Directional solidification

3.1.5 Directional solidification

The exacting mechanical demands made upon gas tur- bine blades have led to the controlled exploitation of columnar crystal growth and the development of direc-

tional solidification (DS) techniques for superalloys. 2 Figure 3.6 A single-crystal blade embodying a DS-starter block and helical constriction and a test plate facilitating As the turbine rotor rotates, the hot blades are subject

orientation check by XRD to extremely large centrifugal forces and to thermal . excursions in temperature. Development has proceeded

in two stages. First, a wholly columnar grain struc- have provided a useful means for predicting the grain ture, without grain boundaries transverse to the major

morphology of DS alloys.

axis of the blade, has been produced during preci- In the second, logical stage of development, a single- sion investment casting by initiating solidification at

crystal (SC) turbine blade is produced by placing a

a watercooled copper chill plate in the mould base geometrical restriction between a chilled starter block and then slowly withdrawing the vertically-positioned

and the mould proper (Figure 3.6). This spiral selector mould from the hot zone of an enclosing furnace. Most

causes several changes in direction and ensures that of the heat is removed by the chill plate. A restricted

only one of the upward-growing columnar crystals in number of crystals is able to grow parallel to the major

the starter block survives. A typical production unit axis of the blade. Transverse grain boundaries, which

is illustrated in Figure 3.7. The orientation of every have been the initiation sites for intergranular creep

blade is checked by means of a computerized X-ray failures in equiaxed blade structures, are virtually elim-

diffraction procedure (e.g. the SCORPIO system at inated. Grain shape is mainly dependent upon (1) the

Rolls-Royce). In the fcc nickel-based superalloys, the thermal gradient G L extending into the melt from the

favoured [1 0 0] growth direction coincides with the melt/solid interface and (2) the growth rate R at which

major axis of the blade and fortunately offers the best this interface moves. Graphical plots of G L versus R

overall mechanical properties. For instance, the modu- lus of elasticity is low in the h1 0 0i directions; conse-

1 Like all separation processes, even so-called chemical quently, thermal stresses are reduced and the resistance separations, it is essentially a physical process.

to thermal fatigue enhanced. If full three-dimensional 2 Directional solidification of high-temperature alloys was

control of crystal orientation is required, a seed crystal pioneered by F. L. VerSnyder and R. W. Guard at the

is precisely located close to the chill plate and gives General Electric Company, Schenectady, USA, in the late

the desired epitaxial, or oriented, overgrowth. The pro- 1950s; by the late 1960s, DS blades were being used in gas

duction of single-crystal turbine blades by directional turbines of commercial aircraft.

solidification (DS) techniques has increased blade life

Structural phases: their formation and transitions 47 form during initial solidification, the sensitivity of the

growth rate to orientation usually results in one of the crystals swamping the others and eventually forming the entire growth front. The method is particularly use- ful for seeding crystals of a predetermined orientation.

A seed crystal is placed adjacent to the polycrystalline sample in the boat and the junction is melted before commencing the melting/solidification process. Wire specimens may be grown in silica or heat-resistant glass tubes internally coated with graphite (Aquadag). In a modern development of these methods, the sam- ple is enclosed in an evacuated silica tube and placed in a water-cooled copper boat; passage through a high- frequency heating coil produces a melt zone.

Most solidification techniques for single crystals are derived from the Bridgman and Czochralski methods. In the former, a pure metal sample is loaded in a vertical mould of smooth graphite, tapered to a point at the bottom end. The mould is lowered slowly down

Figure 3.7 Schematic diagram of directional solidification

a tubular furnace which produces a narrow melt zone. plant for turbine blades .

The crystal grows from the point of the mould. In the Czochralski method, often referred to as ‘crystal- pulling’, a seed crystal is withdrawn slowly from the

substantially and has enabled operating temperatures to

surface of a molten metal, enabling the melt to solidify

with the same orientation as the seed. Rotation of the phy of alloy design. Previously, certain elements, such

C, thus improving engine efficiency. It has also had a far-reaching effect upon the philoso-

be raised by 30

crystal as it is withdrawn produces a cylindrical crystal. as carbon, boron, hafnium and zirconium, were added

This technique is used for the preparation, in vacuo, in order to strengthen grain boundary surfaces. Unfor-

of Si and Ge crystals.

tunately, their presence lowers the incipient melting Crystals may also be prepared by a ‘floating-zone’ point. The DS route dispenses with the need for them

technique (e.g. metals of high melting point such as W, and permits beneficially higher temperatures to be used

Mo and Ta). A pure polycrystalline rod is gripped at during subsequent heat-treatment. The DS approach

the top and bottom in water-cooled grips and rotated requires to be carefully matched to alloy composition;

in an inert gas or vacuum. A small melt zone, pro- it is possible for it to lower the creep strength of certain

duced by either a water-cooled radio-frequency coil superalloys.

or electron bombardment from a circular filament, is passed up its length. High purity is possible because

3.1.6 Production of metallic single crystals

the specimen has no contact with any source of con-

for research

tamination and also because there is a zone-refining action (Section 3.2.4.4).

Development of highly-specialized industrial-scale Methods involving grain growth in the solid state (2) techniques, such as the directional solidification

depend upon the annealing of deformed samples. In (DS) of turbine blades and the production of

the strain-anneal technique, a fine-grained polycrys- silicon, germanium and various compounds for

talline metal is critically strained approximately 1–2% semiconductors, owes much to expertise gained over

many years in producing small single crystals for elongation in tension and then annealed in a moving- fundamental research. Several methods originally

gradient furnace with a peak temperature set below developed for metals have been adapted for ceramics.

the melting point or transformation temperature. Light Experiments with single crystals of metals (and

straining produces very few nuclei for crystallization; ceramics and polymers) have a special place in the

during annealing, one favoured nucleus grows more history of materials science. The two basic methods

rapidly than the other potential nuclei, which it con- for preparing single crystals involve (1) solidification

sumes. The method has been applied to metals and from the melt and (2) grain growth in the solid state.

alloys of high stacking-fault energy, e.g. Al, silicon- In the simplest version of the solidification method,

iron, (see Chapter 4). Single crystals of metals with the polycrystalline metal to be converted to a single

low stacking-fault energy, such as Au and Ag, are crystal is supported in a horizontal, non-reactive boat

difficult to grow because of the ease of formation (e.g. graphite) and made to to freeze progressively

of annealing twins which give multiple orientations. from one end by passing an electric furnace, with

Hexagonal metals are also difficult to prepare because its peak temperature set about 10 °

deformation twins formed during straining act as effec- ing point, over the boat. Although several nuclei may

C above the melt-

tive nucleation sites.

48 Modern Physical Metallurgy and Materials Engineering