ABSTRACT
Most metals and alloys have a negative temperature coefficient of modulus of elasticity; they lose stiffness when heated. Some ferromagnetic materials, however, exhibit markedly different behavior, which can be utilized to make constant-modulus alloys. The alloy can be joined by welding, brazing, or soldering if the surfaces to be joined have been thoroughly cleaned. Since the expansivity of the iron-nickel alloys can be changed by changing the amount of nickel, it is possible to make alloys that can match the expansion of many glasses—an important consideration in hermetic sealing. All iron-nickel alloys of the Invar class are magnetic at room temperature but lose their magnetism at higher temperatures, becoming paramagnetic. NI-SPAN-C alloy 902 is an age-hardenable alloy whose outstanding characteristic is a controllable thermoelastic coefficient (TEC). A formula has been developed for predicting the heat treatment required to produce the desired TEC for a specific lot of material.
