Microstructure and mechanical properties of Nb–Si alloys fabricated by spark plasma sintering

This paper deals with microstructural evolutions and mechanical properties of Nb-Si binaries containing dual-phase Nb/Nb5Si3 with Nb to Nb5Si3 fraction ratios of 90:10,80:20,70:30 and 50:50,prepared by spark plasma sintering(SPS).Dense Nb/Nb5Si3 samples with a relative density larger than 99.5% were obtained by SPS processing.The SPS samples consist of the Nb and Nb5Si3 phases with less than 3% fraction of NbO oxide.Hv at room temperature,and compressive strength at 1150℃ and 1250 1C of the bulk SPS alloys increase monolithically by enhancing fraction of the stiffening Nb5Si3 phase.For example,0.2% yield strength,σ0.2,increases from 175 MPa to 420 MPa at 1150℃ and from 110 MPa to 280 MPa at 1250℃,when the Nb5Si3 fraction increases from 10% to 50%.It is interesting that the fracture toughness,KQ,of the bulk SPS samples seems not to be sensitive to phase fraction.Heat treatment,however,plays a key role on the KQ as compared with that of the as-sintered state,at the corresponding Nb5Si3 fraction and considerably improves the KQ by about 100% for samples with the Nb5Si3 fractions of 10%-30%,and by about 50% for the sample with 50% Nb5Si3 fraction.     

Effects of minor B additions on tensile strength,fracture toughness and oxidation resistance of Nb–Si based alloys

The effects of minor additions of B on microstructure, ambient tensile strength, fracture toughness and high-temperature oxidation resistance of Nb–Si based alloys were investigated. The added contents were designed as 0.05, 0.10 and 0.20 ?at.%. The results show constituent phases in 0.05B and 0.20B alloys were Nbss, α-Nb₅Si₃; while only 0.10B alloy consisted of γ-Nb₅Si₃. Minor B was prioritized into Nbss and redundant one dissolved into silicide enhanced volume fraction of the silicide. With increasing concentrations of boron, the microstructure was refined at first and then coarsened, while the tensile strength was enhanced remarkably. Compared with other two alloys, the 0.10B alloy containing γ-Nb₅Si₃ shows both the best fracture toughness and oxidation resistance. The important roles of γ-Nb₅Si₃ in balancing overall properties are emphasized.     

Effects of alloying and high-temperature heat treatment on the microstructure of Nb–Ti–Si based ultrahigh temperatur e alloys

The phase co mpositions, microstructure and especi ally phase i nterfaces in the as-cast and heat-treated Nb– Ti–Si based ultrahigh temperature alloys have been investigated. It is shown that β(Nb,X)5Si3 and γ(Nb,X)5Si3 are the primary p hase s in the Nb–22Ti–16Si–5Cr–5Al (S1) (at%) and Nb–20Ti–16Si–6C r–4Al–5Hf–2B–0.06Y (S2) (at% ) alloys, respectively. The Nb solid solution (Nbss) is the primary phase in Nb–22Ti–14Si–5Hf–3Al–1. 5B –0.0 6Y (S3) (at%) alloy . An orientation relationship between Nbss and γ(Nb,X)5Si3 was determine d to be (1-10)Nb//(101-0)γ and [111]Nb//[0001]γ in the as-cast S2 and S3 alloys. Some original β(Nb,X)5Si3 transfor med into α(Nb,X)5Si3 because Al and Cr diffused from the β(Nb,X)5Si3 to Nbss during heattreatment at 1500 °C for 50 h in the S1 alloy. Mean while, Ti diffused from Nbss to β(Nb,X)5Si3, which induced a Ti to generate near the interface between Nbss and Ti-rich β(Nb,X)5Si3. The orientation relationship between the newl y-formed a Ti and previous Nbss was (110 )Nb//(1-10-1) αTi and [001]Nb//(12-3-1)αTi. Among the ( Nb,X)5Si3 phases , the contents o f Cr and Al in β(Nb,X)5Si3 are n earl y the same as those in γ(Nb,X)5Si3 but obviously hi gher than those in the α(Nb,X)5Si3, where as the content of Si in α(Nb,X)5Si3 is nearly the same a s that in γ(Nb,X)5Si3 but higher than that in the β(Nb,X)5Si3     

Effect of trace Zn, P and Mg additions on microstructure and mechanical properties of Nb-Si-Ti alloys

The effects of Zn,P and Mg additions on the microstructure and mechanical properties of Nb-22Ti-3Si alloys were studied. The phases of Nbss and Nb_3Si presented in Nb-22Ti-3Si(AC1),Nb-22Ti-3Si-0.2Zn(AC2) and Nb-22Ti-3Si-0.2Mg alloys(AC3). The Nb-22Ti-3Si-0.2P(AC4) alloy consisted of Nbss,Nb_3Si network and eutectic cell of Nbss/α-Nb_5Si_3.By the addition of Zn,the Nb_3Si network was broken and the volume fraction of Nbss increased from 92%to 96%.The values of fracture toughness of the alloy AC2 at ambien...     

Behaviors and interactions of strengthening and constraint for ductile Nbss matrix in Nb–Si based alloys

The ductile matrix in composites could be strengthened by incorporating brittle inclusions, while the constraint effect for matrix would become more obvious and detrimental to material toughening. The balance between strengthening and toughening was crucial to achieve excellent comprehensive mechanical properties. Upon the basis of microstructure of Nb–Si in situ composites, a unit rectangular cell containing elastic and isotropic Nb₅Si₃ particles was modeled, the Niobium solid solution (Nb_ss) matrix according with the Ramberg-Osgood constitutive relation was recognized as elastic-plastic. For Nb–Si based alloys, the effects of volume fractions of Nb₅Si₃ reinforcements, stress states and interfacial characteristics on the stress-strain field, flow localization and damage evolution were investigated by the finite element method (FEM). Further, the normalized work of rupture (χ) was calculated quantitatively as a function of volume fractions of Nb₅Si₃ phase as well as yield strength of Nb_ss matrix. These results would contribute to acquire more fundamental understanding of the strengthening and constraint effects which were exerted by Nb₅Si₃ reinforcements to ductile Nb_ss matrix, also an instructive guidance for the optimum design of Nb–Si based alloys.     

High-throughput synthesis of oxidation-resistant Nb–Si based alloy thin film by magnetron co-sputtering

A high-throughput method was applied to study oxidation behavior of Nb–Si based alloy using composition spread alloy film as combinatorial libraries. An extended range of composition gradients of Nb–Si based alloy film was deposited by (multi)magnetron co-sputtering. The as-deposited film was composed of amorphous phase. Cr₂Nb, Nb₅Si₃ and Nb_ss could be detected after annealing treatment. After oxidation at 1250 ?°C for 10 ?min and 20 ?min, the film composition space was divided into three regions of the distinct oxide scales. The compositions for establishment of the different oxides on Nb–Si based alloy was defined efficiently across the entire composition space by the combinatorial libraries.     

Improvement of oxidation resistance of Nb–Ti–Si based alloys with additions of Al,Cr and B at different temperatures

The effects of Cr, Al and B addition on the microstructure and high-temperature oxidation behaviors (at 1200, 1250 and 1300 ?°C) of Nb–Ti–Si based alloys were investigated. The results showed that the addition of Cr stabilized α-Nb₅Si₃, while Al promoted the formation of β-Nb₅Si₃ and adding B promoted the formation of γ-Nb₅Si₃. Among the three elements, Al and Cr were beneficial to oxidation resistance at 1200 ?°C, and B was favorable to the oxidation resistance at 1300 ?°C. At 1250 ?°C, Al and B had the same effects on the improvement of oxidation resistance. The ratio of these alloying elements might play an important role in enhancing oxidation resistance. The oxidation resistance of the three kinds of silicides was compared, and the sequence was: γ-Nb₅Si₃> β-Nb₅Si₃> α-Nb₅Si₃. To predict the effects of the investigated alloying elements on the oxidation resistance of Nb–Ti–Si based alloys in a wider range of concentration, an artificial neural network (ANN) model was established, showing excellent accuracy and generalization ability. With the instructions of the ANN model, the oxidation resistance can be optimized with less additions of alloying elements.     

高温均匀化对Nb-Ti-Si-Cr基超高温合金组织和成分分布的影响

研究了不同均匀化温度对Nb-Ti-Si-Cr基超高温合金组织和元素分布的影响;结果发现随着高温均匀化处理温度的升高,硅化物的组织形貌由共晶首先转变为细长条状,随后长大为长条状,Laves相Cr_2Nb的形貌由针状转变为粗大的不规则形貌,最后溶解。随着高温均匀化温度的升高,Ti在初生相Nbss中的含量呈现升高的趋势,而Cr在初生相Nbss中的含量呈现先升高后降低的趋势。     

Experimental studies and modeling for the transition from internal to external oxidation of three-phase Nb-Si-Cr alloys

A modified oxidation model, which assumes the cylindrical-like morphology of Cr_2Nb phase, has been proposed to examine the conditions to form external scale of three-phase Nb-base alloys in this paper. The oxidation behavior of three-phase Nb-base alloys was tested at 1250 ℃ for 1 h and 7 h, and the experimental results were compared with the simulated results. The results show that the external CrNbO_4 scale can be achieved by lengthradius ratio and volume fraction increasing for Cr_2Nb phase. The simulated results are consistent with the experimental results.     

Microstructures and mechanical properties of directionally solidified multi-element Nb-Si alloy

Nb-Si system in-situ composites with nominal composition of Nb-21Ti-16Si-7Cr-3Al-2Hf(mole fraction,%) were prepared by directional solidification technology in an optical floating zone furnace at the growth rates of 3,5,8 and 12 mm/h.The microstructures and mechanical properties were investigated as a function of the growth rate.The results show that the solidification microstructures consist of Nb solid solution(Nbss),α-Nb_5Si_3 andβ-Nb_5Si_3.Silicides with high aspect ratio grow continuously and straig...