A silvery-white metal, zirconium (symbol Zr), has a specific gravity of 6.5 and a melting point of about 1850°C. It is more abundant than nickel, but is difficult to reduce to metallic form as it combines easily with oxygen, nitrogen, carbon, and silicon. The metal is obtained from zircon sand by reacting with carbon and then converting to the tetrachloride, which is reduced to a sponge metal for the further production of shapes. The ordinary sponge zirconium contains about 2.5% hafnium, which is closely related and difficult to separate. The commercial metal usually contains hafnium, but reactor-grade zirconium, for use in atomic work, is hafnium-free.
Commercially pure zirconium is not a high-strength metal, with a tensile strength of about 220 MPa, elongation 40%, and Brinell hardness 30, or about the same physical properties as pure iron. But it is valued for atomic-construction purposes because of its low neutron-capture cross section, thermal stability, and corrosion resistance. It is employed mostly in the form of alloys but may be had in 99.99% pure single-crystal rods, sheets, foil, and wire for superconductors, surgical implants, and vacuum-tube parts. The neutron cross section of zirconium is 0.18 barn, compared with 2.4 for iron and 4.5 for nickel. The cold-worked metal, with 50% reduction, has a tensile strength of about 545 MPa, with elongation of 18% and hardness of Brinell 95. The unalloyed metal is difficult to roll, and is usually worked at temperatures to 482°C. Although nontoxic, the metal is pyrophoric because of its heat-generating reaction with oxygen, necessitating special precautions in handling powder and fine chips resulting from machining operations.
The metal has a close-packed hexagonal crystal structure, which changes at 862°C to a body-centered cubic structure that is stable to the melting point. At 300 to 400°C the metal absorbs hydrogen rapidly, and above 200°C it picks up oxygen. At about 400°C it picks up nitrogen, and at 800°C the absorption is rapid, increasing the volume and embrittling the metal.
The metal is not attacked by nitric (except red fuming nitric), sulfuric, or hydrochloric acids, but is dissolved by hydrofluoric acid. Zirconium powder is very reactive, and for making sintered metals it is usually marketed as zirconium hydride, ZrH2, containing about 2% hydrogen, which is driven off when the powder is heated to 300°C. For making sintered parts, alloyed powders are also used. Zirconium copper (containing 35% zirconium), zirconium nickel (with 35 to 50% zirconium), and zirconium cobalt (with 50% zirconium), are marketed as powders of 200 to 300 mesh.
Properties
In addition to resisting HCl at all concentrations and at temperatures above the boiling temperature, zirconium and its alloys also have excellent resistance in sulfuric acid at temperatures above boiling and concentrations to 70%. Corrosion rate in nitric acid is less than 1 mil/year at temperatures above boiling and concentrations to 90%. The metals also resist most organics such as acetic acid and acetic anhydride as well as citric, lactic, tartaric, oxalic, tannic, and chlorinated organic acids.
Relatively few metals besides zirconium can be used in chemical processes requiring alternate contact with strong acids and alkalies. However, zirconium has no resistance to hydrofluoric acid and is rapidly attacked, even at very low concentrations.
Uses
Small amounts of zirconium are used in many steels. It is a powerful deoxidizer, removes the nitrogen, and combines with the sulfur, reducing hot-shortness and giving ductility. Zirconium steels with small amounts of residual zirconium have a fine grain, and are shock resistant and fatigue resistant. In amounts above 0.15% the zirconium forms zirconium sulfide and improves the cutting quality of the steel.
A noncrystalline metal that reportedly has twice the strength of steel and titanium, has been developed. The material, known as Vitrel-loy, is an alloy composed of 61% zirconium, 12% titanium, 12% copper, 11% nickel, and 3% beryllium. Its yield strength is 1900 MPa, compared with 800 MPa for titanium alloy, Ti-6% Al-4% V, and 850 MPa for cast stainless steel.
Fracture toughness is said to be 55 MPa-m1/2, the same as high-strength steel but half that of titanium. Its resistance to permanent deformation is said to be two to three times higher than that of conventional metals. The density of Vitrelloy is 6.1 g/cm3 between cast titanium at 4.5 g/cm3 and cast stainless steel at 7.8 g/cm3. The material is particularly recommended for aerospace applications because of its surface hardness of 50 HRC. Cast titanium and steel are both tested at 30 HRC.
The beneficial properties of the alloy are ascribed to its noncrystalline structure. Because there are no patterns or grains within the structure, weak areas caused by grain boundaries are eliminated.
An advanced machinable ceramic that may be used to produce thermal shock-resistant components for aerospace, automotive, electrical, heat treating, metallurgical, petrochemical, and plastics applications up to 1550°C has been introduced. The new material (AremcoloxTM 502-1550) is based on the zirconium phosphate system (Ba1+xZr4P6-2xSi2xO24) and is especially unique because of its low coefficient of thermal expansion (CTE) of 0.5 x 10-6 in./in.°F. This characteristic sets the material apart from standard ceramic materials such as alumina and zirconia which have CTEs of 4.0 x 10-6 and 2.5 x 10-6, respectively.
A low CTE ensures that as a component is thermally cycled the mechanical stress induced through expansion and contraction does not cause the part to crack. This feature enables engineers to adapt the material to high thermal shock applications, such as combustion and heater systems, that were not previously feasible.
Additional properties and applications of the machinable ceramic include their use as molds, optical stands, microwave housings, engine parts, and applications in which high mechanical strength, hardness, and low porosity are required. A low-density version of the material (502-1550 LD) is recommended for use as brazing fixtures, induction heating liners, rocket nozzles, and high-temperature gauges, tooling, and structures. The material is easily machined using carbide tooling and no postfiring is required.
Alloys
Zirconium alloys generally have only small amounts of alloying elements to add strength and resist hydrogen pickup. Zircoloy 2, for reactor structural parts, has 1.5% tin, 0.12% iron, 0.10% chromium, 0.05% nickel, and the balance zirconium. Tensile strength is 468 MPa, elongation 37%, and hardness Rockwell B89; at 316°C it retains a strength of 206 MPa.
Zirconium alloys can be machined by conventional methods, but they have a tendency to gall and work-harden during machining. Consequently, tools with higher than normal clearance angles are needed to penetrate previously work-hardened surfaces. Results can be satisfactory, however, with cemented carbide or high-speed steel tools. Carbide tools usually provide better finishes and higher productivity.
Mill products are available in four principal grades: 702, 704, 705, and 706. These metals can be formed, bent, and punched on standard shop equipment with a few modifications and special techniques. Grades 702 (unalloyed) and 704 (Zr-Sn-Cr-Fe alloy) sheet and strip can be bent on conventional press-brake or roll-forming equipment to a 5t bend radius at room temperature and to 3t at 200°C. Grades 705 and 706 (Zr-Cb alloys) can be bent to a 3t and 2.5t radius at room temperature and to about 1.5t at 200°C.
Small amounts of zirconium in copper give age-hardening and increase the tensile strength. Copper alloys containing even small amounts of zirconium are called zirconium bronze. They pour more easily than bronzes with titanium, and they have good electric conductivity. Zirconium-copper master alloy for adding zirconium to brasses and bronzes is marketed in grades with 12.5 and 35% zirconium. A nickel-zirconium master alloy has 40 to 50% nickel, 25 to 30% zirconium, 10% aluminum, and up to 10% silicon and 5% iron. Zirco-nium-ferrosilicon, for alloying with steel, contains 9 to 12% zirconium, 40 to 47% silicon, 40 to 45% iron, and 0.20% max carbon, but other compositions are available for special uses. SMZ alloy, for making high-strength cast irons without leaving residual zirconium in the iron, has about 75% silicon, 7% manganese, 7% zirconium, and the balance iron. A typical zirconium copper for electrical use is Amzirc. It is oxygen-free copper with only 0.15% of zirconium added. At 400°C it has a conductivity of 37% IACS, tensile strength of 358 MPa, and elongation of 9%. The softening temperature is 580°C.
Zirconium alloys with high zirconium content have few uses except for atomic applications. Zircoloy tubing is used to contain the uranium oxide fuel pellets in reactors because the zirconium does not have grain growth and deterioration from radiation. Zirconia ceramics are valued for electrical and high-temperature parts and refractory coatings. Zirconium oxide powder, for flame-sprayed coatings, comes in either hexagonal or cubic crystal forms. Zirconium silicate, ZrSi2, comes as a tetragonal crystal powder. Its melting point is about 1649°C and hardness about 1000 Knoop.
Zirconium Beryllides
Intermetallic compounds, ZrBe13 and Zr2Be17, have good strengths at elevated temperatures. ZrBe13 is cubic, density 2.72 g/cm3, melting point 1925°C; Zr2Be17 is hexagonal, density 3.08 g/cm3, melting point 1983°C; parts can be formed by all ceramic-forming methods plus flame and plasma-arc spraying. Materials are subject to safety requirements for all beryllium compounds.
These intermetallics, because of their greater densities (BeO = 1.85 g/cm3), contain more beryllium atoms per unit volume than beryllia, a decided advantage for compact, beryllium-moderated nuclear reactors.
Zirconium Carbide
Zirconium carbide, ZrC2, is produced by heating zirconia with carbon at about 2000°C. The cubic crystalline powder has a hardness of Knoop 2090, and a melting point of 3540°C. The powder is used as an abrasive and for hot-pressing into heat-resistant and abrasion-resistant parts.
Zirconium Diboride
Zirconium diboride (ZrB2) has a density of 6.09 g/cm3 and a hexagonal (AlB2) crystal structure with a melting point of 3040°C.
Zirconium diboride is oxidation resistant at temperatures <1000°C and reacts slowly with nitric, hydrochloric, and hydrofluoric acids. It reacts with aqua regia and hot sulfuric acid, as well as with fused alkalies, carbonates, and bisulfates. Zirconium diboride has a typical room temperature electrical resistivity of 9.2 x 10-6 Q cm and is superconductive at temperatures less than 2 K. Consolidation of ZrB2 powder into parts is accomplished by hot pressing or pressureless sintering.
Similar to titanium diboride, ZrB2 is wet by molten metals but is not attacked by them, making it a useful material for molten metal crucibles, free-formed nozzles, electric discharge machining electrodes, Hall-Heroult cell cathodes, and thermowell tubes for steel refining. This last use is one of the largest uses of zirconium diboride. Other uses for ZrB2 include electrical devices, refractories, and applications where high oxidation resistance is required.
Others
Zirconium oxychloride, ZrOCl2 8H2O, is a cream-colored powder soluble in water that is used as a catalyst, in the manufacture of color lakes, and in textile coatings. Zirconium-fused salt, used to refine aluminum and magnesium, is zirconium tetrachloride, a hygroscopic solid with 86% ZrCl4. Zirconium sulfate, Zr(SO4)2 4H2O, comes in fine, white, water-soluble crystals. It is used in high-temperature lubricants, as a protein precipitant, and for tanning to produce white leathers. Soluble zirconium is sodium zirconium sulfate, used for the precipitation of proteins, as a stabilizer for pigments, and as an opacifier in paper. Zirconium carbonate is used in ointments for poison ivy, as the zirconium combines with the hydroxy groups of the urushiol poison and neutralizes it. Zirconium hydride has been used as a neutron moderator, although the energy moderation may be chiefly from the hydrogen.
