5.5C - Why alloys are often stronger than pure metals
A particle model helps explain why alloying often makes a metal harder to deform. You will compare regular layers in a pure metal with the disturbed arrangement in an alloy, and recognise what this simplified model leaves out.
Why pure metals can change shape
A pure metal contains only one element. Its particles can be modelled as same-sized spheres in regular layers. When a force is applied, layers can slide past one another, allowing many pure metals to be bent, hammered or rolled.
Metallic bonding is strong electrostatic attraction between positive metal ions and delocalised electrons, which move throughout the metal. This attraction remains as layers slide, helping the metal stay together while its shape changes. Malleability does not mean the metal has weak bonds.
How added atoms hinder sliding
Alloy
An alloy is a mixture of a metal with one or more other elements.
Added atoms are often a different size from those in the pure metal. They distort the regular arrangement, making it harder for layers to slide. More force is then required for permanent deformation, so the alloy is often stronger and harder.
[DIAGRAM: asset_name: pure_metal_and_alloy_layer_model: Pure metal and alloy layer model - diagram 1; asset_slug: 124_5_5c_using_models_why_converting_pure_metals_into_alloys_often_increases_the_st_diagram1; file: 124_5_5c_using_models_why_converting_pure_metals_into_alloys_often_increases_the_st_diagram1_imagegen_v1.png; recommended_method: deterministic_drawn; description: Exact GCSE particle-model comparison in NovaLearn monochrome style. Left panel shows a pure metal as regular rows of same-sized atoms with an applied force arrow and a layer sliding arrow. Right panel shows an alloy as mixed different-sized atoms disrupting the rows, with an applied force arrow and blocked layer movement. Labels must state: same-sized atoms, regular layers, layers slide; different-sized atoms, distorted layers, layers do not slide easily. Deterministic drawing is required because atom sizes, layer arrangement and arrow meanings are assessed ideas.]
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