Yoshito Takemoto

Structural stability and mechanochemical activity of titanium nitride prepared by mechanical alloying

Pure titanium nitride (TiN) was synthesized by mechanical alloying (MA). In order to prevent the contamination from the MA vial and atmospheric gas, the MA steel vial was replaced with a titanium vial and atmospheric gas was deoxidized using sponge titanium heated to 623 K. The mechanochemical activity during MA was estimated from the gas purification temperature. The investigation of thermal and pressure stability by thermal treatment and hot isostatic pressing (HIP) revealed that titanium nitride, TiN, was stable on heating to 1173 K under a vacuum, but became unstable under a high pressure, 100 MPa.

In situ observation of stress-induced martensitic transformation and plastic deformation in TiNi alloy

Stress-induced martensitic transformation and plastic deformation in TiNi alloy have been investigated using transmission electron microscope with an in situ deformation technique. It was found that martensite bands were formed and grew with increasing strain and shrank with decreasing strain in tension. In the second deformation stage, a high density of looped dislocations was formed in the reverse-transformed matrix, while in the first deformation stage dislocations were not formed in the reverse-transformed matrix. The formation of dislocation in the second deformation stage is considered as bearing a relation to reverse martensitic transformation and twinning dislocations lying inside the stress-induced martensite.

Nitriding of dilute Mo-Ti alloys at a low temperature of 1373 K

Abstract For Mo-0.5 mass% Ti and pure Mo alloy nitrided in a NH 3 gas at a relatively low temperature of 1373 K, microstructural observations through optical and transmission electron microscopes, X-ray diffraction analysis and hardness measurements were carried out. A surface nitriding layer with very high hardness of approximately Hv ~ 1800 consisted of two Mo-nitride regions: an outer one of γ-Mo 2 N and an inner one of β-Mo 2 N. The inward diffusion of nitrogen is a rate-controlling process in the growth of the surface nitriding layer. In Mo-Ti alloy additionally an internal nitriding layer with relatively high hardness of Hv ~ 800 was formed beneath the surface nitriding layer. Such high hardness in the internal nitriding layer was found to result from the uniform dispersion of extremely fine plate-like particles of titanium nitride. The particles are approximately 0.4 nm thick and have coherent strain field in the matrix.

Antiphase domain boundary tubes in plastically deformed Fe-Al alloys with B2 ordered structure

Fe-Al alloys with 25 to 50 at%Al, typical B2 long-range ordered alloys, are interesting because stacking faults, which had been considered not to exist in body-centered cubic materials, were observed in some of them like Fe-35 at%Al B2 alloys. The formation of the stacking faults was suggested to be due to the vacancy agglomeration under the shear stress induced by the quenching. The purpose of the present paper is to describe the observation of antiphase domain boundary (APB) tubes in several alloys with different compositions by the method of transmission electron microscopy (TEM), and to reveal how the probability of observing the APB tubes varies with the Al concentration of the alloys. Up to date, the APB tubes have only been observed by means of TEM, and the formation model of cross-slip and annihilation cannot explain the high density of the tubes shown in the TEM images. The present paper also discusses the APB tubes as being a result of the film effect of the TEM specimens.

Influence of atomization gas on coating properties under Ti arc spraying

Abstract An atomization gas used in arc spraying has a great influence on the nitriding process of titanium. Hence, we investigated the chemical composition of titanium coatings reflecting the nitriding process with the use of air, N 2 and Ar as the atomization gas. The ratios of contents (at.%) for nitrogen and oxygen in the coatings were 3:1 (air), 6:1 (N 2 ) and 2:1 (Ar), and the oxygen contents were almost the same. The peak height ratios of TiN 0.3 to TiN in XRD rose with decrease of the nitrogen content. The TiN peaks of all of the coatings were slightly shifted to higher angles by the oxygen, and their intensity was influenced only by the nitrogen content. In addition, β-Ti and Fe 2 Ti were detected in XRD from the surface of thin coatings, and an alloyed layer in the interface region between coating and substrate was observed in SEM. The Ar atomized coating had the thickest alloyed layer. N 2 atomization is efficient in the TiN formation. Ar atomization restrains the reaction of titanium with the nitrogen and oxygen, and promotes the chemical bonding between the coating and substrate.

Model for Predicting Recrystallization Behavior of Cold Rolled Extralow Carbon Steel Sheets

Microstructual observations indicate that the recrystallization of cold rolled extralow carbon steel sheets occurs due to the abnormal growth of selected subgrains in recovered subgrain microstructures. The authors measured the orientations of a recovered microstructure of a cold rolled extralow carbon steel sheet by SEM-EBSP and classified the deformed grains into several types due to the orientation and its scattering degree. In this study, a model for predicting the recrystallization behavior of cold rolled extralow carbon steels has been developed by applying the model of Humphreys modified to the grain of each type.A good agreement between the experimental and calculated results using the model developed was obtained.

Alloy composition dependency of plastic deformation behavior in biaxial compressions of Ti-Nb alloys

ABSTRACT Crystal structure of titanium alloy changes from alpha (hexagonal close-packed) to beta (body centered cubic) with increase of beta stabilizer content. This change of structure strongly influences on the plastic deformation behavior of titanium alloys, because it not only induces changes of slip systems but also activates martensitic transformation and deformation twinning. However, most of past studies on titanium alloys have been focused on the development of specific functionalities induced by alloy designing, and few research works have been reported on metal workability under multi-axial stress conditions, which is key factor to appl y titanium alloys for engineering products. In this study, uniaxial and biaxial compression tests of titanium-niobium alloys with various niobium contents have been performed to clarify the influence of beta stabilizer content on the pl astic behavior under compressive stress conditions. The titanium-niobium alloys were solution treated and then quenched from be ta region to obtain metastable structures. The resultant stress-strain relations together with microscopic observations of texture revealed that the influence of niobium contents on the predominant plastic deformation mechanisms and thus on the hardening phenomena. The equi-plastic work contours obtained by uniaxial and biaxial compression tests also implied the crystal structure dependency of anisotropic hardening, which was evaluated quantitatively by means of Hills anisotropic yield criterion. The results will provide information on the versatile constitutive relations of titanium allo ys containing beta stabilizer elements, that is important to prove the performance of products manufactured by compressive metal working processes such as forging and extrusion. Keywords: Plasticity, Metastable titanium-niobium alloy, Biaxial compression, Plastic deformation mechanism, Alloy composition dependency, Beta stabilizer, Deformation twinning

Precipitation and Precipitation Hardening Behavior of V and/or Cu Bearing Middle Carbon Steels

In this study, the precipitation and precipitation hardening behavior of a 0.3%V and 2%Cu bearing middle carbon steel has been investigated in comparison with that of a 0.3%V bearing steel and a 2%Cu bearing steel. The precipitation treatment was carried out isothermally at 600°C.The amount of the precipitation hardening of the 0.3%V and 2%Cu bearing steel is nearly equal to the sum of the precipitation hardening of the 0.3%V bearing steel and the 2%Cu bearing steel In the 0.3%V bearing steel, precipitates were observed in rows, which indicates the occurrence of the interphase precipitation while precipitates observed in the 2%Cu bearing steel were randomly dispersed. In the V and Cu bearing steel, randomly dispersed precipitates were not observed where there were aligned precipitates. In the paper, the different precipitation behavior of the three steels is discussed.

Influence of Thermal History on Microstructure and Mechanical Properties of Steels for Hot Stamping

Hot stamping is an attractive method to produce extra high strength automotive components. In the conventional hot stamping, the furnace heating is employed and the heating rate is quite low. To improve the productivity of the hot stamping technology, the reduction of time for the heating process is required. In this study, the influence of the heating rate in a range up to 200°C/s, heating temperatures between 650°C and 950°C and cooling condition on microstructure and mechanical properties of 0.22% C -3%Mn steel has been investigated. The steel is a promising material for the highly productive new hot stamping technology because this steel transformed into martensite from austenite even at cooling in free air. The specimens heat-treated at a high heating rate and for short holding time at the heating temperature just above Ac3 show significantly fine martensite microstructure and a good strength-toughness balance. In this paper, the α→ γ transformation behavior and the γ→ α transformation behavior after inter-critical annealing are discussed to explain the evolution of the microstructures and mechanical properties.

EXAFS and diffraction studies of athermal agitation effects on the local structure of amorphous Ni75Si8B17 alloy

Abstract EXAFS spectroscopy and X-ray diffraction studies were carried out to reveal the effects of athermal and thermal agitation on the local atomic structure in a splat-cooled amorphous Ni75Si8B17 alloy. The agitation was added by either milling or shock-loading. It was found that the as-splat-cooled amorphous alloy is composed of such tetrahedron molecules as having distorted NiSi bond distance and that such structure was changed, by the athermal agitation, into the activated one which is composed of the molecules with single bond metallic distances and has a little larger intermolecular spacings.