MATS1192 Chap.4 Diffusion, Solid Solutions and Alloys
Diffusion, Solid Solutions and Alloys
Diffusion is random atomic motion that produces net redistribution toward equilibrium when a concentration difference exists. Individual atoms do not march directly from high concentration to low concentration. Instead, many thermally activated jumps create a statistical flux that smooths the composition profile.
Higher temperature raises the frequency of successful jumps, while additional time lets redistribution proceed farther.
Two common solid-state paths are vacancy diffusion and interstitial diffusion. A host or substitutional atom can exchange with a neighbouring vacancy. A sufficiently small atom can jump between interstitial sites.
Both require thermal energy to break and reform local bonds, and both depend on suitable structural opportunities. This connection makes defect knowledge essential to diffusion reasoning.
Alloy composition can be expressed by mass percent using component mass divided by total alloy mass. That overall composition must not be confused with phase composition when more than one phase is present.
Within a single solid solution, a substitutional solute occupies normal host sites, while an interstitial solute occupies gaps. Relative atomic size is important, but site prediction also depends on structure and chemistry.
Complete substitutional solid solubility is favoured when the elements have similar atomic sizes, the same preferred crystal structure, compatible electronegativity and compatible valence.
These are favourable conditions rather than a substitute for the phase diagram. Once the solubility limit is exceeded, excess solute forms another phase, perhaps a second solid solution or an intermetallic compound. Temperature can change the limit, so every solubility statement should include the conditions.
What this chapter covers
- 01
Random atomic jumps and net movement toward equilibrium
- 02
Temperature and time effects on redistribution
- 03
Vacancy and interstitial diffusion mechanisms
- 04
Mass percent as component mass over total mass
- 05
Substitutional and interstitial solid solutions
- 06
Factors favouring complete substitutional solubility
- 07
Solubility limits and formation of additional phases
Worked example · free
Calculate alloy mass percentages
- +1The total mass is 90 plus 6 plus 4 = 100 g.
- +1Titanium mass percent is 90 divided by 100 times 100 = 90 wt%.
- +1Aluminium is 6 divided by 100 times 100 = 6 wt%, and vanadium is 4 divided by 100 times 100 = 4 wt%.
- +1The check is 90 plus 6 plus 4 = 100 wt%, so all component masses were included once.
Key terms
- Diffusion
- Thermally activated atomic movement that produces net redistribution toward equilibrium.
- Vacancy mechanism
- Atomic movement by exchange with an adjacent vacant lattice site.
- Interstitial mechanism
- Movement of a small atom between available interstitial positions.
- Solid solution
- A single solid phase containing dissolved solute atoms.
- Solubility limit
- The maximum solute content retained in a particular phase under stated conditions.
- Mass percent
- Component mass divided by total mass and multiplied by one hundred.
Diffusion, Solid Solutions and Alloys FAQ
Do atoms move straight down a concentration gradient?
No. Individual thermal jumps are random. A concentration difference makes the aggregate statistics unequal, producing a net flux toward a more uniform composition even though single paths wander.
Why does temperature accelerate diffusion?
A higher temperature gives a larger proportion of atoms enough thermal energy to overcome the local barrier for a jump. Successful jumps therefore occur more frequently over the same time.
Is a small solute always interstitial?
No. A small radius makes an interstitial position more plausible, but the actual site also depends on available gaps, bonding, crystal structure and the resulting lattice strain.
What happens above a solubility limit?
The excess component cannot remain entirely within the original solid solution under those conditions. A second phase forms, and its composition can differ from both the first phase and the overall alloy.
Which quantities must stay separate in an alloy problem?
Overall alloy composition describes the whole sample. Phase composition describes the chemistry inside one phase, while phase fraction describes how much of that phase is present. They are not interchangeable.
Exam move
Link every diffusion statement to a jump mechanism, a source of available sites and a thermal condition. Sketch random atomic paths and explain why the population still develops a net direction. For alloys, perform mass-percent calculations until the denominator and sum check are automatic. Then classify a solute site from geometry without claiming unlimited solubility.
Use the phase diagram to settle how much solute can remain in one phase and what appears beyond the limit. Finish with the processing implication: time and temperature control how closely the material approaches equilibrium. Compare pairs of diffusion treatments by changing only temperature or time, then explain the difference through the number of successful atomic jumps.
Draw a vacancy before and after one exchange so the atom direction is not confused with the apparent vacancy direction. In composition drills, calculate total batch mass first, retain units through every numerator and make the percentages close to one hundred. Follow the arithmetic with two separate decisions: where a solute can sit and how much can remain in one phase.
The first uses atomic geometry; the second requires the temperature-dependent phase evidence. This separation prevents a small-radius observation from becoming an unsupported complete-solubility claim.
