A duplex eutectic silicon alloy including 30-70 weight % silicon, 10-45 weight % nitrogen, 1-40 weight % aluminum, and 1-40 weight % oxygen has a eutectic structure comprising a β′-sialon phase and an ο′-sialon phase. The alloy is produced by controlling cooling at a rate of 50° C. or less per minute in combustion synthesis. A ductile sintered product capable of replacing steel in various applications can be produced by placing a compact composed of a powder of the alloy in a sintering furnace which can supply a heat quantity at least ten times the heat capacity of the compact; and sintering the compact at a pressure at least as great as atmospheric pressure, within a nitrogen atmosphere in which the silicon gas mole fraction is 10% or more, and at a temperature within the range from 1400° C. to 1700° C.