An ultrafine-grained maraging steel was fabricated by cold rolling and aging of iron-nickel-manganese lath martensite. Transmission electron microscopy was used to study precipitation behavior at grain boundaries during isothermal aging at 753 K. Relatively coarse precipitates were found at triple junctions of grain boundaries after aging for 0.36 ks. During further aging, precipitates coarsen at triple junctions at the expense of fine precipitates embedded at ultrafine grains. Mass-conservative dissolution of matrix precipitates occurs homogeneously as a result of which the precipitate-free zone around coarse particulates is eliminated. The phenomena of exclusive precipitation at grain boundary triple junctions and homogenous dissolution of matrix precipitates are attributed to the reduced length scale in the ultrafine-grained microstructure.
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