Plastic Deformation of Polycrystalline Metals

The direction of slip varies from one grain to another as a result of random crystallographic orientations. Variation in grain orientation is also clear from the difference in the alignment of the slip lines. During deformation, mechanical integrity and coherency are maintained along the grain boundaries. The grain boundaries do not come apart or open up. Slip lines

The manner in which the grains distort as a result of gross plastic deformation: Polycrystalline metals are stronger than their single-crystal equivalents, which means greater stresses are required for the slip to occur. This is mainly due to geometrical constraints imposed on the grains during deformation.

Mechanisms of Strengthening in Metals Macroscopic plastic deformation corresponds to the motion of large numbers of dislocations. Therefore strengthening of metals relies on this simple principle: Restricting or hindering dislocation motion renders a material harder and stronger. The strengthening mechanisms for a single phase metals are discussed here, which are by Grain size distribution Solid solution alloying Strain hardening

Strengthening by grain size reduction: Adjacent grains normally have different crystallographic orientations and a common grain boundary as shown below: During plastic deformation, slip or dislocation motion must take place across this common boundary, which acts as a barrier to dislocation due to two major reasons: Crystallographic misorientation of the grains Atomic disorder within a grain boundary resulting in discontinuity of slip planes.

If the grain boundary is a high angle boundary, it may also possible to observe stress concentration ahead of slip plane in one grain activating new dislocations. Fine grained material is harder and stronger simple due to greater total grain boundary area compared to coarse grained material. For many materials: Hall-Petch equation d=average grain diameter σ0 and ky are constants for a particular material. This equation is not valid for both very large and extremely small grain size materials. The toughness of the alloy also increases as grain size decreases.

Small angle grain boundaries are not very effective in interfering with the slip process because of the slight misalignment across the boundary. Twin boundary can block the slip effectively and increase the strength of the material. Boundaries between two phase systems are also effective in preventing the movement of dislocations and this is important for strengthening more complex alloys. Solid-Solution Strengthening: This is simply alloying the metals with impurity atoms, which is solid solution (interstitial or substitutional). High purity metals are always softer and weaker than alloys composed of the same base metal. This is because the impurity atoms that go into solid solution impose lattice strains on the surrounding host atoms. Lattice strain between dislocations and impurity atoms result and dislocation movement is restricted. This is illustrated as follows:

The impurity atoms tend to diffuse to and segregate around dislocations in a way so as to reduce the overall strain energy. small impurity atoms creating tensile strain. large impurity atoms creating compressive strain.

The resistance to slip is greater when impurity atoms are present because the overall lattice strain must increase if a dislocation takes place away from them. This requires a greater stress to be applied to initiate plastic deformation. This is evidenced by the enhancement of strength and hardness as shown below:

Strain hardening: It is a phenomenon whereby a ductile metal becomes harder and stronger as it is plastically deformed. It is also work hardening or cold working. Most metals strain harden at room temperature. Degree of plastic deformation is expressed as percent cold work: A0 is the original area of the cross section that experiences deformation Ad is the area after deformation. Initial yield strength is lower than the new yielding strength after plastic deformation. Therefore the meaterial is stronger as it is plastically deformed.

The influence of cold work on stress-strain behavior of a low C steel: The strain hardening is a result of increasing dislocation numbers due to plastic deformation. The dislocation density in a metal increases with cold work and the average distance between dislocations decreases. It is observed dislocation-dislocation interactions are repulsive and therefore the motion of a dislocation is hindered by the presence of other dislocations. Therefore, the imposed stress necessary to deform the material increases with cold work.

Recovery, Recrystallization and Grain Growth Plastic deformation of a polycrystalline metal at T lower than its absolute melting temperature may result in: change in grain shape strain hardening increase in dislocation density The properties and structures may revert back to the precold worked states by appropriate heat treatment (annealing treatment). This process takes place at elevated temperature and recovery, recrystallization and grain growth are the major processes. 1. Recovery: Some fraction of the energy expended in deformation is stored in the metals as strain energy. During recovery, some of this energy is relieved by dislocation motion which is the result of enhanced atomic diffusion at elevated temperature. There will be reduction in the number of dislocations and new dislocation configurations with low strain energies are produced.

2) Recrystallization: is the formation of new strain-free and equiaxed grains with low dislocation densities and they have characteristic of the precold-worked condition. The driving force for the formation of new grains is the difference in the internal energy of strained and unstrained one. Recrystallization of cold-worked material is used to refine the grain structure. Brass Initial stage of recrystallization after heating 3 s at 5800C. Cold worked grain structure small grains at the beginning of recrystallization

Following up stages of recrystallization: Complete recrystallization (8 s at 5800C) Grain growth after 15 min at 5800C and 10 min at 7000C.

During recyrstallization, the mechanical properties changed as a result of cold working are also restored to their precold worked values. constant heat treatment time is 1 hour

The temperature at which recrystallization just reaches completion in 1 h is called recrystallization temperature. Thus, the recrystallization temperature for the brass alloy is about 4500C. T of recrystallization=1/3-1/2 of the absolute melting temperature of the metal or alloy. Of course T of recrystallization also depends on the amount of prior cold work and purity of the alloy. Increasing the percentage of CW enhances the rate of recrystallization and decreases the T of recrystallization. The rate of crystallization approaches a constant or limiting value at high deformations. This value is reported in the literature as the T of recrystallization. Below the critical deformation there is no recrystallization.

Recrystallization proceeds more rapidly in pure metals than alloys Recrystallization proceeds more rapidly in pure metals than alloys. Alloying raises the T of recrystallization.

Grain growth: Following up recrystallization, strain free grains continue to grow at elevated temperature. As the grain size increases, total energy reduces and this is major drive for grain formation. Grain growth occurs by the migration of grain boundaries. Some of them grow, while the others shrink. Boundary motion is just a short range diffusion of atoms from one side to other.

Dependence of diameter to T: For many polycrystalline materials grain diameter (d) varies with time according to: diameter at t=0, K and n are constants. Dependence of diameter to T: This is because diffusion is faster at high T.