Creation of high-temperature heat-resistant alloys based on refractory matrices and natural composites

The scientific, technical and technological aspects in the field of creating new high-temperature materials for the hot tract parts of gas turbine engines (GTE) with operating temperatures exceeding those existing in the GTE are considered. Investigated more refractory metal materials to create new high-temperature alloys used in the manufacture of working and nozzle blades and other parts of promising gas turbine engines based on Co – Cr, Pt – Al, Nb – Si, Mo – Si – B systems. In Co – Cr alloys, heat resistance is mainly ensured by hardening the Co matrix, including dispersed precipitates of the carbide phase (TaC) and the boride phase Cr2B. In alloys of the Pt – Al system, due to the doping of Cr, Al, Ti, Re ... and precipitates of the coherently embedded Pt3Al phase. In eutectic alloys of the Nb-Si system, this is due to complex hardening of the Nb solid solution and Nb5Si3 silicide, as well as the natural compositional structure. In Mo – Si – B alloys, high strength is achieved by doping a-Mo solid solution and the formation of intermetallic phases Mo3Si, Mo5SiB2, carbides Mo2C, TiC. Compositions were selected, analysis of their smelting methods was carried out, including directed crystallization, which provides a natural compositional structure, mechanical properties at room and high temperatures, oxidation resistance were evaluated, structural features were investigated, information was provided on technological equipment and the possibility of obtaining parts in various ways. It is shown that, depending on the composition of the selected matrix, the working temperature of heat-resistant alloys can increase to 1300 – 1500 °C, which significantly exceeds the existing nickel heat-resistant alloys. It is concluded that the materials under study are promising for use in aircraft engine building and the aerospace industry.

Fine-grained dual-phase high-entropy ceramics derived from boro/carbothermal reduction

In the current work fine-grained dual-phase, high-entropy ceramics (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2-(Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C with different phase ratios are prepared from powders synthesized via a boro/carbothermal reduction approach, by adjusting the content of B4C and C in the precursor powders. Phase compositions, densification, microstructure, and mechanical properties have been investigated. Due to the combination of pinning effect and the boro/carbothermal reduction approach, the average grain sizes (0.5–1.5 µm) of the dual-phase high-entropy ceramics, were much smaller as compared with previously reported values. The dual-phase high-entropy ceramics with 15 mol% boride phase exhibit the highest Vickers hardness (24.21 GPa) and fracture toughness (3.2 MPa•m1/2).

The influence of chromium as a diffusive additive in the boronizing treatment of AISI 4140 steel on the corrosion resistance of the coating evaluated by Electrochemical Impedance Spectroscopy (EIS)

A large variety of protective coating is being used in industrial applications to improve the resistance of the metallic substrates against corrosion. The pack-cementation method for boronizing and borochromizing is effective to produce extremely hard and corrosion resistant thick coatings and, additionally, is a low-cost and simple technique. In the present study, AISI 4140 steel specimens underwent boronizing and afterwards chromizing by the pack-cementation method using B4C as boron source and Fe-Cr as chromium source, respectively. In both treatments the appropriate activators were used. After chromizing the boronized substrate, a mixed boride phase FeCrB was formed, as it was confirmed by X-ray Diffractometry (XRD). The boronized and the borochromized specimens were subjected to Electrochemical Impedance Spectroscopy (EIS). From the analysis of the frequency response of the coating systems (Bode and Nyquist display), the conclusion that the borochromized specimens were significantly more corrosion resistant was extracted. Finally, data of optical and electron microscopy contribute to the validity of the conclusions.

Materials for Gas-Thermal Spraying, Obtained by Diffusion Alloying from Powders Based on Austenitic Steels

The article presents experimental studies of composite powder materials for plasma spraying, obtained by the method of diffusion doping of powder materials based on austenitic steels. It is indicated that the main factors forming the diffusion layer on a powder material are a composition of the required saturating medium, treatment temperature and duration of chemical and thermal exposure. Creation of single-phase diffusion layers is possible only in the case of a minimum level of temperature-time characteristics during heat treatment. This is also facilitated by the use of media with a low concentration of boron and introduction of additives inhibiting saturation process (such as carbon, aluminum, silicon) into a saturating mixture of powder. Structure and composition of powders have been thoroughly investigated with the help of X-ray microanalysis that has made it possible to study location of elements contributing to powder alloying and micro-durametric characteristics. A component of high-boride phase is increasing due to higher degree of powder material alloying. Significant changes in phase composition, as well as the chemical one, are noticeable in diffusion processing of the following alloyed powder materials: РR-Х18N9, РR-Х18N10, РR-Х18N15. Free carbon being displaced by boride into a transition zone creates dispersed complex carbide compounds with chromium. This is confirmed by distribution nature of carbide-forming components in a powder particle. All carbide-forming elements have characteristic concentration peaks-bursts in contrast to non-carbideforming silicon. Silicon is practically not present in the considered FeB phase and it is found only in a very small amount in the studied Fe2B phase; it is pushed aside by high-boride phases to a sublayer. The change in microhardness of the FeB and Fe2B phases under study is associated with dissolution of corresponding alloying elements in them and distortions of a crystal lattice in borides. A similar phenomenon is also characteristic for saturation while using boron or while making chemical and thermal treatment of alloyed steels, it has been noted in a number of studies. The increase in microhardness of a particle nucleus during its boriding is caused by displacement of carbon and alloying elements by growing front of boride phases. A core zone moves with an increased microhardness to a particle core while increasing temperature mode and time of boronization and up to realization of the effect with counter diffusion.

Microstructure, Mechanical Properties, Abrasive Wear, and Corrosion Behavior in Molten Zinc of Boride-Based Coatings in Situ Synthesized by an HVOF Spraying Process

Hot-dip galvanizing has been used for anti-corrosion of various steel products; however, the corrosion of molten zinc in the galvanizing industry is the key problem to be solved. Three kinds of Mo–B–Co–Cr, Mo–B–Ni–Cr, and Ti–B–Co–Cr mixture powders were deposited on the surface of a 316L stainless-steel substrate by a HVOF spraying method to prepare MoB/CoCr, MoB/NiCr, and TiB/CoCr coatings. The microstructure, mechanical properties, abrasive wear, and corrosion behavior in molten zinc of the in situ synthesized boride-based coatings were investigated. The experimental results showed that MoB/NiCr coating with a denser microstructure had the lowest porosity (0.811%). The in situ synthesized boride-phase compositions of MoB/CoCr, MoB/NiCr, and TiB/CoCr coatings were CoMoB and CoMo2B2, NiMo2B2, and TiB2, respectively. The MoB/NiCr coating had the highest boride content among the coatings. The presence of binary (TiB2) or ternary boride phases (CoMoB, CoMo2B2, and NiMo2B2) with their excellent mechanical properties could obviously increase the microhardness values in the coatings. The in situ synthesized borides in the coatings also could improve the wear resistance properties; MoB/NiCr coating with shallower grooves and smaller craters/pits had the smoothest worn surface and the lowest weight loss (6.8 ± 0.84 mg) among the coatings. After immersion test in molten zinc for 360 h, no presence of zinc or intermetallic compounds in the three kinds of the coatings (MoB/CoCr, MoB/NiCr, and TiB/CoCr), and the element compositions of the three kinds of coatings after the immersion test were the same as the as-sprayed coatings. Compared to the other coatings, MoB/NiCr coatings had the higher durability in molten zinc.

INVESTIGATION OF POSSIBILITY TO PRODUCE HIGH-STRENGTH BORON ALUMINUM SHEETS WITHOUT HOMOGENIZATION AND QUENCHING OPERATIONS

Al–Cu–Mn (Zr) alloys feature high strength and processability without any thermal treatment operations. Al–2%Cu–1,5%Mn–2%B and Al–2%Cu–1,5%Mn–0,4%Zr–2%B alloys were obtained in order to investigate the possibility of producing a aluminum boroncontaining alloy in the form of high-strength sheet rolled stock without thermal treatment. Melting was performed in the RELTEK induction furnace with intense melt stirring to eliminate sedimentation of boride refractory particles. Melting temperature was 950– 1000 °С. Melt was poured into 40×120×200 mm graphite casting molds. Calculation methods (Thermo-Calc) were used to demonstrate that manganese forms complex borides with aluminum and zirconium at a melting temperature while there is enough manganese in liquid and there is practically no zirconium left. Experimental methods (electronic scanning microscopy and electron microprobe analysis) proved the formation of the complex AlB2Mn2 boride, however, manganese remained in a solid solution is enough to form the Al20Cu2Mn3 phase particles in the amount up to 7 wt.%. In the alloy with zirconium, boron stimulates primary Al3Zr crystal separation and, therefore, zirconium content left in the aluminum solid solution is not sufficient for hardening. It is shown that it is possible to produce thin-rolled steel with a thickness of less than 0,3 mm with uniformly distributed clusters of the boride phase with a particle size of less than 10 μm. A high level of strength up to 543 MPa is reached without the use of hardening and aging due to the precipitation of Al20Cu2Mn3 phase dispersions during hot deformation (t =450 °C).

Analysis of Boride Phase Composition in High Boron Alloyed Stainless Steel Containing Titanium

Hard and brittle (Fe,Cr)2B phase caused by excess boron in high boron alloyed stainless steel has adverse effects on the hot working performance and mechanical properties of material. Adding Ti into high boron alloyed stainless steel can improve the type, morphology and distribution of boride phase. The results show that TiB2 phase with petals or small block shape forms after adding Ti into high boron alloyed stainless steel, and as the increase of Ti content, TiB2 phase replaces (Fe, Cr)2B gradually. Moreover, the petal-like TiB2 phase becomes smaller and more granular after high temperature deformation, and the segregation of matrix composition is significantly weakened by the formation of TiB2 phase.

Comparative Studies on Wear Behaviour of Sintered 316L Stainless Steels Loaded with h-BN and MoS2

Mechanical properties and wear behavior of stainless steel embedded with different solid lubricants were investigated. Hexagonal boron nitride (h-BN) and molybdenum disulfide (MoS2)-embedded 316L stainless steels (SS316L/h-BN and SS316L/MoS2) were prepared by powder metallurgy method. Various h-BN and MoS2 contents (10, 15 and 20 vol%) were mixed with 316L stainless steel powders and then sintered at 1200°C in H2 atmosphere for 60 min. The experimental results showed that small boride phase and h-BN powder occupied the pores in the microstructure of SS316L/h-BN composite whereas the MoS2 second phase occupied the pores of the sintered 316L matrix in the microstructure of SS316L/MoS2 composite. The addition of h-BN decreased the sintered density and hardness whereas that of MoS2 gave the opposite effect. Dry sliding wear behavior of composites was investigated by using pin-on-disc test rig at the sliding speeds of 0.1 and 0.2 m/s and the applied load of 3 N. The results showed that the MoS2 composites had higher wear resistance than the h-BN composite but the h-BN composite yielded a better friction reduction.