Direct imaging of short-range order and its impact on deformation in Ti-6Al

Abstract

Chemical short-range order (SRO) within a nominally single-phase solid solution is known to affect the mechanical properties of alloys. While SRO has been indirectly related to deformation, direct observation of the SRO domain structure, and its effects on deformation mechanisms at the nanoscale, has remained elusive. Here, we report the direct observation of SRO in relation to deformation using energy-filtered imaging in a transmission electron microscope (TEM). The diffraction contrast is enhanced by reducing the inelastically scattered electrons, revealing subnanometer SRO-enhanced domains. The destruction of these domains by dislocation planar slip is observed after ex situ and in situ TEM mechanical testing. These results confirm the impact of SRO in Ti-Al alloys on the scale of angstroms. The direct confirmation of SRO in relationship to dislocation plasticity in metals can provide insight into how the mechanical behavior of concentrated solid solutions by the material's thermal history.

Fig. 1. TEM bright-field imaging of deformed Ti-6Al with different thermal history.

(A) Sample aged to promote SRO; the blue arrows mark the in-band dislocation pairs. (B) Sample quenched after homogenization.

Fig. 2. Energy-filtered DF imaging of SRO domains.

(A) The energy-filtered DF image from superlattice diffraction shows distinguishable SRO domains. (B) Enlarged DF image with identified SRO domains marked by the red circles; the circles are scaled larger than the measured radius. (C) Energy-filtered $[2\overline{1}\overline{1}0]$ diffraction pattern. The orange arrows indicate the positions of the diffuse DO₁₉ superlattice diffraction peaks. The red circle marks the objective aperture position used in (A). (D) Size distribution of the identified SRO domains from three DF images (enlarged images are shown in fig. S7); the error bars were assigned according to the SD among three similar images.

Fig. 3. Comparison of the mechanical response of Ti-6Al in the quenched and SRO-aged conditions.

(A) True stress-strain curves of the quenched and SRO-aged samples. (B) Enlarged true stress-strain curves near yielding of both materials; the 0.2% offset yield strength is marked on the plot. (C and D) Indentation stress-strain curves of as-quenched sample and SRO-aged sample, respectively. Different indentations are marked with different colors.

Fig. 4. Energy-filtered DF images of planar dislocation slip bands.

(A) Bright-field image showing the relation of a slip band and the imaging area. The slip band is marked and shows weak-fringing contrast in the zoomed-in image. (B) Energy-filtered DF image at the same position of (A) showing the lack of contrast in the slip band.

Fig. 5. Diffraction analysis of TEM in situ compression test on a nanopillar made from an SRO-aged sample.

(A) DF image, diffraction pattern, and superlattice diffraction intensity line plot from before the in situ compression test. The position of the line plot is marked on the diffraction pattern. (B) DF image, diffraction pattern, and superlattice diffraction intensity line plot from after the in situ compression test. The position of the line plot is marked on the diffraction pattern. After the deformation, the diffuse superlattice peaks decreased beneath the noise level, presumably caused by the destruction of SRO at the tip region due to the dislocation activities.