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Chapter 7: Problem 35

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: A cold worked metal has elongated grains with a high dislocation density, leading to increased strength and hardness but reduced ductility. A cold worked and recrystallized metal has a more equiaxed, smaller, and uniform grain structure with lower dislocation density, offering a balance between strength, hardness, and ductility.

Step by step solution

01

Introduction

Cold working is a process where metals are deformed at room temperature, whereas recrystallization is a heat treatment process where the cold-worked metal is heated to a temperature high enough to allow new grains to form without causing melting. The grain structure of a material directly affects its mechanical properties such as strength, ductility, and toughness.
02

Grain Structure of Cold Worked Metal

When a metal is cold worked, the applied force causes the existing grains to deform, leading to an elongation of the grain structure in the direction of the applied force. This results in an increase in dislocations, which are defects in the crystal lattice of a material. The increased dislocation density causes the grains to strain harden, which increases the strength and hardness of the material while reducing its ductility. However, excessive dislocation density can lead to material failure because the metal becomes brittle and more prone to crack propagation.
03

Grain Structure of Cold Worked and Recrystallized Metal

When a cold worked metal is heated to an appropriate temperature, recrystallization occurs. New grains start to nucleate from the existing grains' boundaries, growing until they consume the entire cold-worked grain structure. The new grains are typically more equiaxed (having equal dimensions in all directions) and have lower dislocation density. This process helps to restore the metal's ductility without significantly sacrificing its strength and hardness. Recrystallized grain structure is also more uniform and has smaller grains compared to the cold-worked grain structure.
04

Summary

In conclusion, a cold worked metal has elongated grains with a high dislocation density, resulting in increased strength and hardness at the expense of reduced ductility. On the other hand, a cold worked and recrystallized metal has a more equiaxed, smaller, and uniform grain structure with lower dislocation density, providing a balance between strength, hardness, and ductility.

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Most popular questions from this chapter

Briefly explain why HCP metals are typically more brittle than FCC and BCC metals.

An undeformed specimen of some alloy has an average grain diameter of \(0.040 \mathrm{~mm}\). You are asked to reduce its average grain diameter to \(0.010 \mathrm{~mm}\). Is this possible? If so, explain the procedures you would use and name the processes involved. If it is not possible, explain why.

Briefly explain why small-angle grain boundaries are not as effective in interfering with the slip process as are high-angle grain boundaries.

For each of edge, screw, and mixed dislocations, cite the relationship between the direction of the applied shear stress and the direction of dislocation line motion.

The critical resolved shear stress for iron is 27 MPa (4000 psi). Determine the maximum possible yield strength for a single crystal of Fe pulled in tension.
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