Reactive electric-discharge sintering of TiN-TiB2

Powder mixtures based on titanium and its dihydrides are sintered by electric discharge in a certain current mode to produce TiN + TiB₂ composites. It is established that the use of tetragonal titanium hydride as the starting material results in the complete synthesis leading to a TiN + TiB₂ composite. The higher shrinkage rate results from the higher purity of titanium dihydride, which, in its turn, increases the partial pressure of hydrogen in the reaction area and the reaction rate. Tetragonal titanium hydride is more preferable than cubic hydride and metal titanium. The microstructure of alloys resulting from the sintering of hydride-based mixtures is more homogenous: it includes equiaxial grains with the mean size of about 1 µm.     

The preparation of low-porosity specimens of titanium and zirconium hydrides by the method of reactive sintering

Summary The conditions of preparation of dense specimens of titanium and zirconium hydrides by the method of reactive sintering were investigated. It was established that, as a result of reactive sintering, it is possible to obtain dense specimens of the titanium hydride TiH₂ with a density of up to 2.95 g/cm³ (residual porosity of 23–24%) and a hydrogen content of up to 63 at.% and the zirconium hydride ZrH₂ with a density of up to 4.35 g/cm³ (residual porosity of 21%) and a hydrogen content of up to 66.3 at.%.     

Swelling during sintering of titanium alloys based on titanium hydride powder

Abstract

Compacts were prepared by pressing titanium and titanium hydride powders mixed with nickel powder and sintering under vacuum. Severe swelling was observed only for compacts based on TiH₂ powder. Pressure changes in the vacuum furnace, dilatometry results and mass loss data all indicate that dehydrogenation of TiH₂ powder compacts occurs at lower temperature than any significant sintering. Swelling appears to have been caused by a contaminant in the TiH₂ powder rather than hydrogen. The onset of severe swelling during heating was associated with the formation of liquid phase as the solidus was crossed. However, some swelling appears to take place under solid state sintering conditions. Various results indicate that the mechanism of swelling is high gas pressure within closed pores. Large pores appear to form by breakage of ligaments between small pores followed by opening of the pore. It appears that the use of (uncontaminated) TiH₂ powder in place of Ti powder would allow the benefit of lower green porosity to be retained during sintering to achieve low sintered porosity.     

SOME PROBLEMS IN THE POWDER METALLURGY OF TITANIUM

Abstract

An investigation is described of problems encountered in the conventional powder metallurgy of titanium, with particular reference to the pressing and sintering characteristics of titanium and titanium hydride powders and to sources of contamination. It is concluded that, for success to be achieved in the production of fully dense sintered material by the present method without incurring serious contamination, commercial titanium would have to be available as powder conforming to strict purity requirements.     

Effect of interaction between the structural components of W-Co-Sn pseudo-alloys on the kinetics of their compaction during liquid-phase sintering

Compaction kinetics during liquid-phase sintering of W-Co-Sn powder composites containing 90 mass% refractory component and 10 mass% readily-melting component is studied. It is established that compaction kinetics depends markedly on cobalt content in the melt. Specimens with a cobalt content up to 3 mass% at 1200 °C (in the nonisothermal heating stage) undergo an increase in volume, and then they are compacted at a rate typical for liquid-phase sintering. The nonuniform nature of compaction is observed with an increase in cobalt in the test composites. Specimens with a cobalt mass fraction of more than 2% (cobalt content with three-phase equilibrium) experience considerable additional growth due to formation of the intermetallic compound W₆Co₇ whose decomposition temperature exceeds the liquid-phase sintering temperature.     

THE POWDER METALLURGY OF TITANIUM–TIN ALLOYS CONTAINING UP TO 15% TIN

Abstract

Density measurements and data on the mechanical properties of titanium–tin alloys, fabricated by powder metallurgy from sodium-reduced titanium, are reported.

Low porosities were achieved after a single pressing and sintering operation. A typical set of preparation conditions involved compacting at 40 tons/in.² and sintering, in vacuum, at 1300° C. for 1 hr. With tin contents above 5%, evaporation of tin during sintering could prove troublesome.

The presence of tin strengthened titanium markedly. The addition of 15% tin increased the ultimate tensile stress at room temperature by ～90%, with a corresponding, though less marked, reduction in ductility.     

Suitability of nickel as alloying element in titanium sintered in solid state

Abstract

The suitability of nickel as an alloying element in titanium alloys produced using the blended elemental powder metallurgy approach has been explored. Nickel initially accelerates sintering, providing greater densification at lower temperature than observed for unalloyed titanium. However, it provides only a minor improvement in the density achieved after long sintering times or at high solid state sintering temperatures. Swelling is observed under liquid phase sintering conditions. The highest density was achieved by sintering at just below the solidus temperature. Nickel also accelerates the Ostwald ripening of the pore structure and the conversion of open porosity into closed porosity.     

氢化钛的动态分解行为与规律

通过DSC/TG的热分析试验,研究氢化钛升温过程中分解的动力学规律,利用Coast-Redfern积分法计算了分解过程的动力学参数。结果表明,氢化钛热分解的开始温度为510℃,分解过程中总质量损耗率达3.15%,其中565～660℃温度范围内的质量损耗率占总质量损失的50%左右,分解过程中生成了比氢化钛热稳定性更高的TiHx(0.7相似文献   

Structure formation in a titanium-nickel alloy of eutectic composition

Conclusions The mechanism of structure formation in a titanium-nickel alloy of eutectic composition during sintering consists in the dissolution of nickel particles in a titanium matrix, which leads to the formation of the intermetallic compound TiNi and generation of pores, in TiNi Ti₂Ni transition, and in the appearance, at -Ti/Ti₂Ni contacts, of a liquid, which crystallizes in the form of a eutectic at a temperature below 1228°K and undergoes eutectoid decomposition at a temperature below 1043°K. Produced by sintering in the presence of a solid and a liquid phase, the alloy is characterized by high density, strength, and hardness. Its angle of contact on diamond during heating to 1523°K is 72°. In order to be able to use such an alloy in diamond-containing composites, it will be necessary to lower its sintering temperature and increase its adhesion to diamond.Translated from Poroshkovaya Metallurgiya, No. 1(253), pp. 66–71, January, 1984.     

Effects of dispersal and mechanical boron alloying on the thermal stability of hydride phases in the Ti-B-H system

Methods have been applied from scanning electron microscopy, hydrogen thermal desorption, XRD, and differential thermal analysis on the effects of grain size and alloying with boron as regards the thermal stability and decomposition temperatures of hydride phases in mechanical alloys in the Ti-B-H system. The alloys were prepared by high-energy processing for 50 h in a planetary ball mill with mixtures of TiH_1.9 + 9 mass% B + 13 mass% Ti and also with TiH_1.9 + 50 mass% TiB₂ at speeds of 1000 rpm, in addition to mixtures of TiH_1.9 + 40 mass% B and TiH_1.9 + 50 mass% TiB₂, which were treated for 20 min at speeds of 1680 rpm. The dispersal on mechanical treatment and the addition of boron to the titanium hydride powder have substantial effects on the thermal stability. The processing of the mixture TiH_1.9 + 9 mass% B + 13 mass% Ti lasting 50 h in argon gave temperatures for the dissociation of the Ti(B, H)_x hydride phase in the mechanical alloy lower by 300 deg than the decomposition temperature for the initial titanium hydride TiH_1.9. The mechanisms have been identified for the effects of the dispersal and boron alloying on the thermal stability of the titanium hydride.     

Synthesis of Fine Titanium Powders Via Solution Route

Abstract

Titanium, because of its light weight, high specific strength, corrosion resistance and biocompatibility is a demanding material for aerospace, chemical processing industries and biomedical applications. Titanium powders produced from titanium sponge, hydride-dehydride processes and by a variety of centrifugal atomisation techniques from liquid metals are relatively coarse. If fine size titanium powders can be produced, then the grain size in the sintered titanium will be small thereby contributing to the high strength of the product. In the present investigation, an attempt has been made to synthesize fine titanium powders from titanium dioxide. The later is allowed to form a complex, titanium catecholate, in the presence of ammonium sulphate and concentrated sulphuric acid. The complex is filtered, washed with cold isopropyl alcohol and dried. Titanium hydride is prepared by heating the titanium catecholate in the temperature range 500-600°C in hydrogen atmosphere. The powder obtained is crushed ground and reheated at temperatures upto 950°C in vacuum. The product is rapidly cooled from this temperature to obtain titanium powders. The powder characteristics such as particle size and crystallization have been evaluated and these results are reported and discussed.     

Effect of Fe addition on sintering behaviour of titanium powder

Abstract

The effect of iron on the sintering behaviour of titanium powder was investigated from two aspects: (1) diffusional homogenisation of iron; (2) densification of Ti-5Fe alloy. Under the present process conditions (heating rate of 5 K min^-1 and iron content 5 wt-%), iron dissolved into the titanium matrix thoroughly before the first eutectic temperature; potential liquid phase did not appear. The addition of iron enhances the sinterability of titanium alloys because the mobility of titanium atoms is accelerated by the rapid diffusion of iron. Most sintering shrinkage is achieved during the heating stage from 950 to 1250°C. Based on the diffusion creep mechanism of Nabarro-Hering, the result can be explained as a combination of the diffusion coefficient D and inherent local sintering stress σ, and the dissolution of iron in titanium is expected to reduce the creep strength of the Ti matrix at high temperatures due to its very fast diffusion rate. The effect of iron on the microstructure of Ti-5Fe alloy is also discussed. The formation of a Widmanstättenlike microstructure in Ti-5Fe alloy can be attributed to a β stabilising effect and a high diffusion rate of iron during furnace cooling.     

Study on bulk Sm2 Fe17 Nx sintered magnets prepared by spark plasma sintering

Abstract

Sm₂Fe₁₇N_x sintered magnets were fabricated by spark plasma sintering (SPS) technique. Effects of sintering pressure, sintering temperature and heating rate on the magnetic properties and crystal structures of the magnets were investigated. The results showed that the density of the magnet was obviously improved with increasing sintering pressure, but the coercivity dropped simultaneously because Sm₂Fe₁₇N_x decomposed into SmN, α-Fe and N₂. The coercivity decreased rapidly when sintering temperature was above 200°C under 1 GPa sintering pressure, which indicated that high pressure promoted the decomposition of Sm₂Fe₁₇N_x even at low temperature. In addition, the decomposition could not be effectively restrained even if the heating rate reached 450°C min^?1.     

Processing and Pore Growth Mechanisms in Aluminum Gasarites Produced by Thermal Decomposition

Gasarites are a subclass of metallic foams that have a cylindrical pore morphology created by directional solidification of metals saturated with a gas. Thermal decomposition is an alternative process in which the soluble gas is delivered by decomposition of a particulate gas source. Aluminum gasarites formed through decomposition of titanium and zirconium hydrides were studied to both replicate the results of a previous study and discern pore-formation mechanisms. Replication of the previous study was not achieved, and additional processing enhancements were required to produce gasarite pore morphologies. For the first time, zirconium hydride was utilized to produce gasarites, with porosity levels and pore sizes lower than that from titanium hydride. Maximum average porosity levels of 10 and 6 pct were observed for titanium hydride and zirconium hydride, respectively. Pore-formation mechanisms in aluminum gasarite foams created via thermal decomposition of titanium and zirconium hydrides were evaluated through metallographic analysis and scanning electron microscopy. Definitive evidence of gas–metal eutectic pore growth was not found, but pore morphological characteristics and chemical analysis of particulate at pore surfaces support direct gas evolution from the hydride particles as a contributor to pore formation and growth.     

Porous titanium processed by powder injection moulding of titanium hydride and space holders

Abstract

Biocompatibility, bone-like mechanical properties, and good bone-to-implant anchorage are current requirements for permanent implants. Porous titanium can satisfy these requirements provided that sufficient porosity, large enough pores and interconnections allowing bone ingrowth can reliably be obtained with controlled processes. In the present work, porous parts are processed from titanium hydride based feedstocks containing space holders. Two formulations have been developed: a feedstock with a polyethyleneglycol based binder and NaCl space holders, and a feedstock with a paraffin based binder and PMMA space holders. Depending on the sintering conditions, porosity levels between 30 and 60% and open porosity between 10 and 40% are obtained, with pore sizes in the range 50–500 μm. The microstructure, porosity and mechanical properties of porous titanium sintered at various temperatures have been characterised by scanning electron microscopy and compression tests.     

Densification kinetics of W-Fe-Sn pseudoalloys in liquid phase sintering

The densification kinetics in liquid phase sintering of W-Fe-Sn powder composites containing 90 wt.% of a refractory component and 10 wt.% of a low-melting component is strongly dependent on the iron content in the melt. As the iron content in composites increases, the concentration dependence of their densification shows a maximum. Samples containing more than 1.5 wt.% Fe grow intensively because of the formation of W₆Fe₇ intermetallide whose decomposition temperature is higher than that of liquid phase sintering. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 5–6 (455), pp. 22–29, 2007.     

INFLUENCE OF ADSORBED FORMIC AND ACETIC ACID VAPOURS ON SINTERED PROPERTIES OF COPPER POWDER

Abstract

The paper discusses an original method of activating the sintering of copper powder. The technique is based on the interaction of formic or acetic acid vapour with copper powder, which results in adsorption of one or more of the decomposition products. It is shown that the adsorbed layers enhance densification, hardness, and tensile strength. The study includes observations on the effect of activator content, sintering atmosphere, temperature, time, and powder particle size.     

Comparison of sintering of titanium and titanium hydride powders

Abstract

The cold compaction and vacuum sintering behaviour of a Ti powder and a Ti hydride powder were compared. Master sintering curve models were developed for both powders. Die ejection force, green strength and green porosity were lower for hydride powder than for Ti powder, all probably resulting from reduced cold welding and friction during compaction. For sintering temperatures above ～1000°C, most of the difference in the sintered density of Ti and hydride is explained by assuming equal densification, while taking into account the lower green porosity of compacts made from hydride powder. However, there is evidence that particle fracture during compaction also contributes to increased sintered density for hydride powder. The Ti powder conformed to a master sintering curve model with apparent activation energy of 160 kJ mol^?. The activation energy for Ti hydride also appeared to be about 160 kJ mol^?, but the model did not fit the experimental data well.     

Production of titanium grade 4 components by powder injection moulding of titanium hydride

Abstract

Recent developments are presented on powder injection moulding of titanium from metal hydride powders and binders composed of polyethylene, paraffin wax and stearic acid. The feasibility of using this route to process fit for purpose, complex parts is assessed. Titanium hydride offers a low cost solution compared with pure titanium powders. Feedstocks for powder injection moulding were prepared in a sigma mixer. Tensile test specimens and demonstration parts were injection moulded. Solvent debinding in heptane was followed by thermal debinding and dehydrogenation under argon. Titanium parts were sintered at 1200°C under argon. Sintered parts exhibit a linear shrinkage of about 20%, good shape preservation and reproducibility. The yield strength (519 MPa), ultimate tensile strength (666 MPa), elongation to fracture (15%) and interstitial content measured by quantitative analysis meet the requirements for titanium grade 4.     

Hydrogen embrittlement of α titanium: In situ tem studies

The effect of hydrogen on fracture in the h.c.p. α Ti-4 wt % Al alloy and the role of titanium hydride in the fracture process have been studied by deforming samples in situ in a high-voltage electron microscope equipped with an environmental cell. Two fracture mechanisms have been observed in a gaseous hydrogen environment at room temperature; one is fracture by localized plastic deformation enhanced by the presence of hydrogen, the other is a brittle fracture of the stress-induced titanium hydride which precipitates at elastic singularities. The local stress intensity determines which mechanism predominates. At high stress intensities the crack propagates by the process of hydrogen enhanced localized plasticity, while at low stress intensities titanium hydrides form in the vicinity of crack tips and crack propagation proceeds though the hydrides.