Nano-sized Superlattice Clusters Created by Oxygen Ordering in Mechanically Alloyed Fe Alloys

Abstract

Creating and maintaining precipitates coherent with the host matrix, under service conditions is one of the most effective approaches for successful development of alloys for high temperature applications; prominent examples include Ni- and Co-based superalloys and Al alloys. While ferritic alloys are among the most important structural engineering alloys in our society, no reliable coherent precipitates stable at high temperatures have been found for these alloys. Here we report discovery of a new, nano-sized superlattice (NSS) phase in ball-milled Fe alloys, which maintains coherency with the BCC matrix up to at least 913 °C. Different from other precipitates in ferritic alloys, this NSS phase is created by oxygen-ordering in the BCC Fe matrix. It is proposed that this phase has a chemistry of Fe3O and a D03 crystal structure and becomes more stable with the addition of Zr. These nano-sized coherent precipitates effectively double the strength of the BCC matrix above that provided by grain size reduction alone. This discovery provides a new opportunity for developing high-strength ferritic alloys for high temperature applications.

Figure 1. Indentation hardness of

(A) unalloyed Fe (orange) and (B) Fe-1at.% Zr (blue) versus annealing temperature for 1 hour annealing time. The solid squares indicate the observed hardness and the open squares indicate the hardness predicted by the Hall-Petch relationship for ball-milled Fe given by Jang and Koch. Hardness and grain size data are by Darling et al.. The asterisk (*) indicates the 913 °C annealed samples analyzed by TEM in the present work. (C) Hardness comparison between the samples in the present work, our prior study and some other strengthened ferritic alloys with similar grain-size: markers in red (unalloyed Fe samples in present work and prior study1929); markers in blue (ball-milled Fe-alloys222324); markers in green (ODS172125); markers in black (solute and precipitates strengthened steels262728).

Figure 2. SAED pattern, dark and bright field image of the unalloyed Fe sample.

(A) SAED pattern by tilting α-Fe to [001] showing {100} superlattice reflections (marked with circle). (B) dark-field image using the 010 superlattice reflection marked in (A) showing dispersed phase 4.4 ± 0.9 nm in size. (C) [001] HRTEM image showing clustering of ordered superlattice phase (2-5 nm in size). Dashed lines mark approximate boundaries of a few clusters.

Figure 3. Comparison of the EELS O K-edge spectra from an NSS cluster (blue solid line) and the adjacent α-Fe matrix (red dash line) in the unalloyed Fe sample, revealing the oxygen enrichment of the NSS cluster.

Figure 4

(A) Zr elemental map in a grain of the Fe-1at.% Zr alloy recorded in STEM/EDS mode (probe size: 1 nm) showing Zr-enriched clusters with a size of ~5 nm. (B) EDS spectrum from Zr-enriched cluster showing a high oxygen content. (C) EDS spectrum from Fe matrix showing a lower oxygen content and no Zr.

Figure 5

(A) Proposed Fe₃O unit cell [D0₃ (A₃B) structure)], which contains 12 Fe atoms and 4 O atoms and has a space group symmetry of Fmm. The inset shows orientation relationship between Fe₃O unit cell and α-Fe unit cell, i.e. α-Fe [100]// Fe₃O [100], α-Fe [010]// Fe₃O [010], α-Fe [001]// Fe₃O [001]. (B) Simulated [001] diffraction pattern of the proposed Fe₃O structure (here Fe₃O {020} reflections correspond to BCC {010} superlattice reflections). (C) Experimentally observed [001] HRTEM image (unalloyed Fe sample) compared with the simulated HRTEM image of [001] projection of the proposed Fe₃O structure (inset within an orange dotted box), showing good agreement. The [001] structure projection of Fe₃O is shown in corner (left bottom). (D) Differential charge density map of the Fe₃O unit cell in (10) plane without Zr. (E) Differential charge density map of Fe₃O unit cell in (10) plane with one Fe atom replaced by Zr. The unit of charge density is e/ų.