In situ TEM heating studies on the phase transformation of metastable phases in Fe–Cr–B alloy spray coatings

The Fe–Cr–B alloy thermal spray coatings contain a mixture of metastable phases such as nanocrystalline α-(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 α-(Fe,Cr) crystallites started. At higher temperatures (∼820 K), Cr2B borides started to precipitate through the matrix. When the annealing temperature was increased further to 960 K, the α-(Fe,Cr) grains in the matrix were coarsened rapidly and intragranular precipitation of Cr2B-type borides was consequently triggered.