The influence of the volume fraction of the long-period stacking ordered (LPSO) phase on the strengthening mechanisms acting in Mg/LPSO two-phase extruded alloys is discussed by focusing on compression tests of Mg₉₄Zn₂Y₄ and Mg₉₂Zn₃Y₅ alloys. An increase in the LPSO phase volume fraction increases the yield stress of these alloys, but the magnitude of the increase is not monotonic with the volume fraction. For deformation parallel to the extrusion direction, the rate of increase in the yield stress shows two large gaps between the Mg_99.2Zn_0.2Y_0.6/Mg₉₇Zn₁Y₂ and Mg₉₂Zn₃Y₅/Mg₈₉Zn₄Y₇ alloys. This is derived from the change in strengthening mechanisms. The upper gap between Mg₉₂Zn₃Y₅/Mg₈₉Zn₄Y₇ is derived from the change in the strengthening mechanism between short-fiber reinforcement and the simple rule of mixtures. The lower gap between Mg_99.2Zn_0.2Y_0.6/Mg₉₇Zn₁Y₂ corresponds to the existence of a short-fiber strengthening mechanism or not. As the volume fraction of the LPSO phase decreases, the magnitude of kink-band strengthening to the yield stress decreases; however, it is still effective even in alloys with a low volume fraction of the LPSO phase.

Fig. 9 Variations in the strengthening factors affecting the yield stresses of Mg/LPSO two-phase extruded alloys, in the various R10 extruded alloys deformed in the 0° orientation at (a) RT, (b) 200 and (c) 300°C.

Energy shortages and resource depletion due to population growth have become social problems, and there are other problems related to the global environment, such as global warming due to the burden of economic development on the environment. One method of solving these problems from an engineering point of view is to reduce the fuel consumed by transportation equipment such as automobiles. Mg alloys have attracted attention for their potential to address this problem.¹⁾ However, their low strength is a serious issue for practical applications. Recently, Mg alloys containing a long-period stacking ordered (LPSO) phase have been the focus of researchers since it overcomes the low-strength drawback by acting as an effective strengthening component. The dominant deformation mode in the LPSO phase was identified as $(0001)\langle 11\bar{2}0\rangle $ basal slip.^2–5) Under conditions where the operation of basal slip is difficult, the formation of deformation kink bands, on which basal dislocations are arranged almost perpendicular to the slip plane, acts as an effective deformation mode.^2–11)

Initially, the development of Mg/LPSO two-phase alloys was reported for rapidly solidified materials.¹²⁾ However, considering the widespread practical application, it is necessary to study materials that are easier to prepare for practical applications, such as those prepared by rolling and extrusion.^13–32) Regarding Mg/LPSO two-phase extruded alloys, we analyzed the microstructure and texture of Mg₉₇Zn₁Y₂ (at%) alloys containing the LPSO phase with a concentration of 24 vol%^14,20) and Mg₈₉Zn₄Y₇ (at%) alloys containing the LPSO phase with a concentration of 86 vol%.^33,34) The mechanical properties were examined, and the strengthening mechanisms and the factors controlling these mechanisms were discussed. For the Mg₉₇Zn₁Y₂ alloy, the main strengthening mechanisms are short-fiber reinforcement by the LPSO phase and significant grain refinement of the Mg matrix.^14,20) In contrast, for the Mg₈₉Zn₄Y₇ alloy, strengthening is mainly induced by the alignment of the LPSO phase (texture effect) and kink-band strengthening.^33,34) It was suggested that the predominant strengthening mechanism differs depending on the alloy.

There is a large difference in the volume fractions of the LPSO phase for the Mg₉₇Zn₁Y₂ and Mg₈₉Zn₄Y₇ alloys. However, the deformation behaviors and strengthening mechanisms of alloys having compositions between these two alloys are insufficiently understood. Therefore, in this study, Mg₉₄Zn₂Y₄ and Mg₉₂Zn₃Y₅ alloys, which have compositions between the Mg₉₇Zn₁Y₂ and Mg₈₉Zn₄Y₇ alloys, are investigated. The relationships among the microstructure, crystallographic texture, mechanical properties, and deformation microstructure were examined. According to the results, we quantitatively considered the effects of the LPSO-phase volume fraction and extrusion ratio on the mechanical properties and strengthening mechanisms of Mg/LPSO two-phase extruded alloys in a unified manner.

Master ingots with compositions of Mg₉₄Zn₂Y₄ and Mg₉₂Zn₃Y₅ (at%) were prepared by high-frequency induction melting in a carbon crucible, followed by casting. Columnar as-cast ingots with a diameter of 29 mm and a length of 70 mm were then extruded at a temperature of 450°C in air at a ram speed of 2.5 mm/s. Four extruded alloys with different reduction ratios of the cross-sectional area, A₀/A_e (A₀: cross-sectional area of the raw material billet; A_e: cross-sectional area of the extruded bar), of 2:1, 3:1, 5:1, and 10:1 were prepared to examine the variation in the kink-band strengthening effect with the extrusion ratio. Hereafter, these extruded alloys are called the R2, R3, R5, and R10 extruded alloys, respectively. The central region of the extruded alloy along the radial direction, i.e., at least 1 mm from the surface, was used for mechanical testing.

The microstructures of the specimens were observed using optical microscopy (OM, OLYMPUS BX51M) and scanning electron microscopy (SEM, JEOL JSM-7800F). For OM observation, the surface of the specimen was mechanically polished by emery paper, alumina slurry and colloidal silica slurry, and finally etched with a solution containing 4.2 g of picric acid, 10 ml of distilled water, 70 ml of ethyl alcohol, and 10 ml of acetic acid. In the SEM observation, cross-section polisher (JEOL IB-19500CP) was used for the final surface treatment. The crystallographic textures of the extruded alloys were analyzed from the patterns obtained by electron backscatter diffraction (EBSD, TSL solutions).

The mechanical properties of the specimens were examined by compression tests. Rectangular specimens with dimensions of approximately 2 mm × 2 mm × 5 mm were cut using electrodischarge machining. Two loading directions were selected for testing: the first parallel to the direction of extrusion and the second inclined at an angle of 45° with respect to the direction of extrusion. Hereafter, they are referred to as the 0° and 45° orientations, respectively. The tests were conducted in a temperature range of 20 (room temperature: RT) to 400°C in vacuum at a nominal strain rate of 1.67 × 10⁻⁴ s⁻¹. Deformation markings were observed using OM with Nomarski interference contrast.

Figures 1(a), (f) show OM images of the microstructures of the as-cast Mg₉₄Zn₂Y₄ and Mg₉₂Zn₃Y₅ alloys. In addition, Figs. 1(b)–(e) and 1(g)–(j) show the variations in the microstructures caused by extrusion as a function of the extrusion ratio for these alloys. As shown in Figs. 1(a), (f), the LPSO-phase and Mg grains show characteristic thin plate-like and round dendrite-like shapes, respectively, in the as-cast alloy. In a previous study, it was reported that the wide interface of the plate-like shape of an LPSO-phase grain is parallel to (0001).²⁾ Plate-like LPSO grains are randomly oriented in the as-cast alloy. In a previous study, the crystal structures of these LPSO phases were confirmed to be of the 18R type.³⁵⁾ In this study, the crystal structure of the rhombohedral 18R LPSO phase is indexed by hexagonal notation by considering a unit cell whose volume is three times larger than that of the rhombohedral cell. This allows us to more easily understand the crystal geometry of the LPSO phase. In the as-cast alloys shown in Figs. 1(a), (f), the volume fractions of the LPSO phases in the Mg₉₄Zn₂Y₄ and Mg₉₂Zn₃Y₅ alloys are 39% and 61%, respectively.

Fig. 1

OM images of the microstructures of the (a), (f) as-cast alloys and (b)–(e), (g)–(j) extruded alloys as a function of the extrusion ratio observed on longitudinal sections along the extrusion direction: (a)–(e) Mg₉₄Zn₂Y₄ and (f)–(j) Mg₉₂Zn₃Y₅ alloys.

After extrusion, the microstructure significantly varied depending on the extrusion ratio. The plate-like shapes of the LPSO-phase grains were rotated to align along the extrusion direction. The directionality of the alignment along the extrusion direction increased as the extrusion ratio increased. In contrast, significant recrystallization occurred in the worked Mg grains during extrusion. Table 1 lists the average Mg grain sizes of the Mg₉₄Zn₂Y₄ and Mg₉₂Zn₃Y₅ alloys. Some worked Mg grains remained in the R2 extruded alloy in Mg₉₄Zn₂Y₄. However, almost all of the worked grains were transformed into fine recrystallized grains in the other alloys, and the grain diameter was almost independent of the extrusion ratio. In contrast to Mg, no dynamic recrystallization occurred in the LPSO phase, even in R10 alloys, owing to its high thermal stability.³⁾ Although recrystallization did not occur, the flat interfaces of the LPSO-phase grains were significantly bent during the extrusion process. This is because of the formation of kink bands.^33,34) This kink-band formation is expected to contribute to the strengthening of the extruded alloys, as discussed later.

Table 1 Average size of the Mg grains in the Mg₉₄Zn₂Y₄ and Mg₉₂Zn₃Y₅ alloys for different extrusion ratios.

The variation in the microstructure was further examined in detail using crystallographic texture analysis. Figures 2(a), (b) show the crystal orientation maps of the Mg and LPSO phases in the Mg₉₄Zn₂Y₄ and Mg₉₂Zn₃Y₅ alloys, analyzed on the transverse sections of the extruded alloys by SEM-EBSD. In addition, the corresponding pole figures for (0001) and $\{ 10\bar{1}0\} $ are shown in Figs. 3(a), (b). Focusing on the LPSO phase, the intensities of the (0001) poles tend to be located at the outer circumference of the pole figure as the extrusion ratio increases. That is, the (0001) planes are aligned parallel to the extrusion direction. This is in good agreement with the observed microstructure shown in Fig. 1, where the plate-like interfaces of the grains are aligned parallel to the extrusion direction. As the extrusion ratio increases, the intensities of the $\{ 10\bar{1}0\} $ poles at the center, i.e., along the extrusion direction, become significant. These features are almost the same as those observed for the Mg₈₉Zn₄Y₇ extruded alloy, which contains 86-vol% LPSO phase.³³⁾ In contrast, the recrystallized Mg grains show considerably randomized texture. The drastic weakening of the texture in the recrystallized Mg grains is the same as that observed for the Mg₉₇Zn₁Y₂ extruded alloy, which contains 24-vol% LPSO phase.¹⁴⁾ The existence of the LPSO phase enhances the grain refinement of the Mg matrix phase during extrusion by inducing a stress concentration around it; the so-called particle-stimulated nucleation (PSN) mechanism is activated.¹⁴⁾ The PSN assists the randomization of texture.¹⁴⁾ In addition, the addition of Y in Mg is known to contribute to the weakening of the texture.^36–38)

Fig. 2

Crystal orientation maps showing the variations in the texture developed in the (a) Mg₉₄Zn₂Y₄ and (b) Mg₉₂Zn₃Y₅ alloys by extrusion at different extrusion ratios. All maps were acquired by SEM-EBSD analysis on transverse sections with respect to the extrusion direction. The upper and lower images show the crystal orientation maps for the Mg and LPSO phases, respectively.

Fig. 3

Corresponding variations in the {0001} and $\{ 10\bar{1}0\} $ pole figures with the extrusion ratio along the transverse direction shown in Fig. 2 for the (a) Mg₉₄Zn₂Y₄ and (b) Mg₉₂Zn₃Y₅ alloys. Upper and lower images show the pole figures for Mg and LPSO phases, respectively.

The variations in the strengths of the alloys with the extrusion ratio were examined using compression tests. Figures 4(a), (b) show the temperature dependencies of the yield stresses of the Mg₉₄Zn₂Y₄ and Mg₉₂Zn₃Y₅ extruded alloys for deformation in the 0° orientation, i.e., parallel to the extrusion direction. For both alloys, the yield stress is drastically increased by extrusion. More precisely, the increase in the yield stress of the R2 extruded alloy compared to that of the as-cast alloy is significant (an increase of approximately 1.8 times), whereas the rate of increase in the yield stress during a further increase in the extrusion ratio is much smaller compared to the drastic increase between the as-cast and R2 alloys. A high yield stress of approximately 250–350 MPa is maintained up to ∼200°C, irrespective of the extrusion ratio for both alloys. The decrease in the yield stress is small even at 300°C, but it drastically decreases at 400°C, accompanied by a decrease in the dependence of the yield stress on the extrusion ratio.

Fig. 4

Temperature dependencies of the yield stresses of the (a) Mg₉₄Zn₂Y₄ and (b) Mg₉₂Zn₃Y₅ as-cast and extruded alloys for different extrusion ratios measured by compression tests in the 0° orientation.

Figures 5(a), (b) show the stress–strain curves obtained from compression tests. In addition, Figs. 5(c), (d) show the variations in the work-hardening rate, Δσ/Δε, measured at 2% plastic strain as a function of the extrusion ratio and test temperature. The equivalent strain applied by extrusion shown along the horizontal axes of the graphs was previously evaluated.¹⁵⁾ At RT and 200°C, the work-hardening rate of the as-cast alloy exhibits a relatively high value of ∼2.5 GPa/100% at RT. The work-hardening rate is slightly lower for the extruded alloys than that of the as-cast alloy, but a high value of ∼1.5–2.0 GPa/100% is maintained for all alloys. The change in the work hardening rate is small between RT and 200°C. However, it largely decreases to ∼1 GPa/100% at 300°C for the extruded alloys, although a high value is maintained for the as-cast alloys. At 400°C, the work-hardening rate is low for all the alloys.

Fig. 5

(a), (b) Typical stress–strain curves from compression tests of the (a) Mg₉₄Zn₂Y₄ and (b) Mg₉₂Zn₃Y₅ as-cast and extruded alloys deformed in the 0° orientation. (c), (d) Variations in the work-hardening rate measured at 2% plastic strain as a function of the temperature and extrusion ratio for the (c) Mg₉₄Zn₂Y₄ and (d) Mg₉₂Zn₃Y₅ alloys.

To clarify the influence of the volume fraction of the LPSO phase on the strengthening behavior by extrusion, the yield stresses of various R10 extruded alloys are compared. Figures 6(a), (b) show the temperature dependence of the yield stress for deformation in the 0° and 45° orientations. The data for Mg₈₉Zn₄Y₇ (LPSO: 86%),³³⁾ Mg₉₇Zn₁Y₂ (LPSO: 24%),^14,20) and Mg_99.2Zn_0.2Y_0.6 Mg solid-solution alloys (LPSO: ∼3%) are also plotted for comparison. That is, the volume fraction of the LPSO phase increases by approximately 20% in each alloy. A strong orientation dependence of the yield stress is observed for all alloys containing the LPSO phase, i.e., except for the Mg_99.2Zn_0.2Y_0.6 alloy. This demonstrates that the LPSO phase strongly affects the yield stress and anisotropic deformation behavior of the two-phase alloys.

Fig. 6

Temperature dependencies of the yield stresses of the R10 extruded alloys compressed in the (a) 0° and (b) 45° orientations. The data for the Mg₉₇Zn₁Y₂ and Mg₈₉Zn₄Y₇ extruded alloys are taken from previous papers.^14,20,33) In addition, the yield stress of the Mg_99.2Zn_0.2Y_0.6 extruded alloy was newly measured and plotted for comparison.

For both orientations, an increase in the volume fraction of the LPSO phase increases the yield stress, but the magnitude differs depending on the loading orientation. For the 45° orientation, the yield stress for Mg₈₉Zn₄Y₇ is approximately two times larger than that for Mg_99.2Zn_0.2Y_0.6, while it is more than three times larger for the 0° orientation. In addition, the rate of increase in the yield stress with increasing LPSO-phase volume fraction is not monotonic for the 0° orientation. The rate of increase in the yield stress shows two large gaps between the Mg_99.2Zn_0.2Y_0.6/Mg₉₇Zn₁Y₂ and Mg₉₂Zn₃Y₅/Mg₈₉Zn₄Y₇ alloys.

Figure 7 shows the typical deformation microstructures after ∼5% plastic deformation. It was not possible to perform a detailed analysis of the Mg grains to determine the operative deformation mode owing to their very refined microstructure, but the LPSO-phase grains could be observed in detail. For the 45° orientation, slip traces parallel to the plate-like interface of the LPSO-phase grains are frequently observed, as indicated by the blue open arrows, demonstrating the operation of basal slip. This results in a lower yield stress. For the 0° orientation, the basal slip traces are accompanied by the formation of beak-like deformation bands, as indicated by the red arrows in the figures. In a previous study of Mg₈₉Zn₄Y₇, the deformation bands were identified as deformation kink bands.³³⁾ The basal fiber texture developed in the LPSO phase by extrusion reduces the Schmid factor (SF) for basal slip, resulting in an increase in the yield stress accompanied by the formation of kink bands. The details of the deformation mechanisms are discussed in the next section.

Fig. 7

OM images showing the typical deformation microstructures introduced in the specimens by the compression tests to ∼5% plastic strain as a function of the temperature, extrusion ratio, and loading orientation: (a) Mg₉₄Zn₂Y₄ and (b) Mg₉₂Zn₃Y₅ alloys.

As a notable finding of this study, the rate of increase in the yield stress with increasing LPSO-phase volume fraction is not monotonic for the R10 extruded alloys deformed in the 0° orientation. The rate of increase in the yield stress showed two large gaps between Mg_99.2Zn_0.2Y_0.6/Mg₉₇Zn₁Y₂ and Mg₉₂Zn₃Y₅/Mg₈₉Zn₄Y₇. We have previously proposed two different strengthening mechanisms for Mg/LPSO two-phase alloys: kink-band strengthening in Mg₈₉Zn₄Y₇ (86-vol% LPSO-phase alloy) and short-fiber-like strengthening in Mg₉₇Zn₁Y₂ (24-vol% LPSO-phase alloy). In this study, the fusion of two strengthening mechanisms is suggested to explain the present results in a unified way, focusing on alloys with an extrusion ratio of 10, i.e., alloys in which the LPSO phases are well aligned along the extrusion direction.

First, the evaluation of the yield stress of the Mg₈₉Zn₄Y₇ alloy with an LPSO-phase volume fraction of 86% is reconsidered. In a previous study on Mg₈₉Zn₄Y₇,³³⁾ the influence of a small amount of Mg phase (14%) was omitted. In this study, the contribution of Mg is precisely reconsidered using the simple rule of mixtures for the strength as follows:   

Using σ_f = ∼496 MPa, the yield stresses of the Mg₉₄Zn₂Y₄ and Mg₉₂Zn₃Y₅ R10 extruded alloys, considering the rule of mixtures in eq. (1), are evaluated to be 336 and 400 MPa, respectively. These values are largely overestimated compared to the experimentally measured values of 301 and 341 MPa for the Mg₉₄Zn₂Y₄ and Mg₉₂Zn₃Y₅ alloys, respectively (Fig. 6). This indicates that the application of the simple rule of mixtures is inappropriate for these alloys. Instead, the short-fiber reinforcement model was applied, as considered for the Mg₉₇Zn₁Y₂ alloy.¹⁴⁾ For the estimation of the yield stress of the alloy with short-fiber reinforcement, σ_f in eq. (1) is modified to $\bar{\sigma }_{f}$ using the following equations:^39,40)   

As another analysis, according to a model for the Mg₈₉Zn₄Y₇ alloy,³³⁾ the contribution of the LPSO phase to the strengthening of the Mg₉₇Zn₁Y₂, Mg₉₄Zn₂Y₄, and Mg₉₂Zn₃Y₅ extruded alloys during deformation in the 0° orientation was considered in detail. That is, the contribution of the reinforcing LPSO phase was subdivided into the base stress, strengthening by texture, and strengthening by kink bands.

The base stress for the LPSO phase corresponds to the yield stress in its as-cast alloy with random texture. This can be quantitatively estimated using the Hall–Petch relationship previously estimated for the LPSO-phase alloy.⁴¹⁾ Then, the volume fraction of the LPSO phase in each alloy was multiplied by the evaluated base stress.

The strengthening effect by the texture was evaluated from the SEM-EBSD results shown in Figs. 2 and 3. From the observations of the deformation microstructures shown in Fig. 7, $(0001)\langle 11\bar{2}0\rangle $ basal slip and the formation of deformation kink bands were identified as the predominant deformation mechanisms for the LPSO phase for the 0° orientation. To consider the operation of basal slip, the average value of its SF for the LPSO-phase grains, SF_ave, was estimated from an analysis of the crystal orientation maps.¹⁴⁾ The average SFs for basal slip in the 0° and 45° loading orientations for the LPSO phases of the Mg₉₄Zn₂Y₄ and Mg₉₂Zn₃Y₅ extruded alloys are shown in Figs. 8(a), (c). The average SF for the 45° loading orientation shows a high value of ∼0.32 and it is almost unchanged, i.e., independent of the extrusion ratio for both alloys. The average SF drastically decreases as the extrusion ratio increases for the 0° orientation owing to the development of basal fiber texture. This is one factor contributing to the increase in the yield stress. Quantitatively, this effect can be estimated by dividing the critical resolved shear stress (CRSS) for basal slip evaluated for the as-cast alloy by the average SF for basal slip in the 0° orientation.³³⁾ Finally, the evaluated value was multiplied by the volume fraction of the LPSO phase. In contrast, the variations in the SF for basal slip with the loading orientation are much smaller for Mg grains, as shown in Figs. 8(b), (d), owing to the weakened texture shown in Fig. 3.

Fig. 8

Average Schmid factors for basal slip in the (a), (c) LPSO-phase and (b), (d) Mg grains in the extruded alloys deformed in the 0° and 45° loading orientations, as evaluated by the crystal orientation maps shown in Fig. 2 and 3: (a), (b) Mg₉₄Zn₂Y₄ and (c), (d) Mg₉₂Zn₃Y₅ alloys.

In addition to the texture effect, the introduction of deformation kink bands during extrusion is expected to contribute to the increase in the yield stress. This effect can be evaluated using the yield stresses of the extruded alloys deformed in the 45° orientation. For the Mg₈₉Zn₄Y₇ alloy, the apparent CRSS for basal slip in the as-cast alloy is ∼45 MPa at RT, but it increases to ∼87 MPa after extrusion for the R10 alloy. This increase in the CRSS is derived from kink-band strengthening.³³⁾ Because a kink-band boundary is introduced nearly perpendicular to the basal slip plane,^2–9) it acts as a strong obstacle against the motion of basal dislocations. The importance of kink-band strengthening has recently been extended to Mg alloys containing LPSO nanoplates.^42,43) The magnitude of “kink-band strengthening” for the 0° orientation can be quantitatively evaluated by dividing the apparent CRSS evaluated for the 45° orientation by the average SF for basal slip in the 0° orientation, and compared them to that in as-cast alloy,³³⁾ since many kink bands are introduced during the extrusion process. Note that the apparent CRSS variations measured for the Mg₈₉Zn₄Y₇ alloy were also used when estimating the kink-band strengthening for the Mg₉₇Zn₁Y₂, Mg₉₄Zn₂Y₄, and Mg₉₂Zn₃Y₅ alloys, as a rough estimate. This is because the apparent CRSS evaluated for the 45° orientation for the Mg₉₇Zn₁Y₂, Mg₉₄Zn₂Y₄, and Mg₉₂Zn₃Y₅ alloys, shown in Fig. 6(b), is largely influenced by the Mg grains. Therefore, the effect of kink-band strengthening in the LPSO phase cannot be precisely evaluated by using them.

Figure 9(a) shows the evaluation results of the classification of the contribution of each strengthening mechanism to the deformation of R10 extruded alloys at RT. The evaluated base stress, texture strengthening component, and kink-band strengthening component for the LPSO phase are indicated by gray, blue, and red bars, respectively, in addition to the stress carried by the Mg grains, shown in green. The results demonstrate that kink-band strengthening exists not only in the Mg₈₉Zn₄Y₇ alloy but also in other alloys in which the volume fraction of the LPSO phase is smaller. For the Mg₈₉Zn₄Y₇ alloy, the contribution of kink-band strengthening is largely overestimated compared to the experimentally evaluated yield stress, as indicated by the red dotted line. This is because kink bands are newly formed as an additional deformation mode during the compression tests. The magnitude of kink-band strengthening decreases as the LPSO-phase volume fraction decreases, but it is still effective even for the Mg₉₄Zn₂Y₄ extruded alloy with 39-vol% LPSO phase, i.e., kink-band strengthening is effective even in the alloys with low volume fraction of the LPSO phase. In the Mg₉₇Zn₁Y₂ alloy with 24-vol% LPSO phase, however, the red region in bar graph was diminished. Note that this does not directly indicate that the contribution of kink-band strengthening itself is vanished. In the present evaluation shown in Figs. 9–11, the yield stress is evaluated under the consideration of “simple rule of mixture”. Therefore, in the alloys in which the yield stress is governed by the “short-fiber reinforcement strengthening”, the components of the contribution of LPSO phase, i.e., the base stress shown as gray bars and strengthening by texture shown as blue bars are somewhat overestimated. Therefore, it is undoubtfully shown that kink-band strengthening is actually effective in Mg₉₄Zn₂Y₄ and Mg₉₂Zn₃Y₅ alloys, but it is difficult to precisely classify the contribution of strengthening mechanisms in the short-fiber reinforced alloys, especially in Mg₉₇Zn₁Y₂ at the present state. Further consideration is required to clarify them in more details.

Fig. 9

Variations in the strengthening factors affecting the yield stresses of Mg/LPSO two-phase extruded alloys, in the various R10 extruded alloys deformed in the 0° orientation at (a) RT, (b) 200 and (c) 300°C.

Figures 9(b), (c) show the variations in the contributions of the strengthening mechanisms with temperature for the R10 alloys. At 200°C, the contributions of the strengthening mechanisms are almost the same as those for deformation at RT for all alloys, and kink-band strengthening still effectively contributes. However, the contribution of kink-band strengthening to the yield stress at 300°C is reduced, especially in the Mg₈₉Zn₄Y₇ alloy with high LPSO-phase volume fraction. One reason for this is that the effect of kink-band strengthening itself becomes drastically weaker at 300°C, which was confirmed in a study using an LPSO single-phase alloy.³⁴⁾ In addition, other accommodation mechanisms related to the operation of non-basal slip and local grain boundary sliding may occur, decreasing the strengthening effect of the LPSO phase. For Mg₉₄Zn₂Y₄ and Mg₉₂Zn₃Y₅ alloys, the sum of the three strengthening components for the LPSO phase and the contribution to strengthening by the Mg matrix grains is lower than the experimentally obtained yield stress. That is, other unexpected strengthening components, shown by the purple bar in the graph, may exist at high temperatures.

Figures 10 and 11 show the variations in the contributions of the strengthening mechanisms with the temperature and extrusion ratio for the Mg₉₄Zn₂Y₄ and Mg₉₂Zn₃Y₅ alloys, respectively. For the Mg₉₂Zn₃Y₅ alloy, kink-band strengthening seems to be effective, even for alloys with low extrusion ratios and high temperatures, as shown in Fig. 11. For the Mg₉₄Zn₂Y₄ alloy, kink-band strengthening is very small or completely diminished, especially for the low-extrusion ratio alloys deformed at low temperatures. Instead, the unexpected strengthening component indicated by the purple bar contributed to the yield stress. One candidate for extra strengthening in alloys with a low extrusion ratio is the influence of work-hardened Mg grains. For the Mg₉₄Zn₂Y₄ alloy with a low volume fraction of the LPSO phase, some worked Mg grains remained in the alloys with a low extrusion ratio, as shown in Fig. 2. This may induce extra hardening due to a high density of dislocations, i.e., an increase in intrinsic work hardening. In addition, Mayama et al. recently examined the deformation behavior and strengthening mechanisms acting in as-cast Mg/LPSO alloys and reported that the Mg/LPSO two-phase interface plays a significant role as an obstacle to dislocation glide.³⁵⁾ A similar effect is expected in the present extruded alloys in deformation at low temperatures. However, the extra strengthening mechanisms at high temperatures have not yet been clarified. As a plausible influence, the variation in high-temperature deformation mechanism depending on the microstructure is supposed. In the previous research focused on the short-fiber reinforcement by LPSO phase,²⁰⁾ the occurrence of localized grain-boundary sliding was supposed at high temperature above 300°C, that would reduce the interface strength between the LPSO phase and Mg matrix grains. The decreases in extrusion ratio and LPSO volume fraction may reduce the formation frequency of tiny recrystallized grains along the LPSO phase, and it might vary the accommodation process related to grain boundary sliding. This may be affected to the “unexpected strengthening behavior”. However, this is still speculation. Further studies focusing on the strain-rate dependence of the flow stress etc. are required to elucidate them.

Fig. 10

Variations in the strengthening factors with the extrusion ratio for Mg₉₄Zn₂Y₄ alloys deformed in the 0° orientation at (a) RT, (b) 200°C, and (c) 300°C.

Fig. 11

Variations in the strengthening factors with the extrusion ratio for Mg₉₂Zn₃Y₅ alloys deformed in the 0° orientation at (a) RT, (b) 200°C, and (c) 300°C.

In this study, the variations in the yield stress with the extrusion ratio and test temperature were examined for the Mg₉₄Zn₂Y₄ and Mg₉₂Zn₃Y₅ alloys. The influence of the volume fraction of the LPSO phase on the strengthening mechanisms of the Mg/LPSO two-phase extruded alloys was discussed on the basis of previously reported results. The conclusions are as follows:

This work was supported by Japan Society for the Promotion of Science (JSPS) KAKENHI for Scientific Research on Innovative Areas “MFS Materials Science” (Grant Numbers: JP18H05478, JP18H05476 and JP18H05475), and partly supported by JSPS KAKENHI JP18H05254, JP21K18826. This work was also supported by Japan Science and Technology Agency (JST), CREST (grant number JPMJCR2094), Japan, and Iketani Science and Technology Foundation, Japan.

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