添加Sn元素對(duì)Zr-16Nb-xTi (x=4 wt%，16 wt%)合金彈性模量及磁化率的影響

1 Introduction

Due to their superior mechanical properties, biomedical alloys, such as Ti-based alloys, are widely used in clinical medicine [1-6]. Biomaterials have been subjected to increasing performance demands in recent years, due to the constant improvement of medical standards and the rapid development of medical equipment. The most important requirements for innovative biomaterials are low elastic modulus and magnetic susceptibility, as well as biocompatibility [7]. Although several Ti-based alloys meet mechanical requirements, their high magnetic susceptibility and poor biocompatibility limit their application areas in clinical medicine [8-9]. As a result, in recent years, greater attention has been paid to Zr alloys, which have lower magnetic susceptibility and elastic modulus, as well as good biocompatibility, compared to Ti alloys [10-14]. However, there are only a few reports of Zr-based alloys that meet all of the above requirements. A major problem is that α″ martensite phase, which can significantly improve alloy performance, is rarely formed in Zr-Nb and Zr-Nb-Ti systems. OKABE et al [15] first reported α″ martensite phase in the Zr-Nb system with appropriate Sn addition. Furthermore, the addition of Sn has been reported to reduce the magnetic susceptibility of Zr-Nb alloy [13]. Recently, XUE et al [16] presented Zr-16Nb-xTi alloys with outstanding mechanical and magnetic properties, particularly in Zr-16Nb-4Ti and Zr-16Nb-16Ti, and the β+α′ phase constitution is well aligned with the key requirements. Inspired by their research, we explored the possibility of obtaining lower elastic modulus and magnetism by adding appropriate Sn, created Zr-16Nb-xTi-ySn (x=4 wt%, 16 wt%,  y=4 wt%, 6 wt%) alloys, measured their mechanical and magnetic properties, observed their microstructures, and established a correlation between Sn addition and elastic modulus, mechanical strength, as well as magnetic susceptibility of these alloys.

2 Experimental

High purity Zr (99.99%), Nb (99.99%), Ti (99.99%) and Sn (99.99%) particles were selected as raw materials for the preparation of the designed alloys. In order to prevent the volatilization of Sn, Sn was wrapped in other raw materials and smelted in a high-vacuum non-consumable arc melting furnace. Each sample was smelted at least 7 times. The chemical compositions of designed alloys determined by energy dispersive spectrometry (EDS) are shown in Table 1.

Table 1  Chemical compositions of Zr-16Nb-xTi-ySn alloys as determined by EDS( wt% )

Alloy Zr Nb Ti Sn

Zr-16Nb-4Ti-4Sn 75.11 73.58 63.15 61.19

Zr-16Nb-4Ti-6Sn 16.19 16.06 16.40 16.13

Zr-16Nb-16Ti-4Sn 4.48 4.19 16.13 16.32

Zr-16Nb-16Ti-6Sn 4.22 6.17 4.32 6.36

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The ingots were heated to 1000 °C in a vacuum tube furnace for 2 h, before being quenched with ice water. The phase constitution of these alloys was detected by X-ray diffraction analyzer Rigaku D/Max 2500 at room temperature. Test samples with dimensions of 10 mm×10 mm×1 mm were cast. The microstructure was observed by the Tecnai G2 F20 transmission electron microscope at 200 kV. Tensile tests were performed on Instron 3369 mechanical testing system with an initial strain rate of 3.33×10-2 mm/s. Test samples have gauge length of 25 mm. An extensometer was used for accurate strain measurement. The magnetic susceptibility     of these alloys was measured by the superconducting quantum interferometer-vibrating sample magnetometer at room temperature. The magnetic field was set from -3 T to 3 T, which is consistent with the clinical MRI diagnosis magnetic field.

3 Results and discussion

It is clear that acicular and parallel phases were shown in dark-field TEM images (Figure 1). The selected area electron diffraction (SAED) from Zr-16Nb-4Ti-4Sn and Zr-16Nb-16Ti-4Sn alloys can be found in Figure 1. Combined with the SAED patterns, we confirm the secondary phase is α″ martensite phase (Orthorhombic structure). Figure 2 illustrates XRD patterns of Zr-16Nb-xTi-ySn alloys, with only β phase (Body centered cubic structure) identified. The absence of an apparent α″ martensite phase in XRD patterns can be ascribed to relatively high β phase fraction and small α″ martensite phase size in Zr-16Nb-xTi-ySn alloys. According to the study of XUE et al [16], Zr-16Nb-4Ti and Zr-16Nb-16Ti alloys shared the identical secondary phase of α′ martensite phase, but with the addition of Sn, α′ martensite phase was replaced by α″ martensite phase which is structurally closer to β phase. Meanwhile, as Sn concentration increases, the fraction of α″ martensite phase decreases, indicating that Sn can prevent martensite transition from α″ to α′ phase.

Figure 1  Dark-field TEM images of the martensite phase morphology in the Zr-16Nb-xTi-ySn alloys based on different areas: (a) Zr-16Nb-4Ti-4Sn; (b) Zr-16Nb-4Ti-6Sn; (c) Zr-16Nb-16Ti-4Sn; (d) Zr-16Nb-16Ti-6Sn; (e-f) α″ martensite phase selected area electron diffraction (SAED) patterns from (a) and (c)

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Figure 2  XRD patterns of Zr-16Nb-xTi-ySn alloys

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As shown in Figure 3(a), Zr-16Nb-4Ti-6Sn has the lowest elastic modulus (47.3 GPa) while Zr-16Nb-16Ti-6Sn has the highest elastic modulus (52.6 GPa); Zr-16Nb-4Ti-4Sn has the similar elastic modulus with Zr-16Nb-16Ti-4Sn, as seen in Table 2. However, XUE et al [16] reported that Zr-16Nb-16Ti has a much higher elastic modulus than Zr-16Nb-4Ti because the later has more α′ martensite phase. Thus, with Sn addition, a more α′ martensite phase was transformed into α″ martensite phase in Zr-16Nb-16Ti, but Zr-16Nb-4Ti has a less α′ martensite phase compared to Zr-16Nb-16Ti, so a large change in elastic modulus occurred in Zr-16Nb-16Ti, causing a similar elastic modulus between Zr-16Nb-4Ti-4Sn and Zr-16Nb-16Ti-4Sn. HAO et al [17] reported that α'' martensite phase has almost the same elastic modulus compared with β phase, which is also lower than that of α′ martensite phase.

Figure 3  Mechanical properties of the Zr-16Nb-xTi-yS alloys: (a) Elastic modulus; (b) Yield strength, ultimate tensile strength and elongation

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Table 2  Mechanical properties of solution treated Zr-16Nb-xTi-ySn alloys

Alloy Elastic modulus/GPa Yield strength/MPa Ultimate tensile strength/MPa Elongation/%

Zr-16Nb-4Ti-4Sn 49.3 662.6 679.3 4.54

Zr-16Nb-4Ti-6Sn 47.3 689.1 695.7 10.99

Zr-16Nb-16Ti-4Sn 50.2 737.7 758.8 5.60

Zr-16Nb-16Ti-6Sn 52.6 821.7 836.9 6.53

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The α'' martensite content reduces as Sn is added to 6%, although the change in elastic modulus in Zr-16Nb-4Ti and Zr-16Nb-16Ti is not consistent. Specifically, the modulus of the              Zr-16Nb-4Ti-6Sn alloy is decreased compared to the Zr-16Nb-4Ti alloy, while the Zr-16Nb-16Ti-6Sn is increased compared to Zr-16Nb-16Ti alloy. HAO et al [18-19] reported that the effect of Sn content on the elastic modulus of Ti-Nb alloy is non-linear and complex, especially in coexisting with β stabilizing element such as Nb. The mechanical properties of alloys can be found in Table 2. It can be found in Figure 3(b) that with Sn content increased, the yield strength and ultimate tensile strength of alloys increased, and the elongation of alloys increased, except Zr-16Nb-4Ti-6Sn. Based on TEM results of Zr-16Nb-4Ti-6Sn, it has the least martensite phase and the most β phase, which lead to higher elongation. All alloys have yield strengths greater than 660 MPa and ultimate tensile strengths greater than 679 MPa, with the Zr-16Nb-16Ti-6Sn having a maximum yield strength of 821.7 MPa and ultimate tensile strength of 836.9 MPa. The solid solution reinforcement of additional elements is mainly responsible for the increase in alloy's yield and tensile strength. Solid solution strengthening is caused by the interaction between solute atoms and dislocations. The addition of solute atoms causes local distortion of the lattice, thus hindering the movement of dislocations and improving strength.

As shown in Figure 4(a), the magnetizing curves of Zr-16Nb-xTi-ySn alloys preserve a straight line, suggesting that Sn addition has no effect on the para-magnetism of Zr-16Nb-xTi alloys. Figure 4(b) illustrates magnetic susceptibility of these alloys, from which, Zr-16Nb-16Ti-4Sn has the highest magnetic susceptibility (1.7702×10-6 cm3/g), while Zr-16Nb-4Ti-6Sn has the lowest value (1.2241×10-6 cm3/g). The magnetic susceptibility values of Zr-16Nb-4Ti-4Sn and Zr-16Nb-16Ti-6Sn are 1.5271×10-6 cm3/g and 1.2843×10-6 cm3/g, respectively. Based on the work of NOMURA et al [20], the phase magnetic susceptibility of Zr-Nb binary alloys was as follows: χβ>χα'>χω. However, they did not mention about the order of α″ martensite phase. In our study, with Sn addition, none of Zr-16Nb-xTi-ySn alloys were detected with α′ martensite phase, and ω phase. Only β and α″ martensite phases were observed in these alloys.

Figure 4  Magnetic properties of Zr-16Nb-xTi-ySn alloys at room temperature: (a) Magnetizing curves; (b) Magnetic susceptibility

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By comparing Zr-16Nb-4Ti-4Sn and Zr-16Nb-4Ti-6Sn, Zr-16Nb-16Ti-4Sn and Zr-16Nb-16Ti-6Sn alloys, we can find that magnetic susceptibility increases with α″ martensite phase content, meanwhile, the magnetic susceptibility of Sn element is also an important factor as it is just 3.0×10-8 cm3/g. It is worth noting that these alloys with the same Sn content yet display different trends. Though Zr-16Nb-16Ti-6Sn possesses a more α″ martensite phase and Ti content than Zr-16Nb-4Ti-4Sn, the pretty low magnetic susceptibility of Sn creates a huge effect to alloys, magnetic susceptibility, resulting in lower value for Zr-16Nb-16Ti-6Sn. Compared with Zr-16Nb-4Ti and Zr-16Nb-16Ti without Sn, the mass magnetic susceptibility of the alloy is obviously reduced, and the biggest changes occurred in Zr-16Nb-16Ti and Zr-16Nb-16Ti-6Sn. The value decreased from 2.1727×10-6 cm3/g to 1.2843×10-6 cm3/g. The extremely low magnetic susceptibility of the Sn has a greater influence on alloys, which can effectively reduce the magnetic susceptibility.

4 Conclusions

Sn addition is the dominant reason for the decrease of Zr-16Nb-xTi (x=4 wt%, 16 wt%) alloys, elastic modulus and magnetic susceptibility by adjusting alloys phase constitution. Sn inhibits α′ martensite phase formation, and promotes β to α″ martensite phase transformation, which helps effectively reduce the elastic modulus of alloys. In addition Sn can obviously reduce the magnetic susceptibility of alloys because it has a relatively low magnetic susceptibility.

Therefore, with proper Sn addition, Zr-16Nb-xTi (x=4 wt%, 16 wt%) alloys, “stress shielding” effects and artifacts during MRI diagnosis can be further alleviated.