Picking up where we left off, here’s where the tiny size of the interstitial atom comes into play! Because it is so small, it is only weakly “bonded” into the BFM atomic structure and is thus able to quickly diffuse through that atomic structure. This happens when the brazing temperature has increased to the point where the spacing between the big substitutional atoms is large enough for those small interstitial atoms to escape from their locations and move (diffuse) through the BFM’s atomic structure. They escape completely from the BFM into the base metals that are being joined together by that BFM.
Boron is an example of a small interstitial type of atom, which, when added into some nickel-based BFMs, will significantly lower the melting temperature (solidus) of those BFMs. Notice in Figure 2 that it only takes a little bit of boron (about 4% of the alloy’s composition) to form a low-temperature eutectic with nickel (Ni). Therefore, boron has the double advantage on not only being a highly effective “temperature-depressant” (temperature-lowering) ingredient, but it is also a tiny interstitial atom (which is thus only weakly bonded in the nickel-based BFM’s atomic structure).
Remember, the boron was added into the BFM as a “temperature depressant” to lower its melting point. Therefore, does it not seem logical that when the boron leaves (i.e., diffuses away from) the BFM, the melting point of the BFM should go back up? In fact, that does actually happen. And because boron is an interstitial type of atom, it can diffuse quite rapidly away from the BFM, thus significantly increasing its melting temperature. In fact, if you were to hold it at brazing temperature long enough for enough boron to diffuse away, there would no longer be enough boron left in the BFM chemistry to keep it molten (liquid) at that brazing temperature, and ITS will have occurred!
Therefore, three important things are needed for ITS to successfully occur during a brazing process: (1) a temperature-depressant ingredient that very significantly lowers the melting temperature of the alloy into which it is added; (2) the quantity needed of that ingredient to accomplish this temperature-lowering must be small; and (3) the atomic-size of that ingredient must be small (i.e., it must be an interstitial atom). Boron meets all three of those criteria when added to nickel, as shown in both Figures 1 and 2.
The nickel-based BFM classified as BNi-2 by the American Welding Society (AWS) in their specification, AWS A5.8, and by SAE International in their Aerospace Material Specification (AMS) as AMS 4777, is a good example of a BFM that can be isothermally solidified. This BFM alloy contains about 3.5-4.0% boron in its chemistry, and if this BFM is held long enough at brazing temperature, a sufficient amount of boron will diffuse out of the BFM to allow ITS to occur. This usually requires a minimum of about 30-45 minutes or longer at brazing temperature. The suggested brazing temperature is at least 100°F (50°C) above the published liquidus temperature for that specific BFM.
It’s very important to note that once ITS has occurred, the brazed joint will not be able to be re-melted unless heated to temperatures that are many hundreds of degrees (both F and C) above the initial brazing temperature, sometimes as high as the melting temperatures of the base metals being joined.
Isothermal solidification (ITS) can be a significant benefit to the brazing process if properly understood and implemented. As shown in this article, the three characteristics of the metallurgical addition to the BFM chemistry that are needed to bring about ITS are such that it is primarily limited to nickel-based BFMs containing about 3-4% boron in their chemistries. I have not personally seen ITS occur in any other type of BFM other than those boron-containing nickel-BFMs, but I’m always willing to learn. If any reader knows of any other type of BFM in which they have personally experienced ITS, please let me know.