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Chapter 3: Problem 57

Explain why the properties of polycrystalline materials are most often isotropic.

Short Answer

Expert verified
Answer: The properties of polycrystalline materials are most often isotropic due to the random orientation, size, and shape of the grains, the presence of grain boundaries, and the averaging of individual grain properties over all possible orientations. This leads to a material that exhibits uniform properties in all directions on a macroscopic scale.

Step by step solution

01

Understanding Polycrystalline Materials

A polycrystalline material is composed of many individual crystals, known as grains or crystallites, which are randomly oriented with respect to each other. These grains are usually microscopic and vary in size and shape. The properties of polycrystalline materials are influenced by the crystal structure of the individual grains, the size and shape of the grains, and the orientation of these grains with respect to each other.
02

Explaining Isotropic Properties

An isotropic material is one that has the same properties in all directions. Due to the random orientation of grains within a polycrystalline material, the properties of the material tend to be averaged out over all possible grain orientations. This means that no one specific orientation of the grains will have a dominant effect on the overall properties of the material, resulting in a macroscopically isotropic system.
03

Grain Boundaries

Another important aspect of polycrystalline materials is the presence of grain boundaries, which are regions where two grains meet. These boundaries can have a significant impact on the mechanical, electrical, and thermal properties of the material. Grain boundaries often act as barriers to dislocation movement, impeding the transfer of stress between grains and reducing the overall strength of the material. However, since grain boundaries occur randomly throughout the material and their properties are averaged, their influence on the overall isotropic properties of the material becomes less significant.
04

The Role of Average and Randomization

The macroscopic isotropic properties of polycrystalline materials can be attributed to the averaging and randomization of the individual properties of the grains. Since the grains have various orientations, sizes, and shapes, it is challenging to attribute specific properties to any one grain or orientation. It is the collective effect of these grains averaged over all possible orientations that results in isotropic properties.
05

Conclusion

In summary, the properties of polycrystalline materials are most often isotropic due to the random orientation, size, and shape of the grains, the presence of grain boundaries, and the averaging of individual grain properties over all possible orientations. This leads to a material that exhibits uniform properties in all directions on a macroscopic scale.

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Most popular questions from this chapter

The metal iridium has an FCC crystal structure. If the angle of diffraction for the (220) set of planes occurs at \(69.22^{\circ}\) (first-order reflection) when monochromatic \(\mathrm{x}\)-radiation. having a wavelength of \(0.1542 \mathrm{~nm}\) is used, compute (a) the interplanar spacing for this set of planes and (b) the atomic radius for an, iridium atom.

Show for the body-centered cubic crystal structure that the unit cell edge length \(a\) and the atomic radius \(R\) are related through \(a=4 R / \sqrt{3}\)

Within a cubic unit cell, sketch the following directions: (a) \([110]\) (e) \(\left[\begin{array}{ll}1 & 1\end{array}\right]\) (b) \([\overline{1} \overline{2} 1]\) (f) \([\overline{1} 22]\) (c) \([0 \overline{1} 2]\) (g) \([123]\) (d) \([133]\) (h) \([103]\)

Rhodium has an atomic radius of \(0.1345 \mathrm{~nm}\) and a density of \(12.41 \mathrm{~g} / \mathrm{cm}^{3}\). Determine whether it has an FCC or BCC crystal structure.

(a) Derive the planar density expression for the HCP (0001) plane in terms of the atomic radius \(R\). (b) Compute the planar density value for this same plane for magnesium.
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