas molybdenum causes significant hardening but has little effect on stiffness.
Metallurgists have long known that molybdenum is a better hardener than Fe. But actual data on how the hardness varies with composition have been difficult to come by because the traditional approach of studying one alloy at a time is expensive and time-consuming. The combinatorial approach is much more efficient, he says. COMBINATORIAL METALLURGY shows off two "diffusion multiples"--assemblies of different metallic components that are welded together and heated for prolonged periods to promote the formation of intermetallic compounds through interdiffusion. At right is a cross-sectional view of one such diffusion multiple consisting of Inconel 706, nickel, molybdenum, and iron components. Author also has mapped out IN706-Mo-Fe phases and evaluated their properties. This kind of multicomponent phase diagram can be used directly for designing new alloys and for improving the computational design of materials, he says. Author and his coworkers have studied many other alloy systems using this combinatorial approach and have uncovered promising new phases that may lead to new commercial alloys. He firmly believes that the diffusion method is a quicker and less expensive way to search for useful alloy compositions.
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