Specialized metallurgical processes – compositions for use therei – Processes – Producing or purifying free metal powder or producing or...
Reexamination Certificate
2001-09-07
2004-03-02
Wyszomierski, George (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or purifying free metal powder or producing or...
C075S369000, C075S619000
Reexamination Certificate
active
06699305
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to the production of metals and their alloys, particularly including refractory metallic alloys such as titanium and zirconium aluminides and amorphous metals.
BACKGROUND OF THE INVENTION
As the fourth-most plentiful metal in the earth's crust, titanium is relatively abundant in nature (e.g., as rutile-TiO
2
and ilmenite-FeTiO
3
, and has highly useful properties. However, this refractory metal is unfortunately relatively expensive to extract and reduce from its ores, and difficult to fabricate into useful products in view of its high melting point, sometimes requiring use of film or powder metallurgy techniques such as hot isostatic processing of a powdered or thin film form. It is difficult to purify, and even more expensive to prepare in powder form suitable for advanced powder metallurgical manufacturing processes.
Titanium is conventionally produced by reduction of titanium tetrachloride with magnesium metal in a steel batch retort (the “Kroll process”). A significant part of the high cost of titanium as a result of the inefficiency and batch nature of the Kroll process which is currently used for its manufacture. This process produces crude titanium “sponge” which may be intimately contaminated with magnesium chloride and titanium subchlorides, as well as impurities in the magnesium reducing agent. The crude titanium “sponge” which the Kroll process produces, requires costly vacuum arc refining to produce refined titanium ingots which are suitable for manufacturing use. Subsequent grinding and/or plasma particulation of the refined ingot to produce uniform powders for powder metallurgy and composite manufacture is also relatively expensive.
Titanium forms alloys and intermetallic compounds of significant technical importance. Titanium alloys, and especially titanium aluminides, are important, but costly, materials for aerospace components for propulsion and power. The relatively low density of titanium and titanium alloys, combined with their high specific stiffness, high strength, high corrosion resistance and relative toughness, are particularly desirable in aerospace systems. The efficiency of high-performance propulsion systems and turbines is limited by the high temperature capabilities of materials used for engine components. Relatively lightweight gamma-TiAl based intermetallic alloys have desirable strength to weight and other properties, particularly in comparison with the heavier titanium and nickel-base alloys currently used in combustion and compressor sections of engines. A two-phase (TiAl+Ti
3
Al), structure distributed as fine or coarse lamellar microstructures including the &agr;2 (Ti3Al), orthorhombic (Ti
2
AlNb) and &ggr; (TiAl) classes of alloys may be particularly optimal for some applications. More sophisticated titanium and TiAl reinforced composite aerospace components, such as advanced SiC-fiber-reinforced titanium alloy aeroengine and structural components, are under development in many countries (including the U.S., France, the U.K. and China). Such advanced composites utilize expensive Ti or TiAl powders and/or foils in their manufacture. [see, e.g., Z. X. Guo, “Towards Cost Effective Manufacturing Of Ti/SiC Fibre Composites And Components”, Materials Science and Technology, Vol. 14, pp. 864-872 (1998)].
Zirconium and its alloys are of particular use to the nuclear power industry, and chemical and materials industries, and for amorphous metal compositions. The corrosion resistance, mechanical properties and neutron transparency of Zirconium, make Zirconium-based alloys important materials for containing or alloying with uranium fuel, and for the construction of critical components of nuclear reactors. Zirconium also has a wide variety of other uses, as a getter in vacuum tubes, as an alloying agent in steel, in surgical appliances, photoflash bulbs, explosives, fiber spinnerets, and lamp filaments, and as a superconductor (with niobium) to make superconductive magnets. As a refractory metal, Zirconium can be difficult to shape and work. However, a variety of Zirconium-aluminum and similar alloys may be quenched to an amorphous, ductile state. For example, see U.S. Pat. No. 5,980,652, describing amorphous Zr—Al alloys which have significant malleability in their amorphous form. Such amorphous Zirconium alloys typically include aluminum, together with metals such as Fe, Co, Ni or Cu which promote amorphous phase formation. Bulk glass-forming metals based on Ti, Al, Zir and/or Fe which can retain their amorphous state without extremely fast cooling rates typically have three to five or more metallic components with a large atomic-size mismatch to facilitate a high packing density without crystallization. They generally form liquid melts with a small free volume and high viscosity which are energetically close to the crystalline state, because of their high packing density and short-range order, which results in slower ecrystallization kinetics and improved glass forming ability [R. Busch, “The Thermophysical Properties of Bulk Metallic Glass-Forming Liquids”,
JOM,
52 (7) (2000), pp. 39-42. A wide variety of Ti, Al, Zr, and Fe-based glass-forming alloys, such as La—Al—Ni, Zr—Ni—Al—Cu, and Zr—Ti—Cu—Ni—Be, exhibit very good bulk glass-forming ability with high thermal stability in the supercooled glass state, and low critical cooling rates [A. Inoue, et al.,
Mater. Trans. JIM
31 (1991), p. 425; T. Zhang, et al.,
Mater. Trans. JIM,
32 (1991), p. 1005; A. Inoue et al.,
Mater. Trans. JIM,
32 (1991), p. 609; A. Peker and W. L. Johnson,
Appl. Phys. Lett.,
63 (1993), p. 2342; all cited references incorporated hereby reference]; Zr
41.2
Ti
13.8
Cu
10.0
Ni
12.5
Be
22.5
(V1) has a very low critical cooling rate of about 1 K/s, which is 5-6 orders of magnitude lower than some earlier metallic glass-forming systems. The difference in Gibbs free energy between an undercooled metal alloy glass and the corresponding crystallized alloy is the driving force for crystallization. When it is low, as in bulk glass forming alloys, glass-forming ability is high as has been done for alloys such as Zr—Ti—Cu—Ni—Be, and Cu—Ti—Zr—Ni. The Gibbs free energy difference for such “stable” glass-forming alloys may be only 2-4 Kilojoules per mole, normalized to the melting temperature of the respective alloy, even when cooled to temperatures as low as
⅓ the crystalline melting temperature of the alloy. The metal glass formers with the lowest critical cooling rates have smaller (e.g., less than
2 kJ/mole) Gibbs Free Energy differences than do the glass formers with higher critical cooling rates. The small driving force for crystallization of such bulk metal glass mixtures results from their small free volume, and their short-range order in the supercooled liquid, because the variety of atoms with different sizes in the mixture permits effective packing in the glassy state.
Amorphous alloys containing zirconium and titanium have excellent intrinsic corrosion resistance and mechanical properties, but unfortunately have been very expensive. Powder preparation for powder metallurgy manufacturing is also very expensive.
Zirconium is not scarce in nature, but is expensive to extract and reduce from its ores, because of its very high reactivity and high melting point. It is also difficult to purify magnesium chloride byproduct, and even more expensive to prepare in powder or alloy form suitable for advanced powder metallurgical manufacturing processes. Uniform alloy formation can also be an expensive processing step. Zirconiun occurs chiefly as a silicate in the mineral zircon (ZrSiO
4
), and as an oxide in the mineral baddeleyite. Zirconium is produced commercially by reduction of chloride with magnesium (the Kroll Process), as well as other methods. Hafnium is invariably found in Zirconium ores, and the separation of Hf from Zr is difficult. Commercial-grade Zirconium accordingly contains from 1 to 3% Hafnium.
Efforts have been made to directly produce titanium powders by reduc
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