Experimental Investigation of the Ti-Nb-Sn Isothermal Section at 1173 K

Isothermal section of Ti-Nb-Sn at 1173 K was experimentally studied by back-scattered electron, electron probe microanalysis and X-ray diffraction analysis. Solid solution phase β(Ti, Nb), liquid Sn and eight intermetallic compounds Ti3Sn, Ti2Sn, Ti5Sn3, Ti6Sn5, Nb6Sn5, Nb3Sn, Ti3Nb2Sn2 and Ti3NbSn coexisted. Four ternary phase regions Ti3Sn + Ti3NbSn + β(Ti, Nb), Ti3NbSn + Ti3Nb3Sn2 + Ti3Sn, Ti2Sn + Ti3Sn + Ti3Nb3Sn2 and Ti6Sn5 + Ti3Nb3Sn2 + Nb3Sn were experimented. In addition, the proper composition range of the single phase was suggested. All the detected Ti-Sn and Nb-Sn compounds have a remarkable solubility along the isoconcentration of Sn. β(Ti, Nb) has a relatively large solution while liquid Sn has a little in the isothermal section.


Introduction
Titanium alloys have characteristics of high strength, super-elasticity and good biocompatibility.They are promising materials in many fields especially in a biomedical aspect.Nb and Sn are alloy elements of biocompatibility, and do not increase the martensitic transformation temperature of metastable β titanium alloys [1][2][3][4][5][6].It is necessary to know the phase equilibrium in Ti-Nb-Sn ternary system.Therefore, the knowledge of the phase diagram is of great importance in the design and development of titanium alloys.
The information of Ti-Sn, Nb-Sn and Ti-Nb binary systems can be easily found in recent studies.Ti-Sn has been systematically studied [7][8][9][10][11][12][13][14].The version of Ti-Sn system (Figure 1a) as given by Yin et al. was taken because the coordinates of eutectic L Ø (β-Ti) + Ti 3 Sn correspond better to the version of the system presented in these studies.It is also suggested that the thermodynamic model agrees well with the experimental phase diagram [10].There are nine phases: liquid, β-Ti (bcc), α-Ti (hcp), β-Sn (bct), α-Sn, Ti 3 Sn, Ti 2 Sn, Ti 5 Sn 3 and Ti 6 Sn 5 .The composition range of solution phase β-Ti and α-Ti in Ti-Sn system is 0-7.5 at.% Sn and 0-17.0 at.% Sn, respectively.The solid solubility of Sn in Ti 3 Sn and Ti 2 Sn is 23.0-25.0at.% and 32.7-35.9at.%, respectively.In addition, Ti 6 Sn 5 has two crystal structures: orthorhombic α-Ti 6 Sn 5 and hexagonal β-Ti 6 Sn 5 , the later one being found when the temperature was higher than 943 K [8,9].description fitting in some ternary system [16].Three solution phases: liquid, β-Nb and β-Sn as well as three intermediate compounds: Nb3Sn, Nb6Sn5 and NbSn2 exist in it.The solubility of Nb3Sn and Nb2Sn is 19.2-26.7 at.% Sn and 65.3-66.7 at.% Sn, respectively, and compound Nb6Sn5 is a stoichiometric compound.The Ti-Nb phase diagram (Figure 1c) comes from the model proposed by Zhang et al. [17], and the thermodynamic description agrees well with the experimental results.It contains three stable phases: liquid, β(Ti, Nb) and α-Ti.According to the previous work [18], the Ti-Nb-Sn system has been investigated at 973 K below 50 at.% Sn.Ternary compound Ti3Nb3Sn2 was found in the isothermal section at 973 K.It was also reported in some literatures [19][20][21].The crystal structure is same to Nb3Sn [22].Meanwhile, the crystal structure data of phases in the Ti-Sn, Nb-Sn and Ti-Nb can be found in [14,[22][23][24].The purpose of this work is to evaluate the phase relations and compositions of Ti-Nb-Sn at 1173 K by means of the equilibrated alloy method (experimental assessment of two-phase and three-phase equilibria).Based on the phase diagram information, the relation of microstructure, processing and mechanical properties of titanium alloys can better understood.

Experimental Details
Twenty three Ti-Nb-Sn alloys were studied to evaluate the phase diagram of the ternary system.Each sample was prepared to have the weight of 6 g by pure Ti of 99.99 wt.%, Nb of 99.98 wt.% and Sn of 99.999 wt.%.They were melted in an arc furnace with nonconsumable tungsten electrode on a water-cooled copper platform in an Ar atmosphere purified by Ti-melted.The buttons were returned no less than four times to reach the homogeneity in the melting process.The specimens were According to the previous work [18], the Ti-Nb-Sn system has been investigated at 973 K below 50 at.% Sn.Ternary compound Ti 3 Nb 3 Sn 2 was found in the isothermal section at 973 K.It was also reported in some literatures [19][20][21].The crystal structure is same to Nb 3 Sn [22].Meanwhile, the crystal structure data of phases in the Ti-Sn, Nb-Sn and Ti-Nb can be found in [14,[22][23][24].The purpose of this work is to evaluate the phase relations and compositions of Ti-Nb-Sn at 1173 K by means of the equilibrated alloy method (experimental assessment of two-phase and three-phase equilibria).Based on the phase diagram information, the relation of microstructure, processing and mechanical properties of titanium alloys can better understood.

Experimental Details
Twenty three Ti-Nb-Sn alloys were studied to evaluate the phase diagram of the ternary system.Each sample was prepared to have the weight of 6 g by pure Ti of 99.99 wt.%, Nb of 99.98 wt.% and Sn of 99.999 wt.%.They were melted in an arc furnace with nonconsumable tungsten electrode on a water-cooled copper platform in an Ar atmosphere purified by Ti-melted.The buttons were returned no less than four times to reach the homogeneity in the melting process.The specimens were encapsulated in clean quartz tubes under an argon atmosphere to prevent oxidation and other contaminations at high temperature.The 23 alloys were annealed at 1173 K for 1440 h to reach the phase equilibrium and quenched in cold water.
The quantitative element analysis was carried out by EPMA JXA-8230 at a voltage of 20 kV and a current of 20 nA.The specimens for quantitative analysis were polished with diamond paste until scratches disappeared.

Results and Discussion
Twenty three Ti-Nb-Sn alloys were prepared to determine the isothermal section of Ti-Nb-Sn system at 1173 K.All phases found in the two-phase and three-phase equilibrium alloys and their compositions determined by SEM and EPMA are summarized in Table 1.In addition, most of the constituent phases are further confirmed by XRD patterns.3a. Figure 2b is the microstructure of Alloy 16 (Ti 7.5 Nb 50.1 Sn 42.4 ).Following similar analytical procedures, two phases coexisted: a gray solidification phase Nb 3 Sn and gray matrix phase Nb 6 Sn 5 .The results agree with the XRD diffractogram of alloy 16 in Figure 3b.
Figure 2c is the microstructure of Alloy 17 (Ti 32.0 Nb 17.3 Sn 50.7 ).It displays a bright gray phase liquid Sn with some holes and the gray matrix phase Ti 6 Sn 5 coexisted.It agrees with the XRD patterns of alloy 17 in Figure 3c.
Figure 2d-g features the microstructure of three phase region alloys.Figure 2d is the microstructure of Alloy 18 (Ti 61.5 Nb 19.7 Sn 18.8 ).Two precipitates dark gray phase Ti 3 Sn and gray stripy phase β(Ti, Nb), as well as a bright gray matrix phase Ti 3 NbSn were observed.Figure 2e shows the microstructure of Alloy 19 (Ti 50.7 Nb 21.1 Sn 28.2 ).There are two solidification phases, gray reticular phase Ti 3 NbSn and black gray stripy phase, as well as bright gray matrix phase Ti 3 Nb 3 Sn 2 coexisted in the alloy.
Figure 2f shows the microstructure of Alloy 20 (Ti 50.6 Nb 17.9 Sn 31.5 ), which displays two precipitates containing dark gray stripy phase Ti 3 Sn and bright gray reticular phase Ti 2 Sn, as well as a gray matrix phase Ti 3 Nb 3 Sn 2 observed.Meanwhile, the XRD diffractogram of Alloy 20 is identical in Figure 3d.Compared with Alloy 19, Alloy 20 contains two same phases, but the size of solidification phases is small and slender.The size of the solidification phase is related to the content of the phase in the annealed alloy at 1173 K, while, the shape is determined by the crystal structure of phase.
Figure 2g is the microstructure of Alloy 23 (Ti 32.3 Nb 37.1 Sn 30.6 ).A little blocky dark gray phase Ti 3 Nb 3 Sn 2 , bright reticular phase Ti 6 Sn 5 and a gray matrix phase Nb 3 Sn are observed.The result is consistent with the XRD diffractogram of Alloy 23 in Figure 3e.
Figure 2a shows the microstructure of Alloy 12 (Ti26.7Nb49.6Sn23.7).A gray phase with some crack phase Ti3Nb3Sn2 and bright matrix phase Nb3Sn are observed.The results are confirmed with the XRD diffractogram of Alloy 12 in Figure 3a. Figure 2b is the microstructure of Alloy 16 (Ti7.5Nb50.1Sn42.4).Following similar analytical procedures, two phases coexisted: a gray solidification phase Nb3Sn and gray matrix phase Nb6Sn5.The results agree with the XRD diffractogram of alloy 16 in Figure 3b.  Figure 2c is the microstructure of Alloy 17 (Ti32.0Nb17.3Sn50.7).It displays a bright gray phase liquid Sn with some holes and the gray matrix phase Ti6Sn5 coexisted.It agrees with the XRD patterns of alloy 17 in Figure 3c.
Figure 2d-g features the microstructure of three phase region alloys.Figure 2d is the microstructure of Alloy 18 (Ti61.5Nb19.7Sn18.8).Two precipitates dark gray phase Ti3Sn and gray stripy phase β(Ti, Nb), as well as a bright gray matrix phase Ti3NbSn were observed.Figure 2e shows the microstructure of Alloy 19 (Ti50.7Nb21.1Sn28.2).There are two solidification phases, gray reticular phase Ti3NbSn and black gray stripy phase, as well as bright gray matrix phase Ti3Nb3Sn2 coexisted in the alloy.
Figure 2f shows the microstructure of Alloy 20 (Ti50.6Nb17.9Sn31.5),which displays two precipitates  The solubility of the equilibrium phases in the Ti-Nb-Sn ternary system at 1173 K is elucidated based on experimental analysis.The phase boundaries along the Ti-Sn, Ti-Nb and Nb-Sn sides can be obtained from the binary phase diagrams.In addition, the solubility of the equilibrium phases in Ti-Sn, Ti-Nb and Nb-Sn binary at 1173 K systems also given as follows.The solubility of Sn in β(Ti), Ti3Sn, Ti2Sn and liquid Sn in Ti-Sn system is 0-11.1 at.%, 23.8-25.0at.%, 33.3-34.9at.% and 79.2-100.0at.%, respectively.The solubility of Sn in β(Nb), Nb3Sn and liquid Sn is 0.0-3.7 at.%, 25.0-27.4at.% and 99.8-100.0at.%, respectively.The compounds Ti5Sn3, Ti6Sn5 and Nb6Sn5 are regarded as stoichiometric compounds.The equilibrium phase in Ti-Nb at 1173 K contains only one solid solution phase β(Ti, Nb).
The solubility of Sn in β(Ti, Nb) solid solution phase can be determined by compositions of the equilibrium phases of the typical annealed alloys, and the maximum value is 3.7-11.1 at.%, which is a relative large solubility.Liquid Sn is located at the Sn rich side, and the boundary composition range is 79.2-0.2 at.% Sn from Ti side to Nb side, based on Alloy 17 and Ti-Sn and Nb-Sn binary phase diagrams, which is a smaller solubility.
The solubility of Ti3Sn is found to be 22.1-25.5 at.% Sn and 0.0-13.6at.% Nb.The solubility of Ti2Sn extended along the isoconcentration of Sn, which is about 33.3-36.1 at.% Sn and more than 12.2 at.% Nb.Binary compounds Ti6Sn5 and Nb6Sn5 are treated as stoichiometric compounds for the very narrow homogeneity range according to experimental results, and the solubility extent of the two compounds are along the isoconcentration of Sn.The maximum solubility of Ti6Sn5, Nb6Sn5 was found to be no less than 19.3 at.% Nb and 7.4 at.% Ti, respectively, according to the equilibrium phase compositions of alloys 16 and 17.The solubility of Nb3Sn is measured to be about 0.0-24.9at.The solubility of the equilibrium phases in the Ti-Nb-Sn ternary system at 1173 K is elucidated based on experimental analysis.The phase boundaries along the Ti-Sn, Ti-Nb and Nb-Sn sides can be obtained from the binary phase diagrams.In addition, the solubility of the equilibrium phases in Ti-Sn, Ti-Nb and Nb-Sn binary at 1173 K systems also given as follows.The solubility of Sn in β(Ti), Ti 3 Sn, Ti 2 Sn and liquid Sn in Ti-Sn system is 0-11.1 at.%, 23.8-25.0at.%, 33.3-34.9at.% and 79.2-100.0at.%, respectively.The solubility of Sn in β(Nb), Nb 3 Sn and liquid Sn is 0.0-3.7 at.%, 25.0-27.4at.% and 99.8-100.0at.%, respectively.The compounds Ti 5 Sn 3 , Ti 6 Sn 5 and Nb 6 Sn 5 are regarded as stoichiometric compounds.The equilibrium phase in Ti-Nb at 1173 K contains only one solid solution phase β(Ti, Nb).
The solubility of Sn in β(Ti, Nb) solid solution phase can be determined by compositions of the equilibrium phases of the typical annealed alloys, and the maximum value is 3.7-11.1 at.%, which is a relative large solubility.Liquid Sn is located at the Sn rich side, and the boundary composition range is 79.2-0.2 at.% Sn from Ti side to Nb side, based on Alloy 17 and Ti-Sn and Nb-Sn binary phase diagrams, which is a smaller solubility.
The solubility of Ti 3 Sn is found to be 22.1-25.5 at.% Sn and 0.0-13.6at.% Nb.The solubility of Ti 2 Sn extended along the isoconcentration of Sn, which is about 33.3-36.1 at.% Sn and more than 12.2 at.% Nb.Binary compounds Ti 6 Sn 5 and Nb 6 Sn 5 are treated as stoichiometric compounds for the very narrow homogeneity range according to experimental results, and the solubility extent of the Metals 2016, 6, 60 7 of 9 two compounds are along the isoconcentration of Sn.The maximum solubility of Ti 6 Sn 5 , Nb 6 Sn 5 was found to be no less than 19.3 at.% Nb and 7.4 at.% Ti, respectively, according to the equilibrium phase compositions of alloys 16 and 17.The solubility of Nb 3 Sn is measured to be about 0.0-24.9at.% Ti and 21.7-27.4at.% Sn.The solubility of Nb in compound Ti 5 Sn 3 can be deduced to be no more than 7.9 at.% Nb, based on equilibrium phase of Alloy 16.Overall, all of the detected binary compounds has a considerable solution along isoconcentration of Sn.This indicates that a certain amount of Ti atoms are replaced by Nb atoms in Ti-Sn compounds, and some Nb atoms are substituted by Ti atoms in Nb-Sn compounds.
Two ternary compounds Ti 3 Nb 3 Sn 2 and Ti 3 NbSn are found in the isothermal section.The solubility of ternary compound Ti 3 Nb 3 Sn 2 is measured to be about 21.7-43.2at.% Nb and 21.5-26.6at.% Sn, based on the compositions of equilibrium phase of typical alloys.The solubility of Ti 3 NbSn is about 20.2-28.5 at.% Nb, which is regarded as the stoichiometric compound according to the experimental results, and the extending is along the isoconcentratation of Sn.
The proposed isothermal section of Ti-Nb-Sn ternary system at 1173 K in this research was determined and is presented in Figure 4.Meanwhile, the tie-lines of the two-phase alloys by the experimental measurement were given in the isothermal section.The estimated equilibria are represented by dotted lines, which are named "tentative", as they were not confirmed by XRD, SEM and EPMA analysis.The proposed isothermal section of Ti-Nb-Sn ternary system at 1173 K in this research was determined and is presented in Figure 4.Meanwhile, the tie-lines of the two-phase alloys by the experimental measurement were given in the isothermal section.The estimated equilibria are represented by dotted lines, which are named "tentative", as they were not confirmed by XRD, SEM and EPMA analysis.The XRD, SEM and EPMA results confirm 10 stable phases: two solution phases, six binary compounds and two ternary compounds in the isothermal section.They are β(Ti, Nb), liquid Sn, Ti3Sn, Ti2Sn, Ti5Sn3, Ti6Sn5, Nb6Sn5, Nb3Sn, Ti3Nb2Sn2 and Ti3NbSn.Four ternary phase fields in the isothermal section were experimental measured, and they are Ti3Sn + Ti3NbSn + β(Ti,Nb), Ti3NbSn + Ti3Nb3Sn2 + Ti3Sn, Ti2Sn + Ti3Sn + Ti3Nb3Sn2 and Ti6Sn5 + Ti3Nb3Sn2 + Nb3Sn.

Conclusions
The isothermal section of Ti-Nb-Sn ternary system at 1173 K was determined by the means of equilibrated alloy method.One single phase region, 14 binary phase regions and four ternary phase regions were experimentally measured.The probable composition range of all single phases was given.The characteristic of the system is a relatively large region of the β(Ti, Nb) continuous bcc solid solution, and a small region of liquid Sn.The maximum solubility of Sn in β(Ti, Nb) is 3.7-11.1 at.%.All the detected Ti-Sn and Nb-Sn binary compounds have a remarkable solid solution in the isothermal section.This means that a certain amount of Ti atoms were replaced by Nb atoms in Ti-Sn compounds, and some Nb atoms were substituted by Ti atoms in Nb-Sn compounds.There are two ternary compounds Ti3Nb3Sn2 and Ti3NbSn, both of which have a little solid solution.The solubility of Ti3Nb3Sn2 is measured to be about 21.7-43.2at.% Nb and 21.5-26.6at.% Sn, and the solubility of Ti3NbSn is about 20.2-28.5 at.% Nb.

Conclusions
The isothermal section of Ti-Nb-Sn ternary system at 1173 K was determined by the means of equilibrated alloy method.One single phase region, 14 binary phase regions and four ternary phase regions were experimentally measured.The probable composition range of all single phases was given.The characteristic of the system is a relatively large region of the β(Ti, Nb) continuous bcc solid solution, and a small region of liquid Sn.The maximum solubility of Sn in β(Ti, Nb) is 3.7-11.1 at.%.All the detected Ti-Sn and Nb-Sn binary compounds have a remarkable solid solution in the isothermal section.This means that a certain amount of Ti atoms were replaced by Nb atoms in Ti-Sn compounds, and some Nb atoms were substituted by Ti atoms in Nb-Sn compounds.There are two ternary compounds Ti 3 Nb 3 Sn 2 and Ti 3 NbSn, both of which have a little solid solution.The solubility of Ti 3 Nb 3 Sn 2 is measured to be about 21.7-43.2at.% Nb and 21.5-26.6at.% Sn, and the solubility of Ti 3 NbSn is about 20.2-28.5 at.% Nb.

Metals 2016, 6 , 60 7 of 9 %
Ti and 21.7-27.4at.% Sn.The solubility of Nb in compound Ti5Sn3 can be deduced to be no more than 7.9 at.% Nb, based on equilibrium phase of Alloy 16.Overall, all of the detected binary compounds has a considerable solution along isoconcentration of Sn.This indicates that a certain amount of Ti atoms are replaced by Nb atoms in Ti-Sn compounds, and some Nb atoms are substituted by Ti atoms in Nb-Sn compounds.Two ternary compounds Ti3Nb3Sn2 and Ti3NbSn are found in the isothermal section.The solubility of ternary compound Ti3Nb3Sn2 is measured to be about 21.7-43.2at.% Nb and 21.5-26.6at.% Sn, based on the compositions of equilibrium phase of typical alloys.The solubility of Ti3NbSn is about 20.2-28.5 at.% Nb, which is regarded as the stoichiometric compound according to the experimental results, and the extending is along the isoconcentratation of Sn.

Table 1 .
Compositions of the equilibrium phases in multiphase alloys.

Table 1 .
Cont.Figure 2a shows the microstructure of Alloy 12 (Ti 26.7 Nb 49.6 Sn 23.7 ).A gray phase with some crack phase Ti 3 Nb 3 Sn 2 and bright matrix phase Nb 3 Sn are observed.The results are confirmed with the XRD diffractogram of Alloy 12 in Figure