Papers by Author: Yasutaka Iguchi

Abstract: Calcium phosphate films were coated on commercially pure titanium substrates by radiofrequency magnetron sputtering using β-tricalcium phosphate targets. The films consisted of amorphous calcium phosphate and oxyapatite phases. Immersion tests of the films were carried out in Hanks’ solution and PBS(-), and apatite formation and calcium ion elution from the films were investigated. The titanium cylinders coated with calcium phosphate films were implanted into the mandibles of beagle dogs. These results suggest that coating with calcium phosphate improves the biocompatibility of titanium implants with bone tissue.

Abstract: Surface hardening treatment of titanium materials in use of CO gas was studied including investigation of post heat treatment under vacuum. C.P. titanium, α+β type SP-700 alloy with Ti-4.5%Al-3%V-2%Mo-2%Fe and β type alloy with Ti-15%Mo-5%Zr-3%Al were used. Surface hardening was conducted by heating these materials at 1073K for 21.6ks in Ar-5%CO gas. Subsequently, specimens subjected to surface hardening were heated at 1073k for various time periods under vacuum. While the maximum surface hardness value was the largest in C.P. titanium and the least in SP-700 alloy, hardening layer thickness was the thickest in β type alloy and the thinnest in C.P. titanium. Surface hardening in C.P. titanium was brought about by solid solution hardening due to oxygen and carbon. Enrichment of these elements in the surface layer of both titanium alloys caused continuous variations of the microstructure such as β to α+β, or their volume fractions in the surface hardening layer. Post heat treatment at 1073K increased the maximum surface hardness and hardening layer thickness with an extension of the heating time in C.P. titanium, but the surface maximum hardness decreased continuously in β type titanium alloy. Post heat treatment could remove the thin oxide layer formed by surface hardening treatment.

Abstract: New α+β type titanium alloy with Ti-4.5Al-6Nb-2Mo-2Fe was developed on the basis of using biocompatible elements and eliminating the cytotoxic ones such as Vanadium, while achieving the desirable mechanical properties such as appropriate strength, cold workability and low superplastic forming (SPF) temperature. The present study was conducted to investigate the effect of yttrium addition of less than 0.05% into this alloy on static and under superplastic deformation grain growth behavior. The new alloy bar manufactured by α+β processing and annealed at 1073K yielded extremely fine two-phase microstructure with α grain size around 2μm. Specimens were heated at temperatures of 1048, 1073 and 1098K and kept for times between 3.6 to 172.8KS. Yttrium forms in-situ Y2O3 particles, and the presence of these particles yield finer two phase microstructure due to their retardation effect on β phase grain growth. Grain growth behavior during hot deformation was investigated by hot compression test in use of a hot working simulator of THERMEC-Master Z. Strain rate was varied from 2×10-2 to 2×10-4S-1 and strain was 0.69. Grain size of both α and β phases increased with a reduction of strain rate, and Y2O3 particle was also effective to retard grain growth under hot deformation. It was confirmed from comparison of grain growth during isothermal heating with and without hot deformation that grain growth was much accelerated by deformation. All of these results were discussed based on grain growth mechanism or model for two-phase microstructures as well as superplastic deformation mechanism.

Abstract: Currently new continuous casting processes such as thin slab caster or strip casting are industrialized or under developing in the world steel makers. In these casting processes, cooling rate after solidification becomes much faster compared with thick slab caster, and hot rolling mill connected directly with casting machine tends to be installed. The present study was conducted to investigate variations of austenitic grain size and micro segregation with cooling rate after solidification and also direct hot deformation conditions in austenite immediately after solidification in HSLA steels. HSLA steels were 0.15%C-0.25%Si-1.50%Mn, 0.028%Nb and 0.028%Nb-0.015%Ti with the same basic compositions. A hot working simulator of THERMECMASTER-Z was used, and the center part of tensile specimen set up in this machine was partially or fully levitation-melted by induction heating under argon gas atmosphere. After melting, specimens were cooled at cooling rate from 0.4K/s to 40K/s, and this range covered cooling rates after solidification in heavy thick slab caster and strip casting. Direct hot tensile straining in austenite after solidification was conducted at strain rates from 1.4×10-3s-1 to 2.6s-1, corresponding to an extracting speed in a respective caster. The increase of cooling rate refined continuously as cast austenitic grain size, and it was enhanced in micro alloyed steels. Micro segregation such as Mn was improved by faster cooling. Direct straining after solidification markedly refined austenitic grain size through dynamic or static recrystallization occurring depending on strain rate.

Abstract: Wear behavior of titanium materials such as C.P.Ti, Ti-6Al-4V and Ti-6Al-7Nb, was studied in simulated body fluids by means of the pin-on-disk type wear testing. The mass loss in wear testing increased with increasing the sliding distance. The a+b type titanium alloys, Ti-6Al-4V and Ti-6Al-7Nb, exhibited higher wear resistance than the C.P.Ti. Average size of the wear debris was 4.5 µm and 3.7 µm for C.P.Ti and the a+b type titanium alloys, respectively. The elution of metallic constituents into 1 mass% lactic acid solution was detected after the wear test.