Precipitation hardening, or age hardening, provides one of the most widely used mechanisms for the strengthening of metal alloys. The strongest aluminum alloys (2xxx, 6xxx and 7xxx) are produced by age hardening.
In order for an alloy system to be able to be precipitation-strengthened, there must be a terminal solid solution that has a decreasing solid solubility as the temperature decreases. The precipitation-hardening process involves three basic steps: solution treatment, quenching and aging.
Precipitation hardening, or age hardening, is a widely used mechanism for strengthening metal alloys. The strongest aluminum alloys (2xxx, 6xxx, and 7xxx) are produced through this process, which involves three basic steps: solution treatment, quenching, and aging.
The foundation for precipitation hardening was established by early work at the U.S. Bureau of Standards on Duralumin. Discovered accidentally by Wilm between 1903 and 1911, age hardening quickly became significant commercially under the trade name Duralumin.
The strength and hardness of some metal alloys, such as aluminum, copper-tin, certain steels, nickel-based super-alloys, and titanium alloys, can be enhanced by forming small, uniformly dispersed second-phase particles within the original phase matrix. These particles act as obstacles to dislocation movement, strengthening the alloys.
An alloy system must have a terminal solid solution with decreasing solid solubility as temperature decreases. The Al-Cu (Duralumin) phase diagram demonstrates this, where a large decrease in solid solubility occurs from 550°C to 75°C.
Figure 1: The aluminum-rich end of the Al-Cu phase diagram showing the three steps in the age-hardening heat treatment and the microstructures that are produced.
The process involves three basic steps:
Paul D. Merica and coworkers' research on Duralumin revealed significant findings:
Step | Description |
---|---|
Solution Treatment | Heating the alloy above the solvus temperature to form a homogeneous solid solution (α). |
Quenching | Rapid cooling to form a supersaturated solid solution (αSS). |
Aging | Heating the supersaturated α to form finely dispersed precipitates that impede dislocation movement. |
The strongest aluminum alloys (2xxx, 6xxx, and 7xxx) are produced by age hardening. Fine dispersion of precipitates is formed through appropriate heat treatment.
The general model for decomposition and precipitation sequences in aluminum alloys is:
a0 (SSSS) → GP zones → θ'' → →θ' → θ or, more fully:
a0 (SSSS) → α1 + GP zones → α2 + θ'' → α3 + θ' → α4 + θ
The three main mechanisms of strengthening through age hardening are:
Precipitation reactions in Al-Cu alloys are complex:
A comprehensive description of the simultaneous operation of multiple precipitation phenomena is still lacking. Understanding the competitive precipitation of several phases on various nucleation sites and predicting fracture toughness in complex situations remains a challenge.
Precipitation hardening is a crucial process for strengthening aluminum alloys, involving solution treatment, quenching, and aging. The mechanisms and effects of this process have been extensively studied and applied to develop high-strength aluminum alloys used in various industries. However, challenges remain in fully understanding and modeling the complex interactions in precipitation hardening.
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