Abstract
Atom probe tomography (APT) and transmission electron microscopy (TEM)/scanning transmission electron microscopy (STEM) have been used correlatively to explore atomic-scale local structure and chemistry of the exactly same area in the vicinity of growth front of a long-period stacking ordered (LPSO) phase in a ternary Mg–Al–Gd alloy. It is proved for the first time that enrichment of Gd atoms in four consecutive (0001) atomic layers precedes enrichment of Al atoms so that the formation of Al6Gd8 clusters occurs only after sufficient Al atoms to form Al6Gd8 clusters diffuse into the relevant portions. Lateral growth of the LPSO phase is found to occur by ‘ledge’ mechanism with the growth habit plane either {1overline{1}00} or {11overline{2}0} planes. The motion of ledges that give rise to lateral growth of the LPSO phase is considered to be controlled by diffusion of Al atoms.
Highlights
Mg–M–RE (M: metal, RE: rare-earth elements) alloys such as those in Mg–Zn–Y and Mg–Al–Gd systems have received considerable attention as a new class of structural materials for many possible engineering applications due to their low weight, high strength and good ductility[1,2,3]
We investigate variations of atomic structures and elemental distributions on an atomic scale occurring at the interface region between the long-period stacking ordered (LPSO)/OD and Mg phases in a Mg–Al–Gd alloy by correlative atom prove tomography and transmission electron microscopy (TEM)/scanning transmission electron microscopy (STEM) made in the exactly same regions, in order to better understand how chemical modulation synchronizes with structural modulation in the growth sequence of the ‘synchronized’ LPSO phase in Mg with better 3D spatial resolution
An interference-fringe TEM image formed with reflections of the [000l] systematic row and Atom probe tomography (APT) maps for Al and Gd in a 15-nm-thick slice selected from the whole maps obtained from an identical needle specimen of the LPSO/OD phase in the Mg–Al–Gd systems are shown in Fig. 1a–c, respectively
Summary
Mg–M–RE (M: metal, RE: rare-earth elements) alloys such as those in Mg–Zn–Y and Mg–Al–Gd systems have received considerable attention as a new class of structural materials for many possible engineering applications due to their low weight, high strength and good ductility[1,2,3]. STEM with angstrom spatial resolution, in particular Z(atomic number)-contrast imaging by high-angle annular dark-field (HAADF)-STEM has been provided various structural and chemical information of LPSO/ OD phases on an atomic scale such as stacking sequences, enrichment of M/RE atoms and their in-plane ordering to form M 6RE8 clusters[4,5,6,7,8,9,10,11] This technique has its own limitation arising from the fact that the obtained atomic-resolution image is just a two-dimensional projection along a particular crystallographic direction so that 3D information lacks inevitably unless additional imaging along other directions is made. We investigate variations of atomic structures and elemental distributions on an atomic scale occurring at the interface region between the LPSO/OD and Mg phases in a Mg–Al–Gd alloy by correlative atom prove tomography and TEM/STEM made in the exactly same regions, in order to better understand how chemical modulation synchronizes with structural modulation in the growth sequence of the ‘synchronized’ LPSO phase in Mg with better 3D spatial resolution
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