Disconnections and Laves (C14) precipitation in high-Cr ferritic stainless steels

Abstract

Even though it is widely accepted that the Laves phase is a promising strengthener for chromium-containing stainless steels, there is little information available on its microstructural evolution and growth. Therefore, in this study, we analysed the behaviour of Laves (C14) precipitates in high-Cr ferritic stainless steel, Fe-20Cr-2Mo-0.5Nb (at%), annealed at 1073 K for different time periods. A high density of (Fe,Cr)2(Mo,Nb) precipitates was formed in the α matrix during annealing. Morphological evolution in the Laves phase followed the sequence of spheroidal → faceted ellipsoidal → faceted needle-like → faceted plate-like. The Laves phase is related to the α matrix by two sets of orientation relationships, viz. OR-1: [1¯1¯2]α/[12¯10]L, [111]α/[01¯10]L, and (1¯10)α/(0001¯)L and OR-2: [11¯0]α/[0001]L, [113]α/[1¯1¯20]L, and (3¯3¯2)α/(11¯00)L. The habit plane of the Laves phase was found to deviate slightly from the (3¯3¯2)α/(11¯00)L plane. Interfacial defects, namely disconnections (bD, h) and super disconnections (mbD, mh), observed experimentally along the habit plane, were consistent with theoretical results based on OR-2 as the reference coordinate frame. The Burgers vector bD exhibited an edge component byD (0.1539 nm) parallel to Y/[113]α/[1¯1¯20]L and a small normal component bzD (0.0747 nm) perpendicular to the terraces; meanwhile, the overlap step height h = d(11¯00)L. The morphological evolution of Laves precipitates was described based on the lateral motion of disconnections and super disconnections on {3¯3¯2}α/{11¯00}L terraces in the 〈113〉α/〈1¯1¯20〉 direction. Arrays of the disconnections (bD, h) were found to be capable of accommodating the misfit strain on {3¯3¯2}α/{11¯00}L terraces.