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Jan Šerák - One of the best experts on this subject based on the ideXlab platform.

  • Structural characteristics and thermal stability of Al–5.7Cr–2.5Fe–1.3Ti alloy produced by powder metallurgy
    Journal of Alloys and Compounds, 2008
    Co-Authors: Dalibor Vojtěch, Alena Michalcová, J. Pilch, Petr Šittner, Jan Šerák, Pavel Novák
    Abstract:

    Abstract Structure, room temperature- and elevated temperature mechanical properties of the Al–5.7Cr–2.5Fe–1.3Ti alloy are studied in the presented paper. The alloy was prepared by the powder metallurgy including Melt Atomization into rapidly solidified powder followed by hot extrusion. The alloy shows a fine-grained structure (average grain size of 1 μm) and a relatively high tensile strength of 380 MPa. Variations in structure, hardness and tensile properties due to the long-term annealing at 400 °C/200 h are negligible, suggesting the excellent thermal stability of the alloy. Moreover, its tensile properties at 350 °C exceed the commercial Al–Si alloy commonly used in elevated temperature applications.

  • Structure and Properties of PM Nano-Crystalline Al-Cr based Alloys
    Materials Science Forum, 2007
    Co-Authors: Dalibor Vojtech, Jan Verner, Alena Michalcová, Jan Šerák, Frantisek Simancik, Martin Balog, Juraj Nagy
    Abstract:

    In the presented paper, properties of Al-Cr-Fe-Ti alloy produced by powder metallurgy (PM) are described. Rapidly solidified powder alloy was prepared by the pressure nitrogen Melt Atomization. The granulometric powder fraction of less than 45 μm was then hot-extruded. Microstructure of the as-extruded material comprised recrystallized α-Al grains and spheroids of intermetallic phases. Tensile strength of the investigated material was similar to that of a conventional casting Al-Si alloy commonly used in elevated temperature applications. Excellent thermal stability of the PM Al-Cr based material, which much exceeded the elevated temperature casting alloy, was proved by room temperature tensile tests after long-term annealing at elevated temperature. Reasons for the observed thermal stability of the investigated PM alloy are discussed.

  • Mechanism and Kinetics of Plasma Nitriding of the Nb-Alloyed PM Tool Steel
    Defect and Diffusion Forum, 2007
    Co-Authors: Pavel Novák, Dalibor Vojtěch, Jan Šerák, Michal Novák, Barbora Bártová
    Abstract:

    The aim of this work was to describe the mechanism and kinetics of plasma nitriding of a Nb-containing PM (powder metallurgy) tool steel. Material containing 2.5 wt.% C, 3.3% Si, 6.2% Cr, 2.2% Mo, 2.6% V, 2.6% Nb and 1.0% W was prepared by nitrogen Melt Atomization and hot isostatic pressing. Heat-treated steel (quenching from 1100 °C, triple tempering at 550 °C for 1h) was plasma nitrided at temperatures ranging from 470 °C to 530 °C / 30 - 180 min. Light microscopy, TEM, SEM and WDS were used to study the nitrided steel. It has been shown, that nitriding at 470°C leads to the formation of thin layers composed only of a diffusion zone containing nitrogen-rich martensite and fine nitride precipitates, no layer of nitrides is formed on the surface. Nitriding is probably controlled by the nitrogen diffusion in martensite to the material or by the processes in the nitriding atmosphere at this temperature. Nitriding at the temperature of 500°C and more leads to the formation of a continuous layer of nitrides and carbonitrides on the surface that limits further nitrogen diffusion. Niobium, as a prospective element in tool steels, was not found to play a role in the formation of the nitrided layer directly. Niobium replaces vanadium in very thermodynamically stable primary MC carbides. This results in higher vanadium content in others less stable carbides and in the matrix. Due to this effect, higher portion of vanadium can precipitate as VC carbides and VN nitrides during heat treatment and nitriding, respectively.

  • Pulsed-plasma nitriding of a niobium–alloyed PM tool steel
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2004
    Co-Authors: Pavel Novák, Dalibor Vojtěch, Jan Šerák
    Abstract:

    Abstract The aim of this work is to describe the processes during plasma nitriding of Nb-containing, powder metallurgy prepared (PM) tool steel. PM steel was studied containing 2.5% C, 3.3% Si, 6.2% Cr, 2.2% Mo, 2.6% V, 2.6% Nb and 1.0% W, produced by Melt Atomization and hot isostatic pressing. The hardened and tempered steel was plasma nitrided at temperatures ranging from 470 to 530 °C. The nitriding time was 30–180 min. Optical microscopy, electron microscopy, XRD, EDS, WDS and hardness measurements were used to study the steel's properties, microstructure, phases and chemical composition. The nitriding temperature of 470 °C was found to be unsuitable for practical use because the layers prepared at this temperature were non-uniform. The compound layer formed by M 2,3 (C,N) and M 4 N was found after nitriding at temperatures of 500 and 530 °C for at least 60 min. The effect of niobium on the formation of this layer was studied.

  • Rapid Cooling of Alloys - Importance, Technology and Utilization
    Chemicke Listy, 2004
    Co-Authors: Dalibor Vojtech, Jan Verner, Barbora Bártová, Jan Šerák
    Abstract:

    The process of rapid cooling of metal alloys and its utilization in technical practice are the topics of the review. The most significant features of the rapidly solidified alloys are described in detail. These features which can be conveniently used in designing materials with improved mechanical properties and high performance have been of great interest for solid- state physicists. In addition, the four industrial processes of rapid cooling that are the most widely utilized until the present time are characterised: Melt Atomization by a pressure medium, planar flow casting, Melt spraying and surface reMelting. The last part of the paper is devoted to a detailed description of the planar flow casting facility designed and constructed at the Institute.

Pavel Novák - One of the best experts on this subject based on the ideXlab platform.

  • Structural characteristics and thermal stability of Al–5.7Cr–2.5Fe–1.3Ti alloy produced by powder metallurgy
    Journal of Alloys and Compounds, 2008
    Co-Authors: Dalibor Vojtěch, Alena Michalcová, J. Pilch, Petr Šittner, Jan Šerák, Pavel Novák
    Abstract:

    Abstract Structure, room temperature- and elevated temperature mechanical properties of the Al–5.7Cr–2.5Fe–1.3Ti alloy are studied in the presented paper. The alloy was prepared by the powder metallurgy including Melt Atomization into rapidly solidified powder followed by hot extrusion. The alloy shows a fine-grained structure (average grain size of 1 μm) and a relatively high tensile strength of 380 MPa. Variations in structure, hardness and tensile properties due to the long-term annealing at 400 °C/200 h are negligible, suggesting the excellent thermal stability of the alloy. Moreover, its tensile properties at 350 °C exceed the commercial Al–Si alloy commonly used in elevated temperature applications.

  • Structural Evolution of Al-Cr Alloy during Processing
    Solid State Phenomena, 2008
    Co-Authors: Dalibor Vojtěch, Alena Michalcová, Pavel Novák
    Abstract:

    Aluminium-chromium based alloys are promising candidates for manufacture of light components exposed to elevated temperatures. The work describes properties of Al-6.0wt.%Cr- 2.1wt.%Fe-0.5wt.%Ti alloy. The rapidly solidified powder was prepared by the pressure nitrogen Melt Atomization. The powder was then subject to heat treatment in order to investigate solid state phase transformations. Compaction of the powder was carried out by hot extrusion after preheating at 450 °C. Microstructure, phase composition and structural transformations on heat treatment were investigated in the as-atomized powder, as well as in the as-extruded alloy. It is found that metastable state of the rapidly solidified powder is characterized by presence of quasi-crystalline phases and supersaturated solid solution. Heating before and during the hot extrusion induces decomposition of the supersaturated solid solution and quasicrystalline to crystalline phase transformations. The hot extruded alloy has a refined recrystallized structure that remains very stable aven after long-term annealing at 400 °C. Mechanical properties of the extruded alloy are discussed in terms of strengthening mechanisms.

  • Mechanism and Kinetics of Plasma Nitriding of the Nb-Alloyed PM Tool Steel
    Defect and Diffusion Forum, 2007
    Co-Authors: Pavel Novák, Dalibor Vojtěch, Jan Šerák, Michal Novák, Barbora Bártová
    Abstract:

    The aim of this work was to describe the mechanism and kinetics of plasma nitriding of a Nb-containing PM (powder metallurgy) tool steel. Material containing 2.5 wt.% C, 3.3% Si, 6.2% Cr, 2.2% Mo, 2.6% V, 2.6% Nb and 1.0% W was prepared by nitrogen Melt Atomization and hot isostatic pressing. Heat-treated steel (quenching from 1100 °C, triple tempering at 550 °C for 1h) was plasma nitrided at temperatures ranging from 470 °C to 530 °C / 30 - 180 min. Light microscopy, TEM, SEM and WDS were used to study the nitrided steel. It has been shown, that nitriding at 470°C leads to the formation of thin layers composed only of a diffusion zone containing nitrogen-rich martensite and fine nitride precipitates, no layer of nitrides is formed on the surface. Nitriding is probably controlled by the nitrogen diffusion in martensite to the material or by the processes in the nitriding atmosphere at this temperature. Nitriding at the temperature of 500°C and more leads to the formation of a continuous layer of nitrides and carbonitrides on the surface that limits further nitrogen diffusion. Niobium, as a prospective element in tool steels, was not found to play a role in the formation of the nitrided layer directly. Niobium replaces vanadium in very thermodynamically stable primary MC carbides. This results in higher vanadium content in others less stable carbides and in the matrix. Due to this effect, higher portion of vanadium can precipitate as VC carbides and VN nitrides during heat treatment and nitriding, respectively.

  • Pulsed-plasma nitriding of a niobium–alloyed PM tool steel
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2004
    Co-Authors: Pavel Novák, Dalibor Vojtěch, Jan Šerák
    Abstract:

    Abstract The aim of this work is to describe the processes during plasma nitriding of Nb-containing, powder metallurgy prepared (PM) tool steel. PM steel was studied containing 2.5% C, 3.3% Si, 6.2% Cr, 2.2% Mo, 2.6% V, 2.6% Nb and 1.0% W, produced by Melt Atomization and hot isostatic pressing. The hardened and tempered steel was plasma nitrided at temperatures ranging from 470 to 530 °C. The nitriding time was 30–180 min. Optical microscopy, electron microscopy, XRD, EDS, WDS and hardness measurements were used to study the steel's properties, microstructure, phases and chemical composition. The nitriding temperature of 470 °C was found to be unsuitable for practical use because the layers prepared at this temperature were non-uniform. The compound layer formed by M 2,3 (C,N) and M 4 N was found after nitriding at temperatures of 500 and 530 °C for at least 60 min. The effect of niobium on the formation of this layer was studied.

Dalibor Vojtěch - One of the best experts on this subject based on the ideXlab platform.

  • Properties of the thermally stable Al95Cr3.1Fe1.1Ti0.8 alloy prepared by cold-compression at ultra-high pressure and by hot-extrusion
    Materials Characterization, 2012
    Co-Authors: Dalibor Vojtěch, Alena Michalcová, F. Průša, K. Dám, P. Šedá
    Abstract:

    An Al{sub 95}Cr{sub 3.1}Fe{sub 1.1}Ti{sub 0.8} (in at.%) alloy was made into rapidly solidified powder by Melt Atomization. The powder was compacted by two processes: 1) uni-axial cold compression at an ultra-high pressure of 6 GPa and 2) hot extrusion at 480 Degree-Sign C. The structures, mechanical properties and thermal stability of both materials were compared with the commercial AlSi{sub 12}Cu{sub 1}Mg{sub 1}Ni{sub 1} (in wt.%) casting alloy, which is generally considered to be thermally stable. It was found that cold compression at ultra-high pressure created a compact and porosity-free material, which was similar to the material that was prepared with the commonly used hot extrusion method. The Vickers hardness, compressive strength and compressive yield strength of the cold-compressed alloy were 161 HV, 680 MPa and 547 MPa, respectively, which were higher than the values obtained for the hot-extruded and casting alloys. The thermal stability of the hot-extruded Al{sub 95}Cr{sub 3.1}Fe{sub 1.1}Ti{sub 0.8} alloy was excellent because its mechanical properties did not change significantly, even after 100 h of annealing at 500 Degree-Sign C. The mechanical properties and thermal stability of the investigated materials were discussed in relation to their structures and diffusivities of the alloying elements. - Highlights: Black-Right-Pointing-Pointermore » The Al{sub 95}Cr{sub 3.1}Fe{sub 1.1}Ti{sub 0.8} alloy was prepared by compression at an ultra-high pressure of 6 GPa. Black-Right-Pointing-Pointer The resulting material was dense and porosity-free. Black-Right-Pointing-Pointer The material had high hardness of 161 HV and a compressive strength of 680 MPa. Black-Right-Pointing-Pointer The material had excellent thermal stability at 500 Degree-Sign C.« less

  • Structural characteristics and thermal stability of Al–5.7Cr–2.5Fe–1.3Ti alloy produced by powder metallurgy
    Journal of Alloys and Compounds, 2008
    Co-Authors: Dalibor Vojtěch, Alena Michalcová, J. Pilch, Petr Šittner, Jan Šerák, Pavel Novák
    Abstract:

    Abstract Structure, room temperature- and elevated temperature mechanical properties of the Al–5.7Cr–2.5Fe–1.3Ti alloy are studied in the presented paper. The alloy was prepared by the powder metallurgy including Melt Atomization into rapidly solidified powder followed by hot extrusion. The alloy shows a fine-grained structure (average grain size of 1 μm) and a relatively high tensile strength of 380 MPa. Variations in structure, hardness and tensile properties due to the long-term annealing at 400 °C/200 h are negligible, suggesting the excellent thermal stability of the alloy. Moreover, its tensile properties at 350 °C exceed the commercial Al–Si alloy commonly used in elevated temperature applications.

  • Structural Evolution of Al-Cr Alloy during Processing
    Solid State Phenomena, 2008
    Co-Authors: Dalibor Vojtěch, Alena Michalcová, Pavel Novák
    Abstract:

    Aluminium-chromium based alloys are promising candidates for manufacture of light components exposed to elevated temperatures. The work describes properties of Al-6.0wt.%Cr- 2.1wt.%Fe-0.5wt.%Ti alloy. The rapidly solidified powder was prepared by the pressure nitrogen Melt Atomization. The powder was then subject to heat treatment in order to investigate solid state phase transformations. Compaction of the powder was carried out by hot extrusion after preheating at 450 °C. Microstructure, phase composition and structural transformations on heat treatment were investigated in the as-atomized powder, as well as in the as-extruded alloy. It is found that metastable state of the rapidly solidified powder is characterized by presence of quasi-crystalline phases and supersaturated solid solution. Heating before and during the hot extrusion induces decomposition of the supersaturated solid solution and quasicrystalline to crystalline phase transformations. The hot extruded alloy has a refined recrystallized structure that remains very stable aven after long-term annealing at 400 °C. Mechanical properties of the extruded alloy are discussed in terms of strengthening mechanisms.

  • Mechanism and Kinetics of Plasma Nitriding of the Nb-Alloyed PM Tool Steel
    Defect and Diffusion Forum, 2007
    Co-Authors: Pavel Novák, Dalibor Vojtěch, Jan Šerák, Michal Novák, Barbora Bártová
    Abstract:

    The aim of this work was to describe the mechanism and kinetics of plasma nitriding of a Nb-containing PM (powder metallurgy) tool steel. Material containing 2.5 wt.% C, 3.3% Si, 6.2% Cr, 2.2% Mo, 2.6% V, 2.6% Nb and 1.0% W was prepared by nitrogen Melt Atomization and hot isostatic pressing. Heat-treated steel (quenching from 1100 °C, triple tempering at 550 °C for 1h) was plasma nitrided at temperatures ranging from 470 °C to 530 °C / 30 - 180 min. Light microscopy, TEM, SEM and WDS were used to study the nitrided steel. It has been shown, that nitriding at 470°C leads to the formation of thin layers composed only of a diffusion zone containing nitrogen-rich martensite and fine nitride precipitates, no layer of nitrides is formed on the surface. Nitriding is probably controlled by the nitrogen diffusion in martensite to the material or by the processes in the nitriding atmosphere at this temperature. Nitriding at the temperature of 500°C and more leads to the formation of a continuous layer of nitrides and carbonitrides on the surface that limits further nitrogen diffusion. Niobium, as a prospective element in tool steels, was not found to play a role in the formation of the nitrided layer directly. Niobium replaces vanadium in very thermodynamically stable primary MC carbides. This results in higher vanadium content in others less stable carbides and in the matrix. Due to this effect, higher portion of vanadium can precipitate as VC carbides and VN nitrides during heat treatment and nitriding, respectively.

  • Thermal Stability of Rapidly Solidified Alloys of Aluminium with Transition Metals
    Materials Science Forum, 2006
    Co-Authors: Dalibor Vojtěch, Jan Verner, Barbora Bártová, Karel Saksl
    Abstract:

    Rapidly solidified (RS) Al-TM (TM = transition metal) alloys are perspective materials from scientific, as well as technological point of view. Generally, they are produced by the Melt Atomization or by the Melt spinning. Subsequent compaction is commonly performed by the hot extrusion. Since transition metals, such as Cr, Fe, Ni, Zr, Ti, Mn and others, have low diffusion coefficients in solid aluminium (lower by several orders of magnitude than those of common alloying elements like Cu, Si, Mg, Zn etc.) the RS Al-TM alloys are characterized by a high thermal stability. In this paper, several RS Al-TM (TM = Cr, Fe, Ti, Mn, Ni) alloys prepared by the Melt spinning and Melt Atomization are compared to commercially available 2xxx, 6xxx and 7xxx wrought alloys. The main structural features of both RS and wrought alloys are described. The RS alloys are characterized by the presence of micro and nano-scale crystalline and/or quasi-crystalline phases and supersaturated solid solutions. The elevated-temperature behaviour is compared for both groups of materials. The thermal stability of the investigated materials is determined by room temperature hardness measurements after various annealing regimes and a high thermal stability of the RS alloys is demonstrated. The microstructural changes and phase transformations occurring in the investigated materials upon heating are described. In the Al-TM alloys, very slow decomposition of the supersaturated solid solutions, precipitation and decomposition of the metastable quasi-crystalline phases occur.

Alena Michalcová - One of the best experts on this subject based on the ideXlab platform.

  • Properties of the thermally stable Al95Cr3.1Fe1.1Ti0.8 alloy prepared by cold-compression at ultra-high pressure and by hot-extrusion
    Materials Characterization, 2012
    Co-Authors: Dalibor Vojtěch, Alena Michalcová, F. Průša, K. Dám, P. Šedá
    Abstract:

    An Al{sub 95}Cr{sub 3.1}Fe{sub 1.1}Ti{sub 0.8} (in at.%) alloy was made into rapidly solidified powder by Melt Atomization. The powder was compacted by two processes: 1) uni-axial cold compression at an ultra-high pressure of 6 GPa and 2) hot extrusion at 480 Degree-Sign C. The structures, mechanical properties and thermal stability of both materials were compared with the commercial AlSi{sub 12}Cu{sub 1}Mg{sub 1}Ni{sub 1} (in wt.%) casting alloy, which is generally considered to be thermally stable. It was found that cold compression at ultra-high pressure created a compact and porosity-free material, which was similar to the material that was prepared with the commonly used hot extrusion method. The Vickers hardness, compressive strength and compressive yield strength of the cold-compressed alloy were 161 HV, 680 MPa and 547 MPa, respectively, which were higher than the values obtained for the hot-extruded and casting alloys. The thermal stability of the hot-extruded Al{sub 95}Cr{sub 3.1}Fe{sub 1.1}Ti{sub 0.8} alloy was excellent because its mechanical properties did not change significantly, even after 100 h of annealing at 500 Degree-Sign C. The mechanical properties and thermal stability of the investigated materials were discussed in relation to their structures and diffusivities of the alloying elements. - Highlights: Black-Right-Pointing-Pointermore » The Al{sub 95}Cr{sub 3.1}Fe{sub 1.1}Ti{sub 0.8} alloy was prepared by compression at an ultra-high pressure of 6 GPa. Black-Right-Pointing-Pointer The resulting material was dense and porosity-free. Black-Right-Pointing-Pointer The material had high hardness of 161 HV and a compressive strength of 680 MPa. Black-Right-Pointing-Pointer The material had excellent thermal stability at 500 Degree-Sign C.« less

  • Structural characteristics and thermal stability of Al–5.7Cr–2.5Fe–1.3Ti alloy produced by powder metallurgy
    Journal of Alloys and Compounds, 2008
    Co-Authors: Dalibor Vojtěch, Alena Michalcová, J. Pilch, Petr Šittner, Jan Šerák, Pavel Novák
    Abstract:

    Abstract Structure, room temperature- and elevated temperature mechanical properties of the Al–5.7Cr–2.5Fe–1.3Ti alloy are studied in the presented paper. The alloy was prepared by the powder metallurgy including Melt Atomization into rapidly solidified powder followed by hot extrusion. The alloy shows a fine-grained structure (average grain size of 1 μm) and a relatively high tensile strength of 380 MPa. Variations in structure, hardness and tensile properties due to the long-term annealing at 400 °C/200 h are negligible, suggesting the excellent thermal stability of the alloy. Moreover, its tensile properties at 350 °C exceed the commercial Al–Si alloy commonly used in elevated temperature applications.

  • Structural Evolution of Al-Cr Alloy during Processing
    Solid State Phenomena, 2008
    Co-Authors: Dalibor Vojtěch, Alena Michalcová, Pavel Novák
    Abstract:

    Aluminium-chromium based alloys are promising candidates for manufacture of light components exposed to elevated temperatures. The work describes properties of Al-6.0wt.%Cr- 2.1wt.%Fe-0.5wt.%Ti alloy. The rapidly solidified powder was prepared by the pressure nitrogen Melt Atomization. The powder was then subject to heat treatment in order to investigate solid state phase transformations. Compaction of the powder was carried out by hot extrusion after preheating at 450 °C. Microstructure, phase composition and structural transformations on heat treatment were investigated in the as-atomized powder, as well as in the as-extruded alloy. It is found that metastable state of the rapidly solidified powder is characterized by presence of quasi-crystalline phases and supersaturated solid solution. Heating before and during the hot extrusion induces decomposition of the supersaturated solid solution and quasicrystalline to crystalline phase transformations. The hot extruded alloy has a refined recrystallized structure that remains very stable aven after long-term annealing at 400 °C. Mechanical properties of the extruded alloy are discussed in terms of strengthening mechanisms.

  • Structure and Properties of PM Nano-Crystalline Al-Cr based Alloys
    Materials Science Forum, 2007
    Co-Authors: Dalibor Vojtech, Jan Verner, Alena Michalcová, Jan Šerák, Frantisek Simancik, Martin Balog, Juraj Nagy
    Abstract:

    In the presented paper, properties of Al-Cr-Fe-Ti alloy produced by powder metallurgy (PM) are described. Rapidly solidified powder alloy was prepared by the pressure nitrogen Melt Atomization. The granulometric powder fraction of less than 45 μm was then hot-extruded. Microstructure of the as-extruded material comprised recrystallized α-Al grains and spheroids of intermetallic phases. Tensile strength of the investigated material was similar to that of a conventional casting Al-Si alloy commonly used in elevated temperature applications. Excellent thermal stability of the PM Al-Cr based material, which much exceeded the elevated temperature casting alloy, was proved by room temperature tensile tests after long-term annealing at elevated temperature. Reasons for the observed thermal stability of the investigated PM alloy are discussed.

Stephen D. Ridder - One of the best experts on this subject based on the ideXlab platform.

  • High-speed cinematography of gas-metal Atomization
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2004
    Co-Authors: Jason Ting, Jeffery Connor, Stephen D. Ridder
    Abstract:

    Abstract A high-speed cinematographic footage of a 304L stainless steel gas Atomization, recorded at the National Institute of Standard and Technology (NIST), was analyzed using a discrete Fourier transform (DFT) algorithm. The analysis showed the gas Atomization process possesses two prominent frequency ranges of Melt oscillation (pulsation). A low-frequency oscillation in the Melt flow occurring between 5.41 and 123 Hz, with a dominant frequency at 9.93 Hz, was seen in the recirculation zone adjacent to the Melt orifice. A high-frequency Melt oscillation range was observed above 123 Hz, and was more prominent one Melt-tip-diameter downstream in the Melt Atomization image than upstream near the Melt tip. This high-frequency range may reflect the Melt Atomization frequency used to produce finely atomized powder. This range also included a prominent high frequency at 1273 Hz, which dominated in the image further away downstream from the Melt tip. This discrete high-frequency oscillation is most probably caused by the aeroacoustic “screech” phenomenon, intrasound (