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Ball milling of Al-based alloys to obtain amorphous-nanocrystalline structure

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Considering a high strength to weight ratio of Al-based alloys as well as outstanding properties of metallic materials in a glassy state, amorphous aluminum alloys have attracted considerable attention due to their potential in structural applications for transportation and aviation industry[1÷8]. Metastable phases in amorphous or quasicrystalline state can induce two to three times higher strength as compared with those processed through precipitation/age-hardening in crystalline Al‑alloys [1, 2]. The first formation of amorphous single phase in Al‑based alloys containing more than 50 at. % Al was found in 1981 for Al-Fe-B and Al-Co-B ternary alloys [1], but they were very brittle and hence have not attracted much attention. Since then, glass forming ability has been determined in a number of Al-based alloys consisting of Al + transition metal + rare-earth elements, processed mainly by rapid solidification or gas atomization methods [8]. It has been also found that ductility in aluminum alloys can be improved when a few nanometer size crystals are embedded in the amorphous matrix [7]. Choi et al. [9] reported tensile fracture strength as large as 1980 MPa for an amorphous alloy containing about 18% Al nanocrystals - this strength was nearly 1.6 times higher than for the fully amorphous alloy. Later, Kawamura et al. [3] attained a bulk compressive strength of 1420 MPa by hot compaction of gas-atomized amorphous Al85Ni5Y8Co2 powder with nanocrystalline dispersed amorphous matrix. Among many techniques of synthesizing novel materials including nanocrystalline or amorphous products there are melt spinning, gas atomization and similar rapid quenching methods [2] but mechanical alloying (MA) by high-energy ball milling is a convenient solid state synthesis alternative for them. It gives the opportunity of obtaining various phases in the material without need to melt pure elements of the alloy. Furthermore, in the one pro[...]

Structural investigation of Mg-3Ca, Mg-3Zn-1Ca and Mg-3Zn-3Ca as cast alloys

  Magnesium alloys made of Mg-Zn-Ca system are interesting, because of possible application as bioresorbable cardiovascular stents or orthopaedic implants [1÷3]. During the last ten years, rapid growth of research in the application of magnesium and its alloys as biomaterials has been observed [4÷6]. Usage of magnesium based bone implants instead of those made of titanium or steel allows to avoid the removal surgery. Mg is the lightest of all structural metals with density close to those typical for cortical bone (1.75÷2.1 g/cm3). Other material parameters, like Young’s modulus (~45 GPa) are also similar [3]. Moreover, Mg is considered as biocompatible and non-toxic material and has been shown to increase the rate of new bone formation - it is an important ion in the formation of the biological apatites [3]. It was reported that the adult person normally consumes about 300÷400 mg of magnesium every day and an excess of Mg2+ is excreted through the urine [7, 8]. Magnesium is a cofactor for many enzymes and stabilized the structures of DNA and RNA [7, 8]. It is worth noticing that calcium and zinc are also recognized as biocompatible elements [1, 9]. A lot of studies have been performed on rare elements or/and Al containing alloys [10, 11], but these additions increase the cost of possible implant, and biocompatibility of RE is doubtful. An addition of Al can influence human nerves and induces Alzheimer disease [12]. From the metallurgical point of view, alloys made of Mg-Zn-Ca system can undergo solid-solution hardening and Ca is believed to be an effective grain refiner [13÷15]. In spite of possible benefits from magnesium based bone implants, there are a few important questions, which remain open up to date. There are problems with precise control of corrosion rate, which is usually very rapid and connected with hydrogen evolution. Rapid release of H2 in a high amount may cause inflammation process or even death [16]. Thu[...]

Ball milling amorphization and consolidation of NiTiZrNb and NiNbTiZrCoCu alloys

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Amorphous nickel rich alloys have been obtained in recent years by rapid quenching from the liquid state or by mechanical alloying in a planetary mill [1÷9]. One advantage of the latter method is that it makes it possible to obtain new materials from powders of different elements, which are immiscible in the liquid state. Binary NiTi alloys subjected to ball milling or severe plastic deformation can be obtained amorphous [1÷3]. The tendency to form an amorphous structure depends on the relative values of the deformation temperature and martensite start (Ms) temperature. Lowering of the deformation temperature in the range below the martensite finish temperature facilitates amorphization [3]. Mechanical alloying of Ni60Nb20Zr20 alloy [4] involves two consecutive amorphization reactions, leading first to the amorphization reaction between Ni and Zr layers and on further milling consequently a Ni-Nb amorphous phase forms. The resulting two amorphous phases homogenize at longer milling times. Multicomponent nickel base alloys can be obtained as bulk glass at composition of Ni50Co10Nb20Ti10Zr10 (at. %) alloys with reported large supercooled liquid region of more than 40 K formed by copper-mold casting. The alloys with 5 and 10 at. % cobalt possess the highest glass-forming ability [5]. Mechanically alloyed Ni57Zr20Ti18Al5 alloy powders synthesized by high-energy ball milling have shown a complete amorphization after 5 h of milling, even broader supercooled region of 56°C and crystallization temperature above 500°C [6]. Substitution of aluminum by silicon [7] also allowed to obtain amorphous Ni57Zr20Ti20Si3 powders by mechanical alloying of pure Ni, Zr, Ti, Si, and ceramic powder mixture; the metallic alloy amorphized after 5 hours milling indicating a good glass forming ability. In [8] Amorphous Ni59Zr20Ti16Sn5 alloys were fabricated by melt spinning and by mechanical alloying (MA) techniques. Differences in crystallization temperat[...]

Microstructure and properties of ball milled and hot compacted powder of 7055 aluminium alloy

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7xxx series (Al-Zn-Mg-Cu) aluminum alloys are widely used in the aircraft industry due to their low density, high strength and good workability [1, 2]. Their strengthening increases with the concentration of Zn and is associated with higher density of very fine precipitates of metastable η′-phase enriched with Zn and Mg. The high solute (about 8 wt. % of Zn) alloy designated AA 7055 (ALCOA) evokes the highest strength aluminium alloys produced by ingot metallurgy and is applied as upper wing skin materials in commercial aircraft [3]. The 7055 composition processed using the T77 temper provides a microstructure at and near grain boundaries that is resistant to both intergranular fracture and interglanular corrosion. Aluminium based materials produced by powder metallurgy (PM) processing offer a number of interesting opportunities for high strength applications. Powder metallurgy enables to fabricate high quality parts close to final dimensions with refined microstructure as compared with these produced by the conventional ingot metallurgy [4, 5]. The ball milling applied before the compaction allows obtaining a very fine microstructure and the extension of the solid solubility limits of the elements added to the alloy [6]. It results in improved mechanical and corrosion properties of the compacted products. PM technology provides more homogenous distribution of the precipitates and reduces the particle size that makes corrosion more uniform [7]. The aim of the present investigation was to study the effect of ball milling and hot pressing on the microstructure and properties of milled and compacted 7055 aluminium alloy powder. Exp erimental details The mixtures of elemental powders of aluminum, zinc, magnesium, copper and zirconium were used as starting materials to yield (wt. %) Al - 8% Zn - 2% - Mg - 2.3% Cu - 0.2% Zr compositions corresponding to 7055 commercial aluminium alloy. The ball milling of the powder was [...]

Impact of strain rate on Cu mechanical properties

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Materials with ultrafine-grained (UFG) structure have been studied in the last few years because of their unique properties. The main feature of UFG metals is grain size diameter which is below as 1 μm. Considering that grain size reduces to nanometer range, the materials exhibit unique mechanical and physical properties. They have high strength and wear resistance, good ductility at room temperature and superplasticity at elevated temperature [1, 2]. At the same time they have demonstrated properties as a decrease in the elastic moduli, the decrease of the Curie temperature, enhanced diffusivity and improved magnetic properties [1, 3, 4]. The severe plastic deformation methods have been applied to UFG materials formation. The ECAP, ECAP-BP, HPT, ARB are well known technologies nowadays and have been successfully used to structure formation with grain size ~70÷500 nm [5÷7]. The unique properties of UFG metals are connected with specific microstructures features. The UFG microstructure created during SPD processes is formed by dislocations arrangement - “dislocation cell structure“ having mostly low angle boundaries [8]. Based on Valiev’s study [1], during metal processing via SPD great amount of dislocations is introduced to material resulting in high level of internal stresses and elastic distortion of crystal lattice near a boundary. Consequently, the grains boundaries are in the non-equilibrium state and deformation mechanism as grain boundary sliding and grain rotation would be enhanced. The final UFG structure contains huge amount of grain boundaries with mainly high-angle misorientations [9]. The small grain size and great density of defects (as dislocations, vacancies, triple junctions) in UFG materials cause higher strength properties achievement. At the same time, some experimental results show occurrence of superplasticity at lower temperature as well as at high strain rate in UFG metals [10, 11[...]

Microstructure and chemistry of Pb-Sn solder/ENIG interconnections

  The recent directive of EU concerning the restriction of the use of certain hazardous substances, like lead, in electrical and electronic devices, does not apply to such an equipment as missiles, battlefield computers, satellites space probes, computers installed in aircraft, production and processing lines cranes, lifts, conveyor transport, cars, commercial vehicles, aircraft, trains, boat systems, hydraulic excavators, fork-lift road maintenance equipment, harvester, pacemakers, solar arrays and watt balances [1]. One of the very important issue is the plating system of electroless nickel with immersion gold (ENIG) which has been widely used to finish solder pads of printed circuit boards (PCBs), as well as ball-grid array (BGA) and flip chip substrates in many mentioned above devices [2÷4]. The goal of the present study was to provide more details about the microstructure and chemistry of the solder joints on ENIG finish obtained with widely used Pb-Sn alloy. EXPERIMENTAL STUDY Copper pad (35 m thick) with 4÷6 m of deposited Ni-P layer and 0.075 to 0.125 m thick plating of immersion gold was covered with Pb-Sn solder paste (Alpha Metals, 62Sn36Pb2Ag, wt %). The Pb-Sn/ENIG samples produced in such a way were subjected to the sessile drop method by contact heating procedure described in [6] at 503 K for 5 minutes. Then, samples were crosssectioned and examined using the FEI E-SEM XL30 Scanning Electron Microscope (SEM) equipped with the EDAX energy X-ray dispersive spectrometer (EDS). The thin foils for the transmission electron microscopy (TEM) observations were all cut using the Quanta 3D Focused Ions Beam (FIB). The TEM studies were performed on the TECNAI G2 FEG super TWIN (200 kV) microscope equipped with High Angle Annular Dark Field (HAADF) detector integrated with the EDS manufactured by EDAX. RESULTS AND DISCUSSION The SEM image of the cross-sectioned plating after interaction with Pb-Sn so[...]

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