Diffusion Concepts and Applications

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Questions and Answers

What is interstitial diffusion primarily characterized by?

Solvent atoms diffusing at a slower rate

Small atoms fitting into interstitial positions (correct)

Atoms migrating to vacant positions

Equal movement of solute and solvent atoms

Why does interstitial diffusion occur more rapidly in metal alloys compared to vacancy diffusion?

Interstitial atoms are smaller and more mobile (correct)

Vacancy diffusion requires more energy

There are more vacancies than interstitial positions

Interstitial atoms are larger and less mobile

In the context of Fick’s 1st law, what does the term 'flux' refer to?

Time-dependent diffusion profiles

Rate of mass transfer per unit area (correct)

Concentration change over time

Total mass of diffused atoms

What statement accurately describes steady-state diffusion?

Concentration profile remains constant over time Signup and view all the answers

Which of the following mechanisms describes the exchange of solute and solvent atoms during inter-diffusion?

Atomic substitution Signup and view all the answers

Which type of diffusion is characterized by the movement of vacancies in one direction and solute atoms in the opposite direction?

Inter-diffusion Signup and view all the answers

What does the presence of more interstitial positions than vacancies in most metal alloys indicate?

Greater probability of atomic movement through interstitial diffusion Signup and view all the answers

What role do solute and solvent atom size play in diffusion processes?

Size differences affect the movement mechanism and rate of diffusion Signup and view all the answers

What is primarily observed when comparing the diffusion rates of two metals in a diffusion couple?

The lower melting point metal diffuses faster. Signup and view all the answers

What role does the inert marker play in the diffusion anneal process?

It is insoluble in both materials and indicates the diffusion front. Signup and view all the answers

Which of the following mechanisms is responsible for vacancy diffusion?

Atoms jumping into adjacent voids. Signup and view all the answers

In the example of the Kirkendall effect, which metal diffuses faster into the other?

Zinc diffuses faster into copper. Signup and view all the answers

What are the two dominant mechanisms for metallic diffusion?

Interstitial diffusion and vacancy diffusion. Signup and view all the answers

What condition must be met for an atom to diffuse from one lattice site to another?

An adjacent lattice site must be vacant. Signup and view all the answers

Which statement best describes the nature of the energy required for atomic diffusion?

It is vibrational in nature. Signup and view all the answers

What is a consequence of the vacancy diffusion mechanism at elevated temperatures?

Formation of pores in the material. Signup and view all the answers

What characterizes steady-state diffusion?

Constant flux throughout the medium Signup and view all the answers

According to Fick's first law, what is the relationship between flux and concentration gradient?

Flux is directly proportional to the concentration gradient Signup and view all the answers

What does the diffusion coefficient (D) depend on?

Temperature and the nature of the diffusing species Signup and view all the answers

In steady-state diffusion, how does the flux behave over time?

It is constant and independent of time Signup and view all the answers

What does the negative sign in Fick's first law indicate?

Diffusion occurs from high to low concentration Signup and view all the answers

What is a characteristic of non-steady-state diffusion?

There is a net accumulation or depletion of the diffusing species Signup and view all the answers

Which of the following scenarios is most likely an example of steady-state diffusion?

Salt dissolving at a constant rate in water Signup and view all the answers

Which statement accurately represents Fick’s second law?

It pertains to non-steady-state diffusion situations Signup and view all the answers

What is the primary driving force behind the process of diffusion?

Concentration gradients Signup and view all the answers

What happens to the diffusion rates of two metals when they have different rates of diffusion?

The boundary shifts towards the faster diffusing metal. Signup and view all the answers

What is the effect of diffusion in heat-treated materials?

It allows for improved hardness and resistance to fatigue. Signup and view all the answers

What does the term 'self-diffusion' refer to?

The process where atoms of the same type exchange positions. Signup and view all the answers

In a diffusion couple involving copper and nickel, what happens at elevated temperatures?

Copper and nickel atoms migrate into each other. Signup and view all the answers

How does the presence of carbon atoms affect the surface of steel during diffusion?

It improves the hardness of the steel surface. Signup and view all the answers

What key factor is necessary for a material to undergo diffusion?

The existence of a concentration gradient. Signup and view all the answers

What is a significant observation made in a diffusion experiment with copper and nickel?

Atoms of copper and nickel exchange positions with each other. Signup and view all the answers

Study Notes

Diffusion Definition

Movement of individual molecules, driven by random motion and concentration gradients.
Examples: Perfume spreading, ink dissolving, smoke dispersing.
Occurs from high to low concentration regions.

Importance of Studying Diffusion

Necessary for understanding material transport through atomic motion.
Essential in heat treatment for improving material properties.
Heat treatment temperature and time can be predicted using diffusion rate calculations.
Example: Case hardening steel involves diffusing carbon or nitrogen to the outer surface.

Diffusion Couples

Two different materials joined for observation of diffusion.
Example: Copper and Nickel diffusion couples, heated to a specific temperature.
Interdiffusion: Atoms migrate from high concentration regions in an alloy.
Self-diffusion: Atoms within a pure metal exchange positions.

Kirkendall Effect

Occurs when diffusion rates of two metals differ.
Boundary shifts towards the faster diffusing metal.
Inert marker: an insoluble material used to track the boundary shift.
Example: Zinc diffuses faster into Copper than vice versa, creating a Brass layer.

Diffusion Mechanisms

Atoms move from one lattice site to another.
Requires an empty adjacent site and sufficient energy to break bonds.
Two main mechanisms in metals:

Vacancy Diffusion

Atom moves to an adjacent vacancy.
Rate depends on vacancy concentration, which increases with temperature.
Both self-diffusion and interdiffusion can occur through this mechanism.
Example: Carbon diffusing in Iron (steel) where carbon atoms are smaller than Iron atoms.

Interstitial Diffusion

Smaller atoms move between interstitial spaces.
Occurs faster than vacancy diffusion because interstitial sites are more abundant.
Example: Hydrogen, carbon, nitrogen, and oxygen diffusing in metal lattices.

Fick's First Law: Steady-State Diffusion

Concentration profile doesn't change with time.
No net gain or loss of diffusing species in the material.
Flux is constant.

Fick's Second Law: Non-Steady-State Diffusion

Concentration gradient changes over time.
Flux and concentration gradient vary with time.

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Description

Explore the fundamental principles of diffusion, including its definition, importance in material science, and specific examples like diffusion couples and the Kirkendall Effect. This quiz will help you understand how diffusion impacts material transport and properties.