Primary Silicide Research Articles

Ultra-fine Nbss/Nb5Si3 in situ composites with remarkable properties prepared by ultrasonic melt treatment

In order to refine coarse silicide and improve mechanical performance of ternary Nb-20Si-5Mo alloy, ultrasonic vibration treatment for ultra-high temperature melt is successfully applied. The results show that the vibration can improve the fluidity and reduce the shrinkage defects. The hypereutectic microstructure transforms into the fine rod-like eutectic. Due to the solute redistribution in ultrasound field, the Mo-rich layer at Nbss and β-Nb5Si3 phase boundary and the semi-coherent relation can be observed. Mo tends to dissolve in Nbss the matrix phase and inhibit the peritectic reaction. The average size of primary silicide is refined from 49.1 to 2.2 µm with 95.52% refinement efficiency, the compressive strength and fracture toughness of 100 s ultrasonic treated specimen improved 6.84% and 50.75% compared with the as-cast specimen, respectively. The heightened constitutional supercooling and the accelerated nucleation caused by cavitation effect are the main reasons of grain refinement.

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.

Effects of Mo and Zr composite additions on the microstructure, mechanical properties and oxidation resistance of multi-elemental Nb-Si based ultrahigh temperature alloys

• Adding Mo and/or Zr suppresses α(Nb,X) 5 Si 3 formation whilst promotes γ(Nb,X) 5 Si 3 formation. • Zr addition ameliorates room temperature fracture toughness, while further Mo addition degrades it. • Adding Mo/Zr enhances, and cooperatively adding Mo and Zr further increases compressive strength at 1250 ℃. • The alloy containing Mo and Zr shows the best oxidation resistance at 1250 °C. The Nb-Si based alloys adding Mo, Zr and Mo-Zr respectively were prepared by vacuum non-consumable arc melting. The alloys’ microstructure and comprehensive performances including microhardness, room temperature fracture toughness as well as compressive strength and oxidation resistance at 1250 °C were evaluated systematically. The results show that adding Zr in multi-elemental Nb-Si based alloys changes the microstructure from eutectic to hypereutectic, while further adding Mo in Zr-containing alloy decreases the content of primary silicide blocks. Both the single and composite additions of Mo and Zr in the alloys suppress the formation of α(Nb,X) 5 Si 3 whilst promote the formation of γ(Nb,X) 5 Si 3 . Both the dissolved Mo (mainly in Nbss) and Zr (primary in γ(Nb,X) 5 Si 3 ) have the effect of solid solution strengthening and improve the microhardness of phases. The room temperature fracture toughness of the alloys is ameliorated by Zr addition, while is reduced by further Mo addition. Alloying with Mo or Zr alone can enhance, and composite alloying with Mo and Zr continues to increase the compressive strength at 1250 °C of the alloys. The alloy with composite additions of Mo and Zr shows the best oxidation resistance at 1250 °C due to the formation of a denser and well adhesive inner layer of scale and the improvement of the oxidation resistance of silicides under the synergistic effects of Mo and Zr.

Directional solidification of hypereutectic Nb-Si-Ti alloy: Influence of drawing velocity change on microstructures

In this study, the microstructural characteristics of directionally solidified (DS) hypereutectic Nb-Si based alloys were investigated. The abrupt acceleration and deceleration of directional solidification were designed to explore the influence of drawing velocity change on microstructure. Results show that the microstructure can be changed by abrupt deceleration, which provides a new possibility to control the properties of DS Nb-Si based materials. The abrupt decelerated solidification for the 17 at.% Si composition is able to scatter the primary silicide slabs, and irregular octahedral primary silicides are finally formed. For the 16 at.% Si composition, the solidification with a constant velocity of 100 mm/min, pseudo-eutectic can be obtained, in which the primary silicides can be eliminated within a certain distance of 4 mm. However, due to the inheritance of the microstructure, the pseudo-eutectic can not be obtained at the same drawing velocity by acceleration from 10 mm/s to 100 mm/s. The directionally solidified microstructure of hypereutectic Nb-Si alloy consists of eutectic and halo microstructures (primary silicides and Nbss shell). Two different growth modes of halo microstructures were identified by EBSD investigations.

Effect of Heat-Treatment on Microstructure and Mechanical Behavior of Directionally Solidified Nb-Ti-Si Based Alloy

With the melting temperature of 2323 K and withdrawing rate of 100 μm/s, Nb-Ti-Si based ultrahigh temperature alloy has been directionally solidified with the use of crucibles. The directionally solidified (DS) specimens were subsequently heat-treated in two ways: either at 1723 K/50 h (HT1) or at 1623 K/50 h + 1723 K/50 h + 1373 K/50 h (HT2). XRD, SEM and EDS have been employed to investigate the influence of heat-treatments on the microstructures and fractography of the directionally solidified alloys. The results show that after heat-treatment, the volume fraction of the large-size primary silicides decreases. Both kinds of heat-treatments could effectively alleviate or even eliminate the segregation in the alloys. The original boundaries between the (Nb, X ) 5 Si 3 + Nbss eutectic cells in the DS specimens have thoroughly disappeared after heat-treatments. Specifically, silicides have been more evenly distributed after HT2 than HT1. Compared with DS specimens, the average room temperature fracture toughness of the specimens has increased by 12.3% (19.2 MPa·m 1/2 ) while the average tensile strength has increased by 26.6% and the maximum value reaches 933.2 MPa after HT2. These improvements could be mainly attributed to the more effective dispersion strengthening of (Nb, X ) 5 Si 3 silicide particles and the shape change as well as size increase of the ductile Nbss phases after HT2.

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.

Influence of Thermal Treatment on the Structure and Cyclic Crack Resistance of Ti–10.3Al–3.0Zr–1.2Si Alloy

We study the influence of the modes of thermal treatment on the structure, phase composition, and cyclic crack resistance of preliminarily forged Ti–10.3Al–3.0Zr–1.2Si alloy at room and high (700°С) temperatures in air. It is shown that the procedure of hardening guarantees the best values of fatigue threshold ΔKth and cyclic fracture toughness ΔKfc at 20° and 700°С, which is caused by the formation of a martensitic structure, decrease in the silicon content of the titanium matrix, increase in the size of primary silicides Ti5Si3, and the appearance of secondary silicides. It is shown that the increase in the aluminum content of Ti–Al–Zr–Si alloys from 8 to 10.3 % results in a noticeable decrease in the cyclic crack resistance due to the influence of atmospheric oxygen and the precipitation of Ti3Al aluminides (of the α2-phase).

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.

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.

Microstructure evolution and room temperature fracture toughness of an integrally directionally solidified Nb–Ti–Si based ultrahigh temperature alloy

Integrally directional solidification of an Nb–Ti–Si based ultrahigh temperature alloy was performed at 2000 °C with withdrawal rates varying from 2.5 to 100 μm s−1. The directionally solidified microstructure was composed of primary silicides and eutectic colonies that distributed evenly and aligned erectly along the growth direction. Both the average diameter and the lamellar spacing of eutectic cells decreased with increasing withdrawal rate. The room temperature fracture toughness was improved significantly by integrally directional solidification.

The melting diagram of the Ti-corner of the Ti–Zr–Si system and mechanical properties of as-cast compositions

Phase equilibria in the Ti-corner of the Ti–Zr–Si system were studied by the methods of metallography, differential thermal analysis, X-ray and microprobe analyses of as-cast samples. The results are given in the way of the melting diagram and of polythermal sections at 10 and 20at.% Si. At 1300°C invariant four-phase equilibrium of eutectic type L↔〈β-Ti〉(β)+〈Ti5Si3〉(Z)+〈(Ti,Zr)2Si〉 (S2) was shown to occur in the concentration interval studied. The location of the eutectic point E was determined to take place at ∼78Ti–11Zr–11Si. The ternary compound S2 formed from the melt by peritectic reaction L+Z→S2. Microhardness of the primary silicide grains and of eutectics microconstituents has shown that zirconium additions decrease the strength of the Z-phase and of the β+Z eutectic, while they do not change the strength of the S2-silicide and of the β+S2 eutectic. Based on the results of hot hardness measurements, below 550°C the potential for attractive high-temperature structural properties is controlled by refinement of the microstructure, while above this temperature the α↔β transformation temperature of Ti limits the application.

Characterization of Laser Deposited Niobium and Molybdenum Silicides

AbstractRecent advances in laser deposition technology have made the production of advanced composite materials more technically feasible. By utilizing the unique characteristics of the laser deposition process, materials can be made that are difficult to produce by conventional methods. Structural silicide materials hold promise for high temperature applications, unfortunately, their low toughness has prevented their practical use [1-4]. The introduction of a second phase can greatly increase their low temperature mechanical properties. Laser direct deposition can be used to deposit these materials near net shape and produce an in-situ alloy that has the desired structure for improving properties. In-situ alloying was accomplished by using an Optomec LENS™ (Laser Engineered Net Shaping) machine to deposit elemental niobium-silicon and molybdenum-silicon-boron powder blends. The resultant microstructures showed a homogeneous structure with a primary silicide phase surrounded by a continuous eutectic phase. These deposits were characterized using scanning and transmission electron microscopy, x-ray diffraction, and by energy dispersive spectroscopy.

Microstructure study of a γ-TiAl based alloy containing W and Si

The phase transformations and modulated microstructures in the cast alloy Ti-47Al-2W-0.5Si(at.%) have been studied. It has been shown that microstructure is sensitive to heat treatment and composition. The structure in the alloy heat treated at 1300 degrees C mainly consists of coarse lamellar, fine lamellar, equiaxed gamma grains and beta phase. Homogenizing treatment at 1400 degrees C results in nearly fully-lamellar structure which shows colony-boundary gamma phase grains. The observations showed that small increases in W content result in changes in the amount and morphology of beta phase. For the alloy containing 2.1 at.% W, the beta phase has been observed in three types of morphology: blocky beta phase containing gamma needles and fine silicide particles inside; rod-like beta phase and fine needle-like beta phase. For the alloy variant containing 1.5 at.% W, only a very small amount of rod-like beta phase appeared. Different mechanisms are proposed to explain the occurrence of three types of beta phase, in that W may stabilize it by extending the primary beta phase to lower temperature as well as by yielding new phase zones. A few primary silicide particles have been found in the interdendrite region in the alloys containing more than 0.5 at.% Si, while coherent secondary silicides generally nucleated at the interfaces. (C) 1997 Elsevier Science S.A.

Angular correlation between grains of metastable TiSi2

TiSi2 is the primary silicide candidate as interconnect material in very largy scale integrated (VLSI) devices because of its low resistivity (15μΩ-cm) and relatively low processing temperature. While formation of TiSi2 from Ti-on-Si reaction couples can be accomplished easily and quickly at anneal temperatures above 550°C, below ∽650°C TiSi2 forms in the metastable C49 (base-centered orthorhombic; a=3.62Å, b=13.76Å, and c=3.605Å) 12-atom-per-unit-cell crystal structure with a characteristic resistivity of 65μΩ-cm. To achieve the low-resistivity C54 (face-centered orthorhombic; a=8.24Å, b=4.78Å, and c=8.54Å) phase, anneals above ∼650°C are required. It has been suggested that the decrease in resistivity of TiSi2 from the C49 phase to the C54 phase is a result of the reduced number of microstructural defects (defect separation changes from ∼30Å to 5μm) associated with the change of crystal structure. An understanding of the arrangement of the microstructural defects in C49 is needed to correlated the electrical properties of C49 and C54 correctly.

Features of the structure and properties of high-silicon corrosion-resistant steels and their weld joints

1. Intergranular corrosion in austenitic high-silicon steels is caused by the precipitation in tempering at the boundaries of the recrystallized grains of a dispersed phase consisting of carbides, silicides, and carbosilicides. In this case the primary silicides may serve as bases for formation of dispersed silicides and carbides. 2. In the austenitic-ferritic steels the formation of silicide phase in tempering occurs at the austenite-ferrite interface into the depth of the ferrite phase. In this case the intergranular corrosion resistance increases. 3. Stabilization of the austenitic steels with niobium also causes an increase in their intergranular corrosion resistance.