Modelling of Microstructural Evolution of Titanium During Diffusive Saturation by Interstitial Elements

Abstract

coefficient of nitrogen or oxygen in titanium Panasyuk, Metin, Kofstad, . This is why investigations experimental and theoretical aimed at elucidating the kinetic regularities and peculiarities of the distribution of interstitial elements in a surface layer, which determines the changes of its physicomechanical characteristics, are urgent.Diffusive processes determine changes of properties of surface layers of the structural materials in many cases, for example, in the process of their thermochemical treatment or in the conditions of operation at high temperature. However, the diffusion in solids is often accompanied by the structural phase transformations. These processes are interconnected and interdependent diffusion of the elements can stimulate structural phase transformations, and the latter change the conditions of diffusion. It is difficult to describe these processes analytically. Titanium, which undergoes the polymorphic transformation at, is interesting for such theoretical and experimental investigations, in particular its high-temperature interaction with nitrogen or oxygen. Due to high affinity of these elements with titanium nitride or oxide layer forms and grows on the surface. Unlike many alloying elements, in particular vanadium, molybdenum, which are -stabilizers, above mentioned interstitial elements are -stabilizers, which can stimulate structural phase transformations in titanium. The microstructural evolution during ↔ phase transformation as a result of migration of -stabilizers is presented in Malinov et al.,. However, the authors did not take into consideration the role of nitrogen as -stabilizer in the structural transformations. It was demonstrated Matychak, in the studies of interconnection of nitrogen diffusion and structural phase transformations during high-temperature nitriding that, in particular, under the rarefied atmosphere, the continuous nitride layer on the surface was absent for a long time. The aim of work is• to establish the kinetic peculiarities of interaction of titanium with the interstitial element " nitrogen or oxygen at the temperature lower and higher than temperature of allotropic transformation T ↔• to investigate experimentally and model analytically the process of diffusive saturation of -titanium with the interstitial element from a rarefied atmosphere taking into account the surface phenomena• to model the interdependence of the processes of external supply of the interstitial element to the surface and its chemosorption with diffusive dissolution and segregation on defects, caused by the chemical interaction with the titanium atoms• to estimate the influence of temperature-time parameters of treatment on the depth of diffusion zone and change of its microhardness• to establish the kinetic peculiarities of diffusive saturation of titanium with the interstitial element caused by the structural phase transformations.Titanium Alloys -Advances in Properties Control 46

Microstructure and Phase Transformation of Pure Titanium During Nitriding Process by High Density Plasma

“…It can be observed that the decreasing of gas pressure and the increasing of DC bias voltage plays an important role in the formation of compound layer, e.g. the concentration of TiN fine precipitation increased from 1.71% to 8.71% during the process at lower pressure and DC bias voltage of -600 V. The most important factor that determines the formation of TiN as the compound layer is the effectiveness of diffusion process onto the surface of titanium specimen that occurs in the nitriding process [3].…”

“…It should be noted here, that the use of low-pressure discharge of the nitriding process can enhance the formation of TiN due to the high density of plasma ions in the low gas pressure. If nitrogen with high concentration diffused rapidly to the surface of the material, saturation will be more quickly formed, so that there will be a bond between the titanium atoms with the nitrogen atoms to form a compound layer of TiN [3,20]. In addition, the use of high DC bias voltage will also greatly affect the effectiveness of diffusion processes because the accelerated ions and fast neutral have high energy at higher voltage [17].…”

“…That situation was clearly evident in Figure 7 that the peak intensity of -Ti at [112 ത 0] plane decreased significantly followed by the increasing of the peak intensity of TiN compound correspond with [200] plane. This condition occurs due to the saturation condition just started on the lasted process about 4 h, which is characterized by the low concentration of TiN [3].…”

“…For the last few decades, titanium has attracted tremendous interest in many fields of industry, such as aerospace, industrial, consumer applications, and production of implants [1][2][3]. It because titanium is a lightweight metal owing to such properties as low density, high corrosion stability, and biocompatibility [1,4,5].…”

“…The nitriding process of titanium typically involves a high temperature of 700-1000 C which can harden the surface material up to 1300 HV [5,11]. Plasma nitriding is a thermo-chemical method that can control the phase formation and the depth of the nitrided layer [3][4][5]13]. It also requires short periods of nitriding time and can avoid oxidation, but the high temperature in the process can reduce the fatigue strength and cause detrimental micro-structural changes in titanium substrates [5].…”