Nbss Dendrites Research Articles

Rapid directionally solidified microstructure characteristic and fracture behaviour of laser melting deposited Nb–Si–Ti alloy

Aiming at achieving fine and directionally-solidified microstructure of Nb–Si based alloy, Nb, Si and Ti powder particles were utilized as the raw materials, and laser melting deposition (LMD) experiments were conducted with 1500 ​W and 2000 W laser power, respectively. The microstructure characteristic, micro-hardness, and indentation fracture toughness were examined by scanning electron microscope (SEM), X-ray energy dispersive spectrometer (XEDS) and X-ray diffraction (XRD). The results showed that the two kinds of as-deposited Nb–17Si–23Ti alloy samples were mainly composed of NbSS, (Nb, Ti)3Si and Ti-rich NbSS, and the microstructure presents rapid directionally-solidified characteristic, in which the <100> crystallographic direction of NbSS and <110> crystallographic direction of (Nb, Ti)3Si tend to be parallel to the LMD building direction. With the laser power increasing from 1500 ​W to 2000 W, the microstructure became more oriented and homogeneous. For the sample at 2000 W, the dendrite spacing was only about 1–2 ​μm, and NbSS and (Nb, Ti)3Si couples grow alternately and they are parallel to the building direction strictly. In the meantime the micro-hardness and indentation fracture toughness reached 1075 HV and 20.3 ​MPa·m1/2, respectively, indicating significant improvement by the directional growth of fine NbSS and (Nb, Ti)3Si phases, and the dimple fracture mode of the NbSS dendrites.

Microstructural Evolution and Room Temperature Fracture Toughness of a Nb-Ti-Si-Cr-Al-Hf-Y Alloy Prepared by Directional Solidification

The Nb-24Ti-12Si-14Cr-2Al-2Hf-0.1Y (at.%) alloys were fabricated by directional solidification with selected withdrawal rate 1.2 and 18 mm/min, followed by a heat treatment at 1375 °C for 10 h. The microstructure of directional solidified samples were composed of NbSS, Cr2Nb and eutectics (NbSS+Nb5Si3), aligning with the growth direction. After heat treatment, the NbSS in the eutectic structures and NbSS dendrites were connected to form the matrix, and the silicide and Cr2Nb tended to spheroidize. The sample prepared by higher withdrawal rate plus heat treament shows higher average KQ values. The results suggested that the Nb-Si based alloy showed higher room-temperature fracture toughness when the microstructure consists of continuous NbSS distributed with finer Nb5Si3 and Cr2Nb.

Microstructural characterization and mechanical properties of Nb–Ti–C–B in-situ composites with W addition

Nb–Ti–C–B in-situ composites with W addition were fabricated by arc-melting method under an argon atmosphere. The microstructure and mechanical properties of the composites at room and high temperature were investigated, and the strengthening mechanism of W-added composites was discussed in detail. Microstructural observation shows that the composites contain primary Nbss dendrites and interdendritic eutectic network, and W addition refines primary Nbss and coarsens boride phase. The hybrid (Nb,Ti,W)C and (Nb,Ti,W)B second-phases are formed in the eutectic network of W-added composites. The hardness, room- and high-temperature compression properties were evaluated to determine the strengthening effect of W-added composites. The results indicate that the hardness and compressive strength of the composites are improved by W addition, which increases with increasing W content. Though the compressive strength of the composites decreased at 1200°C, the increased W content was beneficial to high temperature strength improvement. The strengthening effect of W-added composites is mainly attributed to solid solution and grain refinement strengthening.

Microstructural characteristics of Nb–Si based ultrahigh temperature alloys with B and Hf additions

Four Nb–Si based ultrahigh temperature alloys with compositions of Nb-22Ti-16Si-5Cr-3Al-mHf-nB ((m, n) = (0, 0), (0, 2), (4, 0) and (4, 2), respectively) (at.%) were prepared by vacuum non-consumable arc melting and then heat-treated at 1450 °C for 50 h. The effects of B and Hf additions on the phase selection, phase stability, microstructure and microhardness of these alloys under both as-cast and heat-treated conditions have been investigated. The results show that the microstructures of all the four alloys are composed of primary silicide blocks, Nbss dendrites together with one or two types of eutectic colonies around. However, the crystal structures of silicides, types of eutectic and amounts of constituent phases have obviously varied with B or Hf addition. Moreover, a low melting point three-phase eutectic is also observed in Hf-containing alloys. After 1450 °C/50 h heat-treatment, the microstructural uniformity of the alloys has been significantly ameliorated as well as their equilibrium phases have been basically obtained, and also some phase transformation reactions have occurred. The microhardness of the constituent phases present in the alloys is dependent on their types or crystal structures.

Effects of Cr and Hf additions on the microstructure and properties of Nb silicide based ultrahigh temperature alloys

Four Nb silicide based ultrahigh temperature alloys with compositions of Nb–22Ti–16Si–3Al–2B–xHf–yCr ((x, y)=(0, 0), (0, 5), (4, 0) and (4, 5)) (at%) were prepared by vacuum non-consumable arc melting. The effects of Cr and Hf additions on the phase selection, microstructure, room-temperature fracture toughness and oxidation resistance at 1250°C of the alloys have been investigated. The results show that all the four alloys are comprised of primary silicides and Nbss dendrites together with one or two types of eutectic. The Cr addition does not change the crystal structures of 5-3 silicides but imposes a significant influence on the amount, size and morphology of the constituent phases. The Hf addition suppresses the formation of α(Nb,X)5Si3 but promotes the formation of γ(Nb,X)5Si3. The combined additions of Cr and Hf also cause the formation of Cr2(Nb,X) containing three-phase eutectic. The room-temperature fracture toughness of the alloys decreases with Cr addition while increases with Hf addition. The oxidation-resistant performance of the alloys has been ameliorated by Cr addition, while it has been significantly degraded by Hf addition in the absence of Cr. Furthermore, the formation mechanism of the scale has been illustrated.

Microstructure of Nb–Ti–Cr–Si based ultrahigh temperature alloy processed by integrally directional solidification

The Nb–Ti–Cr–Si based ultrahigh temperature alloy was integrally directionally solidified with the using of special ceramic crucibles. The effects of different withdrawing rates on the microstructure and the solidification path in a Nb–Ti–Cr–Si based ultrahigh temperature alloy were investigated. The results showed that the microstructure was composed of primary niobium solid solution (Nbss) dendrites, Nbss/(Nb,X)5Si3 (here X represents Ti and Hf elements) eutectic colonies and fine Ti rich phase layer between Nbss dendrites and Nbss/(Nb,X)5Si3 eutectic colonies when the withdrawal rates vary from 2·5 to 10 μm s−1. However, the directionally solidified microstructures were composed of primary Nbss dendrites, Nbss/(Nb,X)5Si3 eutectic colonies and finer (Ti,Nb)ss/Nb3Si/Cr2Nb colonies colonies when the withdrawal rate was reached and exceed than 20 μm s−1. The primary Nbss and Nbss/(Nb,X)5Si3 eutectic aligned the growth direction when the withdrawal rates vary from 2·5 to 20 μm s−1; however, the discontinuous (Nb,X)5Si3 plates were present when the withdrawal rates vary from 50 to 100 μm s−1. The solidification path was studied according to the microstructure of solid/liquid interface.

Microstructure Evolution in Nb-12Si-22Ti-14Cr-2Al2Hf Alloy Fabricated by Directional Solidification

AbstractIn this work, the microstructure evolution of the Nb-12Si-22Ti-14Cr-2Al-2Hf alloy (at.%) fabricated by liquidmetal-cooled directional solidification (LMC) was investigated and the solidification behaviors were discussed. The results revealed that the growth rate was of crucial in controlling the microstructure of this alloy and the higher growth rate was beneficial to the formation of metastable phase Nb3Si. The microstructure of non-melted region consisted of NbSS dendrites, NbSS/Nb5Si3 eutectics and Ti-rich NbSS/Cr2Nb eutectics while the microstructure of quasi-steady-state growth region of this alloy consisted of NbSS dendrites, Nb3Si laths and Ti-rich NbSS/Cr2Nb eutectic. Nb3Si had the advantage of nucleation at medium/high growth rate. Therefore, the microstructure evolution occurred in this study was owing to the increase of growth rate.

Effects of B addition on the microstructure and properties of Nb silicide based ultrahigh temperature alloys

Four Nb silicide based ultrahigh temperature alloys with compositions of Nb–22Ti–16Si–5Cr–4Hf–3Al–xB (x = 0, 2, 5 and 10, respectively) (at%) were prepared by vacuum non-consumable arc melting. The effects of B content on the phase selection, microstructure, oxidation resistance at 1250 °C, room-temperature fracture toughness and microhardness of the alloys were investigated. The results showed that the microstructures of the four alloys were all comprised of primary silicide blocks, Nbss dendrites and eutectic colonies. However, the crystal structures or types of silicides and the amounts of constituent phases obviously varied with increase in B content in the alloys. The oxidation resistance of the alloys was significantly ameliorated by B addition. The room temperature fracture toughness of the alloys was improved with 2 at% B addition but degraded with 5 or 10 at% B addition. The microhardness of Nbss rose slightly with increase in B content in the alloys, while that of silicides was dependent on their crystal structures, types and concentrations of alloying elements.

Effect of Alloying Elements on Nb-Rich Portion of Nb-Si-X Ternary Systems and In Situ Crack Observation of Nb-Si-Based Alloys

To find a new route for microstructure control and to find additive elements beneficial for improving high-temperature strength, a systematic investigation is performed on hypoeutectic Nb-15 at. pct Si-X ternary alloys containing a transition element, Fe, Co, Ni, Cu, Ru, Rh, Pd, Re, Os, Ir, Pt, or Au. Information on phase equilibrium is classified in terms of phase stability of silicide phases, α Nb5Si3, Nb4SiX, and Nb3Si, and the relationship between microstructure and mechanical properties both at room temperature and high temperature is investigated. All the additive elements are found to stabilize either α Nb5Si3 or Nb4SiX but destabilize Nb3Si. A microstructure of Nbss/α Nb5Si3 alloy composed of spheroidized α Nb5Si3 phase embedded in the Nbss matrix is effective for toughening, regardless of the initial as-cast microstructure. Also the plastic deformation of Nbss dendrites may effectively suppress the propagation of longer cracks. High-temperature strength of alloys is governed by the deformation of Nbss phase and increases with higher melting point additives.

The Effects of the Withdrawal Rate and Heat Treatment on the Microstructure of Directionally Solidified Nb-14Si-24Ti Alloy

AbstractIn this work, an Nb-14Si-24Ti alloy (at.%) was prepared by liquid-metal-cooled directional solidification (DS) and then heat treated (HT) at 1450°C for 100 h. The effects of the withdrawal rate and heat treatment on the microstructure were investigated. The results show that the DS samples consisted of NbSS dendrites, Nb3Si blocks and eutectic NbSS/Nb3Si. As the withdrawal rate increased, the microstructure became finer, the volume fraction of eutectic NbSS/Nb3Si increased and the morphology of eutectic cell transformed from petaloid to regularly arranged granular morphology. The microstructure of the HT samples consisted of NbSS and Nb3Si. The metastable phase of Nb3Si did not discompose through the eutectoid reaction. The dendritic NbSSconnected through diffraction and the secondary dendrite arm got coarser. The boundary of eutectic NbSS/Nb3Si became blurry and the black Ti-rich phase of the DS samples in this region disappeared. The Nb3Si connected to form the continuous matrix. The granular NbSS of eutectics spheroidized and the lamellar NbSS of eutectic interconnected and became more bending distributed uniformly in the Nb3Si matrix.

The microstructure evolution of directionally solidified Nb-22Ti-14Si-4Cr-2Al-2Hf alloy during heat treatment

To obtain a homogenous and optimizing microstructure, non-equilibrium directionally solidified (DS) samples at high withdrawal rate were heat treated (HT). The heat treatments were carried out at 1500 °C for 3–100 h. The DS and HT samples consisted of NbSS, α-Nb5Si3 and γ-Nb5Si3, and the DS morphology altered significantly after heat treatment. The well developed NbSS dendrites were compromised and the size decreased from more than 100 μm to less than 60 μm when the heat treated time was 6 h. On the other hand, the NbSS in eutectic interconnected and meanwhile connected with the dendrites, and the frequency of small size got lower with the heat treated time increasing. When heat treated for 100 h, the size of NbSS was comparatively uniform and concentrate in the region of 5–20 μm with a value of 63% for Ostwald ripening. The dendric and eutectic morphologies exited in the DS sample disappeared, turning into a net-work structure. The components of primary Nbss dendrites and Nbss in the intercellular regions were homogenized after heat treated for 3 h. However the enrichment of Ti, Cr and Hf in γ-Nb5Si3 compared to α-Nb5Si3 could not be eliminated by homogenizing treatment even for 100 h.

Effect of Mo on microstructure and mechanical properties of Nb–Ti–C–B multiphase alloy

The effect of Mo on the microstructure and mechanical properties of Nb–Ti–C–B multiphase alloy was investigated. The alloy without Mo contains the primary Nb solid solution (Nbss) dendrites and interdendritic eutectic network containing Nbss, (Nb,Ti)C and (Nb,Ti)B. In Mo-added alloys, the solubility of Mo is found in Nbss and (Nb,Ti)B. Mo addition refines the primary Nbss dendrites and promotes the coarsening of boride phase, but does not obviously change the volume fraction of primary Nbss and total secondary particles. The hardness, compression properties and fracture toughness were evaluated to determine the strengthening and toughening effect of Mo-added alloys at room temperature. The results indicate that the hardness and compressive yield strength increase with increasing Mo content, but Mo addition decreases the room temperature fracture toughness.

Microstructure and room-temperature fracture toughness of directionally solidified Nb–Si–Ti–Cr–Al–Hf alloy

The Nb–12Si–22Ti–14Cr–2Al–2Hf alloy (at%) was directionally solidified at 1750°C with withdrawal rates of 0.24mm/min, 1.2mm/min, 6mm/min, 18mm/min, 30mm/min, 50mm/min, 70mm/min and 100mm/min, and subsequent heat treatment was carried out at 1375°C for 10h. The microstructures and room-temperature fracture toughness of the directionally solidified and heat treated samples were investigated. The results showed that instead of the microstructure consisting of NbSS dendrites, NbSS+Nb5Si3 eutectics and Cr2Nb+Ti-rich NbSS eutectics, the microstructure composed of NbSS dendrites, Nb3Si laths and Cr2Nb+Ti-rich NbSS eutectics was observed when the withdrawal rate was higher than 30mm/min. After the heat treatment, the Nb3Si transformed into extremely fine NbSS and Nb5Si3, and the NbSS dendrites changed to be remarkably continuous. The room-temperature fracture toughness could reach a maximum of 15.7MPa·m1/2 for the alloy withdrawn at 50mm/min plus heat treatment. It is suggested that the microstructure with fine intermetallic phases (Nb5Si3 and Cr2Nb) and continuous NbSS would be of great benefit to the room-temperature fracture toughness.

Microstructure and mechanical properties of directionally solidified and heat treated Nb–14Si–24Ti–2Cr alloy

In this work, the directional solidification of an Nb–14Si–24Ti–2Cr alloy (at-%) was performed at 1900°C with withdrawal rates of 1·2, 6 and 18 mm min−1, and the alloy was then heat treated (HT) at 1450°C for 100 h. The microstructures and mechanical properties of the directionally solidified and HT alloy were investigated. The phases present in the sample were confirmed to be NbSS dendrites, lath-like Nb3Si, eutectic NbSS+Nb3Si and Ti enriched areas. As the withdrawal rate increased, the effects of directional solidification became more evident, and the microstructure became more refined. After the heat treatment, the microstructure was characterised by NbSS dendrites, residual Nb3Si and lamellar eutectic NbSS+Nb3Si, which was decomposed from the lath-like Nb3Si. An increase in the withdrawal rate resulted in a decrease in the room temperature fracture toughness of the HT alloy. However, the room temperature compressive strength of the HT alloy increased to 1593·9 MPa with increasing withdrawal rate, which was twice the value of the compressive strength before heat treatment.

Effect of high temperature treatments on microstructure of Nb-Ti-Cr-Si based ultrahigh temperature alloy

To investigate the effects of homogenizing and aging treatments on the microstructure of an Nb-Ti-Cr-Si based ultrahigh temperature alloy, coupons were homogenized at 1 200-1 500 °C for 24 h, and then aged at 1 000 °C for 24 h. The results show that the heat-treated alloy is composed of Nb solid solution (Nbss), (Nb,X) 5Si 3 and Cr 2Nb phases. With the increase of heat-treatment temperature, previous Nbss dendrites transformed into equiaxed grains, and petal-like Nbss/(Nb,X) 5Si 3 eutectic colonies gradually changed into small (Nb,X) 5Si 3 particles distributed in Nbss matrix. A drastic change occurred in the morphology of the Laves phase after homogenizing treatment. Previously coarse Cr 2Nb blocks dissolved during homogenizing at temperature above 1 300 °C, and then much finer and crowded Cr 2Nb flakes precipitated in the Nbss matrix in cooling. Aging treatment at 1 000 °C for 24 h led to further precipitation of fine particles of Laves phase in Nbss matrix and made the difference in concentrations of Ti, Hf and Al in Nbss, (Nb,X) 5Si 3 and Cr 2Nb phases reduced.

真空感应熔炼Nb-16Si-22Ti-2Hf-2Cr-2Al合金的组织和性能

The melt with a nominal composition of Nb-16Si-22Ti-2Hf-2Cr-2Al was poured in a ceramic shell mould with a temperature gradient of about 4℃/mm, and the ingot with the dimension of 60 mm×170 mm was obtained. The relationship between the microstructure and mechanical properties was measured, and the effects of the silicide on fracture toughness at ambient temperature and compression strength at high temperature were analyzed. It is revealed that the microstructure of the alloy consists of Nb solid solution and silicides, and the cooling rate can obviously change primary phase and constituent phases. The volume fraction of Nb_3Si formed at rapid cooling rate is significantly increased, and Nb_5Si_3 phase is formed with decreasing in cooling rate, whereas the volume fraction of the eutectic colonies and Nbss dendrites increase evidently, especially the second dendrite arm. Fine and uniform eutectic colonies are contributed to the ambient tensile strength, while the coarse primary Nb_3Si decreases the tensile strength but improves the compression strength of the alloy. When the microstructure is mainly composed of fine (Nb_(SS)+Nb_5Si_3) eutectic colonies, the tensile strength and elongation of the alloy reach 449 MPa and 0.3%, respectively. When lath-like Nb_3Si phase has 80μm in width and 50% in volume fraction as well as its long axis is parallel to compression direction, the compression strength of the alloy at 1250℃is about 650 MPa.

Microstructure and Mechanical Properties of an Advanced Niobium Based Ultrahigh Temperature Alloy

The microstructure and mechanical properties including room temperature fracture toughness Kq, tensile strengthσb and elongationδ at 1250°C of the Nb based alloy directionally solidified in an electron beam floating zone melting (EBFZM) furnace have been evaluated. The microstructure is primarily composed of Nb solid solution (Nbss), α-(Nb)5Si3 and (Nb)3Si phases. After directional solidification with the moving rate of electron beam gun R being respectively 2.4, 4.8 and 7.2 mm/min, the primary Nbss dendrites, Nbss + (Nb)5Si3/(Nb)3Si eutectic colonies (lamellar or rod-like) and divorced Nb silicide plates align along the longitudinal axes of the specimens. When R = 2.4 mm/min, the best directional microstructure is obtained. Directional solidification has significantly improved theσb at 1250°C and Kq. The maximumσb occurs for the specimens with R = 2.4 mm/min and is about 85.0 MPa, meanwhile, the Kq is about 19.4 MPam1/2.