Atomic Scale Structure and Chemical Composition across Order-Disorder Interfaces

Abstract

Through a combination of aberration-corrected high-resolution scanning transmission electron microscopy and three-dimensional atom probe tomography, the true atomic-scale structure and change in chemical composition across the complex order-disorder interface in a metallic alloy has been determined. The study reveals the presence of two interfacial widths, one corresponding to an order-disorder transition, and the other to the compositional transition across the interface, raising fundamental questions regarding the definition of the interfacial width in such systems. DOI: 10.1103/PhysRevLett.102.086101 PACS numbers: 68.35.Àp, 68.37.Ma, 68.37.Vj Interfaces play an important role in determining several properties in multiphase systems [1][2][3]. Atomic-scale renditions of the interface structure have been primarily accomplished through high-resolution transmission electron microscope (HRTEM) phase-contrast images (and simulations) that allow for identification of the positions of atomic columns and structural defects at the interface [4,5]. Recent advances in HRTEM and HRSTEM have been enabled by the development of spherical aberration correctors for the electron probe as well as the image [6][7][8].In parallel, developments in first-principles based electronic structure calculations and larger scale atomistic modeling tools permit more accurate simulations of the interface structure [9,10]. Recent atomistic simulations of the interface between an ordered precipitate and disordered matrix in a nickel-base superalloy indicate that the interface is not abrupt at the atomic scale [9]. While the implications of these results on the microstructural stability [11][12][13] and high temperature mechanical properties [14][15][16][17][18][19] are substantial, there is no clear experimental evidence confirming these atomistic simulations. The limitations associated with the resolution of experimental techniques have so far prevented direct atomic-scale imaging and interpretation of the structural and compositional transition across such interfaces. Techniques such as 3D Atom Probe (3DAP) Tomography have enabled detailed exploration of nanometer-scale elemental partitioning across interphase interfaces, including the = 0 interface in nickel base alloys [20][21][22][23][24][25][26]. In addition, developments in aberrationcorrected HRSTEM now permit Z contrast imaging (arising from the differences in atomic numbers) and interpretation at atomic resolution [27][28][29]. Our study provides direct experimental evidence at the atomic scale of the true structure and composition of the = 0 interface in a nickel base alloy by coupling two advanced and complementary techniques: aberration-corrected HRSTEM and 3DAP.This work was carried out on the nickel base superalloy Rene' 88 DT [nominal composition 55.63Ni-18.02Cr-13.00Co-4.74Ti-4.45Al-2.48Mo-1.21W-0.46Nb (in at %)], with superior creep and fatigue properties for turbine disk applications [14][15][16]. The typical microstructure consists of a disordered fcc matrix with ordered ...

A1-L12 interfacial free energies from data on coarsening in five binary Ni alloys, informed by thermodynamic phase diagram assessments

“…The first evidence for this was reported by Harada et al [39]. This early observation has been confirmed by atomistic modeling [40], recent experimental observations using modern atom-probe tomography [41] and high-resolution transmission electron microscopy [42]. A reconciliation of these findings, i.e., the independence of the rate constants on f e and the diffuse c/c 0 interface, provided the stimulus for a new theory of coarsening by Ardell and Ozolins [43], who also showed that the interface is not only diffuse, but also quite ragged in structure.…”

Study of γ/γ′ interfacial width in a nickel-based superalloy by scanning transmission electron microscopy

“…However, their original image (the non-filtered image) shown as Fig. 1(b) in reference [3] does not show high quality so that there is too much difference between the non-filtered image and the filtered image as shown in Fig. …”

“…To nickel base superalloys which have perfect creep and fatigue properties and have been widely used as materials of turbine blades [2], interfaces determine the strengthening capacities in high temperature. By means of high resolution scanning transmission electron microscopy (HRSTEM) and 3D atom probe (3DAP) tomography, Srinivasan et al [3] proposed a new point that in nickel base superalloys there exist two different interfacial widths across the γ/γ´ interface, one corresponding to an order-disorder transition, and the other to the composition transition. We argue about this conclusion in this comment.…”

Coarsening behaviour and interfacial structure of γ′ precipitates in Co-Al-W based superalloys

“…A study of alloy René 88DT using APT and atomic resolution Z-contrast scanning transmission microscopy (STEM) by Srinivasan et al [18] has revealed two markedly different interfacial widths corresponding to the compositional and the order-disorder (structural) gradients. This has raised the fundamental question about the treatment of the γ/γ interface in mathematical models of physical processes such as precipitate coarsening and dislocation-precipitate interactions.…”