Materials Science Forum Vols. 654-656

Abstract: The self-accommodation microstructure of a -titanium shape memory alloy (SMA) was investigated by conventional transmission electron microscopy (CTEM) observation. There were two distinct minimum units for the self-accommodation microstructure. One was the V-shaped pair of two habit plane variants that were bounded by the {111}TypeI twin as reported in some previous studies. The other was the bundle of the two habit plane variants that were bounded by the <211>TypeII twin. The later one had not been recognized as the self-accommodation microstructure in -titanium SMAs.

Abstract: This study aims at producing porous Ti filled with biodegradable materials for biomedical implants by means of spark plasma sintering method (SPS). To improve bone fixation and to obtain appropriate Young’s modulus as a medical implant material, we applied -tri calcium phosphate (-TCP) to the Ti-based composite. Ti/-TCP powder mixtures were sintered by SPS under applied stress of 45MPa with various temperatures and holding time. Vickers hardness (Hv) of obtained composite increased with increasing the holding time up to 10 min, and saturated hardness was approximately 750 Hv, which is extremely higher than that of bulk Ti. Hardness also increased as sintering temperature increased up to 1473 K. From the results of microstructure observations by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDXS), O- and P- containing Ti surrounded around Ti particle, and O diffused into Ti particle to a certain extent. X-ray diffraction results indicated several kinds of Ti-O and/or Ti-P formed in the specimen. Results indicated that it is the brittle phases formed during sintering that increased the hardness.

Abstract: Oxyapatite, amorphous calcium phosphate, and double-layered calcium phosphate coating films were fabricated on mirror-polished commercially pure titanium (CP Ti) and blasted Ti-6Al-4V alloy substrates by radiofrequency (RF) magnetron sputtering; the properties of these films were evaluated in vivo and in vitro. The bonding strength between the calcium phosphate films and the Ti substrates was higher than 50 MPa. This value is higher than the bonding strength reported in the case of plasma-sprayed calcium phosphate coating films fabricated on Ti substrates. The removal torque of screw-type blasted Ti-6Al-4V alloy implants in the femurs of Japanese white rabbits increased with the duration of implantation, and the removal torque values of the coated implants was observed to be higher than those of the non-coated implants. In vitro and in vivo studies indicate that coating Ti implants with calcium phosphate films using RF magnetron sputtering is effective in improving the bone compatibility of Ti implants. Finally, the factors that should be considered in fabricating biomedical coating films were discussed.

Abstract: The equal channel angular pressing (ECAP) results in ultrafine-grained (~200–500 nm) Ti with superior mechanical properties without the harmful alloying elements, which is beneficial for medical implants. To improve the bioactivity of Ti surface, Ca/P-containing porous titania coating were prepared on the ultrafine-grained Ti and coarse-grained Ti by micro-arc oxidation (MAO) in Ca/P based solution. The phase identification, thickness, composition and morphology of the coatings were analyzed. The amounts of Ca, P and Ca/P ratio of the MAO coating formed on ultrafine-grained Ti were higher than those for coarse-grained Ti samples. Hydroxyapatite and α-Ca3(PO4)2 phases appeared in MAO coating formed on ECAP-treated Ti with increasing reaction time of 20 min (E20). Incubated in simulated body fluid within 2 days, bone-like apatite was completely formed on E20 surface, as evidenced of preferable bioactivity.

Abstract: The surface modification of commercially pure titanium (CP Ti) by pack cementation treatment at 973 K using tetracalcium phosphate (Ca4(PO4)2O, TTCP) slurry was investigated. An HAp phase and a CaTiO3 phase were observed on the reaction layer of the CP Ti substrate after pack cementation treatment at 973 K for 86.4 ks. TTCP powder decomposed to HAp and CaO, and CaO reacted with TiO2 to form CaTiO3. The reaction layer on the CP Ti substrate consisted of inner and outer layers and the particles were in the outer reaction layer. The pores observed on the reaction layer were formed by the detachment of particles from the outer layer. The bonding strength of the reaction layer was 68.1 MPa. Apatite completely covered the surface of the pack-cementation-treated CP Ti after immersion in Kokubo solution for 21.6 ks; such rapid apatite formation suggests that pack cementation treatment improves the biocompatibility of titanium.

Abstract: The phase and morphology of precipitates in heat-treated Co-28Cr-6Mo-xC (x = 0.12, 0.15, 0.25, and 0.35mass%) alloys were investigated. The as-cast alloys were solution-treated in the temperature range of 1473 to 1623 K for 0 to 43.2 ks. Complete precipitate dissolution was observed in all four alloys, each of which had different carbon contents. The holding time for complete dissolution was greater for alloys with greater carbon content. The curve representing the boundary between the complete- and incomplete-dissolution conditions for each alloy is C shaped. Under the incomplete precipitate dissolution conditions of the Co-28Cr-6Mo-0.25C alloy, an M23C6 type carbide and a π-phase (M2T3X type carbide with β-Mn structure) were observed at 1548 to 1623 K, and starlike precipitates with a stripe pattern and with a dense appearance were both observed; the former comprised the M23C6 type carbide + γ-phase, and the latter was the π-phase. In contrast, only a blocky-dense M23C6 type carbide was observed at 1473 to 1523 K.

Abstract: In this study, the carbide dissolution and formation of biomedical Co-28Cr-6Mo-0.25C-1Si (1Si) and Co-28Cr-6Mo-0.25C-1Mn (1Mn) alloys occurring during solution treatment and aging were investigated. The addition of Si or Mn markedly affected the behavior of precipitates during heat treatment. The alloy with added Si required a longer solution treatment time for complete precipitate dissolution as compared to the alloy with added Mn. Blocky M23C6-type carbide was observed during solution treatment at 1448–1523 K. Starlike precipitates were observed in the 1Si and 1Mn alloys at 1523–1548 K and 1523 K, respectively. The starlike precipitates exhibited two types of morphologies: dense and stripe patterned. The starlike precipitates with dense and stripe-patterned appearances were a -phase and M23C6¬-type carbide + metallic -phase, respectively. An M23C6-type carbide and an -phase were formed during the aging of both the 1Si and the 1Mn alloys. In addition, an intermetallic -phase was detected during the aging of the 1Si alloy at 1373 K.

Abstract: Thermomechanical processes combined with reverse transformation from -hcp and M2N) dual-phase to -fcc phase of biomedical Co-Cr-Mo-N alloys were examined. According to XRD analysis, the reverse transformation from  to  is completed after heat treating at 1223 to 1373 K without deformation, while after heat treating at 1223 and 1273 K contained residual M2N particles in the  matrix. After the subsequent hot compression at condition at 1273 K and strain rate less than 1 s-1, and 1223 K and strain rate less than 10 s-1, ultra fine grains less than 1 m with residual M2N were obtained. Therefore, it is suggested that the residual M2N suppressed further growing of DRXed grain. The obtained microstructure with ultra fine grain dispersed with residual M2N was obtained to demonstrate both grain refining strengthening and precipitation strengthening.

Abstract: In recent years, there has been an increasing interest in magnetic shape memory alloys (MSMAs) due to their unique ability to produce very large output strains and rapid response frequency. NiMnCoIn is a new-type MSMAs in which a reversible magnetic-field-induced phase transformation is observed. The microstructural evolution in the process of dynamic recrystallization in polycrystalline Ni45Co5Mn36.7In13.3 was studied in the present paper. The experimental results showed that the high deformation temperature and slow strain rate were necessary to achieve perfect dynamic-recrystallizing microstructure in Ni45Co5Mn36.7In13.3 alloy. Precipitates with two sizes were observed. The content of Co in precipitates was higher than that in the matrix alloy, while the content of In was lower than that in the matrix alloy.

Abstract: In this study, Mg-xCa (x = 0.5, 1.0, 2.0, 5.0, 10.0, 15.0 and 20.0 %, wt.%, hereafter) and Mg-1Ca-1Y alloys were investigated as new biodegradable bone implant materials. The compressive strength, ultimate strength and hardness of the Mg-Ca alloys increased, whilst the corrosion rate and biocompatibility decreased, with the increase of the Ca content in the Mg-Ca alloys; higher Ca content caused the Mg-Ca alloy to become brittle. Solutions of simulated body fluid (SBF) and modified minimum essential media (MMEM) with the immersion of Mg-xCa and Mg-1Ca-1Y alloys showed strong alkalisation. The yttrium addition to the Mg-Ca alloys does not improve the corrosion resistance of the Mg-1Ca-1Y alloy as expected compared to the Mg-1Ca alloy. It is suggested that Mg-Ca alloys with Ca additions less than 1.0 wt.% exhibited good biocompatibility and low corrosion rate.