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

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 (similar to 0.02 mass%). The coefficients of thermal expansion (CTEs) of the as-deposited Invar Fe-Ni alloys were approximately 9 to 11 ppm/degrees C and were larger than those of pyrometallurgically produced Invar alloys. When the alloys were heat-treated at 400 to 500 degrees C, their CTEs drastically decreased to approximately 5 ppm/degrees C. Furthermore, upon heat treatment at 600 degrees C, the CTEs reached approximately 2 to 4 ppm/degrees 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 degrees 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 degrees 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 degrees 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. (C) 2016 Elsevier Ltd. All rights reserved.