# Interstitial alloys > Small atoms in the spaces stop the layers slipping ## Objectives - Describe the crystal lattice of a pure metal and how its layers slip on hammering (fig 1-5 a, b). - Explain how small atoms in the spaces of the lattice make an interstitial alloy harder (fig 1-5 c). - List the physical properties that change in an interstitial alloy. - Explain why steel is harder than pure iron, and tell steel apart from cementite. ## 1. Alloys and their types ## 2. Soft iron, hard steel Predict: what does adding the small carbon atoms do to iron? - It makes iron softer - It makes iron harder ✓ - Nothing: there is too little carbon Adding carbon makes iron harder. The small carbon atoms sit in the spaces of the iron lattice and prevent the movement of its layers, so steel is harder than pure iron, which is relatively soft. [ar-EG] الـ carbon بيخلّي الـ iron أجمد. ذرات الـ carbon الصغيرة بتقعد في فراغات الـ lattice بتاع الـ iron وبتمنع حركة طبقاته، فالـ steel بيبقى أجمد من الـ pure iron اللي هو relatively soft. ## 3. Inside a pure metal Crystal lattice The regular arrangement of atoms in a solid. In a pure metal, the atoms are arranged in compact close rows. Pure iron, like other metals, is built this way. All its atoms are the same size. ## 4. Hammer the pure metal Recall the metallic bond: layers of atoms can slide over each other. Try it. Hammer (0 = no, 1 = hit) On hammering, a layer of metal atoms can slip over the other layers. The metal changes shape easily. ## 5. Small atoms in the spaces Interstitial alloy An alloy formed by introducing atoms of an element with a small atomic size into the spaces of the crystal lattice of a pure metal. The small atoms prevent the movement of the metal layers. So the metal becomes harder. When carbon is added to iron, carbon atoms replace iron atoms. ## 6. Pure iron or steel? Choose with or without carbon, then hammer. Compare what the hammer does to each metal. Carbon atoms added (0 = no, 1 = yes) Hammer (0 = no, 1 = hit) The layers slip: pure iron is soft. The layers are blocked: steel is harder than pure iron. ## 7. More than hardness The small atoms increase the hardness. They also change other physical properties of the metal: Interstitial alloy: the physical properties of the metal change (book p.19) malleability ductility melting point electric conductivity magnetic properties Book example: the alloy of iron and carbon, steel. ## 8. Same elements, another type Iron and carbon can also combine chemically into cementite, $\ce{Fe3C}$. That makes it a different type. Intermetallic alloy An alloy whose elements combine chemically into a compound. You will study it soon. ## 9. Check yourself Metal M is soft. A few atoms of element Z, much smaller than M atoms, are put into the spaces of its lattice. What happens? - Nothing changes: the rows are the same - M gets softer: Z atoms help the layers roll - Z atoms take the places of some M atoms - M gets harder: Z atoms block the layers from slipping ✓ M gets harder. Z atoms are much smaller than M atoms, so they enter the spaces of the lattice (an interstitial alloy) and block the layers from slipping on hammering. [ar-EG] الـ M بيبقى أجمد. ذرات الـ Z أصغر بكتير من ذرات الـ M، فبتدخل في فراغات الـ lattice (يعني interstitial alloy) وبتمنع الطبقات إنها تنزلق لما تتطرق. ## 10. Summary Interstitial alloy in one chain Pure metal: atoms in compact close rows. On hammering, a layer slips over the others. Interstitial alloy: atoms with a small atomic size enter the spaces of the lattice. They block the layers: the metal gets harder, and other physical properties change. Steel (Fe + C) is interstitial. Cementite, $\ce{Fe3C}$, is intermetallic. Give a reason: steel is harder than pure iron. Carbon atoms, with a small atomic size, enter the spaces of the iron lattice. They prevent the movement of its layers. Can you explain, with a drawing, why steel is harder than pure iron? Chapter: Transition Elements (https://yaani.ai/ar/learn/chemistry/chem-s3-en.ch1) Source: https://yaani.ai/ar/lesson/chem-s3-en.lesson-alloy.interstitial