Elsevier

Intermetallics

Volume 132, May 2021, 107152
Intermetallics

Twinning and orientation relationships of the monoclinic θ-Al45(Mn,Cr)7 phase in an Al–Mg alloy containing Mn

https://doi.org/10.1016/j.intermet.2021.107152Get rights and content

Highlights

  • Two types of twins with different twin directions are found in the θ-Al45(Mn,Cr)7 particles.

  • One is (1¯11) twin with the (1¯11) plane serving as the twinning plane, and the shear direction is η1 =[12¯3] QUOTE with , this (1¯11) twin is a compound twin.

  • The other is (1¯10) orientation domain with (1¯10) plane acting as the coherent plane, and is verified to be type II twin.

  • Multiple orientation relationships between the FCC structure α-Al matrix and the θ-Al45(Mn,Cr)7 particle were observed and disscussed.

Abstract

The spherical-like θ-Al45(Mn,Cr)7 particle having a monoclinic structure is observed in an Al–Mg alloy using TEM. Two types of twins with different twin directions are found in these θ-Al45(Mn,Cr)7 particles. One type is shown to be a (111) twin with (111) plane serving as the twinning plane and the shear direction is η1 = [123]; the other type is seen to be a (110) orientation twin with (110) plane acting as the coherent plane. At least six different orientation relationships between the θ-Al45(Mn,Cr)7 particles and the α-Al matrix are observed. These relatively complicated orientation relationships are attributed to the pseudo ten-fold symmetric building block of the θ phase viewed along the [101] axis. The possible formation mechanisms of these twins are discussed based on a crystallographic consideration.

Introduction

Aluminum alloys are among the most important engineering materials for modern civilization. Due to their optimal strength-to-weight ratio, combining lightweight with high strength, Al alloys are found in many applications especially in the mobility sector. Preserving resources and reduce costs of production requires continuous optimization. As the structure-property relationships originate at the atomic scale, it is necessary to study these materials at an ever-increasing level of detail.

It has been reported that submicron dispersoids in an Al–Mg alloy are important to promote hot ductility since these fine and stable dispersoids can be utilized to pin grain or sub-grain boundaries, increase the recrystallization temperature [1], and improve the strength and ductility in general [[2], [3], [4]]. In recent decades numerous efforts were made to study these manganese-rich and iron-rich dispersoids in Al–Mg alloys, i.e. in an AA5083 aluminum alloy – one of the highest strength alloys among the non-heat-treatable Al alloys [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]]. Usually, these dispersoids show various morphologies, i.e. spherical-like, rhomboidal, and rod- or lath-like, however the majority is reported to be rod-like Al6(Fe,Mn) phase with an orthorhombic structure [[6], [7], [8], [9], [10]]. Other dispersoids, such as the ν-Al11Mn4 phase having a triclinic structure [7], or the β-Al3Mg2 phase with a cubic structure [8] and the μ-Al4Mn phase with a hexagonal structure [3] were occasionally reported.

According to the Al–Cr binary [11,12] and the Al–Mn–Cr ternary phase diagrams [[13], [14], [15], [16]], when the content of manganese and chromium reaches a certain extent, a θ-Al45(Mn,Cr)7 phase (also referred to as Al13Cr2 or ′θ′) is formed during the homogenization at temperature of 560–720 °C. The θ-Al45(Mn,Cr)7 phase, which has a monoclinic structure with lattice constants of a = 2.5196 nm, b = 0.7574 nm, c = 1.0949 nm and β = 128.72° (space group C2/m by JCPDS#29-0014) exists as the second phase in the form of dispersoids in the α-Al matrix of such aluminum alloys (commonly referred to as ′α′) that contain Mn and Cr. To date, no attention has been paid to this θ-Al45(Mn,Cr)7 phase until recently the θ-Al45(Mn,Cr)7 phase was observed in an AA5083 aluminum alloy with H116 temper condition for the first time [17]. This recent work highlighted both (111) twin and (110) orientation twin present in the θ-Al45(Mn,Cr)7 phase [17], but no in-depth characterization and discussion regarding the twin elements in the θ-Al45(Mn,Cr)7 phase and the orientation relationship with the α-Al matrix which is not only important for Al–Mg alloy processing, but also the fundamental understanding of structure-property relationships.

In this paper, morphology and crystallographic features of twins and orientation domains in the θ-Al45(Mn,Cr)7 phase are presented in the edge-on state by tilting the specimen around the common/special reciprocal vectors using transmission electron microscopy (TEM). The major effort focuses on the twins in this base-centered monoclinic structure and its orientation relationship with the α-Al matrix to better understand the nature of the θ-Al45(Mn,Cr)7 particle phase.

Section snippets

Experimental

The Al–Mg alloy (Al–4.58Mg–0.08Si–0.07Mn–0.15Fe–0.095Cr, in wt.%) used in this study was hot-rolled at above 500 °C and subsequently annealed and cooled to room temperature (H112 temper). It is noted that this is different from the previously mentioned H116 tempered condition [17].

For TEM specimen preparation, cylindrical rods having a diameter of 3 mm were extracted from the bulk Al–Mg alloy by electro-discharge machining and thereafter cut into discs with a thickness of 0.3 mm. These discs

Morphology of the θ particles

The STEM image in Fig. 1a highlights the microstructure of an Al–Mg alloy in the H112 tempered condition with some dispersoids (dark) embedded in an α-Al matrix (bright). Two types of dispersoids are identified based on their different morphology: Plate- or rod-like η-Al5(Mn,Cr) phase (marked with A), and spherical-like θ-Al45(Mn,Cr)7 phase (marked with B). In the remainder of the manuscript, θ-Al45(Mn,Cr)7 particles are the focus of this study.

The length of the rod-like η-Al5(Mn,Cr) particle

Formation of θ-Al45(Mn, Cr)7 particles and its twins

According to the Al–Cr binary phase diagram [11,12] and Al–Cr–Mn phase diagram [[13], [14], [15], [16]], it is likely that there is a eutectic reaction L→Al6Mn + θ occurring in the temperature range of 560-700 °C, suggesting that the θ-Al45(Mn,Cr)7 particles are mostly formed during solidification, thermo-mechanical treatment, and/or annealing. Upon solidification, particles with dimensions in the range of about 100 nm to 1 μm crystallize from the liquid in eutectic or pseudo-peritectic

Conclusions

  • (1)

    Both twin and an orientation domain (orientation twin) were found in the θ-Al45(Mn,Cr)7 particles. The twin was shown to be a (111) or (111) twin with the (111) or (111) plane serving as the twinning plane, another orientation domain was seen to be a (110) domain with the (110) plane serving as the coherent plane. Also, stacking faults and composite-twins were also observed in θ-Al45(Mn,Cr)7 particles.

  • (2)

    For the case of (111) twin, the twinning plane K1 = (111) with twin direction η1 = [12

Authors' contributions

X. X. carried out all TEM experiments and prepared the manuscript.

H. L. participated in TEM data analysis and assisted manuscript preparation.

F. V. participated in discussions and contributed to the manuscript.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was financially supported by the Natural Science Foundation of Guangdong province, China (grant No. 2020A1515010023) and Foshan, grant No. 1920001000412).

References (31)

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