Cold-pressing and vacuum arc melting of γ-tial based alloys

eta (β) solidifying γ-TiAl intermetallic alloys of nominal composition Ti-48Al, Ti-48Al-2Nb, Ti-48Al-2Nb-0.7Cr alloys have been cold pressed and vacuum arc melted. The Al loss was due to compaction method used prior to the melting technique, since it was evident after compaction that Al particles migrated to the surface in contact with the die facets after cold pressing

Structural and gas sensing properties of TiO₂-based (Sn, Mg) nano-structures induced by mechanical milling and annealing

Philosophiae Doctor - PhDTitanium oxynitride has attracted research interest for the fact that it is a bioactive non-toxic material. It is suitable for surface coating of biomaterials and in other applications such as anti-reflective coatings, while oxygen-rich titanium oxynitride has been applied in thin film resistors and photocatalysis. Two common types of titanium oxynitrides are TiOₓNᵧ. and TiO₂-ₓNᵧ. In this work, titanium mixed metals oxynitrides (Ti-TiO₂, Mg-TiO₂ and Mg-Sn-TiO₂) were synthesized for the first time using ball milling (BM) and annealing processes. Their structural, morphological, surface, optical, and gas sensing properties were studied in detail. Structural analyses showed that upon milling a pure TiO₂ phase, tetragonal to orthorhombic phase transformation occurred. However, when milling TiO₂ mixed with Mg, Sn and Ti no evidence of the transformation was observed. Furthermore, scanning electron microscopy, transmission electron microscopy and atomic force microscopy showed that the milling process promotes particle refinement. The gas sensing analyses also demonstrated that the sensing response of the TiO₂, Mg-TiO₂ and Mg-Sn-TiO₂ materials improved upon milling. Moreover, the Mg-TiO₂ showed improved sensing compared to pure TiO₂ due to incorporation of Mg, which might have resulted in a decrease of charge carrier concentration. The Mg-TiO₂ sensing materials showed fast response-recovery time of ~32 s and ~48 s, respectively, as well as high selectivity to NH₃ gas compared to other gases (H₂, and CH₄). In addition, the improved response observed for the milled samples is due to increased surface area and pore diameter, providing more active sites for the target gas and allowing more gas adsorption with an increase in point defects related to oxygen vacancies (Vo), which are the most favorable adsorption sites for oxygen species and thus can enhance the possibility of interaction with gas molecules. A combination of photoluminescence, x-ray photoelectron spectroscopy, vibrating sample magnetometer and sensing analyses demonstrated that a direct relation exists between the magnetization, sensing and the relative occupancy of the Vo present on the surface of TiO₂ nanoparticles. Therefore, based on these finding we conclude that the milling process promotes particle refinement, resulting in an increased BET surface and partial breaking of Ti–O bonds on the TiO₂ surface layer, which results in the formation of oxygen vacancies in the TiO₂ lattice, therefore anticipating improved sensing response.National Research Foundatio

Effect of Mechanical Milling and Cold Pressing on Co Powder

Absract: Cobalt (Co) is a transition metal used in electronics, magnetic recording [1], and hard materials [2–4]. Through thermal [5] and mechanical treatment [6–9], Co metal undergoes the allotropic HCP to FCC phase transformation. The current literature shows that metastable FCC phase is induced by ball milling (BM) [6], cold pressing [10], and thermal treatments [11]. It has been shown in the previous investigations that Co milled with W, V, and C powders forms the complex FCC Co-rich carbide with large lattice parameters [12]. Apart from the FCC phase, Co-based alloys undergo amorphization via electrodeposition [13, 14] and mechanical alloying (MA) [15–17] and during irradiation processes [18, 19]. It is worth noting that in the case of milling, the XRD peaks of nonmetallic elements such as Si in a mixture with Co vanish rapidly when subjected to BM and ultimately lead to amorphization [17]. Similar behaviour occurs when a mixture of Co and metallic elements is subjected to milling [15, 16, 20]..

Effect of Mechanical Milling and Cold Pressing on Co Powder

Absract: Cobalt (Co) is a transition metal used in electronics, magnetic recording [1], and hard materials [2–4]. Through thermal [5] and mechanical treatment [6–9], Co metal undergoes the allotropic HCP to FCC phase transformation. The current literature shows that metastable FCC phase is induced by ball milling (BM) [6], cold pressing [10], and thermal treatments [11]. It has been shown in the previous investigations that Co milled with W, V, and C powders forms the complex FCC Co-rich carbide with large lattice parameters [12]. Apart from the FCC phase, Co-based alloys undergo amorphization via electrodeposition [13, 14] and mechanical alloying (MA) [15–17] and during irradiation processes [18, 19]. It is worth noting that in the case of milling, the XRD peaks of nonmetallic elements such as Si in a mixture with Co vanish rapidly when subjected to BM and ultimately lead to amorphization [17]. Similar behaviour occurs when a mixture of Co and metallic elements is subjected to milling [15, 16, 20]..

Powder characteristics blending and microstructural analysis of a hot-pack rolled vacuum arc-melted gamma-tial-based sheet

In the quest for cost-effective fabrication processes capable of producing sound γ-TiAl products, the microstructure and mechanical properties of a modified second-generation hot-rolled γ-TiAl-based alloy with nominal composition Ti-48Al-2Nb-0.7Cr-0.3Si were investigated in this work. The alloy was fabricated using a processing route that involved uniaxial cold-pressing of powders and vacuum arc re-melting. Prior to the cold pressing, the elemental powder characteristics, such as particle sizes and morphologies, were blended to minimise porosity in the compact that might be inherited by the final ingot. A hot-pack rolling process was carried out directly from the melted button-ingot using a conventional two-high rolling mill to produce a 4 mm-thick sheet. The relative density results of both as-compacted and as-melted alloy parts showed a significant reduction of porosity in the alloy. In addition, both the optical and the scanning electron microscopy micrographs of the rolled sheet revealed a typical 'duplex' microstructure with a mean grain size of about 9 urn. Moreover, the results from a room-temperature indentation plastometry test of the hot-rolled sheet indicated good mechanical properties with recorded yield strength of about 600 MPa, an ultimate tensile strength of about 850 MPa, and a true plastic strain of about 3%

Comparison of metastable phases induced by heat treatment of unmilled and milled cobalt powders

Abstract The sintered and water quenched compact samples were prepared from unmilled and milled Co powders. Characterisation was performed by differential scanning calorimetry, scanning electron microscopy and X-ray diffraction techniques. Several metastable phases were obtained upon sintering and quenching. However, more metastable phases were induced on quenching the milled–sintered samples due to introduction of large number of defects in addition to those induced by milling. Micro hardness values for unmilled sintered samples were the lowest while those of 30 h milled-sintered samples were the highest. The current study reveals that the two FCC metastable phases obtained by quenching unmilled powder were similar to those found in milled–sintered samples

Fabrication of AgCu/TiO2 nanoparticle-based sensors for selective detection of xylene vapor

The design and fabrication of innovative nanostructured materials that could display improved sensitivity, selectivity, and rapid response/recovery characteristics still present significant scientific challenges. Herein we report the timely selective detection of xylene vapour in benzene, toluene, ethylbenzene (BTE) and acetone vapours at low operating temperatures using an n-type AgCu/TiO2 nanoparticlebased sensor. Switching from p-type to n-type conductivity was observed at higher AgCu loadings. The findings showed that sensor switching was not temperature- or gas-dependent. Among the AgCu loaded on TiO2 nanoparticles, n-type 0.5% AgCu loaded on TiO2 displayed a remarkable response (Rg/Ra E 33.2) toward xylene vapour at 150 1C. The sensor exhibited superior selectivity

Deformation and fracture behaviour of the g-TiAl based intermetallic alloys

The b-solidifying g-TiAl intermetallic alloys of nominal composition Ti-48Al (binary alloy), Ti-48Al-2Nb (ternary alloy), Ti-48Al-2Nb-0.7Cr (quaternary alloy), and Ti-48Al-2Nb-0.7Cr-0.3Si (quinary alloy) (in at.%) were developed. The materials produced were tensile tested at room temperature. The as-cast microstructures and fracture surfaces of the tensile tested specimens were examined using conventional metallographic methods. Microstructural examination indicated that the alloys were comprised of lamellar structures (a2+g) embedded in columnar dendritic cores in the as-cast condition. However, the quinary alloy contained a Ti5Si3 second phase. The alloys exhibited no detectable ductility during tensile deformation, indicating the brittleness of all the materials. The fracture surfaces revealed that the alloys failed by translamellar fracture with correspondingly few cleavage facets.The Department of Science and Technology (DST) and the CSIR.http://www.saimm.co.za/journal-papersam2022Materials Science and Metallurgical Engineerin

Low-cycle fatigue behaviour of titanium-aluminium-based intermetallic alloys : a short review

DATA AVAILABILITY STATEMENT: Data sharing not applicable.Over the past decade, relentless efforts have brought lightweight high-temperature γ-TiAl-based intermetallic alloys into real commercialisation. The materials have found their place in General Electric’s (GE) high bypass turbofan aircraft engines for the Boeing 787 as well as in the PW1100GTF engines for low-pressure turbine (LPT) blades. In service, the alloys are required to withstand hostile environments dominated by cyclic stresses or strains. Therefore, to enhance the fatigue resistance of the alloys, a clear understanding of the alloys’ response to fatigue loading is pivotal. In the present review, a detailed discussion about the low-cycle fatigue (LCF) behaviour of γ-TiAl-based alloys in terms of crack initiation, propagation and fracture mechanisms, and the influence of temperature and environment on cyclic deformation mechanisms and the resulting fatigue life has been presented. Furthermore, a comprehensive discussion about modelling and prediction of the fatigue property of these alloys with regard to the initiation and propagation lives as well as the total fatigue life has been provided. Moreover, effective methods of optimising the microstructures of γ-TiAl-based alloys to ensure improved LCF behaviour have been elucidated.Thuthuka National Research Foundation and the APC was funded by the Council of Scientific and Industrial Research (CSIR).https://www.mdpi.com/journal/metalsMaterials Science and Metallurgical EngineeringSDG-07:Affordable and clean energySDG-09: Industry, innovation and infrastructur