Manufacturing and Microstructural Evolution of Mechanuically Alloyed Oxide Dispersion Strengthened Superalloys

Mechanical alloying is a process in which mixtures of powders are severely deformed until they form atomic solutions. Inert oxides can also be introduced to form a dispersion of fine particles which help strengthen the consolidated product. Significant quantities of iron and nickel‐base alloys, with unusual properties, are produced commercially using this process. The total true strain during mechanical alloying can be as large as 9; there is proof that this leads to mixing on an atomic scale and to the development of a uniform grain structure which is sub‐micrometer in size. Following mechanical alloying, the particles are consolidated using standard powder metallurgical techniques. The consolidated metal has a large stored energy, approaching 1 J g^–1. This ought to make it easy to induce recrystallisation, but in practice the alloys fail to recrystallise except at very high temperatures close to melting. On the other hand, the recrystallisation temperature can be reduced dramatically by slightly deforming the consolidated product prior to heat treatment. It is in this context that the solution formation, microstructure and mechanical properties of such alloys are reviewed here.     

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.     

Development of nanocrystalline Fe–Co alloys using mechanical alloying

Abstract

Nanostructured alloys have considerable potential as soft magnetic materials. In these materials a small magnetic anisotropy is desired, which necessitates the choice of cubic crystalline phases of Fe, Co, Ni, etc. In the present work, Fe–50 at.-%Co alloys were prepared using mechanical alloying (MA) in a planetary ball mill under a controlled environment. The influence of milling parameters on the crystallinity and crystal size in the alloys was studied. The particle size and morphology were also investigated using SEM. In addition, a thermal treatment was employed for partial sintering of some of the MA powders. The crystal size in both MA powders and compacted samples was measured using X-ray diffraction. It was shown that the crystal size could be reduced to less than 15 nm in these alloys. The nanocrystalline material obtained was also evaluated for magnetic behaviour.     

Mechanical alloying – the development of strong alloys

Abstract

Mechanical alloying is a solid-state process for making alloys by high-energy milling, under conditions such that constituent powders are repeatedly fractured and welded together and ever more intimately mixed. After subsequent consolidation at elevated temperature, the alloys can be shaped by rolling, forging, and machining. The process is used to incorporate a fine dispersion of ceramic particles. Mechanically alloyed nickel-base superalloys, combining a dispersion of yttria with conventional precipitation strengthening, have achieved higher strength at 900–1100°C than directionally solidified and single-crystal alloys, and are being used for gas-turbine vanes and blades. Mechanically alloyed ferritic stainless steel, with outstanding strength and corrosion resistance at temperatures as high as 1300°C, has been produced as sheet, tube, plate, rings, and forgings. Mechanically alloyed aluminium alloys also offer higher strength, e.g. in as-forged thick sections of Al–Mg–Li alloy.

MST/567     

Capability of mechanical alloying and SPS technique to develop nanostructured high Cr,Al alloyed ODS steels

Abstract

Oxide dispersion strengthened (ODS) Fe alloys were produced by mechanical alloying (MA) with the aim of developing a nanostructured powder. The milled powders were consolidated by spark plasma sintering (SPS). Two prealloyed high chromium stainless steels (Fe–14Cr–5Al–3W) and (Fe–20Cr–5Al+3W) with additions of Y₂O₃ and Ti powders are densified to evaluate the influence of the powder composition on mechanical properties. The microstructure was characterised by scanning electron microscope (SEM) and electron backscattering diffraction (EBSD) was used to analyse grain orientation, grain boundary geometries and distribution grain size. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) equipped with energy dispersive X-ray spectrometer (EDX) were used to observe the nanostructure of ODS alloys and especially to observe and analyse the nanoprecipitates. Vickers microhardness and tensile tests (in situ and ex situ) have been performed on the ODS alloys developed in this work.     

Dynamic recrystallisation in hot deformed oxide dispersion strengthened MA956 and MA957 steels

Abstract

The hot deformation behaviour of the oxide dispersion strengthened stainless steels MA956 and MA957 has been studied. The alloys are made by a mechanical alloying process which leaves them in a very fine grained, cold deformed state immediately after consolidation. It is found that deformation is accompanied by dynamic recrystallisation, even when the deformation temperatures are far less than the ordinary recrystallisation temperatures of the two alloys. These and other results on the strength and anisotropy of the alloys are interpreted in terms of their microstructures.

MST/3043     

Formation of amorphous Ni-Zr alloy powder by mechanical alloying of intermetallic powder mixtures and mixtures of nickel or zirconium with intermetallics

Mechanical alloying was used to synthesize Ni_xZr_1–x alloys from mixtures of intermetallic compound powders, and also from mixtures of intermetallic compound powders and pure elemental powders. The mechanically alloyed powders were amorphous in the range 0.24 x 0.85. This range is larger than amorphous alloys produced by the melt-spinning technique and mechanical alloying of elemental crystalline powders. Two-phase mixtures of the amorphous phase and the corresponding crystalline terminal solid solution were formed in the range 0.10 x 0.22, and x=0.90. It is found that the morphological development during mechanical alloying of these powders is different from mechanical alloying using only pure ductile crystalline elemental powders. The thermal stability has been investigated. The enthalpy and activation energy of crystallization for Ni-Zr amorphous powders prepared by mechanical alloying are lower than those for melt-spun samples of the same composition. The crystallization temperature of the mechanically alloyed Ni-Zr amorphous powders is higher than that of meltspun samples in the composition range Ni₂₀Zr₈₀ to Ni₃₃Zr₆₇ and Ni₄₀Zr₆₀ to Ni₆₀Zr₄₀. The presence of tiny crystallites as nucleation centres and high oxygen levels in the mechanically alloyed amorphous alloys might be responsible for the differences in crystallization behaviour. A new crystalline metastable phase was observed during crystallization studies of Ni₂₄Zr₇₆ amorphous powder.     

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.     

钨基高比重合金的制备研究进展综述

综述了当前国内外钨基高比重合金制备的研究进展,介绍了WHAs制备工艺中存在的问题和当前的研究方向.采用机械合金化制取纳米预合金粉是WHAs制备取得突破性进展的新工艺之一;活化烧结和二步烧结可显著改善其力学性能;纳米粉末烧结技术的关键就是在得到全致密合金的同时,保持材料的纳米结构才能对性能作出很大的贡献.     

The sequence of phase formation during mechanical alloying of chromium and silicon powders

The sequence of phase formation during mechanical alloying of chromium and silicon powders has been studied using high-energy ball milling of mixtures of elemental powders with different Si/Cr atomic ratios. X-ray diffactometry and transmission electron microscopy have been utilized to identify the phases and to characterize the microstructure of the powders. All four equilibrium phases in the Cr-Si system can form. With a Si/Cr atomic ratio equal to or higher than 3/5, CrSi₂ is always the first phase to form, and then CrSi₂ can react with chromium to form CrSi or Cr₅Si₃, depending on the Si/Cr atomic ratio. This is similar to the sequence of phase formation during annealing of multilayer chromium and silicon thin films. However, with low Si/Cr atomic ratio close to 1/3, Cr₃Si is the first and only phase to form during mechanical alloying.     

Amorphization behaviour in mechanically alloyed Ni—Ta powders

Amorphization behaviour of NixTa100–x alloy powders synthesized by mechanical alloying mixtures of pure crystalline Ni and Ta powders with a Spex high energy ball mill was studied. The mechanically alloyed powders were amorphous for the composition range between Ni10Ta90 and Ni80Ta20. This range is larger than amorphous alloys prepared by the rapid-quenching process or by electron-gun deposition technique. A supersaturated nickel solid solution formed for Ni-rich composition. The thermal stability has been investigated by differential thermal analysis. The crystallization temperature of amorphous Ni—Ta powders was proportional to the Ta content, and the activation energy of amorphous Ni—Ta powders exhibited a maximum near the eutectic composition. It is found that the amorphization rate at the early stage of the mechanical alloying process was faster in the intermediate compositions than those at both Ni- and Ta-rich compositions.     

Fabrication of ultrafine W – Cu powders by mechanical alloying

Abstract

Ultrafine composite powders of W – 15 wt-%Cu, W – 25 wt-%Cu, and W – 35 wt-%Cu have been fabricated by mechanical alloying. The effects of type of mill, process control agent, temperature of milling, and ball/powder ratio on the final products have been evaluated. The results show that the planetary ball mill possesses a higher impact energy intensity than that of the vibratory ball mill. The optimum milling time is confirmed by the formation of a nanocrystalline microstructure in the planetary ball mill after optimisation of the milling parameters. A steady state between cold welding and fracture is attained with a milling time of up to 25 h in the planetary ball mill under optimised conditions. Crystallites with sizes of 7 – 8 nm for W – Cu composite powders have been obtained after 25 h of ball milling. The powders obtained after mechanical alloying have been characterised in terms of their size, shape, phase constitution, and microstructural features using X-ray diffraction and scanning electron microscopy.     

Effect of fabrication process on the microstructure and dynamic compressive properties of SiCp/Al composites fabricated by spark plasma sintering

The effect of fabrication process on the microstructure and dynamic properties of SiC_p/Al composites was studied in this paper. Pure Al matrix composites reinforced with 20 vol.% SiC particles were fabricated by spark plasma sintering, and the pre-blended powders were prepared by two different processes. One was to mix the powders in conical flask by using a mechanical stirrer, and the other was the mechanical alloying process by using a planetary ball mill. The sintering temperature was also explored. The conventional split Hopkinson pressure bar was used to test the dynamic properties of these composites. The results show that the sintering temperature significantly affects the consolidation of the composites. The composites, which have not been fully densified, have very loose microstructure and poor mechanical properties. Mechanical alloying process can improve the microstructure and mechanical properties of the composites. These composites are rate dependent, their strengths increase with increasing strain rates.     

Nanocrystalline AlCoFeNiTiZn high entropy Alloy: Microstructural,Magnetic, and thermodynamic properties

AlCoFeNiTiZn high entropy alloy was successfully produced in powder form by the mechanical alloying process. The ball-milled alloyed product was characterized by X-ray diffractometry, scanning electron microscopy, energy dispersive spectroscopy, and transmission electron microscopy techniques, which indicated that after 120 h of milling, the solid solution was formed as predicted by thermodynamic calculations. Mechanical alloying began to form the BCC phase almost at 30 h and the FCC phase after about 30 h. Nucleation and growth were the processes involved in the formation of these phases, as shown by the Johnson-Mehl-Avrami kinetic model. Sintering was then used to fabricate the alloy in bulk metallic form. The powders were cold pressed and sintered after 120 h of mechanical alloying using a tube furnace with a controlled atmosphere at 500 °C. A similar FCC + BCC phase mixture was present after sintering. The sintered sample also contained minor amounts of Gahnite (ZnAl₂O₄) spinel material. DSC analysis revealed that recrystallization occurred at 280 °C. The as-milled and as-sintered alloys exhibit semi-hard magnetic properties measured by vibrating sample magnetometer (VSM), with saturation magnetization values of 39.14 and 65.78 emu/g, respectively.     

Other applications of superalloys

Abstract

It is the intention in this paper to put into context the development of high-temperature alloys to their present position in non-gas-turbine applications, and to identify new alloy systems capable of improving performance in hostile industrial environments. The rise of superalloys from ferritic steels to the current γ′-hardened nickel-base materials, the best of which experience strength limitations above l000°C, is traced. Desired increases in temperature capability are possible with oxide dispersion strengthened powder alloys, which were originally developed for gas turbine usage and are now being produced on a large tonnage basis by the mechanical alloying (MA) process. The MA technique and commercial alloys are described and examples given of the replacement of more conventional materials by fabricated MA components in a diversity of industries. MA alloys exhibit combinations of strength and corrosion resistance capable of meeting many industrial demands for economic improvements to processing capabilities and efficiencies.

MST/525     

Fundamental Formation Laws of Phase Composition, Structure and Properties of Aluminium Materials Manufactured by Mechanical Alloying

The present investigation objective is to develop the technological process for strengthened aluminium alloys formation of different density based on mechanical alloying. Standard aluminium and magnesium powders were used as initial materials. Lithium was introduced in free state and binded in hydroxide LiOH. Granulated composition with mean particle size 0.2 ... 0.3 mm is the product of mechanical alloying. Thermal treatment of granulated compositions and hot compaction of semiproducts initiate phase transformations which contribute to thermodynamic equilibrium state of the alloys. Alloy base is characterized by micro- or small crystalline structure and presents a solid solution of magnesium and lithium in aluminium strengthened by nanocrystalline inclusions of aluminium oxides, magnesium and aluminium carbides which results in high strength of said alloys at low and high temperatures.     

Structural evolution during mechanical milling and subsequent annealing of Cu–Ni–Al–Co–Cr–Fe–Ti alloys

This study reports the structural evolution of high-entropy alloys from elemental materials to amorphous phases during mechanical alloying, and further, to equilibrium phases during subsequent thermal annealing. Four alloys from quaternary Cu_0.5NiAlCo to septenary Cu_0.5NiAlCoCrFeTi were analyzed. Microstructure examinations reveal that during mechanical alloying, Cu and Ni first formed a solid solution, and then other elements gradually dissolved into the solid solution which was finally transformed into amorphous structures after prolonged milling. During thermal annealing, recovery of the amorphous powders begins at 100 °C, crystallization occurs at 250–280 °C, and precipitation and grain growth of equilibrium phases occur at higher temperatures. The glass transition temperature usually observed in bulk amorphous alloys was not observed in the present amorphous phases. These structural evolution reveal three physical significances for high-entropy alloys: (1) the annealed state of amorphous powders produces simple equilibrium solid solution phases instead of complex phases, confirming the high-entropy effect; (2) amorphization caused by mechanical milling still meets the minimum criterion for amorphization based on topological instability proposed by Egami; and (3) the nonexistence of a glass transition temperature suggests that Inoue's rules for bulk amorphous alloys are still crucial for the existence of glass transition for a high-entropy amorphous alloy.     

Fabrication of low-cost Ti-1Al-8V-5Fe by powder metallurgy with TiH2 and FeV80 alloy

The low-cost Ti-1Al-8V-5Fe (Ti-185) alloy with a high strength is prepared by cold-compaction-and-sintering powder metallurgy process with low-cost titanium hydride (TiH₂) powders and FeV₈₀ master alloy powders. The use of simple technique process and cheap alloying elements can lead to the cost reduction for titanium alloys. The thermal decomposition of TiH₂-1Al-8V-5Fe is analyzed by thermal gravimetric analyses and differential scanning calorimetry simultaneous thermal analyzer. The shrinkage behavior of TiH₂-1Al-8V-5Fe during the sintering process is employed by the high-sensitivity dilatometer system. The microstructure of sintered Ti-185 consists of β-phase and lamellar α-phase. The results show that the sintered Ti-185 alloys have the relative density of 97.8%, homogeneous composition, and fine grains. The yield strength and the hardness are 1461?MPa and 40.1?±?1.0 HRC (unit of Rockwell hardness), which are better than that of as-cast Ti-185.     

Evolution of microstructure in laser clad Fe–Cr–Mn–C alloy

Abstract

The laser surface cladding technique was used to form in situ Fe–Cr–Mn–C alloys on AISI 1016 steel substrate. In this process, mixed powders containing Cr, Mn, and C in the weight ratio 10: 1 : 1 were delivered using a screw feed, gravity flow, carrier gas aided system into the melt pool generated by a 10 kW CO₂ laser. This technique produced an ultrafine microstructure in the clad alloy layer. The microstructure of the laser surface clad region was investigated by optical, scanning and transmission electron microscopy, and X-ray microanalysis techniques. Microstructural study showed a high degree of grain refinement and an increase in solid solubility of alloying elements which, in turn, produced a fine distribution of complex types of carbide precipitates in the ferrite matrix because of the high cooling rate. An alloy of this composition does not show any martensitic transformation or retained austenite phase.

MST/356     

Mechanically-driven Amorphization in Metallic Systems

The mechanism of mechanically-driven amorphization was extensively surveyed in various systems.It was concluded that the amorphization could occur in the systems of Cu-Zr with a negative heat ofmixing (-ΔH) and Cu-Ta with a positive heat of mixing (+ΔH) by mechanical alloying of elementalpowders.Such amorphization could also be started from an intermetallic compound Cu_xZr_y with nochemical reaction involved.The energy storage by mechanical attrition should be the driving forcefor the amorphization.The atomic distribution function and nuclear resonance spectroscopic studiesshow that the mechanical alloying provides a true alloying on an atomic level.The alloys formed areof a characteristic structure common to the rapidly quenched amorphous alloys.