Dynamic compressive property of extruded VW75-Ti (wt.%) alloy under high strain rates

The dynamic compressive properties of extruded VW75 and VW75+Ti alloys were investigated by using a split Hopkinson pressure bar (SHPB) at 25°C under high strain rates of 1, 600-2, 700 s-1 in the present work, and the microstructure evolution after deformation was inspected by OM, SEM, and EBSD. The results demonstrated that the maximum stress value rises with the escalating strain rate in the course of impact, and the true stress-true strain curves of the two alloys exhibit strain strengthening and positive strain effect. The addition of Ti particles refines alloy grains, increases fracture strength, and decreases the twinning ratio.


Introduction
Magnesium alloys offer the benefits of low density, high specific strength, and high specific stiffness, and are employed in the automotive industry, aerospace, rail transit, and other fields [1] .With the expansion of magnesium alloy application fields, the service environment of magnesium alloys has become increasingly complex, and magnesium alloy components are very vulnerable to high-speed impact during the use of automobile bodies, wheel hubs, armor plate missiles, etc. Studying the dynamic deformation behavior of magnesium alloys is essential [2] .
Magnesium matrix composites, leveraging the benefits of both the magnesium matrix and reinforcement, have garnered significant attention.The reinforcement phases are mainly particles, fibers, and whiskers.Ceramic particles are in a brittle phase and have a weak deformation ability in coordination with the magnesium matrix.Fiber reinforcements have poor wettability with magnesium matrix, so it is usually necessary to coat the fiber surface.Affected by the surface van der Waals force, whisker reinforcement is not easily dispersed, and the whisker orientation is random, resulting in an increased cracking tendency of magnesium matrix composites [3] .Due to the inherent deficiency of ceramic particles, fibers, and whiskers as reinforcement, finding more suitable reinforcement is the focal point of current research.Some scholars have found that adding metal particle reinforcement with good plasticity and good wettability with magnesium matrix can enhance the comprehensive properties of the alloy.Typical metal particles include Ti particles, high entropy metal particles, Cu particles, etc. Ti has low density, a close-packed hexagonal structure with a magnesium matrix, and excellent wettability with magnesium melt [4] .At the same time, there is no brittle intermetallic compound formation between Ti and Mg, so it is very suitable to use as a strengthening phase of metal composites.Kurt and Kurnaz [5] found that the addition of titanium particles can significantly refine alloy grains and enhance the strength of magnesium alloys, but there are few investigations on the high-speed impact resistance of magnesium alloys reinforced by Ti particles.Therefore, this paper carried out a study on the high-speed impact resistance behavior of VW75 magnesium alloys reinforced by Ti particles, hoping to further expand the application range of magnesium matrix composites.

Experiment
VW75 and VW75+Ti (wt.%) alloys were chosen as the research subjects for this study.The VW75 magnesium alloy is melted and mixed with Ti particles at higher melting temperatures (between 993 K and 1,023 K).Mechanical stirring speeds up the dispersion of Ti particles and quickly cools and obtains the ingot of Ф120×350 mm.After homogenizing the as-cast VW75 and VW75+Ti alloys for 24 h at 793 K, isothermal extrusion was performed to obtain a size of 25 mm bars.During the process, the extrusion ratio and extrusion speed are 23 and 0.6 mm/s, respectively.
Ф8×6 mm cylindrical sample was chosen as the impact test sample along the direction of extrusion.Using a SHPB (Figure 1), high-speed impact tests were carried out on the extruded VW75 and VW75+Ti alloys at 25°C.Extruded VW75 and VW75+Ti alloys were taken through a series of dynamic deformation tests at 25℃ with SHPB at high strain rates ranging from 1,600 s -1 to 2,700 s -1 , with the direction of extrusion aligning to the direction of load-bearing.The microstructure and fracture morphology were evaluated by using SEM, OM, and EBSD.The EBSD sample electrolytic polishing electrolyte was 20% nitric acid-alcohol solution and electrolyzed for 30 s under the conditions of a DC power supply voltage of 20 V and -30℃.

Results and discussion
The true stress-true strain curves and macroscopic morphology of VW75 and VW75+Ti alloy at highspeed impact are displayed in Figure 2. From Figure 2 (a), with increasing strain rate, the samples are continuously compressed and deformed, and the strain variable is continuously increased until the samples finally break into three pieces along the direction of 45°.The true stress-true strain curves of the two alloys' impacts at various strain rates are given in Figures 2 (b) and (c).The samples' maximum stress values gradually rise during the impact process as the strain rate rises, demonstrating both the positive strain effect and the strain strengthening effect.VW75 alloy specimens fracture at a strain rate of 2,700 s -1 , in which the fracture strain and fracture strength are 22% and 435 MPa, respectively.VW75+Ti alloy specimens fracture at a strain rate of 2,400 s -1 , in which the fracture strain and fracture strength are 17% and 449 MPa, respectively.According to Figure 2 (d)-Figure 2 (f), at the same strain rate, the true stress-true strain curves of the VW75+Ti alloys are higher than those of the VW75 alloys.VW75+Ti alloys break sooner than VW75 alloys, with higher breaking stress and smaller breaking strains.The addition of Ti particles increases fracture strength and decreases fracture plasticity of VW75 alloy.Figure 3 shows the longitudinal cross-section microstructure of VW75 and VW75+Ti alloys before and after dynamic deformation.The alloy microstructure after extrusion is equiaxial with no twins.The average grain sizes of VW75 alloy and VW75+Ti alloy are 46.6 μm and 24.6 μm respectively, as shown in Figures 3 (a) and (d).The incorporation of Ti particles significantly refined the alloy grains in Figures 3 (g) and (h).When the specimen was deformed according to the microstructure of high-speed impact, the number of twins increased with increasing strain rate in Figures 3 (b), (c), (e), and (f) and that of the VW75 + 2Ti alloys was less than that of the VW75 alloys under the same strain rate.
Slip and twinning play a decisive role in coordinated deformation at room temperature and high strain rate deformation [6] .When the sample is impacted by a high strain rate, twinning plays a major role, and twins are generated in the crystal.With the generation of twins, the mother crystal is divided into several pieces by twins, which increases the number of interfaces in the sample and becomes more serious.The interaction between twins and dislocation becomes more intense, and with the increase of the deformation rate, the percentage of twins increases.
The fracture morphology of the two alloys following impact is depicted in Figure 4, and the fracture characteristics of the alloy are mixed with toughness and brittleness.According to Figures 4 (b) and (e), the fracture surfaces of the two alloys are smooth and show a large number of step morphologies.There are some bright tearing ridges and tongue patterns on the step plane.There are small dimples in part of the fracture in Figures 4 (c) and (f), and in Figures 4 (a) and (d), there are fish scale structures.The fish scale structure appears on the edge of the smooth platform region, because under the condition of a high strain rate, the deformation of the material generates an enormous quantity of heat, causing the adiabatic temperature inside the material to rise and causing the temperature in the local area to exceed the melting point instantaneously, resulting in a melting phenomenon, and forming a heat softening zone.At the same high strain rate, the VW75+Ti alloy has more fish scale structure than the VW75 alloy.Cracks start at the edge of the thermal softening zone, expand rapidly, and finally lead to sample fracture.Therefore, the VW75+Ti alloy breaks before the VW75 alloy.The mechanism by which Ti particles affect the high-speed impact resistance of VW75 alloys is depicted in Figure 5.The VW75 alloys showed a 47% refinement in grain size during high-temperature extrusion with the Ti particle addition, and the grain boundaries increased.When VW75+Ti alloy is impacted by a high strain rate, the grain boundary obstructs the dislocation movement and increases the work-hardening ability, and the fracture strength of the alloy increases, but the fracture strain decreases.At the same time, during the impact process, with the grain boundary area growing, the coordination deformation ability between grains is improved, which can partially absorb energy.When subjected to high strain rate impacts, the grains are prone to rotation and the proportion of twinning decreases.Therefore, there are few twins in the VW75+Ti alloy, and the addition of Ti particles can reduce the twinning to some extent.

Conclusions
In this paper, the effect of Ti particle addition on the high-speed impact resistance of VW75 alloy was studied by using SHPB.
(1).The true stress-true strain curves of VW75 and VW75+Ti alloys have no obvious yield point and show strain hardening and normal strain effects.At the same strain rate, true stress-true strain curves of the VW75+Ti alloy are higher than those of the VW75 alloy.
(2).Ti granules on the VW75 alloy had a grain refinement effect, and the grain size average dropped from 46.6 μm to 24.6 μm.After high-speed impact, twins are formed inside the grain, the quantity of twins in VW75+Ti alloy is small, and Ti particles reduce the formation of twins.
(3).The two kinds of alloy fracture present ductile and brittle mixed fracture characteristics.The addition of Ti particles to refined alloy grains increases the dynamic fracture strength of the alloy.

Figure 2 .
Figure 2. Macroscopic morphology and true stress-true strain curves of VW75 and VW75+Ti.Figure3shows the longitudinal cross-section microstructure of VW75 and VW75+Ti alloys before and after dynamic deformation.The alloy microstructure after extrusion is equiaxial with no twins.The average grain sizes of VW75 alloy and VW75+Ti alloy are 46.6 μm and 24.6 μm respectively, as shown in Figures3 (a) and (d).The incorporation of Ti particles significantly refined the alloy grains in Figures3 (g) and (h).When the specimen was deformed according to the microstructure of high-speed impact, the number of twins increased with increasing strain rate in Figures3 (b), (c), (e), and (f) and that of the VW75 + 2Ti alloys was less than that of the VW75 alloys under the same strain rate.Slip and twinning play a decisive role in coordinated deformation at room temperature and high strain rate deformation[6] .When the sample is impacted by a high strain rate, twinning plays a major role, and twins are generated in the crystal.With the generation of twins, the mother crystal is divided into several pieces by twins, which increases the number of interfaces in the sample and becomes more serious.The interaction between twins and dislocation becomes more intense, and with the increase of the deformation rate, the percentage of twins increases.The fracture morphology of the two alloys following impact is depicted in Figure4, and the fracture characteristics of the alloy are mixed with toughness and brittleness.According to Figures4 (b) and (e), the fracture surfaces of the two alloys are smooth and show a large number of step morphologies.There are some bright tearing ridges and tongue patterns on the step plane.There are small dimples in part of the fracture in Figures4 (c) and (f), and in Figures4 (a) and (d), there are fish scale structures.The fish scale structure appears on the edge of the smooth platform region, because under the condition of a high strain rate, the deformation of the material generates an enormous quantity of heat, causing the adiabatic temperature inside the material to rise and causing the temperature in the local area to exceed the melting point instantaneously, resulting in a melting phenomenon, and forming a heat softening zone.At the same high strain rate, the VW75+Ti alloy has more fish scale structure than the VW75 alloy.Cracks start at the edge of the thermal softening zone, expand rapidly, and finally lead to sample fracture.Therefore, the VW75+Ti alloy breaks before the VW75 alloy.

Figure 4 .
Figure 4. Fracture morphology after impact of extruded VW75 and VW75+Ti alloy.The mechanism by which Ti particles affect the high-speed impact resistance of VW75 alloys is depicted in Figure5.The VW75 alloys showed a 47% refinement in grain size during high-temperature extrusion with the Ti particle addition, and the grain boundaries increased.When VW75+Ti alloy is impacted by a high strain rate, the grain boundary obstructs the dislocation movement and increases the

Figure 5 .
Figure 5.Effect of Ti particles on VW75 alloy.