A thermally responsive damper for opening the flue of a gas-fired apparatus when gas firing occurs and for otherwise closing the flue to prevent unnecessary energy losses utilizes a low cost, high flexivity, thermostat metal adapted to operate in the corrosive flue environment, the thermostat metal having a high expansion layer of a corrosion-resistant austentic stainless steel which does not transform as it is worked during manufacture of the thermostat metal and a low expansion layer of an alloy of 36 percent nickel and the balance iron. The thermostat metal also has a thin layer of a stabilized, high chromium, ferritic stainless steel on the outer surface of the lower expansion layer for protecting the low expansion layer from corrosion in the flue gasses and for permitting the achievement of high flexivity in the thermostat metal while also permitting economical manufacture of the composite material.