Cr Nb Alloys Research Articles

Synergistic effects of trace silicon, calcium and cerium on the microstructure and properties of a novel Cu–Cr–Nb–Si–Ca–Ce alloy

In this paper, a novel Cu-0.60Cr-0.20Nb-0.05Si-0.02Ca-0.02Ce (CCNSCC, wt.%) alloy is designed. By comparing the differences in the properties and microstructure with the Cu-0.60Cr-0.20Nb (CCN, wt.%) alloy under the same process, the effects of trace Si, Ca, and Ce elements on the CCNSCC alloy are disclosed. After undergoing cold rolling by 80% and aging at 450 °C for 60 min, the CCNSCC alloy exhibits a tensile strength of 524 MPa and a softening temperature of 600 °C. These values represent an increase of 104 MPa and 90 °C respectively compared with the CCN alloy. After a multi-stage combined thermo-mechanical treatment including preaging at 400 °C for 30 min, the tensile strength and electrical conductivity of the CCNSCC alloy reach 618 MPa and 75.2%IACS respectively. Microstructure analysis shows the existence of rod-like and polygonal Cr8Nb3Si particles, as well as globular, ellipsoidal, and crescent-shaped (Ca,Ce)Cu5 particles. During the process of aging, the precipitation of the Cr phase occurs at the phase boundaries of Cr8Nb3Si particles. Additionally, significant quantities of fine Cr particles at the nanoscale surround the Cr8Nb3Si particles, exerting a beneficial influence on the strength and electrical conductivity of the CCNSCC alloy. Besides, the CCNSCC alloy has finer Cr particle size and lower volume fraction of recrystallized grains after aging in comparison to the CCN alloy. This indicates that the addition of trace Si, Ca, and Ce elements significantly inhibits the growth of Cr particles and occurrence of recrystallization. Plenty of dislocations are observed in different states of the CCNSCC alloy, and their interaction with the Cr precipitates is the primary factor contributing to the excellent mechanical properties of the CCNSCC alloy. The findings of this study provide valuable insights for improving the properties and microstructure of Cu–Cr–Nb alloys.

Laser additive manufacturing of Cu–Cr–Nb alloys by using elemental powder

Laser additive manufacturing of Cu–Cr–Nb alloys by using elemental powder

The Effects of Al Addition on the Oxidation Behavior of Ni–Fe–Cr–Nb Alloys at 800 °C in Air

The Effects of Al Addition on the Oxidation Behavior of Ni–Fe–Cr–Nb Alloys at 800 °C in Air

The role of the post-heat treatment on the Cr-based precipitates and related room temperature mechanical properties of the sintered Nb-lean Cu–Cr–Nb alloy

Nb-lean Cu–Cr–Nb alloys have been widely applied in electrical industries as the structural conductive material owing to their excellent combination of the mechanical and electrical properties at room temperature. In this work, the influence of temperature during fast sintering and post-heat treatment strategies on the precipitate evolution and room temperature mechanical properties of the Cu–3Cr-0.5Nb (wt.%) alloy was investigated. The increasing sintering temperature will improve the density and mechanical properties of the alloy without any obvious grain growth during this non-equilibrium process, while the post-aging at lower and higher temperatures can effectively control the formation and coarsening process of the Cr-based precipitates. Systematic characterization was then conducted on the microstructural evolution, especially the altering precipitates’ size and distribution, in the alloy at both as-sintered and the aged conditions, which is correlated to its room temperature tensile properties. Lower aging temperature will induce the nucleation of the new Cr2Nb precipitates without causing a significant coarsening of the existing precipitates like aging at higher temperature. Therefore, the low-temp. aged sample exhibits the highest tensile strength and elongation due to the enhanced Orowan strengthening mechanism induced by its unique precipitate microstructure.

Achieving high strength and ductility of a metastable β-titanium alloy via coupling thermomechanical processing and heat treatments

A metastable β-titanium Ti–Al–Mo–V–Cr–Nb alloy exhibits a synergistic increasing in strength and ductility via coupling thermomechanical processing and heat treatment in present work. Synergistic effect of the thermomechanical processing and heat treatment on microstructure evolution and mechanical properties of the metastable β-Ti alloy was systematically investigated. α+β rolled sample sheet consumes less time to finish recrystallization, and leads to more uniformly distribution and smaller size of β grains than β rolled sample after same solution treatment. Primary α phase in α+β rolled sample hinders the growth of β grains during solution treatment and the spacing distance of α lamella in α+β rolled sample is smaller than β rolled sample after aging treatment, enhancing the strength significantly. The small spacing distance of α lamella is due to an increase in precipitation density. The small size and homogeneous distribution of β grains provides moderate ductility. After solution treatment at 770 °C for 2 h and aging treatment at 525 °C for 6 h, α+β rolled samples exhibit higher ultimate tensile strength (1550 MPa) and better ductility (6.5 %) than β rolled sample. The relationship between the average grain size of β grains, the average spacing distance of α lamella, and the yield strength of the alloy is quantitatively calculated using mathematical methods. The mechanisms of synergistic enhancement of strength and ductility in the alloy is qualitatively analyzed. These findings facilitate manufacturing high-performance metastable β-Ti alloys at an industry scale using commercially available thermomechanical processing.

Enhanced hydrogen storage properties of Ti–Cr–Nb alloys by melt-spin and Mo-doping

Ti–Cr–Nb hydrogen storage alloys with a body centered cubic (BCC) structure have been successfully prepared by melt-spin and Mo-doping. The crystalline structure, solidification microstructural evolution, and hydrogen storage properties of the corresponding alloys were characterized in details. The results showed that the hydrogen storage capacity of Ti–Cr–Nb ingot alloys increased from 2.2 wt% up to around 3.5 wt% under the treatment of melt-spin and Mo-doping. It is ascribed that the single BCC phase of Ti–Cr–Nb alloys was stabilized after melt-spin and Mo-doping, which has a higher theoretical hydrogen storage site than the Laves phase. Furthermore, the melt-spin alloy after Mo doping can further effectively increase the de-/absorption plateau pressure. The hydrogen desorption enthalpy change ΔH of the melt-spin alloy decreased from 48.94 kJ/mol to 43.93 kJ/mol after Mo-doping. The short terms cycling test also manifests that Mo-doping was effective in improving the cycle durability of the Ti–Cr–Nb alloys. And the BCC phase of the Ti–Cr–Nb alloys could form body centered tetragonal (BCT) or face center cubic (FCC) hydride phase after hydrogen absorption and transform to the original BCC phase after desorption process. This study might provide reference for developing reversible metal hydrides with favorable cost and acceptable hydrogen storage characteristics.

Effect of fabrication methods on the microstructure and properties of a Cu–Cr–Nb alloy

In comparison to other Cu-based alloys, the high-temperature strength of the studied Cu-4.06Cr-1.25Nb alloy fabricated by the atmospheric melting method was improved significantly. After homogenization at 950 °C for 4 h, hot rolling by 80%, solid solution treatment at 940 °C for 4 h, cold rolling by 50%, and aging at 400 °C for 2 h, the tensile strengths of the Cu-4.06Cr-1.25Nb alloy fabricated by spark plasma sintering (SPS) at 20 °C, 400 °C, 450 °C, and 500 °C were 535 MPa, 256 MPa, 212 MPa, and 165 MPa, respectively. While they were lower than the alloys fabricated by atmospheric melting at high temperatures, such as at 400 °C (285 MPa), 450 °C (266 MPa), and 500 °C (230 MPa), which could be explained by the grain boundaries strengthening of refined grains. At room temperature, the studied alloy fabricated by SPS had many refined grains, resulting in high tensile strength. However, the grain-refined/coarsened boundaries became the crack sources and the crack propagation paths, which were not conducive to the high-temperature strength. These findings are helpful in developing high heat-resistant Cu–Cr–Nb alloys.

Evolution of the Microstructures and Properties of Cu‐xCr‐0.4Nb Alloys

The microstructures and properties of Cu‐xCr‐0.4Nb alloys were investigated by changing Cr composition and cooling mode. The as‐cast microstructures and fracture morphologies were observed by SEM. The morphology and size of the precipitated phase were observed by TEM. The hardness, electrical conductivity, and tensile strength of the resulting alloys were also tested. For Cu‐xCr‐0.4Nb alloys cooled in furnace, Cr2Nb particles were formed near the eutectic regions, and destroy the continuity of the eutectic regions. The Cu–Cr–Nb alloys casted in copper mold can significantly increase the solid solubility of solute elements. The microstructures are finer cellular crystals without the segregation of Cr2Nb particle. It makes the microstructures more homogeneous and greatly improves the hardness. After aging, Cr and Cr2Nb particles with the scale of 5 ≈ 10 nm were precipitated in the copper matrix. The hardness, conductivity and tensile strength can reach to 142HB, 90.52% International Annealed Copper Standard (IACS) and 427 MPa, respectively. In addition, after the alloy being softened, the Cr2Nb particles were found near the Cr particles by TEM, which inhibited the high‐temperature coarsening of precipitated Cr phase. The softening temperature of Cu‐1.0Cr‐0.4Nb alloy reaches 600 °C.

Microstructures and mechanical properties of CrNb, CrNbTi, and CrNbTaTi concentrated refractory alloys

The microstructures and mechanical properties in compression (25 °C, 1000 °C and 1200 °C) of CrNb, CrNbTi and CrNbTaTi are reported. The CrNb alloy consisted of a Cr2Nb Laves phase matrix and Nb-rich BCC precipitates. After additions of Ti (CrNbTi alloy) the microstructure inverts to a Ti-rich BCC matrix, with Laves phase precipitates inside the matrix phase. Further additions of Ta to CrNbTi refined and decreased the volume fraction of the Laves phase particles in the CrNbTaTi alloy. CrNb exhibited limited ductility at 25 °C and 1000 °C, but became very ductile at 1200 °C and offered the highest compressive strength at all studied temperatures. CrNbTi and CrNbTaTi had better ductility, but suffered from a reduction in compressive strength.

Copper-based alloys for structural high-heat-flux applications: a review of development, properties, and performance of Cu-rich Cu–Cr–Nb alloys

ABSTRACT This review examines the development and current state of Cu-rich Cu–Cr–Nb alloys commonly referred to as GRCop or Glenn Research copper alloys with emphasis on Cu–8Cr–4Nb (at%), or GRCop-84, and Cu–4Cr–2Nb, or GRCop-42. Recent additive manufacturing efforts have increased interest in GRCop alloys, and full-scale hardware has been fabricated using AM techniques and practical hot-fire tests have been conducted, but structure–property relationships are still under development. The development, processing, and current microstructure-property relationships of GRCop alloys are reviewed along with comparisons to similar high-heat-flux Cu alloys including NARloy-Z, GlidCop Al-15, AMZIRC, Cu–1Cr–0.1Zr, and Cu–0.9Cr. The review concludes with an assessment of future prospects for GRCop alloys and overview of advantages provided by additive manufacturing.

Effect of Nb addition on Fe-7Cr-Nb and Fe-15Cr-Nb metastable alloy formation and their thermal stability

Thermal stability of any structural nanomaterial is extremely important for its prospective high temperature applications. Here, we have investigated the influence of Nb content (up to 1at%) on the thermal stability of mechanically alloyed nanocrystalline Fe-Cr alloys for its possible nuclear first wall reactor application. The alloys were prepared by mechanical milling for 25 h in a high purity argon atmosphere (purity <10 ppm O2) at room temperature. The milling was performed in hardened steel grinding media using SPEX 8000M high energy mill and the powder to ball mass ratio was kept at 1:10. Evolution of disordered Fe-Cr-Nb solid solutions has been analyzed through estimating free energy (Gibbs) change as per Toop’s model, variation of precise lattice parameter and TEM-SAED study. The Fe-15%Cr-Nb alloys showed effective thermal stability due to the addition of only 0.25%Nb. The crystallite size of the stabilize alloy was ∼58 nm after annealing at 1000 °C. TEM grain size analysis also corroborated the same. The corresponding microhardness value was found to be quite high (7.8 GPa) which also correlated the stabilizing effect. The grain boundary segregation by oversize Nb atoms along with solid solution hardening by Cr is identified to play the pivotal role for such attractive thermal stability.

The serrated flow and recrystallization in dispersion hardened Cu–Cr–Nb alloy during hot deformation

The hot isothermal deformation of dispersion hardened Cu–Cr–Nb alloy was studied by varying strain rate and temperature during compression testing. The compressive stress-strain curves showed oscillations in the flow stress typical of type A, type B and type C serrations which are typical of Portevin-Le Chatelier effect. The serrated flow was attributed to the pinning of mobile dislocations by Cr2Nb precipitates. The presence Cr2Nb precipitates was also found to affect the dynamic recrystallization process during hot isothermal hot deformation.

Microstructure and Hot Corrosion Properties of Surface Plasma Alloyed Ti2AlNb-Based Alloys

Some unsatisfactory characteristics of Ti2AlNb-based alloys restrict its practical application, such as the insufficient high temperature oxidation, corrosion resistance and poor wear resistance. The double glow plasma surface chromizing can improve their wear resistance, however the hot corrosion properties of surface plasma chromised Ti2AlNb-based alloys were little investigated. The microstructure, composition and surface hot corrosion morphologies of Ti2AlNb-based alloy were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD of alloying layer indicated that the Cr-Nb alloy phase was the main composition, including the compact and uniform Cr2Nb phase, which was benefit to improve the hot corrosion properties of alloy.

Nanoquasicrystalline Al–Fe–Cr–Nb alloys produced by powder metallurgy

Nano-quasicrystalline Al–Fe–Cr based alloys produced by rapid solidification processes exhibit high strength at elevated temperatures. Nevertheless, the quasicrystalline particles in these systems become unstable at high temperature limiting the industrial applications. In early works, it was observed that the use of Nb or Ta increases the stability of the Al–Fe–Cr quasicrystalline phase delaying the microstructural transformation to higher temperatures. Thus, these nano-quasicrystalline Al-based alloys have become promising new high strength material to be used at elevated temperatures in the automotive and aero-nautical industries. In previous works, nano-quasicrystalline Al–Fe–Cr–Nb based alloys were obtained by rapid solidification using the melt-spinning technique. In order to obtain bulk alloys for industrial applications other fabrication routes such as powder production by gas atomization followed by compaction and extrusion are required. In the present work, the production of Al–Fe–Cr–Nb based alloys by powder atomization at laboratory scale was investigated. The powders obtained were sieved in different ranges of sizes and the microstructures were characterised by means of X-ray diffraction, scanning and transmission electron microscopy, and energy dispersive of X-ray analysis. Mechanical properties have been measured by compression tests at room temperature and at 250 C. It was observed that a very high temperature is required to produce these alloys by gas atomization; the icosahedral quasicrystalline phase can be retained after the atomization in powder sizes typically under 75 lm, and also after the extrusion at 375 C. The extruded bars were able to retain a very high strength at elevated temperature, around 60% of the yield stress at room temperature, in contrast with the 10–30% typically obtained for many commercial Al alloys

Effect of powder milling on mechanical properties of hot-pressed and hot-rolled Cu–Cr–Nb alloy

Abstract The present study is on the effect of mechanical milling of gas-atomized powders on mechanical properties of the hot-pressed and subsequently hot-rolled Cu–8 at% Cr–4 at% Nb alloy with a microstructure consisting of pure copper matrix hardened by intermetallic Cr 2 Nb precipitates. The mechanically milled powders result in lower sintering temperature during hot pressing compared to the atomized powders. The hot-pressed samples made from the milled powder exhibit significantly higher hardness and tensile strength, but lower ductility and electrical conductivity compared to that made from the un-milled gas-atomized powders. The hot rolling results in marginal increase in strength, but significant increase in ductility compared to the hot-pressed alloy for both the milled and atomized powders and it is attributed to the decrease in size of the pores and amount of porosities after hot rolling.

Spark plasma sintering of dispersion hardened Cu–Cr–Nb alloy powders

Abstract The densification behavior of dispersion hardened Cu–Cr–Nb alloy powders has been studied by spark plasma sintering using varying stress and temperature. The densification has been strongly inhibited by the presence of intermetallic Cr 2 Nb precipitates. The mechanically milled powders with disk shaped nature have resulted in higher relative density as compared to spherical shaped gas atomized powders in all the experiments. The hardness and compressive yield strength of the sintered alloy samples have been found to have a linear relationship with relative density and it was independent of process parameters for both the atomized and milled powders.

Enhancement of high temperature ductility of hot-pressed Cu–Cr–Nb alloy by hot rolling

Effect of hot rolling of the order of 90% reduction in thickness on the enhancement of ambient and high temperature mechanical properties of the vacuum hot-pressed dispersion hardened Cu–Cr–Nb alloy samples is studied. The vacuum hot-pressed samples are made by consolidation of alloy powders. The ambient and high temperature mechanical properties of the hot-pressed and hot-rolled samples are evaluated. Though the hot-rolled and hot-pressed materials exhibit comparable strength, high temperature ductility of the hot-rolled sample has greatly improved as compared to hot-pressed alloy. This attributes to the fragmentation of prior particle boundary network of the sintered materials and reduction in pore size and porosity level during hot rolling.

Densification and characterisation of W–Cr–Nb alloys prepared by sintering of mechanically alloyed nanocrystalline powders

Sintering of (W1−xCrx)90Nb10 (x = 0·3, 0·5 and 0·6) alloys, prepared through mechanical alloying of elemental powder blends, has been investigated. Relative density of >98% could be achieved by sintering at 1790°C for 5 h. Characterisation by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy has shown the presence of W rich solid solution and Cr2Nb phases in the microstructures of the sintered alloys. The densification of the W–Cr–Nb alloys is possible at relatively lower temperature due to use of nanostructured powder raw material obtained through high energy milling and liquid phase sintering promoted by the presence of Cr2Nb having relatively lower melting point (1730°C). The matrix grain size of the sintered alloys is decreased, whereas hardness is increased noticeably with the amount of Cr2Nb in the microstructure.

High temperature oxidation behavior of W–Cr–Nb Alloys in the Temperature Range of 800–1200 °C

A comparative study of high temperature oxidation behavior of the (W1−xCrx)90Nb10 (x=0.3, 0.5 and 0.6) alloys has been carried out at 800°C, 1000°C and 1200°C in static air. Microstructural study of the alloy samples conventionally sintered from the nanocrystalline elemental powders of W–Cr–Nb alloys shows the presence of a mixture of W-solid solution (Wss) and Cr2Nb phases. Comparison of the results of isothermal and cyclic oxidation tests shows the damage to be greater during the latter tests. The oxidation resistance during exposure at 800°C, 1000°C and 1200°C is found to be the highest for the (W0.4Cr0.6)90Nb10 alloy. Formation of WO3 appears to be responsible for poor oxidation resistance of (W0.7Cr0.3)90Nb10 alloy at 1200°C. On the other hand, growth of continuous Cr2WO6 scale appears to have significant role in protection against oxidation for the other two alloys at 1200°C. The formation of oxide scales containing Cr2WO6+NbWO5.5 and Cr2WO6+Nb2O5.3WO3 is found to be responsible for protection against oxidation at 800°C and1000°C, respectively.

Effect of powder oxidation on densification and properties of vacuum hot pressed Cu–Cr–Nb alloy

The densification behavior of oxidized Cu-8 at% Cr-4 at%Nb alloy powders by vacuum hot pressing and subsequent mechanical properties of alloy compares well with that of fresh powders with no oxides and hydrogen reduced powders sintered using same conditions. The reason for such observation is the presence of high vacuum and carbon atmosphere due to graphite die at the furnace temperature helping in the decomposition of oxide layer. Reduction of oxides of copper controls the overall densification process. A new mechanism for the densification of air-oxidized powders during vacuum hot pressing is also put forward. Present work indicates that reduction of powders is not needed if consolidated by vacuum hot pressing.