Nb Solid Solution Matrix Research Articles

Microstructural evolution and mechanical behaviour of Nb–10Hf–15W alloy doped with Si

The microstructural evolution and mechanical properties of Nb–10Hf–15W–(0/1/3/5)Si (at.-%) alloys were investigated. The as-cast 0Si and 1Si alloys had a single Nb solid solution (NbSS) phase, while the 3Si and 5Si alloys consisted of NbSS matrix and the eutectic of NbSS plus Nb3Si. After heat treatment, the microstructure contained primary NbSS and NbSS + α-Nb5Si3 eutectoid in 3Si and 5Si alloys. Fracture toughness of as-cast and heat-treated samples at room temperature monolithically decreased with increasing Si content. However, the α-Nb5Si3 phase improved compressive strength of the NbSS alloy at 1250 and 1350°C, with no degradation in the ductility or formability of the NbSS/Nb5Si3 microstructure. Finally, the failure mechanisms of the NbSS/α-Nb5Si3 microstructure at room and high temperatures were discussed.

Distribution of Substitutional Alloying Elements and Interstitial Impurities in In Situ Multicomponent Composites Based on the Nb–Si System

The distribution of substitution alloying elements in the structure of hexagonal silicide Nb5Si3 of the γ-modification, which serves as a reinforcing phase of the composite, and in the Nb solid solution matrix of a high-temperature natural composite based on Nb–Si is studied. Studies were carried out on samples in a cast state and after homogenizing thermal treatment. A structural model of the γ-modification of silicide Nb5Si3 is constructed. On the basis of the crystal-chemical analysis of the structure of hexagonal silicide, possible causes of phase instability of the composite after thermal treatment are revealed.

Precipitation of γNb5Si3 in Nb-Si based ultrahigh temperature alloys

Hexagonal γNb5Si3 plates have precipitated from Nb solid solution in heat-treated Nb-Si based ultrahigh temperature alloys. The orientation relationships (ORs) and interfacial structures between γNb5Si3 precipitates and Nb solid solution matrix have been investigated by high-resolution transmission electron microscopy (HRTEM). The ORs have been determined as [111]Nb//[0001]γ and (101¯)Nb//(11¯00)γ with a near coherent interface. The interfacial structure has been interpreted with the Coincidence site lattice/Displacement shift complete lattice (CSL/DSC) model, forming an array of phase boundary dislocations (PBDs) on the interface. PBDs with Burgers vectors of b→=13<1¯21¯>Nb on (111)Nb plane might promote the precipitation of γNb5Si3.

An enhanced thermoelectric figure of merit for Sr(Ti0.8Nb0.2)O3 based on a Ruddlesden–Popper-polytype-induced microstructure

Platelet crystals based on Ruddlesden–Popper (RP) polytypes prepared by molten-salt synthesis were used to influence the microstructural development of Nb-doped SrTiO3 (STO;Nb) perovskite and, consequently, to change the thermoelectric properties. Electron microscopy revealed intergrowth of RP-polytypic phases within the STO;Nb solid-solution matrix phase. The morphology of the grains varied from isotropic to anisotropic, containing polytypic faults up to 100μm in length. Such a microstructure increased the thermoelectric figure of merit by 50% at 650°C, due to substantially enhanced electrical conductivity, slightly increased Seebeck coefficient and lowered thermal conductivity.

Alloying effects on the fracture toughness of Nb-based silicides and Laves phases

Nb-based in situ composites are characterized by a large volume fraction of silicides and Laves phase particles in an Nb solid solution matrix. The intermetallic hard particles can adversely affect the fracture resistance of the composites. Recent work has shown that certain alloy additions can improve the fracture resistance of Nb-based silicides, Laves phases and the Nb solid solution. In this paper, the effects of alloy additions on the fracture toughness of Nb-based silicides and Laves phases are summarized. Theoretical calculations have been performed to determine the influence of alloy additions on the bond order, unstable stacking energy, and the Peierls–Nabarro energy for selected slip systems in the intermetallics. The theoretical results will be used in conjunction with experimental data to elucidate the possible roles of alloy additions in the slip process and the fracture resistance in Nb-based silicides and Laves phases and to compare the observed behaviors against those in Ti-based silicides and Laves phases.

Effects of tungsten addition on tensile properties of a refractory Nb−18Si−10Ti−10Mo−xW (x=0,5, 10 and 15 mol.%)in-situ Composites at 1670 K

To investigate the effect of tungsten addition on mechanical properties, we prepared refractory (62−x)Nb−18Si−10Mo−10Ti−xW (x=0, 5, 10 and 15 mol.%)in-situ composites by the conventional arc-casting technique, and then explored the microstructure, hardness and elastic modulus at ambient temperature and tensile properties at 1670 K. The microstructure consists of relatively fine (Nb, Mo, W, Ti)5Si3 silicide and a Nb solid solution matrix, and the fine eutectic microstructure becomes predominant at a Si content of around 18 mol.%. The hardness of (Nb, Mo, W, Ti)5Si3 silicide in a W-free sample is 1680 GPa, and goes up to 1980 GPa in a W 15 mol.% sample. The hardness, however, of Nb solid solution does not exhibit a remarkable difference when the nominal W content is increased. The elastic modulus shows a similar tendency to the hardness. The optimum tensile properties of the composites investigated are achieved at W 5 mol.% sample, which exhibits a relatively good ultimate strength of 230 MPa and an excellent balance of yield strength of 215 MPa, and an elongation of 3.7%. The SEM fractography generally indicates a ductile fracture in the W-free sample, and a cleavage rupture in W-impregnated ones.

Fracture toughness and fatigue crack growth in rapidly quenched Nb-Cr-Ti In situ composites

In situ composites based on the Nb-Cr-Ti ternary system were processed by rapid solidification in order to reduce the size of the reinforcing intermetallic phase. Two-phase microstructures with small Cr2Nb particles in a Nb(Cr, Ti) solid solution alloy matrix were produced for several compositions that previous work showed to produce high toughness composites in cast materials. The fracture and fatigue behaviors of these composites were characterized at ambient temperature. The results indicate that the fracture resistance increases with a decreasing volume of Cr2Nb particles. Fracture toughnesses of the rapidly solidified materials with their smaller particle sizes were lower than for conventionally processed composites with larger particles of the intermetallic compound. The fatigue crack growth rate curves exhibit steep slopes and a low critical stress intensity factor at fracture. The lack of fracture and fatigue resistance is attributed to the contiguity of the intermetallic particles and the absence of plastic flow in the Nb solid solution matrix. The matrix alloy appears to be embrittled by (1) the rapid solidification processing that prevented plastic relaxation of residual stresses, (2) a high oxygen content, and (3) the constraint caused by the hard Cr2Nb particles.

The fracture toughness of niobium-based,in situ composites

The fracture resistance of Nb-Cr-Ti alloys orin situ composites of three different compositions, Cr2Nb, and a Nb-10Siin situ composite was studied at ambient temperature. The crack-tip deformation and fracture behaviors were characterized using near-tip measurement techniques and fractographic analyses. The relevant fracture and toughening mechanisms were identified and related to the microstructure. Despite fracture by a combination of cleavage and slip band decohesion, the Nb solid-solution alloy exhibited a resistance-curve behavior with a relatively high toughness and local ductility. The source of toughness was modeled and explained in terms of a cracking process that involved alternate slip band decohesion and cleavage. Thein situ composites, on the other hand, exhibited cleavage fracture but considerably lower toughness with little or no resistance-curve behaviors. The difference in the fracture behavior appears to arise from two factors: (1) the presence of a high constraint in the Nb solid-solution matrix in thein situ composites, and (2) the lack of plastic flow associated with cleavage of the constrained Nb solid-solution matrix.

Phase transformations in an in situ Nb-reinforced Nb3Al intermetallic composite

Abstract The development of two-phase Nb/Nb 3 Al in situ composite microstructures by thermal treatment in a Nb-6wt. %Al alloy, processed through powder-metallurgy techniques, is examined in detail using transmission and scanning electron microscopy. Observations reveal that the precipitation of Nb 3 Al in a heavily dislocated Nb solid solution matrix initiates at grain boundaries and progresses along the 〈110〉 and 〈211〉 directions in the matrix and Nb 3 Al precipitate, respectively; the precipitates eventually fuse into small, elongated grains with 1–10 μm dimensions. The evolution of the in situ composite microstructure from the hot-pressed equiaxed structure proceeds by a diffusion-controlled nucleation and growth transformation and not by a massive transformation. The lamellar microstructure of the alloy displays a five-fold increase in toughness over unreinforced Nb 3 Al primarily due to crack bridging and plastic deformation associated with the ductile Nb phase.

Nitridation of niobium alloys: interface effects

In this study metal matrix composites were fabricated by nitridation of solute-rich NbTi and NbZr alloys to form large volumes of nitrides. Studies of the kinetics nitride formation and of the nature of the matrix-nitride interface were carried out. Nb-47wt%Ti and Nb-47wt.%Zr alloys were nitrided for different times and temperatures to nitride preferentially the solute phases into Ti- and Zr-rich nitrides. X-ray diffraction analysis confirmed the formation of the nitrides in the Nb solid solution matrix, and microhardness measurements explicitly indicated the hardening effect to be due to the formation of the nitrides. Orientation relationships were obtained between the matrix and the nitrides, but the habit planes were not defined.