Effect of Initial Microstructure Prior to Extrusion on the Microstructure and Mechanical Properties of Extruded AZ80 Alloy with a Low Temperature and a Low Ratio

Zhang et al. Chinese Journal of Mechanical Engineering (2023) 36:72 https://doi.org/10.1186/s10033-023-00901-w ORIGINAL ARTICLE Chinese Journal of Mechanical Engineering Open Access Effect of Initial Microstructure Prior to Extrusion on the Microstructure and Mechanical Properties of Extruded AZ80 Alloy with a Low Temperature and a Low Ratio Hang Zhang1, Haipeng Li1, Rongguang Li1, Boshu Liu1*, Ruizhi Wu2, Dongyue Zhao2 and Shanshan Li1* Abstract Magnesium (Mg) alloys are the lightest metal structural material for engineering applications and therefore have a wide market of applications. However, compared to steel and aluminum alloys, Mg alloys have lower mechanical properties, which greatly limits their application. Extrusion is one of the most important processing methods for Mg and its alloys. However, the effect of such a heterogeneous microstructure achieved at low temperatures on the mechanical properties is lacking investigation. In this work, commercial AZ80 alloys with different initial microstructures (as-cast and as-homogenized) were selected and extruded at a low extrusion temperature of 220 °C and a low extrusion ratio of 4. The microstructure and mechanical properties of the two extruded AZ80 alloys were investigated. The results show that homogenized-extruded (HE) sample exhibits higher strength than the cast-extruded (CE) sample, which is mainly attributed to the high number density of fine dynamic precipitates and the high fraction of recrystallized ultrafine grains. Compared to the coarse compounds existing in CE sample, the fine dynamical precipitates of M g17(Al, Zn)12 form in the HE sample can effectively promote the dynamical recrystallization during extrusion, while they exhibit a similar effect on the size and orientation of the recrystallized grains. These results can facilitate the designing of high-strength wrought magnesium alloys by rational microstructure construction. Keywords Magnesium alloy, Low temperature and low ratio extrusion, Bimodal grain structure, Dynamic precipitate, Dynamic recrystallization 1 Introduction In structural applications, wrought magnesium alloys are very attractive in terms of weight reduction, but their mechanical strength is relatively low at ambient temperatures [1–4]. In the recent works, researchers find that *Correspondence: Boshu Liu Shanshan Li 1 School of Mechanical and Power Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China 2 Key Laboratory of Superlight Materials & Surface Technology, Ministry of Education, Harbin Engineering University, Harbin 150001, China there is a synergistic effect between the coarse grains with a strong basal texture and submicron-sized grains with a weak texture, resulting in an increase in strength [5–7]. The idea of achieving the heterogeneous microstructures with high mechanical properties has received significant attention [8–11]. For example, a bimodalgrained AZ91 alloy prepared by hard-plate rolling exhibits a high ultimate tensile strength (UTS) of 370 MPa and a high elongation of 24% compared with the fine-grained AZ91 alloys [12]. A Mg-7Y-3Zn (wt.%) alloy with multimodal microstructure developed by extrusion shows a high UTS of 385 MPa and an elongation of 7% [13]. However, the strengthening mechanisms for the high strength of Mg alloys with heterogeneous microstructure still © The Author(s) 2023. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Zhang et al. Chinese Journal of Mechanical Engineering (2023) 36:72 Page 2 of 11 lack systematic investigation. The strengthening effect from grain refinement is suggested to be enhanced with increasing volume fraction of recrystallized grains, but the decreasing volume fraction of deformed grains with strong texture will result in a loss of strength. Besides, the size and distribution of second-phase particles also play vital role on the strength of Mg alloys with heterogeneous microstructure. The precipitates not only directly influence the strengthening effect from second phase, but also affect the formation of heterogeneous microstructure [14–18]. Liao et al. reported that the precipitates with an average size of 1 μm in a Mg-1Gd-1Y-1Zn alloy affected the heterogeneous microstructure by particle stimulated nucleation (PSN) mechanism [15]. Zou et al. reported that the nano-sized precipitates (50 nm) in a Mg-5Zn-1Mn alloy suppressed dynamic recrystallization (DRX), resulting in a bimodal grain size distribution [16]. The influence of precipitates on DRX behavior depends on the deformation temperature because the interaction of dislocations and particles is temperature-dependent [17]. Thus, it is necessary to clarify the role of grain refinement, texture, and second phase on the strength of Mg alloys with different heterogeneous microstructure. Extrusion is one of the most important processing methods for Mg and its alloys. Extrusion temperature and ratio play critical roles in the grain refinement of extruded Mg alloys. For example, the extrusion temperatures and ratio of AZ91 alloys are commonly controlled over 240 °C and 11, respectively, to achieve a homogeneous micron-scale microstructure [19–22]. The decreased extrusion temperature and extrusion ratio are suggested to reduce the recrystallization degree of extruded alloys and lead to a heterogeneous microstructure [23–26]. However, the effect of such a heterogeneous microstructure achieved at low temperatures on the mechanical properties is lacking investigation. Based on these works, we choose a commercial cast AZ80 alloy as initial material and design an extrusion process with low temperature (220 °C) and low ratio (4) to achieve heterogeneous microstructures. The extrusion ratio is chosen based on our previous works on extruded Mg alloys [19, 27], the selected ratio of 4 is suitable to achieve heterogeneous microstructure and ultrafine recrystallized grains. We compare the microstructure and mechanical properties of homogenized-extruded (HE) and cast-extruded (CE) AZ80 alloys. The role of grain refinement, texture, and second phase on the strength of AZ80 alloys are discussed in detail. coupled plasma atomic emission spectrometry (ICPA (...truncated)