Sengo Kobayashi

Effects of ω-phase precipitation on β → α, α transformations in a metastable β titanium alloy

Abstract Effects of ω phase precipitation on β→α, α′′ transformations in a metastable β titanium alloy have been studied by means of transmission electron microscopy. Isothermal holding at 300°C for 100 min after the solution-treatment at 1000°C yielded homogeneously distributed spherical ω phase particles of ∼10 nm in diameter. With raising the holding temperature, ω phase particles of the same orientation variant precipitated as groups. At temperatures above 350°C, α phase laths nucleated at the ω/β interfaces and grew into both the matrix and ω particles with consuming ω phase particles. Thus, the α/ω Takemoto and the ω/β Burgers relationships were held simultaneously, and the orientation of α laths nucleating at an ω/β interface was determined uniquely. In the specimens quenched directly from β single-phase region, α′′ martensite plates nucleated preferentially at β grain boundaries. The α′′ formation, however, was accelerated by a short time holding at temperatures between 600 and 300°C, but was largely retarded by the progress of ω phase precipitation.

Current status of ultra-fine grained W–TiC development for use in irradiation environments

Ultra-fine grained (UFG) W–TiC with a high purity matrix of low dislocation density is expected to exhibit improve resistance to irradiation with neutrons and helium ions and the room temperature mechanical properties. Aiming at such UFG W–TiC with the desired microstructure, powders of W with 0.25–0.8 wt% TiC additions were subjected to mechanical alloying (MA) and hot isostatic pressing (HIP), where purified H2 and Ar were used as the MA atmosphere. Microstructural observations and room- and high-temperature mechanical tests were performed for UFG W–TiC before and after neutron irradiation to a fluence of 2×1024  n m−2 at 873 K. It is shown that the MA atmosphere significantly affects grain refinement, room-temperature strength and high-temperature tensile plasticity of UFG W–TiC. W–0.5TiC with H2 in MA (W–0.5TiC–H2) shows a larger strain rate sensitivity of flow stress, m, of 0.5~0.6 at temperatures from 1673 to 1973 K, which is a feature of superplastic materials. Whereas W–0.5TiC–Ar shows a smaller m value of approximately 0.2. No radiation hardening is recognized in UFG W–0.5TiC–H2 and W–0.5TiC–Ar.

Current status of nanostructured tungsten-based materials development

Nanostructured tungsten (W)-based materials offer many advantages for use as plasma facing materials and components exposed to heavy thermal loads combined with irradiation with high-energy neutron and low-energy ion. This paper first presents the recent progress in nanostructured toughened, fine grained, recrystallized W materials. Thermal desorption spectrometry apparatus equipped with an ion gun has been installed in the radiation controlled area in our Center at Tohoku University to systematically investigate the effects of displacement damage due to high-energy neutron irradiation on hydrogen isotope retention in connection with the nano- or micro-structures in W-based materials. In this paper, the effects of high-energy heavy ion irradiation on deuterium retention in W with different microstructures are described as a preliminary work with the prospective view of neutron irradiation effects.

A new characterization method of the microstructure using the macroscopic composition gradient in alloys

A new experimental method to determine the phase boundary and phase equilibria is accomplished by using the transmission electron microscopic observation of alloys having the macroscopic composition gradient. The various phase boundaries,i.e., the coherent binodal and spinodal lines and incoherent binodal line, are distinctly determined for the Cu-Ti alloy system. Furthermore, the equilibrium compositions at the interface of precipitate/matrix are experimentally obtained for various particle sizes, and therefore, the Gibbs-Thomson relation is verified. It is expected that the composition gradient method proposed in the present study will become an important experimental method for microstructural characterization.

Thermal shock response of fine- and ultra-fine-grained tungsten-based materials

In this work, the focus is on the thermal shock analysis of fine- and ultra-fine-grained tungsten-based materials doped with 0.5–1.1 wt% TiC that showed in previous studies improved ductility at low temperatures and also performed well when exposed to neutrons and ions (H/He). Herein, the resistance of the material to crack formation is evaluated by applying edge-localized mode-like loads (n=100) with an energy density of 1 MJ m-2 at various temperatures up to 150 °C by means of an electron beam facility. The results indicate that the cracking threshold is significantly reduced even down to room temperature when the oxygen content is reduced and the combination of grain size, TiC particle size and distribution of TiC-particles to almost each grain boundary reaches its optimum. This is achieved by a post-manufacturing treatment of the material at 1650 °C using the materials superplasticity caused by grain boundary sliding at this temperature, which changes the materials microstructure from ultra-fine grains surrounded by weak grain boundaries to fine grains with significantly strengthened grain boundaries.

Isothermal decomposition of δ-ferrite in a 25Cr-7Ni-0.14N stainless steel

Abstract Isothermal decomposition processes of δ-ferrite in a 25Cr–7Ni–0.14N stainless steel have been investigated by means of transmission electron microscopy and Vickers hardness measurement. The specimens solution-treated at 1400°C were isothermally transformed at various temperatures between 450 and 600°C. Fine Cr 2 N platelets precipitate homogeneously in all the temperature range investigated. The orientation relationship between the Cr 2 N and the ferrite matrix can be expressed as; [101] δ // ⌊1 2 10⌋ Cr 2 N and ( 1 01) δ //(0001) Cr 2 N , the habit plane being ( 1 00) δ // ( 1 2 1 2) Cr 2 N . Spherical Cu particles precipitate in the temperature range between 550 and 600°C. These Cu particles transform from bcc to 9R structure when their diameter reaches a critical size. Austenite formation is observed at temperatures above 550°C. Occasionally, the austenite particles exhibit twinned structure, and often nucleate on Cr 2 N platelets. R-phase precipitates after prolonged aging at 600°C.

Variation of Hardness with Microstructure Evolutions in Metastable β Titanium Alloy TIMETAL®LCB

Microstructure evolutions and hardness variation during aging in metastable β titanium alloy TIMETAL®LCB have been examined. In as-quenched specimen after solution treatment, athermal ω phase formed. Isothermal ω phase formed during aging at 623 K, and α phase formed after precipitation of isothermal ω phase during aging at 673 and 773 K. Alpha phase nucleated at isothermal ω phase, and had the orientation relationship with β and ω, (111)β//(0001)ω//(1120 )α and [10 1 ]β//[1120 ]ω//[0001]α . During aging at 873 K, α phase formed without isothermal ω phase. Hardness increased with increasing aging time and decreasing aging temperature, partly because number density of isothermal ω or α precipitates increased with increasing aging time and decreasing aging temperature.

Evaluation of microstructures in alloys having a macroscopic composition gradient

In the macroscopic composition gradient (MCG) method, it is important to ensure that the MCG itself does not influence the phase transformation and microstructure behavior of the MCG alloy. The following two points should be theoretically and experimentally assessed to (1) ensure that the macroscopic composition profile is unchanged during aging at the lower temperature and (2) clarify the influence of the MCG on phase transformation, as it may influence microstructure formation even if the MCG profile does not change during aging. On the basis of experimental and theoretical investigations, 10 at. %/µm is considered the maximum composition gradient which does not influence microstructure formation.

Effect of Heat Treatments of Bioactive Nacre on HAp Formation in SBF

Formation of hydroxyapatite (HAp) in simulated body fluid (SBF) on heated nacre has been examined. Nacre is known as composite layer of aragonite platelets and organic materials. Nacre was obtained from the shell of Akoya pearl oyster after removing its prismatic layer. The nacre was heated up to 300°C in air and then soaked in SBF. Nacre heated at 300°C lost iridescent color and became brittle, implying that organic materials which plays a role as glue between aragonite platelets mostly disappeared by heating at 300°C. Formation of HAp particles on nacre in SBF was easier than that on pure Ti. Maximum formation rate of HAp particles was obtained on the nacre heated at 200°C. The amount of HAp particles formed on the nacre heated at 300°C is the smallest. The organic materials in nacre play a critical role for HAp formation on nacre in SBF.

Precipitation of β' Phase in a Low Cost Beta Titanium Alloy

Precipitation behavior of a metastable β Ti alloy, Ti-6.8Mo-4.5Fe-1.5Al(masss%), during two-step aging has been investigated. Supersaturated β phase after solution treatment decomposes into β+ω phases during the first aging at 300°C for 2ks. β′ phase is observed during the second aging at 500°C for 50s. It is suggested that reverse transformation of the ω phase to β phase during the second aging at 500°C for 50s gives rise to solute-lean and –rich regions in the β phase, and that β′ phase is formed in the solute-rich region. β′ phase being coherent with β phase has larger lattice parameter than β phase. β′ phase acts as nucleation site for α phase, and accelerates the nucleation of α phase.