Extra Low Interstitial Alloy Research Articles

Cytocompatibility of various metal and development of new titanium alloys for medical implants

The cytocompatibility of new Ti alloys not containing Al and V such as Ti–15%Zr–4%Nb–4% Ta–0.2%Pd–0.05%N–0.2%O and Ti–15%Sn–4%Nb–2%Ta–0.2%Pd–0.2%O were investigated by comparing with those of pure Ti grade 2 and Ti–6%Al–4%V ELI (Extra Low Interstitial) alloys. The relative growth ratio for the medium extraction of Al containing very little V ions (less than 0.02 mass ppm) was too low as compared to that of V free Al extraction. However, mixed powder extraction such as Ti+Zr+Nb+Ta and Ti+Sn+Nb+Ta exhibited evidently no effect on the relative growth ratio of L929 cells. The relative growth ratios of both L929 and MC3T3-E1 cells for the new Ti alloys were almost equal to 1 and were slightly higher than pure Ti grade 2 and Ti–6%Al–4%V ELI alloys. Also, the influence of extracting with Eagle's minimum essential medium (MEM) solution for L929 cells and ultra pure water for MC3T3-E1 cells on the relative growth ratio of cells were examined. Further, wear tests were conducted with the new Ti alloys, pure Ti grade 2 and Ti–6%Al–4%V ELI alloy disks against apatite ceramics and alloy pins in Eagle's MEM solution. The relative growth ratios of L929 and MC3T3-E1 cells were estimated with these wear test solutions. The relative growth ratio of L929 cells were almost equal to 1 for the new Ti alloys and pure Ti grade 2 against apatite ceramics pin up to 10 5 cycles. However, the relative growth ratios of L929 and MC3T3-E1 cells sharply decreased for Ti–6%Al–4%V ELI disk against apatite ceramics pin from 3×10 4 wear cycles. It was observed that the pH of the Eagle's MEM solution gradually increased as the number of wear cycles increased in all the cases of alloy disk against apatite ceramics pin. Also, the V ion concentration increased as the number of wear cycles increased. This result was also similar for MC3T3-E1 cells. Further, the relationship between metallic concentration in the medium extracted with Ti, Al, V, Zr, Sn, Nb, Ta, and Pd powders and the relative growth ratios of L929 and MC3T3-E1 cells were evaluated. A similar dependence of the concentration of vanadium ion and the relative growth ratios of L929 and MC3T3-E1 cells were seen as in the case of the wear tests.

Cytocompatibility of new Ti alloy without Al and V by evaluating the relative growth ratios of fibroblasts L929 and osteoblasts MC3T3-E1 cells

The cytocompatibility of new Ti alloys not containing Al and V such as Ti-15Zr-4Nb-4Ta-0.2Pd-0,05%N-0.2%O and Ti-15Sn-4Nb-2Ta-0.2Pd-0.2%O were investigated by comparing with those of pure Ti grade 2 andTi-6%Al-4%V ELI (extra low interstitial) alloys. The relative growth ratios of both L929 and MC3T3-E1 cells for the new Ti alloys were almost equal to 1 and were slightly higher than pure Ti grade 2 and Ti-6Al-4V alloys. Also, the influence of extracting conditions with E-MEM solution and ultrapure water on the relative growth ratio of cells were examined. Furthermore, the relationship between metallic concentration in the medium extracted with Ti, Al, V, Zr, Sn, Nb, Ta and Pd powders and the relative growth ratios of L929 cells were evaluated. The relative growth ratio for the medium extraction of Al, containing very little V ions (less than 0.02 ppm) tend to be too low as compared to that of V free Al extraction. However, extraction of Ti, Zr, Sn, Nb and Ta powders, and mixtures of powders such as Ti + Zr + Mb + Ta and Ti + Sn + Nb + Ta had evidently no effect on the relative growth ratio of L929 cells.

Corrosion Resistance and Mechanical Properties of New Titanium Alloys for Centrifugally Cast Dental Implants

Ti-15 mass%Zr-4%Nb-4%Ta-0.2%Pd-0.2%0-0.05%N and Ti-15%Sn-4%Nb-2%Ta-0.2%Pd-0.2%O alloys without V and Al elements for surgical implantation were cast by vacuum arc-melting in response to recent concerns about the long term safety of Ti-6%Al-4%V extra low interstitial alloy (ASTM standard for surgical implantation). After the alloys were forged in their and α + β regions, specimens 30 mm in diameter and 13 mm in height were arc-melted in an argon gas atmosphere with a titanium casting machine for dental use using a tungsten electrode. The molten alloys were cast into a graphite mold by centrifugal force. The cast microstructures of the Ti-Zr and Ti-Sn alloys consisted of α' martensitic structure. To examine of the centrifugal alloys properties for dental application, the corrosion resistance and mechanical properties of the alloys were investigated by the anodic polarization in 5 mass% HCl and 1 mass% lactic acid solutions, the measurements of micro-Vickers hardness and the tensile test at room temperature. The anodic polarization properties of the Ti-Zr and Ti-Sn alloys cast in a 1% lactic acid solution are almost the same as those of Ti-Zr and Ti-Sn alloys annealed at 973 K for 7.2 ks after α+β forging. A rapid increase in current density resulted from low potential in a 5% HCl solution in comparison with the annealed Ti-Zr and Ti-Sn alloys. The passive films on the Ti-15%Zr-4%Nb-2%Ta-0.2%Pd and the Ti-15%Sn-4%Nb-2%Ta-0.2%Pd alloys in a 1% lactic acid solution consisted of TiO 2 , ZrO 2 , SnO 2 , Nb 2 O 5 , Ta 2 O 5 and PdO or Pd as demonstrated by X-ray photoelectron spectroscopy (XPS). Micro-Vickers hardness does not change from the surface to the centeral part in castings. The strength of the cast Ti-Zr and Ti-Sn alloys was higher than those of the annealed Ti-Zr and Ti-Sn alloys. However, the ductility of the cast alloys was lower than the annealing ones.

Corrosion Resistance of Implant Alloys in Pseudo Physiological Solution and Role of Alloying Elements in Passive Films

The corrosion resistance of implant alloys was investigated by anodic polarization measurements in a pseudo physiological solution. Role of alloying elements in passive film formed on implant alloys by the anodic polarization was also examined with X-ray photoelectron spectroscopy (XPS). Passivity zones of SUS316L stainless steel and Co–Cr alloys are narrow, and a rapid increase in the current density is observed from a low potential as compared to Ti alloys. The passive current density of Ti-15%Zr-4%Nb-2%Ta-0.2%Pd alloy (mass%) is very low. Effects of protein and dissolved oxygen with 90%N2+5%CO2+5%O2 gas bubbling on anodic polarization curves are not detected. The passive film formed on the Ti-6%Al-4%V extra low interstitial alloy consists mainly of TiO2, Al2O3 and small amounts of V oxide. Passive films on the Ti-15%Zr-4%Nb-2%Ta-0.2%Pd and Ti-15%Sn-4%Nb-2%Ta-0.2%Pd alloys consist of TiO2, ZrO2, SnO2, Nb2O5, Ta2O5 and PdO or Pd. XPS spectra from the Co–Cr alloy and SUS316L stainless steel show peaks of Cr2O3, CoO, Fe oxide, Mo oxide, Ni, Co, Fe, Mo and Cr. As Ni, V and Co are strong cytotoxic elements, it was found that biocompatibility of SUS 316L, Co–Cr, Ti-6%Al-4%V ELI alloys become lesser after a long time use in the living body. The thickness of the passive film on Ti alloys is thinner than that of Co–Cr alloy. Passive film formed on the Ti-15%Zr-4%Nb-2%Ta-0.2%Pd alloy is strong and stable.

遠心鋳造した生体用新Ti合金の耐食性と室温強度

Ti-15 mass%Zr-4%Nb-4%Ta-0.2%Pd-0.2%0-0.05%N and Ti-15%Sn-4%Nb-2%Ta-0.2%Pd-0.2%O alloys without V and Al elements for surgical implantation were cast by vacuum arc-melting in response to recent concerns about the long term safety of Ti-6%Al-4%V extra low interstitial alloy (ASTM standard for surgical implantation). After the alloys were forged in their and α + β regions, specimens 30 mm in diameter and 13 mm in height were arc-melted in an argon gas atmosphere with a titanium casting machine for dental use using a tungsten electrode. The molten alloys were cast into a graphite mold by centrifugal force. The cast microstructures of the Ti-Zr and Ti-Sn alloys consisted of α' martensitic structure. To examine of the centrifugal alloys properties for dental application, the corrosion resistance and mechanical properties of the alloys were investigated by the anodic polarization in 5 mass% HCl and 1 mass% lactic acid solutions, the measurements of micro-Vickers hardness and the tensile test at room temperature. The anodic polarization properties of the Ti-Zr and Ti-Sn alloys cast in a 1% lactic acid solution are almost the same as those of Ti-Zr and Ti-Sn alloys annealed at 973 K for 7.2 ks after α+β forging. A rapid increase in current density resulted from low potential in a 5% HCl solution in comparison with the annealed Ti-Zr and Ti-Sn alloys. The passive films on the Ti-15%Zr-4%Nb-2%Ta-0.2%Pd and the Ti-15%Sn-4%Nb-2%Ta-0.2%Pd alloys in a 1% lactic acid solution consisted of TiO 2 , ZrO 2 , SnO 2 , Nb 2 O 5 , Ta 2 O 5 and PdO or Pd as demonstrated by X-ray photoelectron spectroscopy (XPS). Micro-Vickers hardness does not change from the surface to the centeral part in castings. The strength of the cast Ti-Zr and Ti-Sn alloys was higher than those of the annealed Ti-Zr and Ti-Sn alloys. However, the ductility of the cast alloys was lower than the annealing ones.

生体用合金の擬似体液中での耐食性と不動態皮膜中の合金元素の役割

The corrosion resistance of implant alloys was investigated by anodic polarization measurements in pseudo physiological solution. Role of alloying elements in passive film formed on implant alloys by the anodic polarization was also examined with X-ray photoelectron spectroscopy (XPS). Passivity zones of SUS316L stainless steel and Co-Cr alloys are narrow, and a rapid increase of current density results from the low potential in comparison to Ti alloys. The passive current density of Ti-15%Zr-4%Nb-2%Ta-0.2%Pd alloy (mass%) is very low. Effects of protein and dissolved oxygen with 90%N2+5%CO2+5%O2 gas bubbling on anodic polarization curves are not detected. The passive film formed on the Ti-6%Al-4%V extra low interstitial alloy consist mainly of TiO2, Al2O3 and small amounts of V oxide. Passive films on Ti-15%Zr-4%Nb-2%Ta-0.2%Pd and Ti-15%Sn-4%Nb-2%Ta-0.2%Pd alloys consist of TiO2, ZrO2, SnO2, Nb2O5, Ta2O5 and PdO or Pd. XPS spectra from the Co-Cr alloy and SUS316L stainless steel show peaks of Cr2O3, CoO, Fe oxide, Mo oxide, Ni, Co, Fe, Mo and Cr. As Ni, V and Co are strong cytotoxic elements, it was found that biocompatibility of SUS316L, Co-Cr, Ti-6%Al-4%V ELI alloys become lesser after a long time use in the living body. The thickness of the passive film on Ti alloys is thinner than that of Co-Cr alloy. Passive film formed on Ti-15%Zr-4%Nb-2%Ta-0.2%Pd alloy is strong and stable.

Effect of Alloying Elements on Anodic Polarization Properties of Titanium Alloys in Acid Solutions

The effect of Zr, Sn, Nb, Ta, Pd, N and O on the corrosion resistance of titanium alloys for medical implants was investigated by measuring the anodic polarization curves at 310 K in 5 mass% H2SO4 and 5%HCl solutions deaerated by high-purity N2 gas bubbling. The critical current density for passivation (Ic) and passive current density at 0.6 V versus SCE of the titanium alloys markedly decreased with increasing Zr, Nb, Ta, and Pd contents in the range from 5 to 20, 4 to 8, 2 to 4 and 0 to 0.2%, respectively, although Sn had an adverse effect in the range from 10 to 20 mass%. The effect of N and O on Ic was undetected. The log (Ic) value decreased linearly with increasing bond order (\barBo) value representing the covalent bond strength between Ti and the alloying elements. The passive film formed on Ti-15%Zr-4%Nb-2%Ta-0.2%Pd alloy in the 5% H2SO4 solution consisted mainly of TiO2, ZrO2, Nb2O5 and Ta2O5, as demonstrated by the electron spectroscopy for chemical analysis (ESCA). The anodic polarization properties of Ti-15%Zr-4%Nb-2%Ta-0.2%Pd and Ti-15%Sn-4%Nb-2%Ta-0.2%Pd alloys were superior to those of the Ti-6%Al-4%V extra low interstitial alloy and SUS316L stainless steel.

Subsurface crack initiation in high cycle fatigue of Ti5Al2.5Sn extra-low interstitial alloy at liquid helium temperature

Subsurface fatigue crack initiation in a Ti5Al2.5Sn extra-low interstitial alloy at liquid helium temperature was characterized. The subsurface crack initiation occurred at low stress level and neither defects nor foreign material were detected at the subsurface initiation site. The subsurface crack initiation site was composed of “small grains” of diameter about 5–10 μm. The concentration of aluminium at the initiation site was much lower than in the bulk. From the viewpoint of the microstructure this alloy had partly a two-phase region (small α grain region). It was composed of small α grains with low aluminium content and an iron-enriched β phase at their grain boundaries. The size, morphology and composition of the subsurface crack initiation site fitted those of the small α grain region. Hence it is concluded that the subsurface crack was initiated in the small α grain region.

Transmission electron microscopy study of high cycle fatigue deformation in Ti5Al2.5Sn extra-low interstitial alloy at cryogenic temperatures

The dislocation structure developed in an α-type Ti5Al2.5Sn extra-low interstitial alloy under various cyclic stresses at cryogenic temperatures was investigated by transmission electron microscopy. When the maximum stress was above the yield strength, coplanar dislocation arrays were distributed over α grains. Prism slip occurred predominantly and a pyramidal slip system was also activated. As the maximum stress was decreased, dislocation pile-ups became less homogeneously distributed. The dislocation arrays on the prism plane passed through grain boundaries. When the maximum stress was below about 75% of the yield strength, where the present alloy showed subsurface fatigue crack initiation, dislocations were piled up and blocked at grain boundaries in the small α grain region. Slip occurred predominantly along prism planes and the Burgers vector of the dislocation was <σ>.

Investigation of the fracture mechanism of Ti-5AI-2.5Sn at cryogenic temperatures

Fractography and metallographic sectioning were used to investigate the influence of microstructure on the fracture mechanism and fracture toughness (KIc) of normal interstitial and extra low interstitial (ELI) Ti-5Al-2.5Sn at 20 K (-423°F) and 77 K (-320°F). Plates of each grade were mill annealed at 815°C (1500°F) followed by either air or furnace cooling. These variations in composition and cooling rate resulted in differences in the volume fraction and internal structure of the dispersed β phase and in the ordering of the α matrix. The ELI alloys were tougher than the normal interstitial plates. KIc of the furnace-cooled ELI plate was 25 pct lower than that of the air-cooled ELI material. Variations in cooling rate had no influence of KIc of the normal interstitial alloys. Fractography showed that a large portion of the fracture surfaces were covered with elongated dimples commonly called “flutes.” Metallographic sections of specimens deformed at 77 K showed that these features form at the intersections of slip bands or deformation twins with grain or twin boundaries. Ordering and higher interstitial levels increase the local strain in slip bands resulting in void nucleation at lower macroscopic strains and lower KIc values.