Detailed Structures and Formation Mechanisms of Well-Known Al10RE2Mn7 Phase in Die-Cast Mg-4Al-4RE-0.3Mn Alloy

doi:10.1007/s40195-018-0819-0

Abstract

Abstract:

The detailed structures and the corresponding formation mechanisms of the well-known Al₁₀RE₂Mn₇ phase in the conventional die-cast Mg-4Al-4RE-0.3Mn alloy were thoroughly investigated using transmission electron microscopy (TEM). The results indicate that the Al₁₀RE₂Mn₇ phase ordinarily contains both normal $(11\overline{2} 1)$ twins and orientation twins. Both detailed TEM observations and density functional theory calculations indicate that the Al₁₀RE₂Mn₇ phase is transferred from the Al₈REMn₄ phase following an orientation relationship as $[010]_{{{\text{Al}}_{ 8} {\text{REMn}}_{ 4} }} / /[\overline{1} 101]_{{{\text{Al}}_{ 1 0} {\text{RE}}_{ 2} {\text{Mn}}_{ 7} }}$ and $(101)_{{{\text{Al}}_{ 8} {\text{REMn}}_{ 4} }} / /(11\overline{2} 0)_{{{\text{Al}}_{ 1 0} {\text{RE}}_{ 2} {\text{Mn}}_{ 7} }}$. Moreover, forming orientation twins in the Al₁₀RE₂Mn₇ phase is attributed to the blurry regions at incoherent twin boundaries in the Al₈REMn₄ phase. Finally, these formed orientation twins result in the $(11\overline{2} 1)$ twins in the Al₁₀RE₂Mn₇ phase.

Key words: Transmission electron microscopy, Phase transformation, Intermetallic phase, Twin boundary, Orientation relationship

Qiang Yang, Shu-Hui Lv, Fan-Zhi Meng, Kai Guan, Bai-Shun Li, Xu-Hu Zhang, Jing-Qi Zhang, Xiao-Juan Liu, Jian Meng. Detailed Structures and Formation Mechanisms of Well-Known Al₁₀RE₂Mn₇ Phase in Die-Cast Mg-4Al-4RE-0.3Mn Alloy[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(2): 178-186.

Figures/Tables 10

Fig. 1 a BF-TEM image of a typical lumpy particle, corresponding SAED patterns of b B segment, c C segment, respectively. The corresponding planar spacings for (h1k1i1l1) and (h2k2i2l2) diffraction spots are shown in c

Fig. 2 BF-TEM image of representative interlaced lath-shaped particles, with the inset being the magnified image of region highlighted by a purple quadrate

Fig. 3 SAED patterns from local regions indicated by area A a, area B b, area C c, area D d, area E e, area F f in Fig. 2

Fig. 4 a HAADF-STEM images of interlaced lath-shaped Al-RE-Mn phase, b EDS mappings of Al, La, Ce and Mn elements for region indicated by yellow dotted quadrate in a, c 2D graphs via MATLAB of intensity values of Al, La, Ce and Mn elements in b, respectively

Fig. 5 a BF-TEM image along with corresponding SAED pattern of segment 1 in Fig. 2a, b-f HR-TEM images along with corresponding FFT patterns from local regions. The orange dotted lines highlight the (110) plane, the absinthe-green dotted lines highlight the CTB, the blue dotted lines schematically highlight the phase boundaries, the green dotted lines highlight the ITB, and the green dotted circles highlight the blurry region at the ITB

Fig. 6 a BF-TEM image of Al10RE2Mn7 phase in interlaced lath-shaped particle, b corresponding SAED pattern along with pattern intensity values along orange and purple dotted lines in SAED pattern, and c corresponding HR-TEM image along with FFT pattern, also with FFT patterns A-C from local regions of A-C, respectively, indicated by yellow quadrates in c (d: spacing of two adjacent diffraction spots as indicated by intensity peaks)

Fig. 7 HR-TEM image along with t corresponding FFT patterns of an orientation twin in Al10RE2Mn7 phase. The bilateral Al10RE2Mn7 domains of the orientation twin stand with fully consistent orientation

Fig. 8 Atomic configurations of Al10RE2Mn7 and Al8REMn4 compounds

Table 1 Calculated lattice parameters, total energies and formation enthalpies for both Al10RE2Mn7 and Al8REMn4 structures as shown in Fig. 8

Phase	Lattice parameter (nm)	E_total (eV)	ΔH (eV/atom)
Al₁₀RE₂Mn₇	a = 0.874, c = 1.297	- 346.656	- 2.165
Al₈REMn₄	a = 0.889, c = 0.526	- 150.168	- 1.898

Fig. 9 Expansive HR-TEM image of Fig. 5c and corresponding FFT patterns from local regions of C-F, where the purple dotted lines marked the trace of $(10\overline{1} 1)_{{{\text{Al}}_{ 1 0} {\text{RE}}{}_{ 2}{\text{Mn}}_{ 7} }} / /(101)_{{{\text{Al}}_{ 8} {\text{REMn}}_{ 4} }}$. In addition, the part of the line in the region highlighted by green dotted circle was highlighted by white dotted line

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Detailed Structures and Formation Mechanisms of Well-Known Al₁₀RE₂Mn₇ Phase in Die-Cast Mg-4Al-4RE-0.3Mn Alloy

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