Lattice Defects of Cold-drawn and Aged Fe-36wt%Ni Alloys and Effects of Additions of C and V on Hardness and Thermal Expansion

Fe-36wt%Ni invar alloy has been widely used for precision parts where dimensional changes with temperature are unfavorable. In the viewpoint of industrial application, high-strength with low-thermal-expansion alloys are in increased demand for structural members such as power cables and electronic devices. Additions of alloying elements to and cold working on Fe-Ni alloys are effective methods that enable the alloys to strengthen, although these modifications also influence the magnitude of thermal expansion. In this study, effects of additions of 0.2wt%C and/or 0.8wt%V on hardness and thermal expansion of cold-drawn and aged Fe-36wt%Ni invar alloys are reported. Hardness in solution-treated and as-drawn conditions increased with addition of C, and, when both C and V were added, hardness of the alloy reached its maximum after aged at 650°C. Besides an increase in hardness with addition of any of C and V, thermal expansion below Tc became smaller in as-drawn condition than in solution-treated condition. Measurements of positron lifetimes revealed that vacancies introduced by cold-drawing were annealed out through aging up to 300°C and dislocation motion was activated at 500°C. Both vacancies and dislocations, especially C-dislocation complexes, seemingly influence the decrease in thermal expansion. While graphite precipitation in the C-added invar alloy showed no hardening, addition of C and V to invar alloy resulted in secondary hardening by vanadium carbide precipitation when aged over 500°C, which realized high-hardness and low-thermal-expansion.