2023 Volume 64 Issue 9 Pages 2333-2336
We investigated the alloy composition of Mg–Ca–Zn alloys suitable for rapidly solidified powder metallurgy (RS PM) processing and tried to develop biomedical Mg–Ca–Zn RS PM alloys with high strength, high ductility, and high corrosion resistance. The alloy composition suitable for RS PM processing was Mg–1.0Ca–0.5Zn (at%) in ternary Mg–Ca–Zn alloys. An Mg–1.0Ca–0.5Zn RS PM alloy with combined additions of 0.1 at% Y and 0.03 at% Mn exhibited excellent performance with a tensile yield strength of 376 ± 17 MPa, an elongation of 14.1 ± 3.1%, and a corrosion rate of 0.46 ± 0.04 mm·year−1 when immersed in HBSS, resulting in the achievement of our target properties. Especially the yield strength of the Mg–1.0Ca–0.5Zn–0.1Y–0.03Mn RS PM alloy developed in this study was higher than that of previously reported Mg–Zn–Ca(–Mn–Zr) or Mg–Ca–Zn(–Mn–Zr) ingot metallurgy (IM) alloys, which were produced by single extrusion, double extrusion or severe plastic deformation (SPD) of cast ingots.
Comparison of the tensile yield strength and elongation of the biomedical Mg–1.0Ca–0.5Zn–0.1Y–0.03Mn (at%) rapidly solidified powder metallurgy (RS PM) alloys developed in this study, with various high strength Mg–Zn–Ca(–Mn–Zr) or Mg–Ca–Zn(–Mn–Zr) ingot metallurgy (IM) alloys. The RS PM alloy developed in this study has higher yield strength than previously developed biomedical high-strength Mg–Zn–Ca(–Mn–Zr) or Mg–Ca–Zn(–Mn–Zr) IM alloys.
Magnesium alloys are expected to be used as biomedical materials for bone plates and other implants because they have a Young’s modulus close to that of human bone, which reduces the negative effects of stress shielding, and they have biocompatibility and bioabsorbability.1–7) Mg–Ca, Mg–Ca–Mn, Mg–Zn–Ca, Mg–Sr, Mg–Zn–Sr alloys with Ca and Sr, which have the bone regeneration effect, have already been developed, but the yield strength and corrosion resistance of these alloys are not sufficient for practical use.8–13) These alloys are fabricated by ingot metallurgy (IM) processing where cast ingots are wrought plastically by extrusion or rolling. The rapidly solidified powder metallurgy (RS PM) processing provides features such as grain refinement, expansion of the solid solution limit, homogenization of the microstructure, and formation of nonequilibrium phases.14) It has been reported that the RS PM processing improves the tensile yield strength and the corrosion resistance as compared with IM processing in Mg–Zn–Y and Mg–Al–Ca alloys.15–22)
It is well known that Ti–6Al–4V (mass%) alloy having a yield strength of 825 MPa or higher, an elongation of 10% or larger and a Young’s modulus of 110 GPa is a typical biomedical material.1,23,24) For the same elastic limit strain as the Ti–6Al–4V alloy, the tensile yield strength of magnesium alloys with a Young’s modulus of approximately 45 GPa must be at least 335 MPa. It is also well known that Mg–4Y–3Nd–0.4Zr (mass%) (WE43) alloy is a typical biomedical magnesium alloy.7) According to our measurement, the corrosion rate of the WE43 alloy is 0.50 mm·year−1 when immersed in Hanks’ Balanced Salt Solution (HBSS). Accordingly, magnesium alloys having a tensile yield strength of 335 MPa or higher, an elongation of 10% or larger and a corrosion rate of 0.5 mm·year−1 or lower when immersed in HBSS need to be developed.
In this study, we investigated the alloy composition of Mg–Ca–Zn alloys suitable for RS PM processing, and tried to develop biomedical Mg–Ca–Zn RS PM alloys with high strength, high ductility, and high corrosion resistance, with a yield strength of over 335 MPa, an elongation of over 10%, and a corrosion rate of less than 0.5 mm·year−1 when immersed in HBSS.
Master alloy ingots were prepared by the high-frequency induction melting of pure Mg (99.99 mass%), Ca (99.9 mass%), Zn (99.9 mass%), Y (99.5 mass%) and Mn (99.9 mass%) metals. The molten alloys were maintained at 1023 K for 15 min in an Ar atmosphere and then cast into a mild steel mold. The rapidly solidified (RS) ribbons were prepared using a single-roller melt-spinning method with a roll-circumferential velocity of 42 m·s−1. The RS ribbons were collected in air after exposure to dry air, immediately transferred to a glove box, where they were compacted into copper capsules with an outer diameter of 29 mm in a high-purity argon gas atmosphere. Degassing was then performed at 573 K for 15 min. Consolidation was conducted by hot extrusion at an extrusion ratio of 10, an extrusion temperature of 623 K, and a ram speed of 2.5 mm·s−1. For comparison, ingot metallurgy (IM) alloys were produced by extrusion of cast ingots with a diameter of 29 mm under the same extrusion conditions as the RS ribbon consolidation.
Tensile tests were performed using an Instron testing machine (Instron, Model 5584) at room temperature and at an initial strain rate of 5 × 10−4 s−1. The gauge sections of the tensile specimens were 2.5 mm and 15 mm in diameter and length, respectively. The tensile axis was placed along the extrusion direction. A 0.2% proof strength was used as the yield strength. The tensile properties were measured for three specimens and averaged. The corrosion rate was evaluated by an immersion test in Hanks’ balanced salt solution (HBSS, SIGMA-ALDRICH, H1387) for 168 h. The composition of the HBSS is shown in Table 1. The dimension of immersion test specimens was 4 mm × 20 mm × 2 mm. The surface area of the immersion test specimen was 2.56 cm2. During the immersion test, the pH of the HBSS solution was maintained at 7.0–7.4 using a pH sensor and CO2 gas. The corrosion rate, PM [mm·year−1], was calculated by the weight change of the specimens. For the immersion test, 450 ml of the solutions was used per measurement and the temperature of the solution was kept at 310 K.
The microstructure was investigated by an X-ray diffractometer (XRD; Bruker, D8 DISCOVER), a scanning electron microscope (SEM; JEOL, JIB-4601F) equipped with an energy-dispersive X-ray spectroscopy (EDS) system, and a scanning transmission electron microscope operating at 200 kV (STEM/TEM; JEOL, 2100F). The crystallographic orientations of the alloys were analyzed by electron backscatter diffraction (EBSD) analysis using orientation imaging microscopy (OIM; TSL Solutions K.K.).
Figure 1 shows the dependence of yield strength and elongation of binary Mg–xCa (at%) alloys on Ca addition. The yield strength increased and the elongation decreased with increasing Ca content. The RS PM alloys exhibited higher strength and higher ductility than the IM alloys, and exhibited a yield strength of 305 ± 3 MPa and an elongation of 14.0 ± 0.7% at 1.0 at% Ca, indicating that both high strength and high ductility were achieved.
Tensile yield strength and elongation of the Mg–xCa (at%) alloys produced by RS PM and IM processing.
Figure 2 shows the yield strength, elongation and corrosion rate when immersed in HBSS for the ternary Mg–1.0Ca–xZn (at%) alloys. The yield strength decreased with increasing Zn addition, while the elongation increased up to 0.5 at% Zn. The corrosion rate of the IM alloys increased monotonically with increasing Zn content, while the corrosion rate of the RS PM alloys decreased significantly with increasing Zn content up to 0.5 at% and increased slightly thereafter. The RS PM alloys exhibited higher strength, higher ductility and higher corrosion resistance than the IM alloys, with the best balance of yield strength, elongation and corrosion resistance at 0.5 at% Zn. The tensile yield strength, the elongation and the corrosion rate when immersed in HBSS of the Mg–1.0Ca–0.5Zn RS PM alloy were 318 ± 1.8 MPa, 19.5 ± 0.2% and 0.34 ± 0.02 mm·year−1, respectively.
(a) Tensile yield strength and elongation, and (b) corrosion rate when immersed in HBSS of the Mg–1.0Ca–xZn (at%) alloys produced by RS PM and IM processing.
XRD patterns of the ternary Mg–1.0Ca–xZn (at%) RS PM alloys showed that α-Mg and C14-type Mg2Ca phases were formed when the amount of Zn added was 1 at% or less, but Mg6Zn2Ca3 compound was formed in addition to the α-Mg and Mg2Ca phases when the amount of Zn added was above 2 at%. The Mg–1.0Ca–0.5Zn RS PM alloy was found to be duplex, with α-Mg and Mg2Ca phases, in which the grain size of the α-Mg phase and the particle size of the Mg2Ca were approximately 1.2 ± 0.1 µm and 180 ± 20 nm, respectively.
3.2 Trace addition of Y and Mn to Mg–1.0Ca–0.5Zn RS PM alloyIt has been reported that adding a trace amount of Y to Mg alloys improves mechanical properties and corrosion resistance.25) It has also been reported that a trace addition of Mn to Mg alloys improves corrosion resistance.26) Therefore, we tried adding 0.1 at% Y and 0.03 at% Mn to the Mg–1.0Ca–0.5Zn RS PM alloy. As shown in Fig. 3, the Mg–1.0Ca–0.5Zn–0.1Y–0.03Mn RS PM alloy exhibited excellent properties, namely a yield strength of 376 ± 17 MPa, an elongation of 14.1 ± 3.1% and a corrosion rate of 0.46 ± 0.04 mm·year−1 when immersed in HBSS, resulting in achieving the target properties of this study.
(a) Engineering stress-strain curves, and (b) corrosion rate when immersed in HBSS of the Mg–1.0Ca–0.5Zn and Mg–1.0Ca–0.5Zn–0.1Y–0.03Mn (at%) RS PM alloys.
A Mg3Zn3Y2 phase was detected in addition to the α-Mg and Mg2Ca phases in the XRD pattern of the Mg–1.0Ca–0.5Zn–0.1Y–0.03Mn RS PM alloy. Figure 4 shows SEM micrograph, STEM image and IPF map of the Mg–1.0Ca–0.5Zn–0.1Y–0.03Mn RS PM alloy. EBSD analysis reveals that the α-Mg matrix has a grain size of about 1.3 ± 0.1 µm, where fine Mg2Ca and Mg3Zn3Y2 particles are homogeneously distributed. STEM observation shows that the size of the Mg2Ca and Mg3Zn3Y2 particles is 130 ± 11 nm and 35 ± 5 nm, respectively. The excellent properties can be attributed to the fine α-Mg grains and homogeneous dispersion of fine Mg2Ca and Mg3Zn3Y2 particles.
SEM micrograph, STEM image and IPF map of the Mg–1.0Ca–0.5Zn–0.1Y–0.03Mn (at%) RS PM alloy.
Figure 5 show the comparison of the tensile yield strength and elongation of the Mg–1.0Ca–0.5Zn–0.1Y–0.03Mn RS PM alloy developed in this study with previously reported biomedical Mg–Zn–Ca(–Mn–Zr) and Mg–Ca–Zn(–Mn–Zr) IM alloys having a tensile yield strength of over 300 MPa and an elongation of over 5%, which were produced by single extrusion, double extrusion or severe plastic deformation (SPD) of cast ingots.2,8–13) It is clear that the Mg–1.0Ca–0.5Zn–0.1Y–0.03Mn RS PM alloy developed in this study has higher strength than the Mg–Zn–Ca(–Mn–Zr) and Mg–Ca–Zn(–Mn–Zr) IM alloys.
Mg–Ca–Zn alloys fabricated by RS PM processing showed higher strength, higher ductility and higher corrosion resistance as compared to those fabricated by IM processing. The alloy composition suitable for the RS PM processing was Mg–1.0Ca–0.5Zn (at%). RS PM alloys with combined additions of 0.1 at% Y and 0.03 at% Mn exhibited excellent performance with a tensile yield strength of 379 MPa, an elongation of 15%, and a corrosion rate of 0.49 mm·year−1 when immersed in HBSS, resulting in the achievement of our target properties. Especially, the yield strength of the Mg–1.0Ca–0.5Zn–0.1Y–0.03Mn RS PM alloy developed in this study was higher than that of previously reported biomedical Mg–Zn–Ca(–Mn–Zr) and Mg–Ca–Zn(–Mn–Zr) IM alloys.
This work was partially supported by JSPS KAKENHI for Scientific Research A (JP20H00312), JST SICORP V4 (JPMJSC2019) and the Light Metal Educational Foundation, Inc.
