In order to promote the application of magnesium (Mg) alloys in aircraft and space structural components, many efforts have been devoted to develop ultrahigh strength Mg alloys [[1], [2], [3], [4], [5]]. Mg-Y-Ni alloys containing long period stacking ordered (LPSO) phase have received extensive attention due to their excellent mechanical properties [[6], [7], [8], [9]]. The research on phase equilibrium of Mg-Y-Ni alloys indicates that 18R-LPSO, 14H-LPSO and 10H-LPSO can be observed at 400 °C [10]. In addition to LPSO phase, Mg₂₄Y₅ phase is formed when Y/Ni atomic ratios are 2-2.5, and Mg₂Ni phase is formed when Y/Ni atomic ratios are 0.25-1 [10]. In the as-cast Mg-Y-Ni alloys fabricated by conventional casting process, bulk 18R-LPSO phase is formed at grain boundaries when Y/Ni atomic ratios are between 0.6 and 5; Mg₂₄Y₅ phase is formed in the alloy with Y/Ni = 5, and Mg₂Ni phase is formed when Y/Ni atomic ratios are 0.6-1.25 [11]. Meanwhile, there are a few lamellar 14H-LPSO phases in the Mg matrix when Y/Ni atomic ratios are less than or equal to 2 [11].

Compared with Mg-Y-Zn wrought alloys, Mg-Y-Ni wrought alloys exhibit superior mechanical properties [12,13]. The Mg-18.9Y-6.5Ni (wt%) alloy containing 65% volume fraction of LPSO phase exhibits a tensile yield strength (TYS) of 460 MPa, an ultimate tensile strength (UTS) of 526 MPa and an elongation to failure of 8% at room temperature after processed by rolling and subsequent annealing [14]. The high strength and moderate ductility are due to the formation of basal texture in the LPSO phase and submicron recrystallized grains, respectively [14]. The extruded Mg-20.1Y-13.3Ni (wt%) alloy composed primarily of LPSO phase exhibits a compressive yield strength of 602 MPa, an ultimate compressive strength of 763 MPa and an elongation to failure of 7.8% at room temperature [12]. These ultrahigh strength Mg-Y-Ni alloys contain high concentration of rare earth elements (RE), which significantly increases the cost and limits their applications. Therefore, it is necessary to develop ultrahigh strength Mg-Y-Ni alloys with lower RE content.

In this research, the effect of Y/Ni atomic ratio on microstructure and mechanical properties of as-cast and as-extruded Mg-Y-Ni alloys was investigated in order to develop ultrahigh strength Mg-Y-Ni alloys containing relatively low concentration of RE by engineering the types of second phases.

Mg-Y-Ni alloys with different Y/Ni atomic ratios were prepared by permanent mold direct-chill casting [15]. The chemical compositions of the alloys were analyzed by X-ray fluorescence (XRF) spectroscopy and the results are listed in Table 1. The as-cast alloys with dimension of Φ42 mm × 35 mm were preheated at 420 °C for 15 min and then extruded at 420 ℃ with an extrusion ratio of 12:1 and a constant ram speed of 6 mm/min. The cylindrical extrusion rods with a diameter of 12 mm and length of 200 mm were obtained. The phase analysis of the alloys was performed using X-ray diffractometer (XRD). The microstructures of the as-cast and as-extruded alloys were investigated by ZEISS-Supra55 field-emission scanning electron microscope (SEM) and FEI-Talos F200x transmission electron microscope (TEM). The volume fraction of second phases was measured by Image-Pro software using at least 5 SEM images that were taken at random locations of each alloy. The texture was examined by an Aztec-HKL electron backscattered diffraction (EBSD) system, and the data were analyzed with Channel 5 software. Tensile specimens with a gauge dimension of 18 mm × 5 mm × 2 mm were machined along the extrusion direction. The tensile tests were performed on an Instron 5569 electronic universal testing machine with a crosshead speed of 1 mm/min at room temperature.

The XRD patterns of the as-cast Mg-Y-Ni alloys with different Y/Ni atomic ratios are shown in Fig. 1, indicating that the second phases in the as-cast Mg-Y-Ni alloys are LPSO and Mg₂Ni. When Y/Ni atomic ratios are greater than 1, the alloys are composed of α-Mg and LPSO phase. When Y/Ni atomic ratios are less than or equal to 1, Mg₂Ni phase is formed in addition to the LPSO phase.

Fig. 2 shows the SEM images of the as-cast Mg-Y-Ni alloys. It can be seen that large amounts of bulk phase with gray contrast are observed at grain boundaries of the alloys. Eutectic intermetallic compound with white contrast is formed between the bulk phase in the alloys with Y/Ni≤1 (Fig. 2(b) and (c)). In addition, the plate-shaped phase is also observed in the matrix of the alloys.

Fig. 3(a)-(d) shows the TEM bright field images, selected area electron diffraction patterns and composition analysis of the second phases in the as-cast alloys. The bulk phase is identified as 18R-LPSO phase with a composition of Mg-5.6Y-4.1Ni (at.%) and its Y/Ni atomic ratio is close to 4:3 (Fig. 3(a) and (b)). The eutectic intermetallic compound is determined to be Mg₂Ni phase (Fig. 3(c) and (d)). The nanoscale plate-shaped phase parallel to each other can be observed in the Mg matrix (Fig. 3(e)), which is confirmed to be stacking fault with stacking order of ABCA (Fig. 3(f)). Elemental mappings in Fig. 3(g) show that the stacking fault is enriched with Y and Ni solute atoms, which are generally considered to be γ' phase [5,[16], [17], [18]].

The volume fraction of the second phases located at grain boundaries in the as-cast Mg-Y-Ni alloys with different Y/Ni atomic ratios is shown in Table 2. As the Y/Ni atomic ratio decreases, the fraction of Mg₂Ni phase increases, and its morphology changes from long strip to typical lamellar eutectic morphology (Fig. 2(b) and (c)), while the fraction of LPSO phase decreases. Compared with α-Mg and LPSO phase, Mg₂Ni phase forms finally during solidification [19,20]. In the Mg96Y2Ni2 alloy with Y/Ni = 1, the Ni content is slightly excessive for producing LPSO phase, so that only a trace amount of Ni element participates in the formation of Mg₂Ni phase. Therefore, a small amount of Mg₂Ni phase with long strip morphology is formed in the Mg96Y2Ni2 alloy (Fig. 2(b)). As the Y/Ni ratio decreases, the concentration of Ni in final solidified regions is increased with increasing relative content of Ni in the Mg97Y1Ni2 alloy, resulting in a great amount of Mg₂Ni phase with lamellar eutectic morphology. When the Ni content is unchanged, the fraction of LPSO phase mainly depends on whether the Y/Ni atomic ratio of the alloys is close to 4:3, since LPSO phase is composed of RE₈TM₆ with RE/TM atomic ratio of about 4:3 [[21], [22], [23]]. Consequently, in the three Mg-Y-Ni alloys, Mg96Y3Ni2 alloy with Y/Ni atomic ratio closest to 4:3 contains the largest amount of bulk LPSO phase.

The γ' phase has not been observed in the previously reported Mg-Y-Ni alloys [[6], [7], [8], [9], [10], [11], [12], [13], [14]], while this phase is usually observed in Mg-Gd-Y-Zn-Zr [5], Mg-Y-Zn [16,17], Mg-Gd-Zn [18] alloys after heat treatment. The present Mg-Y-Ni alloys are prepared by permanent mold direct-chill casting so that the cooling rate is relatively fast, leading to high solid solubility of Y and Ni in the Mg matrix, which may promote the precipitation of γ' phase during solidification. The γ' phase can be regarded as a basal stacking fault (SF) I₂ enriched with solute atoms. Substituting a Mg atom by an alloying atom with larger atomic radius will reduce the energy of basal stacking fault [24,25]. On the contrary, the replacement of a Mg atom by an alloying atom with smaller atomic radius will increase the energy of basal stacking fault. It is reported that the addition of Y or Zn exhibits the opposite effect on the basal stacking fault energy, but the simultaneous addition of Zn and Y decreases the basal stacking fault energy dramatically [26]. The atomic radius is 0.180 nm for Y, 0.124 nm for Ni and 0.160 nm for Mg. Therefore, it is reasonably speculated that the simultaneous addition of Ni and Y decreases the basal stacking fault energy, leading to the precipitation of γ' phase. In the present research, the γ' phase is only observed in the Mg96Y2Ni2 and Mg97Y1Ni2 alloys, suggesting that the γ' phase is formed in the alloys with a suitable Y/Ni atomic ratio. Table 3 shows the compositions of the matrix in the as-cast Mg-Y-Ni alloys. As can be seen that the concentration of Y in the matrix of Mg95Y3Ni2 alloy is much higher than that of Ni. It is reported that the effect of reducing SF energy becomes weaker with increase of the solute concentration of Y [25]. Therefore, it can be considered that a higher Y/Ni ratio results in an excessively high concentration of Y in the matrix of Mg95Y3Ni2 alloy, which is unfavorable for the formation of γ' phase. While the Y/Ni ratios in the matrix of Mg96Y2Ni2 and Mg97Y1Ni2 alloys are closer to the RE/TM ratios in γ' phase reported in Mg-Gd-Zn alloy (Gd:Zn = 1:1) and Mg-Gd-Y-Zn-Zr alloy (RE:Zn = 1.2:1) [18,27], which may facilitate the precipitation of γ' phase.

Fig. 4(a)-(c) shows the SEM images of the three alloys with different Y/Ni atomic ratios after preheating at 420 °C for 15 min prior to extrusion. As can be seen that no significant changes can be observed except for the γ' phase. In the alloy with Y/Ni>1, γ' phase is not formed in the matrix even after preheating (Fig. 4(a)), while large amounts of γ' phase are formed in the alloys with Y/Ni≤1 after preheating (Fig. 4(b) and (c)). Fig. 4(d)-(f) shows the TEM bright field image and the HADDF-STEM images of LPSO phase and γ' phase in Mg96Y2Ni2 alloy after preheating. It can be clearly seen that γ' phase has the stacking order of ABCA (Fig. 4(e)), and the 18R-LPSO phase remains in the preheated alloy.

Fig. 5 shows the SEM images of the as-extruded Mg-Y-Ni alloys with different Y/Ni atomic ratios along extrusion direction (ED). The microstructure of the as-extruded alloys consists of second phases, dynamic recrystallized (DRXed) regions and non-dynamically recrystallized (non-DRXed) regions. The LPSO and γ' phases in all the extruded alloys are distributed along the extrusion direction. The eutectic Mg₂Ni phase is broken into micron-sized particles, dispersing in the particle bands along the extrusion direction. In the alloys with Y/Ni>1, there is no γ' phase in the non-DRXed regions (marked by the red dotted line in Fig. 5(a)). However, in the alloys with Y/Ni≤1, the non-DRXed regions contain a large amount of γ' phase (marked by the green dotted line in Fig. 5(b) and (c). Additionally, the DRXed grains in the alloys with Y/Ni>1 are obviously coarser than those in the alloys with Y/Ni≤1.

Fig. 6(a)-(c) shows the inverse pole figure (IPF) maps of the as-extruded Mg-Y-Ni alloys along extrusion direction (ED). The colored regions represent Mg matrix, and the LPSO phase located at grain boundaries could not be indexed and appears black in the figure. It can be seen that the Mg matrix is composed of the non-dynamically recrystallized (non-DRXed) regions with strong texture and dynamic recrystallized (DRXed) regions with weak texture. The fraction of non-DRXed regions and DRXed regions, and DRXed grain size of the as-extruded alloys are shown in Table 4. The DRX ratio of Mg matrix is the fraction of DRXed regions divided by the fraction of Mg matrix. Table 4 shows that the DRX ratio of the matrix decreases first and then increases with the decreasing Y/Ni atomic ratio. The extruded Mg96Y2Ni2 alloy with Y/Ni = 1 has the lowest DRX ratio. The DRXed grain size is above 1 μm in the alloys with Y/Ni>1; while the DRXed grain size is below 1 μm in the alloys with Y/Ni≤1.

Fig. 6(d)-(i) shows the inverse pole figures along the extrusion direction of the as-extruded Mg-Y-Ni alloys. As shown in Fig. 6(d)-(f), all the extruded alloys exhibit (0001) <10 $\bar{1}$ 0> basal texture. With decreasing Y/Ni atomic ratio, basal texture intensity first increases and then decreases, and as-extruded Mg96Y2Ni2 alloy with Y/Ni = 1 has the largest basal texture intensity. The basal texture becomes stronger with decreasing DRX ratio, as shown in Table 4. Fig. 6(g)-(i) shows the inverse pole figures of the DRXed regions along the extrusion direction of the as-extruded Mg-Y-Ni alloys. The DRXed regions also exhibit <10 $\bar{1}$ 0> texture component, but its intensity is extremely weak. Therefore, the alloys with a lower DRX ratio show stronger basal texture.

In order to illustrate the effect of the second phases on dynamic recrystallization behavior, the microstructures at the position of 10 mm and 0 mm below the die-entrance of partially extruded alloys are observed, as shown in Fig. 7. For Mg95Y3Ni2 alloy with Y/Ni>1, coarse DRXed grains can be observed near the bulk LPSO phase (Fig. 7(a)). For the alloys containing γ' phase (Mg96Y2Ni2 alloy and Mg97Y1Ni2 alloy with Y/Ni≤1), fine DRXed grains can be observed at the interfaces of Mg/LPSO and Mg/Mg₂Ni (Fig. 7(b) and (c)). This indicates that the bulk LPSO phase and the eutectic phase Mg₂Ni can promote DRX during initial stage of extrusion through particle-stimulated nucleation mechanism [28]. Consequently, the Mg95Y3Ni2 alloy containing a large amount of bulk LPSO phase exhibits higher DRX ratio. In particular, eutectic phase Mg₂Ni is crushed into micron-sized particles during extrusion due to its hard and brittle characteristics. As the extrusion continues, the broken particles will promote dynamic recrystallization more effectively. As indicated by the blue arrows in Fig. 7(d), DRXed grains can be observed around the broken Mg₂Ni particles. As a consequence, the Mg97Y1Ni2 alloy containing the largest amount of Mg₂Ni phase has the highest DRX ratio.

As shown in Fig. 7(b), a few of DRXed grains are observed at the kinking boundaries of the γ' phase, indicating that the kinking boundaries can act as favorable sites for recrystallization nucleation. In the as-cast Mg-Y-Ni alloys with random crystallographic orientation, kinking of the γ' phase can only be activated in some grains with their basal planes parallel to compressive direction [29]. It has been reported that the nanoscale lamellar γ' phase hinders DRX by restricting the DRX nucleation and the migration of the DRXed grain boundaries [30,31]. This leads to the finer DRXed grain size of less than 1 μm in the as-extruded Mg97Y1Ni2 and Mg97Y2Ni2 alloys containing large amounts of γ' phase. The lowest DRX ratio in the as-extruded Mg96Y2Ni2 alloy is also ascribed to the restriction of DRX by high volume fraction of γ' phase. On the other hand, a large number of DRXed grains promoted by Mg₂Ni particles in the Mg97Y1Ni2 alloy causes the dissolution of γ' phase, which leads to the decreased amount of γ' phase in the as-extruded Mg96Y1Ni2 alloy (Fig. 5(c)).

Fig. 8(a) shows the tensile stress-strain curves of as-extruded Mg-Y-Ni alloys. The strength of the extruded alloys increases first and then decreases with the decreasing Y/Ni atomic ratio, and the optimal mechanical properties can be obtained in the alloy with Y/Ni = 1 (Fig. 8(b)). The as-extruded Mg96Y2Ni2 alloy exhibits excellent mechanical properties with tensile yield strength of 465 MPa, ultimate tensile strength of 510 MPa and elongation to failure of 7.2%. As shown in Fig. 8(c), the as-extruded Mg96Y2Ni2 alloy with only 6.7 wt% RE addition exhibits ultrahigh strength, which is comparable to those of the age-hardened Mg-Gd-Y [1,2] and Mg-Gd-Y-Zn [[3], [4], [5]] wrought alloys with much higher concentration of RE (12-16.6 wt%). It is noted that the present as-extruded Mg96Y2Ni2 alloy containing large amounts of bulk LPSO phase and nanoscale γ' phase shows a higher tensile yield strength than Mg-Y-Zn [[32], [33], [34], [35], [36]] and Mg-Y-Ni [9,14] alloys containing mainly LPSO phase.

The LPSO-reinforced Mg-Y-Ni alloys can be considered as metallic composites composed of the hard LPSO phase and the soft Mg matrix, and the Mg matrix possesses a bimodal microstructure consisting of non-DRXed regions and DRXed regions. LPSO can strengthen Mg alloys by load transfer from Mg matrix to the LPSO phase [37,38]. Therefore, the strength of Mg alloys increases with the amount of LPSO phase. The LPSO phase with volume fraction of 50.3% makes a great contribution to the ultrahigh strength of the as-extruded Mg96Y2Ni2 alloy. The non-DRXed regions with basal plane parallel to the extrusion direction are hard-oriented grains, which restrict the activation of basal slip during tensile loading and strengthen the Mg-Y-Ni alloys effectively [37]. Meanwhile, refining the DRXed grains is effective for strengthening the wrought Mg alloys [37]. As a result, the largest proportion of non-DRXed regions and finer DRXed grains in the as-extruded Mg96Y2Ni2 alloy can strengthen the alloy effectively. In addition, the γ' phase is an effective strengthening phase and its strengthening effect is dependent on the mean lamellar spacing [27,39]. In non-DRXed regions of the Mg-Y-Ni alloys with Y/Ni≤1, the nano lamellar γ' phase with nano-spacing can effectively strengthen the Mg-Y-Ni alloys. Therefore, the ultrahigh strength of the as-extruded Mg96Y2Ni2 alloy is mainly ascribed to the large amounts of bulk LPSO phase, nano lamellar γ' phase with nano-spacing in the non-DRXed regions, the largest proportion of non-DRXed regions and finer DRXed grains.

The as-extruded Mg95Y3Ni2 alloy has the highest fraction of LPSO phase, but its yield strength is the lowest among the three Mg-Y-Ni alloys, which is mainly attributed to the softer Mg matrix due to the higher DRX ratio and coarse DRXed grains in the as-extruded Mg95Y3Ni2 alloy. The lowest fraction of LPSO phase and small amounts of γ' phase leads to the low strength of the as-extruded Mg97Y1Ni2 alloy. The low strain hardening rate of the as-extruded Mg97Y1Ni2 alloy is mainly ascribed to the highest ratio of ultrafine DRXed grains with grain size less than 1 μm. The higher amount of Mg₂Ni phase may increase the strength of the as-extruded Mg97Y1Ni2 alloy, but the dispersed Mg₂Ni particles with narrow spacing may act as microcrack initiation sites [40], which may deteriorate the ductility of the alloys.

The present research indicates that Mg-Y-Ni alloys mainly composed of LPSO phase and γ' phase have great potential as ultrahigh strength Mg alloys. The mechanical properties of the alloys can be further improved by regulating the volume fraction, size and distribution of LPSO phase and γ' phase.

(1) With decreasing Y/Ni atomic ratio, the main second phases in as-cast Mg-Y-Ni alloys change from LPSO to LPSO+γ', LPSO+γ'+Mg₂Ni. Meanwhile, the fraction of LPSO phase is decreased, and the fraction of Mg₂Ni phase is increased. The γ' phase only appears in the alloys with Y/Ni≤1 and its amount is increased significantly during the preheating prior to extrusion.

(2) The LPSO and γ' phases are distributed along extrusion direction, while the Mg₂Ni phase is broken into particles after extrusion. With decreasing Y/Ni atomic ratio, due to the inhibition of DRX by γ' phase, the as-extruded Mg96Y2Ni2 alloy with Y/Ni = 1 has the lowest DRX ratio and finer DRXed grains, leading to the strongest basal texture. The as-extruded Mg95Y3Ni2 alloy with Y/Ni>1 containing LPSO phase exhibits higher DRX ratio and coarse DRXed grains. In the as-extruded Mg97Y1Ni2 alloy with Y/Ni<1, the increased amount of Mg₂Ni particles promote DRX, leading to the highest DRX ratio and weak basal texture.

(3) The as-extruded Mg96Y2Ni2 alloy with Y/Ni = 1 exhibits tensile yield strength of 465 MPa, ultimate tensile strength of 510 MPa and elongation to failure of 7.2%, which is attributed to the synergistic effect of large amounts of LPSO phase, nanoscale γ' phase and the strongest basal texture. The LPSO phase is the main strengthening phase in the alloy. Large amounts of lamellar γ' phase in the as-extruded Mg96Y2Ni2 alloy restrict DRX, leading to the strongest basal texture. In addition, the nano lamellar γ' phase with nano-spacing also can strengthen the alloy obviously.