Effect of yttrium addition on dynamic mechanical properties, microstructure, and fracture behavior of extrusion-shear ZC61 + xY (x = 0, 1, 2, 3) alloys

Highlights

• An extrusion-shear method that combines hot extrusion with the equal channel anger pressing was used to prepare Mg alloys.

• The dynamic compression properties were improved with Y addition due to secondary phases strengthening and grain refinement.

• The pyramidal slip and extension twinning are dominated deformation modes to coordinate plastic deformation of Mg alloys.

• The absorption energy density increases with Y addition lead to the fracture mode from brittle fracture to ductile fracture.

Abstract

In this study, extrusion-shear ZC61 + xY (x = 0, 1, 2, 3) magnesium (Mg) alloys were prepared by a novel severe plastic deformation method, i.e., extrusion-shear, which integrates direct extrusion with equal channel angular pressing. The effect of yttrium (Y) addition on dynamic mechanical properties, microstructure, and fracture behavior of ZC61 + xY (x = 0, 1, 2, 3) alloys was investigated by Split-Hopkinson Pressure Bar setup under a strain rate of 2100 s⁻¹ along extrusion direction. With the increase in the content of added Y from 0 to 3 wt%, due to the strengthening of secondary phases and grain refinement, the yield strength of Mg alloys was enhanced from 142 to 342 MPa, and the ultimate compression strength of Mg alloys was improved from 582 to 701 MPa. During dynamic compression, the <c + a > pyramidal slip and extension twinning were the dominant deformation modes, which coordinated plastic deformation of ZC61 + xY (x = 0, 1, 2, 3) alloys. Analysis of the fracture behavior of four Mg alloys samples indicated that the fracture mode of ZC61 alloy was the brittle and melt mixed-mode; however, the fracture behavior of ZC61 + xY (x = 1, 2, 3) alloys was the brittle and ductile mixed fracture. With the increase of Y content from 1 to 3 wt%, the absorption energy density was observably increased, which was mainly responsible for the change of fracture mode from brittle fracture to ductile fracture.

Introduction

Magnesium (Mg) and its alloys, as the lightest metallic structure materials, have low density and high specific strength. Mg alloys have been utilized in several fields such as automobile, defense, and aerospace industry [[1], [2], [3]]. Light Mg alloys are extensively used in automobile, which may be subjected to dynamic loadings such as shock, impact, and vibration. Over the past decades, a large number of studies mainly focused on the mechanical properties and deformation mechanism of Mg and its alloys under static and quasi-static loading. However, dynamic loading may be very different from static and quasi-static loading. Therefore, more attention should be paid to the investigation of the dynamic loading of Mg and its alloys.

Few studies have recently reported that the action time during dynamic deformation is shorter than that during quasi-static loading, leading to lack of time to cause diffusion of heat into samples in dynamic deformation process [4]. Moreover, the mechanical behavior, deformation mechanism, and fracture behavior are very different in Mg and its alloys under dynamic loading. These previous studies mainly concentrated on the mechanical property, deformation mechanism, absorption energy density, and fracture behavior of Mg and its alloys, including pure Mg, AZ series, AM series, ZK series, and rare-earth-containing Mg alloys, under dynamic loading (high strain rate loading) [[4], [5], [6], [7], [8]]. Based on mechanical behavior, the mechanical properties and deformation characteristics of pure Mg were studied by Li et al. They found that yield strength, flow stress, and ductility in dynamic compression loading were obviously higher than those in quasi-static compression test [4]. Owing to the formation of extension twinning, the true stress–true strain curves of Mg and its alloys generally exhibit sigmoidal behavior during dynamic loading [5]. Moreover, the Mg alloy showed positive strain rate sensitive behavior. Based on the deformation mechanism, some researches showed that main deformation mechanisms under high strain rates loading were extension twinning and basal slip [6]. Dixit et al. found that the extension twinning and non-basal dislocation were activated under compressed state at 1000 s⁻¹ strain rate along extrusion direction (ED) to coordinate the plastic deformation for pure Mg. At the same time, the <c + a > dislocations significantly increased with increasing strain [5]. Based on absorption energy density, Zhang et al. studied MCT AZ31B under high strain rate compression in the range of 1326–5107 s⁻¹, indicating that basal-type textures and basal and non-basal slip were mainly responsible for deformation behavior [7]. Moreover, it was also shown that the energy absorption capacity of MCT AZ31B was very high due to conspicuous dynamic recrystallization and brittle–ductile transition under high strain rate loading. Based on fracture behavior, a ductile fracture characterized by micro-dimples was observed in special areas at a high strain rate. In contrast, fracture behavior was found to be a typical brittle fracture under quasi-static loading [8].

Yttrium (Y) element is a rare earth element, which can enhance the mechanical properties and weaken the deformation texture in Mg alloys. Moreover, previous studies showed that the properties of Mg-Zn-Cu alloys are more excellent than those of binary Mg-Zn alloys [[9], [10], [11], [12], [13], [14]], and many studies have also reported that Cu addition can significantly enhance the yield strength and elongation of Mg-6Zn-0.6Zr alloys [[9], [10], [11], [12]]. According to reports by Zhu et al. and Wang et al., Mg-6Zn-xCu-0.6Zr alloys have better yield strength, ultimate tensile strength, and elongation, when the added Cu content is in the range of 0.5–1.0 wt.-% [[10], [11], [12]]. When Y element was added into Mg-Zn-Cu alloys, some new secondary phases were formed. However, over the past decade, investigation on Mg-Zn-Cu-Y alloy has rarely been done. Bai et al. [15] studied the effects of Y content on microstructure and mechanical behavior of Mg-Zn-Cu-Zr alloys. It was found that grain size was adequately refined with Y addition in as-cast ZCK630 alloys. Moreover, they also discovered that the continuous networks of grain boundary W-phases resulted in obvious deterioration of the mechanical properties of as-cast ZCK630 alloy. In the past several years, effect of addition of Y element on the mechanical behavior and microstructures of Mg-Zn-Y alloy has been extensively studied. Luo et al. also reported that Mg₃YZn₆ (I-phase), Mg₃Y₂Zn₃ (W-phase), and Mg₁₂ZnY (X-phase) were often formed under the different Y to Zn mole ratios in Mg-Zn-Y alloys [16]. The I-phase is a stable quasicrystalline phase, which plays a major role in strengthening the alloy due to its high hardness [17,18]. In general, the W-phase is not considered as a very useful strengthening phase. Nonetheless, it can effectively pin the movement of dislocation and dispersion-strengthening of alloys due to its diffusive distribution in the matrix after hot extrusion [19]. Previous results pointed out that the stable, coherent interface between X-phase and matrix was not favored for voids and/or microcracks nucleated during deformation, thus the X-phase is significantly important for improving the strength and ductility of Mg alloy [20].

In present study, the extrusion-shear (ES) ZC61 + xY (x = 0, 1, 2, 3) alloys were compressed to fracture using a Split-Hopkinson pressure bar (SHPB) along ED under dynamic loading at a strain rate of 2100 s⁻¹. The dynamic compression properties, strengthening mechanism, deformation mechanism, and dynamic fracture behavior of these alloys were analyzed by electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) system equipped with an energy dispersive spectroscopy (EDS) system, and X-ray diffraction (XRD). The effect of Y addition on dynamic compression properties and fracture behaviors of ZC61 + xY (x = 0, 1, 2, 3) alloys was comprehensively investigated in this study.