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The ªInvar effectº was discovered in the Fe-Ni system by Guillaume in 1896. [1] It is characterized by abnormally small and nearly constant coefficients of thermal expansion (CTEs) in a certain temperature range. The Invar effect, as far as it is understood today, is due to a compensation of thermal expansion by a large positive, spontaneous volume magnetostriction on heating temperature increases near the Curie point. In the Fe-Ni system, the effect occurs over a wide composition range (30-50 wt.-%Ni) with the microstructure playing a decisive role: the alloy must be single-phase fcc (c-phase), which is metastable at room temperature for Ni concentrations above 30 at.-%. [2,3] The CTE goes through a minimum at a concentration of 36 wt.-% Ni, whereas the Curie temperature rises continuously with increasing Ni content. [4] Besides Fe-Ni, several other systems containing Fe or Mn exhibit the Invar effect and are of practical interest. [5] Invar alloys are also of great interest in the form of thin films when dimensional stability at varying temperatures is required. In addition, they offer the benefit of reduced thermal stresses in the construction of micro-devices using incremental deposition methods. [6] Invar films have successfully been deposited in earlier studies. [7,8] However, the characterization of the films was restricted to the magnetic behavior and the determination of the CTE, while the mechanical behavior of thin Invar films, to the knowledge of the authors, has not been addressed in previous work. In this note, the results of first investigations into the mechanical stress-temperature behavior of thin Fe-36 wt.-% Ni films will be reported.The first and second stress-temperature cycles of the Fe-Ni film deposited at room temperature are shown in Figure 1. The room temperature stress after deposition amounts to more than 500 MPa, which is a high value but not unreasonable for a thin metallic film. Upon heating, the tensile stress decreases at first along a thermo-elastic line. At 120 C the stress departs from the linear behavior, giving rise to an undulation with a minimum at about 480 MPa and a subsequent maximum at about 520 MPa. Upon further heating (above the Curie temperature of 220 C) the stress decreases again along a thermo-elastic line with different slope until the maximum test temperature of 400 C is reached. During cooling the stress increases monotonically and reaches a room temperature value of nearly 600 MPa. It is remarkable that the difference to the room temperature stress at the beginning of the test reflects very nearly the stress increase during the undulation of the heating cycle. This and the fact that the second temperature cycle superimposes exactly on the cooling curve of the first cycle supports the conclusion that the stress undulation during the first heating is due to an irreversible mechanism such as grain growth.During the second (and subsequent) cycle(s), the stresstemperature curves coincide for heating and cooling; they are hence fully reversible and do not show any ...