# Intermetallic alloys

> The elements combine chemically into a compound


## Objectives

- State that in an intermetallic alloy the elements combine chemically to form a compound.
- Show that a formula such as $\ce{Fe3C}$ disobeys the valence law.
- Recall the book's examples: duralumin (Al-Ni, Al-Cu), $\ce{Au2Pb}$ and cementite $\ce{Fe3C}$.
- Tell intermetallic alloys apart from interstitial and substitutional alloys.

## 1. Alloys and their types




## 2. Same elements, same type?

Predict: is cementite also an interstitial alloy?

- No: it is substitutional
- No: in cementite, Fe and C combine chemically ✓
- Yes: same elements, same type

Cementite is not an interstitial alloy: in $\ce{Fe3C}$, iron and carbon combine chemically into a compound with a fixed formula, so it is an intermetallic alloy. Steel, made of the same elements, is the interstitial one.

[ar-EG] الـ cementite مش interstitial alloy: في الـ $\ce{Fe3C}$ الـ iron والـ carbon بيتحدوا كيميائيًا ويكوّنوا compound ليه formula ثابتة، فهو intermetallic alloy. أما الـ steel، اللي من نفس العناصر، فهو الـ interstitial.
## 3. A compound, not a mixture

Intermetallic alloy An alloy in which the elements combine with each other chemically to form a chemical compound. An intermetallic alloy is a physical mixture. A fixed formula means a fixed ratio of atoms. In steel, the amount of carbon can vary.


## 4. Worked: test Fe₃C with valency

The book adds: its chemical formula disobeys the valence law. Test $\ce{Fe3C}$ with the swap method. No valency of iron gives $\ce{Fe3C}$. The formula disobeys the valence law. Note: the swap pairs a metal with a non-metal. Two metals, as in $\ce{Au2Pb}$, cannot be paired by valency at all.


## 5. Your turn: metal X

So what does $\ce{X3C}$ tell you?

- It obeys the valence law
- It is a physical mixture of X and C
- It disobeys the valence law, so X and C form an intermetallic compound ✓

Valency predicts $\ce{X2C}$, but the real formula is $\ce{X3C}$, so it disobeys the valence law. A real formula that valency cannot explain is the sign that X and C form an intermetallic compound.

[ar-EG] الـ valency بتتوقع $\ce{X2C}$، لكن الـ formula الحقيقية $\ce{X3C}$، يعني مش ماشية على الـ valence law. والـ formula الحقيقية اللي الـ valency مش قادرة تفسّرها علامة إن X و C بيكوّنوا intermetallic compound.
## 6. The book's examples

In these alloys, the metals are not in the same group of the periodic table. Careful: the group alone does not decide the type. The test is a chemical compound with a fixed formula.


## 7. Three types, one sort

Sort all the alloys you have met. Ask: spaces, places, or a compound? Which type is each alloy?


## 8. Check yourself

Metals P and Q, from different groups, form an alloy with the fixed formula $\ce{P3Q}$. Valency cannot explain this formula. What type is it?

- Interstitial
- Not an alloy: a physical mixture
- Substitutional
- Intermetallic ✓

It is an intermetallic alloy: P and Q are metals from different groups that combine chemically into a compound with a fixed formula that disobeys the valence law.

[ar-EG] ده intermetallic alloy: الـ P والـ Q metals من groups مختلفة، بيتحدوا كيميائيًا ويكوّنوا compound ليه formula ثابتة مش ماشية على الـ valence law.
## 9. Summary

Intermetallic alloy: the elements combine chemically to form a chemical compound. Its formula disobeys the valence law: no valency of iron gives $\ce{Fe3C}$. The metals are not in the same group of the periodic table. Book examples: duralumin (Al-Ni, Al-Cu), $\ce{Au2Pb}$, cementite $\ce{Fe3C}$. Book question (p.25): "Type of alloys in which the elements bind together chemically is ... alloy." Answer: intermetallic. Back to the hook: can you explain why steel and cementite are different types, though both are Fe + C? In steel, the amount of carbon can vary: it is an interstitial alloy. In cementite, Fe and C combine chemically to form a compound with the fixed formula $\ce{Fe3C}$: an intermetallic alloy.



Chapter: Transition Elements (https://yaani.ai/ar/learn/chemistry/chem-s3-en.ch1)
Source: https://yaani.ai/ar/lesson/chem-s3-en.lesson-alloy.intermetallic