Titanium hardness

Reaction-bonded titanium nitride (RBTN) ceramics are like RBSN made from a porous green shape of titanium powder that is reacted with nitrogen to titanium nitride (TiN) at temperatures up to 1000°C. Here the titanium hardly increases in molar volume when nitrided and the initial porosity remains the same but the gas permeability of a pressed titanium tablet is increased after it has been converted to titanium nitride. If the titanium powder particles are too large, the reaction stops after passivation of the metal surfaces the TiN formed at the surface is a diffusion barrier that stops the reaction. A fractal powder morphology of the starting metal (such as can be obtained from gas-phase preparation) is a very suitable reactant for complete reaction at modest temperatures. 

An alloy ideally should be homogeneous, but in practise it can contain segregations, for example hard alpha in titanium. Beeause of their different mechanieal properties sueh segregations can be the origin of eracks when the component is operated near to its temperature and stress limits. 

If the normal carbonate is used, the basic carbonate or white lead, Pb(OH),. 2PbCO,. is precipitated. The basic carbonate was used extensively as a base in paints but is now less common, having been largely replaced by either titanium dioxide or zinc oxide. Paints made with white lead are not only poisonous but blacken in urban atmospheres due to the formation of lead sulphide and it is hardly surprising that their use is declining. 

Natural titanium is reported to become very radioactive after bombardment with deuterons. The emitted radiations are mostly positrons and hard gamma rays. The metal is dimorphic. The hexagonal alpha form changes to the cubic beta form very slowly at about 88O0C. The metal combines with oxygen at red heat, and with chlorine at 550oC. 

The i5p-titanium(IV) atom is hard, ie, not very polarizable, and can be expected to form its most stable complexes with hard ligands, eg, fluoride, chloride, oxygen, and nitrogen. Soft or relatively polarizable ligands containing second- and third-row elements or multiple bonds should give less stable complexes. The stabihty depends on the coordination number of titanium, on whether the ligand is mono- or polydentate, and on the mechanism of the reaction used to measure stabihty. 

The four most important carbides for the production of hard metals are tungsten carbide [12070-12-17, WC, titanium carbide [12070-08-5] TiC, tantalum carbide [12070-06-3J, TaC, and niobium carbide [12069-94-2] NbC. The binary and ternary soHd solutions of these carbides such as WC—TiC and WC—TiC—TaC (NbC) are also of great importance. Chromium carbide (3 2) [12012-39-0], molybdenum carbide [12011-97-1], MoC, and... 

The response of titanium-aluminum powder mixtures in a 3 1 molar ratio was investigated under the same shock-loading conditions as in the nickel aluminides. Such mixtures are especially interesting in that the shock impedances of the materials are approximately equal and both are relatively hard and difficult to deform. In addition to any chemical differences, such materials should prove to be difficult to mix with the shock conditions. 

Carbide-based cermets have particles of carbides of tungsten, chromium, and titanium. Tungsten carbide in a cobalt matrix is used in machine parts requiring very high hardness such as wire-drawing dies, valves, etc. Chromium carbide in a cobalt matrix has high corrosion and abrasion resistance it also has a coefficient of thermal expansion close to that of steel, so is well-suited for use in valves. Titanium carbide in either a nickel or a cobalt matrix is often used in high-temperature applications such as turbine parts. Cermets are also used as nuclear reactor fuel elements and control rods. Fuel elements can be uranium oxide particles in stainless steel ceramic, whereas boron carbide in stainless steel is used for control rods. 

Since discovering and making use of the piezoelectric effect in naturally occurring crystals such as quartz and Rochelle salts, scientists have produced a wide range of piezoelectric materials in the laboratoi y. An early example is barium titanate, used in an electrical component called a capacitor. Currently, most piezoelectric materials are oxide materials based on lead oxide, zirconate oxide, and titanium. These very hard piezoelectric materials are termed piezoceramics. 

Materials with hard oxides, including stainless steels and aluminum and titanium alloys, are particularly susceptible to this form of attack. In steel, it is also known as false Brinelling because of the high surface hardness that can be created in work-hardening grades. 

Fig. 1.51 Crevice corrosion resulting from the crevice produced between the gasket and the flange of a titanium pipe used for conveying a hot hypochlorite solution. The attacked areas are coated with a hard deposit of titanium oxides, whilst the unattacked area of metal outside...

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