Engineering Materials Chapter 4

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Questions and Answers

What is the simplest point defect found in crystalline solids?

Grain boundary

Self-interstitial

Vacancy (correct)

Dislocation

How are crystalline defects classified?

According to weight and atomic basis

According to geometry and dimensions (correct)

Based on their chemical composition

By their thermal properties

What is the effect of vacancies in crystalline solids?

They decrease the density of the material

They increase the order of the crystal

They increase the entropy of the crystal (correct)

They eliminate lattice distortion

Which defect involves 'extra' atoms positioned between atomic sites?

Self-interstitial Signup and view all the answers

What occurs during the mixing of water and alcohol on a molecular scale?

Creation of a liquid solution Signup and view all the answers

In terms of geometry, point defects in a material can include which of the following?

Point, line, or plane defects Signup and view all the answers

Which of the following statements is true about crystalline defects?

Most real materials have errors in perfection. Signup and view all the answers

What term is used to describe the phenomenon associated with the formation of solid from a liquid?

Crystallization Signup and view all the answers

What is required for charge neutrality in a compound containing Al3+ and Mg2+ ions?

Two Al3+ ions can fill every three Mg2+ vacant sites. Signup and view all the answers

Which defect involves both cation and anion vacancies paired together?

Schottky Defect Signup and view all the answers

In the context of dislocations, what does the Burgers vector, b, represent?

The magnitude of lattice distortion. Signup and view all the answers

Which statement correctly describes an edge dislocation?

It is characterized by an extra half-plane of atoms. Signup and view all the answers

What is a characteristic feature of a screw dislocation?

It results in a spiral stacking of crystal planes. Signup and view all the answers

What happens in the formation of a Frenkel defect?

A cation is displaced from its lattice site. Signup and view all the answers

In a nonstoichiometric compound like iron oxide, how are the cation sites occupied?

Both Fe2+ and Fe3+ ions occupy the sites with some vacancies. Signup and view all the answers

What defines a linear defect in a crystal structure?

A one-dimensional misalignment of atoms. Signup and view all the answers

Which metal has the highest atomic radius listed in the data?

Ag Signup and view all the answers

In the context of solid solutions, what is a key characteristic of interstitial solid solutions?

The smaller atom fits into the interstices of the host lattice. Signup and view all the answers

What is the formula for calculating atom percent from weight percent?

$c = \frac{n m_1}{n m_1 + n m_2} \times 100$ Signup and view all the answers

Which alloy is specified by the composition 60% Cu and 40% Ni by weight?

Cu60Ni40 Signup and view all the answers

The substitution solid solution involving NiO and MgO occurs among which types of ions?

Ni2+ and Mg2+ Signup and view all the answers

Which element has the lowest atomic radius from the provided data?

Cr Signup and view all the answers

What typically causes distortion in solid solutions when the sizes of constituent elements differ?

Difference in atomic size Signup and view all the answers

What is the molar mass used to calculate the moles of Cu if 600g of the alloy contains 60% Cu?

63.55 g/mol Signup and view all the answers

What is indicated by the larger grain size number in ASTM grain size classification?

Smaller grains are present. Signup and view all the answers

How is the number of grains per square inch at 100x magnification calculated?

N = 2n - 1 Signup and view all the answers

What does the term 'grain boundary' refer to in materials science?

Planar imperfections between crystalline regions. Signup and view all the answers

Which characteristic is true for non-crystalline oxides compared to crystalline oxides?

They maintain short-range order but lose long-range order. Signup and view all the answers

What is represented by the polyhedra in the Bernal model of an amorphous metal structure?

The connectivity between adjacent atoms. Signup and view all the answers

What is a significant feature of grain boundaries when viewed under optical microscopy?

They may be revealed as dark lines. Signup and view all the answers

What does the ASTM define in relation to grain size?

A system to assign numbers based on average grain sizes. Signup and view all the answers

In terms of grain size measurement, what is the significance of the magnification factor?

It affects how grains are counted and classified. Signup and view all the answers

What effect does etching have on grain boundaries?

It changes the way they reflect light. Signup and view all the answers

What characterizes a tilt boundary in crystalline structures?

Adjacent grains tilted by a few degrees. Signup and view all the answers

Which of the following observations can be made about polycrystalline specimens?

They can exhibit grains observable by the unaided eye. Signup and view all the answers

At what magnification is the grain-size number G typically defined?

At 100x magnification. Signup and view all the answers

What is an effect of polishing on the examination of materials?

It removes surface features like scratches. Signup and view all the answers

Which statement about optical microscopy is correct?

It can be used for examining grain boundaries at high magnifications. Signup and view all the answers

How does crystal orientation affect etching?

Different crystal planes have varying reflectance due to etching. Signup and view all the answers

What determines the distance between isolated edge dislocations in a tilt boundary?

The length of the Burgers vector and the tilt angle. Signup and view all the answers

Study Notes

Crystal Defects and Noncrystalline Structure

Crystalline materials usually assume perfect order; however, real materials often exhibit crystalline defects or lattice irregularities.
Defects in materials can range from atomic scale to the size of many lattice sites and are classified by geometry (point, line, plane) and dimensions.

Liquid Solutions and Alloys

Mixing of substances like water and alcohol occurs on a molecular scale; solutions can be defined based on weight or atomic basis.
Similar principles apply to metal mixtures known as alloys.

Crystallization and Solidification

Crystallization involves grain formation through nucleation.

Point Defects in Solid State

Point defects are primarily vacancies (empty atomic sites) and self-interstitials (extra atoms between sites).
Vacancies are crucial for increasing entropy and randomness in crystals, existing in all crystalline solids.

Composition Specifications

Composition is often specified in weight percent (wt.%) or atom percent (at.%).
Calculating atom percent involves the formula involving masses and number of moles of components.

Interstitial and Substitutional Solid Solutions

Interstitial solid solutions, like carbon in iron, can only occur with small atoms due to size mismatch.
Substitutional solid solutions, such as NiO in MgO, maintain anionic arrangements and require charge neutrality in ionic compounds.

Defects in Ceramic Structures

Frenkel defects occur when a cation is displaced from its site.
Schottky defects involve paired vacancies of cations and anions.

Linear Defects: Dislocations

Dislocations are linear defects characterized by misalignment of atoms.
Edge dislocation features an extra half-plane of atoms, while screw dislocation results from shear deformation with a spiral stacking of crystal planes.

Grain Boundaries and Optical Microscopy

Grain boundaries arise from adjacent crystalline grains tilted relative to each other.
Optical microscopy can visualize grains up to about 2000X magnification; surface features are removed through polishing and etching reveals crystal orientation.

ASTM Grain Size Measurement

The ASTM system assigns grain size numbers from 1 to 10, where higher numbers indicate smaller grains.
Grain size is calculated using the number of grains per unit area at a specified magnification.

Amorphous Materials

Non-crystalline oxides maintain short-range order but lack long-range ordering, leading to a glassy structure.
The Bernal model illustrates amorphous metal structures as irregular polyhedral arrangements without repetitive stacking.

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Description

Explore the nuances of crystalline defects and noncrystalline structures in engineering materials. This quiz delves into the imperfections that exist in most real materials, challenging the assumption of perfect order discussed in previous lectures. Test your understanding of these critical concepts in materials science.