Interfacial structure and mechanical properties of hot-roll bonded joints between titanium alloy and stainless steel using niobium interlayer

The hot-roll bonding was carried out in vacuum between titanium alloy and stainless steel using niobium interlayer. The interfacial structure and mechanical properties were analyzed. The results show that the plasticity of bonded joint is improved significantly. When the bonding temperature is 800 °C or 900 °C, there is not intermetallic layer at the interface between stainless steel and niobium. When the bonding temperature is 1000 °C or 1050 °C, Fe-Nb intermetallic layer forms at the interface. When the bonding temperature is 1050 °C, cracking occurs between stainless steel and intermetallic layer. The maximum strength of -417.5 MPa is obtained at the bonding temperature of 900 °C, the reduction of 25% and the rolling speed of 38 mm/s, and the tensile specimen fractures in the niobium interlayer with plastic fracture characteristics. When the hot-roll bonded transition joints were TIG welded with titanium alloy and stainless steel respectively, the tensile strength of the transition joints after TIG welding is -410.3 MPa, and the specimen fractures in the niobium interlayer.

Formation of carbonitride precipitates in hardfacing alloy with niobium addition

Niobium, as the most effective second-phase forming element, was added in the Fe-Crl3-C-N hard- facing alloy to get carbonitride precipitates. Morphology and composition of carbonitride in the hardfacing alloy were studied by optical microscopy, scanning electron microscopy, and electron probe microanalyzer. The ther- modynamics and the effect on the matrix of the formation of carbonitride were also discussed. It was found that niobium carbonitrides are complex Nb（C, N） precipitate distributed on grain boundary and matrix of the hardfacing alloy. Under as-welded condition, primary carbonitride particles were readily precipitated from the hardfacing alloy with large size and morphology as they were formed already during solidification. Under heat treatment condi- tion, a large number of secondary carbonitrides can pre- cipitate out with very fine size and make a great secondary hardening effect on the matrix. As a result, addition of niobium in the hardfacing alloy can prevent the formation of chromium-rich phase on grain boundaries and inter- granular chromium depletion.

Effect of niobium alloying level on the oxidation behavior of titanium aluminides at 850°C

This work addresses the alloying of titanium aluminides used in aircraft engine applications and automobiles. The oxidation resistance behavior of two titanium aluminides of α2 ＋ γ （Ti3Al ＋ TiAl） and orthorhombic Ti2NbAl, recognized as candidates for high-temperature applications, was investigated by exposure of the alloys for 100 h in air. Thus, oxidation resistance was expressed as the mass gain rate, whereas surface aspects were analyzed using scanning electron microscopy in conjunction with energy-dispersive X-ray spectroscopy, and the type of oxidation products was analyzed by X-ray diffraction and Raman spectroscopy. The orthorhombic Ti2NbAl alloy was embrittled, and pores and microcracks were formed as a result of oxygen diffusion through the external oxide layer formed during thermal oxidation for 100 h.

Niobium alloying effect in high carbon equivalent grey cast iron

The effect of niobium on the formation of NbC phase and solidification structure in high carbon equivalent grey cast iron was investigated.The experimental results indicated that an increase in the niobium content is favorable to refining the graphite and eutectic cell;and the pearlite lamellar spacing is reduced.Based on the thermodynamic calculation the formation of NbC is prior to the eutectic reaction.The reduction in the pearlite lamellar spacing is mainly attributed to the decrease of eutectic temperature with the addition of niobium.Additionally,properties including hardness and wear resistance were improved after the addition of niobium.

Plasma Niobium Surface Alloying of Pure Titanium and its Oxidation at 900 ℃

A niobium-modified layer on pure titanium surface was obtained by means of double glow plasma surface alloying technique. The modified layer was uniform, continuous, compact and well adhered to the substrate. The niobium composition in the modified layer decreased gradually from the surface to the substrate. The oxidation behavior of the niobium-modified layer was investigated and com- pared with the untreated surface at 900 ~C for 100 h. Characterization of the layers was performed using X-ray diffraction and scanning electron microscope, respectively. The test results show that the oxidation behavior of pure titanium was improved by niobium alloying process. Niobium has a positive influence on the oxidation resistance.

Static oxidation behaviour of silicide coating on niobium alloy at high temperature

A silicide coating was prepared on the surface of the Nb521 alloy by the complex pack cemented method. The oxidation resistance properties of the present coating were examined by the static oxidation tests at 1 700 ℃ in air. The compositions and the microstructures of the coating before and after test were characterised and analysed through scanning electron microscopy(SEM),X-ray diffraction analysis(XRD),energy dispersive X-ray spectrometry(EDS) and electron probe microanalysis(EPMA),respectively. The present silicide coating can provide an effective protection for the Nb alloy for 25 h at 1 700 ℃ in air. The results show that the oxidation kinetics of the present silicide coating is parabolic. The diffusion of Si leads to the phase transformation and evolution during the oxidation.

THE ROLE OF NIOBIUM IN NICKEL-BASED SUPERALLOYS AND CHARACTERIZATION OF PM ALLOY EP741NP

The role of niobium in nickel-based superalloys is reviewed. The importance of niobium as a strengthener is discussed. New developments in nickel-based superalloys are also briefly mentioned, including some results that show improved resistance to sulfidation by niobium. Research results from a current program on the role of niobium in the Russian powder metallurgy alloy EP741NP are presented. Future research plans on the role of niobium in superalloys are also discussed.

Diffusion bonding behaviour of β-γTiAl alloys containing high niobium with Ti interlayer by spark plasma sintering

High niobium β-γ TiAl alloy(HNBG) was diffusion bonded using spark plasma sintering with pure Ti as interlayer. The joint microstructural evolution, growth kinetics and mechanical properties were investigated. The joint included three diffusion zones. The β/B2 phase formed in the Zone Ⅰ, α_(2)phase in the Zone Ⅱ, and β-Ti and α-Ti phases in the Zone Ⅲ. The thickness of β/B2 phase, the average grain size of α_(2)phase and the amount of β-Ti phase increased with the increase of bonding temperature or bonding time. The growth activation energies of β/B2 and α_(2)phases were 582 and 253 kJ/mol, respectively. The joint acquired at 1000 °C, 10 min and 10 MPa showed the maximum shear strength of 308 MPa. Fracture mainly occurred along the interfaces between Zone Ⅰ and HNBG alloy, and between Zone I and Zone Ⅱ. Fracture mechanism of the joint was characterized by brittleness rupture along the phase boundary.

Powder mixed electrochemical discharge process for micro machining of C103 niobium alloy

This work demonstrates the viability of the powder-mixed micro-electrochemical discharge machining(PMECDM) process to fabricate micro-holes on C103 niobium-based alloy for high temperature applications.Three processes are involved simultaneously i.e.spark erosion,chemical etching,and abrasive grinding for removal of material while the classical electrochemical discharge machining process involves double actions i.e.spark erosion,and chemical etching.The powder-mixed electrolyte process resulted in rapid material removal along with a better surface finish as compared to the classical microelectrochemical discharge machining(MECDM).Further,the results are optimized through a multiobjective optimization approach and study of the surface topography of the hole wall surface obtained at optimized parameters.In the selected range of experimental parameters,PMECDM shows a higher material removal rate(MRR) and lower surface roughness(R_(a))(MRR:2.8 mg/min and R_(a) of 0.61 μm) as compared to the MECDM process(MRR:2.01 mg/min and corresponding Raof 1.11 μm).A detailed analysis of the results is presented in this paper.

Fatigue small crack growth threshold determination of a high-Nb TiAl alloy at different temperatures by in-situ observation

The purpose of this paper is to estimate the fatigue crack growth threshold of a high-Nb TiAl alloy at the different temperatures based on scanning electron microscopy （SEM） in-situ observation. The results indicated that the fatigue crack growth threshold △Kth of a nearly lamellar high-Nb TiAl alloy with 8% Nb content at room temperature and 750℃ was determined as 12.89 MPa.m^1/2 and 8.69 MPa.m^1/2, respectively. The effect of the elevated temperature on the fatigue crack growth threshold cannot be ignored. At the same time, the early stage of fatigue crack propagation exhibited multicrack initiation and bridge-link behavior.

Near-net shape processing of spherical high Nb-TiAl alloy powder by gelcasting

Spherical Ti-45A1-8.5Nb-（W,B,Y） alloy powder prepared by an argon plasma process was near-net shape by gelcasting. In the non-aqueous system, methaerylate-2-hydroxy ethyl, toluene, benzoyl peroxide, and N,N-dimethylaniline were used as the monomer, solvent, initiator, and catalyst, respectively. To improve sintering and forming behaviors, many additives were included in the suspension. The concentrated suspension with a solid loading of 70vo1% was prepared. The high Nb-TiA1 powder was analyzed by electron microscopy and X-ray diffraction. It was found that the green bodies had a smooth surface and homogeneous microstructure, exhibiting a bending strength as high as 50 MPa. After sintering at 1480℃ for 2 h in vacuum, uniform complex-shaped high Nb-TiA1 parts were successfully produced.

Microstructure evolution of as-cast Nb-Ti-C alloys

Nb-Ti-C alloys with different Ti contents(4%-16%,mole fraction)were fabricated by vacuum non-consumable arc-melting method.The results indicate that the alloys contain niobium solid solution(Nb_(ss)),carbides(Nb_(2)C,(Nb,Ti)C)and eutectic of Nb_(ss)/MC.Carbides in the Nb-Ti-C alloys change from Nb_(2)C to(Nb,Ti)C with the increase of titanium content. Microstructures in the as-cast state include primary plate-like Nb_(2)C,eutectic of Nb_(ss)/Nb_(2)C,Nb_(ss) and secondary needle-like Nb_(2)C in Nb-5Ti-4C alloy,and other microstructures are primary Nb_(ss),eutectic of Nb_(ss)/(Nb,Ti)C.Morphology of carbides are changed apparently through adding different Ti contents,and morphology of Nb_(ss) changes from the cellular structure to dendrite.

Effect of Mn on microstructure of as-cast Ti47A18Nb(1～2)Mn alloys

The microstructures of Ti47Al8NbxMn (mole fraction, %) alloy with x=1 or 2 were studied on as-cast, as-HIPped and on heat-treated samples. The results showed that Mn element is β phase stabilizer and promotes the formation of β(B 2) phase. B 2 phase exists at the dendrite cores of the as-cast microstructures. This phase is metastable and can completely decompose into α and(or) γ during 1 200 ℃, 200 MPa, 4 h HIP process. In the as-HIPed alloy with 1% (mole fraction) Mn addition, the α 2+γ lamellae structure is interrupted and decomposed resulting in grain refined near gamma microstructure. The α 2+γ lamellae structure near interdendritic regions decomposes almost completely into γ grains, but near dendrite cores incompletely into γ grains with a small amount of α 2 particles or needles around or in it. However, compared with the alloy with 1% (mole fraction) Mn, more α 2 phase is retained in the alloy with 2% (mole fraction) Mn. α 2 phase contains more Mn and is more stable in the alloy with 2% (mole fraction) Mn than in that with 1%(mole fraction) Mn. This makes its grain refinement more difficult. The amount of α 2 phase decreases following (1 250 ℃, 7 h-1 150 ℃, 15 h) for 3 cycles heat treatment.

NEWLY-DEVELOPED AND CHEAP NANOCRYSTALLINE Fe-Cu-Nb-Mo-Si-B ALLOYS FOR SWITCHING MODE POWER SUPPLY

The synthetical soft magnetic properties were reported for newly-developed nanocrystalline Fe73.5Cu1Nb0.9 Mo2.1Si13.5B9 and Fe73.5Cu1Nb0.5 Mo2.5Si13.5B9 alloys. The levels of high-frequency losses of the new alloys are P3/100k=612 and 670 kWm-3, P2/200k=880 and 973kWm-3, P2/500k=4300 and 4600 kWm-3, P0.5/1000k=860 and 920 kWm-3, respectively. They are significantly lower than those of the superior power Mn-Zn ferrite H7c4. The dependence of core loss on frequency and amplitude flux density has been analyzed. The practical applications of the new alloys to switching mode power supplies with the output power of 1 and 2kW were reported.

Sulphidation of Fe-Nb,Co-Nb and Ni-Nb Alloys in H_2-H_2S at 700℃

The corrosion of alloys based on Fe,Co and Ni containing 15 and 30 wt-% Nb was studied at 700℃ in H_2-H_2S mixtures providing 10^(-8) atm S_2.The Fe and Co-based alloys formed duplex scales with an outer layer of pure base-metal sulphide and an inner layer containing both metals.The Nb addi- tions were not able to produce exclusive Nb sulphidation but yielded a significant decrease of the corrosion rates for both types of alloys with respect to the pure metals even though they were still much higher than that of pure Nb.The Ni-base alloys corroded rapidly forming a single complex scale layer,mainly due to the appearance of a liquid Ni-S solution.

TEMPERATURE DEPENDENCE OF EFFECTIVE MAGNETIC ANISOTROPY OF Fe-Nb-B ALLOY

The correlation between the effective magnetic anisotropy 〈κ〉 and the temperature T, 〈κ〉-T curve, for the Fe83Nb6B11 alloys as-cast and annealed at 460-580°C was investigated. The experimental results of the 〈κ〉-T curves for nanocrystalline samples were explained using the theory of exchange interaction between α-Fe grains.

Microstructure and magnetostrictive performance of NbC-doped <100>oriented Fe-Ga alloys

The〈100〉oriented Fe83Ga17 alloy rods with various NbC contents less than 1at%were prepared by the directional solidification method at a growth rate of 720 mm·h^-1. Low NbC-content was found to affect the oriented grain growth and slightly improve the〈100〉ori-entation. Flat grain boundaries in the alloys with low NbC contents less than 0.2at%became greatly curved at higher NbC contents, and a large amount of Nb-rich precipitates were observed in the alloys with high NbC contents. Small amounts of NbC, less than 0.2at%, resulted in an increase in magnetostrictive strain due to the improvement of the〈100〉orientation, and a high magnetostrictive strain value of 335 ×10^-6 under a pre-stress of 15 MPa was obtained in the 0.1at%NbC-doped alloys. The magnetostrictive performance obviously decreased with the NbC addition higher than 0.5at%, and the strain sensitivity under no pre-stress was lower than that in the binary Fe？Ga alloy.

Niobium Extraction from Baotou Iron Ore Containing Niobium

The experimental results of niobium enrichment in laboratory and in blast furnace in Baotou Iron and Steel Corporation were reported.The effects of temperature,constituents of slag and silicon content in hot metal on niobium recovery were studied and the results have been verified by blast furnace operation.Optical and electronic microscopy and EPMA was observed that at the metal-slag interface there is NbC deposit layer obstructed Nb entering into the metal,By raising the temperature of hearth and agitating the bath,the reduc- tion of niobium oxide in slag could be speeded up,the solubility of Nb in hot metal could increase and the deposited NbC layer became thinner and even disappeared,in result,Nb recovery was increased.A compre- hensive extraction process combining ore dressing with pyro-hydro-metallurgy was suggested for Baotou niobium resource which was difficult to be concentrated formerly.The methods of increasing niobium recovery were also suggested.

High-Temperature Sulfidation of Mo-Nb Alloys

The sulfidation rates and sulfide scales of pure Mo, pure Nb, and Mo Nb alloys are studied. By comparing the rates, an alloy with best sulfidation resistance is found. In addition, the sulfidation kinetics of Mo-50 %Nb is studied in detail.