Amorphization and nanocrystalline Nb3Al intermetallic formation during mechanical alloying and subsequent annealing

In this paper the formation as well as the stability of Nb₃Al intermetallic compounds from pure Nb and Al metallic powders through mechanical alloying (MA) and subsequent annealing were studied. According to this method, the mixture of powders with the proportion of Nb-25 at% Al were milled under an argon gas atmosphere in a high-energy planetary ball mill, at 7, 14, 27 and 41 h, to fabricate disordered nanocrystalline Nb₃Al. The solid solution phase transitions of MA powders before and after annealing were characterized using X-ray diffractometry (XRD). The microstructural analysis was performed using scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM). The results show that in the early stages of milling, Nb(Al) solid solution was formed with a nanocrystalline structure that is transformed into the amorphous structure by further milling times. Amorphization would appear if the milling time was as long as 27 h. Partially ordered Nb₃Al intermetallic could be synthesized by annealing treatment at 850 °C for 7 h at lower milling times. The size of the crystallites after subsequent annealing was kept around 45 nm.     

Synthesis and structural characterization of nanocrystalline (Ni,Fe)3Al intermetallic compound prepared by mechanical alloying

Synthesis of (Ni, Fe)₃Al intermetallic compound by mechanical alloying (MA) of Ni, Fe and Al elemental powder mixtures with composition Ni₅₀Fe₂₅Al₂₅ was successfully investigated. The effects of Fe-substitution in Ni₃Al alloy on mechanical alloying process and on the final products were investigated. The structural changes of powder particles during mechanical alloying were studied by X-ray diffractometry, scanning electron microscopy and microhardness measurements. At the early stages, mechanical alloying resulted in a Ni (Al, Fe) solid solution with a layered nanocrystalline structure consisting of cold welded Ni, Al and Fe layers. By continued milling, this structure transformed to the disordered (Ni, Fe)₃Al intermetallic compound which increased the degree of L1₂ ordering upon heating. In comparison to Ni–Al system, Ni (Al, Fe) solid solution formed at longer milling times. Meanwhile, the substitution of Fe in Ni₃Al alloy delayed the formation of Ni (Al, Fe) solid solution and (Ni, Fe)₃Al intermetallic compound. The microhardness for (Ni, Fe)₃Al phase produced after 80 h milling was measured to be about 1170HV which is due to formation of nanocrystalline (Ni, Fe)₃Al intermetallic compound.     

The effect of Ti addition on alloying and formation of nanocrystalline structure in Fe–Al system

In this work, the effect of Ti addition on alloying and formation of nanocrystalline structure in Fe–Al system was studied by utilizing mechanical alloying (MA) process. Structural and morphological evolutions of powder particles were studied by X-ray diffractometry, microhardness measurements, and scanning electron microscopy. In both Fe₇₅Al₂₅ and Fe₅₀Al₂₅Ti₂₅ systems MA led to the formation of Fe-based solid solution which transformed to the corresponding intermetallic compounds after longer milling times. The results indicated that the Ti addition in Fe–Al system affects the phase transition during mechanical alloying, the final crystallite size, the mean powder particle size, the hardness value and ordering of DO₃ structure after annealing. The crystallite size of Fe₃Al and (Fe,Ti)₃Al phases after 100 h of milling time were 35 and 12 nm, respectively. The Fe₃Al intermetallic compound exhibited the hardness value of 700 Hv which is significantly smaller than 1050 Hv obtained for (Fe,Ti)₃Al intermetallic compound.     

Synthesis of nanocrystallite by mechanical alloying and in situ observation of their combustion phase transformation in Al3Ti

Elemental powders of stoichiometric Al₃Ti were mechanically alloyed (MA) in order to investigate the phase formation during the milling process. Furthermore the stability of MA powders were studied under transmission electron microscopy (TEM). The results indicate that a supersaturated Al(Ti) solid solution with nanocrystalline size has been formed after mechanical alloying for 360 ks in consuming the elemental powders of Al and Ti and no further phase transformation can be detected upon longer milling. The MA powders are unstable being irradiated by electron beams under the TEM observation, exothermically forming various intermetallic compounds. The combustion phase transformation processes and products are depending on the time of mechanical alloying. The structural changes and phase transformations during both mechanical alloying process and annealing process were also characterized by using X-ray diffraction measuring.     

Sintering behavior of mechanically alloyed Ti-48Al-2Nb aluminides

Ti-Al intermetallics have been produced using mechanical alloying technique. A composition of Ti-48Al-2Nb at % powders was mechanically alloyed for various durations of 20, 40, 60, 80 and 100 h. At the early stages of milling, a Ti (Al) solid solution is formed, on further milling the formation of amorphous phase occurs. Traces of TiAl and Ti₃Al were formed with major Ti and Al phases after milling at 40 h and beyond. When further milled, phases of intermetallic compounds like TiAl and Ti₃Al were formed after 80 hours of milling and they also found in 100 h milled powders. The powders milled for different durations were sintered at 785°C in vacuum. The mechanically alloyed powders as well as the sintered compacts were characterized by XRD, FESEM and DTA to determine the phases, crystallite size, microstructures and the influence of sintering over mechanical alloying.     

Microstructure development in Nb3Sn(Ti) internal tin superconducting wire

The authors have studied the phase formation sequences in a Nb₃Sn ‘internal tin’ process superconductor. Heat treatments were performed to convert the starting materials of tin, Ti–Sn, copper and niobium, to bronze and Nb₃Sn. Specimens were quenched at different points of the heat treatment, followed by metallography to identify the phases present and X-ray microtomography (XMT) to investigate the void volume and distribution. An unexpected observation of the microstructure development was the uphill diffusion of tin during the Cu–Sn reactive diffusion. Some defects likely to affect the superconducting performance of the wires were observed. Microscopy revealed the presence of a Ti–Sn intermetallic compound displacing the niobium filaments, and XMT revealed the formation of long pores in the longitudinal direction. Two types of pore formation mechanism, in addition to Kirkendall pores, are proposed. The phase and microstructure development suggests that low-temperature heat treatment (below 415 °C) will have significant influence on optimising the final superconducting properties.     

Nb3Sn material development in Russia

In the USSR and later in Russia, the main activities in technical superconductivity were concentrated in the institutes that belonged to the Ministry of Atomic Energy (Minatom). The development of new technologies shortly transferred to the large-scale industrial production of NbTi and Nb₃Sn superconductors in early 1970s. Two main technologies for multifilamentary Nb₃Sn strands were under investigation during that time – bronze-process and internal tin method. More than 25 ton of Nb₃Sn bronze-processed strands were produced for the fabrication of 90 ton of conductors for application in the magnet system of first in the world fusion facility (tokamak T-15) with magnet system based on the intermetallic compound. The characteristics of these strands and conductors have been briefly described. The requirements for the Nb₃Sn strands constantly increased and the main R&D on the enhancement of critical current density have been reviewed. For bronze-processed strands the increase of the tin content in large ingots was the crucial factor. The artificial doping of niobium filaments by niobium–titanium alloy was invented, which enabled to improve the workability of Nb₃Sn strands, with enhanced critical current density in high fields. For internal tin Nb₃Sn strands the main R&D were concentrated on the optimization of the layouts of the strand and on the multistage heat treatment because of the inevitable liquid phase formation which could result in severe distortion of the geometrical arrangement of the filaments and even in destruction of the whole strand. The main results of these investigations have been presented. The corresponding impact of these R&D on the design of bronze-processed and internal tin strands has been analyzed. The quantitative estimations of the grain size were made for bronze-processed and internal tin strands. It was shown that in bronze-processed and internal tin strands subjected to the standard ITER heat treatment characterized by two stages at 575 °C and 650 °C, the variation of Nb₃Sn grain size in the range of 30–300 nm could be observed. The correlations of microstructure and superconducting properties have been discussed. The ITER connected activities in Russia on the development of Nb₃Sn strands, which met the HP-II specification, have been outlined. The results of the ITER Model Coil Program have shown a degradation of the critical current of large cable-in-conduit conductors (CICC) built with Nb₃Sn strands. For this reason, the investigation on the strain dependence of critical current density in Nb₃Sn strands of different designs is of high interest and priority. The R&D on development of bronze-processed and internal tin Nb₃Sn strands with enhanced, by the nanostructured Cu–Nb material, mechanical strength have been reviewed.     

Thermodynamic aspects of nanostructured Ti5Si3 formation during mechanical alloying and its characterization

Mechanical alloying (MA) was used to produce Ti₅Si₃ intermetallic compound with nanocrystalline structure from elemental powders. The structural changes and characterization of powder particles during milling were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), particle size analyser (PSA) and microhardness measurements. MA resulted in gradual formation of disordered Ti₅Si₃ intermetallic compound with crystallite size of about 15 nm after 45 h of milling. Also a thermodynamic analysis of the process was carried out using Miedema model. The results showed that in the nominal composition of Ti₅Si₃ intermetallic phase (X _Si ?=?0·375), formation of an intermetallic compound has the lowest Gibbs free energy rather than solid solution or amorphous phases. So the MA product is the most stable phase in nominal composition of Ti₅Si₃. This intermetallic compound exhibits high microhardness value of about 1235 HV.     

Formation mechanism of NbSi2–Al2O3 nanocomposite subject to mechanical alloying

Silicide compounds such as NbSi₂ have many desirable properties such as high melting point, high resistance to oxidation and suitable electrical conductivity. However, they have limited practical use because of low ductility. To overcome this limit, we produced NbSi₂ based nanocomposite containing Alumina second phase by an exothermic reaction between Al and Nb₂O₅ in mechanical alloying of Al–Nb₂O₅–Si system. Structural and phase evolution throughout milling were investigated by using X-ray diffraction and microscopy methods. It followed that after 10 h of MA, the reaction between Al and niobium oxide began in a gradual mode and after around 40 h of milling; the reaction was successfully completed. The final product consisted of NbSi₂ intermetallic compound and nanocrystalline Al₂O₃ with a grain size of 15 and 45 nm, respectively. Microhardness and fracture toughness of nanocomposite were also measured which are greater than NbSi₂ intermetallic. As the result of this research we showed that high strength together with increased ductility could be gained in nanocomposite compounds.     

Thermodynamic analysis of NiTi formation by mechanical alloying

Disordered B2-NiTi intermetallic phase was produced from a mixture of Ni and Ti powders by mechanical alloying (MA). X-ray technique was used for phase analysis. The results indicated that Ni(Ti) solid solution can be formed earlier and changed to disordered B2-NiTi intermetallic phase after 60 h of MA. A thermodynamic analysis of the process was then carried out using Miedema model. The results showed that there is a thermodynamic driving force in Ni–Ti binary to form solid solution at all compositions and amorphous phase in the composition range X_Ni: 0.05–0.95 where X_Ni is mole fraction of Ni. However, the stable phase which has the minimal Gibbs free energy is solid solution compared to amorphous phase at all compositions. The results of MA were compared with thermodynamic analysis and it was indicated that the product of MA is the most stable phase in Ni–Ti binary system.     

Production of Fe3Al–TiC nanocomposite by mechanical alloying of FeTi2–Al–C powders

Abstract

The aim of the present work was to produce Fe₃Al/TiC nanocomposite by mechanical alloying of the FeTi₂30Al10C60 (in at-%) powder mixture. The morphology and the phase transformations in the powder during milling were examined as a function of milling time. The phase constituents of the product were evaluated by X-ray diffraction (XRD). The morphological evolution during mechanical alloying was analysed using scanning electron microscopy (SEM). The results obtained show that high energy ball milling, as performed in the present work, leads to the formation of a bcc phase identified as Fe(Al) solid solution and an fcc phase identified as TiC and that both phases are nanocrystalline. Subsequently, the milled powders were sintered at 873 K. The XRD investigations of the powders revealed that after sintering, the material remained nanocrystalline and that there were no phase changes, except for the ordering of Fe(Al), i.e. formation of Fe₃Al intermetallic compound, during the sintering process.     

Structural evolution of the Ti70Ni15Al15 powders prepared by mechanical alloying

Amorphization and crystallization behaviors of Ti₇₀Ni₁₅Al₁₅ powders during mechanical alloying (MA) and subsequent heat treatments are studied. Amorphous phase that cannot be obtained in the rapidly quenched ribbon is formed in the powders after MA for 60 h. Upon continuous heating of the amorphous powders in DSC, two exothermic events are observed. The first exothermic event corresponds to the crystallization of the amorphous matrix into a supersaturated α-Ti phase of hexagonal close-packed structure. The growth kinetic of the α-Ti phase is sluggish, resulting in the formation of nanostructured α-Ti matrix. The second exothermic event corresponds to the solid state transformation of the meta-stable α-Ti into the equilibrium phases, Ti₂Ni and Ti₃Al. Using the amorphous powders, Ti-based bulk materials with novel microstructures can be developed for structural applications.     

High temperature strength at 1773 K and room temperature fracture toughness of Nbss/Nb5Si3 in situ composites alloyed with Mo

High temperature compressive strength at 1773 K and room temperature fracture toughness have been studied in terms of microstructure, phase stability and solid solution hardening in Nb-Si-Mo in situ composites consisting of niobium solid solution and Nb₅Si₃. Molybdenum addition stabilizes the -Nb₅Si₃ phase and makes unstable Nb₃Si phase in the in situ composite. It is found that molybdenum has a strong effect to increase the yield stress of the present in situ composite at 1773 K due to solid solution hardening. Yield strength depends not only on chemical composition and volume fraction but also the Nb₅Si₃ phase itself. Room temperature fracture toughness is very sensitive to microstructure and the content of ternary alloying element, but not to the volume fraction of constituent phases within the composition ranges investigated. It is suggested that plastic deformation of Nb solid solution and interface decohesion is responsible for high fracture toughness in this alloy system. Details are discussed in relation to microstructural features and Molybdenum alloying.     

Phase transformations of Ni-15 wt.% B powders during mechanical alloying and annealing

The formation of Ni-B binary intermetallic compounds was investigated by mechanical alloying (MA) of the Ni-15 wt.% B (≈ Ni-48 at.% B) powder mixture and subsequent heat treatment. It was found that an interstitial Ni(B) solid solution was formed at the early stage of milling, followed by the formation of Ni₃B intermetallic compound after 25 h of milling. On further milling, the Ni₃B transformed to Ni₂B and o-Ni₄B₃ (orthorhombic). Phase transformation during heating of Ni(B) solid solution phase up to 800 °C could be represented by Ni(B) → Ni₃B → Ni₂B. Other intermetallics can be formed by heat treatment of Ni(B) solid solution at temperatures above 800 °C.     

Structural evolution during mechanical alloying and annealing of a Nb-25at%Al alloy

Mixtures of pure elemental Al and Nb powders of Nb-25at%Al composition was mechanically alloyed, and structural evolution during high energy ball milling has been examined. Al dissolved in Nb from the early stage of the ball milling, and amorphization became noticeable after longer than five hours of milling. However the dissolution of Al in Nb was not completed before the amorphization. No intermetallic phase formed during the mechanical alloying. Before complete amorphization, metastable nitride of Nb_4.62N_2.14 (i.e., -NbN) with hexagonal structure has formed in nanocrystalline size through nitrogen incorporation from ambient environment. The lattice parameter of Nb increased significantly (up to 3.3433 Å after 5 hours of milling) during the milling. Upon annealing above 950 °C, Nb₂Al became the dominant feature with the -NbN, and Nb₃Al did not form from the samples milled at ambient environment. Nb₃Al appeared only from a sample milled at Ar environment. Structural evolution during mechanical alloying of the Nb-Al system is critically dependent the upon milling environment.     

Microstructure and oxidation behaviour of TiAl(Nb)/Ti<Subscript>2</Subscript>AlC composites fabricated by mechanical alloying and hot pressing

TiAl-based intermetallic matrix composites with dispersed Ti₂AlC particles and different amounts of Nb were successfully synthesized by mechanical alloying and hot pressing. The phase evolution of Ti–48 at%. Al elemental powder mixture milled for different times with hexane as a process control agent was investigated. It was found that after milling for 25 h, a Ti(Al) solid solution was formed; also with increase in the milling time to 50 h, an amorphous phase was detected. Formation of a supersaturated Ti(Al) solid solution after 75 h milling was achieved by crystallization of amorphous phase. Addition of Nb to system also exhibited a supersaturated Ti(Al,Nb) solid solution after milling for 75 h, implying that the Al and Nb elements were dissolved in the Ti lattice in a non-equilibrium state. Annealing of 75 h milled powders resulted in the formation of equilibrium TiAl intermetallic with Ti₂AlC phases that showed the carbon that originates from hexane, participated in the reaction to form Ti₂AlC during heating. Consolidation of milled powder with different amounts of Nb was performed by hot pressing at 1000°C for 1 h. Only the presence of γ-TiAl and Ti₂AlC was detected and no secondary phases were observed on the base of Nb. Displacement of γ-TiAl peaks with Nb addition implied that the Nb element was dissolved into TiAl matrix in the form of solid solution, causing the lattice tetragonality of TiAl to increase slightly. The values for density and porosity of samples indicated that condition of hot pressing process with temperature and pressure was adequate to consolidate almost fully densified samples. The isothermal oxidation test was carried out at 1000°C in air to assess the effect of Nb addition on the oxidation behaviour of TiAl/Ti₂AlC composites. The oxidation resistance of composites was improved with the increase in the Nb content due to the suppression of TiO₂ growth, the formation and stabilization of nitride in the oxide scale and better scale spallation resistance.     

Nanocrystalline Al/Al12(Fe,V)3Si alloy prepared by mechanical alloying: Synthesis and thermodynamic analysis

Al–Al₁₂(Fe,V)₃Si nanocrystalline alloy was fabricated by mechanical alloying (MA) of Al–11.6Fe–1.3V–2.3Si (wt.%) powder mixture followed by a suitable subsequent annealing process. Structural changes of powder particles during the MA were investigated by X-ray diffraction (XRD). Microstructure of powder particles were characterized using scanning electron microscopy (SEM). Differential scanning calorimeter (DSC) was used to study thermal behavior of the as-milled product. A thermodynamic analysis of the process was performed using the extended Miedema model. This analysis showed that in the Al–11.6Fe–1.3V–2.3Si powder mixture, the thermodynamic driving force for solid solution formation is greater than that for amorphous phase formation. XRD results showed that no intermetallic phase is formed by MA alone. Microstructure of the powder after 60 h of MA consisted of a nanostructured Al-based solid solution, with a crystallite size of 19 nm. After annealing of the as-milled powder at 550 °C for 30 min, the Al₁₂(Fe,V)₃Si intermetallic phase precipitated in the Al matrix. The final alloy obtained by MA and subsequent annealing had a crystallite size of 49 nm and showed a high microhardness value of 249 HV which is higher than that reported for similar alloy obtained by melt spinning and subsequent milling.     

Formation mechanism of Fe<Superscript>3</Superscript>Al and FeAl intermetallic compounds during mechanical alloying

Fe and Al elemental powder mixtures with composition Fe⁷⁵Al²⁵ and Fe⁵⁰Al⁵⁰ were mechanically alloyed in a planetary ball mill under different conditions. The structural changes of powder particles were studied by x-ray diffractometery and scanning electron microscopy. Mechanical alloying of Fe⁷⁵Al²⁵ and Fe⁵⁰Al⁵⁰ first produced a fine Fe/Al layered microstructure which transformed directly to the corresponding intermetallic compounds, Fe³Al and FeAl, with nanocrystalline structure at longer milling time. No intermediate phase, i.e. solid solution, was formed during mechanical alloying as a precursor to the intermetallic phase. The rate of mechanical alloying process was found to be dependent on milling variables such as rotation speed of mill, ball-to-powder weight ratio and number of milling balls.     

Effect of Fe content on the mechanical alloying and mechanical properties of Al-Fe alloys

Al-Fe alloys with Fe contents ranging from 5 to 12 wt% are produced by a double mechanical alloying process (DMA) which consists of a first step of mechanical alloying (MA1) applied to elemental Al and Fe powders, with subsequent heat treatment of MA1 powders to promote the formation of Al-Fe intermetallic phases, and a second mechanical alloying step (MA2) to refine the intermetallic phase, and consolidation of the produced powders by combination of degassing and hot extrusion. The effect of Fe content on the process, as well as on the mechanical properties of the extruded alloys, has been extensively studied. The alloys produced by this process show excellent tensile strength and stiffness at room and elevated temperatures due to the strengthening of Al by intermetallics, as well as to the stabilization of the structure by inert dispersoids.     

Fabrication of Nb3Sn superconductors by the solid-liquid diffusion method using Sn rich CuSn alloy

Superconducting composite wires having thick Nb₃Sn layers (? 20 μm) and high current carrying capacities were fabricated by the diffusion reaction between Nb (solid) and Sn rich CuSn alloy (liquid): the solid-liquid diffusion method. Composite wires with a fine inner core of Cu 12 at % Sn alloy surrounded by Nb were produced by cold drawing and heat treated at about 700°C. The Sn rich intermetallic compounds which formed initially were transformed to Nb₃Sn in 50 ～ 100 h, as the Cu concentration in the CuSn alloy core increased due to the consumption of Sn. The process produced thick Nb₃Sn layers, in comparison with the bronze method, because of the high Sn content in CuSn alloy core. The mechanism of enhanced Nb₃Sn formation by Cu was also studied, and it was clarified that the Cu in CuSn alloy lowers the activity of Sn so that the formation of Sn poor intermetallic compounds Nb₃Sn becomes advantageous in the diffusion reaction as compared with other Sn rich compounds.