Hideyuki Kuwahara

Ion-nitriding of an Fe-19 wt % Cr alloy

The ion-nitriding behaviour of an Fe-18.75wt% Cr alloy was investigated at 803 K under constant plasma conditions. Both a thin surface layer ofγ′-Fe4N and an internal-nitriding layer were observed. The nitride formed in the internal-nitriding layer was found to be CrN, rather than Cr2N. The hardness of the nitriding layer rises to Hv=1200 due to small CrN precipitates. The growth rate of the internal nitriding layer, in the present alloy is controlled by a nitrogen diffusion process in the matrix metal,α-iron. Because such ion-nitriding behaviour is analogous to that of internal-oxidation, the growth rate of nitriding was discussed according to the rate equation to that of internal-oxidation. The nitrogen diffusion in the present alloy is scarcely affected by the CrN precipitates.

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.

Behavior of magnesium in Hank's solution aimed to trabecular pattern of natural bone

An ideal artificial bone is expected to grow together with other natural bones with aid of osteoblast cells and to fade out into other natural bones at the same rate of restructuring natural bone. Magnesium is thought to be one of candidate materials, since it has a potential to enhance natural bone growth and to homogenize the implanted artificial bodies with natural bone. In the present study, we are concerned with the formation of trabecular pattern in the natural bone to consider how to reconstruct this pattan in the artificial bone made from magnesium. For that purpose, a series of experiments were perfonned to observe the chemical behavior of dipped magnesium plate and cellular magnesium in Hanks solution. A magnesium specimen is annealed at 773 - 803 K for various periods in an atmosphere to homogenize its microstructure. Mass change of magnesium is estimated by immersing it in Hanks solution. It is well known that magnesium is easily corroded by chlorine ion. Both x-ray diffraction and energy dispersed x-ray analyses were carried out in order to identify a reaction product and its chemical composition. Mass of a magnesium specimen, which was annealed at 803 K for 32.4 ks or 14.4 ks, increases after immersing it into Hanks solution for 4.5 18 Ms (1255 h). Furthermore, the cellular magnesium, which was annealed at 803 K for 1.8 ks, fanned a reacted layer with around 80 μ in thickness and it contained Mg, Ca, P, and a little bit of CI.

Normal and anormal microstructure of plasma nitrided Fe-Cr alloys

Plasma nitriding behavior of Fe-Cr alloys has been studied at temperatures in the range of 773–873 K in order to provide basic knowledge for microstructure design of nitrided layers and to improve the wear resistance. In the nitriding temperature of 773 K, typical microstructure of nitrided layers was observed as reported elsewhere. However, anormal microstructure of nitrided layers was observed under a nitriding condition, at 873 K for 176.4 ks (49 h). In Fe-13Cr alloy, nitrided layer showed stripe-pattern, each sub-layer of which has different chromium content. Nitrided layer hardness increased gradually from the specimen surface to the nitriding front before dropping drastically to the same level as matrix hardness. The stripe-pattern was also observed for Fe-3Cr alloy at the vicinity of nitriding front for the same nitriding conditions. On the other hand, nitrided layers in Fe-8Cr and Fe-19Cr alloys are composed from different sub-layers, containing different concentration of chromium. These phenomena cannot be explained only by nitrogen diffusion process during the nitriding.

Plasma nitriding of Fe-18Cr-9Ni in the range of 723–823 K

To clarify the mechanism of plasma nitriding, we examined the optical microstructure, the hardness, the precipitation, and the concentration of dissolved nitrogen in Fe-18Cr-9Ni nitrided using plasma in the range of 723–823 K. Compared with ammonia-gas nitriding, the features of plasma nitriding are the formation of small chromium-nitride precipitates (CrN), the absence of an externally nitrided layer, the high concentration of dissolved nitrogen, and the high hardness (HV=1200). The diffusion coefficient of nitrogen in the present alloy was determined using the growth rate of the internally nitrided layer, based on calculations used in internal oxidation. Plasma- and gas-nitriding were also compared with respect to the growth rate of the nitrided layer.

Mechanical properties of Ag-Ni super-laminates produced by rolling

Abstract Multilayer materials which have the layer thickness with nano-scale or meso-scopic scale have been produced by the repeated process of diffusion bonding and rolling. We call these multilayers ‘super-laminates’. The strength of Ag-Ni super-laminates increases with the decreasing layer thickness. The ultimate tensile strength reached 1000 MPa for the laminates which have a layer thickness 673 K due to the structural change. This repeated rolling process may be expected to produce super high strength materials with ductility by introducing the proper heat treatment.

Effect of alloying element content on ion-nitriding behavior of Fe-Ti alloys

The ion-nitriding behavior of iron alloys with a titanium content of between 1.07 and 2.58 wt.% was investigated in the α-phase region. The behavior was found to be analogous to the internal oxidation behavior of iron alloys: An internal-nitriding layer, where small TiN precipitates are dispersed, as well as a very thin surface layer of γ′-Fe4N were formed. A parabolic rate law holds for growth of the internal-nitriding layer. The kinetics of growth of the internalnitriding layer is discussed according to the rate equation of internal-oxidation, giving the diffusion coefficient of nitrogen, DNapp, in the layer. The measured DNapp decreases as the volume fraction of TiN, f, increases, indicating that the diffusion of nitrogen is apparently inhibited by the existence of TiN precipitates. Furthermore, the diffusion coefficient of nitrogen in α-iron was evaluated by extrapolating DNapp to f=0, being in good agreement with that reported previously. The f-dependence of DNapp is discussed in terms of the effective area for diffusion of nitrogen in α-iron.

Ion-nitriding behaviour of Fe-Ti alloys in the α-phase region

The ion-nitriding behaviour of four iron alloys containing between 0.11 and 1.48 wt% titanium was investigated in theα-phase region to discuss kinetics of the growth of the nitriding layer. The ion-nitriding experiments have been made at 823 K. Two nitriding layers were observed: a thin surface layer which mainly consists of Fe4N; an internal nitriding layer beneath the surface layer, where the nitride formed was found to be TiN. The growth of the internal nitriding layer is controlled by a diffusion process of nitrogen in the matrix metal. The apparent diffusion coefficient of nitrogen in the nitriding layer, evaluated using the rate equation proposed for internal oxidation, increases linearly with the volume fraction of titanium nitride. Furthermore, by excluding the effect of the titanium nitride from the apparent diffusion coefficient, the diffusion coefficient of nitrogen inα-iron was calculated, being in good agreement with that reported so far. In addition, the increase in hardness in the internal nitriding layer has been discussed.

Ammonia gas nitriding of Fe-18Cr-9Ni alloy at lower than 823 K

An Fe-18Cr-9Ni alloy, which it had not previously been possible to nitride at temperatures below 873 K, was found to form nitrides in an ammonia gas atmosphere at temperatures as low as 823 K after annealing at low hydrogen pressure at 1473 K. Microstructure and hardness were examined on cross-sections of the nitrided specimens. An internal nitriding layer had formed beneath an external nitriding layer on the specimen surface. Vickers hardness was above 1000 throughout the internal nitriding layer. The nitrides formed at the specimen surface and in the internal nitriding layer were identified using grazing incidence X-ray diffraction and ordinary X-ray diffraction methods, respectively. The external nitriding layer, which was about 6 to 10 μm thick, formed on the surface, which consisted of ɛ-Fe2−3N, γ′-Fe4N, and CrN. Two types of chromium nitride were precipitated by ammonia gas nitriding of the present alloy: CrN in the external nitriding layer and Cr2N in the internal nitriding layer.

Effect of plasma on nitriding of Fe-18Cr-9Ni alloy

The plasma nitriding behaviour of Fe-18Cr-9Ni alloy was compared with gas nitriding. The alloy was nitrided under the following conditions: specimen temperature: 823 K, nitriding time: mainly 108 ks, total pressure: 0.4–0.7 kPa, mixture ratio of N2 and H2∶ 0.25, discharge voltage: 350–450 V, current: 0.8–1.1 A. Formation of a surface layer of iron nitrides was not observed. Formation of a homogeneous internal nitriding layer, consisting of small precipitates of CrN and the γ-phase matrix, was, however, noted. The lattice constant at the specimen surface was smaller than that at greater depth. This may have been because the sputtering effect decreased the dissolved nitrogen content at the specimen surface. The sputtering of iron nitrides at the specimen surface by the plasma was experimentally confirmed through γ′-Fe4N formation on Si beside an alloy specimen. The characteristics of the plasma nitriding mentioned above are discussed in relation to the sputtering.