Age hardening process
The strength and hardness of some metal alloys may be enhanced by the formation of extremely small, uniformly dispersed particles of a second phase within the original phase matrix; this must be accomplished by phase transformations that are induced by appropriate heat treatments.
What is age hardening?
Age hardening, also known as precipitation hardening, is a heat treatment process used to increase the strength and hardness materials, particularly metals and alloys. This process increases the yield strength of the material, making it harder and more durable.
This process is particularly effective for materials that contain alloying elements capable of forming finely dispersed secondary phase particles.
Steps of age hardening process
The process involves three main steps: solution treatment, quenching, and aging.
1. Solution Treatment
The aim of this step is to create a homogeneous solid solution by dissolving solute atoms into the host matrix. This process involves heating the material to a high temperature, typically within the single-phase region (solution phase) of the phase diagram, where the alloying elements are completely soluble in the base metal. This temperature is high enough to dissolve any secondary phase particles into the primary matrix, forming a single-phase solid solution.
2. Quenching
The aim of this step is to retain the solid solution formed during solution treatment. In this process the material is rapidly cooled (quenched) from the solution treatment temperature to a lower temperature, typically room temperature, using a cooling medium such as water, oil, or air.
The rapid cooling prevents the solute atoms from precipitating out of the solution, trapping them in a supersaturated solid solution. This creates a non-equilibrium state that is essential for the subsequent aging process.
3. Aging
The aim of this step is to precipitate finely dispersed particles within the matrix, which impede dislocation motion and thereby strengthen the material. In this process the quenched material is reheated to a lower temperature than the solution treatment temperature. This process can be performed at room temperature (natural aging) or at an elevated temperature (artificial aging), depending on the specific material and desired properties.
The supersaturated solid solution decomposes, and solute atoms cluster together to form small precipitates. These precipitates hinder dislocation movement within the material, thereby increasing its hardness and strength.
Factors influencing age hardening
There are several factors affect on the age hardening process:
- Alloy Composition - Different alloying elements have varying solubility limits and precipitate formation tendencies, affecting the hardening potential.
- Temperature and Time - The aging temperature and time determine the size, distribution, and coherence of precipitates. Optimal aging conditions maximize strength.
- Quenching Rate - The rate of cooling during quenching influences the supersaturation of the solid solution and subsequent precipitate formation.
Age hardening applications
Age hardening is widely used in various industries, including aerospace industry, automotive industry, and manufacturing tools to enhance the mechanical properties of metals and alloys.
Aerospace Industry - Aluminum alloys (such as 7075 and 2024) are age-hardened to achieve high strength-to-weight ratios, essential for aircraft structures.
Automotive Industry - Age-hardened steels and aluminum alloys are used for structural components to improve safety and performance.
Manufacturing Tools - Precipitation-hardened stainless steels are used to manufacture cutting tools, molds, and dies, offering high hardness and wear resistance.
Mechanisms of strengthening in age hardening
The strengthening effect in age hardening arises from the interaction between dislocations and precipitates:
1. Coherency Strain Hardening
Small precipitates that are coherent with the matrix lattice cause local strains in the lattice, which impede dislocation motion.
2. Orowan Bypassing
As precipitates grow, they may become incoherent, meaning the lattice structures of the precipitate and matrix do not match. Dislocations bypass these larger, incoherent precipitates through a process known as the Orowan mechanism, which requires extra energy and increases the material's strength.
3. Cutting Mechanism
In some cases, very fine precipitates can be sheared by dislocations. This interaction between dislocations and precipitates increases the force required for further dislocation motion.
