Duplex stainless steel is named as such because it has 2 phases in its structure.  Just imagine a marbled cake and that is what you find with the duplex stainless steel. The first phase is the yellow austenitic and the second is the blue ferritic phase. When the steel is melted, it turns to ferritic and as the steel cools down, 50% of the ferritic grain turns into austenitic grains creating that marbled effect.

Duplex stainless steel is a good alternative because it is twice stronger than either ferritic or solid austenitic stainless steel products. It is also ductile and a lot tougher than ferritic although it fails to achieve the same values of the austenitic grades. Its corrosion resistance is almost identical to austenitic stainless steel but at the same time has an almost identical stress corrosion resistance of the ferritic stainless steel. This means it can handle well under elevated and humid temperatures as well as when exposed to chlorides.

Finally, the cost of duplex stainless steel is lower because it contains less nickel and molybdenum.