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Chapter 6: Problem 36

Explain the differences in grain structure for a metal that has been cold worked and one that has been cold worked and then recrystallized.

Short Answer

Expert verified
Answer: The main differences in grain structures between cold worked metals and cold worked-recrystallized metals are the grain boundary shape and dislocation density. In cold worked metals, the grain boundaries are more irregular, the grains are elongated, and the dislocation density is higher, resulting in increased strength and hardness but decreased ductility. In cold worked-recrystallized metals, the grain boundaries are more regular, new strain-free grains are formed, and the dislocation density is lower, leading to decreased strength and hardness but increased ductility.

Step by step solution

01

Define Cold Working

Cold working is a process in which the metal is deformed at a temperature below the metal's recrystallization temperature. This causes the grain structure to change due to the introduction of a large amount of dislocations in the metal structure, which eventually leads to an increase in the metal's strength and hardness properties.
02

Define Recrystallization

Recrystallization is a process that occurs when a cold worked metal is heated to a temperature that allows it to recover its original grain structure. This heating causes the dislocations to rearrange and form new, strain-free grains. As a result, the metal's properties are altered, typically reducing the hardness and strength but increasing the ductility.
03

Cold Worked Metal Grain Structure

When a metal goes through cold working, the grain structure undergoes a significant change. As the material is deformed, dislocations are introduced and the grains become elongated and distorted. The grain boundaries become more irregular, and the overall density of dislocations in the metal increases. This increased dislocation density strengthens the metal and increases its hardness, but also makes it less ductile.
04

Cold Worked and Recrystallized Metal Grain Structure

When a cold worked metal undergoes recrystallization, the grain structure experiences another change. As the metal is heated, new strain-free grains grow in the areas with high dislocation density, replacing the previous distorted grains. This new grain structure has fewer dislocations and more regular grain boundaries. As a result, the metal's hardness and strength are decreased, while its ductility increases.
05

Comparing the Grain Structures

The main differences in grain structures between a cold worked metal and a cold worked and recrystallized metal lie in the grain boundary shape and the dislocation density. The cold worked metal has irregular grain boundaries, elongated grains, and a higher dislocation density, which leads to increased strength and hardness but decreased ductility. On the other hand, the cold worked and recrystallized metal has more regular grain boundaries, new strain-free grains, and a lower dislocation density, which results in decreased strength and hardness but increased ductility.

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Most popular questions from this chapter

(a) What is the driving force for recrystallization? (b) What is the driving force for grain growth?

Briefly explain why HCP metals are typically more brittle than \(\mathrm{FCC}\) and \(\mathrm{BCC}\) metals.

Two previously undeformed cylindrical specimens of an alloy are to be strain hardened by reducing their cross-sectional areas (while maintaining their circular cross sections). For one specimen, the initial and deformed radii are 15 and \(12 \mathrm{~mm}\), respectively. The second specimen, with an initial radius of \(11 \mathrm{~mm}\), must have the same deformed hardness as the first specimen; compute the second specimen's radius after deformation

For a brass alloy, the stress at which plastic deformation begins is \(345 \mathrm{MPa}\) (50,000 psi), and the modulus of elasticity is \(103 \mathrm{GPa}\left(15.0 \times 10^{6}\right.\) psi). (a) What is the maximum load that can be applied to a specimen with a cross- sectional area of \(130 \mathrm{~mm}^{2}\left(0.2 \mathrm{in}{ }^{2}\right.\) ) without plastic deformation? (b) If the original specimen length is \(76 \mathrm{~mm}\) (3.0 in.), what is the maximum length to which it can be stretched without causing plastic deformation?

Experimentally, it has been observed for single crystals of a number of metals that the critical resolved shear stress \(\tau_{\mathrm{crs}}\) is a function of the dislocation density \(\rho_{D}\) as $$ \tau_{\text {crss }}=\tau_{0}+A \sqrt{\rho_{D}} $$ where \(\tau_{0}\) and \(A\) are constants. For copper, the critical resolved shear stress is \(0.69 \mathrm{MPa}\) (100 psi) at a dislocation density of \(10^{4} \mathrm{~mm}^{-2}\). If it is known that the value of \(\tau_{0}\) for copper is \(0.069 \mathrm{MPa}\) (10 psi), compute \(\tau_{\mathrm{crss}}\) at a dislocation density of \(10^{6} \mathrm{~mm}^{-2}\).
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