Diffusion in solid metals is the process by which atoms move through a metal. It enables us to use heat treating to tailor the properties of metals to specific needs. The property changes that occur during heat treating are a result of microstructure changes that occur because of the motion of atoms through metals. Examples of microstructure changes that occur during heat treating are new grain formation and growth during annealing and strengthening of steel using the quench and temper heat treatment.
The microstructure changes that occur during heat treating depend on diffusion. Without it we would not have heat treating as an option for modifying metal properties.
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Diffusion in solid metals is the process by which atoms migrate within the metal's crystal structure, as they jump from one position to an adjacent position. As atoms diffuse through a metal they can rearrange themselves to form new grains, form new phases, or result in other microstructure changes.
The speed of atom motion depends on a few factors, one of which is metal temperature. As a metal's temperature increases, the speed of atom motion increases and the faster the microstructure changes occur. Many high-temperature alloys, i.e. high melting point temperature, must be heated to several hundred degrees Celsius for significant diffusion to occur to bring about microstructure changes that result in the desired properties.
Atoms in a solid metal constantly vibrate on their positions within a metal's crystal structure. In fact, they vibrate up/down, back/forth, and side-to-side about one trillion times per second. When the size (amplitude) of the vibration is large enough, an atom will jump from one position to an adjacent position.
The vibration amplitude depends on an atom's thermal energy. As a metal's temperature increases the atom's thermal energy increases and the vibration amplitude increases. So, the amount of atom jumping increases as the temperature of a metal increases.
Atoms move by a few mechanisms. The primary mechanisms are vacancy and interstitial diffusion. With vacancy diffusion, atoms jump from one position in the metal crystal to an adjacent position that is vacant. Interstitial diffusion applies to atoms that are small enough to fit in the spaces (interstices) between the host metal atoms. Atoms small enough to fit in the spaces between atoms include carbon, boron, hydrogen, and nitrogen. The diagrams below illustrate vacancy and interstitial diffusion.
Diffusion rate refers to the speed of atom motion through a metal. Factors that affect it include metal temperature, the host metal, and the type and size of the atoms diffusing.
Higher temperatures increase the diffusion rate because increased thermal energy provides atoms with the necessary energy to jump to new positions. The temperatures required for significant diffusion to occur depends on the host metal. Higher melting point temperature metals like iron (steel), nickel, and titanium must be heated to several hundred degrees Celsius for a significant diffusion rate. For lower melting point temperature metals like aluminum and zinc, significant diffusion rates occur at a few hundred degrees Celsius and lower.
The size and type of alloying element atoms in a metal influence the ease of movement. Smaller atoms and atoms with similar sizes to the host metal atoms generally diffuse more quickly.
Understanding the diffusion mechanism, the factors that influence diffusion rate, and the effects of diffusion rate on microstructure changes is important for developing and controlling heat treatments that result in the desired microstructure and properties. This knowledge is also helpful for having discussions with supplier about their heat treating processes and troubleshooting quality problems.
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Michael Pfeifer, Ph.D., P.E. is President of Industrial Metallurgists and a metallurgical engineering consultant with more than 30 years of experience in product design, manufacturing, failure analysis, and metallurgy training. He helps clients worldwide solve design and manufacturing problems and is the author of Materials Enabled Designs.
