Chapter 4 Imperfections In Solids Flashcards
Why is a ‘perfect crystal’ considered thermodynamically impossible?
A perfect crystal is thermodynamically impossible because defects lower the energy of a crystal, making it more stable.
How do defects in a crystal structure contribute to its stability?
Defects lower the energy of a crystal, which makes it more stable.
What are the two primary steps involved in the solidification of a molten material, and how do they lead to the formation of a grain structure?
The two steps are the formation of nuclei of the solid phase and the growth of these nuclei into crystals until they meet at crystal boundaries, forming the grain structure.
In the context of materials science, why is the term ‘defect’ not necessarily negative?
A defect is not necessarily negative because it can be intentionally introduced to enhance material properties, such as adding carbon to iron to make steel.
Give three examples of how the intentional introduction of defects can enhance material properties.
The addition of C to Fe to make steel, the addition of Cu to Ni to make thermocouple wires, and the introduction of grain boundaries to strengthen materials.
What are the three main classifications of imperfections in solids, and what distinguishes them?
The three main classifications are point defects, line defects, and area defects. Point defects are zero-dimensional, line defects are one-dimensional, and area defects are two-dimensional.
Explain the difference between a vacancy and a self-interstitial point defect.
A vacancy is a missing atom from a lattice site, while a self-interstitial is an extra atom positioned between atomic sites in the crystal structure.
Why are self-interstitials less common than vacancies in crystal structures?
Self-interstitials are less common because they require a relatively large amount of energy to squeeze an atom into the small void between existing atomic sites.
How do vacancies increase the randomness or entropy of a crystal?
Vacancies increase the randomness because they introduce disorder in the otherwise periodic arrangement of atoms in the crystal.
How does temperature affect the equilibrium concentration of vacancies in a crystal, and why?
The equilibrium concentration of vacancies increases with temperature because higher temperatures provide more energy for atoms to move out of their lattice sites, creating vacancies.
What is the significance of Boltzmann’s constant in the context of vacancy concentration?
Boltzmann’s constant (k) is used in the formula that relates the number of vacancies to temperature and activation energy.
Explain the exponential relationship between defect concentration and temperature as described by the provided formula.
The formula N_v/N = exp(-Q_v/kT) shows that the ratio of the number of vacancies to the number of lattice sites increases exponentially with temperature (T), where Q_v is the activation energy for vacancy formation, and k is Boltzmann’s constant.
How can the activation energy for vacancy formation be determined experimentally?
The activation energy can be determined by measuring the vacancy concentration (N_v/N) at different temperatures, plotting ln(N_v/N) against 1/T and finding the slope of the line.
Given the formula for equilibrium vacancy concentration, what experimental data would be needed to find the activation energy?
Experimental data needed includes measurements of defect concentration and temperature at different points.
How does the number of potential defect sites relate to the number of atoms in a crystal?
Each lattice site in a crystal is a potential vacancy site, so the number of potential defect sites is directly related to the total number of atoms in the crystal.
What is the significance of alloying metals?
Alloying metals allows for the creation of materials with enhanced properties like increased strength and corrosion resistance.
Why are pure metals not often used in engineering applications?
Pure metals are often too soft, weak, or not corrosion-resistant for engineering applications. Alloying provides more desirable properties.
Explain the difference between a solvent and a solute in the context of solid solutions.
In a solid solution, the solvent is the major element (host), and the solute is the minor element (impurity).
How does a solid solution differ from a mixture of different materials?
In a solid solution, the crystal structure of the host element is preserved and a single new phase is formed. In a mixture, different materials maintain their own structures and phases.
What are the two main types of solid solutions, and how do they differ in terms of atomic arrangement?
The two main types are substitutional and interstitial. In substitutional solid solutions, impurity atoms replace host atoms, while in interstitial solid solutions, impurity atoms fit into voids between host atoms.
Describe the two possible outcomes when an impurity element (B) is added to a host element (A).
The two outcomes are a solid solution of B in A, or a solid solution of B in A with particles of a new phase.
Essentially, the initial solid solution is saturated therare a new lattice will be formed
What are the four W. Hume-Rothery rules that govern the degree to which a solute dissolves in a solvent?
The four rules are: 1) the difference in atomic radii should be less than 15%; 2) the elements should be close on the periodic table; 3) the pure metals should have the same crystal structure; 4) metals with higher valences have a greater tendency to dissolve metals with lower valences.
How does the difference in atomic radius affect the solubility of one metal in another?
A difference in atomic radius greater than 15% limits the solubility of one metal in another, due to lattice distortion.
What is the significance of electronegativity in determining the solubility of elements?
Similar electronegativities between solute and solvent favor solid solution formation. Large differences may result in intermetallic compounds.
