J. Cent. South Univ. (2021) 28: 1316-1323

DOI: https://doi.org/10.1007/s11771-021-4699-5

Microstructure and anisotropy of mechanical properties in ring rolled AZ80-Ag alloy

ZHANG Dong-dong(张冬冬)¹, LIU Chu-ming(刘楚明)^{1, 2}, WAN Ying-chun(万迎春)¹,JIANG Shu-nong(蒋树农)¹, ZENG Gang(曾钢)¹

1. School of Materials Science and Engineering, Central South University, Changsha 410083, China;

2. School of Materials Science and Engineering, Hunan University of Science and Technology,Xiangtan 411201, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021

Abstract: The microstructure, anisotropy in tensile strength and tensile creep resistance of the ring rolled AZ80-Ag alloy were studied. The ring exhibited higher strength along rolling direction (RD) than transverse direction (TD). The microstructure characterization and texture analysis demonstrated that the tilted basal texture was responsible for the higher tensile performance along RD. Investigations on the creep anisotropy revealed that the samples along RD had lower creep resistance, higher creep strain and higher steady creep rate at 70-80 MPa, compared with that along TD. The nominal creep stress exponent (n) values, 1.13 for RD and 3.86 for TD, indicated that diffusional creep and dislocation climb were the two corresponding creep mechanisms. During creep of the alloy, Mg₁₇Al₁₂ phase discontinuous precipitations were witnessed and their volume fraction enhanced with increasing stress.

Key words: creep; anisotropy; precipitation; ring rolling; magnesium alloy

Cite this article as: ZHANG Dong-dong, LIU Chu-ming, WAN Ying-chun, JIANG Shu-nong, ZENG Gang. Microstructure and anisotropy of mechanical properties in ring rolled AZ80-Ag alloy [J]. Journal of Central South University, 2021, 28(5): 1316-1323. DOI: https://doi.org/10.1007/s11771-021-4699-5.

1 Introduction

Magnesium (Mg) alloys possess a density of about 25% of steel and 67% of aluminum alloys, making them promising structural materials in applications requiring severe weight reduction, such as automobiles and aerospace components. However, the low symmetry of close-packed hexagonal structure results in the poor formality of Mg alloys, especially at ambient temperature. As a result, most of the currently used Mg alloys are produced via die casting. Nevertheless, the low mechanical properties (tension, compression, creep and wear) make them less competitive in components calling for high material performance. Therefore, quantitative investigations have been focused on wrought Mg alloys to both improve the mechanical properties and develop new processing methods [1, 2].

Mg-Al-Zn alloys are the most used Mg alloys owing to their good ambient temperature tensile property. While the poor elevated-temperature performances in tension and creep tests limited their further application in aerospace. Many efforts have been made to enhance their high temperature performances. Alloying Ca was used to improve the strength of Mg-Al-Zn alloy at various temperatures through generating thermally stable Al₂Ca phas with melting temperature of 1080 °C. Investigation of JIANG et al [3] displayed yield strength (YS) of 275 MPa for extruded Mg-2.32Al-1.7Ca alloy. Addition of Ca could also enhance the elevated temperature ductility of Mg alloys. The hot-rolled AZ31 alloy was revealed to have an increment in ductility from 347 % to 406 % via adding 0.2 wt% Ca [4]. As recently reported, adding Ag was a more effective way to strengthen Mg-Al-Zn alloy because trace of Ag strongly accelerated the precipitation kinetic and enhanced the aging response of the alloy [5].

Ring rolling is a newly developed processing method for Mg alloy in producing ring or cylinder shaped components. In our previous investigations, microstructure, texture, heat treatment, mechanical performances in tension and creep of AZ80-Ag alloys were systematically studied [6, 7]. The ring rolled (RRed) Mg alloys exhibited obvious tensile anisotropy owing to its unique texture. So it deserves investigation to uncover whether the RRed alloy would also exhibit anisotropy in creep resistance. In the present study, we made a systematical investigation on the microstructure, tension and creep anisotropy, microstructure evolution during creep of the RRed AZ80-Ag alloy.

2 Experimental procedures

The RRed AZ80-Ag alloy (Mg-8.10Al-0.48Zn-0.21Mn-0.20Ag (wt%)) was produced with a radial-axial machine at 380 °C, and the schematic diagram is shown in Figure 1(a). The details of the operation principle is described in Ref. [6].

Specimens for tension and creep were machined in the middle region of the ring along rolling direction (RD) and transverse direction (TD). Instron 3369 testing machine and 1 mm/min tension rate were used to evaluate the ambient temperature strength. Creep tests were carried out at 120 °C and 70-90 MPa on a RWS50 creep testing machine. The samples were held at preset temperature for 1 h before starting creep tests.

All samples were sectioned in the middle region of the ring and observed on the RD-TD plane, as revealed by Figure 1(b). Leica optical microscopy (OM) and Quanta 200 scanning electron microscope (SEM) were used for microstructure characterization. Electron backscatter diffraction (EBSD) was carried out on a FEI Helios Nanolab 600i SEM with HKL Channel 5 data acquisition and analysis software. Measurement of macro-texture and the phase pattern was performed on Bruker D8 Discover diffractometer. Samples for the foregoing tests were prepared according to the respective methods in Refs. [6, 7].

Figure 1 Illustrations of processing method (a) [6] and sampling for tests (b)

3 Results and discussion

3.1 Microstructure and tensile property

The microstructure of the alloy is characterized and shown in Figures 2 and 3. As illustrated in Figure 2(a), the alloy revealed a heterogeneous, dynamic recrystallized (DRX) structure with grain sizes varied between 10 and 100 μm. Precipitate particles with spherical shape were observed along the grain boundaries (GBs) as shown in Figures 2(b) and (c). Mg alloys have very different precipitation behaviors during deformation in contrast with in heat treatment. Aging precipitates usually nucleate and coarsen on the habit planes and present shapes of “plate” or “rod”, while the phase particles generated in processing were observed to have no specific orientation relationship with matrix. The so called dynamic precipitates usually have spherical appearance, as witnessed in multi-directional forging [8], equal-channel angular pressing [9] and large-strain rolling [10]. The dynamically precipitated particles were usually formed along with the nucleation of new grains and considered to have hindering effect on grain growth. The element analysis of the precipitates in Figure 2(c) based on EDS revealed existence of Al and Mn in the particles, indicating that Mg₁₇Al₁₂ phase was dynamically precipitated. The Mn element was considered to exist in the primary Al-Mn phase and re-distributed at the GBs after processing. The absence of the peaks of it was attributed to the trace quantity. Al-Mn compound particles in AZ alloys were not considered to affect the alloys’ stress-strain response at high strain rate, but enhance GB sliding at low strain rate [11].

Figure 2 Microstructures of RRed AZ80-Ag alloy:

Figure 3 Inverse pole figure (IPF) map (a) and recrystallized map (b) of RRed AZ80-Ag alloy

The EBSD image, as shown in Figure 3, revealed that the alloy was composed of dynamic recrystallization grains, substructures, and deformed grains with the volume fractions of 71.3%, 18.6% and 10.1%, respectively. In RRed process, a large quantity of dislocations was generated and accumulated at GBs, resulting in a severe local torsion, which would induce DRX through burgling of initial GBs. DRX occurred at GBs usually results in heterogeneous structures with finer grains around initial deformed coarser grains.

We compared the alloy’s tensile mechanical performance at ambient temperature along RD and TD as listed in Table 1. It was obvious that specimens along RD exhibited higher yield strength (YS) and ultimate tensile strength (UTS) than those along TD.

Table 1 Tensile properties of RRed AZ80-Ag alloy with standard deviation

To understand the difference in mechanical property, analysis on the alloy’s micro-texture was investigated based on the EBSD results in Figure 3. The EBSD-based {0001} pole figure is shown in Figure 4, indicating an inclined basal texture: the c axis of some grains was slightly tilted away from normal direction (ND) to TD of the ring. The TD spread is not typical for the present AZ80-Ag alloy, and also reported in alloys containing rare earth elements [12]. DRX resulting from particle-stimulated nucleation as well as shear band nucleation was considered responsible for this spread character [13-15]. Here, we consider the dynamic precipitate particles observed contributed to the generation of the TD spread texture through operation of PSN. The XRD-based {0001} macro-texture shown in Figure 5 also confirmed the texture character, a tilted basal texture with pole slightly inclined away from ND.

Figure 4 EBSD-based {0001} pole figure of RRed AZ80-Ag alloy

Figure 5 XRD-based {0001} pole figure of RRed AZ80-Ag alloy

Schmid factor (SF) distributions for dislocation basal slipping in tension test along RD and TD were calculated based on the EBSD results, as shown in Figure 6. The mean SFs for RD and TD are 0.186 and 0.249, respectively. Since dislocation basal slipping was considered to dominate deformation at ambient temperature [16], the average {0001}SF to some extent, determined the YS, the samples with lower average SF exhibited higher resistance for dislocation slipping. Here, logically,higher YS was obtained in tension along RD.

Figure 6 Distribution of dislocation basal slipping Schmid factor of RRed AZ80-Ag alloy

3.2 Creep performance and creep mechanism analysis

Actually, Mg alloys were usually observed to exhibit anisotropy in various mechanical properties in addition to tensile strength. Therefore, creep behaviors of the alloy were investigated at 120 °C under 70-90 MPa tensile stresses along RD and TD. Creep curves under various conditions were displayed in Figure 7 with the data summarized in Table 2. In this study, all specimens’ creep curves were made up of two stages, deceleration creep and steady creep, due to the limitation of creep time (about 100 h) and relatively low temperature (120 °C). Specimens along the two directions all exhibited decreased creep resistance with increased stress as the creep strain and creep rate obviously went down [17]. Steady creep ratecould be described by [18-20]:

                     (1)

Figure 7 Creep curves of RRed AZ80-Ag alloy

Table 2 Creep data of RRed AZ80-Ag alloy

where A is a material constant; R is the gas constant (8.314 J/(mol·K)); n is the nominal creep stress exponent; Q is the creep activation energy (J/mol); T is absolute temperature (K); σ is the creep stress.

Therefore,

                              (2)

To further differentiate the creep responses along RD and TD, we displayed the relationship betweenand lnσ in Figure 8. The n values were determined to be 1.13 and 3.86 respectively for RD and ND, indicating responding creep mechanisms of diffusional creep and dislocation climb [21-23].

Figure 8 Relationship of RRed AZ80-Ag alloy between and lnσ in different directions

Microstructure of the alloy after creep was characterized and shown in Figure 9. The grains did not reveal any change in average size and shape after creep under 70-90 MPa for 100 h at 120 °C. It is notable that plate-shaped precipitate particles were observed, in addition to the spherical-shaped phase dynamically precipitated shown in Figure 2. The X-ray diffraction pattern shown in Figure 10 revealed only Mg₁₇Al₁₂ phase and α-Mg phase were contained. Therefore, it can be confirmed that the newly formed phase at grain boundaries were discontinuous Mg₁₇Al₁₂ phase. As the Mg₁₇Al₁₂ phase was usually observed after heat treatment above 150 °C, the stress in creep was considered to promote the precipitation even at 120 °C. Stress in creep was reported to affect dislocation slipping and perhaps result in dislocation tangling, then ultimately accelerate Mg₁₇Al₁₂ phase precipitation [24, 25]. Furthermore, as revealed in Figure 9, the volume fraction of Mg₁₇Al₁₂ precipitate particles increased with creep stress, confirming that stress contributed to Mg₁₇Al₁₂ phase precipitation. These precipitates were harmful to improve creep resistance [26].

Figure 9 Microstructures of RRed AZ80-Ag alloy after creep under different pressures for about 100 h along RD (a, b): 70 MPa; (c, d) 80 MPa; (e, f) 90 MPa

The RRed AZ80-Ag alloy was characterized to have a tilted basal texture and presented an obvious anisotropy in creep resistance along RD and TD. The deduced n and Q values indicated distinctly different operative creep mechanisms along the two directions. In addition, precipitation occurred under the function of stress. Therefore, the initial texture and precipitation in creep may be the two factors responsible for the creep anisotropy of the RRed alloy.

Figure 10 XRD diffraction pattern of RRed AZ80-Ag alloy after creep for 100 h at 80 MPa and 120°C

4 Conclusions

In the present investigation, microstructure, anisotropy in tensile strength and creep resistance of the RRed AZ80-Ag alloy are studied.

The RRed alloy is composed of dynamic recrystallization grains, deformed grains, and dynamic precipitates at GBs. Strength along RD is higher than that along TD due to the basal texture inclined to TD. The ring has a better creep resistance along TD than RD, and the operative creep mechanism is diffusional creep and dislocation climb, respectively. Strain induced precipitation is generated during creep and the precipitation kinetic is enhanced with increment in stress.

Contributors

ZHANG Dong-dong provided experimental data, analyzed the measured data, and edited the original draft of manuscript. LIU Chu-ming provided samples, equipment, and funding. WAN Ying-chun provided funding and edited the draft of manuscript. JIANG Shu-nong edited the draft of manuscript and supervised the whole experiment. ZENG Gang analyzed the measured data. All authors replied to reviewers’ comments and revised the final version.

Conflict of interest

ZHANG Dong-dong, LIU Chu-ming, WAN Ying-chun, JIANG Shu-nong, ZENG Gang declare that they have no conflict of interest.

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(Edited by FANG Jing-hua)

中文导读

AZ80-Ag镁合金环轧件的组织及力学性能各向异性

摘要：本文研究了AZ80-Ag镁合金环轧件显微组织及拉伸强度和拉伸蠕变的各向异性。环件在轧向比横向有更高的强度，织构分析结果表明基面织构偏转导致沿轧向有更好的拉伸性能。通过拉伸蠕变试验发现，当蠕变应力在70~80 MPa时，轧向比横向的蠕变抗性差，会产生更高的蠕变应变和稳态蠕变速率。轧向和横向的蠕变应力指数n分别为1.13和3.86，分别对应扩散蠕变和位错攀移蠕变机制。在蠕变过程中，环件会析出不连续Mg₁₇Al₁₂相，并且它们的体积分数随应力增加而增加。

关键词：蠕变；各向异性；析出；环轧；镁合金

Foundation item: Projects(51574291, 51874367) supported by the National Natural Science Foundation of China; Project(2019JJ50787) supported by the Natural Science Foundation of Hunan Province, China; Project(2018M642999) supported by the China Postdoctoral Science Foundation

Received date: 2020-06-17; Accepted date: 2020-09-15

Corresponding author: JIANG Shu-nong, PhD, Associate Professor; Tel: +86-731-88830257; E-mail: shnjiang@csu.edu.cn; ORCID: https://orcid.org/0000-0002-9734-1754