Thermal expansions and mechanical properties of electrodeposited Fe–Ni alloys in the Invar composition range

Abstract

Electrodeposited Invar Fe–Ni alloys with 36 to 40 mass% Ni were prepared from plating baths containing saccharin as a stress reducer and containing various Fe2+ concentrations. The Invar Fe–Ni alloys contained of small amount of S (∼0.02 mass%). The coefficients of thermal expansion (CTEs) of the as-deposited Invar Fe–Ni alloys were approximately 9 to 11ppm/°C and were larger than those of pyrometallurgically produced Invar alloys. When the alloys were heat-treated at 400 to 500°C, their CTEs drastically decreased to approximately 5ppm/°C. Furthermore, upon heat treatment at 600°C, the CTEs reached approximately 2 to 4ppm/°C depending on alloy composition; these CTEs are comparable with those of pyrometallurgically produced alloys. The as-deposited Invar Fe–Ni alloys were mainly composed of metastable bcc phases, resulting in larger CTEs. When the alloys were annealed at 400°C or above, the equilibrium fcc phases became the predominant phases, accompanied by a drastic decrease of the CTEs. The bcc-to-fcc transformation led to a decrease of the CTEs and to thermal contractions. Upon the heat treatment, an S (sulfide) at bcc grain boundaries segregated not as a thin film but as a granular sulfide form at primary bcc grain boundaries in the electrodeposited Invar alloys. Upon heat treatment at 500°C or above, bcc grain eliminated accompanied by fcc grain growth and the granular sulfide agglutinated further. Consequently, the agglutinating granular sulfide was entrapped in the matrix grains or grain-boundary triple-points of transformed fcc grains. In addition, it is considered that these two-type morphologies of the sulfide, as a thin film at grain boundaries or as a precipitate, will be determined by grain growth form during an annealing. Ductile behavior of the electrodeposited Invar Fe–Ni alloys was confirmed, irrespective of whether the alloys were heat-treated. Upon heat treatment at 400 to 500°C, the Invar Fe–Ni alloys exhibited high strength with good ductility, consistent with their low CTE. After the heat treatment, no severe embrittlement of the electrodeposited Invar alloys was observed despite the codeposition of S because the S existed as a granular sulfide, thereby preventing grain-boundary embrittlement.