The constituent common to duralumin and alnico is

Explanation

Constituents of duralumin are: Al, Cu, Mg, Mn.
Constituents of Alnico are: Al, Ni and Co

In 1909, the alloy of duralumin was discovered by Alfred Wilon consisting of 94% Al, 4% Cu, 1% Mg and 1% Mn(Manganese)
Alnico is an acronym referring to a family of iron alloys which in addition to iron are composed primarily of Al, Ni and Co.

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