Why is pure aluminum softer than most aluminum alloys?

Pure aluminum is relatively soft and weak because, in its high-purity state, it lacks the internal structural barriers necessary to impede the movement of dislocations—the line defects within a crystal lattice that allow for plastic deformation.123

In contrast, aluminum alloys are significantly harder and stronger because they incorporate alloying elements that create barriers to this movement. These barriers are distributed throughout the metal matrix through several methods:

• Solid solution hardening: Alloying atoms are dissolved into the aluminum matrix, where they occupy either regular atomic sites or interstitial spaces, disrupting the lattice and hindering dislocation motion.2
• Precipitation hardening: Through specific heat treatment processes, fine, solute-rich particles are formed within the metal. These particles act as obstacles that effectively block the slip planes where dislocations would otherwise travel.2
• Strain hardening: Mechanical processes like rolling or forging generate and tangle dislocations, which inhibits further slip and increases the metal's yield stress.2

By introducing these structural impediments, the inherent ductility of pure aluminum is modified to achieve the high strength required for industrial applications.12