Papers by Keyword: TiH₂

Abstract: Structure properties of Al-Ti-C mother alloy which was prepared by TiH₂ and graphite method were studied by various characterization methods.Results demonstrated that the synthesis process of Al-Ti-C alloy contained three stages.Firstly,TiH₂ and graphite was respectively used as titanium and carbon sources.Secondly,titanium reacted with aluminum and graphite,producing Ti-Al,C-Al,Ti-C compounds.Finally,these compounds formed Al-Ti-C alloy.It was equiaxed or dendrite structure with grain size of 20～40μm.There were needle-like and elongated phases in alloy matrix.There are petal primary crystals containing Ti,C,Fe and Si on grain boundaries.Ti and C distribute evenly along grain boundaries and grains.Ti and C enrichment and segregation occurred close to the primary phase.Cryolites were conducive to wetting of carbon and aluminum, the second phase generated more fully in the grain.

Abstract: The diffusion mechanism of H in metals and metal hydrides is studied particularly at high H₂ pressures. Thin films of Mg and Ti offer a convenient tool to quantify the atomic transport. We show how different parameters of hydrogenation affect the kinetics. At 200°C, the Pd-Mg interface is predominant and a linear regime of hydrogenation is observed, whereas at 300°C a parabolic regime is detected. In Mg, the hydride forms from the surface to the substrate whereas in Ti growth of TiH₂ starts from the substrate. A linear kinetics is seen during hydrogenation of Ti films, which is due to the oxide layer on top, measured to be about 10nm thick. In the studied high pressure regime, the hydrogenation is not pressure dependent any more. Quantitative calculation of the growth rate and the diffusion coefficient of H in the hydrides is presented.

Abstract: Aluminum foam sandwiches (AFS) were successfully prepared by a powder metallurgical method. TiH₂ powders were pre-treated in air at 450°C and 500°C for 2h in order to clarify the effect of oxidation treatment on AFS in comparison to those as-received. The decomposition curves of as-received and oxidized TiH₂ powders were monitored using DSC. The macro-morphology, expansion ratio, interfacial bonding and cell structure of AFS were systematically investigated. The results show that the AFS utilize TiH₂ powders oxidized in air at 450°C for 2h exhibits the best performance, under which conditions the effective bonding between the panel and core, 200% of the expansion rate and uniform pore structure of AFS have been achieved.

Abstract: In this paper, titanium silicon carbide (Ti₃SiC₂) powders were synthesized from TiH₂ as Ti source by pressureless sintering in flowing argon atmosphere without preliminary dehydrogenation. Starting materials are powder mixtures with the mole ratio of 3TiH₂/Si/2C or 3TiH₂/SiC/C. Both kinds of starting materials were sintered in a tube furnace at the temperature range from 1300°C to 1500°C for 10~180min in flowing argon atmosphere. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the phase compositions and morphology of the products after different treatments. It was showed that almost single phase Ti₃SiC₂ powder (94.7 wt.%) can be synthesized by pressureless sintering from 3TiH₂/Si/2C powders at 1400~1425°C for about 180min or from 3TiH₂/SiC/C powders at 1425~1500°C for about 180min. From SEM micrographs, as-synthesized samples were porous. Most plate-like grains were about 5~10 μm in diameter and 1~2 μm in thickness. The speed of temperature increasing is an important factor to affect the purity of as-synthesized Ti₃SiC₂.

Abstract: This paper investigated the self-thickening effects on fabricating close cell aluminum foam directly using semi-solid melt by mechanical stirring without adding other particles. Using the high viscosity character of semi-solid melt as well as mechanical stirring technique, blowing agents (TiH2) could be homogeneously distributed without other thickening additions such as CaO or TiC which were commonly used to thicken aluminum melt in metal foam industry. It was observed that the semi-solid melt could reach a proper condition for the blowing agents to separate and foam after self-thickening process and the solid fraction played a key role in self-thicken effects compared to the stirring time and speed. The self-thickening effects could meet the purity requirement of melt as well as to fabricate aluminum foam with small pores and good structure using precursors. Aluminum foam was a multi-functional material with features of ultra-light, high-toughness, impact resistance, high specific strength, high specific stiffness, heat isolation and fire retardant [1]. The viscosity of melt was extremely important in the process of fabricating aluminum foam. Jin.I and Kenny.LD did some research on the effects of adding solid particles to thicken the melt and their study showed that the best thickening effect could be obtained when the particle size of 5-20 mm and the proportion of 10~20% of liquid aluminum [2]. Miyoshi.T also did some research on the stirring time in the thickening process and showed that adding Ca particles of 1.5~3%(wt) could effectively controlled the viscosity [2]. Research by J.Banhart showed shat 1-5 %(wt) Al2O3 and SiC adding particles could also thicken the semi-solid melt very effectively[3] [4]. Berry.CB and Hall.CG’s research showed that blowing air, oxygen or other gases could also reach thickening effect [2]. There were several methods to thicken the melt but the same ultimate goal was to obtain a suitable melt environment for distribution and foaming of blowing agents. This paper mainly investigated the effects of the self-thickening semi-solid melt on the distribution behavior of blowing agents. Taking the advantage of the high viscosity of semi-solid melt as well as mechanical stirring technique, without adding other thickening particles, blowing agents (TiH2) could be homogeneously distributed in the semi-solid melt. The parameters such as solid fraction, stirring time and speed were investigated in this paper.

Abstract: Being the first step of foaming, bubble heterogeneous nucleation of aluminum foam has remarkable effects on the final cell structure of aluminum foam. The bubble heterogeneous nucleation of foaming Al-9Si and T_iH₂ powder compact has been analyzed. The results obtained are given as follows: (1)The remaining T_iH₂ particles act as the nuclei during heterogeneous nucleation. (2)The nucleation rate increases with the increasing of T_iH₂ content, the decrease of T_iH₂ granularity, the elevation of foaming temperature, and the elongation of foaming time. (3) Only about 5‰ bubble nuclei can survive and develop to the final foam cells. (4)The influence of processing parameters on the nucleation rate descends by the following sequence: foaming temperature elevates 20°C, T_iH₂ granularity decreases from 150μm to 26μm, T_iH₂ content increases 0.5%wt., and the time for foaming extends 10s.

Abstract: Powders metallurgy route was used to produce foams from pure Al powders and pre-alloyed Al alloy powders (Alumix 231) with or without reinforcing elements addition. Both classical Al foam and spherical Al foam were produced with various production parameters and the results obtained have been discussed. SiC particles were used as reinforcing elements and TiH2 powders were used as foaming agent. Prior to compaction process, Al powders, reinforcing element and foaming agent were mixed in a three dimensional turbula for 30 min. Mixed powders were compacted, sintered and deformed and then foamed at 690 oC for spherical foam and 710 oC for classical foam. Effect of volume fraction of reinforcing elements, foaming temperatures and foaming agent amounts on the foaming behavior, pore structure, pore distribution, linear expansion rate, density and wall thickness of the cell were investigated. Experimental studies have shown that 1% foaming agent and 4 % SiC addition was found to be the most suitable for foaming. In general foam produced from Alumix 231 powders exhibited more homogenous pore structure compared to the ones produced from pure Al powders.

Abstract: TiC cermet is widely used for working dies with a high hardness and tool materials. In this research, we attempted to produce submicron sized TiC powders from the ball milled TiH2 and carbon black mixture by thermal treatment. The titanium hydride and carbon composite powders were milled under argon atmosphere for 7 hours at various ball to powder ratios. At the initial stage, an increase in particle size was observed, and graphite phase disappered. The TiC phase of 300nm mean particle size was obtained by milling for 5 hours. As a result, its morphologies were excessively agglomerated. At the heat treating temperature of 500°C, TiH2 phase transformed to Ti completely and the complete TiC of lattice parameter 0.431 nm was formed when the temperature reached 1000°C. Metal matrix composites(MMCs) based on the Fe-TiC system can be synthesized by spark plasma sintering. Specimen formed sintering Fe-TiC powders display a microstructure of uniformly dispersed TiC grain in a continuous metal matrix.