Intermetallic FeTi Research Articles

Mechanism of activation of TiFe intermetallics for hydrogen storage by severe plastic deformation using high-pressure torsion

TiFe, a potential candidate for solid-state hydrogen storage, does not absorb hydrogen without a sophisticated activation process because of severe oxidation. This study shows that nanostructured TiFe becomes active by high-pressure torsion (HPT) and is not deactivated even after storage for several hundred days in the air. Surface segregation and formation of Fe-rich islands and cracks occur after HPT. The Fe-rich islands are suggested to act as catalysts for hydrogen dissociation and cracks and nanograin boundaries act as pathways to transport hydrogen through the oxide layer. Rapid atomic diffusion by HPT is responsible for enhanced surface segregation and hydrogen transportation.

High-pressure torsion of TiFe intermetallics for activation of hydrogen storage at room temperature with heterogeneous nanostructure

TiFe is a potential candidate for the stationary hydrogen storage systems, but it requires initial activation to absorb hydrogen. This study shows that TiFe processed by high-pressure torsion (HPT) absorbs and desorbs 1.7 wt.% hydrogen at room temperature without activation. The absorption pressure decreases from 2 MPa in the first hydrogenation cycle to 0.7 MPa in the latter cycles. The HPT-processed TiFe exhibits heterogeneous microstructures composed of nanograins, coarse-grains, amorphous-like phases and disordered phases with a high hardness of ∼1050 Hv.

Titanium to steel joining by spark plasma sintering (SPS) technology

The bonding of Ti–6Al–4V to low alloy steel (AISI4330) using SPS technique in the 850–950°C temperature range was examined. The formation of a thin (∼1μm) titanium carbide interfacial layer was observed with a thickness only slightly dependent on the joining temperature. This layer separates the joined metals and prevents the formation of Fe–Ti intermetallics in the bonding zone. The maximal tensile strength of the joints (of about 250MPa) was achieved for bonding at 950°C for 3.6ks. The formation of the titanium carbide layer and its evolution are discussed based on the isothermal section of the ternary Fe–Ti–C phase diagram.

Effect of a copper filler metal on the microstructure and mechanical properties of electron beam welded titanium–stainless steel joint

Abstract Cracking in an electron beam weld of titanium to stainless steel occurred during the cooling process because of internal thermal stress. Using a copper filler metal, a crack free joint was obtained, which had a tensile strength of 310 MPa. To determine the reasons for cracking in the Ti/Fe joint and the function of the copper filler metal on the improvement of the cracking resistance of the Ti/Cu/Fe joint, the microstructures of the joints were studied by optical microscopy, scanning electron microscopy and X-ray diffraction. The cracking susceptibilities of the joints were evaluated with microhardness tests on the cross-sections. In addition, microindentation tests were used to compare the brittleness of the intermetallics in the welds. The results showed that the Ti/Fe joint was characterized by continuously distributed brittle intermetallics such as TiFe and TiFe(Cr)2 with high hardness (~ 1200 HV). For the Ti/Cu/Fe joint, most of the weld consisted of a soft solid solution of copper with dispersed TiFe intermetallics. The transition region between the weld and the titanium alloy was made up of a relatively soft Ti–Cu intermetallic layer with a lower hardness (~ 500 HV). The formation of soft phases reduced the cracking susceptibility of the joint.

Dissimilar laser welding of NiTi to stainless steel

Shape memory superelastic alloys as NiTi have mechanical properties and biocompatibility characteristics quite interesting for a set of industries, amongst which is the medical industry. They find applications in tools and devices where frequently joining between them and to other alloys as austenitic stainless steel is required. However, permanent joining of NiTi to stainless steel is problematic due to the formation of Fe–Ti intermetallics. Pulsed laser welding was studied to join thin foils of NiTi in similar and dissimilar joints to stainless steel. Joining NiTi to NiTi was succeeded with no weld defects. When joining NiTi to stainless steel, it was found that the material impinged by the laser determined the weld pool shape and structure, and it was better when the stainless steel foil was placed below the laser, because nickel enrichment of the weld pool was found to minimise cracking. The factors controlling the weld pool composition and, consequently, the quality of the weld were investigated and discussed.

An optimized diffusion database for the disordered and ordered bcc phases in the binary Fe–Ti system

An assessed mobility database for the disordered A2-bcc and ordered bcc-B2 phases in the binary Fe–Ti system has been constructed using the model of Helander and Ågren [T. Helander, J. Ågren, A phenomenological treatment of diffusion in Al–Fe and Al–Ni alloys having B2-BCC ordered structure, Acta Mater. 47 (1999) 1141–1152]. The database is based on the experimental data available in the literature and previously assessed mobilities. In the disordered phases, the model yields a good description of both the anomalous diffusion behavior of Fe and Ti in β-Ti and the effect of ferromagnetic ordering in α-Fe. Contributions from long range ordering are incorporated in the database to describe diffusion in the intermetallic FeTi phase.

Synthesis of FeTi from mixed oxide precursors

A study was carried out on the synthesis of FeTi intermetallics from mixed oxide precursors using the method of electro-deoxidation. Fe 2O 3 and TiO 2 mixed in molar proportions of 1:2 were sintered at temperatures ranging from 900 °C to 1300 °C. The sintered pellet of mixed oxides, connected as cathode, was then electrolyzed in a molten CaCl 2 at 900 °C using a graphite anode at a potential of 3.2 V. The electrolysis yielded the target composition FeTi in substantial amounts only when the sintering temperature was close to or above 1100 °C. The process of deoxidation was followed with the use of interrupted experiments. This has shown that the two-phase structure of Fe 2TiO 5 and TiO 2 in the oxide pellet reacts with the molten salt even before the electrolysis forming CaTiO 3, transforming the rest into a mixture of ilmenite (FeTiO 3) and a spinel phase (Fe 2TiO 4). During electrolysis these complex oxides were converted into simpler ones. Fe is the first element to be produced followed by the intermetallic Fe 2Ti. FeTi evolves quite late in the electrolysis which seemed to have followed the reduction of CaTiO 3. It is shown that interrupted experiments and examination of partially reduced pellets yield considerable information with regard to the sequence of changes that occur during the deoxidation process.

Infrared brazing of Ti–6Al–4V and 17-4 PH stainless steel with (Ni)/Cr barrier layer(s)

Ti–6Al–4V and 17-4PH stainless steel (17-4PH SS) with (Ni)/Cr barrier layer(s) were infrared vacuum brazed using two silver-based braze alloys, 72Ag–28Cu and 63Ag–35.25Cu–1.75Ti (wt.%), respectively. Both Cr (15 μm) and Ni (2 μm)/Cr (15 μm) coated 17-4PH SS plates were evaluated in the study. Introducing (Ni)/Cr barrier layer(s) can effectively inhibit the interfacial reaction between the 17-4PH SS and the molten braze during brazing. Therefore, the formation of brittle interfacial Ti–Fe intermetallics next to the 17-4PH SS is avoided and replaced by Ti–Cu–(Ni) and TiCr 2 intermetallic compounds in the joint. The average shear strength of the infrared brazed joint with Ni/Cr barrier layers is significantly improved up to 233 MPa. Using (Ni)/Cr barrier layers on the 17-4PH SS has the potential of industrial application in brazing Ti–6Al–4V and 17-4PH SS.

Solid state reaction of Fe/Ti nanometer-scale multilayers

The Fe/Ti multilayers 19.0 nm thick with alternating Fe and Ti sublayers thickness ratio of 1:1 were deposited by direct current magnetron sputtering. The Fe/Ti multilayers were in situ submitted to thermal vacuum annealing at the temperatures ranging from 250 °C to 450 °C for the annealing time of 1 h. Structural evolution of Fe/Ti multilayers was detected by X-ray diffraction and high resolution transmission electron microscopy. The diffraction peaks of α-Fe (110) and broaden α-Ti (002) were obtained without any additional diffraction peaks when the Fe/Ti multilayers were annealed at the lower temperatures of 250 °C and 350 °C, which suggested the formation of the solid solution of Fe with Ti due to interdiffusion between Fe and Ti sublayers during the thermal annealing. At the higher annealing temperature of 450 °C, the precipitation of intermetallic FeTi carried out in the Fe–Ti solid solution. The mixture of intermetallic FeTi and α-Fe phase presented in the extended Fe sublayer of the Fe/Ti multilayers. Three distinct exothermic peaks were characterized in the differential scanning calorimetry traces of the Fe/Ti multilayers 19.0 nm thick heated from 25 °C to 630 °C at the four heating rates of 10–40 °C/min. The activation energy for the three peaks is 1.04 eV, 1.43 eV and 1.49 eV, respectively, corresponding to the formation of the Fe–Ti solid solution, the nucleation and growth of the intermetallic FeTi, respectively.

Infrared brazing of Ti-6Al-4V and 17-4 PH stainless steel with a nickel barrier layer

Infrared brazing of Ti-6Al-4V and 17-4 PH stainless steel using the BAg-8 filler metal was performed in this study. A nickel barrier layer 10 µm thick was introduced on the 17-4 PH stainless steel before infrared brazing. For the specimen that was infrared brazed at 800 °C and 850 °C for less than 300 seconds, the Ni layer served as an effective barrier layer to prevent the formation of Ti-Fe intermetallics. Experimental results show that the average shear strength of the joint can be greatly improved for the specimen by Ni plating. Comparing the specimens with and without electroless-plated Ni film, the former has no Ti-Fe intermetallic compound, but interfacial CuNiTi and NiPTi phases are observed in the latter. The fractured location of the joint after the shear test is changed from the interfacial TiFe (without Ni plating) into the TiCu reaction layer (with Ni plating). The plated Ni layer is consumed for the specimen that was infrared brazed at 880 °C for 300 seconds, and its bonding strength is impaired. Consequently, a lower brazing temperature and/or time are still preferred even though a plated Ni barrier layer is applied.

Microstructural evolution of Fe/Ti multilayers submitted to in situ thermal annealing

The Fe/Ti multilayers of nominal bilayer thickness of 4.0, 8.0 and 18.0 nm with alternating Fe and Ti sublayers thickness ratio of 1:1 were deposited by direct current magnetron sputtering on Si(100) substrates. The bilayer thickness of as-deposited Fe/Ti multilayers was measured as a modulation wavelength of 4.8, 8.5 and 19.0 nm, respectively, by small angle X-ray diffraction and cross-sectional transmission electron microscopy (XTEM). The three Fe/Ti multilayers were composed of pure metallic α-Fe and α-Ti according to wide angle X-ray diffraction and selected area diffraction. The Fe/Ti multilayers were in situ submitted to thermal vacuum annealing at the temperatures ranging from 523 K to 723 K for the annealing time of 3 h. With annealing at the lower temperature of 523 K, the intermetallic FeTi appeared in the Fe/Ti multilayers with small modulation wavelength of 4.8 nm. At 623 K, the intermetallic FeTi was formed with modulation wavelength of 8.5 nm. At the higher temperature of 723 K, the intermetallic FeTi was detected with modulation wavelength of 19.0 nm. The modulation wavelength of the Fe/Ti multilayers remained during the thermal annealing. The mixture of intermetallic FeTi and α-Fe phase was observed in the extended Fe sublayer of the Fe/Ti multilayers annealed at 723 K by XTEM, correspondingly the remaining Ti sublayer was obtained as a thinned sublayer. Coexistence of the intermetallic FeTi, α-Fe and α-Ti phases indicated that the dynamic factors have control of the intermixing between the Fe and Ti sublayers in the Fe/Ti multilayers during the thermal annealing.

Phase structure of the Fe–Ti layers produced by the IPD method

This paper is concerned with the synthesis of Fe–Ti layers under the impulse mass and energy transport. The synthesis method employed in the present study was impulse plasma deposition. The synthesis processes were conducted in an apparatus equipped with two independent impulse plasma accelerators operated alternately. The material was an Fe–Ti alloy. The sources of Fe and Ti were the internal electrodes of the impulse plasma accelerators, made of titanium and iron. The phase structure of the Fe–Ti layers was studied using X-ray diffraction and conversion electron Mössbauer spectroscopy (CEMS). The phases identified were α -Fe, α -Ti, β -Ti bcc-Fe(Ti), crystalline intermetallic FeTi, amorphous FeTi and amorphous Fe–Ti–C.

Characterisation of Mg–x wt.% FeTi (x = 5–30) and Mg–40 wt.% FeTiMn hydrogen absorbing materials prepared by mechanical alloying

Composites based on Mg with intermetallic FeTi (up to 30wt.%) and FeTiMn (40wt.%) were prepared by mechanical alloying for evaluation of their hydrogen storage characteristics. In the first case FeTi was prepared by mechanical alloying and subsequently milled with Mg under argon in different proportions. Powders milled under argon had to be activated by remilling under hydrogen. In the second case FeTiMn prepared by melting was milled together with MgH2 under hydrogen. Hydrogen storage characteristics, pressure-composition isotherms and kinetics were investigated under different parameters. Among the FeTi composites, Mg–5wt.% FeTi had the maximum absorption capacity of 5.80wt.% at 300°C and 5.12wt.% at 200°C. At 300°C the initial desorption rate was the highest for Mg–10wt.% FeTi, but its final desorption capacity was less than that of Mg–5wt.% FeTi. None of the compositions showed any desorption at 200°C. In the case of composites with FeTiMn, absorption and desorption temperatures decreased by ball milling and higher storage capacities were achieved. In Mg–40wt.% FeTiMn, the lowest absorption and desorption temperatures were 80 and 240°C, respectively, with the material absorbing 4wt.% hydrogen even at 80°C. The kinetics improved with increasing milling time.

Reactive Diffusion Bonding Between Commercially Pure Titanium and 304 Stainless Steel Using Nickel Interlayer

Diffusion bonding was carried out between commercially pure titanium and 304 stainless steel using nickel as interlayer in the temperature range of 800-900°C for 5.4 ks under 3 MPa load in vacuum. The transition joints thus formed were characterized in optical and scanning electron microscopes. TiNi3, TiNi and Ti2Ni are formed at the Ni-Ti interface whereas, 304 ss-Ni diffusion zone is free from intermetallic compounds at 800 and 850°C processing temperatures. At 900°C, Ni-Ti interface exhibits the presence of α-β Ti discrete islands in the matrix of Ti2Ni and the phase mixture of α-Fe+λ+χ occurs at the ss-Ni interface. Nickel is able to inhibit the diffusion of Ti to 304 ss side up to 850°C; however, becomes unable to restrict the migration of Ti to stainless steel at 900°C. Highest bond strength of 80% of that of titanium has been obtained for the diffusion couple processed at 850°C owing to the better coalescence of the mating surfaces and failure takes place from Ni-Ti interface. At higher joining temperature, the formation of Fe-Ti intermetallics reduces the bond strength and failure in tensile loading occurs from ss-Ni interface.

Non-equilibrium Ti-Fe bulk alloys with ultra-high strength and enhanced ductility

ABSTRACTThe high-strength and ductile hypo-, hyper- and eutectic Ti-Fe alloys were formed in the shape of the arc-melted ingots with the dimensions of about 25–40 mm in diameter and 10–15 mm in height. The structure of the samples consists of cubic Pm 3 m TiFe and BCC Im 3 m β-Ti supersaturated solid solution phase. The arc-melted hypereutectic Ti65Fe35 alloy has a dispersed structure consisting of the primary TiFe phase and submicron-size eutectic structure. This alloy exhibits excellent mechanical properties: a Young's modulus of 149 GPa, a high mechanical fracture strength of 2.2 GPa, a 0.2 % yield strength of 1.8 GPa and 6.7 % ductility. The hard round-shaped intermetallic TiFe phase and the supersaturated β-Ti solid solution result in a high strength of the Ti65Fe35 alloy which in addition has much higher ductility compared to that of the nanostructured or glassy alloys. The reasons for the high ductility of the hypereutectic alloy are discussed.

メカニカルアロイング法により作製したＴｉＦｅ水素吸蔵合金の初期活性化特性

The intermetallic TiFe alloy is a hydrogen storage alloy with a high hydrogen storage capacity, 1.8wt%, and a relatively low cost compared with other alloys. However, the TiFe alloy does not easily start to absorb hydrogen because of the presence of a very stable oxide layers covering the alloy surface. In order to activate the alloy, the alloy needs an intensive initial activation process at high temperatures over 720K under high hydrogen pressures over 6.5MPa. In this study, the intermetallic TiFe alloy was successfully prepared by mechanical alloying of Ti and Fe metals. The formed nano-TiFe alloy with a mixture of nano and amorphous structures was found to have a very easy, activation behavior compared with a standard polycrystalline TiFe alloy.

XAFS study of an intermetallic TiFe 0.95Zr 0.03Mo 0.02 system for CO 2 conversion

An intermetallic TiFe 0.95Zr 0.03Mo 0.02 system used for CO 2 conversion was studied by XAFS spectroscopy. The initial samples prepared by the skull-furnace method were exposed to the below treatments: (1) H 2 absorption (up to ∼1 mol of H 2 per 1 mol of the initial sample); (2) H 2 thermal desorption (up to ∼0.18 mol of H 2 at 180 °C in inert argon atmosphere); (3) hydrogen titration by CO 2 (at 350 °C for 4–6 h). For samples [TiFe 0.95Zr 0.03Mo 0.02]H 2 x ( x⩽0.18), the position of Ti-K edge shifts of about ∼3 eV compared to that of the initial sample. From the XAFS studies of the modified [TiFe 0.95Zr 0.03Mo 0.02] intermetallide, it is established that the interaction of dissolved hydrogen and titanium follows the donor–acceptor mechanism. Hydrogen atoms are located on the straight line connecting two titanium atoms. The one distance between a hydrogen atom and a titanium atom is less than 1.5 Å. It is proposed that the effect of strong binding of hydrogen in the [TiFe 0.95Zr 0.03Mo 0.02]H 2 x intermetallide at a molar concentration of absorbed hydrogen of 0.18 is associated with the formation of such binding.

Diffusion bonding between Ti -6Al -4V alloy and microduplex stainless steel with copper interlayer

In the present study, diffusion bonding was used to join Ti -6Al- 4V alloy to a microduplex stainless steel using a pure copper interlayer. The effects of heating rate and holding time on microstructural developments across the joint region were investigated. After bonding, microstructural analysis including metallographic examination and energy dispersive spectroscopy (EDS), microhardness measurements, and shear strength tests were carried out. From the results, it was seen that heating rate and holding time directly affect microstructural development at the joint, especially with respect to the formation of TiFe intermetallic compounds, and this in turn affects the shear strength of the bonds. A sound bond was obtained with a heating rate of 100 K min -1 and holding time of 5 min, and this was related to the small amount of TiFe intermetallics formed close to the duplex stainless steel side at this bonding condition. Although Ti2Cu and TiFe intermetallics were formed in all specimens, it was seen that the most deleterious intermetallic was TiFe. As the heating rate was decreased and holding time increased the amount of TiFe intermetallics increased, and consequently shear strength decreased. As a result, from the microstructural observations, EDS analysis, microhardness measurements, and shear strength tests, it was concluded that a high heating rate and a short holding time must be used in the diffusion bonding of Ti-6Al- 4V to a microduplex stainless steel when pure copper interlayers are used.

Mössbauer study of titanium implanted -Fe

Bulk -Fe has been implanted with 200 keV Ti ions with doses ranging from 1 × 1017 to 8 × 1017 at. Ti cm-2 and characterized by conversion electron Mössbauer spectroscopy (CEMS). The CEMS spectra recorded for the low dose Ti implantation of the surface region reveal the formation of the crystalline phases (bcc-FeTi solid solution and FeTi intermetallic compound) and of the amorphous Fe-Ti-C phase. For Ti doses exceeding 4 × 1017 at. Ti cm-2 the amorphous FeTi phase is formed, whose relative spectral fraction saturates at about 40% of the total spectral area at 6 × 1017 at. Ti cm-2. At high ion doses the amorphous Fe-Ti-C phase disappears but the bcc-FeTi and crystalline intermetallic FeTi phases persist, beside the amorphous FeTi phase, even at the highest Ti dose used.

The Crystalline-to-Amorphous Transformations in the Ti-Fe and Ti(H<sub>2</sub>)-Fe Systems during Ball Milling

Elemental equiatomic Ti-Fe as well as similar Ti(H 2 )-Fe powder mixtures were mechanically alloyed in ball mill. The structural changes as a function of milling time were investigated by X-ray diffractometry and low temperature Mossbauer spectroscopy. During milling the supersaturated solid solutions β-Ti(Fe) and α-Fe(Ti) in wide range of composition, two intermetallic phases FeTi and Fe 2 Ti, formation and transformation up to quasi-amorphous state were observed. Furthermore in Fe-Ti(H2) system the MA process proceeds much slower and the formation of FeTi and FeTiH x phases and α-Fe(Ti) solid solutions were observed.