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


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.”

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.

(1)    D = Doe-Q/RT

where:
     D = the measure of mobility of a diffusing atom with units of m2/second
     Do = the temperature independent constant of a given diffusing atom in a metallic matrix with units of m2/second
     Q = the activation energy for diffusion with units of eV/atom or J/mole
     R = the gas constant equal to 8.31 J/mole-K or 8.62 x 10-5 eV/atom-K
     T = the absolute temperature in K

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.
 

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