Cold-Worked Structure in Metals and Alloys: 3 Categories | Metallurgy

Cold-working produces structural changes in polycrystalline metals and alloys, which can be broadly classified in three categories: 1. Changes in Shape and Size of Grains 2. Changes in Orientation of Grains 3. Changes in Internal Structure of the Grains.

Category # 1. Changes in Shape and Size of Grains:

Commercial metals and alloys are polycrystalline in nature in which the axes of different crystals are randomly oriented. When plastic deformation is done of the polycrystalline material, the slip should start first in those grains in which the slip system is most favourably oriented with respect to the applied stress.

Each grain does not deform as a separate single crystal as illustrated in simple slip. Since, if this were so, the different grains would then deform in different directions with the result that voids would be created at the grains boundaries (this happens in creep). Each grain deforms in coherence with its neighbouring grains, and more slip systems become operative.

That is why, a polycrystalline metal has higher yield strength as compared to that of its single crystal. Grain boundaries act as barriers to the slip from one grain to the next grain. However, the fact that the continuity of the metal is maintained during plastic deformation, must mean that each grain is deformed into a shape that is dictated by the deformation of its neighbours.

The equiaxed grains, Fig. 7.2 (a), due to displacements along the slip planes on deformation, are elongated in the direction of acting force, Fig. 7.2 (b), i.e., grains are stretched in the direction of main tensile deformation stress-say, in the direction of rolling or wire drawing.

As the amount of plastic deformation increases, grains are elongated more and more, and with the change in the shape of the grains, the slip blocks may be broken into new smaller grains. Fig. 7.2 (c) illustrates two grains formed from one grain of Fig. 7.2 (b). The change in the shape and the size of the grains can be observed under microscope after etching.

Category # 2. Deformation Texture:

When a single crystal is plastically deformed, the slip planes tend to rotate parallel to the axis of the main stress. Even in polycrystalline metal, Fig. 7.2 (a, b, c), on deformation, the individual grains tend to rotate into a common orientation. Say, in tension, the grains rotate in a way that the applied stress axis is towards the operative slip direction (In compression, the applied stress towards the normal to the slip planes).

The appearance of the texture is due to the fact that slip takes place in grains in close-packed directions and normally on close-packed planes, and that the directions regularly rotate relative to the deformation axes of the object. Preferred orientation or texture is the state of severely cold worked metal in which certain crystallographic planes of the grains orient themselves in a preferred manner with respect to the direction of the stress.

In Fig. 7.2 (c), slip planes of different grains tend to become parallel to axis of stress. The preferred orientation is thus gradually developed as the amount of deformation increases, and becomes extensive above about 90 percent reduction in area. Preferred orientation is determined by x-ray methods. Preferred orientation is not normally effected by variables like die-angle, roll diameter, roll speed, etc.

The development of preferred orientation depends on at least following four factors:

a. The type of deformation such as rolling, or wire drawing, etc.

b. The amount of deformation: The texture is more pronounced with larger deformation ≈ 90%.

c. Temperature of deformation.

d. Nature of crystal lattice of metal (i.e., slip, or twinning systems which are available) and the stacking-fault energy.

The simplest deformation texture is obtained by wire drawing or by rolling of a wire. This is also called ‘fibre texture’ as it resembles the arrangement in natural fibrous materials. Table 7.1 gives textures for cold wire drawing and cold rolling of some common metals. In FCC metals (Cu, Al, Ni, Au, Ag, Pb), wire drawing preferably orients <111> cube diagonals along the axis of wire, or some have cube edges <100> along the wire axis.

This is called double-texture. Aluminium and other metals having high stacking-fault energy (where cross slip can take place) favour <111> texture, whereas silver and brass having low stacking- fault energy normally have <100> texture. In BCC metals (Fe, W, Mo), <110> cube-face-diagonals are oriented along the wire axis.

The above cases pertain to crystallographic fibering as it is produced by crystallographic reorientation of grains during the deformation. It is important to distinguish between crystallographic fibering and mechanical fibering—also called flow lines. The mechanical fibering is obtained by the alignment of inclusions, cavities and second phase inclusions in the main direction of mechanical working. Both types of fibering are responsible for producing directional mechanical properties of plastically deformed metals and alloys in shapes such as sheets and rods.

The rolling texture of a sheet is characterised by the crystallographic planes becoming parallel to the surface of the sheet (rolling plane), as well as the crystallographic directions in that plane becoming parallel to the direction of rolling.

In FCC metals and alloys, there are two predominant deformation textures:

(i) Brass-Type Texture:

{110} <112>, i.e., {110} planes are oriented parallel to the rolling plane and the <112> directions of the crystal lattice are parallel to the rolling direction, α-brass shows this texture. Cross-slip of dislocations does not take place.

(ii) Copper-Type Texture:

{112} <111> is shown by heavily cold rolled copper. This texture is preferred in FCC metals if cross-slip of dislocations takes place. Copper has high stacking fault-energy (and thus extended dislocations can constrict) to facilitate cross-slip of dislocations. Aluminium having high stacking-fault energy shows copper-type texture.

In FCC metals and alloys, the transition of copper-type texture to brass-type texture may take place either by lowering the temperature of deformation, or by adding solid solution forming elements which lower the stacking-fault energy, so that cross-slip does not take place. Aluminium has very high stacking-fault energy which cannot be easily reduced, and thus, does not show the transition as an exception.

Higher temperature of deformation promotes constriction of stacking-faults by thermal fluctuations to cause the cross-slip, and change to copper-type texture. At lower temperatures, twinning becomes more prominent, thus, cross-slip becomes less common, i.e., brass-type texture prevails.

In BCC metals no transition takes place by alloying, i.e., texture developed is insensitive to material variables. Here, {100} cube planes are parallel to the plane of rolling, and <110> face-diagonal is along the direction of rolling.

In HCP metals, material variables can effect the deformation texture. Variations in c/a ratio cause changes in deformation texture.

The preferred orientation cannot in general be eliminated even by recrystallisation. A new annealing is texture is obtained which is related to the deformation texture by standard lattice rotation.

Category # 3. Changes in Internal Structure of Crystals:

When a crystalline material is plastically deformed below about 0.3 T_m.p., where T_m.p. is the melting point of material in Kelvin, i.e., when cold-worked, then between 1 and 15% of the work of deformation gets absorbed in the material (rest is lost as heat), which amounts to a maximum of 4 KJ/kg.

The magnitude of the stored energy increases with the melting point, with increase of solute content, with increase of deformation up to a limiting value corresponding to saturation, with decrease of temperature of deformation, with decrease in grain size of metal, with increase in rate of deformation. It also depends on the type of deformation process, e.g., wire drawing, rolling, etc.

This stored energy in cold worked metal is in the form of energy of crystal defects, introduced during plastic deformation, which is given below:

(i) Plastic Deformation Increases the Concentration of Point Defects (Vacancies and Interstitialcy Atoms):

Point defects can be generated by the gliding jogged screw dislocations Fig. 4.74, or by annihilation of edge dislocations of opposite signs in neighbouring planes, Fig. 7.3 (a, b, c, d), or when edge dislocation climbs down, then vacancies are created which move away from dislocations, or due to other methods.

(ii) With the increase of cold-working, the number of stacking-faults increases and thus the density of extended dislocations increases.

(iii) With the plastic deformation, the number of kinks, jogs, dipoles, prismatic loops increase.

(iv) The most important change in the internal structure of deformed metal is the increases in the density of dislocations from 10⁶cm^-2 in annealed state to 10^9-10 to 10 cm^-2 or even more on heavy deformation to 10¹² cm^-2. Thin-film electron-microscopy has revealed some details of patterns of dislocation distribution with increasing deformation.

At low deformations, slip is essentially on primary slip planes and dislocations form coplanar arrays. The distribution of dislocations starts be­coming non-uniform with pile-ups formed in some portions. As deformation increases, cross-slip takes place, and multiplication sources operate resulting in the formation of spatial-dislocation-pile-ups.

This creates high-dislocation-density regions or tangles in which dislocations are intermingled irregularly. As deformation increases further, the number of tangles increases to develop into tangled networks, and the density of dislocations in them also increases. At a strain of around 10%, the cellular structure begins to form. Fig. 7.4 (a) in which dislocation tangles interconnect with one another to form boundaries of regions in which the density of dislocations is very low.

These regions are called cells. The cell size decreases with more strains at low plastic deformation but soon reaches almost a fixed size (0.3 – 3 μm). With more deformation, cellular structure becomes more distinct and more dislocations sweep across the cell areas to join the tangles in the cell walls, i.e., only a few dislocations remain inside, and the rest go to walls, Fig. 7.4 (b).

Cells try to become flat-walled and appear as sub-grains. The actual nature of cold worked structure will depend on a few factors. Basically, the cross-slip and climb of dislocations play important role in the development of cellular structure. The development of cellular structure is less prominent, when the deformation temperature is low, when strain rate of deformation is high, at low amount of deformations, and in metals and alloys with low stacking-fault energies.

Metals with low stacking-fault energy cannot easily form constriction to cause cross slip. Al, Ni, Cu, Au, Fe show cellular structure on deformation, whereas stainless steels, a-brass having low stacking-fault energy either do not show cellular structure or form cellular structure at high amount of deformation.