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MICRO- AND MACROLIQUATION FORMATION DURING THE DIRECTIONAL CONTROLLED SOLIDIFICATION

OF SINGLE CRYSTALS

Sidorov E.V., Solomeina Yu.V. Vladimir State University, Vladimir, Russia

Directional controlled solidification is used to manufacture metallic single crystals. Currently,

two completely different principles of the metallic single crystals production are known: 1.The principle of single crystal growing of pure metals or of alloys without the crystallization

interval based on the theory of the diffusion (concentration) undercooling with the formation and preservation of the planar crystallization front.

2.The principle of single crystal growing of alloys with the crystallization interval through the transition two-phase region.

For investigations three alloy compositions were chosen which differ in crystallization intervals

and character. In fig.1 the phase diagram of the binary system A-B, the alloys compositions and their characteristics are presented, where tℓ is the liquidus temperature, te.s. is the equilibrium solidus temperature, tn.s. is the nonequlibrium solidus temperature, δte is the equilibrium crystallization interval, δtn is the nonequlibrium crystallization interval, ko is the equilibrium distribution coefficient, Mmelt is the alloy fraction which is crystallized owing to the diffusion disintegration, Minteract is the alloy fraction, which is crystallized owing to the diffusion interaction of the liquid with the crystals which were precipitated earlier, ie is the crystallization rate on the liquidus, is is the crystallization rate on the solidus.

Simultaneously, the distribution of the components of these alloys along the height of the grown crystal (macroliquation) and the dendrite cell (microliquation) at the completely nonequilibrum crystallization and during the solidification process through the planar front and the transition two-phase region was calculated.

In figures 2a, 3a, 4a the component distribution along the height of the grown crystal depending on the fraction of alloys 1, 2, 3, being solidified, correspondingly is shown. In figures 2b, 3b, 4b the transition two-phase region and the component distribution in the trunk-cell and in the liquid zone for alloys 1, 2, 3 accordingly are shown (200 magnification).

The results show that in case of the planar crystallization front an improved zone liquation (macroliquation) is formed for all three alloy compositions. This liquation cannot be eliminated by homogenation. If the solidification process proceeds through the transition two-phase region the zone liquation is absent, but the microliquation within the dendrite cell is formed. If we estimate the dendrite liquation with the coefficient kliq=kb /kc , where kb is the alloying component content on the dendrite cell boundary, kc is the alloying component content in the dendrite cell centre, then the maximum microliquation will be revealed in alloy 1 ( ;01,0≈А

liqк 45≈Вliqк ), and the minimum in alloy

3 ( ;03,0≈Аliqк 8,1≈В

liqк ), alloy 2 will have the liquation coefficients ;01,0≈Аliqк 7≈В

liqк . It is possible to eliminate the microliquation in the samples, which were prepared by the homogenizing annealing.

The homogenization process will be more intensive when the dendrite cell size is less and when the difference in concentration within the cell is less. Experiments were carried out with alloys of the antimony – bismuth system. Five compositions with different crystallization intervals and different crystallization character were chosen. These parameters are shown in the table.

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Fig.1 The phase diagram of the A-B system, compositions of alloys and their characteristics

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Fig.2 The distribution of alloy 1 components in the ingot after the solidification through the planar front (a) and in the dendrite cell

(microliquation) at the solidification through the transition two-phase region (b)

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Fig3 The distribution of the alloy 2 components in the ingot after the solidification through the planar front (a) and in the dendrite cell at the solidification through the

transition two-phase region (b)

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Fig4 The distribution of the alloy 3 components in the ingot after the solidification through the planar front (a) and in the dendrite cell at the solidification through the

transition two-phase region (b)

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te, °С tn, °С ко iliq, °С-1 isol, °С-1 Мmelt Мinteract Alloy 1 102 339 0,26-0,32 0,11 0,012 0,71 0,29 Alloy 2 100 129 3,15-4,16 0,007 0,015 0,31 0,69 Alloy 3 62 342 0,2-0,28 0,11 0,01 0,75 0,25 Alloy 4 61 72 4,1-5 0,008 0,03 0,28 0,72 Alloy 5 202 245 0,3; 2,5 0,0125 0,015 0,525 0,475

Alloys were melted in alundum crucibles in the resistance furnace. Then the samples were

cooled at different rates and the dendrite cell size and the dendrite liquation were determined. The results allowed to reveal some regularities of the dendrite structure formation depending on the alloys crystallization character and the cooling rate.

References 1. Tiller W.A., Jackson K.A, Rutter J.W., Chalmers B. The Rebusrition of Solute Atoms

During the Solidification of Metals. Acta Met. 1953. v1, N4, p.428-437. 2. Pikunov M.V., Sidorov E.V. On the manufacturing of cast ingots with single crystal

dendrite structure of alloys based on solid solutions. Izv. AC USSR. Metals. 1990. N3. 69-73. 3. Pikunov M.V., Sidorov E.V., Belyaev I.V. Manufacturing of cast ingots with columnar

and single crystal structure of alloys and growing of pure metal single crystals. Izv. Vuzov. Ferrous Metallurgy. 1998.N5. 60-63.