Page "Alloy" Paragraph 15

Conversely, most heat-treatable alloys are precipitation hardening alloys, which produce the opposite effects that steel does.

When heated to form a solution and then cooled quickly, these alloys become much softer than normal, during the diffusionless transformation, and then harden as they age.

The solutes in these alloys will precipitate over time, forming intermetallic phases, which are difficult to discern from the base metal.

Unlike steel, in which the solid solution separates to form different crystal phases, precipitation hardening alloys separate to form different phases within the same crystal.

These intermetallic alloys appear homogeneous in crystal structure, but tend to behave heterogeneous, becoming hard and somewhat brittle.