Steel is useful because it is strong and cheap, but it is also heavy, so the obvious solution is to alloy steel with a lighter metal:
And the obvious one to choose is aluminium, which is, like iron (steel’s principal component), cheap and abundant. An alloy of iron, aluminium and carbon (steel’s other essential ingredient) is too brittle to be useful. Adding manganese helps a bit, but not enough for aluminium-steel to be used in vehicles.
Dr Kim and his colleagues have, however, found that a fifth ingredient, nickel, overcomes this problem. To a chemist, an alloy is a mixture of materials rather than a true chemical compound. But metals do sometimes react to form real compounds, and one class of these, known as B2 intermetallic compounds (which have equal numbers of atoms of two different metals within them), lies at the heart of Dr Kim’s invention. The nickel reacts with some of the aluminium to create B2 crystals a few nanometres across. These crystals form both between and within the steel’s grains when it is annealed (a form of heat treatment).
B2 crystals are resistant to shearing, so when a force is applied to the new material they do not break. This stops tiny cracks propagating through the stuff, which gives it strength. That strength, allied with the lightness brought by the aluminium, is what Dr Kim was after.
So, no copper, and no greenish-blue hue.
Coat it with titanium and anodize the titanium if you want nifty colours.
Besides high strength and low cost, one of the great advantages of steel is its ductility. Steel generally fails plastically by bending or stretching rather than fracturing. This is a nice feature in large buildings, because incipient failures given warnings.
Aluminum and titanium are expensive, and they only replace steel in uses where weight is a primary consideration, as in airplanes.