Strengthening by dispersing fine precipitates through solution treatment, quenching and aging, so that they obstruct dislocation motion. Used in alloys such as Al-Cu; aging too long causes softening by overaging.
Japanese name: 析出硬化
Precipitation hardening appears in 1 practice question across 1 topic, including Heat treatment and metallic materials.
Precipitation hardening requires a phase diagram in which the solute dissolves well at high temperature and its solubility falls sharply as temperature drops. In the Al–Cu system, the maximum solubility of Cu is about 5.65 wt% at 548 °C, but only a tiny amount dissolves at room temperature. The alloy is therefore solution-treated at about 500 °C to form a uniform solid solution and quenched to carry the supersaturated solid solution to room temperature. Aging at room temperature or at about 100–200 °C then disperses fine precipitates uniformly. Alloys whose solubility hardly changes with temperature cannot be treated this way.
GP zones and θ″ that form early in aging are small and coherent, with atomic rows continuous with the matrix, and dislocations pass by cutting through them. In this case, the larger the precipitates grow, the higher the stress needed to cut them. As the precipitates grow larger and their spacing widens, dislocations switch to the Orowan mechanism, bowing out between them without cutting, and strength falls in inverse proportion to the spacing. Strength peaks at the size where the two mechanisms cross over, which is the peak-aged condition.
The higher the aging temperature, the faster precipitation proceeds and the sooner peak hardness is reached, but the peak hardness is lower. Aging conditions are chosen to balance the required strength against corrosion resistance and toughness. In aluminum alloy temper designations, natural aging after solution treatment is T4, artificial aging close to peak is T6, and further overaging is T7. For 7000-series alloys, T7-type tempers deliberately overaged for better resistance to stress corrosion cracking, at some cost in strength, are used in aircraft.
Precipitation hardening is also used in maraging steels strengthened by Ni₃Ti and similar phases, precipitation-hardening stainless steels such as 17-4PH, nickel-based superalloys strengthened by the γ′ phase, and beryllium copper, the strongest copper alloy. On the other hand, when a precipitation-hardened alloy is used at high temperature or heated by welding, the precipitates coarsen and the alloy overages, losing strength. Parts are designed and materials selected with the softening of weld heat-affected zones and the upper limit of service temperature in mind.
Answer: A. Solution treatment → quench → aging
First, a solution treatment holds the alloy above the solvus to dissolve the Cu, and quenching carries the supersaturated solid solution down to room temperature. Holding it at room temperature (natural aging) or at about 100–200 °C (artificial aging) then forms fine precipitates that are coherent or semicoherent with the matrix, such as GP zones, θ″ and θ′, which obstruct dislocation motion and raise the strength. Aging for too long coarsens the stable θ phase (Al₂Cu) and lowers the strength, which is overaging. Slow cooling after solution treatment precipitates coarse equilibrium phases that do not help strengthening, so the quench is essential.
They are used in almost the same sense. Precipitation hardening is a name focused on the mechanism of strengthening by precipitates, and age hardening a name focused on hardening through the aging treatment.
Natural aging advances precipitation at room temperature, while artificial aging advances it by heating to about 100–200 °C. Artificial aging proceeds faster, and in many alloys it gives higher strength.
The precipitates coarsen and become fewer and farther apart, so dislocations can pass between them more easily. The change of coherent metastable phases into the stable θ phase and the like also reduces the strengthening effect.
Dislocation Hall–Petch relationship Hardenability Martensite Tempering