Heat-Resistant Al-Alloys with Quasicrystalline and L12- Precipitates

We have been developing Al-Mn-Cu based alloys alloyed with minor additions of different elements. Small additions of beryllium enhance the formation of the icosahedral quasicrystalline phase (IQC) during solidification, especially during ageing. Upon solidification, primary IQC-particles may form, with sizes, ranging from 5 to 50 μm. IQC is also present as a part of binary eutectic in the interdendritic regions. More importantly, nanosized quasicrystalline precipitates can form during T5-treatment at temperatures ranging from about 250−450 °C. They are, in fact, metastable precipitates transforming to ternary T-precipitates (Al20Mn3Cu2) phase above 450 °C. The heat resistance can be increased considerably by the addition of Sc and Zr by forming L12-precipitates in spaces between quasicrystalline precipitates. In this paper, we studied three alloys, two Al-Mn-Cu-Be alloys and an Al-Mn-Cu-Be-Sc-Zr alloy. The alloys were produced by vacuum induction melting and casting into a copper mould. We investigated the response of the alloys to different heat treatments and their heat resistance at higher temperatures. It was shown that the alloys could be precipitation strengthened by ageing at 300 °C and 400 °C. The hardness of the alloy stayed at relatively high levels even at 500 °C, while more substantial softening occurred at 600 °C.

The Effect of Niobium Addition on Mechanical Properties and Corrosion Resistance of a Medical Grade SS316L

To improve mechanical properties, especially elongation, of as-cast medical grade 316L stainless steel, niobium (Nb) was introduced into the alloys, followed by solution heat treatment. Alloying was performed using a 250 kg air induction melting furnace with duplex raw materials and ferronickel. Heat treatment using a solution at 1040 oC, with a holding time of 45 minutes, and water quenching was used. The sample was tested using hardness and ultimate tensile machines. Corrosion tests with simulated body fluids were carried out using media with similar corrosion conditions to human blood. Microstructure observations were performed optically. The results show that the addition of Nb increases the hardness of medical grade 316L stainless steel by 6% compared to the unalloyed steel, both before and after heat treatment. The addition of Nb increases the tensile strength by 8% compared to non-heat treated steel and increases the elongation before and after heat treatment by 8% and 5%, respectively. However, the corrosion rate of the material with Nb is higher than without the addition of Nb. Nb as a carbide former improves the mechanical properties of medical grade 316L stainless steel but adversely affects its corrosion resistance

Effect of Ta and W Additions on Microstructure and Mechanical Properties of Tilt-Cast Ti-45Al-5Nb-2C Alloy

The effect of Ta and W additions on microstructure and mechanical properties of tilt-cast Ti-45Al-5Nb-2C (at.%) alloy was investigated. Three alloys with nominal composition Ti-45Al-5Nb-2C-2X (in at.%), where X is Ta or W, were prepared by vacuum induction melting in graphite crucibles followed by tilt casting into graphite moulds. The microstructure of the tilt-cast alloys consists of the α2(Ti3Al) + γ(TiAl) lamellar grains, single γ phase, (Ti,Nb,X)2AlC particles with a small amount of (Ti,Nb,X)C, and β/B2 phase identified only in W containing alloy. The EDS analysis shows that Ta segregates into the carbide particles and reduces dissolution of Nb in both (Ti,Nb,Ta)C and (Ti,Nb,Ta)2AlC phases. The alloying with W reduces Nb content in both carbide phases and leads to stabilisation of β/B2 phase in the lamellar α2 + γ regions. The alloying with Ta and W does not affect the volume fraction of the carbide particles but influences their size and morphology. While the alloying with Ta and W has no significant effect on Vickers hardness and the indentation elastic modulus of the studied alloys, the addition of Ta affects the nanohardness and elastic modulus of the (Ti,Nb,Ta)2AlC phase. The addition of W significantly increases the Vickers microhardness of the lamellar α2 + γ regions.

Synthesis of Nb0.8Hf0.2FeSb0.98Sn0.02 and Hf0.25Zr0.25Ti0.5NiSn0.98Sb0.02 Half-Heusler Materials and Fabrication of Thermoelectric Generators

In this study, half-Heusler (HH) thermoelectric materials Nb0.8Hf0.2FeSb0.98Sn0.02 (p-type) and Hf0.25Zr0.25Ti0.5NiSn0.98Sb0.02 (n-type) were synthesized using induction melting and spark plasma sintering. For alloying, a conventional induction melting technique was employed rather than arc melting, for mass production compatibility, and the thermoelectric properties of the materials were analyzed. The maximum dimensionless figures of merit (zTmax) were 0.75 and 0.82 for the p- and n-type material at 650 oC and 600 oC, respectively. These materials were then used to fabricate generator modules, wherein two pairs of p- and nlegs without interfacial metal layers were brazed on direct bonded copper (DBC)/Al2O3 substrates using a Zrbased alloy. A maximum power of 0.57 W was obtained from the module by applying a temperature gradient of 476 oC, which corresponds to a maximum power density of 1.58 W cm -2 when normalized by the area of the material. The maximum electrical conversion efficiency of the module was 3.22% at 476 oC temperature gradient. This value was negatively affected by the non-negligible contact resistivity of the brazed interfaces, which ranged from 6.63 × 10 -9 Ωm2 to 7.54 × 10 -9 Ω m2 at hot-side temperatures of 190 oC and 517 oC, respectively. The low electrical resistivity of the HH materials makes it especially important to develop a brazing technique for ultralow resistance contacts.

Production and investigation of powders for additive synthesis from a new beta-solidifying TiAl-based alloy

Chemical composition, structure, and technological properties have been investigated for metal powder compositions (MPCs) of a new six-component TiAl-based alloy with Gd microadditions: Ti-31.0Al-2.5V-2.5Nb-2.5Cr-0.4Gd, wt.% (Ti-44.5Al-2V-1Nb-2Cr-0.1Gd, at.%). Three MPCs fractions (10–63, 40–100, 80–120 μm) were produced by electrode induction melting and inert gas atomization technique and targeted for the additive synthesis of parts. It is shown that the chemical composition of the MPCs for the main elements corresponds to that of the electrode. In contrast, a 1.5-fold increase of the oxygen content in the MPCs was observed, which is being the result of natural oxidation of powder particles upon air environment due to developed specific surface. It has been determined that the phase composition of the MPCs (γ+α(α2)+β) differs from the equilibrium phase composition of the electrode (γ+α2)+β0/B2) and corresponds to a rapidly quenched metastable state, which indicates high solidification rates in the atomization process, exceeding critical cooling rates of the alloy. The technological properties, specifically the powder flowability, were found to be improved for 40–100 and 80–120 μm fractions, making them applicable for additive synthesis of parts from the studied alloy by selective electron-beam melting method

Effect of Aluminum Addition with Nitrogen on K-Carbide Formation in Carbon-Mn Steel

An attempt has been made in the present research work to investigate the role and influence of chemical effect of aluminum addition in the experimental steel towards the formation of k-carbides. Two steel grades were made with and without aluminum addition by induction melting furnace and were cast to ingots. Steel A has no aluminum addition and steel B has some aluminum content. These ingots were then solution heat treated on a temperature of 1200°C for 2-hours’ time and were cooled in the air. After that, they were hot rolled to drawn in plate and sheet. The small samples were cut from bulk and were then heat-treated at 800°C for 1 hour and quenched. Microstructure by OM and SEM was captured. In steel A there was no k-carbide present in the matrix and surprisingly in steel B, small fine k-carbides were present this was then confirmed by XRD later. OM, SEM, and TEM analysis revealed that the presence of k-carbides in steel B makes less dense. It was concluded that aluminum in conjunction with nitrogen forms the small nitride particles having a high melting point does not dissolve during the melting and casting such particles are known as AlN or aluminum nitride particles was observed by TEM along with EDS was the main reason to support the formation of k-carbides, these fine nano level k-carbides are orderly distributed in the steel matrix as was shown by XRD peaks.

Extremely Fast and Cheap Densification of Cu2S by Induction Melting Method

The aim of this work was to obtain dense Cu2S superionic thermoelectric materials, homogeneous in terms of phase and chemical composition, using a very fast and cheap induction-melting technique. The chemical composition was investigated via scanning electron microscopy (SEM) combined with an energy-dispersive spectroscopy (EDS) method, and the phase composition was established by X-ray diffraction (XRD). The thermoelectric figure of merit ZT was determined on the basis of thermoelectric transport properties, i.e., Seebeck coefficient, electrical and thermal conductivity in the temperature range of 300–923 K. The obtained values of the ZT parameter are comparable with those obtained using the induction hot pressing (IHP) technique and about 30–45% higher in the temperature range of 773–923 K in comparison with Cu2S samples densified with the spark plasma sintering (SPS) technique.