3218

2*3* Imporfeotion of a metal composition and

struoture*- Chemio.al Conroogition

Chemical composition and structure entirely determine

mechanical and physical properties and, thus, strength and

isirable elements the total content of which is negligible but they can affect deleteriously the required properties* Por some applications, the standard may specify the tolera­ nces in' those elements termed tramp or residual elements, the presence o f which must not exceed the limits specified* Some tramp elements or impurities are particularly injurious and have to bo kept to very muoh finer limits*

- dirty raw charge and scrap covered with sand, rust or corrosion products are used for melting,

- raw materials of unknown ohomical compositon are melted,

-Different kinds of raw materials are mixed at the stor­ age,

-incorrect weighing of charging materials,

- the charge is composed erroneously,

V

•• the charts is о<'трояне, erroneously,

-there are some error in the calculation of a metal

waste at the melting in the furnace*

- melting operations are imperfect.

To prevent this kind of the defect it is necessary*

-to control and analyse all raw materials in spite of

•to observe the order of maintenance of melting furnaces_ as well as the prescribed techanological regime of melt­ ing a metal,

•to analyse the metal chemical composition during melting and prior to pouring*

Alloy

Composttio/i

Fig* 2*30* ghase diagram for jthe nickel-copper a ll system* Microstructures shown are obtained by very slow solidification*

Fig«2*31* Phase diagram of the aluminium-rich end of the aluminium-copper'fifijase diagram* Microstructures shown are obtained by very slow solidification*

is freezing increases in composition to 42 per cent copper. Diffusion of copper in the solid eliminates concentration gradients, and the f$$zen portion of tho melt is a solid solution containing 42 per cent copper (a288°C).

fin ally, at the solidus (1249°C) tho last copper liquid, containing 67 per cent copper, freezes to a solid containing

50 per cent copper. The excess copper atoms from the liquid diffuse into the metal to make the whole solid alloy a homogeneous solid solution of 50 per cent nickel, 50 per cent copper.

Metals that are not mutually soluble in all proportions combine as meohanical mixtures to fourm alloys. Alloys of this type are made up of two ehemic ally (and sometimes structurally) different tygtes of crystals^

ф .

he copper-rich crystals which freeze are beta-crystals

of 52*5 per cent copper* Simultaneously with their formation, more aluminium-rich. crystals are able to form, with the result that at the eutectic temperature a mechanical mix­ ture of fine copper-rich crystals and fine aluminium-rich crystals freeze to form an aggregate known as eutectic

The structure is sketched in fig*2*ЗЗцС.

Unfortunatelly (usually) for the metallurgist and

the heat treater,solidification in castings is seldom slow

of casting occurs under non-equilibrium conditions, in which lack of complete diffusion in the solid state results in

coring* In both two examples,the outer surface <bf the dend­

rites would have had a considerably .higher copper content than the center, the dendrite would have been “gored*1* Por example, for the aluminium-copper alloy containing 10

per cent copper( the last case), the. composition o f Jbhe center of the dendrites can be as low as 1 or 2 per cent copper*

-improve casting ductility, especially in ingots which

are to be subsequently worked , or

-dissolve a maximum amount of certain desirable constit­ uents in the lattice structure iqo make the alloy amena-

ble'^Subsequent hardq^ng treatments#

An example of solution treatment for the latter purpose might be chosen from nearly any ane of the higher-

-strength aluminium, magnesium, or,copper-base alloys, typical of these is the aluminium -45 per cent copper

are much faster than at room temperature, the rate of solution and the elimination of concentration gradients will occur in a reasonable length of time (8 to l6hours)# When the alloy is completely solutiorfeed, it is quenched (oooled rapidly ) to room temperature to retain the homo­ geneous, single-phase structure#

In certain alloys macrosegregation can take place# Macrosegregation is a chemical irregularity of an alloy in different parts of a solid casting# It is usually known as body or major segregation#

The process of cooling and freezing a cast alloy in the mould begins from the outer surface of the cast­ ing an&then is continuing in the direction to the wall centeri