Cryogenics and Bat Performance  by Ron D. Noebe, Ph.D. and Mark McDowell, Ph.D.

In recent years it seems that the softball companies come up with some gimmick every season to try and set their products apart from the competition and get you to shell out additional bucks. In past years it has been the hype surrounding "new" alloys used to produce bats. These are really commercially available aerospace aluminum (Al) alloys but given "new" names to make them sound more exotic. This year the rage is "cryogenics". There are really two important issues with cryogenics. What is really happening to the structure of the bat during the cryogenic treatment and what effect this may have on properties and performance.

According to the manufacturer, the effect of the cryogenic treatment on the structure of a bat is depicted in the Figure 1 below, from the latest Softball Sales catalogue.

This figure, however, is misleading at best. In part (A) of Figure 1, the caption reads that before the cryogenic process the molecules in the bat are in a random arrangement, which is far from fact. A molecule is the smallest neutral particle of a substance capable of independent existence. In a metal that means the individual atoms, not groups of two or three atoms as shown in the picture. Furthermore, the atoms would not be randomly arranged. If they were you would have a gas, which is the closest state of matter that Figure (A) represents. In all metallic solids, the atoms already exist in some type of structured crystalline arrangement at any temperature below the metals melting point. Even above the melting point of the metal when it becomes liquid, the atoms would look more like the second picture, Figure 1B, than the first one.

In the case of aluminum, the atoms arrange themselves in what is known as an fcc (face centered cubic) structure. In other words, if you picture a cube, there would be an atom at each corner and in the center of each face of that cube. This structure would be repeated over and over until you had sufficient atoms to form the object you have in hand. Therefore, the microstructure of any Al bat would look more like Figure 1C above, no matter what temperature the bat is experiencing.

However, there are still some problems with the Figure 1C. This diagram portrays an ordered or alternating arrangement of apparently two different kinds of atoms, one large and one small. All Al atoms have the same weight and size, which is what defines it as an Al atom. Therefore this figure doesn’t adequately describe the structure of Al either, at least not the alloys that are used in softball bats. The three figures above are very misleading.

 That means we need to start from scratch and explain the structure of Al before we can see how cryogenics may effect this structure. First we need to make a distinction between an Al alloy and pure Al. Something made out of pure aluminum would consist of only Al atoms. Softball bats are made of Al alloys. Other elements are added to Al to improve various properties of the metal, such as strength for softball bats, making them Al alloys. A more accurate representation of an Al alloy is shown in Figure 2 below. The pink atoms represent Al and the blue represent an alloying addition, such as Zinc, (Zn). The two cubes of atoms in the middle of Figure 2 are expanded at the side of the figure so that it is easier to see the location of the various atoms. (The layers of atoms are not separated in real life but exist in a close packing arrangement as the cubes exemplify.) Part (A) represents a solid solution alloy and part (B) represents the formation of a precipitate. In part (A) the blue (Zn) atoms are more or less randomly dispersed in the aluminum matrix. This is known as a solid solution alloy. This is analogous to putting sugar in your coffee; the sugar dissolves in the coffee and is randomly in solution.

When you put too much sugar in your coffee, not all of it will go into solution. The excess precipitates out and will settle at the bottom of your coffee cup. The same thing can occur in metals. If you add too much alloying addition (Zn), it will precipitate out into Zn-rich areas known as precipitates. The atomic arrangement in Part B represents the formation of a Zn-rich precipitate in the Al alloy. Because of the small number of atoms in our model, there is only one precipitate shown. In your bat, there would be many precipitates as the cubic cell was repeated billions of times in order to represent all the atoms in your entire bat.

In terms of strength, a fine dispersion of precipitates is usually better than a solid solution alloy. That is why metals are given heat treatments--to optimize the size and spacing of the precipitates. Heat treatment can be used to manipulate the precipitate structure in a metal because the solubility of the various alloying additions increases with temperature. In other words more Zn can go into solution at high temperatures than at low temperatures. Therefore, to optimize the structure of an Al alloy you would heat it up to a very high temperature, force all the Zn into solution and then cool the Al alloy down. As you cool the aluminum matrix down, the solubility for the Zn decreases and Zn-rich precipitates are formed.

In reality, Al alloys used to make softball bats are more complicated than these simplified diagrams and usually contain Magnesium, (Mg) and Copper, (Cu) as well as Zn. (More details on the composition of various bats may be presented at a later time if anyone is interested.)

                                                  (A)

*   The first problem has to do with diffusion kinetics. It is true that as the temperature decreases, the solubility of the alloying additions decreases, and thus, a greater volume fraction of precipitates will form. But there are practical limits to this behavior. Below a certain temperature, the atoms will move around so slowly in the metal that they will not be able to form precipitates. Thermodynamically speaking, more precipitates should form by cooling the matrix to below room temperature. However, in reality the precipitates will not form because of the diffusion kinetics. The resulting microstructure is then known as supersaturated solid solution.

*  Furthermore, if some additional precipitates formed at the cryogenic temperatures, due to the decrease in solubility of the alloying addition with temperature, then when the sample was heated back to room temperature (where you would use the bat) those extra precipitates would again dissolve. They would go back into solution because you have now heated the sample back to a higher temperature where the solubility for the alloying addition is higher.

 Therefore, metallurgically speaking, the cryogenic treatment would have no effect on the microstructure of your bat and if there is no change in structure, there can be no changes (for better or worse) in the properties of the bat. The only effect in the long run is that you will be out $25.

Are you being fleeced with this treatment? Certainly. Is it strictly intentional or an honest mistake? Who knows. It all boils down to many softball players thinking or perceiving every new product as a wonder "DRUG" or quick fix for success. The only real quick fix is you working hard at gaining strength as well as flexibility. Practice, practice, and more practice are the only ways to achieve any long term success!!! Too many players still think that it is the bat doing the hitting and not the batter.