Materials Science and Engineering A304–306 (2001) 321–326 In situ TEM heating studies on the phase transformation of metastable phases in Fe–Cr–B alloy spray coatings H.W. Jin a , C.G. Park a,* , M.C. Kim b a Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, South Korea b Research Institute of Industrial Science and Technology, Pohang 790-600, South Korea Abstract The Fe–Cr–B alloy thermal spray coatings contain a mixture of metastable phases such as nanocrystalline -(Fe,Cr) solid solution phase supersaturated with boron and the remainder of amorphous phase. In order to investigate the thermal stability and phase transformation of these metastable phases, in situ TEM heating experiment has been performed. Upon an exposure to high temperatures, the metastable phases were thermally decomposed in the following sequences: At temperatures above 670 K, the growth of nanocrystalline -(Fe,Cr) crystallites started. At higher temperatures (∼820 K), Cr 2 B borides started to precipitate through the matrix. When the annealing temperature was increased further to 960 K, the -(Fe,Cr) grains in the matrix were coarsened rapidly and intragranular precipitation of Cr 2 B-type borides was consequently triggered. © 2001 Published by Elsevier Science B.V. Keywords: Fe–Cr–B alloy; Metastable phase; Thermal stability; Microstructure; In situ TEM heating 1. Introduction Metastable phases such as extended solid solution and amorphous phase are usually formed in spray deposited coatings due to a rapid solidification of sprayed splats during the thermal spraying process [1]. An exposure of the spray coatings to a high-temperature environment, either during the post-spraying heat treatment or during the service, often results in the crystallization of amor- phous phase and/or the transformation of the metastable phases. It is greatly important to understand the crystal- lization and/or phase transformation phenomena of these metastable phases, since the mechanical properties of the spray coatings are strongly dependent on the microstructure including the presence and the fraction of these metastable phases. Fe–Cr–B alloy coatings have been reported to exhibit excellent wear and corrosion resistance, which can be at- tributed to the matrix structure, a mixture of amorphous and nanocrystalline phases [2–6]. However, this mixed microstructure of metastable amorphous/nanocrystalline phases has brought complexity in understanding the ther- mal stability of the Fe–Cr–B alloy coatings. The thermal * Corresponding author. Tel.: +82-562-2792139; fax: +82-562-2792399. E-mail address: cgpark@postech.ac.kr (C.G. Park). stability of amorphous and/or metastable materials has been usually investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD), nuclear magnetic reso- nance (NMR), Mossbauer spectroscopy (MES), etc. Most of these methods give only average information in time and in space, which restrains the usefulness of these techniques in such materials having complex and inhomogeneous structure. In situ heating experiment in an electron microscope is much advantageous method to collect direct information on the thermal stability of multi-phase materials with complex microstructure. Various transformation behavior of materi- als, such as crystallization of amorphous alloy, nucleation, growth, grain morphology and grain size distribution, can be obtained dynamically as a function of temperature with an excellent spatial resolution (∼0.1 nm). In the present study, investigations have been focused on the phase transforma- tion behavior of metastable phases in thermally sprayed Fe–Cr–B alloy. 2. Experimental procedure Fe–Cr–B alloy coatings about 500 m thickness were prepared by using a detonation gun thermal spraying. Ex- cess fuel gas condition was used in the present study to 0921-5093/01/$ – see front matter © 2001 Published by Elsevier Science B.V. PII:S0921-5093(00)01525-2