strengthening mechanisms
If a metal is weak because dislocations glide easily, then to make it stronger you must make dislocations harder to move. That single idea is the key to this whole field. Every practical way of strengthening a metal is a way of putting obstacles in a dislocation's path, a bumpier road for the wrinkle you are trying to walk across the rug.
There are four classic mechanisms, and they differ only in the kind of obstacle. Grain-size reduction packs in more grain boundaries, walls where slip has to stop and restart. Solid-solution strengthening dissolves foreign atoms that strain the lattice and snag passing dislocations. Strain (work) hardening deliberately deforms the metal so its own dislocations multiply and tangle, jamming each other. Precipitation (age) hardening grows a fine dust of hard particles that dislocations must cut through or bow around. Often two or three are used together, so a real alloy is strengthened many ways at once.
The honest catch runs through all four: blocking dislocation motion raises strength but usually lowers ductility, because the same easy slip that made the metal weak also let it bend without breaking. Notably, one lever barely moves: Young's modulus (stiffness) is set by atomic bonding and hardly changes with any of these treatments. You can make steel five times stronger by heat treatment and cold work, yet its stiffness stays near 200 GPa. Strength and stiffness are different properties.
Modern high-strength steel combines all four mechanisms: fine grains, alloying atoms in solution, cold-worked dislocation tangles, and precipitated carbides; each adds to the yield strength, and the design job is getting strength without giving away too much toughness.
The four levers are additive in principle; real alloys pull several at once.
These mechanisms raise strength (yield and tensile) and hardness; they do not meaningfully change Young's modulus, which depends on bonding, not on obstacles to slip.