Nbss Phase Research Articles

The synergistic mechanism of ultrasonic and heat treatment on the composite structure and mechanical properties of Nb-Si-Zr based in-situ composites

Ultrasonic assisted solidification (UST), heat treatment and alloying are all effective methods to control the microstructure of materials and improve their properties. In this paper, the Nb-16Si-20Zr-2C-xW composites were prepared using ultrasonic assisted solidification technology of high temperature melt (≥1800℃) and heat treatment. The influence mechanism of ultrasonic, the W element, and the synergistic regulation mechanism of ultrasonic and heat treatment on the material were studied. Nb-16Si-20Zr-2C-xW(UST) alloys are mainly composed of Nbss phase and γNb5Si3 phase, and their microstructures are mainly divided into strong active zone(SZ) and weak active zone(WZ). In the Nb-Si superalloy melt treated by ultrasonic wave, the area near the ultrasonic sound source is the strong active zone, while the area far away from the ultrasonic sound source is the weak active zone. The features of faceted primary phases and non-faceted primary phases in the SZ and WZ are explained by the equation of undercooling degree at different interfaces and effect of ultrasonic on component undercooling. As the W element content in the alloys (UST) increases, the content of the Nbss phase increases, the median diameter of the primary γNb5Si3 phase decreases, and the room temperature fracture toughness and compressive strength show an upward trend. When the addition amount of W element reaches 2 at%, there are more crack deflections, crack bridging and secondary cracks in the crack propagation path. Through the special arrangement of primary γNb5Si3 phase in the ultrasonic field, the energy density of ultrasonic waves in the melt is calculate to be 33.5 kW/m2. The microstructure of Nb-16Si-20Zr-2C-xW alloys(UST+HT) became more uniform and spheroidize. It is worth mentioning that there are plenty of long strip γNb5Si3 phases with different orientations in the Nb-16Si-20Zr-2C-0.1W(UST+HT) alloy and the long strip γNb5Si3 phases with the same orientation are arranged in a periodic manner. This special composite structure induces crack deflections and branching cracks when cracks propagation, so the room temperature fracture toughness of Nb-16Si-20Zr-2C-0.1 W (UST+HT) alloy reaches 15.66 MPa·m1/2.

Additively manufactured Nb-Ti-Si based alloy: As-built and heat-treated conditions

This work aims to fully characterise the Nb-Ti-Si based alloy (Nb-26Ti-16Si-2.2Al-2Cr), processed by the electron-beam powder-bed-fusion in both as-built and heat-treated conditions, to elucidate the microstructure-property relationships. The as-built condition has [001]-oriented columnar grains of the Nb3Si phase with the Nbss phase dispersed throughout the microstructure. The microhardness is 645.2 ± 6.7 HV0.5, and the indentation fracture toughness shows distinct directionality: 7.7 MPa·m1/2 in the horizontal direction compared to 5.3 MPa·m1/2 in the vertical direction. Both properties are comparable to the cast version. The directionality is attributed to the underlying mechanisms such as crack bridging, arrest, and micro-crack formation. By contrast, in the heat-treated condition, the alloy exhibits a dual-phase microstructure (Nbss and Nb5Si3 phases) with near-equiaxed grain shape due to the Nb3Si phase decomposition. The fracture toughness increases to 12.1 MPa·m1/2, at the expense of a reduced microhardness of 564.4 ± 15.0 HV0.5.

Microstructure and Mechanical Properties of the Powder Metallurgy Nb-16Si-24Ti-2Al-2Cr Alloy.

The Nb-16Si-24Ti-2Al-2Cr alloy was prepared by plasma rotating electrode process (PREP) technology and the hot-pressing (HP) method, and the effects of sintering temperature on the microstructure, mechanical properties and fracture behavior were investigated. The HP alloys sintered at temperatures below 1400 °C are composed of Nbss (Nb solid solution), Nb3Si and Nb5Si3 phases. When the sintering temperature reaches 1450 °C, the Nb3Si phase is completely decomposed into Nbss and Nb5Si3 phases. Meanwhile, the microstructure coarsens significantly. Compared with the cast alloy, the HP alloy shows better mechanical properties. The fracture toughness of the alloy sintered at 1400 °C reaches 20.2 MPa·m1/2, which exceeds the application threshold. The main reason for the highest fracture toughness is attributed to the decomposition of large-sized brittle Nb3Si phase and the formation of a fine microstructure, which greatly increases the number of phase interfaces and improves the chance of crack deflection. In addition, the reduction in the size and content of silicides also reduces their plastic constraints on the ductile Nbss phase.

Effects of TaB2 addition on microstructure and mechanical properties of Nb[sbnd]Si based alloys

The effects of TaB2 addition on the Nb-16Si-22Ti-3Al alloy were investigated in this study. The results showed that the Nb-16Si-22Ti-3Al alloy consisted of the Nbss phase, α-Nb5Si3 phase, Nb3Si phase, and Tiss phase. With the addition of TaB2 increased from 0 to 0.25 at. %, the Nb3Si disappeared and the eutectic transformation reaction occurred (Nb3Si → Nbss + α-Nb5Si3), which made the volume fraction of the Nbss phase increased from 38.6% to 50.3%. With the TaB2 addition further increasing to 2.0 at. %, the α-Nb5Si3 became the primary phase, and the microstructure transformed from the Nbss + α-Nb5Si3 eutectic structure to the primary α-Nb5Si3 and (Nbss + γ-Nb5Si3) hypereutectic structure. The alloy with 0.25 at. % TaB2 addition had the highest fracture toughness of 10.2 MPa•m1/2 due to the refinement of microstructure and the increment of ductile Nbss. The maximum compressive strength of TaB2 added alloy at room temperature reached 2587.25 MPa in 2.0TaB2 specimen, owing to the solid solution strengthening of Ta and B, and the increment of the silicides content. Additionally, by comparing the fracture toughness of alloys with TaB2 addition obtained by three-point bending (SENBs) and indentation methods, respectively, the feasibility of estimating the fracture toughness of NbSi based alloys by the indentation method is demonstrated with an error of below 20%, which provided a basis for subsequent rapid performance characterization.

Microstructure and fracture toughness of a Nb–Ti–Si-based in-situ composite fabricated by laser-based directed energy deposition

Laser-based directed energy deposition (LDED), a significant integrated preparation technique combining material design and production, has been progressively used to manufacture Nb–Ti–Si-based in-situ composites. However, it is commonly known that Nb–Si-based in-situ composites have low room-temperature fracture toughness. Herein, we have designed a novel Nb–40Ti–10Si–5Al–2V (at.%) composite, the microstructure of LDED-built composite is made up of Nbss and γ-Nb5Si3, the formation of γ-Nb5Si3 precipitate in the Nbss phase with an orientation relationships (ORs) of [1110]γ//[011]Nbss, (10 1‾ 0)γ//(011)Nbss and (1 2‾ 11)γ//(200)Nbss. The composite has Nbss, Nb3Si and γ-Nb5Si3 phases after heat treatment at 1400 °C for 30 h, and δ-Nb11Si4 precipitate has formed in the Nbss phase with an ORs of [100]Nbss//[010]δ, (010)Nbss//(010)δ. The composite processing Nb–40Ti–10Si–5Al–2V (at.%) has outstanding toughness (21.62 MPa·m1/2). It has ascertained that the room-temperature fracture toughness of Nb–Si-based in-situ composite is positively impacted by the high concentration of Ti.

Acoustic streaming and cavitation effect on microstructure evolution of Nb-Si-Ti alloy treated by ultrasound

Heat and mass transfer play a critical role in grain refinement by ultrasound treatment. In order to reveal the mechanism of ultrasonic in Nb-Si molten metal, a 3-D model was simulated on the acoustic, temperature and flow fields with different acoustic frequencies and displacement amplitudes by Comsol Multiphysics. Simulation results show that a new harmonic wave appears with frequency increasing, and the acoustic streaming effect is significant at 20 kHz. As the displacement amplitude increases from 10 μm to 20 μm, the maximum velocity varies from 0.01 m/s to 0.03 m/s. The distributions of three fields are dependent on frequency, and the displacement amplitude determines acoustic pressure value and fluid velocity. The displacement amplitude has a positive effect on the acoustic streaming effect improvement in the melt. In the UST ingots with different displacement amplitudes, the size of the broken dendrites decreases from 57.5 μm to 10 μm with the displacement amplitude lower than 16 μm, by 82.6%. γ-Nb5Si3 phase increases and the primary Nbss phases are elongated, coarsened and spheroidized with the increase of the displacement amplitudes. The simulation results of the flow directions can be verified by the elongated Nbss phase. The acoustic streaming effect promotes heat and mass transfer and the sluggish eutectoid reaction in Nb-Si-Ti alloy during the ultrasound treatment.

Optimization of Zr and Mo element on room-temperature fracture toughness and the integrity of oxide film

In this paper, Zr and Mo elements are introduced into the Nb–Si based alloy to improve the room-temperature fracture toughness and oxidation resistance at high-temperature environment. Nb–16Si–24Ti-xZr-yMo (x = 4, 8; y = 3, 6) alloys were prepared by arc melting and their room-temperature fracture toughness and oxidation resistance at 1250 °C is investigated. Results show that the addition of Zr and Mo element promotes the formation of lamellar Nbss-(Nb, X)5Si3 eutectic structure, which is beneficial to the fracture toughness. The room-temperature fracture toughness increases with the content of alloying element. The KQ value of Nb–16Si–24Ti–8Zr–6Mo alloy reaches 14.88 MPa ·m1/2, which is 72% higher than that of the Nb–16Si–24Ti original alloy. The oxidation resistance decreases due to the presence of more Nbss phase and the holes left by Mo oxide. However, the generation of ZrO2 and the volatilization of Mo oxide relieve the internal stress of oxide film and improve the integrity and adhesion of the oxide layer.

Mechanical properties and microstructure evolution of NbSiZr based alloy

To improve the mechanical properties of Nb-Si-Zr based superalloys at room temperature, the effect of a trace amount of Cr, C, Hf, Ta and Sc elements on the solidification processes, phase composition, mechanical properties and fracture behavior of Nb-Si-Zr based superalloys are studied by vacuum non-consumable arc-melting. The results show that the based alloy consists Nbss phase, γ-(Nb,X)5Si3 phase and Zr-rich γ-(Nb,X)5Si3 phase. However, there is no Zr-rich γ-(Nb,X)5Si3 phase in 20Zr-2C and 20Zr-4Hf alloys, indicating that a trace amount of C or Hf elements accelerates the diffusion of Zr element, and improves the segregation of the Nb-Si-Zr based alloys for the later stage of the superalloy solidification. The Nbss fraction in the matrix alloy (20Zr alloy) is 60%. C element increases the Nbss phase fraction by 4%, and Cr and Sc elements reduce the Nbss phase fraction by 4% and 6%, Hf and Ta elements reduce the Nbss phase by 1% and 2% compare with the matrix alloy (20Zr alloy). C element is dissolved in Nbss phase and γ-(Nb,X)5Si3 phase, while Cr element is more dissolved in Nbss phase and Sc element is more dissolved in Zr-rich γ-(Nb,X)5Si3 phase. The toughness of 20Zr-2 C alloy is the highest, and its KQ value reaches 14.05 MPa·m1/2, compared with the matrix alloy (20Zr alloy) increased by 23.46%. The enhancement of Nbss phase fraction and petal shaped microstructure make the crack propagation more difficult, thus increasing the KQ value of the 20Zr-2C alloy. The compressive properties of 20Zr-4Cr alloy are the best, and its compressive strength reaches 1807.3 MPa, compared with the matrix alloy (20Zr alloy) increased by 17.09%. The reason is that a trace amount of Cr element increases the fraction of γ-(Nb,X)5Si3 phase.

Effect of deformation parameters on microstructure evolution, flow softening behavior and nanoindentation properties of Nb–24Ti–16Si–2Al–4Fe–5V alloy

The microstructural evolution and the flow softening behavior of Nb–24Ti–16Si–2Al–4Fe–5V alloy during hot deformation under different processing parameters were investigated. Results show that the as-cast Nbss/β-(Nb, X)5Si3 eutectic structure disappears and becomes the Nbss matrix and β-(Nb, X)5Si3 blocks after hot deformation, wherein the continuous Nbss matrix consists of fine recrystallized grains. With the increase of deformation temperature, the β-(Nb, X)5Si3 particles evolve from irregular to quadrilateral shape. The refinement mechanism of the β-(Nb, X)5Si3 phase can be divided into silicide splitting and continuous dynamic recrystallization (CDRX). Moreover, the decrease of strain rate makes the DRX degree of Nbss increase, while the influence on that of β-(Nb, X)5Si3 is relatively less. The flow softening behavior of the alloy is mainly controlled by the CDRX behavior of the Nbss phase, and the recrystallization refinement can improve the hardness (H) of the Nbss phase. After deformation, the H values of Nbss will increase with the increase of recrystallization degree, while the H value of β-(Nb, X)5Si3 phase will increase with the decrease of strain rate.

Anisotropy of mechanical properties in directionally solidified Nb-Si alloys at room temperature compression

The room-temperature compressive strength and deformation mechanisms in directional solidified Nb-Si alloy with different orientations were studied. The Electron Backscattered Diffraction (EBSD) experiment further explained that the orientation relationship between the Nbss phase and the α-Nb5Si3 phase is [001] Nbss // [001] α-Nb5Si3, (110) Nbss // (110) α-Nb5Si3, and the corresponding relationship between the material crystal structure and macroscopic spatial coordinates was established. The anisotropy of mechanical properties and failure mode in the alloys are attributed to the phase structures which are the long rod-like morphology of the α-Nb5Si3 phase growing along [001] and the spatial asymmetry of a single (110) Nbss // (110) α-Nb5Si3 phase interface. The Tsai-Hill criterion was applied to explain the anisotropy of the compressive strength of alloy at an angle to the direction of the heat flow. In addition, phase boundary separation was considered to be an important reason for the higher plasticity exhibited by the [100] compression orientation.

Remarkable fracture toughness of hypereutectic Nb-16Si-20Ti-2ZrC composites with fine primary γ-(Nb,X)5Si3 regulated by TiB2

To meet the requirements of the new generation of aeroengines, Nb-Si-based alloys have been widely studied as candidate materials for the new generation of blade materials; however, the lower ambient fracture toughness limits their applications. In this study, the effects of various contents (0, 0.5, 1, 3 at%) of TiB2 on the microstructure and ambient fracture toughness of the hypoeutectic composite of Nb-16Si-20Ti-2ZrC (base) composite were investigated. Ingots were manufactured via arc melting in an Ar atmosphere. Results demonstrate that the ingots were composed of Niobium based solid solution (Nbss) and γ-(Nb,X)5Si3, and TiC phases precipitate with a TiB2 content of 3 at%. A moderate amount of TiB2 (1 % TiB2 and below) markedly refined the microstructure, and the average phase sizes of Nbss and γ-(Nb,X)5Si3 decreased to 12.5 and 9.8 µm in the Nb-16Si-20Ti-2ZrC-1TiB2 composite, respectively. TiB2 promotes the movement of the eutectic point to the low Si side in the Nb-Si-based composite, transforming from a hypoeutectic (base) alloy to a eutectic alloy and a hypereutectic alloy. The ambient fracture toughness reached 18.6 MPa·m1/2, increased by 22 %. Many microcracks near the primary crack generated by the fine eutectic colony passivate the primary crack, and the deformation ability of the small-sized Nbss phase is enhanced, resulting in many dimples. The ambient fracture toughness of the Nb-16Si-20Ti-2ZrC-1TiB2 composite in this study is the highest among the reported as-cast Nb-Si-based composites, which can promote Nb-Si-based composites to meet the blade material requirements.

Microstructure and mechanical properties of Nb–Si alloy with addition of Mn

The effect of Mn element on the microstructure and mechanical properties of Nb–Si based alloys is studied, and the investigated alloys compositions are Nb–16Si–22Ti–xMn (x = 0, 2, 4, 8 at.%). Solubility of Mn in Nbss phase is higher than that in silicide. This dissolution of Mn in Nbss phase and silicide causes solid solution strengthening. Tiss phase and Mn2Ti phase eutectic structure precipitates between dendrites of 8Mn alloy, which affect room-temperature fracture toughness and compressive strength adversely. After heat treatment, Tiss phase and Mn2Ti eutectic structure almost disappears in Mn-contained alloys (Nb, Ti)3Si phase decomposes and transforms to Nbss phase and α-(Nb, Ti)5Si3 phase. The fracture toughness of heat treated 8Mn alloy has been improved greatly due to Mn solution strengthening in Nbss phase.

Solidification behavior and microstructure evolution of Nb-Si-Mo alloy in ultrasonic field

The ultrasonic irradiation is successfully introduced into Nb-Si-Mo melt. The coarse silicide has been refined significantly. Ultrasonic irradiation can increase the solid solubility of Si in Nbss phase. The vibration attenuates during propagation in transition region, leading to the silicide with specific shape in linear arrangement. The wavelength and introducing efficiency of the standing wave are calculated.

A study on fine Nbss/γ-(Nb,X)5Si3 eutectic with significantly improved room-temperature fracture toughness by added ZrB2

Four NbSi based ultrahigh temperature alloys with compositions of Nb-16Si-20Ti-1ZrC-xZrB2 (x = 0, 0.5, 1, 3) (at.%) were prepared by vacuum non-consumable arc melting. The microstructure evolution, fracture toughness, and fracture behavior of four alloys were investigated. Results showed that four alloys consisted of Nbss, γ-(Nb,X)5Si3, and (Nb,X)3Si phases. The low content of ZrB2 facilitates the disintegration of (Nb,X)3Si, while the high content of ZrB2 stabilizes (Nb,X)3Si. The maze-like Nbss/γ-(Nb,X)5Si3 eutectic structure forms in Nb-16Si-20Ti-1ZrC-0.5ZrB2 alloy. With the increase of ZrB2 content, the room-temperature fracture toughness of Nb-Si-Ti-ZrC-based alloy increases and decreases. When the addition amount of ZrB2 is 0.5 at.%, the room-temperature fracture toughness is the largest of 18.4 MPa·m1/2, and the fracture mode changes from quasi-cleavage fracture to dimple fracture. Compared with the base alloy, it is increased by 28%. It is the highest value of as-cast arc melting that has been reported so far. The maze-like eutectic of the high content of Nbss (54.9 ± 5.9%) phase promotes the formation of microcracks and inhibits the propagation of cracks in Nb-16Si-20Ti-1ZrC-0.5ZrB2 alloy.

Effect of Re addition on microstructure and mechanical properties of Nb-Si-Ti-Al-Cr-Hf ultrahigh temperature alloys

In order to improve the mechanical performance of multicomponent NbSi based alloy, the effects of rare metal element Re addition on microstructure evolution and mechanical performance of Nb-16Si-20Ti-2Al-2Cr-2Hf (at.% as follows) alloys are studied systematically. With the high melting point alloying element Re addition, the primary Nbss phase of Nb-16Si-20Ti-2Al-2Cr-2Hf-xRe transforms from equiaxed structure to dendritic structure due to the component supercooling, and the silicide phase transforms from α-Nb5Si3 to β-Nb5Si3. Moreover, with the increasing Re addition, the microstructure has been refined firstly, then coarsened and finally formed developed dendrite. The element Re preferentially solubilizes in the Nbss phase, when the Re content exceeds 1.0%, it can be detected in silicide phase. The Vickers microhardness and compressive strength of the multicomponent NbSi based alloys have been improved significantly because of the solid solution strengthening by Re addition. And the most obvious improvement is the room temperature fracture toughness increases from 10.33 MPa m1/2 to 14.40 MPa m1/2, as the Re content increases from 0 at.% to 0.5 at.%, which is 39.4% higher than the Re free NbSi based alloy.

Microstructure and mechanical properties of Nb[sbnd]16Si alloys with Zr additions

To optimize the microstructure and fracture toughness of NbSi based alloys, the effect of Zr addition on the microstructure and mechanical properties was investigated in Nb16Si alloy. The results show that Zr element promotes the sluggish eutectoid reaction of (Nb, Zr)3Si → Nbss + α-(Nb, Zr)5Si3, and Nb-16Si-8Zr alloy only contains Nbss and α-(Nb, Zr)5Si3 phases. From the energy point of view, as the Zr content reaches 4 at.%, (Nb,X)5Si3 phase is more stable than (Nb, X)3Si phase. With the increase of Zr content, the fracture toughness increases since high Zr content refines eutectic structure and increases the content of primary Nbss. The thickness of primary Nbss has a strong influence on crack propagation. Most of the cracks can destroy primary Nbss phase with the thickness of (20 ± 2) μm and continue to propagate in the original direction. However, it will deflect when it intersects primary Nbss phase with a thickness of about (30 ± 2) μm. The improvement of compressive properties was caused by solid solution of Zr.

Microstructure characterization of high temperature mechanisms in a Nb–Ti–Si alloy

The deformation mechanisms of an Nb–20Si–25Ti–6Al–3Cr–3Mo alloy deformed in compression at 1000 °C and at a strain rate of 10−4 s−1 have been investigated, using focused ion beam and transmission electron microscopy techniques. The initial microstructure of the alloy was constituted of a solid solution bcc Nbss matrix and of Nb5Si3 phases of β (tetragonal) and γ (hexagonal) crystallographic structures, with also hcp-Ti precipitates. As expected, the only phase which exhibited plastic deformation at 1000 °C was the Nbss matrix. Glide of b→=1/2111 dislocations has been observed, in planes compatible with {112} and {123} planes, or with cross-slip in {110} planes, as classically reported in bcc structures. Conversely, climb contribution has not been evidenced, probably because of rearrangement of the climbing dislocations in sub-boundaries. Fine δ-Nb11Si4 precipitates have been detected in the samples compressed at 1000 °C. Interactions of the δ-Nb11Si4 precipitates with the dislocations have been observed, which can potentially lead to strengthening of the Nbss phase.

Effect of Co on microstructures and mechanical properties for Nb-Si based in-situ composites

In this paper, six alloys with different Co contents (Nb-16Si-20Ti-4Cr-1.5Al-4Hf-4Zr-1Ta-xCo (x = 0, 1, 2, 3, 4 and 5)) were prepared by copper water-cooled crucible arc-melting. The effects of Co on phase selection, microstructure, room-temperature fracture toughness, and compressibility were investigated. The six alloys are composed of Nbss, α-(Nb,X)5Si3, γ-(Nb,X)5Si3, and Cr2(Nb,X) phases. The addition of Co element promotes the precipitation of Tiss phase. Primary β-(Nb,X)5Si3 phase was stabilized at room temperature by adding more than 3% Co. Alloy changes from a near-eutectic to a hypereutectic by the addition of Co. The bulk of primary silicide phase deteriorates the room-temperature fracture toughness. At the same time, Co element solid solution in Nbss phase reduces the Si content in Nbss phase and increases the KQ value of alloy. Therefore, the KQ value of 1Co alloy reaches 13.14 MPa·m1/2. The maximum compressive strength of 5Co alloy is 2155 MPa, which is mainly due to the solid solution strengthening of Co element and the highest silicide phase content.

An as-cast Nb-Si-based alloy with fine-grains and remarkable fracture toughness by minor Sc addition

The effect of Sc addition on the phase composition, microstructure, and room-temperature fracture properties of arc melted Nb-Si-Ti-ZrC-based alloys was systematically investigated. The constituent phases of all alloys are Nbss, γ-(Nb,X)5Si3 and TiC. Sc refines the microstructure, while the grain sizes for the primary γ-(Nb,X)5Si3 and Nbss phases drop from 213 µm and 22.3–23 µm and 5.7 µm, respectively. The content of Nbss and TiC decreases with increasing Sc. The Sc dissolves and then softens Nbss phase. The degree of restraint of Nbss phase decreases with increasing Sc content, therefore causing formation of dimples in the fracture and interface decohesion. Because of the refinement of the primary γ-Nb5Si3 phase, the dimples and interface decohesion created by the low degree of constraint of Nbss phase, the KQ value of Sc3 alloy becomes 16.4 MPa·m1/2. The average grain size of Nbss phase becomes the lowest (5.7 µm), when 0.5 at% Sc is added. The small grain size of Nbss phase deteriorates room-temperature fracture toughness.

Improving primary Nbss morphology and fracture toughness in hypoeutectic Nb-Si-Ti ternary alloy by ultrasonic treatment

Nb-Si ultra-high temperature alloys have been a hot topic in aviation materials, but low fracture toughness limits the application. Ultrasound treatment has a significant advantage on the grain refinement and properties improvement, successfully induced into hypoeutectic Nb-Si-Ti ternary alloys to improve primary morphology and fracture toughness. In this work, Nb-16Si-22Ti alloy was treated by ultrasound of 0 s, 80 s, 120 s, 160 s and 200 s. Meanwhile, the coupled physical fields were simulated in Comsol Multiphysics to reveal the mechanism of ultrasonic action. Results show that γ-Nb5Si3 appears with ultrasound induced, and the solidification path is close to the equilibrium transition. Primary Nbss phases were coarsened and spheroidized after ultrasound treatment, and fracture toughness improved by 69.16%. Simulation results show that the strong acoustic active zone is at the height of 15 mm in the ingot, with maximum sound pressure of 36.3 MPa. The high sound pressure can break dendrites. Maximum fluid velocity reaches 0.03 m/s in the longitudinal section and 0.02 m/s in the transversal section with several circulations. Acoustic streaming and cavitation effects break dendrites and spheroidize Nbss phase, benefit to elements distribution uniformly, equilibrium transition and fracture toughness improvement.