TiZr Null Matrix Alloy
Titanium has a coherent scattering length of -3.44 fm and Zirconium +7.16 fm. When these elements are alloyed in the molar mass ratio 3.44/7.16, i.e. 2.08 Ti / 1 Zr, the total coherent scattering length should equal zero. For 1 mole of Zirconium, adding 2.08 moles of Titanium will produce an alloy with null coherent scattering length.
Titanium has an atomic weight of 47.867 g/mol and Zirconium has an atomic weight of 91.224 g/mol. By weight, we get an alloy with null coherent scattering length by adding 2.08 x 47.867 = 99.56 g of Titanium to 91.224 g of Zirconium. This corresponds to the ratio 52.2% Ti / 47.8% Zr.
In this composition, the TiZr alloy exhibits excellent strength properties whilst retaining neutron transparency (coherent scattering). Room temperature tensile test data are generally [0.2% PS]=545 to 760 Mpa, [UTS]=700 to 920 MPa, [%Elong]=9 to 12, [%RA]=19 to 25, [Young Modulus]=90 to 100 GPa. Its appearance is similar to stainless steel, its density is about 5.23 g/cm3 and its melting point varies between 1540 and 1575°C.
The tensile tests performed in liquid helium and nitrogen give [0.2% PS]=1420 Mpa and [UTS]=1320 MPa at 4K and [0.2% PS]=1090 Mpa and [UTS]=920 MPa at 77K. Above room temperature in argon atmosphere, [0.2% PS]=545 Mpa and [UTS]=690 MPa at 473K and [0.2% PS]=420 Mpa and [UTS]=540 MPa at 773K. The values given above for both the high and low temperatures are based on an alloy with less than 0.1% oxygen content. The range of values for the Young's Modulus, UTS and proof stress (PS) at 20°C are dependant on the relative Oxygen content of the alloy. By weight: 0.05 % for the lower values and 0.15 % for the higher values.
TiZr has a density of 5.23 g/cm3 and is corrosion resistant against liquid alkali metals. The measuring temperatures, however, should be kept below 750°C because the TiZr alloy starts to soften and recrystallization can occur leading to Bragg peaks.
Production & Machining
The alloy is produced in billet form and is prepared using a double vacuum-arc smelting process using nuclear grade sponges of Titanium and Zirconium. All residual impurities are kept to a minimum with the Hafnium content in particular being kept to below 0.004 % by weight. Hf is an important neutron absorber and even though it is a heavier element, its electron configuration makes it practically identical with the element zirconium, and they are always found in the same ores. It is also important that the oxygen content of the ingot is between 0.14 % and 0.18% to produce high tensile strengths.
In the as cast form, the alloy will retain large amounts of residual gas porosity, normally in the form of spherical gas pores. These are significantly reduced by subjecting the alloy to a further process of Hot Isostatic Pressing (HIPing). This involves the material being surrounded by argon gas at a high isostatic pressure of 1850 bar and at a temperature of at least 1000°C. This has the effect of reducing both the size and quantity of voids in the material, producing a dense, well bonded structure.
Vacuum leak tests have been performed on 0.5 mm thick samples and taken down to pressure of 10-5 mbar, demonstrating that any remaining voids are small and do not form any leak path. The alloy has good machining properties, very similar to that of a tough stainless steel. Relatively fast speeds can be used with the respective feeds and depth of cut being kept small. Tipped cutting tools can be employed. A continuous supply of coolant must be provided throughout any machining operation to prevent overheating. All swarf must be removed frequently to avoid the accumulation on the machine.
Nitric-hydrofluoric acid can be used for the pickling or etching of Zirconium. The usual bath employed for metal removal is composed of 25% to 50% by volume Nitric acid, 2% to 5% by volume Hydrofluoric acid and water
Among the companies producing TiZr alloy, one can cite:
- Sophisticated Allows Inc., P.O. Box 2245, Butler, PA 16003 (USA)
- Timet, Titanium Metals Corporation, Henderson Technical Laboratory, P.O. Box 2128, Henderson, NV 89009 (USA)
Because of its intrinsic phyrophoric nature, there is a fire hazard associated with the processing of the material. Its maximum service temperature in air is 200°C, above which the alloy reacts with the gases present and there is a risk of it self igniting. Processes such as welding must be undertaken in a controlled atmosphere of Argon to prevent any such reaction.
- Nitryl fluoride (FN02) will initiate a reaction with Zirconium metal at room temperature to produce a glowing or white incandescence.
- Above 200 QC Zirconium reacts exothermically with halogen gases, fluorine, chlorine, bromine, iodine and halo-carbons, including carbon tetrachloride, Freons and Teflon.
- These reactions with incompatible materials will generate hazardous reaction products such as flammable hydrogen, toxic fumes of nitrogen oxides or corrosive Zirconium halide vapours.
- Titanium enters into thermite type reaction with iron oxides. Explosive reactions have been reported whilst attempting to use Titanium metal or powder in red fuming nitric acid.
- Titanium/Zirconium metal is rapidly dissolved by hydrofluoric acid or hydrofluoric-nitric acid mixtures.
- Titanium/Zirconium alloys are non toxic and safe to handle in solid forms.
- Finely divided process residues such as sludges may contain oil, acids or other harmful process contaminants and should not be ingested, inhaled or allowed to come into contact with the skin or eyes.
- If processed in such a manner as to produce fine dust or metal fume, it is recommended that adequate ventilation be provided.
- As a result of studies covering several Zirconium compounds, Zirconium was concluded to be an element of low toxicity.
There is a considerable fire hazard associated with the processing of Zirconium alloys. The metal has a greater heat of combustion than most metals and when in a finely divided form it may ignite spontaneously at ambient temperatures or be ignited at temperature
- High surface area / Finely divided forms
- Forms of Titanium/Zirconium such as foils, fine wire, turnings, millings and grindings are combustible and any material with a dimension less then 0.05 mm is very easily ignited.
- Once ignited, the alloy will burn fiercely, giving off intense heat and is difficult to extinguish.
- Metal powder or dust, forms an explosive mixture with air or an oxidiser.
- Water reacts with burning Titanium/Zirconium to release Hydrogen. The alloy will bum in an atmosphere of Carbon dioxide or Nitrogen.
- Severe explosions can result from the ignition of Zirconium powder or machining fines containing moisture in the range of 5 % to 10%.
- Solid forms
- Solid Titanium/Zirconium will not ignite.
- Liquid forms
- If the metal is being melted in a water cooled copper crucible it must be noted that molten Titanium/Zirconium and water can react with explosive violence. Precautions should be made to avoid any failure of the cooling system. All adequate protection for personnel must be provided in the event of an explosion.
- A.V. Dobromyslov, N.I. Taluts: "STRUCTURE INVESTIGATION OF QUENCHED AND TEMPERED ALLOYS OF THE Zr-Ti SYSTEM", Phys. Met. Metallogr. 63 1 (1987) 114–120.
- T. Doi, H. Ishida, T. Umezawa: "Study of Nb-Zr-Ti Phase Diagram (Studies of Hard Superconductor, II)", Nippon Kinzoku Gakkaishi 30 (1966) 139–145.
- L. Kaufman, L.E. Tanner: "COUPLED PHASE DIAGRAMS AND THERMOCHEMICAL DESCRIPTIONS OF THE TITANIUM-BERYLLIUM, ZIRCONIUM-BERYLLIUM AND HAFNIUM-BERYLLIUM SYSTEMS", Calphad: Comput. Coupling Phase Diagrams Thermochem. 3 (1979) 91–107.
- lan Bailey, Rutherford and Appleton Laboratory - Titanium/Zirconium null matrix composition data.
- Rob Done, Rutherford and Appleton Laboratory - A report on the Titanium/Zirconium null matrix alloy available at RAL - IID/SEIN3/91.
- BNF Metals Technology Laboratory - High temperature tensile test data on Titanium/Zirconium Zero alloy'.
- Fulmer Research Limited - Report and data information on the HlPing of Titanium/Zirconium alloy.
- Justin R. Carmichael, Oak Ridge National Laboratory - Private communications
- Technology department, Rutherford and Appleton Laboratory - Room temperature and cryogenic temperature tensile test data on Titanium/Zirconium alloy.
- lMl Titanium Limited, product safety information - Material safety data sheets for Zirconium and Titanium.
- Harwell Laboratory, safety data sheet - Precautions to be taken when working with Zirconium or Zirconium alloys - HL80/1503.
- "Nanometer superstructure in some liquid alloys; a neutron diffraction study", ISBN 90-407-1723-0 / CIP, Delft University Press
- S. S. Sidhu, LeRoy Heaton, D. D. Zauberis, and F. P. Campos, J. of Applied Physics 27, 9 (1956) 1040 (doi:10.1063/1.1722538)