Materials Science Forum Vols. 561-565

Abstract: Porous Co-29Cr-6Mo alloy compacts were fabricated and their microstructures were examined. The Co-29Cr-6Mo alloy powders were atomized by using Plasma Rotating Electrode Process (PREP) in an Ar atmosphere. These powders were hot-pressed at 1223 K in Ar or nitrogen and annealed at 1473 K in vacuum or nitrogen. X-ray analysis revealed that the γ phase (f.c.c.) mostly exists in the compact hot-pressed in nitrogen although the ε phase (h.c.p.) is dominant in the compact hot-pressed in Ar. In addition, precipitates are observed at the interdendritic region in the compact hot-pressed in nitrogen. These results suggest that the Co-29Cr-6Mo alloy powders react with nitrogen to stabilize the γ phase. Accordingly, the constituent phase in the Co-29Cr-6Mo alloy is strongly affected by the atmosphere during hot-pressing.

Abstract: The tensile and plain fatigue properties of the β-type titanium alloy, Ti-29Nb-13Ta-4.6Zr alloy (TNTZ), which was subjected to various thermomechanical treatments, and cast TNTZ were investigated in order to judge its potential for biomedical and dental applications. The tensile strengths of TNTZ aged after solution treatment and that aged after cold rolling decrease with an increase in the aging temperature; however, their elongation exhibits an opposite trend. TNTZ composed of the ω phase or the ω and α phases in the β phase exhibits a tensile strength of about 1000 MPa or more. The tensile properties of the cast TNTZ with and without a surface reaction layer is are not significantly different, and are almost identcal to those of as-solutionized TNTZ. The plain fatigue strengths of TNTZ aged after solution treatment and those of TNTZ aged after cold rolling increase with the aging temperature. In particular, TNTZ aged at 723 K after cold rolling exhibits the highest fatigue strength in both the low- and high-cycle fatigue life regions. Further, the plain fatigue limit, which is about 770 MPa, is nearly equal to that of hot-rolled Ti-6Al-4V ELI alloy subjected to aging after solution treatment; Ti-6Al-4V ELI alloy is a representative α+β-type titanium alloy for biomedical applications. The plain fatigue strength of cast TNTZ with a surface reaction layer is considerably less than that of the as-cold-rolled and as-solutionized TNTZ. Consequently, in the low-fatigue life region, the fatigue crack easily occurs at the surface reaction layer, which is brittle, and in the high-fatigue life region, the fatigue crack occurs at the sites of casting defects (shrinkage). The fatigue limits range from 180 MPa to 200 MPa.

Abstract: The surface of Ti-29Nb-13Ta-4.6Zr (TNTZ) subjected to gas nitriding at 1023–1223 K was investigated in comparison with the conventional biomedical titanium alloy, Ti-6Al-4V ELI (Ti64). After gas nitriding, the microstructures near the surface of these alloys were observed by optical microscopy, X-ray diffraction, Auger electron spectroscopy, and X-ray photoelectron spectroscopy. In both alloys, two titanium nitrides (TiN and Ti2N) are formed and the α phase precipitated by gas nitriding. Furthermore, oxygen impurity in the gas nitriding atmosphere reacts with the titanium nitrides; thus, TiO2 is formed at the outermost titanium nitride layer. The surface hardening was also evaluated by Vickers hardness measurement. The Vickers hardness near the surface of TNTZ and Ti64 increases significantly by gas nitriding.

Abstract: Calcium phosphate films were prepared by MOCVD using Ca(dpm)2 and (C6H5O)3PO precursors. The phases, composition and morphology of films changed depending on the molar ratio of Ca to P precursors (RCa/P), total pressure (Ptot) and substrate temperature (Tsub). α-tricalcimu phosphate (α-TCP, α-Ca3(PO4)2) films in a single phase were obtained at Tsub = 1073 K, 0.1 < RCa/P < 0.4 and Tsub = 973 K, RCa/P < 0.4. Hydroxyapatite (HAp, Ca10(PO4)6(OH)2) films in a single phase were obtained at Tsub = 1073 K, 0.8 < RCa/P < 1.0 and Tsub = 973 K, 0.5 < RCa/P < 0.6. Ca-P-O films had a dense and smooth surface. HAp formed within 1 day on the α-TCP film and wholly covered the specimens within 2 weeks in a Hanks’ solution. The surface of CVD-HAp film was covered by precipitated HAp within 6 hours.

Abstract: Rolling texture of α"(C-orthorhombic) martensite phase in Ti-22mol%Nb-3mol%Al β-titanium based shape memory alloy was examined using X-ray pole figure measurement. The alloy was dual phase of α'' and β(bcc) at room temperature (RT) and was cold-rolled with reduction rates of 40%~99% at RT. Stress-induced martensitic transformation occurred during the rolling and then the material was fully transformed into martensite phase. Subscript α" and β indicate martensite and parent phase hereafter. The major texture was close to {411}α"<173>α" when the reduction rate was lower than 60%. This texture is a result of a preferential formation of specific variants which have a higher interaction with the macroscopic deformation. As the reduction rate was increased, ND moved toward {101}α" whereas RD was around <010>α" regardless of the reduction rate. {101}α"<010>α" texture was confirmed when the reduction rate was 99%.

Abstract: Hydrothermal treatment was performed on a titanium plate for its surface modification to enhance its bioactivity. The titanium plate was autoclaved in deionized water (DW) or 0.05 mol/l NaOH solution at 120, 200 and 240 °C for 24 hr. The titanium surfaces autoclaved in DW were homogeneously coated with nano-sized particles and polygonal deposits with the sizes of several hundreds nanometers. The deposits were regarded as anatase according to the results of laser Raman spectroscopy (LRS). In the case of autoclaving in 0.05 mol/l NaOH solution, leaf-like, needle-like and pyramid deposits were observed on titanium surfaces and regarded to consist of anatase or brookite by LRS analysis. The formation of the deposits significantly depended on the autoclaving temperature. The deposits adhered on the titanium surface though a tape-test; the adhesion of the particle was found to be excellent.

Abstract: A Zr-Ta-Nb alloy made of biocompatible alloy elements only and consisted of single β-phase with bcc structure was prepared by conventional solidification. Electron diffraction, transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM) were used to examine microstructural evolutions of the β-Zr-Ta-Nb alloy subjected to different heat treatments. The results show that the metastable β-phase in the as-solidified Zr-Ta-Nb alloy turned to decompose when annealed at 400°C and this led to the formation of a highly strained structure containing nanosized precipitates and local disordered regions. Striking phase separation including the formation of the equilibrium α-phase was found to occur at 600°C. By means of nanosized electron-probe analysis of energy dispersive X-ray spectroscopy, the compositions for the nanosized phases formed due to different heat treatments were analysis.

Abstract: Bone-like hydroxyapatite (HA) coatings were fabricated on titanium implants by a self-assembled technique and biomimetic method. After titanium plates were oxidized in a mixture of H2SO4/H2O2, a hexadecanoic acid self-assembled onto titanium surfaces. Contact angles of water and tricresyl phosphate on the surfaces were measured to characterize the self-assembled monolayer (SAM) and confirm the presence of the functional group. The titanium plates with SAM were used to fabricate HA coatings. In the simulate body fluid (SBF) with and without bovine serum albumin (BSA), Ca2+ and PO4 3- ions could spontaneously deposit onto the titanium surfaces and form bone-like HA coatings. The morphology, component and structure of samples were examined by scanning electronic microscopy, X-ray photoelectron spectroscopy, X-ray diffraction and attenuated total reflection Fourier transform infrared spectroscopy. The results suggested that the SAM can induce the formation of the nano-HA coating with a network and microporous structure. For the biomimetical HA coating induced by HDA-SAM, BSA could modulate the growth of HA crystal and decreased the grain size.

Abstract: Internal friction of Ti-24mol%Nb-3mol%Al (Ti-24Nb-3Al) shape memory alloy with a well-developed texture was investigated by dynamic mechanical analysis (DMA) in a tensile mode with a frequency of 1Hz and a temperature range of 123~423K. Ti-24Nb-3Al alloy fabricated by Ar arc-melting was cold-rolled with a reduction in thickness of 99% and then heat-treated at 1173K for 3.6ks in vacuum to develop a recrystallization texture. The martensitic transformation temperature and internal friction (tan δ) were evaluated under the stress amplitude of 36MPa. The tensile direction was selected to be the rolling direction (RD) and the transverse direction (TD). The specimen was β-single phase with a well-developed {112}β<110>β texture at room temperature and exhibited high damping during martensitic transformation and in the martensite phase. The relationship between internal friction and the tensile direction is discussed on the view point of the crystallography of the transformation.