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Chapter 7: Problem 39

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.

Short Answer

Expert verified
Answer: Yes, it is possible to reduce the average grain diameter of an undeformed alloy specimen from 0.040 mm to 0.010 mm. The procedures and processes involved include a combination of cold working and heat treatment (annealing). Cold working induces deformation in the material and heat treatment allows the nucleation and growth of smaller grains to replace the larger ones.

Step by step solution

01

Understanding grain size reduction

To determine if it is possible to reduce the average grain diameter of the alloy specimen, we need to understand the factors influencing grain size reduction. Grain size reduction can be achieved through various metallurgical processes, such as cold working, hot working, and heat treatment.
02

Cold working and hot working

Cold working and hot working are two commonly used methods for grain size reduction. Cold working is performed below the recrystallization temperature of the alloy, while hot working is performed above the recrystallization temperature. Both techniques involve processes like rolling, forging, and extrusion, which cause deformation in the material, thus reducing the grain size.
03

Heat treatment

Heat treatment processes, such as annealing, are also effective in reducing the grain size. Annealing involves heating the material to a specific temperature and holding it there for a certain period, followed by slow cooling. This allows new and smaller grains to nucleate and grow, which replace the previous larger grains.
04

Deciding the method to use

Considering the available methods for grain size reduction, we can combine cold working (to induce deformation) and annealing (to control the nucleation and growth of new grains). This combination of methods can effectively change the grain size of the alloy specimen.
05

Addressing the feasibility of the reduction

Reducing the average grain diameter from 0.040 mm to 0.010 mm (a 4-fold reduction) is achievable through a combination of cold working and heat treatment (annealing). This is because the mentioned processes can be controlled to achieve the desired grain size.
06

Conclusion

Reducing the average grain diameter of an undeformed alloy specimen from 0.040 mm to 0.010 mm is indeed possible. The procedures to achieve this grain size reduction involve a combination of cold working and heat treatment (annealing).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Metallurgical Processes
Metallurgical processes involve the study and techniques used to extract, refine, alloy, and fabricate metals. Grain size reduction is one such process, significantly impacting the mechanical properties of alloys. Typically, finer grains can enhance strength, ductility, and toughness as they provide more barriers to dislocation movement. The execution of this process requires precise control and understanding of multiple techniques such as cold working, hot working, and heat treatment—each of which manipulates the metal's internal structure in different ways.
Cold Working
Cold working refers to the process of deforming metals below their recrystallization temperature. Unlike hot working, it does not bring about significant atomic diffusion, but it induces defects like dislocations that obstruct the movement of other dislocations, thus strengthening the metal through strain hardening. This deformation typically increases hardness and decreases ductility. Common cold working processes include rolling, drawing, and extrusion. The process also leads to a reduction in grain size as the imposed stress can fragment grains into smaller domains.
Hot Working
On the other hand, hot working is carried out above the recrystallization temperature of the metal—where it becomes pliable and less resistant to deformation without strain hardening. During hot working, processes like rolling, forging, and extrusion are utilized to alter grain structure. The high temperature employed allows for recrystallization to occur concurrently with deformation, which can lead to grain refinement as well as the healing of existing defects.
Heat Treatment
Heat treatment encompasses various procedures that alter the microstructure of a metal to change its mechanical properties. Annealing, quenching, and tempering are examples of heat treatment processes. By heating the metal to specific temperatures and then controlling the cooling rate, new microstructures can be induced. In the context of grain size reduction, controlled heat treatment encourages the formation of small, uniform grains that can better resist deformation.
Annealing
Annealing is a heat treatment process that softens metal, allowing it to be more ductile and less brittle. The process includes three main stages: recovery, recrystallization, and grain growth. Recovery releases stored energy from dislocations, recrystallization nucleates new strain-free grains, and grain growth allows these new grains to consume the deformed grains, stabilizing the structure. By fine-tuning the annealing temperature and duration, the grain size can be effectively controlled, contributing to reduced grain diameter.
Material Deformation
Material deformation in metals can be plastic or elastic. Plastic deformation leads to permanent changes in the grain structure, while elastic deformation is reversible. The processes of cold and hot working involve plastic deformation, where force applied to the metal causes dislocation movements and changes in grain boundaries. The ability to control the amount and type of deformation is crucial in achieving the desired grain size reduction and thus improving the overall properties of the alloy.

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Most popular questions from this chapter

Consider a single crystal of silver oriented such that a tensile stress is applied along a \([001]\) direction. If slip occurs on a (111) plane and in a [ \(\overline{101}\) ] direction, and is initiated at an applied tensile stress of \(1.1 \mathrm{MPa}\) (160 psi), compute the critical resolved shear stress.

One slip system for the BCC crystal structure is \(\\{110\\}(111\rangle\). In a manner similar to Figure \(7.6 b\), sketch a \\{110\\}-type plane for the BCC structure, representing atom positions with circles. Now, using arrows, indicate two different \(\langle 111\rangle\) slip directions within this plane.

The lower yield point for an iron that has an average grain diameter of \(5 \times 10^{-2} \mathrm{~mm}\) is 135 MPa (19,500 psi). At a grain diameter of \(8 \times\) \(10^{-3} \mathrm{~mm}\), the yield point increases to \(260 \mathrm{MPa}\) \((37,500 \mathrm{psi})\). At what grain diameter will the lower yield point be \(205 \mathrm{MPa}(30,000 \mathrm{psi})\) ?

A single crystal of a metal that has the FCC crystal structure is oriented such that a tensile stress is applied parallel to the \([110]\) direction. If the critical resolved shear stress for this material is \(1.75 \mathrm{MPa}\), calculate the magnitude(s) of applied stress(es) necessary to cause slip to occur on the (111) plane in each of the [1\overline{110], [10\overline{1} ] } \text { and } [ 0 1 \overline { 1 } ] \text { directions. }

Consider a single crystal of some hypothetical metal that has the FCC crystal structure and is oriented such that a tensile stress is applied along a [102] direction. If slip occurs on a (111) plane and in a [101] direction, compute the stress at which the crystal yields if its critical resolved shear stress is \(3.42 \mathrm{MPa}\).
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