1940
Titanium
William Justin Kroll (1889–1973)
Imagine that you are an engineer and you are commonly working with both aluminum and steel. Steel has an advantage because it is inexpensive and strong, but it is relatively heavy. Aluminum has an advantage because it is lighter than steel by about 50 percent for the same strength. But it melts at a much lower temperature and it suffers from fatigue faster.
As an engineer you would ask yourself, “Is there a metal that is light like aluminum and strong like steel and keeps its strength at higher temperatures?” The answer would be “titanium.” This is why the SR-71 aircraft is made mostly of titanium. There is no way the SR-71 could get off the ground if it were made of steel—it would be too heavy. There is no way the SR-71 could fly at Mach 3 if it were made of aluminum—the skin of the SR-71 gets too hot and an aluminum SR-71 would disintegrate. Titanium solves the problem. In addition, there are no rust, corrosion, or fatigue problems.
So what’s not to like about titanium? Why aren’t all airplanes made of this metal? The big thing is cost. In 1940, the Kroll process, developed by metallurgist William Justin Kroll, made titanium cheap enough to consider using it outside the lab, but no one has yet come up with a truly inexpensive process for producing titanium. What this means in round numbers is that if a pound of steel costs X, then a pound of aluminum costs 2X and a pound of titanium costs 20X.
The other problem with titanium is its workability. There is not really a way to make cast titanium parts. Machining titanium is slower and more problematic than steel. And welding titanium requires quite a bit of care to avoid faulty or contaminated welds. Sometimes you see titanium jewelry with pretty colors. The cause of those colors is oxygen contamination, which is bad structurally. Any kind of oil (including fingerprints) on the metal can also cause contamination.
Besides high-speed airplanes and rockets, engineers use titanium inside the human body because it is strong, light, and inert. If you have an artificial hip, it is probably titanium.
SEE ALSO Bessemer Process (1855), Hall-Héroult Process (1889), SR-71 (1962).
Pictured: X-ray of a hip prosthetic made of titanium.