Microstructure Development of Directionally Solidified Nb-Ti-Si Based Ultrahigh Temperature Alloy

Integrally directional solidification of an Nb-Ti-Si based ultrahigh temperature alloy was performed in an ultrahigh temperature and high thermal gradient furnace with the use of ceramic crucibles. The microstructural evolution with the withdrawing rate increasing during directional solidification was revealed. The integrally directionally solidified microstructure was composed of couple grown lamellar (Nbss+(Nb,X)5Si3) eutectic colonies and a few hexagonally cross-sectioned (Nb,X)5Si3 columns (X represents...     

定向凝固Nb-Ti-Si基超高温合金的组织与室温条件断裂韧性

采用真空自耗电弧熔炼法制备了Nb-Ti-Si基超高温合金的母合金锭,在2050℃的熔体温度下实现了合金的有坩埚整体定向凝固.测定了电弧熔炼态与定向凝固试样的室温条件断裂韧性,采用SEM,EDS等方法分析了凝固速率V分别为10,20和50 μm/s的整体定向凝固组织、单边切口梁弯曲试样的断口形貌及裂纹扩展路径,并讨论了其断裂机理.结果表明:合金的整体定向凝固组织主要由沿着试棒轴向挺直排列的横截面为多边形的初生(Nb,X)5Si3 (X代表Ti,Hf和Cr元素)棒与耦合生长的层片状Nbss/(Nb,X)5Si3共晶团(Nbss表示铌基固溶体)组成.整体定向凝固显著提高合金的室温条件断裂韧性KQ,且V=50μm/s时的最高,达16.1 MPa·m1/2,较电弧熔炼态试样的KQ提高了50.5％.定向凝固试样中Nbss与(Nb,X)5Si3沿垂直于受力方向的定向排列以及粗糙的Nbss产生的裂纹桥接和偏转,增大了裂纹扩展阻力,从而提高了合金的室温条件断裂韧性.     

DIRECTIONALLY SOLIDIFIED MICROSTRUCTURE OF AN ULTRA-HIGH TEMPERATURE Nb-Si-Ti-Hf-Cr-Al ALLOY

The directionally solidified samples of an ultra-high temperature Nb-Si- Ti-Hf-Cr-Al alloy have been prepared with the use of an electron beam floating zone melting (EBFZM)furnace, and their microstructural characteristics have been analyzed. All the primary dendrites of Nb solid solution (Nbss), euteetic colonies of Nbss plus (Nb, Ti)3 Si//(Nb, Ti)5 Si3 and chains of (Nb, Ti)3 Si/(Nb, Ti)5 Si3 plates align along the growth direction of the samples. With increasing of the withdrawing rate, the microstructure is refined, and the amounts of Nbss (Nb, Ti)3 Si/(Nb, Ti)5 Si3 euteetic colonies and (Nb, Ti)3 Si/(Nb, Ti)5 Si3 plates increase. There appear nodes in the (Nb, Ti)3 Si/(Nb, Ti)5 Si3 plates.     

Nb-Mo-Si共晶合金的定向凝固组织特征

采用光悬浮区熔定向凝固技术制备了Nb-Mo-Si三元共晶合金,研究了试棒旋转速度和抽拉速度对Nbss-β(Nb,Mo)5Si3共晶组织形态的影响。结果表明籽晶棒的旋转对组织形态有显著影响,而料棒的旋转对组织形态的影响并不明显。当抽拉速度是5mm/h,籽晶棒不旋转时,得到了规则的Nbss-β(Nb,Mo)5Si3片层结构;而当籽晶棒旋转时,试棒中心处为先析出的球状Nbss和规则片层组织,沿径向向外,规则的片层组织逐步转变成不规则的碎块状组织。当籽晶棒不旋转,抽拉速度是1mm/h时,组织为不规则共晶组织,Nbss倾向于连成网状;当抽拉速度是3～5mm/h时,形成典型的规则片层组织;当抽拉速度是10mm/h时,具有片层组织的柱状晶和胞状晶共存。此外,从液/固界面特性及过冷度的角度对组织多样性进行了讨论。     

Mechanical alloying behavior of Nb–Ti–Si-based alloy made from elemental powders by ball milling process

Nb-Ti-Si-based alloy powders were prepared by mechanical alloying(MA) of elemental particles.The evolutions of morphology,size,phase constituents,crystallite size,lattice strain,composition and internal microstructure,etc.,of the alloy powders were analyzed by X-ray diffraction(XRD),scanning electron microscopy(SEM),energy-dispersive spectroscopy(EDS),laser particle size analyzer and transmission electron microscope(TEM) analyses.The alloy particles are gradually refined and their shapes become globular with the increase in milling time.The diffraction peaks of Nb solid solution(Nbss) phase shift toward lower29 angles during ball milling from 2 to 5 h,and after that Nbss diffraction peaks shift toward higher 29 angles with the increase in milling time from 5 to 70 h,which is mainly attributed to the alteration of the lattice parameter of Nbss powders due to the solution of the alloying element atoms into Nb lattice to form Nbss.During ball milling process,the decrease in crystallite size and increase in lattice strain of Nbss powders lead to continuous broadening of their diffraction peaks.A typical lamellar microstructure is formed inside the powder particles after ball milling for 5 h and becomes more refined and homogenized with the increase in milling time.After 40-h-ball milling,the typical lamellar microstructure disappears and a very homogeneous microstructure is formed instead.This homogeneous microstructure is proved to be composed of only supersaturated Nbss phase.     

Effects of V and Al on Microstructure and Room Temperature Fracture Toughness of Nb-Si In-situ Composites

The effect of alloying elements V and Al on microstructure and room temperature fracture toughness of an experimental Nb-Si in-situ composite was investigated. The Nb-Si alloys with different amount of V and Al were prepared by non-consumable arc-melting furnace. The experimental results showed that with the addition of V and Al, the microstructure of the Nb-Si materials transformed from Nbss+Nb3Si to Nbss+Nb5Si3 and the lattice parameters of Nbss phase decreased, and the alloying element V promoted the for...     

多相Nb硅化物的断裂韧度和高温力学性能

通过机械合金化和热压烧结制备了4种近理论密度的Nb-xSi-2Fe（x=3,6,10,16）原位复合材料。X射线衍射（XRD）分析表明：制备的复合材料均由铌固溶体（Nbss）相、Nb3S i相、Nb5S i3相和Nb4Fe3Si5相组成。各物相的平均晶粒尺寸为3μm,并且呈等轴状。Nb-Si-Fe复合材料随着S i含量的增加断裂韧度减小,Nb-Si-Fe室温断裂韧度大于11MPam1/2,Nbss相的塑性变形能够减小界面的应力集中延迟裂纹的形核从而改善这些复合材料的断裂韧度。Nb-Si-Fe复合材料在1 300℃时的抗拉强度随S i含量的增加而增加。复合材料在1 300℃时的抗拉强度为112~237 MPa,拉伸延伸率为54%~95%,室温断裂韧度大于11 MPam1/2。     

凝固速率对铌基共晶自生复合材料定向凝固组织的影响

铌基共晶自生复合材料（NBISC）经真空自耗电弧熔炼成母合金锭，采用高温度梯度的电子束区熔装置制备定向凝固的试样，分析其组织特征。结果表明：Nb基固溶体（Nbss）相、（Nb，Ti）3Si相和（Nb，Ti）5Si3相为NBISC材料的主要组成相；在电了束区熔条件下，随着电子枪移动速率的提高，NBISC材料共晶组织变细，组织中片层状的共晶团增多，块状或板条状的（Nb，Ti）3Si／（Nb，Ti）5Si3相尺度减小、数量增多，组织趋于规则、分布更均匀，组织的定向性增强，定向效果显著。     

碳化物对铌基多相合金力学性能和组织结构的影响

采用纳米压痕和原子力显微镜对Nb-21Ti-4C-x Al(x:0,5,10,15 at%,下同)、Nb-35Ti-4C及Nb-25Ti-8C合金Nbss、(Nb,Ti)C、Nb3Al及相界面的力学性能和变形行为进行了研究。研究表明:Ti、Al元素合金化能有效提高Nbss的硬度,且Al强化效果优于Ti;高硬度的碳化物、Nb3Al与Nbss界面结合良好是理想的增强相。热处理后(Nb,Ti)C内部析出二次Nbss,改善了碳化物的变形行为,提高了韧性;热处理后Nbss内部组织均匀化及位错密度降低导致Nbss硬度下降。Ti、Al、C原子含量变化及热处理对Nbss的弹性模量影响不大。     

高温热处理对Nb-Ti-Cr-Si基超高温合金显微组织的影响

为了研究高温均匀化及时效热处理对Nb-Ti-Cr-Si基超高温合金显微组织的影响,对样品进行均匀化处理,于1200-1500°C保温24h,随后于1000°C保温24h进行时效。结果表明,热处理后的组织主要由Nbss、（Nb,X）5Si3和Cr2Nb组成。随着均匀化处理温度的升高,电弧熔炼态的树枝状Nbss转变为等轴状,原先花瓣状的Nbss/（Nb,X）5Si3共晶组织消失,转变为分布于Nbss基体上的小块状（Nb,X）5Si3组织。Cr2Nb的形貌随均匀化处理温度的升高而发生明显变化。当均匀化处理温度达到1300°C以上,原先粗大的Cr2Nb发生溶解,在随后的冷却过程中在Nbss基体上沉淀析出细小、密集的针状Cr2Nb。经高温均匀化和时效复合处理后,Nbss基体上析出更为细小、密集的沉淀相Cr2Nb,使得Nbss、（Nb,X）5Si3和Cr2Nb相中Ti、Hf和Al元素的含量差别缩小。     

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。     

Effect of annealing on the microstructures and Vickers hardness at room temperature of intermetallics in Mo-Si system

The microstructures and Vickers hardness at room tempcrature of arc-melting processed intermetallics of Mo5Si3-MoSi2 hypoeutectic alloy and hypereutectic alloy annealed at 1200℃ for different time were investigated. Lamellar structure consisted of MosSi3 (D8m) phase and MoSi2 (C11b) phase was observed in all the alloys. For Mo5Si3-MoSi2 hypoeutectic alloy, the lamellar structure was found only after annealing and developed well with fine spacing on the order of hundred nanometers after annealing at 1200℃ for 48 h. But when the annealing time was up to 96 h, the well-developed lamellar structure was destroyed. For Mo5Si3-MoSi2 hypereutectic alloy, the lamellar structure was found both before and after annealing. However the volume fraction and spacing of the lamellar structure did not change significantly before and after annealing. The effects of the formation, development and destruction of lamellar structure on Vickers hardness of alloys were also investigated. When Mo5Si3-MoSi2 hypoeutectic alloy annealed at 1200℃ for 48 h, the Vickers hardness was improved about 19% compared with that without annealing and formation of lamellar structure. The highest Vickers hardness of Mo5Si3-MoSi2 hypereutectic was increasing about 18% when annealing at 1200℃ for 48 h.     

Effects of alloying elements on creep of TiAl alloys with a fine lamellar structure

Creep experiments were conducted on five powder-metallurgy TiAl alloys with fine grains (65–80 μm), fine lamellar spacings (0.1–0.16 μm), and different compositions [Ti–47Al (+Cr, Nb, Ta, W, Si)] at temperatures of 760°C and 815°C and stresses from 35 to 723 MPa. Results show that at a given lamellar spacing, replacing 1% Nb (atomic percent) with 1% Ta and replacing 0.2% Ta with 0.2% W induced little effect, but addition of 0.3% Si decreased the creep resistance by a factor of 3–4 under otherwise identical conditions. Field emission TEM was used to characterize the changes of microstructure and alloy element distribution before and after creep. It was found that thinning and dissolution of α₂ lamellae and continuous coarsening of γ lamellae were the main creep processes and the microalloying elements tended to segregate at lamellar interfaces, especially at ledges during creep. The effects of different alloying elements are interpreted in terms of the interaction of alloy segregants with misfit and/or misorientation dislocations at the lamellar interface. That is, the interaction retards the climb of interfacial dislocations and thus the creep process in the case of large segregants (Nb, Ta, W), but facilitates the climb and creep in the case of small segregants (Si).     

粉末冶金碳化物强化铌合金组织演变及其力学行为

采用机械球磨与热压烧结相结合的粉末冶金法对不同球磨时间Nb-35Ti-6Al-5Cr-8V-5C合金的粉末变形行为,微观组织结构和力学行为进行研究。结果表明:随着球磨时间的增加,Nb-35Ti-6Al-5Cr-8V-5C复合粉末中的块状金属颗粒首先变形为片状后在碰撞挤压作用下破碎成絮状,TiC粉末均匀的分布于片状金属粉末表面;Nb-35Ti-6Al-5Cr-8V-5C合金由Nbss和(Nb,Ti)C两相构成,各合金碳化物体积分数均为11%左右,Ti元素主要分布于Nbss晶界和碳化物内,Al、Cr、V元素主要分布于Nbss晶粒内,Nbss和(Nb,Ti)C相尺寸均随球磨时间增加而尺寸减小;Nbss晶粒细化及强化相碳化物弥散化导致合金的室温压缩力学性能和塑性变形能力显著提高,压缩变形后合金Nbss与碳化物具有良好的界面结合能力,但是碳化物内部存在明显的近似平行分布的裂纹;数据对比表明,粉末冶金法制备Nb-35Ti-6Al-5Cr-8V-5C合金的力学性能优于电弧熔炼法。     

Interface microstructure evolution and bonding strength of TC11/γ-TiAl bi-materials fabricated by laser powder deposition

Laser powder deposition was applied to fabricate the Ti–6.5Al–3.5Mo–1.5Zr–0.3Si (wt%)/Ti–47Al–2Cr–2Nb–0.2W–0.15B (at%) bi-material system. The as-deposited TC11 alloy shows a basket-wave-like morphology while the as-deposited γ-TiAl alloy consists of fully α₂/γ lamellar microstructures. Regarding the thermal mismatch between TC11 and γ-TiAl during processing, the interface microstructure evolution was concerned. The transformation pathway was illustrated. It is found that the content changes of Al elements and β-stabilizers Mo, Cr, and Nb are responsible for the evolution of microstructures at the interface. The fracture surfaces are located at the γ-TiAl side. The bi-material shows a brittle-fracture manner, with the ultimate tensile strength of 560 MPa.     

定向凝固对Nb-14Si-22Ti-2Hf-2Al-4Cr合金组织和高低温力学性能的影响

采用光悬浮炉在氩气环境下制备了成分为Nb-14Si-22Ti-2Hf-2Al-4Cr的定向凝固试棒,研究了凝固工艺对合金相组成、组织形貌和高低温力学性能的影响,并与电弧熔炼态的合金进行了比较。结果表明,定向凝固试样相组成与电弧熔炼态相比没有发生变化,均由Nb基固溶体(Nb solid solution, NbSS)和Nb/Si化合物Nb5Si3组成,定向凝固合金NbSS呈枝晶状,硅化物呈板条状沿着生长方向分布。与电弧熔炼态相比,定向凝固速度为15和10 mm/h的合金在1250 ℃的抗压缩强度从电弧熔炼态的290 MPa分别提高到约442和493 MPa;15 mm/h的定向凝固后,合金的室温断裂韧性从电弧熔炼态的12 MPa·m1/2增加到15 MPa·m1/2。通过光悬浮定向凝固法制备的该合金具有优良的高低温力学性能     

Ir–Nb–Si ternary refractory superalloys with a three-phase fcc/L12/silicide structure for high-temperature applications

Ir–Nb binary alloys doped with silicon have been used in this work to attain a three-phase fcc/L1₂/silicide structure. Typical Ir–Nb binary alloys, including a hypoeutectic Ir–10Nb, an eutectic Ir–16Nb, and a hypereutectic Ir–25Nb, were used as alloy bases, and Ir was further replaced by 5 at% Si. With the addition of Si, the microstructures of the Ir–(10–25)Nb–5Si ternary alloys contained three phases: fcc, L1₂, and compounds of Ir and Si (referred to silicide hereafter). Compressive tests from room temperature to 1500 °C showed that the Ir–10Nb–5Si alloy, with a predominant fcc microstructure, always had the highest deformation hardening rate, strength, and ductility; on the other hand, the Ir–25Nb–5Si alloy showed the worst performance. With the silicide in the microstructures, the damage sustained by the Ir–Nb–Si alloys at both room and high temperatures was dominated by interface debonding, which occurred between the fcc and the silicide or the L1₂ and the silicide. It is believed that the interface debonding is an instinct failure mechanism of Ir-based alloys. Additionally, a strong solid-solution hardening effect of Si acting on the fcc phase was found to occur without loss of ductility. A principle in the composition and microstructure design is proposed in this paper for further development of Ir-based alloys with Si addition. This principle is to saturate the fcc phase with Si and other alloying elements so as to achieve maximum solid-solution hardening and tie-in fine silicides homogenously distributed within the fcc by elimination of the grain boundary concentration of silicides.     

钛硅系铸造合金的微观组织演变

研究了钛硅系铸造合金的相组成、微观组织。X射线衍射试验表明,随着硅含量的增加,Ti5Si3相（ζ）的衍射峰越来越明显。同时,合金由固溶体的片状或网蓝片组织逐渐演变为典型的棒状共晶组织。结果表明,钛硅系铸造合金与传统的铸造钛合金有着截然不同的微观组织特征。     

Al含量对Nb-Ti-C合金组织的影响

利用真空非自耗电弧炉制备了不同Al含量的Nb-21Ti-4C(βNb-Ti)合金,研究了Al元素含量变化对Nb-21Ti-4C合金组织和晶格常数的影响。研究表明:铸态条件下合金由枝晶生长的Nb基固溶体(Nbss),碳化物((Nb,Ti)C),Nb3Al和枝晶间Nbss/MC共晶组织构成;Al元素部分固溶进入Nbss和MC中,随着Al含量增加,以Nb3Al相生长在Nbss内部。Al元素含量增加,使得Nbss枝晶先变大后变小。1100℃,24h热处理后,Nbss内部析出针状或片状碳化物和Nb3Al,导致Nbss晶格常数增大;枝晶间碳化物内部Nbss进一步长大,Nb原子的析出和Al元素固溶含量变化导致碳化物晶格常数改变。     

Mechanical properties and fracture behavior of an Nb-Silicide in situ composite

An Nb-Silicide in situ composite with a nominal composition of Nb-16Si-10Ti-10Mo-5Hf (at. %) was fabricated by mechanical alloying followed by hot-pressing sintering. The microstructure consisted of an Nb solid solution, Nb₅Si₃ and a small amount of Nb₃Si. This in-situ composite exhibited good balance of strength between ambient temperature and high temperatures; the ultimate tensile strength was 413 and 496 MPa at room temperature and 1200 °C, respectively. The tensile fracture behavior was dominated by cleavage of the Nbss and Nb₅Si₃ at 1200 °C and lower temperatures. However, the fracture behavior was governed by ductile rupture of Nbss at 1300 °C and higher temperature, which was ascribed to both the increased ductility of Nbss and the decreased interface strength. At 1400 °C and higher temperature, the material exhibited extensive plasticity or superplasticity; the dominant deformation mechanism was grain boundary sliding at 1400 °C and higher temperature.