Keiichi Fukunaga

Corrosion wear fracture of new β type biomedical titanium alloys

Abstract Metallic materials such as stainless steel, Co–Cr alloy, pure titanium and titanium alloys have been used for surgical implant materials. The α+β type titanium alloy such as Ti-6Al-4V ELI has been most widely used as an implant material for artificial hip joint and dental implant because of its high strength and excellent corrosion resistance. Toxicity of alloying elements in conventional biomedical titanium alloys like Al and V, and the high modulus of elasticity of these alloy as compared to that of bone have been, however, pointed out [1] , [2] . New β type titanium alloys composed of non-toxic elements like Nb, Ta, Zr, Mo and Sn with lower moduli of elasticity, greater strength and greater corrosion resistance were, therefore, designed in this study. The friction wear properties of titanium alloys are, however, low as compared to those of other conventional metallic implant materials such as stainless steels and Co–Cr alloy. Tensile tests and friction wear tests in Ringer’s solution were conducted in order to investigate the mechanical properties of designed alloys. The friction wear characteristics of designed alloys and typical conventional biomedical titanium alloys were evaluated using a pin-on-disk type friction wear testing system and measuring the weight loss and width of groove of the specimen.

Fatigue characteristics of ultra high molecular weight polyethylene with different molecular weight for implant material

In order to investigate the effect of molecular weight on fatigue characteristics in the ultra high molecular weight polyethylene (UHMWPE), tension–tension fatigue tests of notched specimens were carried out in the present study. The effects of frequency and stress ratio on the fatigue characteristics were also investigated and fractography was discussed. The fatigue strength does not increase with increasing molecular weight. The fatigue strength might be influenced by the high degree of crystallinity in spite of the decreased tie molecule density in this study. Almost no effect of frequency on the number of cycles to failure can be observed. However, the higher the frequency, the higher the crack tip temperature. The effects of heat and strain rate on the fatigue strength must be considered in polymer materials. At a high stress ratio, the stress–number of cycles to failure (S–N) curves shift to high number cycles to failure side. Both stress amplitude and mean stress influence the fatigue life of UHMWPE.

Fracture characteristics and microstructural factors in single and duplex annealed Ti–4.5Al–3V–2Mo–2Fe

Abstract The influence of microstructure on fracture toughness of single and duplex annealed Ti–4.5Al–3V–2Mo–2Fe alloys was investigated. For the single-annealing treatment between 1103 and 1173 K for 3.6 ks, fracture toughness, J IC , decreases first with the increase of annealing temperature from 1103 to 1123 K and then increases with increasing annealing temperature upto 1173 K. As a result, there is a minimum of fracture toughness at 1123 K. This anomalous relation between fracture toughness and annealing temperature also exists in the duplex-annealed specimens at 993 K for 3.6 ks followed by the above single-annealing treatment. Examining the change of J IC and microstructure with annealing temperature, it can be found that J IC is related to volume fraction, aspect ratio and grain size of primary α phase, width of acicular α phase and prior β grain size. The decrease of fracture toughness in the temperature range from 1103 to 1123 K is mainly due to the decrease of volume fraction of primary α phase, while the increase of fracture toughness above 1123 K is mainly caused by the increase of prior β grain size.

Long crack growth behavior of implant material Ti–5Al–2.5Fe in air and simulated body environment related to microstructure

Abstract The fatigue crack propagation behavior of Ti–5Al–2.5Fe with various microstructures for biomedical applications was investigated in air and in a simulated body environment, Ringers solution, in comparison with that of Ti–6Al–4V ELI and that of SUS 316L stainless steel. The crack propagation rate, d a /d N , of Ti–5Al–2.5Fe in the case of each microstructure is greater than that of the Widmanstatten α structure in Ti–6Al–4V ELI in air whereas d a /d N of Ti–5Al–2.5Fe is nearly equal to that of the equiaxed α structure in Ti–6Al–4V ELI in air when d a /d N is plotted versus the nominal cyclic stress intensity factor range, Δ K . d a /d N of the equiaxed α structure and that of the Widmanstatten α structure in Ti–5Al–2.5Fe are nearly the same in air when d a /d N is plotted versus Δ K . d a /d N of Ti–5Al–2.5Fe is nearly equal to that of SUS 316L stainless steel in the Paris Law region, whereas d a /d N of Ti–5Al–2.5Fe is greater than that of SUS 316L stainless steel in the threshold region in air, when d a /d N is plotted versus Δ K . d a /d N of Ti–5Al–2.5Fe or Ti–6Al–4V ELI is nearly the same in air and in Ringers solution when d a /d N is plotted versus the effective cyclic stress intensity factor range, Δ K eff , whereas d a /d N of Ti–5Al–2.5Fe or Ti–6Al–4V ELI is greater in Ringers solution than in air when d a /d N is plotted versus Δ K .

In-situ TEM observation of electromagnetic field in some real materials

Electromagnetic fields presents in some real materials have been observed using electron holography and a simple method named the Shadow Image Distortion (SID) method which we have developed. The in-situ electron holography observation of the electric field surrounding a ceramic particle showed the rapid degradation of dielectric properties of the particle at an elevated temperature. The cross sectional view of mean electrostatic potential distributions in a silicon device has been observed. In-situ electron holography and SID observations showed the electrostatic potential distribution across a reverse biased p-n junction in a compound semiconductor. The SID method using a dedicated tool allowed single-step imaging of 2D maps of electromagnetic field.