Deformation Is Slip, and Slip Is Sliding
In the Mechanical Properties rung you learned that once a metal is pushed past its yield strength, it deforms plastically — it stays bent when you let go, unlike the elastic stretch that springs back the moment you release it. This guide answers the deeper question that rung left open: what is actually happening inside the metal at that instant? For almost every metal the answer is a single microscopic process called slip, and once you can see it, the whole business of strength suddenly makes sense.
Polish a soft metal mirror-flat, bend it gently, and look at the surface under a microscope: the smooth mirror has broken into a staircase of fine parallel steps, called slip lines. Each step is a place where one block of the crystal has slid bodily over the block beneath it — like a fanned deck of cards — along a particular crystal plane and in a particular direction. Plastic deformation is nothing more than the sum of millions of these little slides. Notice the crucial detail: the crystal is not stretched like taffy. Its atoms keep their neat lattice; they simply shift over by whole atomic spacings, and that is exactly why the new shape is permanent.
The Rug Trick: Why a Dislocation Makes Slip Easy
The naive picture of slip would be brutal: to slide the top half of a crystal over the bottom half by one atom spacing, you break every single bond across the slip plane at the same moment, shove the whole plane over, and let the bonds snap back into place. That would take a colossal stress — and, honestly, it is roughly what a flawless crystal really would need. Real metals slip at a tiny fraction of that stress, and the reason is the dislocation you met back in the Imperfections rung: a line defect, most simply pictured as an extra half-plane of atoms wedged part-way into the lattice.
Here is the analogy that makes it click. Think of shifting a heavy rug across a floor. Drag the whole rug at once and you fight the friction of every square metre of it — exhausting. Instead, kick a small ruck (a wrinkle) into one end and walk that ruck across to the other; at any instant only the little strip of rug under the wrinkle is moving, yet when the ruck runs off the far edge the whole rug has advanced. A gliding dislocation is precisely that ruck. As the dislocation line sweeps across the slip plane, only the one row of bonds at its core is being broken and remade at any moment — one row at a time, never the whole plane. When it exits the far side, the top of the crystal has slipped by exactly one Burgers vector, the fixed atomic step that measures the dislocation.
EDGE DISLOCATION = an extra half-plane of atoms (a "ruck" in the rug)
o o o | o o o the vertical | is the extra half-plane;
o o o | o o o its lower tip is the dislocation core
o o o o o o
- - - - - - <- slip plane
GLIDE: the core hops ONE bond at a time, left ---> right
o o | o o o o o o | o o o o o o | o top block has slid
o o | o o o -> o o o | o o -> o o o o | o over the bottom by
- - - - - - - - - - - - - - - - - - one atom spacing (b)
Only one row of bonds breaks and reforms at each step -- cheap.
Shoving the WHOLE plane across at once would need ~100x the stress.The extra-half-plane kind is an edge dislocation; there is also the screw dislocation, where the crystal is sheared into a spiral ramp instead, and in real metals the two blend into curved loops that expand outward. Either way the mechanism is the same bond-by-bond relay. A caterpillar walks by the identical trick: it never slides its whole underside at once, it passes a hump of contracted body from tail to head. This is the single most important idea in the mechanical behaviour of metals, and it is worth saying plainly — metals are soft and shapeable not despite their defects, but because of them.
Slip Systems: Plane Times Direction, and Why FCC Is Ductile
Slip is fussy about where it happens. A dislocation glides most easily on the crystal's most densely packed planes, and along the most densely packed directions within them — and that specific combination, one plane plus one direction, is called a slip system. The intuition is physical: a close-packed plane is the smoothest, so blocks slide over it with the least atomic bumping; and a close-packed direction is the shortest hop from atom to atom, which makes the Burgers vector — the step — as small as it can be, so each glide is cheap.
This is why the crystal structure a metal happens to have decides how easily it can be shaped. Face-centered-cubic metals — aluminium, copper, gold, nickel, austenitic stainless steel — carry the densest planes possible (the {111} family, sitting at the 0.74 atomic packing factor you computed earlier), with three close-packed directions in each, giving 4 x 3 = 12 easy slip systems spread evenly through the crystal. Because so many good slip systems point in so many directions, an FCC grain can always find several to accommodate whatever shape change is demanded of it. That is the microscopic reason FCC metals are famously ductile — you can roll aluminium into kitchen foil or draw copper into hair-thin wire without it tearing.
The other two common structures tell the honest flip side. Body-centered-cubic metals like iron have no truly close-packed plane (BCC packs to only 0.68), but several nearly-close-packed ones that add up to as many as 48 possible systems; slip works, yet needs a higher stress, so BCC metals tend to be stronger while still workable — and, to be honest, they can turn brittle when cold, the ductile-to-brittle transition you will meet in fracture. Hexagonal-close-packed metals — magnesium, zinc, titanium — do have a close-packed basal plane, but essentially only that one plane slips easily, leaving just about 3 handy systems. Three is too few to satisfy an arbitrary shape change, which is why HCP metals are the awkward ones: prone to cracking when cold-formed, and usually worked hot instead.
Resolved Shear Stress and Schmid's Law
Now for the subtlety promised earlier. Slip needs shear along the slip plane, yet in a tensile test you pull the bar straight — so how much of that straight pull actually lands as shear on a tilted slip plane? You resolve it. Only the component of the applied stress that lies in the slip plane and points along the slip direction can push the dislocation; that surviving component is the resolved shear stress. A plane lying square-on or edge-on to the pull feels almost none; a plane tilted between them feels the most.
The geometry gives Schmid's law: the resolved shear stress is tau_R = sigma times cos(phi) times cos(lambda), where sigma is the applied tensile stress, phi is the angle between the pull axis and the normal to the slip plane, and lambda is the angle between the pull axis and the slip direction. The product cos(phi) times cos(lambda) is the Schmid factor, and it peaks at 0.5 when both angles are 45 degrees — a slip plane tilted at 45 degrees to the pull catches the most shear. A grain begins to slip the instant its resolved shear stress reaches a fixed material threshold, the critical resolved shear stress (CRSS) — a genuine property of the metal, set low by nature, and, as the next guides show, exactly the number every strengthening trick is trying to push up.
A quick worked example makes the lever concrete. Say a single crystal has a CRSS of 5 MPa, and its best-oriented slip system sits at phi = lambda = 45 degrees, so its Schmid factor is cos(45) times cos(45) = 0.707 x 0.707 = 0.5. It starts to slip when sigma times 0.5 = 5 MPa, that is, at an applied stress of just 10 MPa. Now rotate the crystal so that same system is poorly oriented and its Schmid factor drops to 0.25; you now need 20 MPa to start it moving. Same metal, same CRSS, double the yield stress, purely from orientation. In a real polycrystal every grain points a different way, so the favourably oriented grains slip first while their stiff neighbours hold — which is part of why a real yield point is a gradual, slightly fuzzy transition rather than a sharp click.
The Puzzle This Opens, and the Lever It Hands Us
Look hard at what those CRSS numbers just implied. A perfect, dislocation-free crystal should need a shear stress of very roughly G/10 to slide whole planes past one another — for copper, whose shear modulus G is about 45 GPa, that is around 4 to 5 GPa. Yet annealed copper actually begins to slip below 1 MPa of resolved shear. Real metals are not a little weaker than the ideal; they are hundreds to thousands of times weaker, and the entire discrepancy is the dislocation doing its rug-ruck trick. (Honesty check: the exact factor depends on the metal and how you measure it, but even the most conservative comparison leaves real metals at least tens of times below theory.) That gaping gap is the puzzle guide 2 takes apart in full.
Once you accept that a metal is soft because its dislocations glide so freely, the whole strategy for making it stronger writes itself: put obstacles in their way. Every classical strengthening mechanism is just a different roadblock. Pack the crystal with more grain boundaries the dislocations cannot cross — a boundary being the mismatched seam where two crystal patches meet, like floor tiles laid at different angles. Dissolve in foreign atoms that snag them. Tangle them against each other by cold working until the dislocation density is so high they jam — the way kinking a paperclip back and forth makes the bend stiffen and finally resist. Or sprinkle in hard precipitate particles the dislocations must bow around. Guides 3 and 4 build out all four.