Heat Treatment for Nonferrous Metals

The evolution of heat-treatment processes for aluminum and copper
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Diffusion Mechanisms (Part 2: Nonferrous Alloys)

By Daniel H. Herring

Diffusion mechanisms play a key role in the strengthening of many nonferrous alloys, and heat treatments influence the distribution of the key alloying elements that are responsible for the material’s property improvements.

Strengthening Mechanisms

Many nonferrous alloys (e.g., aluminum, titanium) in pure form have (relatively) low strength and cannot be used in applications where resistance to deformation and fracture are important. For example, 1199 aluminum in the “O” condition has a tensile strength of only 45 MPa (6.5 ksi). For structural use, the strongest alloy that meets minimum requirements for other properties (corrosion, ductility, toughness, etc.) is usually the most cost-effective. As such, composition is often selected based on strength requirements.

Strengthening mechanisms in aluminum (Fig 1) include:
  • Point defects (a) – vacancies (a), solute atoms (b), interstitial atoms (c)
  • Stacking faults
  • Grain boundaries (e)
  • Dislocations or line defects (d) – edge, screw
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Figure 1. Types of crystal lattice defects

Solute Atoms into Vacancies

The substitution of base-metal atoms by solute atoms results in lattice distortions and a local increase in crystal-lattice energy. For a successful strengthening alloy, additions must satisfy two criteria:

  • High room-temperature solid solubility
  • Atomic “misfit” to create local compressive or tensile strains

The size of the atom (Fig. 2) determines whether, in a given lattice vacancy, the strain energy is tensile or compressive. The atomic radius comparison between aluminum and common alloying elements can be used as a guide to “potency.”

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Figure 2. Relative size of solute atoms (compared with aluminum)

Diffusion processes in annealing and in other heat-treatment processes (such as precipitation hardening) are governed by temperature-time-related phenomena that have been modeled by the exponential Arrhenius equation (Equation 1), which is used to determine the temperature variation of the rates of diffusion, creation of crystal vacancies and other thermally activated processes.

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Equation 1: Arrhenius equation

Atoms involved in thermally activated processes have to overcome activation energy before they can move by diffusion, an important variable in the Arrhenius equation. A rule of thumb derived from the Arrhenius equation and used by many who study thermally activated processes is that the reaction rate doubles for every 10 degrees Kelvin increase in temperature, assuming that the reaction mechanism (activation energy) remains the same.

Summary

Diffusion mechanisms play a key role in the strengthening of many nonferrous alloys, and heat treatments influence the distribution of the key alloying elements that are responsible for the material’s property improvements.

Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance). He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.

References

1. Herring, Daniel H., Atmosphere Heat Treatment, Volume I, BNP Media, 2014.

 

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