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.

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)