Microstructure of Nb／Nb5Si3 in—situ composites

The Nb-10Si(mole fraction,%)alloy was fabricated using the vacuum arc-melting method and heat-treated at 1850℃ and 1550℃ for2-100h in Ar atmosphere.The microstructure of the alloy has been investigated using Xray diffractometry(XRD),scanning electron microscopy(SEM)equipped with X-ray energy dispersive spectrometry (EDS)and transmission electrom microscopy(TEM).The results show that 1550℃,100h is an optimum heat-treatment condition to acquire the equilibruum Nb Nb5Si3 two-phase microstructure.The microstructure of Nb-10Si alloy in the as-cast condition consists of continuous Nb3Si matrix and dispersed Nb particles,which implies that th alloy is in the metastable equilibrium state.In th case of 1850℃，2h heat-treatment the Nb particles are coarsened evidently.However,in the heat-treatment condition of 1550℃ for 25-100h the growth of Nb particles in unconspicuous.After heattreated at 1550℃，Nd3Si phase transforms into the equilibrium Nb5Si3 and Nb phase with the increase of heat-treatment time gradually.TEM obsevations reveal that the interface of Nb phase Nb5Si3 phase is clean and some twins with about 10nm in width are found.     

Synergistic toughening mechanism in 3Y–TZP/Nb composites

This paper focuses on the study of the flaw tolerance and resistance curve behaviour (R-curve) in 3Y–TZP/Nb composites containing indentation-induced small surface cracks. It has been found that toughening and resistance curve behaviour occurs as a result of the combined effects of crack bridging and transformation toughening. The ZrO₂–Nb composites are described as synergistically toughened composites due to anticipated interactions between crack bridging and stress-induced phase transformation. It has been found that these interactions produce a notable increase in toughness (K_ss ≈ 13 MPa m^1/2) greater than the sum of the combination that would be provided separately by the two types of reinforcement.     

Al对CNTs增强Nb/Nb5Si3复合材料组织和性能的影响

以Nb、Si、CNTs和Al粉末为原料,采用放电等离子烧结法(SPS)原位合成了Nb-20Si-2 mass%CNTs-xAl(x=0、1、2、3、4 mass%)复合材料。利用扫描电镜(SEM)、电子探针微区分析(EPMA)和X射线衍射(XRD)等对复合材料的组织结构进行了分析,研究了Al对CNTs增强Nb/Nb5Si3复合材料的组织和性能的影响。结果表明:Al能部分固溶于Nb5Si3和Nbss相中,多余的Al会与Nb反应生成AlNb3相,较均匀地分布中Nb5Si3相中,并大部分聚集在Nb/Nb5Si3的界面处。Al元素的加入能明显改善Nb/Nb5Si3复合材料的断裂韧性,随着Al含量的增加,复合材料的断裂韧性先升高后降低,其中Al含量为2 mass%时达到最大值,即6.94 MPa·m^1/2,相对于未加时提高了约56.9%。同时Al元素还能有效降低Nb/Nb5Si3复合材料高温氧化速度,提高高温抗氧化性能,而Al元素的加入量越高,其高温抗氧化性能越好。     

Mo effect on the phase stability and room-temperature fracture toughness of Nb/Nb_5Si_3 in-situ composites

Nb/Nb5Si3 in-situ composites are very attractive structural materials because these materials perform a good balance in mechanical properties, including high strength at high temperature (>1000℃) and reasonably high fracture toughness at room temperature. Metastable phase Nb3Si plays an important role in the properties of     

Synthesis of Ti5Si3-x Nb composites by the field-activated combustion method

The influence of an electric field on the combustion synthesis of Ti₅Si₃-x Nb composites (0≤ x ≤0.8) was investigated. Composites where x ≥ 0.8 can only be synthesized in the presence of a field. In the absence of a field, those systems where x=0.8 can result in a non-steady combustion wave causing an incomplete reaction. That is, an unstable wave propagates to the middle of the sample and then becomes extinguished. The wave velocity of the Ti₅Si₃-x Nb composites slightly increases with the imposition of an external electric field across the sample.     

碳纳米管增强Nb/Nb5Si3复合材料的组织和性能研究

采用放电等离子烧结法(SPS)原位合成了多壁碳纳米管(CNTS)增强Nb/Nb₅Si₃复合材料,研究了不同含量的碳纳米管对Nb/Nb₅Si₃复合材料的组织和性能的影响。研究表明：Nb/Nb₅Si₃复合材料的相组成主要为Nb、α-Nb₅Si₃和γ-Nb₅Si₃,当CNTS加入量达到2wt.%时开始出现了新相Nb₄C₃。复合材料的力学性能(压缩强度、断裂韧性)随着碳纳米管含量的增加而增加,加入2wt.%CNTs时达到最大值,压缩强度和断裂韧性提高幅度分别达到56%、31%;随后加入3wt.%CNTs时,压缩强度和断裂韧性都有所降低。复合材料断口的扫描电镜照片表明,复合材料的断裂模式主要为脆性解理断裂并有部分沿晶断裂,复合材料的增韧化作用主要是由于碳纳米管的拔出效应和桥联机制。     

A study of the effects of Hf and Sn additions on the microstructure of Nbss/Nb5Si3 based in situ composites

The phase stability and microstructures of Nb_ss/Nb₅Si₃ based in situ composites alloyed with Hf and Sn have been investigated. The Nb₅Si₃ silicide in the β and γ forms was the primary phase in the Nb–24Ti–18Si–5Cr–5Al–5Hf–2Mo and Nb–24Ti–18Si–5Cr–5Al–5Hf–5Sn–2Mo alloys studied in this work. In the as cast alloys, the formation of the Nb₃Si phase was suppressed by the addition of Hf. The structure of the Nb₅Si₃ phase was mainly affected by the Hf addition. The Hf-rich regions in the 5–3 silicide probably corresponded to the γNb₅Si₃. This silicide was stable up to 1500 °C in the presence of Hf. The Ti solubility in the Hf-rich Nb₅Si₃ was higher than that in the Nb₅Si₃. The Si concentration in the Nb₅Si₃ phase increased slightly and shifted closer to its stoichiometric composition by the addition of Hf. The alloying elements Hf and Sn preferentially substituted for Nb in the Nb₅Si₃ and Nb_ss, respectively. The Sn addition had a significant effect on the niobium solid solution leading to the formation of Sn-rich and Sn-poor parts in the solid solution in the as cast microstructure. In the presence of Sn, the Si solubility in Nb_ss increased considerably whilst the Cr solubility decreased. As a result of the decrease of the Cr solubility in Nb_ss, the Sn addition promoted the formation of Si-rich C14 Cr₂Nb Laves phase and stabilised this phase at 1300 °C. TiN and HfO₂ were formed below the surfaces of the heat treated alloys.     

Effect of Mo addition on the phase stability of β-Nb5Si3 phase

Effect of Mo addition on phase stability of β-Nb₅Si₃ in NbSi₂/Nb₅Si₃ in situ composites has been investigated using X-ray diffraction (XRD). The preferential occupancy of the Mo atoms in the β-Nb₅Si₃ structure was investigated with the Rietveld profile technique. It is suggested that Mo addition stabilizes the β-Nb₅Si₃ phase. The results were explained on the basis of selective substitution of Mo atoms in β-Nb₅Si₃.     

SPS熔铸工艺制备Nb/Nb5Si3原位复合材料

以Nb,Si粉末为原料,采用放电等离子烧结(SPS)技术,原位合成了密实的Nb/Nb5Si3复合材料。利用扫描电镜(SEM)、电子探针微区分析(EPMA)和X射线衍射(XRD)等手段对材料的组织结构进行了分析,并探讨了材料的结构形成机制。结果表明,合成的材料由近球状的Nb颗粒与Nb-Nb5Si3共晶组织组成;SPS过程中产生的放电等离子体使Si粉及Nb颗粒的表面熔化,熔融的Nb和Si在冷却过程中发生共晶反应而形成Nb-Nb5Si3共晶体,而未反应完的Nb颗粒则均匀分布在共晶组织中     

Al对Nb/Nb5Si3复合材料高温抗氧化性能的影响

采用放电等离子烧结制备Nb-20Si、Nb-20Si-10Al和Nb-20Si-15Al合金,并研究其在800℃的氧化行为,利用XRD和SEM分别对合金氧化膜的相组成及形貌进行了观察和分析.结果表明,在Nb-20Si-15Al的氧化膜中存在玻璃态SiO<,2>,使氧化膜平整,抑制了氧的扩散.     

Microstructure and high temperature strength at 1773 K of Nbss/Nb5Si3 composites alloyed with molybdenum

Nb_ss/Nb₅Si₃ composites alloyed with and without Mo, where Nb_ss denotes Nb solid solution, are prepared by various processes such as arc-melting (AM), isothermal forging (IF), directional solidification (DS) and powder metallurgy (PM), followed by heat treatment at 1973 and 2123 K. The microstructures and mechanical properties at 1773 K of the composites are investigated under various processing and heat treatment conditions. In binary alloys the microstructures of AM composites are characterized by a dispersed structure of Nb_ss in Nb₅Si₃ matrix, while those of PM composites are composed of an aggregate structure. In ternary alloys microstructures consist of a maze-like structure for AM composites, an equiaxed structure for IF composites and lamella structure aligned to the growth direction for DS composites. The yield strength at 1773 K of AM binary alloys is higher than that of PM binary alloys. The increase in yield strength at 1773 K of the ternary composites by Mo addition is attributable to solid solution hardening in Nb_ss. The highest yield strength at 1773 K is obtained with an aligned lamella microstructure produced by DS, which may be associated with fine lamella spacing. It is suggested that the yield strength at 1773 K of α-Nb₅Si₃ is superior to that of β-Nb₅Si₃ alloyed with Mo.     

Ta含量对Nb-Nb5Si3共晶合金的组织和压缩性能的影响

采用真空电弧熔炼方法制备了不同Ta含量的Nb-NbsSi3共晶合金,显微组织和压缩性能的研究表明：Ta主要固溶在Nb基体相中,随Ta含量的增加,NbsSi3相逐渐减少;Ta促使β-Nb5Si3相转变为α—NbsSi3相;Ta的加入有利于复合材料室温和高温强度的提高,Ta含量为5％（原子分数）的材料高温压缩强度最高.     

A thermo-gravimetric and microstructural study of the oxidation of Nbss/Nb5Si3-based in situ composites with Sn addition

The effect of Sn addition on the oxidation of the Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf–5Sn (at.%) alloy (JG6) in the as cast (AC) and heat treated (HT) conditions was studied at 800 °C and 1200 °C in static air using thermo-gravimetry and microstructural analysis. The oxidation kinetics, morphology and microstructure of the oxide scale and the microstructure of the bulk of the oxidised alloy were investigated. Oxidation occurred by inward oxygen anion diffusion. The oxidation of JG6 at 800 °C and 1200 °C is compared with the oxidation of Sn-free Nb–Ti–Si–Cr–Al–Mo–Hf alloys and is found to have been improved by the addition of Sn. At 800 °C pest oxidation, which was exhibited by the heat treated Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf alloy (JG4-HT), was eliminated by alloying with 5 at.% Sn. The elimination of pesting at 800 °C is attributed to the nature of the Nb solid solution in the alloy which consists of Sn-rich, Si-rich and Ti lean solid solution usually surrounded by Sn-poor, Si-poor and Ti-rich solid solution. The oxide scales that formed at 1200 °C on JG6 did not separate from the base metal and consisted of Nb₂O₅, TiO₂, SiO₂, HfO₂ and TiNb₂O₇. TiN, instead of TiO₂, and the (Nb,Ti)₅(Sn_1−xSi_x)₃ phase, which is considered as a ternary phase based on Nb₅Sn₂Si, are formed in the diffusion zone of the alloys JG6-AC and JG6-HT after oxidation at 1200 °C. The formation of these phases in the oxidised alloys JG6-AC and JG6-HT controlled the penetration of oxygen into the base material. The better oxidation performance of JG6-AC compared to JG6-HT at 1200 °C is attributed to the formation of Nb₃Sn in the former. It is suggested that the presence of the Sn-poor, Si-poor and Ti-rich Nb_ss in the microstructure is a key to the formation of the Nb₃Sn phase at the scale/diffusion zone interface in the JG6-AC oxidised at 1200 °C.     

烧结温度对放电等离子烧结Nb/Nb5Si3复合材料显微结构的影响

采用放电等离子烧结技术,分别在1500、1600、1680℃下对Nb-20Si试样进行烧结,研究了烧结温度对Nb/Nb<,5>Si<,3>复合材料显微结构的影响.结果表明,烧结温度高于1500℃时,烧结试样由Nb和Nb<,5>Si<,3>两相组成;随烧结温度的升高,烧结试样中Nb颗粒尺寸减小,试样致密度逐渐增大;烧结温度为1680℃时,显微组织中出现大量Nb+Nb<,5>Si<,3>共晶组织.     

烧结温度对SPS Nb/Nb5Si3原位复合材料组织结构的影响

以Nb、Si粉末为原料,采用放电等离子烧结（SPS）技术,原位合成Nb／NbsSi,复合材料,研究了烧结温度对材料组织结构的影响。结果表明,Nb／Nb5Si3复合材料的反应SPS烧结存在一个临界烧结温度Tc=1400℃;当烧结温度低于Tc时,合成的材料致密性较低,且存在Nb3Si中间相;当烧结温度高于Tc时,可获得所需的Nb／Nb5Si3复合材料,而且随着烧结温度的提高,Nb颗粒尺寸变小,材料的致密性高达99．59％。     

Hot corrosion behaviour of Nbss/Nb5Si3 in situ composites in the mixture of Na2SO4 and NaCl melts

Hot corrosion behaviour of Nb–16Si–24Ti–6Cr–6Al–2Hf (at.%) in the mixture of Na₂SO₄ and NaCl melts at 900 °C was studied. The results show that the corrosion kinetics of the alloy fit parabolic law. The oxides consist of a loose and porous outer layer and an internal oxidation zone. Outer oxides are mainly composed of TiO₂, TiNb₂O₇, Nb₂O₅, CrNbO₄ and SiO₂ while the internal oxidation zone is composed of TiO₂. Hot corrosion mechanism of the alloy in the presence of Na₂SO₄ and NaCl deposits is discussed.