Influence of Alloying Elements Content on High Temperature Properties of Ti-V-Cr and Ti-Al-V Series Titanium Alloys: A JMatPro Program Calculation Study

For the issues of high temperature performance affected by the alloying elements content in Ti-V-Cr and Ti-V-Cr alloys, the thermodynamic calculation method based on JMatPro program was applied in this study. The research is mainly focused on the analysis of phase composition, thermodynamic parameters and mechanical properties of Ti-Al-V and Ti-V-Cr series alloys with different element proportions under high temperature environment. Those obtained results show that the proportion of Al in Ti-Al-V alloys has a great influence on the high temperature properties. Increasing the content of Al not only increases the transformation temperature of β single-phase structure and delays the transformation process of α/β microstructure to β single-phase structure, but also helps to improve the high temperature thermal conductivity and elastic deformation resistance of the alloy. In Ti-V-Cr alloys, the influence of V element on high temperature properties is mainly focused on the improvement of thermal conductivity and high temperature deformation properties, while the influence of Cr element is relatively weak. Besides, adding a small amount of Al element to Ti-V-Cr alloy can further improve the thermal conductivity of the alloy. The Young’s modulus of the Ti-V-Cr alloy increases when 0.3%-1% of C element is added. Finally, the effect of Si element on the high temperature elastic deformation of the alloy is relatively weak.


Introduction
The Ti-V-Cr alloys applied in aeroengine is a critical high-pressure compressors materials and has derived typical flame retardant titanium alloys [1] such as Ti40 and TF550.While Ti-Al-V alloys, which is used for body materials, also occupies a large proportion in aircraft manufacturing, because of its excellent specific strength, high temperature resistance and processing plasticity properties [2] .Therefore, the study of Ti-V-Cr and Ti-Al-V alloys is of great significance to the aviation industry.
In the study of these two alloys, the high temperature microstructure and mechanical properties are highly related to structural stability and flight safety.In recent years, the research on Ti-6Al-4V alloy shows that the creep failure was mainly caused by the crystal dislocation, and its surface defects had less effect on fatigue failure, when the high temperature creep rate index was between 3.7 and 5.6 [3] .Meanwhile, the taper ratio of the Ti-6Al-4V alloy is much higher than the edge slip when the high temperature plastic deformation is failed [4] .If the β phase ratio is improved, the high temperature creep failure of Ti-6Al-4V alloy will be reduced [5] .In addition, the oxidation layer of the Ti-6Al-4V alloy in high temperature is usually a three-layer structure, which contains the internal TiO2 layer, the middle Al2O3 layer, the external TiVO4 layer [6] .In the interface of the oxidation layer and the basement interface, the toughness fracture of the Ti-6Al-4V alloy fails when it is about 500-600℃, which can cause the brittle fracture of the Ti-6Al-4V alloy [7] .As the flame retardant titanium alloy, the Ti-V-Cr series alloys has the higher flame retardant performance of the Ti-Al-V series alloys.According to the friction pressure-oxygen concentration (P-Co) relationship, when the Co is higher than 70%, the anti-ignition properties of the Ti-25V-15Cr alloy are about 40% higher than the Ti-6Al-4V alloy [8] .While under the same oxygen pressure, the ignition temperature of Ti-25V-15Cr alloy is much higher than Ti-6Al-4V, because the melting, decomposition and evaporation of V2O5 in the Ti-25V-15Cr alloy could absorb certain amount of heat.Which reduces the heat concentration effect of the specimen surface [9] .
In order to further optimize the performance of Ti-V-Cr and Ti-Al-V series alloys, the element composition design and research under the framework of Ti-V-Cr and Ti-Al-V series alloys have been widely applied.In the Ti-Al-V element system, it was reported that the atomic interaction of the Ti-V atoms is higher than that of Ti-Al and Al-V atomic groups, which reduces the compatibility of Al elements in the system [10] .The Al2O3 produced in the oxidation of Al largely affects the antioxidant properties of Ti-Al-V alloys.If the content of V is too high, it will inhibit the protection and density of Al2O3, thereby reducing the antioxidant properties of the Ti-Al-V alloys [11] .For instance, the depth of the oxidation layer of the Ti-35V alloy could be 1.45 times higher than that of Ti-25V alloy [12] .In addition, the activity of Al atoms in the Ti-Al-V alloy is raised with the decrease of the Ti content and the increase of the V content [13] .For the study of the Ti-25V-15Cr alloy, it seems that the Cr2O3 and V2O5 in TiO2 is produced during the high temperature oxidation process, and this is also one of the key factors for the flame retardant properties [14] .With the increase of V content, it not only helps to improve the overall thermodynamic stability of the alloy [15] , but also helps to reduce the thickness of the oxide film [12] .The increase of the Cr element will cause the rich of Cr at substrate interface, and this will further reduce the combustion speed [16] .Additionally, with the prediction of Supporting Vector Machine (SVM) model, the mechanical properties of Ti-25V-15Cr alloy will be further improved by adding 1.5-1.8% of Al element or 0.15-0.2% of C elements [17] .
The traditional melting, sample preparation, heat treatment, machining, testing and contrastive analysis processes not only have low efficiency and long period, but also increase the research cost and limit the accuracy of element variables.The simulation or methods represented by SVM model [17] , neural network model [18] and thermodynamic model [19] can achieve efficient acquisition of microstructure and properties under different components, and provide an early basis for reducing the screening scope for test verification.For instance, the JMatPro program can not only directly obtain the hot-working microstructure phase diagram, CCT and TTT diagram [20] of different alloys, but also indirectly serve as the initial parameters [21] or comparison verification items of other numerical calculation programs [22] .Besides, it can also be used to optimize the alloy processing technology including heat treatment [23] .
This study adopts the calculation method based on JMatPro program, and takes Ti-V-Cr and Ti-Al-V alloy as the basis of element system.By adjusting the contents of Al, V and V, Cr in Ti-V-Cr and Ti-Al-V alloys, the effects of different elements on microstructure, thermodynamic parameters and high temperature mechanical properties were obtained.Then the trend of high temperature properties was predicted after the composition changes.The research data will be used as the basis for determining the composition ratio of test materials in the next stage, which can effectively shorten the composition screening range, improve the screening accuracy, and optimize the efficiency of the composition optimization design of Ti-V-Cr and Ti-Al-V alloys

Methods
In order to build the prediction trend model of Ti-V-Cr and Ti-Al-V alloys, Al, V, V and Cr elements in this study is set to be floated within the set interval.When one of the alloy elements is adjusted, the other alloy element is still set by the base value of the Ti-25V-15Cr and Ti-6Al-4V.Table 1 is the two alloys of the composition system.In the table, the Al, Si and C element of the Ti-25V-15Cr system is added to the fourth item, which constitutes a quaternary alloy.
Table 1.The parameters Ti-V-Cr and Ti-Al-V alloy systems

Alloy System
Al (%) V (%) Cr (%) Si (%) C (%) In terms of the calculation method, the performance change trend is difficult to obtain by the characterization of tests.Therefore, this study mainly provides the phase diagram, thermodynamic parameters and high temperature mechanical performance results obtained by the JMatPro (Version 7.0) program.
In the JMatPro, the calculation database of Ti-V-Cr and Ti-Al-V alloys was Titanium Alloy database.Then, the Step Temperature and Step Concentration tools of Thermodynamic Properties module were selected.The Extended General tool under the Thermo-Physical Properties module also calculated the phase diagram and thermodynamic parameters of Ti-V-Cr and Ti-Al-V alloy series alloys at different temperatures.In addition, the High Temperature Strength tool under the Mechanical Properties module was used to calculate the high temperature elastic mechanical properties of Ti-V-Cr and Ti-Al-V alloys with different compositions.

Verification of calculated values from JMatPro program
The previous verification reports [24] showed that the tensile strength of Ti-6Al-4V alloy obtained by tensile strength test and JMatPro while at room temperature was about 1028.4MPa and 1019.64MPa, respectively.With the deviation of 0.85%, good agreements between testing and JMatPro results has been proved.In order to further verify two different element systems, Ti-V-Cr and Ti-Al-V alloy, the validity of the high temperature parameter results calculated by JMatPro was verified from the aspects of phase structure, melting point, high temperature proof stress and so on.Figure 1 (a) shows the change of Ti-6Al-4V alloy phase structure at 600-1200℃.Below 900℃, Ti-6Al-4V alloy mainly consists of α/β phase structure, and the α phase begins to transform to β phase with the increase of temperature.At about 960℃, the α/β phase structure is transformed into β single-phase structure, which is basically consistent with the experimental value of phase transition temperature of Ti-6Al-4V alloy under heating condition (980℃) [25] .The Ti-25V-15Cr in Figure 1 (b) conforms to the basic characteristics of stable β-type titanium alloys, showing a β-single phase structure [26] .Therefore, according to the alloy phase structure at high temperatures above 600℃, results of JMatPro are basically consistent with the reported values.  2 respectively reflect the degree of heat absorption and atomic freedom change of Ti-6Al-4V and Ti-25V-15Cr alloys when heated to 2100 ℃ .As shown in Figure 2 (a), while at 1700 ℃ , both the enthalpy and the entropy changes of Ti-6Al-4V alloy step up to a higher value, indicating that atomic thermal diffusion intensifies and the alloy melts into liquid phase.This value is basically close to the actual melting point of Ti-6Al-4V alloy (1660℃ [27] ).In Figure 2 (b), the melting point of Ti-25V-15Cr alloy shown by the step enthalpy change and entropy change is about 1400 ℃ , about 300 ℃ lower than that of Ti-6Al-4V, which conforms to the low melting point of Ti-V-Cr flame retardant titanium alloy and accelerates heat dissipation to achieve flame retardant characteristics [28] .The difference of melting point between Ti-6Al-4V and Ti-25V-15Cr alloys is basically close to the difference of critical combustion temperature [9] .Figure 2. Enthalpy and entropy changes in the temperature range of 100-2100℃, (a): Ti-6Al-4V; (b): Ti-25V-15Cr Finally, in terms of high temperature mechanical properties verification, Figure 3 (a) shows the relationship between the proof stress and temperature for Ti-6Al-4V alloy with an average grain size of 11.3 μm at a constant loading strain rate of 0.005s -1 .At 960℃, Ti-6Al-4V alloy has a β single-phase structure, and the calculated elastic limit stress is 23.13MPa, which is consistent with the true stress-strain curve of Ti-6Al-4V alloy at 960℃ and 0.005s -1 strain [29] .The calculated proof stress of Ti-25V-15Cr alloy in Figure 3 (b) is 432.23MPa when the loading strain rate is constant as 1 s -1 and the average grain size is 135 μm at 950℃.The results are basically close to the stress-strain curve results of Ti-25V-15Cr alloy under the same conditions [30] .The validity of JMatPro program in calculating the mechanical and thermodynamic properties of Ti-6Al-4V and Ti-25V-15Cr alloys at high temperature is verified.

Microstructure change analysis
As the Ti-25V-15Cr alloy belongs to the stable β type titanium alloy, the microstructure is β single-phase structure, and this does not involve the conversion process of α to β phase.This section only discusses the role of Al and V elements in Ti-Al-V system.In the Ti-Al-V system, Al and V belong to α phase and β phase stable elements respectively [10] .Therefore, Al elements and V elements will have an effect on the phase composition of the alloy and the trend of heating phase change.In Figure 4, a fixed V content, with Ti as the equilibrium element of Al, shows the phase structure with 3%, 6%, and 10% Al content.At high temperature, the critical temperatures of Ti-3Al-4V, Ti-6Al-4V and Ti-10Al-4V alloys to complete the transition from α/β structure to β single-phase structure are 880℃, 960℃ and 1020℃, respectively.In other words, with the increase of Al content, the conversion temperature of β single-phase structure is increased, and the conversion process of α to β phase is delayed.In terms of element V, Figure 5 shows the relationship between phase structure and temperature when Ti is the equilibrium element V and the mass fraction of Al is 6%.In the figure, the effect of V content on β phase is opposite to that of Al, so the increase of V content is conducive to the formation of β single-phase structure.
Figure 4. Microstructure change curves of Ti-3Al-4V, Ti-6Al-4V and Ti-10Al-4V Previous studies have shown that β single-phase structures with bcc structure have better slip system and creep properties than α/β structures at high temperatures [5] .As shown in Figure 4 and Figure 5, reducing the content of Al element or increasing the content of V element in Ti-Al-V alloy will lead to lower α/β phase conversion temperature of β single phase and increase the volume fraction of β phase of Ti-Al-V alloy at a certain temperature.

Analysis of thermodynamic parameters
The thermal conductivity of Ti-Al-V and Ti-V-Cr alloy at high temperature is one of the important indicators that affect the ignition/flame retardancy characteristics.If the thermal conductivity of the alloy is high, the heat conduction efficiency will be fast.Then the heat concentration is avoided when the local heat is heated, and the ignition probability [28] and the ignition temperature [9] are reduced.
As shown in Figure 6 (a), increasing the content of Al and V in the Ti-Al-V alloy is conducive to improve the overall thermal conductivity of the alloy.Especially in the temperature range of less than 700℃, the thermal conductivity of Ti-10Al-4V alloy is significantly higher than that of other alloys.In the temperature range above 700℃, the difference of thermal conductivity decreases, but there is still a significant difference.Among them, the inflection point of the thermal conductivity curve of Ti-10Al-4V alloy appears at about 900℃, which is because Ti-10Al-4V alloy has a certain Ti3Al phase at room temperature.With the increase of temperature, the alloy gradually turns into a β single-phase structure, where the inflection point is the temperature point at which the Ti3Al phase completely turns into the β phase.In the Ti-V-Cr alloy as shown in Figure 6 (b), the increase of the V element also shows the effect of helping to improve the overall thermal conductivity of the alloy.On the contrary, Cr content has relatively little effect on the thermal conductivity of the alloy.When the temperature is higher than 600 ℃ , the thermal conductivity curves of Ti-25V-10Cr, Ti-25V-13Cr and Ti-25V-15Cr basically coincide.alloying elements content In addition to the thermal conductivity, another parameter that can reflect the temperature response mechanism of Ti-Al-V and Ti-V-Cr alloys is the heat capacity.The critical temperature for accelerated high-temperature oxidation of Ti-6Al-4V alloy is 900℃ [6] .While at this temperature, the heat capacity of Ti-6Al-4V alloy in Figure 7 (a) is lower than that of Ti-4Al-4V.At 900 ℃ , when Ti-Al-V alloy receives a certain amount of heat due to abnormal friction or heat conduction, the temperature will be higher with more Al content.However, the heat capacity of V is different.When the heat capacity reaches the peak value at Ti-6Al-6V, the heat capacity of which decreases with less V content.As shown in Figure 7 (b), the heat capacity of the Ti-V-Cr alloy is relatively stable when the content of V element is between 0% and 25% at 900℃.In the same range, the heat capacity of the alloy increases linearly and slowly with the increase of Cr content.contents at 900℃ When the temperature is further increased to about 1400 ℃ and 1700 ℃ , Ti-25V-15Cr alloy and Ti-6Al-4V alloy melt into the liquid phase successively.As shown in Figure 8 (a), the effect of V element on the melting point of the alloy is not obvious enough, and the enthalpy change curve basically coincides.In Ti-Al-V alloy system, the influence of Al content on melting point is relatively more obvious.As shown in Figure 8 (b), the melting point of Ti-25V-15Cr alloy does not change significantly when the content of V and Cr increases or decreases to 35% and 10% respectively.However, from the analysis of the enthalpy curve, the potential influence of V element on the melting point of the alloy is less than that of Cr element.This characteristic is consistent with the characteristic of melting point of V element in Ti-Al-V system.

Analysis of elastic deformation properties
The ability of Ti-Al-V and Ti-V-Cr alloys to resist elastic deformation at medium-high temperature is one of the important factors that affect their service performance as aviation materials.The Young's modulus of Ti-Al-V and Ti-V-Cr alloys directly reflects the difficulty of elastic deformation of the alloys.In Figure 9 (a), the calculated value of Young's modulus of Ti-6Al-4V alloy at 25℃ is 114 GPa, which is close to the retrieved value of MatWeb database (113.8GPa).As shown in Figure 9 (a) and (b), the Young's modulus of Ti-Al-V and Ti-V-Cr alloys showed a near linear decline under the heating condition.This is because at low temperatures, the in-situ thermal vibration of atoms is limited and no obvious displacement can be generated.While under the effect of heating and softening, the bond energy is destroyed and the lattice's restriction on deformation is weakened [31] , resulting in a near-linear decrease in Young's modulus.In the aspect of elemental action, the change of V content in Ti-Al-V alloy has little effect on the Young's modulus of Ti-Al-V alloy at medium-high temperature, and its curve is basically overlapping.In the Ti-V-Cr alloy, the Young's modulus of Ti-35V-15Cr alloy at high temperature is significantly higher than that of Ti-25V-15Cr alloy.At the same time, the Cr element also shows the same trend.In Ti-Al-V alloy, the change of Al content has a great influence on the overall Young's modulus of the alloy.Especially the Young's modulus of Ti-10Al-4V alloy reaches a trough value at about 820℃.The second inflection point temperature is 1020℃, which is the critical temperature for Ti-10Al-4V alloy to complete the β single-phase transformation.This is because the increase in the mass fraction of Al increases the proportion of Ti3Al phase [32] , while in other alloys, Ti3Al is less or almost absent, so the inflection point trend is small before completing the α/β to β single-phase conversion, which is approximately shown as an overall near-linear downward trend.Compared with Ti-Al-V alloy, Ti-V-Cr alloy is more regular in the variation trend of Young's modulus after the adjustment of V and Cr element, because it does not involve phase structure transformation.
Finally, in terms of the variation trend of proof stress with the rise of temperature, the Ti-Al-V alloy as shown in Figure 10 (a) has obvious difference in proof stress below 500℃.In which Al content has more obvious influence on the overall proof stress than V element.Generally, the proof stress will be higher with more Al content.When the temperature is higher than 500℃, the in-situ thermal vibration of atoms is intensified, and the lattice limitation is weakened.So the proof stress of Ti-Al-V alloy decreases in this process.Therefore, the influence of Al and V content adjustment on the overall resistance to elastic deformation of Ti-Al-V alloy at high temperature is significantly lower than that at medium and low temperature.As shown in Figure 10 (b), the proof stress of Ti-V-Cr alloy in the environment below 1000℃ is stable, and no obvious inflection point appears.Therefore, its stability of elastic mechanical properties at high temperatures is better than that of Ti-Al-V alloy.In terms of elemental action, the addition of V and Cr elements will bring up the entire proof stress.However, the difference of proof stress is limited in the element range of 5%-10% .In the temperature range from 200℃ to 900℃, the change of proof stress caused by the adjustment of element content does not show a trend of expansion or contraction.Therefore, it is shown that the Ti-V-Cr alloys have good resistance to elastic deformation at high temperature.

Thermal conductivity and Young's modulus of quaternary alloys
At present, the study on the change of element content of Ti-V-Cr alloy is not only limited to the traditional ternary alloy system.For instance, on the basis of Ti-35V-15Cr alloy, Ti-25V-15Cr-0.2Si and Ti-35V-15Cr-Si-C [26] alloys have been developed successively.Therefore, this section is based on Ti-25V-15Cr alloy, and Al, Si and C elements are added in a certain range to form a quaternary alloy.Figure 11 shows the thermal conductivity trends under different element compositions.Since the thermal conductivity of Al and Ti is 210 W/m-K and 17.0 W/m-K respectively, the thermal conductivity of Al element is much higher than that of Ti element.So the higher the Al content is adjusted in Ti-25V-15Cr alloy, the higher the overall thermal conductivity of the alloy will be.added in small amounts, so the impact on thermal conductivity is limited.However, from the numerical comparison, the thermal conductivity of Ti-25V-15Cr-1C alloy is slightly lower than that of Ti-25V-15Cr-0.3C alloy.So the addition of C element could not bring potential improvement for the thermal conductivity.In addition, Figure 12 shows the variation trend of Young's modulus of Ti-25V-15Cr-M alloy in the temperature range of 700℃-1100℃.As shown in the Figure 12 (a), the addition content of Al element 1%-5% and C element 0.3%-1% can have a great influence on the Young's modulus of Ti-25V-15Cr-M alloy at high temperature.The specific influence trend is that the higher the content of Al or C elements is, the greater the overall Young's modulus of the alloy will be.In contrast, the change of Si element content has little influence on the overall Young's modulus of the alloy, and the curve in Figure 12 (b) does not show significant difference.

Conclusion
In this study, Ti-6Al-4V and Ti-25V-15Cr alloys were taken as calculation objects, and the calculated values based on JMatPro thermodynamic calculation program were compared and verified.The validity of JMatPro's calculation of Ti-Al-V and Ti-V-Cr element systems is verified on the basis of the agreement between the calculation results and those of literature and database.Then, the influence of element content adjustment on the high temperature properties of the alloy was calculated and analyzed, and the conclusions were listed as follows: 1) In the Ti-Al-V element system, reducing the content of Al element in the alloy or increasing the content of V element will lead to the reduction of the α/β phase conversion temperature of the β single phase.In the heating state, increasing the content of Al and V is helpful to improve the overall thermal conductivity of the alloy.But in the aspect of heat capacity, the influence trend of Al and V elements is opposite.While at high temperature, the influence of Al and V contents on the elastic deformation resistance of the alloy is weakened. 2) In the Ti-V-Cr element system, when the content of V element is increased to 35%, the thermal conductivity of the alloy at high temperature can be effectively improved, but the heat capacity and melting point are not affected.When the content of Cr is about 10%-15%, the influence on the thermal conductivity, heat capacity and melting point is limited.In terms of high temperature elastic deformation properties, the higher the content of V and Cr are, the higher the Young's modulus and elastic limit stress of the alloy at high temperature will be. 3) In the Ti-V-Cr-M quaternary alloy system, the addition of Al element is helpful to improve the thermal conductivity and Young's modulus of the alloy, but the addition of C element could not improve the thermal conductivity of Ti-V-Cr-M alloy.Finally, Si element has little influence on thermal conductivity and high temperature deformation properties of the alloy.

Figure 1 .
Figure 1.Phase structure in the temperature range 600-1200℃, (a): Ti-6Al-4V; (b): Ti-25V-15Cr When the temperature is further increased to be above 2000℃, the intermediate Ti-6Al-4V and Ti-25V-15Cr alloys also involve the melting process of β single-phase structure to liquid phase transformation.The enthalpy and entropy change values shown in Figure 2 respectively reflect the degree of heat absorption and atomic freedom change of Ti-6Al-4V and Ti-25V-15Cr alloys when heated to 2100 ℃ .As shown in Figure2(a), while at 1700 ℃ , both the enthalpy and the entropy changes of Ti-6Al-4V alloy step up to a higher value, indicating that atomic thermal diffusion intensifies and the alloy melts into liquid phase.This value is basically close to the actual melting point of Ti-6Al-4V alloy (1660℃[27] ).In Figure2(b), the melting point of Ti-25V-15Cr alloy shown by the step enthalpy change and entropy change is about 1400 ℃ , about 300 ℃ lower than that of Ti-6Al-4V, which conforms to the low melting point of Ti-V-Cr flame retardant titanium alloy and accelerates heat dissipation to achieve flame retardant characteristics[28] .The difference of melting

Figure 6 .
Figure 6.Thermal conductivity change values of Ti-Al-V (a) and Ti-V-Cr (b) alloys with differentalloying elements content In addition to the thermal conductivity, another parameter that can reflect the temperature response mechanism of Ti-Al-V and Ti-V-Cr alloys is the heat capacity.The critical temperature for accelerated high-temperature oxidation of Ti-6Al-4V alloy is 900℃[6] .While at this temperature, the heat capacity of Ti-6Al-4V alloy in Figure7(a) is lower than that of Ti-4Al-4V.At 900 ℃ , when Ti-Al-V alloy receives a certain amount of heat due to abnormal friction or heat conduction, the temperature will be higher with more Al content.However, the heat capacity of V is different.When the heat capacity reaches the peak value at Ti-6Al-6V, the heat capacity of which decreases with less V content.As shown in Figure7(b), the heat capacity of the Ti-V-Cr alloy is relatively stable when the content of V element is between 0% and 25% at 900℃.In the same range, the heat capacity of the alloy increases linearly and slowly with the increase of Cr content.

Figure 7 .
Figure 7. Heat capacity changes of Ti-Al-V (a) and Ti-V-Cr (b) alloys with different alloying elementcontents at 900℃ When the temperature is further increased to about 1400 ℃ and 1700 ℃ , Ti-25V-15Cr alloy and Ti-6Al-4V alloy melt into the liquid phase successively.As shown in Figure8(a), the effect of V element on the melting point of the alloy is not obvious enough, and the enthalpy change curve basically coincides.In Ti-Al-V alloy system, the influence of Al content on melting point is relatively more obvious.As shown in Figure8(b), the melting point of Ti-25V-15Cr alloy does not change significantly when the content of V and Cr increases or decreases to 35% and 10% respectively.However, from the analysis of the enthalpy curve, the potential influence of V element on the melting point of the alloy is less than that of Cr element.This characteristic is consistent with the characteristic of melting point of V element in Ti-Al-V system.

Figure 8 .
Figure 8. Relationship between enthalpy change and temperature of Ti-Al-V (a) and Ti-V-Cr (b) alloys with different alloying element content

Figure 9 .
Figure 9. Temperature variation curve of Young's modulus of Ti-Al-V (a) and Ti-V-Cr (b) alloysIn the aspect of elemental action, the change of V content in Ti-Al-V alloy has little effect on the Young's modulus of Ti-Al-V alloy at medium-high temperature, and its curve is basically overlapping.In the Ti-V-Cr alloy, the Young's modulus of Ti-35V-15Cr alloy at high temperature is significantly higher than that of Ti-25V-15Cr alloy.At the same time, the Cr element also shows the same trend.In Ti-Al-V alloy, the change of Al content has a great influence on the overall Young's modulus of the alloy.Especially the Young's modulus of Ti-10Al-4V alloy reaches a trough value at about 820℃.The second inflection point temperature is 1020℃, which is the critical temperature for Ti-10Al-4V alloy to complete the β single-phase transformation.This is because the increase in the mass fraction of Al increases the proportion of Ti3Al phase[32] , while in other alloys, Ti3Al is less or almost absent, so the inflection point trend is small before completing the α/β to β single-phase conversion, which is approximately shown as an overall near-linear downward trend.Compared with Ti-Al-V alloy, Ti-V-Cr alloy is more regular in the variation trend of Young's modulus after the adjustment of V and Cr element, because it does not involve phase structure transformation.Finally, in terms of the variation trend of proof stress with the rise of temperature, the Ti-Al-V alloy as shown in Figure10(a) has obvious difference in proof stress below 500℃.In which Al content has more obvious influence on the overall proof stress than V element.Generally, the proof stress will be higher with more Al content.When the temperature is higher than 500℃, the in-situ thermal vibration of atoms is intensified, and the lattice limitation is weakened.So the proof stress of Ti-Al-V alloy decreases in this process.Therefore, the influence of Al and V content adjustment on the overall resistance to elastic deformation of Ti-Al-V alloy at high temperature is significantly lower than that at medium and low temperature.As shown in Figure10(b), the proof stress of Ti-V-Cr alloy in the environment below 1000℃ is stable, and no obvious inflection point appears.Therefore, its stability of elastic mechanical properties at high temperatures is better than that of Ti-Al-V alloy.In terms of elemental action, the addition of V and Cr elements will bring up the entire proof stress.However, the difference of proof stress is limited in the element range of 5%-10% .In the temperature range from 200℃ to 900℃, the change of proof stress caused by the adjustment of element content does not show

Figure 11 .
Figure 11.Thermal conductivity change curve of Ti-25V-15Cr-M quaternary alloy at high temperature, (a) M=Al; (b) M=Si/C Compared with Al, Si and C are added in small amounts, so the impact on thermal conductivity is limited.However, from the numerical comparison, the thermal conductivity of Ti-25V-15Cr-1C alloy is slightly lower than that of Ti-25V-15Cr-0.3C alloy.So the addition of C element could not bring potential improvement for the thermal conductivity.In addition, Figure12shows the variation trend of Young's modulus of Ti-25V-15Cr-M alloy in the temperature range of 700℃-1100℃.As shown in the Figure12(a), the addition content of Al element 1%-5% and C element 0.3%-1% can have a great