Making High-Tensile-Strength Amalgam Components
- Thursday, 03 January 2008
Instead of spheroids or flakes, wires are used as the solid constituents.
Structural components made of amalgams can be made to have tensile strengths much greater than previously known to be possible. Amalgams, perhaps best known for their use in dental fillings, have several useful attributes, including room- temperature fabrication, corrosion resistance, dimensional stability, and high compressive strength. However, the range of applications of amalgams has been limited by their very small tensile strengths. Now, it has been discovered that the tensile strength of an amalgam depends critically on the sizes and shapes of the particles from which it is made and, consequently, the tensile strength can be greatly increased through suitable choice of the particles.
The term “amalgam” generally denotes an alloy of mercury with one or more other metals (e.g., copper or a copper alloy in the case of dental fillings). Amalgams can also be based on gallium, or gallium alloys, which melt near room temperature. An amalgam is formed by a peritectic reaction in a process called “trituration,” in which the solid metal (e.g., copper) in powder form is ground together with the liquid metal (e.g., gallium). The grinding serves to break the oxide skin on the solid metal particles, enabling wetting of the clean metal surfaces by the liquid metal. The liquid metal reacts with the solid metal to form a new solid that is a composite of the starting solid metal (e.g., Cu) and an intermetallic compound (e.g., CuGa).
Heretofore, the powder particles used to make amalgams have been, variously, in the form of micron-sized spheroids or flakes. The tensile reinforcement contributed by the spheroids and flakes is minimal because fracture paths simply go around these particles. However, if spheroids or flakes are replaced by strands having greater lengths, then tensile reinforcement can be increased significantly. The feasibility of this concept was shown in an experiment in which electrical copper wires, serving as demonstration substitutes for copper powder particles, were triturated with gallium by use of a mortar and pestle and the resulting amalgam was compressed into a mold. The tensile strength of the amalgam specimen was then measured and found to be greater than 104 psi (greater than about 69 MPa).
Proceeding forward from this demonstration of feasibility, much remains to be done to optimize the properties of amalgams for various applications through suitable choice of starting constituents and modification of the trituration and molding processes. The choice of wire size and composition is expected to be especially important. Perusal of phase diagrams of metal mixtures could give insight that would enable choices of solid and liquid metal constituents. For example, phase diagrams have revealed that gallium should form amalgams with iron and nickel (as already demonstrated), as well as zirconium, and titanium. Finally, whereas heretofore, only binary alloys have been considered for amalgams, ternary additions to liquid or solid components should be considered as means to impart desired properties to amalgams.
This work was done by Richard Grugel of Marshall Space Flight Center.