Varian cross-field amplifier brazing failure

Beverly MA company called Tom in to consult on phased-array radar component (Aegis missile system context). Crossed-field amplifier had molybdenum tips brazed to a copper body with Au-18Ni eutectic braze; the 18% nickel was forming Ni₃Mo intermetallic at the moly tip, occasionally breaking off and destroying the electron beam. Tom suggested TLP joint approach. Varian had also independently tried 37Au-3Ni-60Cu and found 180-degree bend ductility; Navy refused the substitution as "cost savings" until Tom relabeled it "transient liquid phase diffusion bonding" in a letter. The Navy accepted the new label.

There are other stories. Varian, a company up here in Beverly, called me in once as a consultant. The dynamics of this are interesting. They say, can you come up, we've got a brazing problem. I go up, this meeting starts about one o'clock in the afternoon, I go in this room, there are about 10 engineers and managers — this is not all that uncommon — and they start telling me about their problem. It's the crossed-field amplifier for the phased array radar system. In this particular case it was the Aegis missile system, but essentially it's the same type of radar system that allows jets to fly very low over the mountains — they have a phased array that tells them, oh, there's a mountain coming up, you better have something automatically to go over it rather than try to go through it.

Tom's mid-1980s consulting case at Varian (North Shore, MA). Gold-nickel braze on copper-to-molybdenum joints in radar crossfield amplifiers failed 5T bend tests because Mo–Ni intermetallics are brittle. Varian had stumbled empirically onto a transient liquid phase diffusion bond using 65Cu-35Au-3Ni; Tom diagnosed it after metallography. He had to write a letter to convince the Navy the cheaper alloy was actually better.

[Tom passes a tensile specimen around the class.] I was hired — this is probably twenty, twenty-five years ago — by Varian, up here on the North Shore. They make what's called a crossfield amplifier. Carl, you know what a crossfield amplifier is — you're an Air Force guy, you should know. When planes are flying very low over the ground, they have to have radar so the pilot doesn't run into a hill. It's a good idea when you're going Mach 1 at 500 feet over the ground — hills get to be a problem. A crossfield amplifier is a little electron beam that is surrounded by a bunch of copper fingers. You put a pulsed electron beam through there, and the field you put on the little copper fingers that run perpendicular to the beam will modulate the electron beam, and give you all kinds of radio waves. So it's basically a transistor for radio waves. The electron beam gives you your power, and the signal you want to put in — which is for phased array radar — will basically amplify, and the electron beam gives you a big signal.