An alloy is a metal made by mixing two or more metals together to make them stronger. How well that mixed metal holds up under extreme heat depends on which metals were combined. You place sheets of nickel, titanium, and steel in a high-temperature dry box oven, aging each sample at five temperatures between 200 and 1000 degrees Celsius for 24 hours per round. When each round ends, a Rockwell hardness tester reveals which alloy keeps its strength the longest.