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Why can a cold-worked metal crack if it is deformed too much?

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Diagram of a zinc-lead blast furnace system showing ore and coke entering through charge hoppers, with labeled furnace, air preheater, blower, taphole for slag and lead bullion, and a connected lead-splash condenser using rotors to separate and cool lead and zinc, with pathways for molten metals and off-gas. — metallurgy A zinc-lead blast furnace and lead-splash condenser. This technology is utilized mainly for producing zinc while managing both lead and zinc from raw materials, a technique that was highly influential in 20th-century smelting.

When a metal undergoes cold working—a process of shaping metal below its recrystallization temperature—it becomes harder and stronger through a phenomenon known as work hardening.

This strengthening occurs because the deformation generates and moves line defects called dislocations within the metal's crystalline structure. As these dislocations move along slip planes, they intersect and form complex tangles that act as obstacles, inhibiting further slippage and increasing the metal's yield stress.12

However, there is a definite limit to the amount of strain a metal can withstand during cold working. If the metal is deformed too much, these internal obstacles become so dense that the material loses its ability to undergo further plastic deformation. Because the metal has become increasingly hard and brittle, it can no longer accommodate the applied stress through slip, leading to fracture or cracking. To prevent this, manufacturers may use annealing to recrystallize the metal, restoring its ductility and allowing for further forming operations.345