Titanium alloys are lightweight, strong, crack-resistant, and fatigue-resistant. This makes them useful in aerospace applications. Unfortunately Titanium is also expensive – around $3 a pound.
Steel is strong and cheap (30 cents a pound), but much heavier than titanium, making it unsuitable for jets. Aluminum is light and cheap (84 cents a pound) but weaker than steel or titanium.
Researchers at Pohang University of Science and Technology in South Korea have created an alloy out of iron, aluminum, carbon, manganese, and nickel that has "the strength and the lightness of titanium alloys but will, when produced at scale, cost a tenth as much."