Structural evolution in nanocrystalline Fe-10 Wt pct Cr alloy powder produced by mechanical alloying—An atomic-force microscopy study

Abstract

A mechanical alloying (MA) method was used to synthesize Fe-10 wt pct Cr alloy powder. The formation of an Fe-Cr solid solution during milling was studied using atomic-force microscopy (AFM), with the help of an atomic-force microscope in acoustic AC (AAC) mode. The AFM amplitude images indicated that the interlamellar spacing in the structure decreased with an increase in the milling time, and finally gave way to a nonlamellar structure. For structures obtained by milling up to 40 hours, AFM phase-contrast images showed regions of inhomogeneity. Surface-topography images of the granular milled powder showed that the powder surfaces were not smooth, but consisted of a typical hills-and-valley structure. The mean height of the hills decreased with an increase in the milling time. Powders milled up to 20 hours showed a structure that contained grains and subgrains. However, as the interlamellar spacing in granules was reduced, the clear definition of the grains disappeared. Only subgrains were observed in powders milled for time intervals ≥40 hours. With the milling time ≥40 hours, the subgrains not only got more and more refined, they also got elongated in the direction of granular flow. The subgrains in the powder milled for 100 hours were found to have an aspect ratio of 2.5 to 3.0; their smaller dimensions varied from 5 to 30 nm.