Koji Hagihara

Possibility of Mg- and Ca-based intermetallic compounds as new biodegradable implant materials.

Mg- or Ca-based intermetallic compounds of Mg2Ca, Mg2Si, Ca2Si and CaMgSi are investigated as possible new candidates for biodegradable implant materials, attempting to improve the degradation behavior compared to Mg and Ca alloys. The reactivity of Ca can be indeed reduced by the formation of compounds with Mg and Si, but its reactivity is still high for applications as an implant material. In contrast, Mg2Si shows a higher corrosion resistance than conventional Mg alloys while retaining biodegradability. In cytotoxicity tests under the severe condition conducted in this study, both pure Mg and Mg2Si showed relatively high cytotoxicity on preosteoblast MC3T3-E1. However, the cell viability cultured in the Mg2Si extract medium was confirmed to be better than that in a pure Mg extract medium in all the conditions investigated with the exception of the 10% extract medium, because of the lower corrosion rate of Mg2Si. The cytotoxicity derived from the Si ion was not significantly detected in the Mg2Si extract medium in the concentration level of ~70 mg/l measured in the present study. For aiming the practical application of Mg2Si as an implant material, however, its brittle nature must be improved.

Strength and deformation mechanism of C40-based single crystal and polycrystalline silicides

Abstract Plastic behavior and anomalous strengthening in various C40-type silicide single crystals are reviewed. The anomalous strengthening occurs in binary and ternary C40-type silicides at high temperatures. MoSi 2 and WSi 2 crystallize in the C11 b structure. Additional Mo and W atoms in NbSi 2 -based silicides may preferentially gather at the superlattice intrinsic stacking fault between two 1/6 〈1 2 10〉 superpartials and form a strong dragging atmosphere. The anomalous strengthening is due to the dragging atmosphere around moving 1/3 〈1 2 10〉 superlattice dislocations. Formation of the atmosphere can improve high temperature strength of NbSi 2 -based silicides with the C40 structure: the anomalous peak temperature is shifted from 1400 to 1600°C, and the peak stress height rises with increasing concentration of Mo and W addition. Attempts to improve the ductility and high-temperature strength of C40-based polycrystalline silicides are also made by controlling the microstructure and species of constituent phase and volume fraction of each phase. Lamellar structure in pseudo binary MoSi 2 /NbSi 2 is produced during the phase transformation from the C40 to C11 b phase after the peritectic reaction. The lamellar structure effectively maintains the good thermal stability and improve the high-temperature strength.

Indentation fracture behavior of (Mo0.85Nb0.15)Si2 crystals with C40 single-phase and MoSi2(C11b)/NbSi2(C40) duplex-phase with oriented lamellae

Abstract Fracture behavior and toughness of C40-structured (Mo0.85Nb0.15)Si2 single crystals and MoSi2(C11b)/NbSi2(C40) duplex-phase crystals with oriented lamellae were investigated by micro-Vickers hardness tests. The indentation fracture toughness in the C40 single-phase showed low values of about 1.0–1.4 MPa m1/2, while fracture toughness was improved in the duplex-phase crystals. Ductile phase toughening by the C11b phase, deflection of crack-propagation at the lamellar boundary and the effect of residual stress field at the crack tip are considered to be responsible for the increment of fracture toughness of MoSi2(C11b)/NbSi2(C40) duplex-phase crystals.

Crystallographic nature of deformation bands shown in Zn and Mg-based long-period stacking ordered (LPSO) phase

Formation of curious deformation bands has been reported as one of the deformation mechanisms occurring in an Mg-based long-period stacking ordered (LPSO) phase. The origin of the deformation band is still unknown, and the possibility of the deformation kink band and/or the deformation twin has been discussed. To clarify this, the crystallographic nature of deformation bands formed in the LPSO phase was examined by scanning electron microscope–electron backscatter diffraction (SEM-EBSD) pattern analysis. The results were compared to those of the deformation kink bands formed in hcp-Zn and deformation twins formed in hcp-Mg polycrystals. The deformation bands in the LPSO phase was confirmed not to exhibit a fixed crystal orientation relationship with respect to the matrix, different from the case shown in the deformation twin. Instead, the deformation band in the LPSO phase showed three arbitrariness on its crystallographic nature: an ambiguous crystal rotation axis that varied on the [0 0 0 1] zone axis from band to band; an arbitral crystal rotation angle that was not fixed and showed relatively wide distributions; and a variation in crystal rotation angle depending on the position even within a deformation band boundary itself. These features were coincident with those observed in the deformation bands formed in Zn polycrystals, suggesting that the formed deformation bands in LPSO phase crystals are predominantly deformation kink bands.

Degradation behavior of Ca–Mg–Zn intermetallic compounds for use as biodegradable implant materials

With the goal of developing new biodegradable implant materials, we have investigated the degradation behavior of (Ca, Mg)-based intermetallic compounds. The degradation behavior of the compounds within the Ca-Mg-Zn system was roughly classified into four groups, and their behaviors were strongly influenced by the compositions of the compounds. For example, the Ca3MgxZn(15-x) compound exhibited a large solubility region with varying the Mg/Zn ratio, and the Ca3Mg12Zn3 phase alloy with the lowest Zn content was rapidly broken apart within 6h of immersion. Alternatively, the Ca3Mg4.6Zn10.4 phase alloy with the highest Zn content retained the bulk shape even after 250 h of immersion. These varying degradation behaviors were ascribed to the difference in the formability of Zn oxide as a protective layer against corrosion on the specimen surfaces, depending on the Zn content. The gained results suggest that there is a feasibility on developing new biodegradable materials based on intermetallic compounds in which the degradation rate can be controlled by their compositions.

Isotropic plasticity of β-type Ti-29Nb-13Ta-4.6Zr alloy single crystals for the development of single crystalline β-Ti implants.

β-type Ti-29Nb-13Ta-4.6Zr alloy is a promising novel material for biomedical applications. We have proposed a ‘single crystalline β-Ti implant’ as new hard tissue replacements for suppressing the stress shielding by achieving a drastic reduction in the Young’s modulus. To develop this, the orientation dependence of the plastic deformation behavior of the Ti-29Nb-13Ta-4.6Zr single crystal was first clarified. Dislocation slip with a Burgers vector parallel to <111> was the predominant deformation mode in the wide loading orientation. The orientation dependence of the yield stress due to <111> dislocations was small, in contrast to other β-Ti alloys. In addition, {332} twin was found to be operative at the loading orientation around [001]. The asymmetric features of the {332} twin formation depending on the loading orientation could be roughly anticipated by their Schmid factors. However, the critical resolved shear stress for the {332} twins appeared to show orientation dependence. The simultaneous operation of <111> slip and {332} twin were found to be the origin of the good mechanical properties with excellent strength and ductility. It was clarified that the Ti-29Nb-13Ta-4.6Zr alloy single crystal shows the “plastically almost-isotropic and elastically highly-anisotropic” nature, that is desirable for the development of ‘single crystalline β-Ti implant’.

Multimodal Microstructure Evolution in Wrought Mg-Zn-Y Alloys with High Strength and Increased Ductility

High strength and ductile Mg96Zn2Y2 (at%) alloys with multi-modal microstructure are developed. Microstructure of the extruded Mg96Zn2Y2 alloy consists of three regions; the dynamically recrystallized -Mg fine-grains region, the hot-worked -Mg coarse-grains region elongated along extrusion direction, and the long-period stacking ordered (LPSO) phase region with kink deformation bands. Bimodal microstructure evolution in -Mg matrix is influenced by the morphology of the LPSO phase in the as-cast state, therefore, the effect of secondary dendrite arm spacing in cast state on the microstructure evolution and mechanical properties of the extruded Mg-Zn-Y alloy is investigated. An increase in the dynamically recrystallized grains improves ductility of the extruded alloys; the effective dispersion of the LPSO phase enhances strengthening of the alloy.

Single-crystal growth and plastic deformation behaviour of a Ti-15Mo-5Zr-3Al alloy for biomedical application

A Ti-15Mo-5Zr-3Al alloy with a bcc structure are promising materials for biomedical application and were examined. The focus of this study was on the effect of heat treatment on microstructure and plastic deformation behaviour using a single crystal. The single crystal was successfully obtained by a floating zone method at a crystal growth rate of 2.5 mm/h. A slip at the dislocation was present irrespective of heat treatment at 573K or 673K for 1.2 ks or 300 ks. The yield stress at the [149] loading axis varied significantly depending on microstructure, especially from the precipitation of the a phase. Al addition suppresses generation of the σ phase and increases the yield stress at the same time.

Microstructure and compressive flow stress of directionally solidified ternary Ni3(Al, Nb) and quaternary Ni3(Al, Nb, Ti) alloys with duplex phase

Abstract The microstructure and compressive flow stress of directionally solidified ternary Ni3 (Al, Nb) and quaternary Ni3 (Al, Nb, Ti) alloys were examined. Three compositions of Ni–16.0 at.%Nb–9.0 at.%Al (Alloy 1), Ni–13.3 at.%Nb–7.5 at.%Al–4.2 at.%Ti (Alloy 2) and Ni– 10.7 at.%Nb–6.0 at.%Al–8.3 at.%Ti (Alloy 3) were selected for investigation. Alloy 1 was composed of the L12 and the D0a phases while the constituent phases varied for the D024 and the D0a phases for Alloys 2 and 3 with Ti content. The definite crystallographic relationship was observed between the D024 and the D0a phases to be (0001)D024// (010)D0a and ∹〉 D024//〈100〉D0a in Alloy 3. Compression tests were conducted along the growth direction in the temperature ranging from room temperature to 1000 °C. Alloy 1 exhibited high yield stress at low temperatures, but it rapidly decreased above 700 °C. Similar temperature dependence of yield stress was observed in Alloy 2, although the onset temperature of a rapid decrease in yield stress was somewhat lower. Alloy 3 with the highest Ti content showed the lowest compressive strength among the three alloys, while relatively good low-temperature ductility was obtained in Alloy 3. Yield stress of Alloy 3 exhibited anomalous strengthening behaviour accompanied by the basal slip in both D024 and D0a phases. Transition in operative slip systems from the basal slip to the prism slip occurred at the peak temperature of yield stress anomaly (600 °C), resulting in a gradual decrease in yield stress. Slip transfer behaviour between the D024 and the D0a phases was briefly discussed.

Temperature and Orientation Dependence of Fracture Behavior of Directionally Solidified Duplex-Phase Crystals Composed of Ni3X-Type Intermetallic Compounds

Fracture behaviors of three directionally solidified (DS) duplex-phase alloys composed of Ni3Nb(D0a)/Ni3Al(L12), Ni6TaAl(D024)/Ni3Al(L12) and Ni3Ti(D024)/Ni3Si(L12) phases, respectively were investigated by three-point bending tests, focusing on temperature and orientation dependence. The temperature-toughness relation showed dissimilar curves depending on alloy. The increasing rate of fracture toughness was the highest in the Ni3Al/Ni3Nb alloy with fine lamellar structure and was the lowest in the Ni3Al/Ni6TaAl alloy with rod-like precipitates. The controlling mechanism for the temperature dependence of fracture behavior of Ni3Al/Ni3Nb alloys was discussed.