HRTEM Observation of a ?113?? Low Angle Tilt Boundary in the Al-Al2Cu(?) Eutectic Composite

Abstract

Particular defect structures appear during directional solidification of the Al(fcc)-Al 2 Cu(q) lamellar eutectic composite, the so-called lamellar faults. Each fault corresponds to terminations of Al and q lamellae. These terminations consist of a continuous succession of three types of ribbon-like interfaces, among which two are low angle boundaries: one is located in the fcc phase and the other in the q intermetallic (C16 standard type, a = 0.6063 nm, c = 0.4872 nm). High resolution transmission electron microscopy (HRTEM) of a q/q low angle tilt boundary with an electron beam parallel to h113iq shows that the core structures of the dislocations are undissociated despite the large magnitude of the possible Burgers vectors. The dislocations are of mixed-type character and oriented along the h113iq direction the closest to the growth axis direction of the eutectic. An unusual defect present in the tilt boundary consists of a dense group of three dislocations separated by nanometric distances. This defect acts like a single dislocation at far field distance. The mechanical equilibrium of such a defect has been studied using isotropic and anisotropic elasticities. It is strongly suggested that two of these dislocations form a dipole, which would indicate that the boundary has not a completely relaxed structure.

Local decomposition induced by dislocation motions inside precipitates in an Al-alloy

“…Dislocation characteristics in the body-centered tetragonal Al 2 Cu crystal have been studied earlier by diffraction contrast analysis and high-resolution imaging2728. Dislocations with Burgers vectors of , [001], and [100] were identified, and faults on (110) and (001) planes were also observed.…”

Crystalline Defects in a B.C.T. Al2Cu(?) Single Crystal Obtained by Unidirectional Solidification along [001]

“…In the literature, possible Burgers vectors attached to perfect dislocations in a C16 lattice were only mentioned for q and Zr 2 Ni. They were proposed from five different experiments: (i) from analyses of low angle tilt boundaries with axes close to [100] present in deformed q lamellae of the eutectic [3]; (ii) by the application of the invisibility criterion in polycrystalline q [11] and in Zr 2 Ni deformed at room temperature by indentation [36]; (iii) optical analysis of slip lines on flat faces of q single crystals plastically deformed above 400 C [23]; (iv) analysis of slip lines on the free surfaces of thin foils in the Al-q lamellar eutectic deformed by in situ TEM experiments at 320 C [6]; (v) from recent HRTEM images of q/q low angle tilt boundaries with the axis h113i, formed at lamellar faults of the eutectic [10].…”

“…The structural and mechanical properties of the Al(a)-Al 2 Cu(q) lamellar eutectic have been largely studied in the past by conventional transmission electron microscopy (TEM) at room and high temperature [1][2][3][4][5][6][7][8][9][10]. Most of these studies involved specimens taken from a unidirectionaly solidified ingot.…”

“…Most of these studies involved specimens taken from a unidirectionaly solidified ingot. They were aimed to understand the relationships between the macroscopic properties of such composites and the nature of their microstructural defects: crystalline defects inside each phase [3,[6][7][8], defects along the heterointerfaces [4,7], defects related to the mosaic substructure of a single grain eutectic [1,[7][8][9][10]. However, if the defects present in the face-centred cubic Al(a) phase are fairly well understood, the Burgers vectors of the dislocations present in the bodycentred tetragonal (b.c.t.)…”

Effect of solidification rate on microstructures and orientations of Al‐Cu hypereutectic alloy in thin crucible

“…. Generally, Al 2 Cu phase grows freely from various crystal planes (110), (310), (200), (211), (112), (202), and (420) . Only two peaks, (110) and (310) appeared at the growth rate of 5 μm/s, as shown in figure .…”