A Ni-Fe-Cr alloy having high strength, ductility and corrosion resistance especially for use in deep-drilled, corrosive oil and gas well environments, as well as for marine environments. The alloy comprises in weight %: 35-55% Ni, 12-25% Cr, 0.5-5% Mo, up to 3% Cu, 2.1-4.5% Nb, 0.5-3% Ti, up to 0.7% Al, 0.005-0.04% C, balance Fe plus incidental impurities and deoxidizers. The alloy must also satisfy the ratio of (Nb-7.75 C)/(Al+Ti)=0.5-9 in order to obtain the desired high strength by the formation of gamma' and gamma'' phases. The alloy has a minimum of 1% by weight gamma'' phase dispersed in its matrix for strength purposes and a total weight percent of gamma'+gamma'' phases being between 10 and 30.