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 -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 -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, D _N ^app , in the layer. The measured D _N ^app 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 -iron was evaluated by extrapolating D _N ^app to f=0, being in good agreement with that reported previously. The f-dependence of D _N ^app is discussed in terms of the effective area for diffusion of nitrogen in -iron.     

First-principles study of the thermodynamic and elastic properties of eutectic Fe-Ti alloys

Ti-Fe alloys covering a broad range of Ti concentrations are studied using quantum-mechanical calculations. Employing density functional theory, we correctly reproduce selected key features of the experimental Fe-Ti phase diagram. Analyzing the electronic structure of the stable phases in detail provides an explanation for the thermodynamic stability in terms of the strong correlation between the composition and density of states at the Fermi energy (DOS(E_F)). Based on this insight, we extend our study on both single-crystalline and polycrystalline elasticity of various Fe-Ti alloys by computing the compositional dependence of homogenized elastic constants. These quantities and their compositional dependence provide a direct explanation for the origin of the ductility and softness of the β-Ti(Fe) phase. Specifically, we find that this phase has an Fe concentration close to a threshold value connected with the onset of mechanical instability. By interlinking thermodynamic and mechanical stabilities we explain the softness and ductility of the β-Ti(Fe) in terms of a reduced mechanical stability that is connected with an increased DOS(E_F) in the β-Ti(Fe).     

Influence of heating in vacuum on low-temperature fracture behavior of carburized Mo-Ti alloys

In order to deduce the state of carbon and its interaction with titanium in the carburized molybdenum-titanium alloys (target titanium content: 0.1, 0.2, 0.5 and 1.0 mass%), changes of low-temperature fracture strength and ductility after heating in vacuum at 1773 K were investigated.

Bend tests were performed at temperatures from liquid nitrogen temperature to room temperature and yield strength, maximum strength and bend angle were calculated in each temperature. In this study the low-temperature fracture strength and ductility of a material were represented by the critical stress and the critical temperature, respectively. Fracture surfaces of the specimens failed at low temperatures were examined by using a scanning electron microscope and crack initiation and propagation characteristics were investigated. The results are summarized as follows.

1. (1) For the alloy with lower titanium content (e.g. Mo-0.1 mass%Ti alloy): the critical stress, the critical temperature and the fracture mode were almost unchanged after heating in vacuum. In this case it is deduced that carbon exists as free carbon and carbides at the grain boundaries. During heating, reduction of free carbon occurred by decarburizing, but it was compensated by the carbon produced by the resolution of carbides in a manner similar to pure molybdenum. Consequently there still remained enough carbon to strengthen the grain boundaries.

2. (2) For the alloy with higher titanium content (e.g. Mo-1.0 mass%Ti alloy): the critical stress remarkably decreased and the critical temperature increased after heating in vacuum. The fracture mode also changed from transgranular to intergranular. In this case it is deduced that carbon exists as free carbon both at the grain boundaries and at the interfaces between titanium-oxides and matrix. During heating, only the reduction of free carbon proceeded and consequently the grain boundaries became short of carbon.

     

Structure of carburized layers of transmission gears of power-saturated vehicles

Metal Science and Heat Treatment - The microstructure of carburized layers of high-stress gears after advanced thermochemical treatments is studied. A variant of technical requirements on the...     

Structure and properties of magnetically soft carburized chromium steel 16Kh-VI

The possibility of using carburized corrosion-resistant steel 16Kh-VI for parts of magnetic systems is estimated from results of an investigation of the properties of diffusion layers and an evaluation of the mechanical characteristics of specimens.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 31 – 33, March, 1996.     

Comparison of Fe-Ti assessments

Five assessments of the Fe-Ti system are compared with experimental information, and a new assessment is carried out with particular attention to the properties of dilute solutions of Ti in liquid Fe. All assessments have their shortcomings, but it is concluded that a more ambitious assessment should not be attempted before more experimental information is available.     

Fabrication of Fe-Ti alloys by pulsed current-assisted reaction from iron, manganese and titanium oxide or titanium hydride

Fe-Ti alloys were prepared by applying a pulsed electric current through compacted mixtures of iron, manganese and titanium oxide (with carbon) or titanium hydride (without carbon). After ball milling the mixture into a fine powder, the mixture was heated up to 1373–1573 K in a carbon mold by applying a pulsed electric current, and the temperature was maintained for 3–10 min. When TiO₂ was used as a starting material, TiC was formed. Fe was alloyed into FeTi and Fe₂Ti, with Ti also observed. When the C content was lowered to 8.136 wt%, the remaining C content after pulsed current application decreased to 4.64 wt%. When using TiH₂ as a starting material, Fe was alloyed into FeTi and Fe₂Ti, and Ti, Mn, Fe, and FeMn₂ were also observed. For a Fe-Ti alloy prepared using titanium hydride as a starting material, hydrogen content vs. absorption time curves were obtained at various numbers of cycles and temperatures.     

Structure and hardness of titanium surfaces carburized by pulsed laser melting with graphite addition

Titanium foils coated with graphite films 20 μm thick were irradiated by means of a pulsed Nd-YAG in order to harden this metal by surface melting and alloying. The relationships between irradiation parameters, microstructure and hardness of the synthesized composite coatings were determined. Four relevant parameters were defined as governing the irradiation processes. The parametric working field of the laser source was investigated next to its periphery. The influences of the relevant irradiation parameters on the microstructure and hardness of the melted zone were deduced from the metallographic analyses and Vickers micro-indentation tests of the cross sections of this composite zone. Such a zone was constituted always with hard titanium carbide and ductile metallic titanium, some times with the presence of lubricating graphite inclusions. The main advantage of such a surface treatment of titanium is to synthesize, under clearly defined irradiation conditions, a self-lubricating composite coating that resists abrasive or adhesive wear.     

Fe-Ti二元体系的热力学重新优化(英文)

为提高Fe-Ti二元系外推到三元或多元体系的能力,应用CALPHAD方法重新优化了该二元系。与前人的优化工作相比,重点放在对两个二元金属间化合物Fe2Ti和FeTi的热力学描述上。因目前普遍采用双亚点阵模型来描述C14_Laves相,所以采用双亚点阵模型来描述Fe2Ti相。通过检验包含Fe-Ti二元系的三元体系Fe-Ti边界上Fe2Ti相的均匀化范围进一步证实了Fe2Ti相的相边界。FeTi相具有BCC_B2晶体结构,因而将其处理成为BCC_A2相的有序相,并且用统一的Gibbs 能函数来描述有序和无序相。另外一个特别关注的方面就是重现这两个化合物的实测热容。计算结果与有关相图和热力学性质实验结果的广泛对比显示两者符合得很好,从而证明了所得热力学描述的有效性。