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Low thermal expansion alloys are often used in microwave packaging, particularly when hermeticity is required. For over 100 years engineers have been playing around with alloys to control their temperature expansion.
Low expansion alloys are often used in microwave housings and glass feedthroughs.
Invar
"Invar" usually refers to the alloy Invar 36. Invar is the original low-expansion iron alloy, and dates to 1896. The word Invar was derived from "invariant" by its inventor, Swiss Charles Édouard Guillaume. For his work he received the Nobel Prize in Physics in 1920. And he now appears in the Microwave Hall of Fame! The applications of low expansion alloys are many, particularly when you consider glass materials have naturally low thermal expansion, and glass and metal are often joined to create things that we need. Observatory telescope mirrors are mounted on Invar supports so that they don't distort with the ambient temperature. Read more about the history of Invar on Carpenter's web site.
Invar's composition is denoted Fe36Ni (36% nickel, balance is iron). Iron nickel alloys have a minimum thermal expansion at 36% nickel, thus the term "Invar 36" refers to this minimum.
Kovar
Kovar is the most popular iron alloy used in packaging. Kovar is very similar to Invar, but with a touch of cobalt: Fe29Ni17Co, with a trace of some other elements thrown in. Kovar is a trademark of Carpenter Technology Corporation, but the trademark has been diluted over the years so that you almost never see the little symbol next to it. Shame on us all for our lack of respect!