Density of alloys

The enrichment of the slow dissolving component, B, in an alloy surface under simultaneous dissolution conditions may be rationalized by a model of alloy dissolution that is based on the simplifying assumptions (1) that a homogeneous solid solution may be described as a heterogeneous dispersion of atomic dimensions with area fraction (surface mole fraction) X j for component j, and (2) that the alloy components dissolve independently. The partial current density ij of an alloy component j will then be given by ij = i -X j, where i is the current density of the pure metal, j, and for a binary alloy A-B, the total current density of alloy dissolution. 

Fig. 2. Calculated and experimental densities of alloys of the Cr-Si system in the CrSi2 region 1) experimental data 2) (calculated) for a substitutional solid solution 3) (calculated) for a substitutional solid solution a) for unannealed single crystals b) annealed.

Also the polarization curve of this sample (III) differs from the others. The current density is significantly higher than the other in terms of both active and passive state. Under the potential about E > 0.5 V, the current density of alloy containing 11% Co is two times higher than for the alloy with a low cobalt content (Table 1). The increase in cobalt content... 

R universal gas constant Poi mass density of alloy component i ... 

If the metal liquid system does not contain any components that provoke the exchange reactions in an electrochemical cell, the EMF method is well suited for such a system. The problem of liquation (or phase separation) is less serious, but the big difference of the sp>ecific density of alloying effects on the rate of establishment of thermodynamic equilibrium and distorts the potentiometric measurements. 

Besides the material based characteristics, the difference of density of the used particle/substrate combination is a very important criterion. The difference of density influences the contrast of the radiographic tests. Tungsten carbides were used as mechanically resistant particles and titanium based alloys as substrate. The substrate material is marked by an advantageous relation of strength to density. This material is often used in aeronautics, astronautics, and for modification of boundary layers. The density of tungsten carbide (15.7 g/cm ) is about 3.5 times higher than the density of titanium (4.45-4.6 g/cm ). 

The actuality of researches of multi-layer objects with high density of materials for airspace technics is substantiate by several firms. The distribution of the heavier in longerons and elements of the screw of the helicopter was determined with the help of RCT. In the filler made of graphite or rubbers the layers of heavier from leaden alloys and the air stratification 1 up to 5 mm wide are revealed with the ratio signal / noise more than 3, even at presence of an external steel cover. 

Density is a particularly important characteristic of alloys used in rotating machinery, because centrifugal stresses increase with density. Densities of the various metals in Table 1 range from 6.1 to 19.3 g/cm. Those of iron, nickel, and cobalt-base superaHoys fall in the range 7-8.5 g/cm. Those alloys which contain the heavier elements, ie, molybdenum, tantalum, or tungsten, have correspondingly high densities. 

Metallurgy. Lithium forms alloys with numerous metals. Early uses of lithium alloys were made in Germany with the production of the lead alloy, BahnmetaH (0.04% Li), which was used for bearings for railroad cars, and the aluminum alloy, Scleron. In the United States, the aluminum alloy X-2020 (4.5% Cu, 1.1% Li, 0.5% Mn, 0.2% Cd, balance Al) was introduced in 1957 for stmctural components of naval aircraft. The lower density and stmctural strength enhancement of aluminum lithium alloys compared to normal aluminum alloys make it attractive for uses in airframes. A distinct lithium—aluminum phase (Al Li) forms in the alloy which bonds tightly to the host aluminum matrix to yield about a 10% increase in the modules of elasticity of the aluminum lithium alloys produced by the main aluminum producers. The density of the alloys is about 10% less than that of other stmctural aluminum alloys. 

Peripheral pitting and etching associated with the low current densities arising outside the main machining zone occur when higher current densities of 45-75 A/cm are appHed. This is a recurrent difficulty when high alloy, particularly those containing about 6% molybdenum, titanium alloys are electrochemicaHy machined. 

The electrolysis is conducted at 90—95°C and an anode current density of about 50 120 A/m when using lead alloy anodes and lead cathodes. Using graphite electrodes, the current density is from 70 100 A/m using titanium anodes and graphite cathodes, the current density is 50 80 A/m (82). 

The electrorefining of many metals can be carried out using molten salt electrolytes, but these processes are usually expensive and have found Httie commercial use in spite of possible technical advantages. The only appHcation on an industrial scale is the electrorefining of aluminum by the three-layer process. The density of the molten salt electrolyte is adjusted so that a pure molten aluminum cathode floats on the electrolyte, which in turn floats on the impure anode consisting of a molten copper—aluminum alloy. The process is used to manufacture high purity aluminum. 

The process is used for ferrous P/M stmctural parts that have densities of at least 7.4 g/cm and mechanical properties superior than those of parts that have been only compacted and sintered. Depending on the appHcation, the porous matrix may be infiltrated only partially or almost completely. Copper-base alloy infiltrants have been developed to minimise erosion of the iron matrix. 

An ingenious method to avoid or reduce segregation of alloying elements involves preparing small spheres of material by the atomization of a Hquid stream through a nozzle to produce a powder. This powder can be compacted, often hot and triaxially by gas pressure, to form a material where, on further heating, the residual pores close by diffusion to approach 100% density. 

Alloys of the P type respond to heat treatment, are characterized by higher density than pure titanium, and are more easily fabricated. The purpose of alloying to promote the P phase is either to form an aE-P-phase aEoy having commercially useful quaUties, to form aEoys that have duplex a- and P-stmcture to enhance he at-treatment response, ie, changing the a and P volume ratio, or to use P-eutectoid elements for intermetallic hardening. The most important commercial P-aEoying element is vanadium. 

High density tungsten alloy machine chips are recovered by oxidation at about 850°C, foUowed by reduction in hydrogen at 700—900°C. Typically, the resultant powders are about 3-p.m grain size and resinter readily. There can be some pickup of refractory materials used in furnace constmction, which must be controUed. This process is important commercially. Eor materials that may be contaminated with other metals or impurities, the preferred recovery process is the wet chemical conversion process used for recovery of tungsten from ores and process wastes. Materials can always be considered for use as additions in alloy steel melting. 

Table 9 Hsts select properties of Co—Cr alloys. It is generally conceded that the casting shrinkage of the cobalt—chromium alloys is greater than that of the gold alloys. The lower density of the base metal alloys provides a weight advantage over the higher-density gold alloys in certain types of bulky restorations. Cobalt—chromium alloys have Knoop hardnesses of 310—415.

Asahi also reports an undivided cell process employing a lead alloy cathode, a nickel—steel anode, and an electrolyte composed of an emulsion of 20 wt % of an oil phase and 80 wt % of an aqueous phase (125). The aqueous phase is 10 wt % K HPO, 3 wt % K B O, and 2 wt % (C2H (C4H )2N)2HP04. The oil phase is about 28 wt % acrylonitrile and 50 wt % adiponitrile. The balance of the oil phase consists of by-products and water. The cell operates at a current density of 20 A/dm at 50°C. Circulated across the cathode surface at a superficial velocity of 1.5 m/s is the electrolyte. A 91% selectivity to adiponitrile is claimed at a current efficiency of 90%. The respective anode and cathode corrosion rates are about mg/(Ah). Asahi s improved EHD process is reported to have been commercialized in 1987. 

Area)(time)(metal density) where weight loss is in mg, area is in in" of metal surface exposed, time is in hours exposed, and density is in g/cm. Densities for alloys can be obtained from theproducers or from various metal handbooks. 

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