1 Introduction

Magnesium alloys represent a compelling choice as lightweight structural materials for applications in the automotive and aerospace sectors, primarily owing to their notable attributes of low density and high specific strength. However, the low formability of Mg alloys at ambient temperature is a significant barrier to their widespread use. This is owing to hexagonal close-packed crystal structure of magnesium that offers only two slip systems for deformation at ambient temperature and resulting in low formability (Ref 1, 2). The strong basal texture observed in wrought magnesium alloys, which arises as a consequence of secondary deformation processes such as extrusion, rolling and sheet forming, gives rise to notable anisotropy in their mechanical properties, thereby impacting their suitability for subsequent applications (Ref 3,4,5,6). When magnesium alloys are extruded, a strong final basal texture is produced with the grains aligned in the {0 0 0 1} plane and \(\left\langle {10\overline{1}0} \right\rangle\) direction parallel to the extrusion direction (Ref 3). For the basal slip, the deformation process in magnesium that is most readily triggered, these types of grain orientations are undesirable. When loads are applied parallel to the longitudinal axis of the extruded rod, the presence of strong basal texture further reduces the ductility of the material.

Extruded Mg alloys also demonstrate the tensile–compressive yield asymmetry. Specifically, the tensile yield strength is significantly greater than the compressive yield strength (Ref 7,8,9,10). The yield asymmetry is caused by the strong basal texture and is associated with twinning related issue. Twinning can support tensile strain along the c-axis in the hexagonal close-packed (HCP) crystal structure of magnesium (Mg), but not compressive strain. Twinning is the preferred deformation mechanism during the compression of extruded bars when the basal planes of the crystal structure align parallel to the direction of extrusion (Ref 11). Asymmetry can be reduced by deliberately suppressing the twinning process, which is possible by maintaining a fine average grain size in the extruded material. Apparently, twinning becomes more and more difficult when the grain size is reduced (Ref 3, 12). Grain coarsening is the result of high processing temperatures in extrusion process. The deformation of Mg alloys is only accomplished at the elevated temperatures since only two slip systems are available for the deformation at room temperature. The inclusion of second-phase particles in the morphology can limit grain coarsening during the secondary deformation process. Effective pinning sites for these boundaries can be provided by particles located near grain boundaries or those created by dynamic precipitation within the matrix during extrusion. As a result, they prevent the rapid increase in grain size in the extruded material (Ref 13, 14).

Alloying is the method used for refinement of grains, enhancement of mechanical properties and reducing yield asymmetry in wrought Mg alloys. Recently, it was demonstrated for wrought magnesium alloy that the individual or combined incorporation of rare earth elements (RE) enhances mechanical properties, weakens the texture and reduces the yield asymmetry. The improvement in mechanical properties and reduction in yield asymmetry have been attributed to \(\left\{ {10\overline{1}2} \right\}\) tensile-twin solid solution hardening and texture weakening (Ref 15,16,17,18,19). Bu et al. (Ref 20) studied the addition of Y and Gd in AZ80 alloy and found enhanced tensile and compression properties. In this case, the yield asymmetry was found to be reduced due to promotion of non-basal slip induced with the Gd, Y addition as well as particle-stimulated nucleation (PSN) mechanism induced by Y and Gd particles.

However, RE-free alloys are attractive because they are less expensive, yet only Ca has been shown to imitate the effects of RE texture weakening (Ref 21, 22). In the cast condition, Ca has demonstrated its effectiveness as a grain refiner in AZ81 alloy, up to a 1% addition. However, beyond this point, Ca induces grain coarsening. Moreover, the incorporation of 1% Ca leads to the enhancement of mechanical strength, with a subsequent decline observed beyond this threshold at room temperature (Ref 23). Li et al. (Ref 24) investigated the impact of adding 2, 4 and 6% Ca to AZ91 alloy following extrusion. Their findings indicate that a 2% Ca addition to AZ91 enhances its mechanical properties while surpassing this amount deteriorates the extruded AZ91-Ca alloy’s performance. Allameh et al. (Ref 25) enhanced the mechanical properties of extruded AZ80-Ca by incorporating 0.5% Ca. Che (Ref 26) incorporated 1% Ca along with Re and Si into AZ91, resulting in improved mechanical properties at higher temperatures. These studies highlight the crucial role of Ca addition in enhancing the mechanical properties of extruded AZ series magnesium alloys. In addition to Ca, the other alkaline earth metal, Sr has proved to be the good candidate for reduction in average grain size along with reduction in yield asymmetry of the Mg alloys. Borkar et al. (Ref 27) studied the effect of Sr addition on Mg-Mn alloy. In his investigation, the Mg-Mn-Sr alloys demonstrated a weaker texture attributed to the emergence of randomly oriented texture components during the extrusion process. The occurrence of texture randomization during recrystallization is attributed to particle-stimulated nucleation (PSN) mechanism occurring at the interfaces of Mg-Sr intermetallic. In addition, M1-Sr compositions are discovered to have enhanced yield stress and % elongation as well as decreased yield asymmetry in his investigation. Similar results were obtained by Sadeghi et al. (Ref 28) with Sr addition in AZ31 alloy.

AZ91 is one of the most widely used commercial Mg alloys. Till date, there have been limited studies in the literature on individual Ca and Sr additions to cast or wrought AZ91 alloy and no studies on combined additions of Ca and Sr to wrought AZ91 alloy, specifically with regard to mechanical properties, texture and yield asymmetry. The principal aim of the current study is to produce high-strength extruded AZ91 with reduced tension–compression yield asymmetry by incorporating Ca and Sr, thereby enhancing its suitability for structural applications. In this work, the AZ91-1Ca-xSr alloys were produced with squeeze casting technique and were further subjected to extrusion. The effect of combined alloying additions on microstructure, texture and mechanical properties including yield asymmetry was investigated. An in-depth characterization was also carried out to determine the recrystallization mechanisms responsible for texture weakening. Additionally, the findings from this study are compared with the existing literature on extruded commercial Mg-Al alloys.

2 Experimental Procedure

Casting of the alloys was carried out using the as-received AZ91 alloy and Mg-20 wt.% Ca and Mg-20 wt.% Sr master alloys. The SwamEquip’s bottom pouring-type stir casting machine was used for casting of alloys. The melting was done by using bottom pouring stir casting electrical resistance furnace in the crucible under the inert gas atmosphere of Ar and SF6. The melting setup was equipped with squeeze casting arrangement to obtained squeeze cast alloys. Blocks of AZ91 alloy underwent a melting process at a temperature of 700 °C, and appropriate amounts of Ca and Sr master alloys were introduced into the molten mixture. In order to ensure uniform distribution of the alloying elements, the melt was stirred up to 10 min. Following this, the melt was poured into the mild steel die having diameter 4 cm and length 30 cm and immediate squeezing was done at 100 MPa pressure for 30 s. The cylindrical cast rods were obtained for four targeted alloy compositions with 1 wt.% Ca and varying quantity of Sr (0.3-1.2 wt.%). The targeted and obtained alloy compositions using spark emission spectrometer along with alloy codes used in the present work are presented in Table 1. Thermodynamic analysis has been conducted using Thermo-Calc software and its TCMG6 database to simulate the formation of equilibrium phases.

Table 1 Targeted and obtained chemical compositions (wt.%) of extruded AZ91-1Ca-xSr Mg alloys
Full size table

The machined squeeze cast billets with 4 cm diameter and 15 cm long were further subjected to hot extrusion process using SwamEquip’s laboratory-scale 150-ton hydraulic press. The extrusions were carried out at 400 °C temperature and 1 mm/s initial ram speed with 4:1 extrusion ratio.

Bulk texture analysis was performed on the extruded rod cross section (perpendicular to extrusion direction) using Rigaku Ultima x-ray diffractometer with four circle goniometer in Schulz back-reflection geometry. Three incomplete pole figures \(\left(0^\circ \le \alpha \le 70^\circ \right)\) of \(\left\{ {10\overline{1}0} \right\},\;\left\{ {0002} \right\} \;{\text{and}}\; \left\{ {10\overline{1}1} \right\}\) poles were indexed with Cu-Kα radiation at the operating condition of 40 kV and 40 mA current. Further, bulk texture analysis was carried out by Resmat TexTools software for the orientation distribution function (ODF) calculation. ODFs were used to recalculate the inverse pole figures (IPF).

The extruded specimens underwent standard metallographic preparation, including polishing, followed by etching with an acetic picral solution composed of 2.5 gms of picric acid, 2.5 ml of acetic acid, 45 ml of ethanol and 5 ml of water for microstructure analysis. The optical microstructure was obtained using Zeiss axiovert a1 inverted microscope. JEOL JSM-7610 F plus field emission scanning electron microscope (FESEM) equipped with electron backscattered diffraction (EBSD) was used for microstructure and microtexture characterization. For EBSD analysis, the specimens were mechanically polished with colloidal silica for 1 hr after final polishing with diamond suspensions (with 9 – 1 µm) and the EBSD maps were generated using 20 kV accelerating voltage with 0.9 µm step size. Later, raw EBSD data were analyzed using HKL channel 5 system software.

The mechanical properties of the various compositions of extruded alloys were evaluated using uniaxial tensile and compression tests on a 100kN MTS-610 universal testing machine (UTM) at room temperature. Tensile tests were conducted on samples with gauge lengths of 24 mm and a diameter of 6 mm, initiating at strain rate of 0.001/s. Compression tests were performed on specimens of 6 mm in diameter and 9 mm in length at a constant strain rate of 0.001/s. The samples were cut from the center of the rod along extruded direction. Figure 1 shows the tensile and compression sample dimensions along with their locations in extruded rod.

Fig. 1
figure 1

Geometry of compression and tensile test specimens (all dimensions are in mm)

Full size image

3 Results

3.1 Bulk Texture

X-ray diffraction (XRD) analysis was used to identify the bulk textures in the extruded samples, and the resulting data is represented in the form of inverse pole figures (IPFs) in Fig. 2. All the alloys in the current study including base alloy AZ91 develop a fiber texture in which prismatic planes {10\(\overline{1 }\)0} are perpendicular to the extrusion direction. The maximum texture intensity of IPF represents the strength of the texture. It can be observed that the base alloy AZ91 produces a strong fiber texture featuring maximum texture intensity of 4.5 (Fig. 2a), which further reduces to 4.1 with the incorporation of 1 wt.% Ca and 0.3 wt.% Sr (Fig. 2b). With an increase in Sr content from 0.3 to 0.9 wt.%, the texture intensity further weakens (Fig. 2c and d), demonstrating the effect of increased Sr content in extruded AZ91. The maximum texture intensity remains constant from 0.9 wt.% (Fig. 2d) to 1.2 wt.% Sr addition (Fig. 2e). Therefore, it can be deduced that the addition of Ca and Sr weakens the texture, and this effect is further pronounced with increasing Sr content in extruded AZ91 alloy.

Fig. 2
figure 2

Inverse pole figures (perpendicular to extrusion direction) of extruded samples: (a) E1 (AZ91), (b) E2 (AZ91-1Ca-0.3Sr), (c) E3 (AZ91-1Ca-0.6Sr), (d) E4 (AZ91-1Ca-0.9Sr), (e) E5 (AZ91-1Ca-1.2Sr)

Full size image

An indirect approach was used to assess texture in extruded alloys in order to better understand the texture. The volume fraction of grains featuring basal planes in the radial direction may be equivalently expressed by the volume fraction of grains possessing prismatic planes perpendicular to the extrusion direction, owing to the development of basal fiber texture. This relationship has been previously discussed by one of the authors in this work, Borkar et al. (Ref 27). Also, the measurement of the volume fraction of the prismatic planes based on generated ODF from texture results is given in the literature by Sadeghi et al. (Ref 28). Figure 3 shows that volume fractions of prismatic planes for all extruded AZ91-1Ca-xSr alloys are less compared to extruded base AZ91 (E1 alloy), which exhibits the weakening of the fiber texture with combined Ca and Sr additions.

Fig. 3
figure 3

Volume fraction of the prismatic planes having less than 20 degrees deviation from extrusion direction

Full size image

3.2 Thermodynamic Calculations

Figure 4 illustrates the outcomes of thermodynamic calculations predicting the equilibrium phases of all five alloys examined in the present study, based on their chemical compositions obtained from Table 1. In Fig. 4(a), the results indicate Mg and Mg17Al12 as major phases in AZ91, while other phases show Mg and Al intermetallic containing Mn and Si in very less quantities. During cooling, Mg matrix begins solidifying at 600 °C, whereas precipitation of Mg17Al12 starts at 355 °C. The addition of 1Ca-0.3Sr to AZ91 results in the evolution of Al2Ca and Al4Sr phases at 542.29 °C and 510.07 °C, respectively. For 1Ca-1.2Sr, a new Mg17Sr2 phase evolves after 500 °C. The volume fraction of Mg17Al12, Al2Ca and Al4Sr at room temperature and 400 °C (extrusion temperature in the present work) is tabulated in Table 2.

Fig. 4
figure 4

Calculated equilibrium phases with different temperatures for (a) AZ91, (b) AZ91-1Ca-0.3r, (c) AZ91-1Ca-0.6Sr, (d) AZ91-1Ca-0.9Sr and (e) AZ91-1Ca-1.2Sr

Full size image
Table 2 Volume fraction of Mg17Al12, Al2Ca and Al4Sr at room temperature and 400 °C obtained from Thermo-Calc prediction (in %)
Full size table

The calculated volume fraction of Al2Ca is nearly consistent for all compositions at room temperature. Regarding the Al4Sr phase, an increase in Sr content leads to an increment in the volume fraction at room temperature. A separate graph depicting the evolution of the Al4Sr phase in all AZ91-1Ca-xSr alloys is presented in Fig. 5. It is observed that the volume fraction of Al2Ca and Al4Sr slightly reduces at 400 °C compared to room temperature due to diffusion in the Mg matrix at high temperatures. However, both phases remain thermally stable even at 400 °C, which is the extrusion temperature for the present study.

Fig. 5
figure 5

Calculated volume fraction of Al4Sr in AZ91-1Ca-xSr alloys

Full size image

Notably, with the addition of Ca and Sr, and subsequently with an increase in Sr content, the volume fraction of Mg17Al12 decreases. In contrast, that of Sr-containing precipitates increases at room temperature. Additionally, Mg17Al12 is not found at 400 °C. Therefore, from thermodynamic calculations, it can be predicted that Mg17Al12 does not participate in recrystallization during extrusion at 400 °C. Due to the thermal stability of Al2Ca and Al4Sr at 400 °C, it can be expected that these intermetallic phases primarily act as heterogeneous sites for nucleation and actively participate in recrystallization and texture weakening through particle-stimulated nucleation (PSN) during extrusion.

3.3 Microstructure of Extruded Alloys and Grain Size Measurement

Figure 6 presents the optical micrographs of all extruded AZ91-1Ca-xSr alloy samples with indication of extruded direction (ED). The microstructure analysis of extruded base AZ91 alloy sample (Fig. 6a) presents a bimodal morphology, featuring fine dynamically recrystallized (DRX) grains nucleated during extrusion and coarser deformed grains extended in the extrusion direction without recrystallization. With the addition of Ca and Sr to base AZ91 alloy, the microstructure consists of Al-Ca and Al-Sr stringers elongated in the extrusion direction as indicated in Fig. 6(b-e) for alloy samples E2 to E5. The microstructures of alloy E2 to E5 are seen to be fully recrystallized with some of the recrystallized grains around Al-Ca and Al-Sr stringers. With 1 wt.% Ca + 0.3 wt.% Sr and 1 wt.% Ca + 0.6 wt.%Sr additions to AZ91 (E2 and E3 alloy), the discontinuous network of stringers is obtained, whereas, with 1 wt.% Ca + 0.9 wt.% Sr and 1 wt.% Ca + 1.2 wt.% Sr additions (E4 and E5 alloy), homogeneous and continuous network of stringers is obtained. The incorporation of Ca and Sr to extruded base AZ91 alloy leads to grain size reduction, which has been later confirmed with EBSD analysis in this manuscript.

Fig. 6
figure 6

Optical micrographs of extruded samples (extrusion direction as indicated): (a) E1 (AZ91), (b) E2 (AZ91-1Ca-0.3Sr), (c) E3 (AZ91-1Ca-0.6Sr), (d) E4 (AZ91-1Ca-0.9Sr), (e) E5 (AZ91-1Ca-1.2Sr)

Full size image

Dynamic recrystallization (DRX) mechanisms in magnesium alloys can be broadly divided into continuous DRX (CDRX), twin-induced DRX (TDRX) and discontinuous DRX (DDRX) (Ref 29, 30). In brief, CDRX and TDRX are prominent mechanisms at temperatures lower than 300 °C; conversely, DDRX takes precedence at temperatures above this threshold. (Ref 31). High deformation temperature in the DDRX process results in a little change in the critical resolved shear stress (CRSS) for basal slip but a large drop in the CRSS for other slip systems, reducing the difference between the two (Ref 32). Consequently, the engagement of an enough number of slip systems to introduce plastic deformation results from the initiation of non-basal slip at elevated temperatures (400 °C in the present work) and hinders the initiation of twin formation. As a result, in the present work, extrusion at 400 °C suppresses the development of twins, and therefore, twins are not found in the optical micrographs of extruded AZ91 and AZ91-1Ca-xSr alloys (Fig. 6), and as dislocation slip takes precedence, it facilitates the expansion of grain boundaries into areas characterized by high dislocation densities. The phenomenon of grain boundary migration, which unfolds during subsequent deformation, progressively augments the misorientation between the parent grain and the bulging subgrain, resulting in the generation of a new strain-free grain. The extruded AZ91-1Ca-xSr alloy contains a significant fraction of DRXed grains, and the unDRXed grains in the alloy have a relatively equiaxed morphology. Here, due to the presence of stringers in extruded AZ91-1Ca-xSr alloys, along with grain boundary bulging, particle-stimulated nucleation (PSN) is a probable mechanism for the formation of recrystallized grains around the stringers.

Figure 7 shows the FESEM micrographs of all extruded alloys, along with EDS spectrums of Al-Ca and Al-Sr precipitates. It is observed from the AZ91 micrograph in Fig. 7(a) that the β-phase network is broken into tiny precipitates which are distributed throughout the microstructure. In contrast, AZ91-1Ca-xSr alloys contain the stringer-like structure of secondary precipitates. The well-developed network of stringers can be observed when Sr levels are increased from 0.3 wt.% (alloy E2) to 1.2 wt.% (alloy E5) (Fig. 7b-e). FESEM micrograph findings exhibit congruence with the outcomes derived from optical micrographs (Fig. 6). With the incorporation of Ca and Sr in the base alloy AZ91 up to the levels added in the present work, the Al2Ca and Al4Sr precipitates are formed and can be identified depending on the atomic number of the elements they contain in the FESEM images in BSE mode as shown in Fig. 7. As a result of the greater atomic number of strontium (38) in comparison with other elements, the Al4Sr intermetallic displays a more luminous appearance as compared to Al2Ca. Figure 7(f) shows the EDS spectrum, confirming the more luminous Al-Sr precipitates and the less luminous Al-Ca precipitates. The formation of these precipitates has been discussed in our previous work on Ca and Sr addition in squeeze cast AZ91 alloy (Ref 33). The incorporation of Ca and Sr into the squeeze cast AZ91 alloy led to the development of a continuous and finer secondary interdendritic network. Additionally, this process led to the emergence of lamellar Al2Ca and Al4Sr phases, while simultaneously suppressing the existence of the β-Mg17Al12 phase (Ref 33). The Al2Ca and Al4Sr are thermally stable precipitates with high melting points compared to β-Mg17Al12 precipitates (Ref 33, 34). Therefore, at an extrusion temperature of 400 °C, thermally stable Al2Ca and Al4Sr precipitates are fragmented in the form of discontinuous stringers and are extended in the direction of extrusion (Fig. 7). The thermal stability of Al2Ca and Al4Sr precipitates at 400 °C is also predicted by thermodynamic calculations in section 3.2 in the present work.

Fig. 7
figure 7

FESEM micrographs of extruded samples in BSE mode (extrusion direction is horizontal): (a) E1 (AZ91), (b) E2 (AZ91-1Ca-0.3Sr), (c) E3 (AZ91-1Ca-0.6Sr), (d) E4 (AZ91-1Ca-0.9Sr), (e) E5 (AZ91-1Ca-1.2Sr); (f) EDS spectrum of marked points in the magnified image of rectangular section in the E5 alloy (AZ91-1Ca-1.2Sr) micrograph

Full size image

The grain size of extruded alloys was determined through EBSD analysis. EBSD scans were obtained for all the extruded alloys, and their inverse pole figure (IPF) maps along with their grain size distribution maps are illustrated in Fig. 8. The initial obtained grain size of the squeeze cast AZ91 alloy is 63.02 µm (Ref 33), and after extrusion, the average grain size is reduced to 10.21 µm (Fig. 8a). With the initial addition of Ca and Sr in alloy E2 (Fig. 8b), the grain size is reduced to 8.89 µm, and the distribution range of the grain diameter narrows (Fig. 8f). With an increase in Sr content, the average grain size is further reduced, showing a more homogeneous grain size distribution compared to the base alloy AZ91. The E2, E3, E4 and E5 alloys show 12.93, 38.68, 39.86 and 41.52% reduction in average grain size compared to the base alloy AZ91 (E1), respectively. Figure 9 illustrates the relationship between the increase in Sr content and the corresponding reduction in grain size. Feng Li et al. (Ref 24) and Guan Shao-kang et al. (Ref 35) made similar observations about the effect of calcium (Ca) addition in extruded AZ91 alloy and strontium (Sr) addition in extruded AZ80 alloy, respectively. Here, the reduction in the grain size of extruded AZ91-1Ca-xSr alloys, compared to the base alloy, is attributed to the grain boundary pinning mechanism resulting from secondary Al2Ca and Al4Sr phases. With increasing Sr levels, the quantity of Al4Sr increases, enhancing the grain boundary pinning effect and leading to a further reduction in grain size.

Fig. 8
figure 8

IPF maps along with the grain size distribution maps for extruded (a) E1 (AZ91), (b) E2 (AZ91-1Ca-0.3Sr), (c) E3 (AZ91-1Ca-0.6Sr), (d) E4 (AZ91-1Ca-0.9Sr), (e) E5 (AZ91-1Ca-1.2Sr) (all maps are plotted perpendicular to the extrusion direction), (f) grain size distribution graph

Full size image
Fig. 9
figure 9

Average grain size plot measured from IPF maps of extruded alloys

Full size image

Figure 10 presents the image quality (IQ) map along with IPF map of extruded AZ91 (base E1 alloy) and AZ91-1Ca-1.2Sr (E5 alloy). Considering IQ and IPF maps, the three types of grains can be identified, viz. deformed grains (mainly in base alloy AZ91), larger-sized recrystallized grains and small-sized recrystallized grains. The progressive shift in color in the deformed grains suggests localized strain and the existence of low-angle grain boundaries. Typically, grains displaying low-angle grain boundaries alongside an internal misorientation spread exceeding 2° are categorized as parent grains exhibiting deformation. The color variation within these grains serves as an indicator of localized deformation. Meanwhile, recrystallized grains are distinguished by their uniform coloration and an internal misorientation spread of less than 2°. White-colored grain boundaries correspond to low-angle boundaries, whereas black-colored grain boundaries signify high-angle boundaries. In the AZ91-1Ca-1.2Sr alloy, the Al2Ca and Al4Sr stringers are elongated in the extruded direction (Fig. 6 and 7) and form a neckless-type structure. The EBSD orientation map portrays the stringers with a representation in black color. Recrystallized grains are observed within the matrix and surrounding the stringers within the microstructure. Distinct categorization of these recrystallized grains is achievable based on their proximity to the stringers.

Fig. 10
figure 10

Electron backscattered diffraction (EBSD) microstructure: (a) IQ map, (b) inverse pole figure map, (c) point-to-origin misorientation line profile graph of extruded AZ91 and (d) IQ map, (e) inverse pole figure map, (f) point-to-origin misorientation line profile graph of extruded AZ91-1Ca-1.2Sr (extrusion direction is horizontal and all maps use IPF colors referring to the extrusion direction)

Full size image

Particle-stimulated nucleation (PSN) is a probable mechanism for the formation of recrystallized grains around the stringers in AZ91-1Ca-1.2Sr alloy. There are recrystallized grains around parent deformed grains observed in the microstructure of extruded base AZ91 alloy as well as in AZ91-1Ca-1.2Sr alloys (not associated with stringers) which are most probably formed by grain boundary bulging mechanism, a type of DDRX mechanism. Extruded AZ91-1Ca-1.2Sr exhibits the reduction in average grain size compared to that of base alloy and with the presence of extensive network of stringers (Fig. 6 and 7), and PSN is the crucial recrystallization mechanism that decides the final texture in this alloy. PSN is also expected to affect the final texture in all alloys E2 to E5 with Ca and Sr additions in this study, and the fraction of grains recrystallized by PSN is expected to increase with increasing Sr addition to AZ91.

To study the grain boundary bulging and PSN in detail, the misorientation degrees are calculated for the recrystallized grains associated with parent deformed grains (in AZ91 alloy) and recrystallized grains associated with stringers (in AZ91-1Ca-1.2Sr alloy) in Fig. 10(c) and (f), respectively. It is noteworthy that when the recrystallized grains surrounding the stringers collided with another grain or stringer, their growth stopped. Figure 10(c) presents the inset of Fig. 10(b) indicating the misorientation between the deformed grain and three recrystallized grains associated with the deformed grains. The misorientation profile plotted across the deformed grain and grain associated with deformed grain (line 1, 2 and 3 in Fig. 10c) shows the characteristic misorientation of 15° to 35°, whereas the misorientation profile plotted across the two grains separated by the stringers (line 1, 2 and 3 in Fig. 10f) shows the characteristic misorientation of 55° to 90°. In Fig. 10(c), the recrystallized grains that have originated from parent deformed grains (for AZ91 alloy) are likely a result of the grain boundary bulging mechanism, as evidenced by their relatively smaller difference in misorientation with those of parent deformed grains. In contrast, the recrystallized grains that originated from stringers in AZ91-1Ca-1.2Sr alloy (Fig. 10f) have wider difference in misorientation with each other, which is a characteristic of PSN mechanism produced by second-phase particles in the microstructure. In this study, PSN is most likely generated by intermetallic particles of Al2Ca and Al4Sr present in the form of elongated stringers parallel to extrusion direction which causes texture weakening in the AZ91 alloy.

3.4 Mechanical Properties

The tensile and compressive properties of extruded alloys are plotted in Fig. 11 and 12, respectively, while the numerical values of these properties are presented in Tables 3 and 4, respectively.

Fig. 11
figure 11

(a) Engineering stress strain curve and (b) its respective values of tensile yield stress (TYS), ultimate tensile stress (UTS) and % elongation for tensile test of extruded alloys at room temperature

Full size image
Fig. 12
figure 12

(a) Engineering stress strain curve and (b) its respective values of compressive yield stress (CYS), ultimate compressive stress (UCS) and % fracture strain for compression test of extruded alloys at room temperature

Full size image
Table 3 Tensile properties of extruded alloys
Full size table
Table 4 Compressive properties of extruded alloys
Full size table

Figure 11 and Table 3 show that both the tensile yield strength (TYS) and ultimate tensile strength (UTS) increase upon the initial incorporation of Ca and Sr (1wt.% Ca-0.3wt.% Sr) to AZ91 alloy. This trend of enhanced tensile strength holds consistent across all extruded AZ91-1Ca-xSr alloys. However, a reduction in % elongation (%El) is observed across all AZ91-1Ca-xSr alloys, exhibiting irregular behavior. E4 alloy (extruded AZ91-1wt.% Ca-0.9 wt.% Sr) shows the good combination of all three tensile properties (TYS, UTS and % El). Specifically, compared to the base AZ91 alloy, the TYS and UTS in the E4 alloy exhibits increases of 15.7% and 10.2%, respectively, while the % El shows a significant reduction of 62.8%. Furthermore, as the Sr content is gradually increased to 1.2%, elongation and UTS degrade.

Figure 12 and Table 4 reveal that the CYS increases with the addition of Ca and Sr to AZ91, whereas the UCS and % fracture strain show irregular behavior. The highest CYS and UCS values are obtained with 1wt.% Ca + 1.2wt.% Sr additions (in E5 alloy). The % fracture strain reduces with the addition of Ca + Sr for all studied compositions compared to base AZ91. The E5 alloy exhibit the balanced combinations of all three compressive properties, with improvement of 27.6% and 4.7% in CYS and UCS, respectively, compared to the base AZ91 (E1 alloy).

It is evident that the difference between TYS and UTS is smaller (Fig. 11) compared to the difference between CYS and UCS (Fig. 14). Being the measure of strain hardening, the difference in yield stress and fracture stress in both tensile and compression graphs shows that the strain hardening is higher in the compression than tension for the extruded AZ91-1Ca-xSr alloys. Furthermore, fracture strain (%) is much greater under compression compared to tension.

Figure 13 presents the yield asymmetry (CYS/TYS) for all five extruded alloys and the graph is plotted based on the values obtained from tensile and compressive testing. It can be observed that the CYS/TYS is increasing with combined Ca and Sr additions for all alloys (i.e., yield asymmetry is reducing). The highest CYS/TYS value is obtained for the extruded AZ91-1%Ca-1.2%Sr alloy (E5 alloy).

Fig. 13
figure 13

Yield asymmetry plot (CYS/TYS) for extruded alloys

Full size image

4 Discussion

4.1 Microstructure and Texture Development

The final microstructure and texture development of the extruded alloys in this work are determined by recrystallization mechanisms taking place during and after deformation. The recrystallization mechanisms are influenced by the composition of the alloy while temperature and strain rate of the extrusion process also have an impact. The extruded microstructure is characterized by grain size and the nature of second phases present which also impact the recrystallization mechanisms and the overall texture that is developed after extrusion.

The AZ91 base alloy exhibits a typical magnesium texture following extrusion, primarily due to the occurrence of preferential basal slip during the extrusion process. At 400 °C, the CRSS for non-basal slip systems reduces and comes closer to the CRSS of basal slip systems. Hence, a significant number of slip systems are initiated to introduce plastic deformation and thus obstruct the formation of deformation twinning. The reduction in texture intensity can be observed in all extruded AZ91-1Ca-xSr alloys (E2 to E5) with increase in Sr content compared to extruded AZ91. This shows that the basal texture is weakened with incorporation of Ca and Sr to base AZ91 alloy.

In AZ91-Ca-xSr extrusions, the presence of Al2Ca and Al4Sr precipitates serves as initiation points for the PSN phenomenon by creating localized variations in strain energy. PSN, in turn, produces grains with random orientations, thus diminishing the overall sharpness of the texture. The base AZ91 alloy mainly shows the recrystallization based on grain boundary bulging. The AZ91-1Ca-xSr alloys with 0.9 wt.% and 1.2 wt.% Sr show lower texture intensity compared to other alloys due to higher propensity of PSN with higher second-phase fraction available for PSN in these compositions. Ca and Sr additions to AZ91 alloy also result in grain refinement of the microstructure since both of these alloying elements have demonstrated their effectiveness as grain refiners for Mg alloys in past studies as well (Ref 24, 25, 27, 28).

4.2 Effect of Ca and Sr Additions on Mechanical Properties

The mechanical behavior of extruded Mg alloys is influenced by different strengthening mechanisms mainly solid solution hardening, grain boundary hardening and precipitate hardening. In the extruded AZ91-1Ca-xSr alloys, the solubility of Ca and Sr in the α-Mg matrix is limited, i.e., solubility of Ca in Mg is 0.05 at.% and Sr in Mg is 0.04 at.% (Ref 36, 37). Hence, within all AZ91-1Ca-xSr alloys, only a minimal portion of calcium (Ca) and strontium (Sr) persists in the form of solute atoms within the α-Mg matrix of the extruded AZ91. Consequently, while the presence of dissolved Ca and Sr in the AZ91 alloys does indeed invoke a solid solution hardening effect, its impact remains relatively modest due to the constrained quantity of dissolved calcium and strontium. Hence, Ca and Sr consume Al from Mg17Al12 and form Al2Ca and Al4Sr precipitates (Ref 33). Due to their high melting points, they remain stable at the extrusion temperature (400 °C) and form stringer-like structure. With 1.2% Sr additions, the dense neckless structure of the stringers can be observed (Fig. 6 and 7). The precipitates of Al2Ca and Al4Sr act as the barriers to the dislocation glide and provide the precipitation strengthening effect which further results in enhancement of mechanical properties. Another important aspect of improved mechanical properties of AZ91-1Ca-xSr alloys is the change in the microstructure, i.e., increase in grain boundary density. The grain size reduction with Ca and Sr addition leads to the improvement in mechanical properties of extruded alloys. The addition of 1 wt.% Ca and 0.3 wt.% Sr (in E2 alloy) leads to a significant reduction in grain size compared to extruded base alloy AZ91 (E1). However, subsequent additions of Sr do not lead to a substantial alteration in grain size. A similar trend is also evident in the mechanical properties in tensile and compression testing. With 1% Ca and 0.3% Sr additions (E2), the TYS is improved by 16.62% compared to base AZ91 alloy (E1), whereas the further increase in Sr from 0.3 to 1.2% (E5) leads to only 0.52% improvement in TYS. Similarly, in the compressive testing, the E2 alloy shows 23.07% enhanced CYS compared to E1, whereas E5 showed only 3.7% enhancement in CYS from E2 to E5 alloy.

The intensities of both grain boundary hardening and precipitate hardening exhibit a noticeable rise as the strontium (Sr) content increases. This phenomenon is attributed to reduced average grain size and the rise in the presence of precipitates associated with higher Sr content. As all extruded AZ91-1Ca-xSr alloys feature a fully dynamically recrystallized (DRXed) grain structure, the average grain size in these alloys is primarily influenced by the rate of grain growth of the newly formed DRXed grains. The increase in the quantity of precipitates contributes to an augmented degree of grain boundary pinning, subsequently enhancing the PSN mechanism. This, in turn, results to the formation of smaller DRXed grains and a subsequent reduction in the average grain size.

For temperatures below 350 °C, CDRX and TDRX are the dominant mechanisms for deformation, while DDRX becomes dominant above this temperature. At lower temperatures, texture weakening occurs due to the formation of a necklace structure of fine grains attributed to CDRX and the simultaneous formation of new DRXed grains at the twin boundaries through the TDRX mechanism. However, in the present work, at 400 °C, a network of fine grains is not observed, as CDRX and TDRX are not feasible at this temperature. In comparison, extrusion at higher temperatures exhibits a stronger texture compared to temperatures below 350 °C. Furthermore, an increase in extrusion temperature results in larger grain sizes for both DRXed and unDRXed grains, reducing TYS and CYS values. Therefore, at higher temperatures, PSN plays a major role in strength enhancement and texture weakening since CDRX and TDRX mechanisms are not applicable (Ref 38, 39).

In this study, the initial AZ91 extruded alloy displays a pronounced basal texture, characterized by the alignment of the majority of the basal planes in parallel with the extrusion direction. Consequently, the AZ91 alloy, with its larger grain size, demonstrates the lowest yield strength (TYS). In cases where a material displays a preferred orientation, the yield asymmetry (CYS/TYS) becomes more pronounced. This is because most magnesium (Mg) alloys tend to undergo twinning along the c-axis of the hexagonal close-packed (HCP) unit cell during tensile loading. The basal texture identified in the AZ91 alloy encourages twinning to occur during compression rather than in tension, particularly in alignment with the extrusion direction. Consequently, AZ91 alloy exhibits the highest yield asymmetry, with a CYS/TYS ratio of 0.68. Additionally, the AZ91 alloy, distinguished by its comparatively larger grain size, exhibits the highest fracture strain under compression, thus indicating a significant contribution of twinning to the deformation behavior within this alloy. The incorporation of Ca and Sr elements reduces the yield asymmetry of extruded AZ91 alloy by introducing randomness to the texture and decreasing the grain size. With increasing Sr content, the values of stress parameters such as tensile yield strength (TYS), ultimate tensile strength (UTS) and compressive yield strength (CYS) gradually increase. This phenomenon is attributed to the process of precipitate hardening caused by the presence of Ca and Sr. Hence, the variations in mechanical properties observed in different AZ91-1Ca-xSr alloys in the present work are primarily determined by their texture and hardening behavior.

4.3 Comparison of Tensile and Compressive Properties of Extruded AZ91-Ca-Sr Alloy with Developed Extruded AZ Series Mg Alloys

Table 5 presents the values of tensile properties of the extruded AZ series Mg alloys in the available literature and shows its comparison with the present work. For better clarity, Fig. 14 depicts a UTS versus elongation graph, incorporating the optimized combinations of our developed alloy and those reported in the literature. The prepared extruded AZ91 alloy in the present work showed superior tensile strength with reasonable percentage elongation compared to previously developed extruded AZ91 without alloy additions. Considering E4 alloy (AZ91-1 wt.% Ca-0.9 wt.% Sr), which showed the highest tensile strength in the present work, also exhibits superior tensile strength compared to almost all developed extruded AZ91 with alloy additions (Ca, Sr, Mn, Sn, Si, REs, etc.) except Mg-xAl-3Ca-0.3Mn alloys (x = 5, 6, 7).

Table 5 Tensile properties comparison with the available literature
Full size table
Fig. 14
figure 14

Comparison of ultimate tensile strength (UTS) of extruded base AZ91 and extruded AZ91-1Ca-0.9Sr alloy developed in the present work with previously reported extruded AZ series alloys

Full size image

Table 6 presents the values of compressive properties of the AZ series Mg alloy in the available literature and shows its comparison with present work. Figure 15 illustrates the UCS versus fracture strain, incorporating optimized combinations of developed alloy in the present work and alloys reported in the literature. The prepared extruded AZ91 alloy in the present work showed all three enhanced compressive properties compared to previously developed extruded AZ91 without alloy additions. Also, the E5 alloy (AZ91-1 wt.% Ca-1.2 wt.% Sr) showed the highest compressive strength with good fracture strain, standing at a very good position among all developed extruded AZ91 with alloy additions. Hence, extruded AZ91-1Ca-xSr alloy developed in the present work are the promising candidates for the structural applications considering developed extruded AZ series Mg alloys.

Table 6 Compressive properties comparison with the available literature
Full size table
Fig. 15
figure 15

Comparison of ultimate compressive strength (UCS) of extruded base AZ91 and extruded AZ91-1Ca-1.2Sr alloy developed in the present work with previously reported extruded AZ series alloys

Full size image

5 Conclusion

In the present work, the AZ91-1Ca-xSr alloys with different compositions were produced with squeeze casting technique followed by hot extrusion. The effect of combined Ca and Sr additions on microstructure, texture and mechanical properties in extruded AZ91-1Ca-xSr alloys was studied. The prevailing recrystallization mechanisms and the role of second phases in the deformation behavior were also subjected to detailed characterization. The conclusions from the study are summarized below:

  1. (1)

    Extruded microstructure of AZ91 shows the bimodal structure consisting of DDRX grains and deformed grains, whereas extruded microstructures of AZ91-1Ca-xSr alloys show fully recrystallized grains consisting of intermetallic Al-Ca and Al-Sr stringers elongated in the extrusion direction. Thermodynamic calculations predict the stability of these precipitates at 400 °C extrusion temperatures, actively participating in recrystallization. The precipitates are found to be discontinuous in alloys with 0.3% Sr and 0.6% Sr while they form continuous necklace-like structure of stringers in 0.9% Sr and 1.2% Sr alloys. Recrystallization occurs at the boundaries of parent grains as well as around the stringers.

  2. (2)

    The average grain size is observed to be reduced with combined incorporation of Ca and Sr to AZ91 after extrusion. The reduction in the grain size of extruded AZ91-1Ca-xSr alloys compared to base alloy is attributed to the grain boundary pinning mechanism due to secondary Al2Ca and Al4Sr phases.

  3. (3)

    In extruded base AZ91 alloy, new grains formed near deformed grains undergo recrystallization through the grain boundary bulging mechanism. In AZ91-1Ca-xSr alloys, new grains associated with Al-Ca and Al-Sr stringers undergo recrystallization through the particle-stimulated nucleation (PSN), while grains away from stringers recrystallize either via the grain boundary bulging or DDRX mechanism.

  4. (4)

    Combined additions of Ca and Sr lead to texture weakening of extruded AZ91 alloy. The Al2Ca and Al4Sr precipitates within AZ91-1Ca-xSr extrusions serve as sites for the initiation of the PSN mechanism by inducing local variations in strain energy. PSN, in response, generates nuclei with random orientations, ultimately leading to a reduction in the overall texture sharpness.

  5. (5)

    Grain size reduction and precipitate strengthening are mainly responsible for enhancement of strength of extruded AZ91-1Ca-xSr alloys. The reduction in yield asymmetry observed in the AZ91-1Ca-xSr alloys can be ascribed to the collective impacts of grain size reduction and weakened texture resulting from PSN mechanism promoted by Al2Ca and Al4Sr intermetallic.

  6. (6)

    E4 alloy (AZ91-1 wt.% Ca-0.9 wt.% Sr) exhibits the highest tensile strength in this study, surpassing the majority of the developed extruded AZ series alloys with various alloy additions. Furthermore, E5 alloy (AZ91-1 wt.% Ca-1.2 wt.% Sr) demonstrates the highest compressive strength and exhibits high fracture strain, standing at a very good position among all developed extruded AZ series Mg alloys with alloy additions. Compared to the literature on individual Ca and Sr additions to extruded AZ91, the combined addition of Ca and Sr to extruded AZ91 yields superior tensile and compressive properties. Hence, the extruded AZ91-1Ca-0.9Sr and AZ91-1Ca-1.2Sr alloys developed in the present work emerge as promising candidates for structural applications, particularly when compared to previously developed extruded AZ series Mg alloys.