General metals and alloys possess a negative temperature coefficient of modulus of elasticity, that is to state their hardness decreases upon heating. They possess positive coefficient of thermal expansion, leading to increase in length upon heating.

The modulus of elasticity (E) of ferromagnetic materials is a complex function of physical characteristics. The initial alloys made for stable modulus were binary iron-nickel compositions. A zero temperature coefficient is received from alloys consisting of 27% to 44% nickel and remaining iron. Such alloys were found to be extremely sensitive to the nominal variations in their chemistry to meet a commercial purpose, for example, a change by 1% nickel shifts the coefficient of 44% alloy by 50 x 10(6) per oF.

Inclusion of chromium to such iron-nickel alloys decreases sensitivity to chemistry but the obtained alloys are still tough to make with the required properties and they need extensive cold reductions. Inclusion of titanium to iron-nickel-chromium mixture produces an alloy with a controlled thermoelastic coefficient, known as Ni Span C 902. The chemistry of this alloy is shown as following:

The required thermoelastic coefficient is then received by cold processing and suitable thermal processing. The cold treatment develops internal strains to provide more negative coefficient. The thermal work reduces strain in the low temperature limits. They also result in complex ordering mechanism that provides more positive coefficient. Heating up to 900oF results in the precipitation of an intermetallic compound of titanium and nickel, extracting nickel from the matrix and making coefficient more positive.

Thermoelastic coefficient

The thermoelastic coefficient or TEC of an alloy is the rate of variation of elastic modulus with variation in temperature. It is described as parts per million per oF. The unpreventable variations in the chemistry between heats of alloy cause nominal variation in elastic properties. These changes are slightly lower than overall accuracy of equipments in which Ni Span C902 is employed. For highly precise applications, the influence of change in chemistry can be adjusted by treatment to receive the required TEC.

Low Frequency Equipments

Low frequency devices often need a low TEC, small mechanical hysteresis and low drift. This is received through cold processing about 35% and heat work at 1100oF to 1200oF for five hours. The whole heats of alloy when receive this processing, obtain a suitable TEC for low frequency applications.

High Frequency Equipments

 Very high precision equipment needs that every material group to undergo pilot test to find the specific heat processing that is essential to develop the specific thermoelastic coefficient needed. Zero TEC is produced by 50% cold processing for 5 hours at 860oF. Increasing the heat processing temperature to 1100oF develops 12 ppm/oF of TEC. The treatment should be directed toward producing high strength, with assistant fatigue resistance and low mechanical hysteresis. The received TEC will be very nominal equivalent to zero.  The processing temperature should be above 600oF to receive stable properties.

Physical Properties

Thermal conductivity

Thermal conductivity for age hardened material at 1260oF for 6 hours are shown as following:

Thermal expansion

 The average coefficients of expansion of one melt of the alloy with different heat processing are shown in the following table:

It can be noticed that after heat processing, no considerable variation occurs in the expansion properties. A stable rate of expansion is maintained over the entire working limit. The temperature at which the expansion rate starts to increase quickly is called as inflection point. This kind of conduct is often shown by low expansion iron-nickel alloys.

Mechanical Properties

Room temperature tensile and hardness data are shown in the following figure:

Low temperature properties

Mechanical Hysteresis

After loading and unloading a spring, the load deflection curves do not coincide, however without exceeding the elastic limit. This variation from linear elastic behavior is known as mechanical hysteresis.

Magnetostrictive properties

 Ni Span C902 alloy has small magnetostrictive properties. Although these are sufficient for few applications like delay lines. Alloy C902 has low mechanical hysteresis about 0.02% received by cold and heat processing. The value of mechanical hysteresis is based on  the highest loading stress that increases with increase in loading stress.

Magnetic Properties

 The saturation magnetization at room temperature is about 5000 gauss. Permeability is influenced by cold and heat processing. In the entire ferromagnetic alloys, the modulus of elasticity is influenced by magnetization. With increase in magnetic intensity, modulus decreases.

Corrosion Resistance

Nickel Span C902 is non-stainless metal and it creates a red brown oxide layer upon exposing to environments. In the industrial and seawater conditions, the corrosion rates are lower than 0.001 inch per year.

Applications of Ni Span C902

The Ni Span C902 alloy is employed in the various precision equipments where elastic materials are used in fluctuating temperatures. The resonant vibrating equipments like electro-mechanical filters, tuning forks and vibrating reeds are general examples of devices that need constant frequency. An alloy with zero thermoelastic coefficients is suitable as a vibrating member or a nominally positive or negative TEC may be selected to compensate for thermal drift caused by other materials in the apparatus.

The alloy is also widely employed in springs. The constant TEC value offers independent deflection of temperature such as Bourdon tubes, aneroid capsules, and geophysical devices, hairsprings for timing equipments, diaphragms and springs for weighing equipments.