Fig.1 Notice that “ductile” and “brittle” are used to describe the type of failure
In my last two articles, we explored the definitions of the words “brazing”, “passivation” and “defect”. Each of these words has also been mentioned and discussed in this column in years past, but I am bringing them out again to help a new generation of brazing personnel to understand them correctly. Another word that needs to be explored once again, because of its misuse by many brazing personnel today, is the word “brittle”.
Particularly when referring to parts that have been brazed with a nickel-based brazing filler metal (BFM), I still hear some people say that those nickel-brazed joints are “brittle” joints, and thus are probably not suitable for certain applications. Be careful! This is not true! Early in my metallurgical training (I am a graduate Metallurgical Engineer from Rensselaer Polytechnic Institute) I learned that “brittle” is not a word describing “a state of being”, but instead, is used to describe a mode of failure, as in the words: “…that joint failed in a brittle manner”.
Look at the photos in Fig. 1. Notice that “ductile” and “brittle” are both used as descriptive terms when looking at failure-modes.
Let me ask you this: When you picture the word “brittle” in YOUR mind, what do you see? I’ll venture to say that most of you will be picturing something that is shattering, or cracking, or breaking. Yes, your mental-picture accurately illustrates what that word is, and what it actually means. I dare say that when you think about the word “brittle” very few of you merely picture a solid piece of metal sitting on a table or bench. And yet, when you call a nickel-brazed joint “brittle” just because it was nickel-brazed, then, in my opinion (based on both my metallurgical training and on my many years of brazing experience), you are truly misusing that word. What you probably mean is that nickel-based brazing filler metals (BFMs) can produce joints that have high-hardness (on the Rockwell scale), and could therefore perhaps perform in a “non-ductile” manner, and which, under certain conditions of shock/stress, etc., might fail, i.e., crack, in a brittle manner. Yes, you would be correct in using those precise types of words. BUT —- as to nickel-brazed joints being, by nature, non-ductile — that is true ONLY under certain conditions — conditions in which the brazed joint had a wide-gap (large joint clearance), thus allowing the formation of a continuous centerline-eutectic down the center of the joint, perhaps resulting in a crack down the center of that joint when put under stress, such as that shown in Fig. 2. These continuous centerline-eutectics have been discussed in great detail in some of my earlier articles on this website, and I suggest you search out those articles by typing in “centerline eutectics” in the “Search” box at the top of this article.