You mentioned that your brazing cycle was such that the molten BFM was held at brazing temperature for 10 minutes. Although you did not mention it in your note to me, I will assume that your actual brazing temperature is approximately 100°F (50°C) above the published liquidus temperature of the BNi-2 alloy. Since the melting range for BNi-2 goes from 1780-1830°F (970-1000°C), your actual brazing temperature is probably around 1930°F (1055°C).
Diffusion of the boron atoms away from the BNi-2 begins as soon as the metal is hot enough to open up the distance between the atoms in the BNi-2 sufficiently to allow the tiny boron atoms to escape. This begins fairly soon after the metal has started to warm up during heating in the furnace, and it has certainly occurred by the time the BFM has reached 1000°F (550°C) when being heated up toward its final brazing temperature. These “wide open spaces” continue until the BFM (and the base metal) have come back down again to well below the same temperatures, giving the boron atoms a huge amount of time to diffuse away from the BFM during that brazing cycle.
Consequently, even a fairly short holding time at brazing temperature will result in a significant increase in the remelt temperature of the BNi-2 brazed joint. That increase is (in my opinion and experience) at least 200°F (100°C) minimum over the actual SOLIDUS temperature of the BFM and probably a lot more (I’m trying to be a bit conservative here).
Thus, if you were to hold the BNi-2 at 1930°F (1055°C) for 10 minutes, that BFM should not begin to even think about remelting until it has reached at least 1980°F (1085°C) or higher. That’s a temperature that is higher than the brazing temperature that you used! And if you were to hold it there for even longer (e.g., 30 minutes), then the remelt temperature might be even higher, such as 2000-2100°F (1095-1150°C).
Obviously, you cannot diffuse ALL the boron out of the joint. It will only diffuse to the extent that the boron is evenly distributed through the joint and base metals on either side of the joint. As mentioned in a previous article, if it is held at brazing temperature long enough, the BFM might actually solidify (while being held at brazing temperature) since enough boron will have diffused away to allow this to happen.