Effects of Al and V on deformation behavior and microscopic mechanism of titanium alloy at ultra-low temperature using molecular dynamics

A uniaxial tensile was executed with the molecular dynamics method, while the interaction between Ti, Al and V was described by hybrid potential. The influences of Al and V elements on the plastic deformation behavior and microscopic deformation mechanism of Ti-Al-V alloy were studied under ultra-low temperatures. The results show that the high content of Al and V elements leads to a sudden decrease of plasticity at 300 K. The plasticity is slightly reduced with increasing Al content when the content of V is 3.3~6.2 wt.% at 77 K. However, with the temperature reduced to 50 K, Al promotes the activation of dislocation when the content of V is high (6.2 wt.%) and hinders the activation of dislocation when V content is low (0.4 wt.%). At high V content (6.2 wt.%), the increase in plasticity with increasing Al content is more pronounced. Therefore, the strength and plasticity of Ti6.5Al6.2V increase with decreasing temperature.


Introduction
Titanium alloys have a high specific strength, low thermal conductivity, and excellent high and lowtemperature properties, and are widely [1][2][3] used in aerospace.At present, Ti6Al4V alloy with α+β biphase structure has been widely used in liquid hydrogen vessels, liquid hydrogen conduits, and other cryogenic environment components [4] .However, it has been found that the stress-strain curve of Ti6Al4V is prone to "zigzag" fluctuations when the deformation temperature is lower than 77 K.The uniform plastic strain capacity of Ti6Al4V decreases significantly (uniform plastic strain <2%) with the further reduction of deformation temperature (for example, 20 K liquid hydrogen environment).This severely limits the application [5] of Ti6Al4V at ultra-low temperatures.
The Al and V elements in Ti6Al4Valloy are α and β stable elements, respectively.At present, the research on the influence of alloying elements in titanium alloy mostly focuses on the influence of alloying element content on the phase composition, phase content, microstructure morphology, and mechanical properties, etc. [6,7] , but the research on the microplastic deformation mechanism of alloying elements lacks quantitative characterization.In addition, there are few reports on the effects of two or more alloying elements on the mechanical properties/microplastic deformation mechanism.For example, Wei et al. [8] conducted cold rolling on Ti-5 at.% to study its phase transformation, deformation mechanism, and relationship.Subsequently, Zhang et al. [9] observed the dynamic twinning process through molecular dynamics and verified the formation mechanism of {1011} twins observed in the experiment.
Based on the above reasons, given Ti6Al4V alloy widely used at ultra-low temperatures, the influence of Al and V alloy elements on the deformation behavior and microscopic plastic deformation mechanism of Ti6Al4V alloy at ultra-low temperatures was studied.It is of great significance to reveal the microscopic mechanism of Ti6Al4V alloy's significant decrease in uniform plasticity at ultra-low temperature and to develop new ultra-low temperature titanium alloys.
In this paper, a uniaxial tensile model of titanium alloy with different contents of Al and V was established by the molecular dynamics method.The influence of Al and V on the deformation behavior and microscopic deformation mechanism of Ti6Al4V alloy under ultra-low temperature deformation was revealed by stress-strain curve, dislocation, and type analysis.

Calculation method and model establishment
LAMMPS was used for all the molecular dynamics simulations in this paper.The atomic model was constructed by Atomsk.All visualization operations and some post-processing were carried out in OVITO.
The uniaxial tensile model is shown in Figure 1, where the lattice parameter a is 2.9527 Å and the axial ratio is 1.5853 for the HCP structure single crystal model.The size of the tensile model is 81.8 Å× 318.3 Å× 82.7 Å, which contains 121856 atoms.[-1100], [0001], [11-20] crystal directions are parallel to x, y, and z, respectively, and tensile loads are applied along the y direction.The content models for the different alloy elements were constructed by randomly replacing the original Ti atoms with Al and V atoms, as shown in Figure 1.Nine components (Al: 4.5~6.5 wt.%; V: 0.4~6.2wt.% [6] ) Ti-Al-V alloy was simulated at 300 K, 77 Kand 50 K, and the contents of Al and V elements are shown in Table .1The hybrid potential was used to describe the interaction between Ti, Al and V elements [2] .The interaction between Ti and Al is described by the Embedded Atom Method (EAM) developed by Zope and Mishin [10] .The interaction between Ti and V is described by the Modified Embedded Atom Method (MEAM) developed by Maisel et al. [11] , and the interaction between Al and V is described by the MEAM potential developed by Shim et al. [12] .
The initial model first underwent energy minimization, then structural relaxation of 100ps (timestep 0.001 ps) under NPT ensemble, followed by stretching along the Y-axis (i.e.[0001]) under NVT ensemble at a strain rate of 109 s -1 .Periodic boundary conditions were used in all directions.

Influence of alloying elements on plastic deformation behavior 3.1.1 Strength and plastic distribution caused by composition and temperature. The stress-strain curves
of Ti-Al-V alloy by molecular dynamics tensile simulation at 300 K, 77 K, and 50 K are shown in Figures 2(a), (b), and (c) at high (6.2 wt.%), medium (3.3 wt.%) and low (0.4 wt.%) V content, respectively.At 300 K temperature, when the content of V is high (6.2 wt.%), the fracture strain increases obviously with the increase of the content of Al, and there is no significant difference in strength.With the decrease of V content, the strength increases from 10.28 GPa to 13.36 GPa, the strength difference caused by the Al element begins to appear, and the difference of fracture strain starts to narrow.When the temperature is reduced to 50 K, the fracture strain decreases with the increase of element Al content, and the effect becomes more obvious with the decrease of element V content.Take the peak value of stress reached for the first time as strength σ1, which is used to characterize the strength of the material, and take the difference between the strain when the stress drops to 80% and the strain when the peak stress, denoting the plasticity of the material as ε1.The strength and plasticity distributions of different component models at different temperatures are shown in Figure 3.The results show that as the temperature drops, the strength of Ti-Al-V alloy increases, the plasticity decreases, and the distribution area shifts from lower right to upper left.In general, when the composition is the same, the strength increases and the plasticity decreases with the decrease of temperature.It is worth noting that Ti6.5Al6.2Vhas low ductility and strength at room temperature, but increases both strength and ductility with decreasing temperature.At 300 K, the plasticity increases with increasing V and Al but decreases to a certain extent at high V and high Al content (Ti6.5Al6.2V).When the temperature decreased to 77 K, the plasticity still increases with the increase of element V content, but at higher V content (3.3~6.2 wt.%), the plasticity decreases slightly with increasing Al.However, when the temperature is further reduced to 50 K, the increase in both V and Al increases the plasticity, and the effect of Al becomes more obvious with higher V content.

Analysis of dislocation length and type
The dislocation type and length during stretching were calculated by a dislocation analysis method in OVITO (DXA).The maximum dislocation length in the stretch model is shown in Figure 5(a), and the fraction of each type of dislocation when the maximum dislocation length is reached is shown in Figure 5  The results show that, in general, the maximum dislocation density in stretching decreases with decreasing temperature.However, in the case of low Al and low V and high Al and high V, there is a larger dislocation density at 50 K than 77 K.At 300 K temperature, the maximum dislocation length in the tensile model is the lowest at low Al and high V, and the dislocation density increases with the increase of Al content and the decrease of V content.However, at 50 K temperature, the influence of Al on the dislocation length showed different trends at different V content levels.Under high V conditions, dislocation length increases with the increase of Al content; but under low V conditions, dislocation length decreases with Al content increasing.At 77 K temperature, the dislocation length is also significantly affected by the content of element V, but not significantly affected by the content of element Al.
In the stretching process, Shockley partial dislocation 1/3<-1100>, mixed dislocation and 1/3<11-20> total dislocation dominate.Figure 5(b) shows the percentage of each type of dislocation to the total dislocation length and the dependent variable when the total dislocation length reaches its peak.The results show that when V content is low, more mixed dislocations are generated at all temperatures, and the peak dislocation length can only be reached at higher stress variables.Under the condition of moderate V content, the model produced more than 1/3<11-20> dislocations during the stretching process.As the temperature decreases, the V content decreases and the Al content decreases, and the stress variable for the dislocation length to reach its peak increases.

Conclusion
In this paper, the effects of temperature and composition on uniaxial tensile of α-phase titanium alloy in the range of 300 K~50 K were studied by molecular dynamics simulation method, and the following conclusions were obtained: (1) With the decrease in temperature, the strength of Ti-Al-V alloy increases, and the plasticity decreases.At 50 K temperature and higher V content (3.3~6.2 wt.%), ductility increases with the increase of Al content, showing an opposite trend to 77 K.
(2) At 50 K temperature and 6.2 wt.% V content, Al promotes dislocation growth.However, at 0.4 wt.% V content, Al retarded dislocation growth.As a result, the strength and plasticity of Ti6.5Al6.2Vincrease with the decrease in temperature.

Figure 1 .
Figure 1.Crystal orientation and element distribution in the uniaxial tensile model (blue is Ti atom, green is Al atom, and red is V atom).

Figure 2 .
Figure 2. Stress-strain curves of each model at 300 K. (a) Variation of Al content under high V element; (b) Variation of Al content under median V element; (c) Variation of Al content under low V element.

Figure 3 .
Figure 3. Yield strength and plasticity distribution of each model at different temperatures.

3. 1 . 2
Influence trends of composition and temperature on strength and plasticity.The influence trend of composition and temperature on strength and plasticity is shown in Figure4.As the content of element V increases, the strength of the model, σ1, decreases.When the content of element V is constant, the strength of the model decreases as the content of element Al increases.It is noteworthy that the softening effect of element Al decreases with the increase of element V.

Figure 5 .
Figure 5. (a) 3D incremental histogram of maximum dislocation length during stretching; (b) The stacked bar chart shows the proportion of each type of dislocation to the total dislocation length fraction when the maximum dislocation length is reached.The broken line chart shows the corresponding strain when the maximum dislocation length is reached.

Table 1 .
. Contents of Al and V elements in each model.