Microstructure and Mechanical Properties of Medical Magnesium Alloy Fabricated by Unequal Channel Angular Pressing

Xiaoping Luo, Mingang Zhang, Daqin Fang, Yaosheng Chai

School of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan, China.
School of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan, China..

DOI: 10.4236/jbnb.2013.42018                

Abstract

A new unequal channel angular pressing (UCAP) procedure is proposed for ultrafine-grained metals and alloys. The microstructures and mechanical properties of Mg-5.8Zn-1.2Y-0.7Zr alloys subjected to unequal channel angular pressing (UCAP) are investigated. It is found that the optimum condition in UCAPed alloy is obtained at 523 K with a largest elongation to failure of 13.1% and ultimate tensile strength of ~400 MPa. Microstructural observations show that the grain size is refined to ~1.0 μm during UCAP. The probable mechanisms for these high mechanical properties are attributed to grain size and destroyed secondary phase strengthening effects and fine precipitates formed during pressing at high temperature by severe shear and plastic deformation.

Keywords

Mg-Zn-Y-Zr Magnesium Alloy; Unequal Channel Angular Pressing; Microstructure; Mechanical Property

Figure 3. X-ray diffraction pattern of the Mg-5.8Zn-1.2Y- 0.7Zr alloy before UCAP.

Figure 4. Optical microstructure of Mg-5.8Zn-1.2Y-0.7Zr alloy after UCAP at 523 K.

plane, the grains are elongated in an inclined direction, and on the Y plane the grains remain reasonably equiaxed, reduced from their initial size, and there is some slip parallel to the Z direction. It is clear from these photomicrographs that the coarse-grains have become fragmented by the high pressure imposed and sheared during UCAP. The microstructure shows the presence of small grains of ~1 - 3 µm diameters together with comparatively coarse grains. Some grain sizes are too small to be distinguished by optical microscopy. At low processing temperature (Figures 4 (x-z)), microstructures are mainly composed of fine recrystallized grains. Moreover, some elongated and unrecrystallised grains along extrusion direction are also observed. These elongated and unrecrystallised grains have high dislocation density. In contrast, with increasing processing temperature, more recrystallized grains appear in microstructures. The increase of processing temperature results in grain growth, on the other hand, coordinated movement among grains is improved because elongated grains are replaced by equiaxed and recrystallised grains. A closer investigation using SEM revealed that the grain size of the UCAP sample is 0.8 um. Figure 5 shows the SEM image of the UCAPed alloy. Secondary phase of the alloy are moved away from grain boundaries and destroyed into smaller particles with irregular shape by UCAP.

To check this possibility, Figure 6 shows the DSC results of the before and after UCAP alloys during the heating process where the material was subjected to a heating rate of 10 K·min⁻¹. The first endothermic peak appeared at about 771 K for before UCAP alloy and 793 K for after UCAP alloys. This peak corresponds to the melting temperature of eutectic phase. The eutectic temperature of UCAP alloys is much higher than the eutectic temperature of as-cast alloy, showing that under high temperature and severe plastic deformation, UCAP refined grain that can lead to the composition of the matrix alloy changed, part of the I-phase is dissolved into the α-Mg and released Zn and formed W-phase in the region with a higher concentration and phase transition temperature significantly improved. The lower eutectic temperature of as-cast alloy is much higher than the eutectic temperature of Mg-Zn binary alloy (613 K). It can be inferred that the addition of yttrium can greatly increase the eutectic temperature compared to Mg-Zn binary alloy. Based on the DSC curves and microstructural examinations, it is concluded that significant grain refinement and breakup of the secondary phases and their dispersive distribution in the Mg matrix by the UCAP severe plastic deformation contributes to the high melting temperature.

Figure 7 shows the TEM morphology and selected area diffraction patterns (SADP). The SADP are taken from the large particles in the UCAP treated alloy, and allow these particles to be identified as I-phase and Wphase, which confirms the XRD results for the as-cast alloy. The particle which has a diffraction pattern showing five-fold symmetry can be identified as I-phase.

Figure 5. SEM image of the Mg-Zn-Y-Zr alloy after UCAP at 523 K.

Figure 6. DSC of Mg-5.8Zn-1.2Y-0.7Zr before and after UCAPed alloy.

Figure 7. TEM images of large destroyed particles in the UCAPed Mg-5.8Zn-1.2Y-0.7Zr alloy at 573 K: (a) I-phase; (b) The selected area diffraction (SAD) patterns of I-phase; (c) W-phase; (d) The selected area diffraction (SAD) patterns of W-phase.

3.2. Mechanical Properties

The small grain sizes and high defect densities inherent in materials processed by UCAP lead to much higher mechanical properties than those of their coarse-grained counterparts. Table 1 shows the mechanical properties of the alloys UCAP processed at different temperatures. UCAP treatment has produced outstanding improvement in both strength and ductility-related characteristics. The mechanical properties depend strongly on the processing temperature. There was an obvious decrease in the yield strength (YS) and the ultimate tensile strength (UTS) for the sample processed at 673 K compared with the sample processed at 473 K. The best combination of both high yield strength and ultimate tensile strength was found

Table 1. Mechanical properties of Mg-5.8Zn-1.2Y-0.7Zr magnesium alloys after UCAP.

at 523 K. The increase in the processing temperature provides a similar effect to that reported in previous studies.

4. Conclusion

Mg-5.8Zn-1.2Y-0.7Zr alloy consists of α-Mg, Mg₃Zn₆Y and Mg₃Zn₃Y₂ phases in the unpressed condition. The microstructure of the cast alloy in the homogenized state was coarse-grained with an average grain size of about 110 µm, processing by UCAP produced considerable grain refinement. Mg₃Zn₆Y phases were destroyed into small particles. These particles played a strengthening role in the UACPed alloy and led to a final ultimate tensile strength of ~400 MPa and sufficient ductility of ~ 13.1%.

5. Acknowledgements

The study was financially supported by both the Key Science and Technology Program of Shanxi Province, China (No. 20090322007), and the project of innovation of Shanxi Province Postgraduates (No. 20100392).

Conflicts of Interest

The authors declare no conflicts of interest.

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