If equilibrium is to be maintained in the alloy, the two phases must continuously adjust their compositions by atomic migration.

Cored structures can be homogenized by annealing.

An ingot may be heated to a temperature just below the solidus temperature where diffusion is rapid.

The temperature must be selected with care because some regions might be rich enough in low melting point metal to cause localized fusion.

6. Join the halves of the sentences in the columns:

The two phases must	contain composition gradients between the core.
Each grain of the α-phase will	adjust their compositions by atomic migration.
Coring in chill-cast ingots is	broken up by deformation.
Dendritic structures are	quite extensive.

 

7. Answer the questions:

1. What is called the α-phase?

Is the diffusion relatively rapid in the liquid phase?

Is the cooling rate of the solid phase often too rapid under industrial conditions?

4. Will each grain of the α-phase contain composition gradients between the core?

Can cored structures be homogenized by annealing?

Why is it necessary to select the temperature of heating with care?

8. Read text B and choose the best title for it:

Dendritic coring.

Cellular microsegregation.

3. Constitutional undercooling.

Make sure that you know the following words and word combinations:

dilute alloy - разбавленный сплав

the slope of the curve for actual temperature - наклон кривой для фактической температуры

Text B

In the case of a solid solution, we have seen that it is possible for solvent atoms to tend to freeze before solute atoms, causing gradual solute enrichment of an alloy melt and, under non-equilibrium conditions, dendritic coring (e.g. Ni–Cu). When a very dilute alloy melt or impure metal freezes, it is possible for each crystal to develop a regular cell structure on a finer scale than coring. The thermal and compositional condition responsible for this cellular microsegregation is referred to as constitutional undercooling. Suppose that a melt containing a small amount of lower-m.p. solute is freezing. The liquid becomes increasingly enriched in rejected solute atoms, particularly close to the moving solid/liquid interface. The variation of liquid composition with distance from the interface is shown in Figure 3.14a. There is a corresponding variation with distance of the temperature TL at which the liquid will freeze, since solute atoms lower the freezing temperature. Consequently, for the positive gradient of melt temperature T shown in Figure 3.14b, there is a layer of liquid in which the actual temperature T is below the freezing temperature TL: this layer is constitutionally undercooled. Clearly, the depth of the undercooled zone, as measured from the point of intersection, will depend upon the slope of the curve for actual temperature, i.e. GL D dT/dx. As GL decreases, the degree of constitutional undercooling will increase.

Figure 3.14. Variation with distance from solid/liquid interface of (a) melt composition and (b) actual temperature T and freezing temperature TL.

 

 

Part IV

 

1. Learn the words:

three-phase equilibria	трехфазные равновесия
the eutectic reaction	эвтектическая реакция
fixed values	фиксированные значения
lead	свинец
tin	олово
straight	прямой
ternary	тройной
in accordance with	в соответствии с
equation	уравнение
approximately	примерно
ratio	соотношение
enriched	обогащенный
to remain	оставаться
to comprise	включать
to deduce	выводить, делать вывод
to consist of	состоять из
precipitate	осадок, оставлять осадок
to obtain	получать
to superimpose	накладывать, впечатывать
sign	знак, признак

 

2. Guess the meaning of the following words and word combinations:

Crystalline phases; to coexist; a collapsed three-phase triangle; become unstable; to be in equilibrium; to form a mechanical mixture; slow cooling; solvus lines.

3. Identify the part of speech and translate the words:

Success, successful, successfully; to enrich, enriched, enrichment; similar, similarly, similarity; to obtain, obtained, obtainable; to sign, signature; to modify, modifier, modification; to compose, composite, composition; to limit, limited, limitation; to discuss, discussion, discussed; to define, definite, definitely.

4. Match the words with their definitions:

the freezing of a melt	затвердевший гиперэвтектический сплав
a mechanical mixture of two solid phases	сплавы с низким содержанием свинца
a hypoeutectic alloy melt	сплавы с низким содержанием олова
primary grains	замораживать, полностью преобразуясь
a solidified hyper-eutectic alloy	первичная фаза
low-lead alloys	первичные зерна
low-tin alloys	расплав гипоэвтектического сплава
freeze by transforming completely	механическая смесь двух твердых фаз
a primary phase	замораживание расплава

 

Text A