INFLUENCE OF THERMOMECHANICAL TREATMENT ON THE EXPANSION BEHAVIOR OF INVAR AND SUPERINVAR

The thermal expansion coefficients (CTE) of invar (Fe36Ni) and Superinvar (Fe33Ni4Co) are considerably decreased by plastic deformation. For superinvar even negative thermal expansion coefficients can be obtained. The deformation induced decrease of the CTE is explained by considering the stress dependence of the magnetovolume effect. By this contribution the thermal expansion becomes anisotropic. For annealing temperatures around 500°C CTE values larger than the equilibrium value are observed for superinvar but not for invar. This effect is proposed to be due to a partial decomposition of the original γ-phase into a Fe-rich bcc phase and a Ni-rich fcc phase. © 1989.