Mechanical properties of a new Fe-Ni-based Invar alloy doped with Ti, V and C

The Fe-36 wt.% Ni binary Invar alloy has low mechanical strength, with an ultimate tensile strength (σUTS) of less than 500 MPa. This greatly limits its practical applications. An increase in mechanical properties was achieved by the development of a new Invar alloy, Fe-32 wt.% Ni-2.7 wt.% (Ti, V, C). Alloying the binary alloy with carbon, titanium and vanadium, followed by deformation and heat treatment, allows increasing the ultimate tensile strength to σUTS =1185 ± 3 MPa, which is more than twice that of classical Invar. At the same time, it retains acceptable ductility (δ = 4.6 ± 0.3 %) and a low coefficient of thermal expansion at room temperature (about 1.2 ×10−6 K−1). The high strength of the new Invar alloy can be explained by the fact that, as a result of alloying followed by deformation and heat treatment, several hardening mechanisms are implemented, namely, dispersion, solid-solution, deformation and grain-boundary ones. However, the study showed that the potential for dispersion strengthening is not realized to its fullest extent due to carbide coarsening during hot forging and incomplete dissolution during further annealing. Further work will focus on optimizing the hot working and heat treatment parameters to prevent carbide growth and increase the strength of the alloy. These results will enable the use of the new Invar alloy in advanced industrial sectors where high accuracy, dimensional stability and the ability to withstand significant mechanical loads are required.

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