plastic deformation
Bend a metal spoon a little and it springs back; bend it far and it stays bent. That staying-bent, will-not-return change is plastic deformation. Unlike elastic stretching, where atoms just lean and snap back, plastic deformation permanently rearranges the atoms into new neighbours, so the shape change sticks even after you let go.
It begins once stress passes the yield strength, and it is what lets us shape metals at all — bending, rolling, forging, stamping, and drawing wire all rely on making metal flow plastically without cracking. Inside the metal it happens mostly by slip: layers of atoms slide over one another along close-packed planes, carried by the gliding of line defects called dislocations. Picture moving a big heavy rug not by dragging the whole thing at once but by pushing a small ruck across it — the rug ends up shifted with far less force. Dislocations do the same for atoms, which is why real metals yield 10 to 100 times more easily than a flaw-free crystal ever would.
As plastic deformation proceeds the metal usually gets stronger and harder but less ductile (work hardening), because the dislocations multiply and tangle, jamming each other. This is genuinely useful — cold-rolling stiffens a sheet — but it has a limit: keep deforming and the metal eventually exhausts its ductility and cracks. Note the everyday word 'plastic' the material is unrelated; here 'plastic' just means permanently mouldable.
A blacksmith hammering hot iron into a horseshoe, or a press stamping a car door from flat sheet, are both deliberately pushing metal past its yield point so it flows and keeps the new shape.
Plastic deformation is permanent shape change by dislocation slip — the basis of all metal forming.
'Stronger' almost always means 'harder for dislocations to move', and that usually costs ductility. A perfect crystal would be far stronger than any real metal — real metals are weak precisely because dislocations glide so easily.