Cu-Nb System Research Articles

Effect of amorphous complexions on plastic deformation of nanolayered composites

Interface-induced dislocation nucleation and interface sliding are two decisive mechanisms to understand plastic deformation of nanolayered composites. Taking the Cu-Nb system as a prototype, with atomic simulations, we reveal the atomic mechanism of dislocation nucleation and interface sliding after introducing the CuNb amorphous complexions with different compositions. Under tension loading, the initial nucleation sites correspond to the localized shearing region, and the amorphous composition can regulate the ordering degree of crystalline/amorphous interfaces. Under shear loading, the pathway of interface sliding corresponds to the low-energy region of the energetic landscape, and the shear resistance increases with the increase of Cu content owing to the enhancement of the sliding barrier. These findings reveal the influence of the amorphous complexions on plastic deformation, and provide theoretical guidance for the design of nanolayered composites.

Heterogenous activation of dynamic recrystallization and twinning during friction stir processing of a Cu-4Nb alloy

An interplay between high degree of shear deformation and deformation-induced heating occurs during friction stir processing (FSP) of metals. In medium-to-low stacking fault energy Cu alloys, this can lead to a complex spatially heterogenous activation of dynamic recrystallization (DRX) and twinning mechanisms. Within the Cu-Nb system, the presence of Nb is further expected to influence the DRX mechanism of the Cu matrix. However, the microstructural changes induced by the co-deformation of Nb during FSP are still not well understood. Therefore, this study uses a combination of multimodal microstructural characterization, solution thermodynamics-based predictions, and computational crystal plasticity simulation to reveal the various microstructural evolution mechanisms that can occur during FSP of a Cu-4at%Nb binary model alloy. The formation of softer DRX zones, and harder shear localization regions are revealed using electron backscatter diffraction, transmission electron microscopy, atom probe tomography, and crystal plasticity modeling.

Use of pure vanadium and niobium/copper inserts for laser welding of titanium to stainless steel

Niobium and vanadium have high metallurgical compatibility with titanium and therefore can be used as inserts to avoid the accumulation of brittle intermetallic phases such as Fe2Ti during the fusion welding of titanium alloys with steels. In the present study, the continuous double pass welding of 1 mm thick Ti-6Al-4V alloy and 316 L stainless steel plates through several mm wide pure vanadium or niobium insert was studied. In case of a vanadium insert, a beam offset on the vanadium was found to produce cold crack formation in vanadium/316 L melted zones containing more than 40 wt.% V despite the absence of σ phase. Whereas a centered beam position and offset on the steel side produced crack-free melted zones still composed of (Fe, V) solid solutions, and exhibiting a ductile fracture in the unmelted vanadium insert at UTS of 493 ± 25 MPa. The niobium insert produced brittle niobium/316 L melted zones with UTS of 160 ± 10 MPa, because of the formation of Fe2Nb and Fe7Nb6 layers at the niobium/316 L interface. The addition of a copper insert between niobium and steel allowed avoiding the development of Fe-Nb brittle intermetallics thanks to the absence of brittle phases in Cu-Nb system. For optimal welding conditions, a ductile fracture occurred in the copper interlayer at maximal UTS of 255 ± 10 MPa.

Effect of Nb Solute Concentration on Crystallite Size Refinement and Strength Enhancement in Mechanically Alloyed Cu-Nb Alloys

Cu-Nb alloys with a Nb concentration range of 0 wt%∼30 wt% were prepared by mechanical alloying (MA) at room temperature. The effects of Nb content on the crystalline refinement process and mechanical properties of the immiscible Cu-Nb system were investigated by X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM) with energy-dispersive X-ray detection (EDX), transmission electron microscopy (TEM) and microhardness measurement. Results show that the completed dissolution of Nb in Cu can be achieved in the samples with Nb content less than ∼11 wt% after 100 h milling, although the equilibrium solubility level is nearly zero. The grain size refinement capability of MA-ed Cu-Nb powder is enhanced with increasing Nb content up to 30 wt%. This is because the susceptibility to recovery process becomes reduced, when the amount of Nb solutes segregated into the dislocation of Cu phase is increased. Cu-30wt%Nb powder milled for 100 h, the average Cu grain size is only ∼6 nm. The microhardness of the samples shows an enhancement with increasing Nb concentration. The main strengthening mechanisms of the MA-ed Cu-Nb alloys are linked to the grain size reduction and the dissolution of Nb into Cu matrix.

Construction of metallurgical interface with high strength between immiscible Cu and Nb by direct bonding method

Cu/Nb joints are promising to be used in resistance to radiation damage because of their outstanding properties. Due to the immiscibility in the Cu-Nb system, metallurgical interface is difficult to construct directly between Cu and Nb. In this paper, a direct bonding method was used to construct the metallurgical interface directly between Cu and Nb rods and prepare Cu/Nb joints. The key point of the method is to anneal Cu-Nb assemblies at the temperature between 92% and 98% of the melting point of Cu (TmCu = 1356 K) for 3 h under a pressure of 106 MPa. The maximum tensile strength and bending strength of the obtained Cu/Nb joints are 222 MPa and 47 MPa, respectively. The Micro-test results show that a diffusion layer with a thickness of about 36 nm forms between Cu and Nb, indicating that a metallurgical interface is successfully constructed between Cu and Nb. The high temperature structure of Cu at the temperature close to the TmCu provides diffusion paths probably. In addition, a thermodynamic model was established for the direct bonding of Cu and Nb. Through the thermodynamic calculations and differential scanning calorimetry (DSC) tests, the storage energies in the Cu and Nb rods are proved to serve as the thermodynamic driving force for the diffusion between Cu and Nb.

New insights on the formation of supersaturated Cu-Nb solid solution prepared by mechanical alloying

The deformation-driven alloying mechanisms in the immiscible Cu-Nb system during mechanical alloying (MA) process have been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) observations. It is found that the dominating alloying mechanism changes at the different stages of milling. Initially, the alloying process is mainly governed by interfaces and dislocations, which supply fast diffusion channels between phases. In the intermediate stage, Nb particles with sizes below 8nm undergo a bcc to fcc allotropic transformation, and the interdiffusion between fcc-Nb and fcc-Cu is promoted due to the similarity of lattice structures. Further milling generates disclination defects, which contribute to the formation of fragments or subgrains with sizes below 5nm. The introduction of such defects and fragments benefits the formation of a complete Cu-Nb solid solution. The formation kinetics of solid solution is analyzed by Johnson-Mehl-Avrami equation, which suggests that the speed of the diffusion of Nb atoms into Cu is very slow.

Determination of Activities of Niobium in Cu-Nb Melts Containing Dilute Nb

The activity coefficients of niobium in Cu-Nb melts were measured by equilibrating solid NbO2 with liquid copper under controlled oxygen potentials in the temperature range of 1773 K to 1898 K (1500 °C to 1625 °C). Either CO-CO2 gas mixture or H2-CO2 gas mixture was employed to obtain the desired oxygen partial pressures. Cu-Nb system was found to follow Henry’s law in the composition range studied. The temperature dependence of Henry’s constant in the Cu-Nb melts could be expressed as follows: $$ \log \gamma_{{{\text{Nb(l}})}}^{0} = \frac{3305}{T} + 0.72. $$ The partial molar excess Gibbs energy change of niobium in Cu-Nb melts can be expressed as follows: $$ \Delta \overline{G}_{\text{Nb}}^{\text{ex}} = 63,280 + 13.2T \left( {\text{J/mol}} \right) $$

La France championne du monde de la consommation de psychotropes… aussi en hémodialyse ?

The deformation-driven alloying mechanisms in the immiscible Cu-Nb system during mechanical alloying (MA) process have been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) observations. It is found that the dominating alloying mechanism changes at the different stages of milling. Initially, the alloying process is mainly governed by interfaces and dislocations, which supply fast diffusion channels between phases. In the intermediate stage, Nb particles with sizes below 8 nm undergo a bcc to fcc allotropic transformation, and the interdiffusion between fcc-Nb and fcc-Cu is promoted due to the similarity of lattice structures. Further milling generates disclination defects, which contribute to the formation of fragments or subgrains with sizes below 5 nm. The introduction of such defects and fragments benefits the formation of a complete Cu-Nb solid solution. The formation kinetics of solid solution is analyzed by Johnson-Mehl-Avrami equation, which suggests that the speed of the diffusion of Nb atoms into Cu is very slow.

Early Stages of Crystallization in Phase-Separated Amorphous Copper-Niobium Alloy Thin Films

The binary Cu-Nb system is rather interesting, because Cu and Nb are virtually immiscible in the solid state, exhibit a relatively strong clustering (phase separating) tendency in the liquid state, and therefore can only be alloyed in a highly metastable form, for example, by vapor quenching techniques such as sputtering. In this article, metastable amorphous Cu-Nb alloy thin films with nominal compositions of Cu-45 at. pct Nb have been deposited by magnetron cosputtering. The amorphous phase in these thin films exhibits nanoscale phase separation in the as-deposited condition. The early stages of devitrification of this amorphous phase have been investigated using three-dimensional atom probe tomography coupled with transmission electron microscopy. The results indicate that devitrification of this phase-separated amorphous phase is initiated by the formation of nanoscale fcc Cu-rich grains uniformly distributed in the amorphous matrix. These nanoscale fcc Cu-rich grains are often faulted and appear to retain some Nb supersaturation. The role of the nanoscale phase separation in the as-deposited amorphous film on the subsequent devitrification is discussed in the context of free energy–composition plots for this system.

Shock-induced reaction synthesis of isomorphous (Cu-Ni) and immiscible (Cu-Nb) compounds

Shock compression of Cu-Ni and Cu-Nb elemental powder mixtures was investigated to study the shock-induced chemical reaction behavior of material systems with very low heats of formation and to synthesize isomorphous, as well as otherwise-immiscible, compounds. Shock loading experiments were performed using a 12-capsule plate-impact recovery fixture, with explosive loading at 0.9 to 1.6 km/s impact velocities. The Cu-Ni powder mixtures revealed formation of an isomorphous Cu-Ni solid-solution alloy with a fine dendritic microstructure, formed via a mechanism involving intense mechanical mixing and melting of elemental reactants. The extent of the reaction was dependent on the shock strength, and the chemistry of the product was observed to depend on the morphology of the powders due to its effect on the crush strength. In the case of Cu-Nb powder mixtures, submicronscale mechanical mixing was observed between the reactants, with possible dissolution of as much as 10 wt pct solute in each component. Complete alloying of copper and niobium was also observed, as indicated by transmission electron microscopy (TEM)/energy-dispersive X-ray (EDS) and X-ray diffraction (XRD) analysis; however, the alloyed region showed the presence of Mo and other impurity elements from the stainless steel capsule. These additives may, in fact, be responsible for stabilizing the ternary compound, in an otherwise immiscible Cu-Nb system.

Alloying in an Immiscible Cu-Nb System upon Solid-State Reaction

ABSTRACTIn an immiscible Cu-Nb system, an amorphous alloy and two metastable crystalline phases were obtained by solid-state reaction of Cu-Nb multilayered films, and the formation of the alloy phases was found to be quite sensitive to the average composition of the films. At Nb concentration of 75at%, amorphization was achieved by 250°C annealing for 50 min, while in the films with compositions of 70 and 80 at% Nb, a simple cubic (a=0.405±0.005nm) and an orthorhombic phase (a=0.421, b=0.334, c=0.291±0.005nm) were observed, respectively. Thermodynamic calculation was conducted for the Nb-Cu alloy phases and the energetic state of the multilayers, which consisted of 9 Cu/Nb bilayers. It turned out that the excess free energy originating from the interfacial atoms could raise the multilayers to an energy level being higher than that of the amorphous or/and metastable crystalline phases both with a convex shape, and thus provided a major driving force for alloy phase formation in such immiscible system.

Wetting characteristics of copper on niobium

Sessile drop experiments have been performed in a temperature range between 1090 and 1300° C, aiming to study the wetting of niobium by liquid copper and the influence of different atmospheres (argon, hydrogen, vacuum), crystallographic orientation, roughness, and oxygen doping of niobium on the wetting angle. At the peritectic temperature in the Cu-Nb system (1090° C) the contact angle is high,θ=67°, denoting poor wetting. With increasing temperature,θ decreases steadily for all samples. The wetting is at its lowest for the oxygen-doped samples and at its best for samples annealed under a hydrogen atmosphere. A new mechanism is suggested and discussed for oxygen degassing of niobium through liquid copper.