Effect of Ti Content on Microstructure and Mechanical Properties of Mg-Sn Alloys Produced by Casting and Hot Extrusion

Figure 2 shows the cross-sectional SEM image of as-cast Mg-5Sn-xTi (x = 0.15, 0.75, 1.5 wt.%) alloys. It is obvious that the microstructure of the as-cast samples was strongly influenced by Ti content. In all samples, α-Mg matrix, eutectic of (Mg₂Sn- α-Mg), tiny Mg₂Sn precipitates, and divorced Mg₂Sn phases can be observed. In addition to aforesaid phases, in the sample with 0.75 and 1.5 wt.% Ti, individual and clustered Ti particles were also observed. From Fig. 2(A) and (B), it is clear that the Mg-5Sn sample contained a relatively coarse dendritic structure together with coarse nonhomogeneous Mg₂Sn phases. With respect to the Mg-5Sn sample, the alloy with 0.15 wt.% Ti showed a different microstructure, in which Mg₂Sn precipitate and dendritic structure became finer and Mg₂Sn phase was distributed more uniformly (Fig. 2C and D).

With the further increase in Ti content (Fig. 2E, F, and G), the dendrite structure became non-uniform and the distribution and size of Mg₂Sn phases became coarser and nonhomogeneous. Moreover, in the case of 0.75Ti and 1.5Ti, clusters of Ti particles caused by the agglomeration of Ti particles during melting and solidification were detected.

It is worth noting that the solute atoms can act as a growth-restricting element and are a crucial factor, which improves grain refinement. Studies conducted by Easton et al. (Ref 28) showed that the powerful segregating ability of Ti causes the formation of constitutionally supercooled (CS) area in solid/liquid interface and thus limits the growth of liquid/solid interface leading to a decrease in grain size of alloy. Three factors of cooling rate, the particle size of nucleate, and segregating power of solute element can affect the grain size of cast alloy. During solidification, strong segregation ability of Ti leads to the formation of rich Ti regions in the liquid/solid interface, which results in intensive constitutional supercooling (CS) in front of the growth interface; this can act as a strong growth-restricting element and promote the formation of the nucleus, which will, in turn, lead to refinement of α-Mg grains. Moreover, Ti as a surface-active element, can hinder the diffusion of elements and cause the constitution supercooling (CS) far from liquid/solid interface.

According to the study performed by Turnbull and Vonnegut (Ref 29) who pointed out the effect of crystallography in grain refinement, the criterion for heterogeneous nucleation is that an effective nucleus should have a lattice mismatch factor of δ ≤ 0.005 − 0.015. Ti and Mg both have the same crystallography; Ti and Mg have a similar crystal structure of hexagonal close-packed (hcp) and very close value of lattice constants. Moreover, the lattice misfit between Ti and Mg (σ_α= 8.05% and σ_c=10.1%) is less than 15%. All the mentioned factors make Ti a powerful heterogeneous nucleant in molten Mg. According to the Mg-Ti binary phase diagram, the peritectic transformation of L+(αTi) = Mg occurs at 650 °C, subsequently, the α-Mg phases nuclei form on the Ti particle surfaces. When Ti content exceeds 0.75 wt.%, the α-Mg grains become coarser that is mainly originated by excessive addition of Ti. Due to the large differences in density of Ti and Mg, the Ti particle segregated during solidification, Moreover the large amount of Ti particles in the Mg-Sn melt would be clustered and increased its size, and thus the segregation and clustering of Ti particles leads to reducing the number of effective heterogeneous nucleation suites and consequently leading to coarsening of α-Mg grains.

By comparing the microstructure of samples in Fig. 2, it is clear that the microstructure of 0Ti, 0.75Ti, and 1.5Ti shows the presence of continuous thin plates of Mg₂Sn phases, which were stretched through grain boundaries, whereas for the ST0.15 sample, this feature is less apparent and Mg₂Sn phases distributed randomly both inside and in the grain boundary of this sample.

Figure 2(G) shows the microstructure of 1.5Ti and the high magnification of Ti particles. The surface of Ti particle undergoes transformation on which EDS analysis was conducted (Fig. 2F) to investigate the transformed area. By surveying the phase diagram of Mg-Ti and thermodynamic data (Ref 30), there is no expectation of reaction between Ti and Mg; however, in the Ti-Sn system, the formation of SnTi₃ on rich Ti corner of binary phase diagram has been reported (Ref 31); comparing the EDS analysis, thermodynamic data and phase diagram, it is expected the Ti particle surface transformed to the SnTi₃.

Due to the significant effect of DAS on the mechanical properties of the alloys, the intercept method was applied to evaluate the relation between DAS of Mg-5Sn-xTi (x = 0.15, 0.75, 1.5 wt.%) alloys and Ti content using Eq 1.where n is the number of dendrite arms measured, V is the magnification (considered 400 in this work), and L is the length of the line. The calculated DAS and grain size value are summarized in Table 2. As confirmed by the results, Ti causes a significant decrease in both DAS and grain size values. The minimum DAS was found for Mg-5Sn-0.15Ti wt.%, whereas with further addition of Ti, no significant decrease in DAS was observed.

Figure 3 shows the EDS line scan analysis and mapping of Ti on the surfaces of the 0.15Ti sample. It can be clearly seen that the Ti is distributed homogenously in the Mg matrix, but the concentration of Ti slightly was higher in grain boundaries. From Fig. 3(A), it can be seen that the Mg₂Sn phase was randomly distributed with higher precipitation on grain boundaries.

Before hot extrusion, all the cast Mg-5Sn-xTi (x = 0.15, 0.75, 1.5 wt.%) alloys were subjected to solution treatment, based on Mg-Sn binary phase diagram (Ref 32). The solution treatment temperature was set to 520 °C with peak temperature holding time of 24 h (see Fig. 1). As it can be observed in Fig. 4, Sn went in solid solution and single supersaturated Mg phase was formed in all samples, indicating that the selected temperature and the holding time of 24 hours were high enough to dissolve all Mg₂Sn phase in the Mg matrix. Subsequent quenching of the sample in water prevented diffusion of Sn through Mg matrix and the solid solution of Mg-Sn remained mainly unchanged through cooling from 520 °C to room temperature. Figure 4 shows the SEM image of solutionized and extruded samples. As can be seen in all samples most of the Mg₂Sn phases went into solid solution. All samples show two distinct gray and white areas, which indicate the Mg-rich and Sn-rich areas, respectively. Sn-rich areas formed during the hot extrusion process as a result of the segregation of Sn in the interdendritic area. Figure 4(D) exhibits that the Ti particles were broken into fragments and aligned along the extrusion direction. As mentioned in Fig. 4 with the solution process all the Ti particles transformed into the SnTi₃ phase according to the EDS analysis (Fig. 4E).

By applying load, during the hot extrusion process, the Mg-5Sn matrix undergoes plastic deformation. Generally in composite materials load transfer occurs from matrix to the hard particles, and thus Ti/SnTi₃ particles endure higher load in comparison with the Mg-Sn matrix. As a result of load transfer, the stress concentration exceeds the fracture strength of The Ti/SnTi₃ particles and leads to particle fracture. During deformation, coarser Ti/SnTi₃ particles tend to fracture more easily than smaller ones. First, a larger matrix/particle interface in coarser particles causes more load transfer from matrix to particles and thus endures higher forces. Second, interstice flows play a crucial role in the strength of the particles. The particle size can affect the size of the flaw, i.e., when the particle size increases, it is statistically more likely to contain more flaws. Thus coarse particles tend to break more easily due to the higher content of flaws compared to the smaller particles.

The XRD pattern of as-cast and extruded Mg-5Sn-xTi (x = 0.15, 0.75, 1.5 wt.%) alloys are shown in Fig. 5. XRD analysis was carried out on polished surfaces perpendicular to the extrusion direction. The XRD patterns show the peak related to the Mg and Mg₂Sn phases; no peak related to the Ti or Ti containing compound was found, which should be attributed to its low volume fraction.

When Mg-5Sn-xTi alloys were solution-treated at 520 °C, most of the Mg₂Sn phase dissolved into the α-Mg matrix; during the hot extrusion, the Mg₂Sn phase precipitated (Fig 4A and C), which was confirmed by XRD results. As can be seen in Fig. 5, Mg₂Sn in the sample with 1.5wt.% Ti shows a more intense peak indicating that Ti facilitates the precipitation of the Mg₂Sn phase during hot extrusion. The XRD pattern of samples (Fig. 5) shows that the intensity of peak corresponding to the prism, basal \(\{0002\}\) and pyramidal \(\left\{ {10\overline{1}1} \right\}\) planes varied by Ti content after the hot extrusion process. The \((101)\) plane displayed a stronger peak intensity as the preferred orientation in extruded 0Ti and 0.15Ti samples, while the 0.75 and 1.5 Ti samples show a more intense \((100)\) plane; thus it can be concluded that Ti contributes to the preferred crystal orientation on \((100)\) plane.

Generally, the second phase or reinforcement can affect the basal texture of the Mg crystal (Ref 33), and in this work, we can conclude that by the addition of Ti to Mg-5Sn alloy during hot extrusion most of the prismatic planes were placed perpendicular to the extrusion direction. On the other hand, the intensity of basal plane was maximum in Mg-5Sn alloy and the addition of Ti led to a decrease in the intensity of the basal plane. The preferred orientation of α-Mg was found to be \((101)\) and \((100)\) planes.

The Lotgering analysis (Ref 34) was used for a semi-quantitative estimation of the degree of orientation of grain or texture volume fraction of Mg-5Sn-XTi alloy. The Lotgering factor (LF) is a comparison of the relative X-ray peak intensity of (hkl) planes with all the observed reflections in the XRD spectrum.

The textured fraction of (002), (100), and (101) planes were calculated from XRD patterns using the Lotgering method (Ref 34) according to Eq 2.where \(P_{{({\text{hkl}})}} = \frac{{I_{{({\text{hkl)}}}} }}{{\sum {I_{{({\text{hkl}})}} } }}\) and \(P_{{0({\text{hkl}})}} = \frac{{I_{{0({\text{hkl)}}}} }}{{\sum {I_{{0({\text{hkl}})}} } }}\), with I_(hkl) and I_0(hkl) denoting the intensity of (hkl) peak corresponding to textured and randomly oriented sample, respectively. The LF was calculated for a 2θ scan from 30 to 80°. The calculated Lotgering factor for Mg-5Sn-XTi presented in Fig. 6 shows that with the increase in Ti content up to 0.75 wt.%, the LF decreases; this trend indicates the decrease in texture fraction of (002) plane. Further addition in Ti content led to a slight increase in (002) plane texture fraction. Recent studies showed that the basal plane in Mg plays an important role in ductility of alloys (Ref 35). The addition of alloying elements may trigger basal texture weakening and lead to an increase in the formability of the Mg alloy. Contrary to the (002) plane, the texture fraction of (100) and (101) planes increased as Ti content increased up to 0.75wt.%. Further addition of Ti led to a slight decrease in texture fraction of (100) and (101) planes. Increasing Ti content from 0.75 to 1.5 wt.% led to a decrease in LF for (002), (100), and (101) planes in comparison to the sample with 0.75wt.% Ti. A decrease in LF of all mentioned planes in the 1.5Ti sample can be related to the fracturing of Ti particles, which affect the flow of the Mg-5Sn matrix during hot extrusion as confirmed by Fig. 4D.

Uniaxial tensile and compression test results and the average UTS, yield strength, (YS) and elongation to failure of samples are shown in Fig. 7 and Table 2, respectively. Figure 7 clearly shows that the Ti content induced tangible change in mechanical properties of Mg-5Sn alloys. The results indicated that the addition of 0.15 wt.% Ti led to improve both UTS and elongation to failure to 287 ± 3 MPa and 5.8 ± 0.4% in comparison with the Mg-5Sn alloy with 269 ± 4 MPa and 5.6 ± 0.5%, respectively. The sample with 0.75wt.% Ti showed the maximum UTS of 296 ± 6 MPa, whereas for 1.5 Ti the lowest UTS and elongation of 263 ± 10 MPa and 3.7 ± 0.6% were obtained, respectively.

The lowest UTS and elongation value were attributed to the sample with 1.5wt.% Ti (1.5Ti), which may be due to the non-uniform distribution of Ti particles in the Mg-5Sn matrix; the agglomerated region of Ti particles is shown in Fig. 2(G) and 4(D). In addition, the presence of second hard particles (either Ti or SnTi₃ phases) in the ductile Mg-5Sn matrix caused a decrease in elongation of samples.

It should be noted that in the case of alloy reinforced by particulate phases two factors of (1) distribution of particles in the soft matrix and (2) matrix-particle interfacial bonding condition affect the mechanical properties of the samples (Ref 36, 37).

From Fig. 7(B), it can be seen that the addition of Ti to Mg-5Sn alloy led to increases in both ultimate compressive strength (UCS) and elongation. The results show that the sample with 1.5wt.% Ti has higher UCS and compressive deformation. Meanwhile, the addition of 1.5wt.% Ti to Mg-5Sn alloy led to an increase in UCS and compressive deformation by about 99.5 and 126%, respectively, whereas the compression yield strength (CYS) decreased by about 15%.

From Table 3, the UCS for 0Ti and 0.15Ti sample was obtained to be 240 ± 6 and 374 ± 4 MPa, respectively, which means that addition of 0.15wt.% of Ti led to increased UCS of Mg-5Sn alloy by ~55.8%; this observation indicates the notable impact of solid solution and grain boundary strengthening induced by Ti on UCS of Mg-5Sn alloy. With further addition of Ti, the UCS and compressive deformation continuously increased, while the CYS slightly decreased. The enhancement of compressive strength can be due to the presence of Ti atom as solute and Ti particles as reinforcement; in fact, Ti and SnTi₃ trigger strengthening mechanisms such as solid solution, grain boundary, and dislocation strengthening. The coefficient of thermal expansion (CTE) mismatch between Ti/SnTi₃ and Mg-5Sn matrix can affect the strength of Mg-5Sn alloy; the CTEs of Ti and Mg are \(10.5 \times 10^{ - 6} K^{ - 1}\) and \(25 \times 10^{ - 6} K^{ - 1}\), respectively. The great difference between thermal expansion of Ti and Mg results in the generation of dislocation, which leads to an increase in the strength of Mg-5Sn alloy. Moreover, Ti particles act as a barrier to dislocation movement resulting in the strengthening of the alloy (Ref 38). On the other hand, in addition to solid solution strengthening mechanism, particle strengthening due to the presence of Ti/SnTi₃ particles led to increased UCS of 0.75Ti and 1.5Ti samples by ~75.4 and ~99.5%, respectively, in comparison with 0Ti sample. As reported in Table 3, as the Ti content is increased, the compressive deformation increased continuously; the 1.5 Ti showed the highest compressive deformation of 18.6 ± 3.3%, which indicated increase of ~126.8% in comparison with 0Ti sample. Recent study conducted by Wang et al. (Ref 24) shows that the Ti can increase the generalized stacking-fault energy (GSFE) of (001) (basal plane), leading to activation of slip on lower GSFE planes, which improves the formability of Mg alloy. As discussed in XRD section, with the increase in Ti content the Lotgering factor of (002) plane decreases, indicating decrease in fraction texture of (002) plane (see Fig. 6) and increase in fraction texture of (100) and (101) planes. This observation suggests improvement of deformability for Mg-5Sn-xTi alloy.

By comparing Fig. 7(A) and (B), the major difference between tensile and compression test results is the considerably higher deformation and ultimate strength of the alloys in compression test. As the Ti content increases, the differences between ultimate strength and deformation in the compression and tensile tests increase. The higher differences related to the 1.5Ti sample for which the UCS and deformation in the compression test are about 2 and 4 times those in the tensile test, respectively. Higher UCS of Mg-5Sn-xTi can be related to the higher deformability of the alloy under compression than tension.

Table 3 shows that with an increase in Ti content, the hardness increases. The highest hardness was related to the 0.75 Ti sample. The results show that the hardness of the 1.5 Ti sample slightly decreased in comparison with the 0.75 Ti sample; this observation can be attributed to the non-uniform distribution of Ti particles together with the fracture of Ti/SnTi₃ particles, which led to a decrease in mechanical properties of the alloy; moreover in sample with 1.5wt.% Ti, the extensive reaction of Ti and Mg-Sn matrix led to unclear matrix/particle interface, which deteriorated the YS and UTS.

The fracture surface of the extruded Mg-5Sn-xTi (x = 0.15, 0.75, 1.5 wt %) alloys after tensile test is illustrated in Fig. 8. As can be observed in Fig. 8(B) and (D), the fracture surface of 0Ti and 0.15Ti samples is similar, while the 0 Ti samples represent more microporosity together with more tear ridges and microcracks initiation and propagation; on the other hand, the 0.15 Ti sample shows more ductile features with more ductile dimple and small tear ridges. The 0.75 and 1.5 Ti alloys show ductile dimple, but in comparison with the 0.15 Ti sample the dimple size is larger; they also show more porosity and a higher density of tear ridges. The lower elongation of 0.75 and 0.15 Ti samples can be attributed to their higher porosity and more brittle fracture features in comparison with the 0Ti and 0.15Ti alloys. By comparing Fig. 7 and 8, it can be seen that the results obtained from tensile and compression tests are consistent with the SEM image of fractured surfaces. 0Ti and 1.5Ti samples show higher deformation in tensile mode and more ductile features as confirmed by Fig. 8(B) and (D). The lower UTS and tensile failure strain of the 1.5Ti sample were related to the higher porosity and breakage of Ti particles during hot extrusion. It is noteworthy to mention that as Ti content increased, the mechanical properties in compression mode increased constantly, which can be attributed to the lower sensitivity of mechanical properties to porosity in compression rather than tensile mode (Ref 39).