Study on preparation and mechanical property of nanocrystalline NiAl intermetallic

Nanocrystalline NiAl materials were fabricated using mechanical alloying and hot-pressing sintering technique. The crystal structural and microstructure of milled powders during mechanical alloying, and the microstructure and mechanical properties of bulk NiAl intermetallic were characterized. The results show that B2 ordered nanocrystalline NiAl powders were successfully synthesized by solid-state diffusion via the gradual exothermic reaction mechanism during mechanical alloying. Scanning electron microscope image confirmed that the powder particles were flat and flake shape in the early stage of milling, but changed to a spherical shape with the crystallite size about 30 nm after the milling. After sintering, the crystal structure of nanocrystalline NiAl intermetallic was assigned to B2 order NiAl phase with the average crystallite size about 100 nm. The nanocrystalline NiAl intermetallic exhibited prominent room temperature compressive properties, such as the true ultimate compressive strength and the fracture strain were 2143 MPa and 32.2%, respectively. The appearances of vein-like patterns on the fracture surface of NiAl intermetallic materials indicated that the fracture mechanism could be characterized as ductile fracture. It can be concluded that higher sintering density and nanocrystalline of NiAl intermetallic were benefited for the improvement of mechanical properties.     

Microstructures and mechanical properties of spark plasma sintered Cu–Cr composites prepared by mechanical milling and alloying

Nanograined Cu–8 at.% Cr composite was produced by a combination of mechanical milling (MM), mechanical alloying (MA) and spark plasma sintering (SPS). Commercial Cu and Cr powders were pre-milled separately by MM. The milled Cu and Cr powders were then mechanically alloyed with as-received Cr and Cu powders respectively. After milling, the powder mixtures were separately subjected to SPS. It was found that pre-milling Cr can efficiently decrease the size of grain and reinforcement, resulting in remarkable strengthening. The grain size of Cu matrix was about 82 nm after SPS. The Vickers hardness, compressive yield strength and compression ratio of the composite were 327 HV, 1049 MPa and 10.4%, respectively. The excellent mechanical properties were primarily attributed to dispersion strengthening of the Cr particles and fine grain strengthening of the Cu matrix. The strong Cu/Cr interface and dissolved Cr atoms can also contribute to strengthening of the composite.     

Mechanically induced self-propagating reaction and consequent consolidation for the production of fully dense nanocrystalline Ti55C45 bulk material

We employed a high-energy ball mill for the synthesis of nanograined Ti₅₅C₄₅ powders starting from elemental Ti and C powders. The mechanically induced self-propagating reaction that occurred between the reactant materials was monitored via a gas atmosphere gas-temperature-monitoring system. A single phase of NaCl-type TiC was obtained after 5 h of ball milling. To decrease the powder and grain sizes, the material was subjected to further ball milling time. The powders obtained after 200 h of milling possessed spherical-like morphology with average particle and grain sizes of 45 μm and 4.2 nm, respectively. The end-products obtained after 200 h of ball milling time, were then consolidated into full dense compacts, using hot pressing and spark plasma sintering at 1500 and 34.5 MPa, with heating rates of 20 °C/min and 500 °C/min, respectively. Whereas hot pressing of the powders led to severe grain growth (~ 436 nm in diameter), the as-spark plasma sintered powders maintained their nanograined characteristics (~ 28 nm in diameter). The as-synthesized and as-consolidated powders were characterized, using X-ray diffraction, high-resolution electron microscopy, and scanning electron microscopy. The mechanical properties of the consolidated samples obtained via the hot pressing and spark plasma sintering techniques were characterized, using Vickers microhardness and non-destructive testing techniques. The Vickers hardness, Young's modulus, shear modulus and fracture toughness of as-spark plasma sintered samples were 32 GPa, 358 GPa, 151 GPa and 6.4 MPa·m^1/2, respectively. The effects of the consolidation approach on the grain size and mechanical properties were investigated and are discussed.     

Mechanical alloying and spark plasma sintering of CoCrFeNiMnAl high-entropy alloy

An equiatomic CoCrFeNiMnAl high-entropy alloy was synthesized by mechanical alloying, and alloying behaviors, microstructure and annealing behaviors were investigated. It was found that a solid solution with refined microstructure of 20 nm in grain size could be obtained after 30 h milling. As-milled powder transformed into a face-centered cubic phase above 500 °C. The as-milled powder was subsequently consolidated by spark plasma sintering at 800 °C, BCC phase and FCC phase coexisted in the consolidated HEA, which had excellent properties in Vickers hardness of 662 HV and compressive strength of 2142 MPa.     

Microstructural evolutions of nickel-aluminide nanocomposites during powder synthesis and thermal spray processes

The synthesis and microstructural evolutions of the NiAl-15 wt% (Al₂O₃–13% TiO₂) nanocomposite powders were studied. These nanocomposite powders are used as feedstock materials for thermal spray applications. These powders were prepared using high and low-energy mechanical milling of the Ni, Al powders and Al₂O₃–13% TiO₂ nanoparticle mixtures. High and low-energy ball-milled nanocomposite powders were also sprayed by means of high-velocity oxy fuel (HVOF) and air plasma spraying (APS) techniques respectively. The results showed that the formation of the NiAl intermetallic phase was noticed after 8 h of high-energy ball milling with nanometric grain sizes but in a low-energy ball mill, the powder particles contained only α-Ni solid solution with no trace of the intermetallic phase after 25 h of milling. The crystallite sizes in HVOF coating were in the nanometric range and the coating and feedstock powders showed the same phases. However, under the APS conditions, the coating was composed of the NiAl intermetallic phase in the α-Ni solid solution matrix. In both of the nanocomposite coatings, reinforcing nanoparticles (Al₂O₃–13% TiO₂) were located at the grain boundaries of the coatings and pinned the boundaries, therefore, the grain growth was prohibited during the thermal spraying processes.     

Dilatometry of attrition milled nanocrystalline titanium powders

The sintering behavior of nanosized titanium powders was investigated by dilatometry. The nanosized Ti powders (40 nm) were produced by the attrition milling of micron sized Ti powders (12 μm) in Ar atmosphere. Sintering was carried out in Ar atmosphere in the temperature range of 450–1250 °C for nanosized Ti and 650–1250 °C for micron sized Ti by heating at 10 °C/min, up to the sintering temperature followed by isothermal holding for 1 h. The nanosized Ti powders exhibited a lower sintering onset temperature, larger shrinkage, larger shrinkage rate, and lower activation energy for sintering as compared to the micron sized Ti powders. The sintered micron sized Ti specimens exhibited both intraagglomerate and interagglomerate porosity while the nanosized Ti specimens exhibited well densified agglomerates (almost no interagglomerate porosity) and large intraagglomerate porosity. In nanosized Ti grain growth was found to take place beyond 700 °C and reached a maximum of 66 nm in samples sintered at 1100 °C.     

Effect of cup and ball types on alumina–tungsten carbide nanocomposite powder synthesized by mechanical alloying

Alumina-based nanocomposite powders with tungsten carbides particulates were synthesized by ball milling WO₃, Al and graphite powders. X-ray Diffraction (XRD) was used to characterize the milled and annealed powders. Microstructures of milled powders were studied by Transmission Electron Microscopy (TEM). Results showed that Al₂O₃–W₂C composite formed after 5 h of milling with major amount of un-reacted W in stainless steel cup. The remained W was decreased to minor amount by increasing carbon content up to 10 wt.%. When milled with ZrO₂ cup and balls, Al₂O₃–W₂C composite was completely synthesized after 20 h of milling with the major impurity of ZrO₂. In the case of stainless steel cup and balls with 10 wt.% carbon, Fe impurity after 5 h of milling (maximum 0.09 wt.%) was removed from the powder by leaching in 3HCl·HNO₃ solution. The mean grain size of the powder milled for 5 h was less than 60 nm. The powder preserved its nanocrystalline nature after annealing at 800 °C.     

Mechanical properties of nanostructured Al2024–MWCNT composite prepared by optimized mechanical milling and hot pressing methods

Nanostructured Al2024–multiwall carbon nanotubes (MWCNTs) composites were produced using optimized mechanical milling and hot pressing methods. Nanostructured Al2024 powder was first prepared through 30 h mechanical milling of the alloy powder. MWCNTs up to 3 vol.% were added to the milled Al2024 powder and milled for different times. Differential thermal analysis (DTA) and X-ray diffraction (XRD) were used to assess the structural changes and thermal behavior during mechanical milling and hot pressing. Hardness and compression tests were applied on bulk samples to evaluate their mechanical properties. Mechanical milling applied on Al2024 powders for 30 h resulted in the grain refinement to ～30 nm. DTA analysis showed an endothermic peak at ～632 °C due to Al2024 melting and an exothermic peak between 645 and 658 °C related to Al and MWCNTs reaction. Mechanical milling of nanocomposite powder for 4 h and following hot pressing at 500 °C under a pressure of 250 MPa for 0.5 h were selected as optimized conditions for bulk nanocomposite preparation. With MWCNTs addition up to 2 vol.%, relative density remained at 98%, and hardness increased to 245 HV. Compressive strength of nanocomposites found a maximum value of 810 MPa at 2 vol.% MWCNTs addition which is 78%, 34% and 12% greater than that for Al2024–O, Al2024–T6 and nanostructured Al2024, respectively.     

Structural and microstructural characterizations of nanocrystalline hydroxyapatite synthesized by mechanical alloying

Single phase nanocrystalline hydroxyapatite (HAp) powder has been synthesized by mechanical alloying the stoichiometric mixture of CaCO₃ and CaHPO₄ powders in open air at room temperature, for the first time, within 2 h of milling. Nanocrystalline hexagonal single crystals are obtained by sintering of 2 h milled sample at 500 °C. Structural and microstructural properties of as-milled and sintered powders are revealed from both the X-ray line profile analysis and transmission electron microscopy. Shape and lattice strain of nanocrystalline HAp particles are found to be anisotropic in nature. Particle size of HAp powder remains almost invariant up to 10 h of milling and there is no significant growth of nanocrystalline HAp particles after sintering at 500 °C for 3 h. Changes in lattice volume and some primary bond lengths of as-milled and sintered are critically measured, which indicate that lattice imperfections introduced into the HAp lattice during ball milling have been reduced partially after sintering the powder at elevated temperatures. We could achieve ~ 96.7% of theoretical density of HAp within 3 h by sintering the pellet of nanocrystalline powder at a lower temperature of 1000 °C. Vickers microhardness (VHN) of the uni-axially pressed (6.86 MPa) pellet of nanocrystalline HAp is 4.5 GPa at 100 gm load which is close to the VHN of bulk HAp sintered at higher temperature. The strain-hardening index (n) of the sintered pellet is found to be > 2, indicating a further increase in microhardness value at higher load.     

Fabrication of transparent mullite ceramic by spark plasma sintering from powders synthesized via sol–gel process combined with pulse current heating

Mullite nanopowders were synthesized by combining the advantages of the sol–gel process with the rapid synthesis provided by pulse current heating. The mullite ceramic with an infrared transmittance of 83–88% in the wavelength range from 2.5 to 4 μm with a fine grain size of 200 nm was obtained by spark plasma sintering at 1350 °C. Due to the high relative density and the small grain size, the hardness and toughness values of the sample reached 17.82 GPa and 3.6 MPa m^1/2, respectively. In contrast, when the mullite powders synthesized in a muffle furnace, an intermediate phase occurred so that the powder synthesis required high crystallization temperatures and resulted in agglomerated particles. Thus, the mullite ceramics required high temperatures for densification. As a result, the optical and mechanical properties of the ceramics were poor due to the low relative density and the elongated grain growth.     

Development of high performance powder metallurgy steels by high-energy milling

High-energy milling is considered to be one of the most efficient techniques for producing materials that have a well-controlled chemical composition and microstructural features that are difficult to obtain using other synthesis routes. In this study, the mechanical alloying technique (MA) was used to develop special powder metallurgy (PM) steels with two different types of properties. The use of this technique is essential for obtaining the target microstructure, which ensures the desired performance of the resulting material. Oxide dispersion strengthened (ODS) ferritic steels were produced using MA based on the prealloyed grade Fe–20Cr–5Al and Fe–14Cr–5Al–3 W steels with the addition of Ti and Y₂O₃ as reinforcements. The incorporation of Y₂O₃ enables a homogeneous dispersion of nano-oxides and nano-clusters in a submicron-grained structure that should enhance the mechanical properties up to 600 °C. In addition, the base alloying system, Fe–Cr–Al (Ti), should enable the development of protective oxide layers through high-temperature treatments, which improves the compatibility with the environment, avoids liquid–metal embrittlement and contributes to the increase of the mechanical response up to 600 °C. Furthermore, microalloyed powders can be obtained using MA and consolidated with a pressure-assisted sintering process in an attempt to control the final grain size, to achieve microalloyed steels, and to achieve an extraordinary balance of properties.     

An investigation of the synthesis,consolidation and mechanical behaviour of Al 6061 nanocomposites reinforced by TiC via mechanical alloying

Nanostructured Al 6061–x wt.% TiC (x = 0.5, 1.0, 1.5 and 2.0 wt.%) composites were synthesised by mechanical alloying with a milling time of 30 h. The milled powders were consolidated by cold uniaxial compaction followed by sintering at various temperatures (723, 798 and 873 K). The uniform distribution and dispersion of TiC particles in the Al 6061 matrix was confirmed by characterising these nanocomposite powders by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), differential thermal analysis (DTA) and transmission electron microscopy (TEM). The mechanical properties, specifically the green compressive strength and hardness, were tested. A maximum hardness of 1180 MPa was obtained for the Al 6061–2 wt.% TiC nanocomposite sintered at 873 K, which was approximately four times higher than that of the Al 6061 microcrystalline material. A maximum green compressive strength of 233 MPa was obtained when 2 wt.% TiC was added. The effect of reinforcement on the densification was studied and reported in terms of the relative density, sinterability, green compressive strength, compressibility and Vickers hardness of the nanocomposites. The compressibility curves of the developed nanocomposite powders were also plotted and investigated using the Heckel, Panelli and Ambrosio Filho and Ge equations.     

Mechanical synthesis of nanostructured titanium–nickel alloys

The methods of mechanically assisted synthesis and mechanical alloying are used to obtain nanostructured TiNi shape-memory alloys. A stoichiometric mixture of Ti and Ni powders was subjected to intense mechanical treatment in a planetary ball mill. It was shown that after 30 h milling, the synthesis of the product with a mean particle size of 20–30 nm proceeded at 550 °C. XRD data show mainly the presence of TiNi, Ti₂Ni and TiNi₃ phases. Prolongation of the milling process up to 40 h leads to direct synthesis of a product with a similar phase composition. SEM and TEM analyses are used to study morphological changes of reagent and product particles in the course of mechanical treatment and after the synthesis of products. The mechanochemical synthesis routes are compared with the traditional method of thermal synthesis. The advantages of the mechanochemical methods of synthesis of nanostructured products and obtaining Ti–Ni powders with a high sinterability are also discussed. It was shown that after cold pressing and sintering at 800 °C, compacts containing even distributed pores with a mean size of 1 μm were obtained. TiNi bodies with similar structural peculiarities are suitable for the purposes of implantology.     

Development of NiFe-CNT and Ni3Fe-CNT nanocomposites by mechanical alloying

NiFe-CNT and Ni₃Fe-CNT nanocomposites were fabricated by high energy mechanical alloying method. X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and optical microscopy were employed for evolution of phase composition, morphology and microstructure of the powder particles. Ball milled powders were heat treated at 500 °C for 1 h to release the milling induced stresses. Bulk samples were prepared by sintering of cold pressed (300 MPa) samples at 1040 °C for 1 h. XRD patterns of powders, as-milled and after annealing at 500 °C did not show any peak related to CNTs or excess phases due to the interaction between CNTs and matrix. SEM micrographs showed that the addition of CNTs caused a reduction of powder particles size. The hardness value of as-milled NiFe and Ni₃Fe powders reach to 660 and 720 HV, respectively. According to optical microscopy evaluations, the amount and size of the porosities of the composites bulk samples decreased in comparison with matrix ones.     

Kinetics of grain growth in nanocrystalline magnesium-based metal–metal composite synthesized by mechanical alloying

Nanocrystalline (nc) Mg–5 wt%–Al and Mg–5 wt%–Al–10.3 wt%–Ti metal–metal composites have been synthesized by mechanical alloying (MA) for 20 h of milling duration. XRD was employed as analysis tool for the structural evolution during MA and thermal stability at elevated temperatures. Kinetics of grain growth of Mg–5 wt%–Al–10.3 wt%–Ti and Mg–5 wt%–Al systems under isothermal and isochronal annealing was investigated. The results showed that the grain growth behavior can be described by the parabolic kinetic equation of grain growth and the activation energy of mechanically alloyed metal–metal composite is higher than that of cast Mg alloy and that of its base metal of Mg–5 wt%–Al.     

A study on preparation of Mo–0.6Ti–0.2Zr–0.02C alloy by mechanical alloying and hot isostatic pressing,and its characterization

Mo–0.6Ti–0.2Zr–0.1C alloy was prepared by mechanical alloying (MA) and subsequently consolidated by powder processing techniques. The pellets prepared from the fine size MA powder showed a high rate of densification during sintering in the temperature range of 1300–1500 °C. Close to theoretical density was attained by hot isostatic pressing (HIP) at 1250 °C and TEM studies revealed the uniform distribution of complex carbide precipitates (<100 nm) in the fine grain microstructure of the consolidated alloy. The alloy consolidated by HIP showed a high hardness of the order of 500 HK due to the presence of the carbides in the fine grain microstructure.     

Microstructure and mechanical properties of bulk highly faulted fcc/hcp nanostructured cobalt microstructures

Nanostructured cobalt powders with an average particle size of 50 nm were synthesized using a polyol method and subsequently consolidated by spark plasma sintering (SPS). SPS experiments performed at 650 °C with sintering times ranging from 5 to 45 min under a pressure of 100 MPa, yielded to dense bulk nanostructured cobalt (relative density greater than 97%). X-ray diffraction patterns of the as-prepared powders showed only a face centered cubic (fcc) crystalline phase, whereas the consolidated samples exhibited a mixture of both fcc and hexagonal close packed (hcp) phases. Transmission electron microscopy observations revealed a lamellar substructure with a high density of nanotwins and stacking faults in every grain of the sintered samples. Room temperature compression tests, carried out at a strain rate of 10^− 3 s^− 1, yielded to highest strain to fracture values of up to 5% for sample of holding time of 15 min, which exhibited a yield strength of 1440 MPa, an ultimate strength as high as 1740 MPa and a Young's modulus of 205 GPa. The modulus of elasticity obtained from the nanoindentation tests, ranges from 181 to 218 GPa. The lowest modulus value of 181 GPa was obtained for the sample with the highest sintering time (45 min), which could be related to mass density loss as a consequence of trapped gases releasing.     

Combined effects of milling and calcination methods on the characteristics of nanocrystalline barium titanate

Barium titanate (BT) nanoparticles were synthesized by high-energy planetary milling technique. Wet milling and dry milling effects were compared. The milled powders were calcined at 1000 °C by microwave and conventional heating methodologies. The calcined powders were characterized by XRD, VP-SEM, EDX, BET, TGA and laser Raman spectroscopy techniques. Dry milled and microwave calcined BT showed the highest extent of tetragonality value of 1.021. VP-SEM micrograph confirmed the tetragonal morphology of microwave calcined BT(s) with an average grain size of 81 nm. Raman Spectra corroborated well with the XRD results. Wet milled and microwave calcined powders showed relatively higher residual hydroxyl ion contamination than that of the dry milled powders. Dry milled and microwave calcined powders showed superior room temperature dielectric constant value with relatively lower dielectric loss tangent value which are very much essential for the fabrication of multi layered ceramic capacitors.     

A study on the mechanochemical behavior of TiO2–Al–Si system to produce Ti5Si3–Al2O3 nanocomposite

In the present study Ti₅Si₃–Al₂O₃ nanocomposite was synthesized by a displacement reaction between Al and TiO₂ in ball milling of TiO₂, Al and Si powders. The effect of milling time and heat treatment temperatures were also investigated. The structural changes of powder particles during mechanical alloying were investigated by X-ray diffraction (XRD). Morphology and microstructure of powders were characterized by scanning electron microscopy (SEM). It was found that after 10 h of MA, the reaction between Al and TiO₂ initiated in a gradual mode and after about 45 h of milling, the reaction was successfully completed. The final product consisted of Ti₅Si₃ intermetallic compound with a crystallite size of 13 nm and amorphous Al₂O₃. Heat treatment of this structure at 1050 °C led to the crystallization of Al₂O₃ and ordering of Ti₅Si₃. The crystallite size of Ti₅Si₃ and Al₂O₃ after annealing at 1050 °C for 1 h remained in nanometer scale. So the final product appeared to be stable upon annealing.     

Preparation and characterization of hydroxyapatite synthesized from oyster shell powders

The ready availability and the low cost of oyster shells, which is composed predominantly of calcium carbonate with rare impurities, along with natural wastes are attractive features for converting the biological material into hydroxyapatite (HA) powders for biomedical applications. The HA powder was synthesized using oyster shell powders and dicalcium phosphate dihydrate (CaHPO₄·2H₂O, DCPD) through ball milling and subsequently heat treatment. The HA was initiated through sintering the 1-h milled sample at 1000 °C for 1 h, while pure HA phase is formed after sintering the 10-h milled sample. The as-prepared samples, obtained after 5 or 10 h of milling and then heat-treating at 1000 °C for 1 h, contain the phase of β-tricalcium phosphate (β-TCP). Moreover, the result of FTIR analysis showed that the as-prepared HA sample is A- and B-type carbonate-containing calcium phosphates. The as-synthesize HA powder containing trace elements Mg and Sr exhibited good crystallinity (96.3%) and high phase-purity.