Microstructure Characterization of Electrodeposited Invar Fe–Ni Alloy
Authors: Nagayama Tomio , Yamamoto Takayo , Nakamura Toshihiro 
Published: December 2020

In Fe–Ni alloys, coefficients of thermal expansion (CTEs) of Fe–Ni alloys with 30 to 40 mass% Ni are small compared to that of Fe (CTE = 12 ppm/K) or Ni (CTE = 13 ppm/K) at near room temperature. In particular, the Fe–Ni alloys with 36 mass% Ni exhibit the lowest CTE (approximately 1 ppm/K) among the Fe–Ni alloys. These Fe–Ni alloys with 30 to 40 mass% Ni are called “Invar alloys.” An Fe–Ni alloy electroplating can be used as the high-throughput Invar alloy fabrication process, furthermore, an Fe–Ni alloy electroforming process can provide precise micrometer-sized three-dimensional structures of Invar alloys with excellent thermal dimensional stability, e.g., MEMS (microelectromechanical systems).

We already have prepared electrodeposited Invar Fe–Ni alloys with 36 mass% Ni from plating baths containing saccharin as a stress reducer and containing Fe2+ and Ni2+ ions with various concentration ratios. The electrodeposited Invar Fe–Ni alloys contained of small amount of S (~0.02 mass%) which was originated from saccharin.

It was confirmed that 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.

In general, electrodeposited fcc Ni and Ni-rich Fe–Ni alloys containing small amounts of S are often embrittled upon annealing. An S (sulfide) in the as-deposited fcc films is dispersed at grain boundaries (2). During recrystallization and grain growth of the fcc grains by heat treatment, the S (sulfide) is extruded into recrystallized an fcc grain boundary with segregating as a thin layer form. The S thin layer coats the fcc grain boundaries and causes grain boundary weakening; therefore, distinct embrittlement occurs, and the embrittlement is promoted by rapid grain growth during annealing (3).

With respect to our electrodeposited Invar Fe–Ni alloys with S, in contrast, ductile behavior 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 (1). Moreover, we observed that the S in the Invar Fe–Ni alloys heat-treated at 600°C existed not as a filmy sulfide but as a granular sulfide at grain boundaries. However, the microstructures containing the granular sulfide generated in the film during heat treatment have not been sufficiently explored.

In this study, we investigated microstructure of the electrodeposited Invar alloys with and without heat treatment in detail. In particular, morphology and composition of the sulfide were evaluated in the Invar alloys with changing of microstrucutre during an annealing.

A galvanostatic electrolysis was conducted for preparation of the Invar Fe–Ni alloy film with 36 mass% Ni from the plating bath (1).

A transmission electron microscopy (TEM) was are used for microstructure characterization of the obtained films. A focused-ion-beam (FIB) system have given choice to fabricate cross-sectional TEM specimen.

From the results of cross-sectional TEM observation of the electrodeposited Invar Fe- 36 mass%Ni alloys with and without heat treatment at 300 to 600°C, it was found that the as-deposited bcc electrodeposited Invar alloys exhibited a columnar microstructure composed of micrometer-sized coarse grains and submicrometer-sized grains.

After the heat treatment at 300 to 400°C, where the proportion of fcc phase increased, the columnar structure became indistinct and the fraction of the submicrometer-sized fine grains increased. Furthermore, no substantial segregated sulfide was detected at the Fe–Ni alloy from elemental mapping by energy dispersive spectroscopy (EDS) in TEM.

Heat treatment at 500°C led to grain growth; the alloys treated at this temperature were fully composed of the fcc phase, accompanied by a decrease in the CTE.

Our TEM results suggest that S in the Invar Fe-36 mass%Ni alloy became highly concentration upon heat treatment at 500°C and formed granular Fe-rich Fe–Ni–S in grain-boundary triple-points.

In our research, the formation of granular sulfide in the film after heat treatment at 500 ° C and its composition were confirmed for the first time.

Upon heat treatment at 600°C, Fe-Ni matrix grains grew up as recrystallized grains, accompanied by grain growth of the Fe–Ni–S compound in the matrix grains or grain-boundary triple-points.

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