Precipitation hardening

Lore & Background

Precipitation hardening relies on changes in solid solubility with temperature to produce fine particles of an impurity phase, which impede the movement of dislocations. The technique exploits supersaturation and involves balancing the driving force for precipitation and thermal activation energy. Classical nucleation theory (CNT) is the most commonly accepted theory behind precipitation. Nucleation occurs at a relatively high temperature (often just below the solubility limit) so that the kinetic barrier of surface energy can be more easily overcome, and particles are then allowed to grow at lower temperature in a process called ageing.

Reader's Guide

Precipitation strengthening is possible if the line of solid solubility slopes strongly toward the center of a phase diagram. While a large volume of precipitate particles is desirable, a small enough amount of the alloying element should be added so that it remains easily soluble at some reasonable annealing temperature. Elements used for precipitation strengthening in typical aluminium and titanium alloys make up about 10% of their composition, though this is not universally accurate as many age-hardenable alloys have significantly less or more than 10%. Two different heat treatments involving precipitates can alter strength: solution heat treating (forming a single-phase solid solution via quenching) and precipitation heat treating (adding impurity particles to increase strength).

Did You Know?

• Unlike ordinary tempering, alloys must be kept at elevated temperature for hours to allow precipitation to take place; this time delay is called 'aging'.
• Solution treatment and aging is sometimes abbreviated 'STA' in specifications and certificates for metals.
• Too little diffusion (under ageing) results in particles too small to impede dislocations; too much (over ageing) results in particles too large and dispersed to interact with most dislocations.
• Some aluminium alloys used for aircraft rivets are kept in dry ice from initial heat treatment until installation, then aged at room temperature to increase strength.