Nanocrystalline NiAl Research Articles

Synthesis and microstructure characterization of nanocrystalline NiAl compound with copper addition by mechanical alloying

Synthesis and microstructure characterization of nanocrystalline NiAl compound with copper addition by mechanical alloying

Microstructural evolution and mechanical properties of Ni/Al reactive nanolaminates with different NixAly intermetallic phases

Nanolaminates made up by alternative Ni and Al nanolayers are widely applied to solder structural materials, due to abundant heat releases when supplied with an energetic input. This work studies the microstructural evolution and mechanical properties of Ni/Al nanolaminates with a wide mole ratio of Ni (MNi, 0.18–0.53), to ascertain the role of NixAly intermetallic phases during heat-treatment. To this end, 3 nanolaminates with Al thickness fixed at ≈110 nm and Ni thickness varying 15–75 nm are fabricated and are heat-treated at 400 °C, to trigger formations of NiAl3, Ni2Al3 and NiAl intermetallic phases. The results suggest an enhanced strength of Ni/Al after heat-treatment. However, their evolution with MNi is complex. The nanolaminate with MNi ≈0.18 is the softest after heat-treatment, due to the dominant plasticity of residual Al phases. The maximized strength is achieved when MNi ≈0.40, in which a novel NiAl3/Ni2Al3 nanolaminate structure is formed. Owing to the brittleness of both phases, the nanolaminate after heat-treatment is fractured by shearing upon uniaxial compression. The best strength-ductility synergy is achieved when Ni mole ratio ≈0.53. In the heat-treated nanolaminate a homogenous nanocrystalline NiAl phase is formed. As a result, the nanolaminate exhibits excellent ductility and intermediate strength among the 3 nanolaminates.

Pulse Plasma Sintering of NiAl-Al2O3 Composite Powder Produced by Mechanical Alloying with Contribution of Nanometric Al2O3 Powder.

NiAl-Al2O3 composites, fabricated from the prepared composite powders by mechanical alloying and then consolidated by pulse plasma sintering, were presented. The use of nanometric alumina powder for reinforcement of a synthetized intermetallic matrix was the innovative concept of this work. Moreover, this is the first reported attempt to use the Pulse Plasma Sintering (PPS) method to consolidate composite powder with the contribution of nanometric alumina powder. The composite powders consisting of the intermetallic phase NiAl and Al2O3 were prepared by mechanical alloying from powder mixtures containing Ni-50at.%Al with the contribution of 10 wt.% or 20 wt.% nanometric aluminum oxide. A nanocrystalline NiAl matrix was formed, with uniformly distributed Al2O3 inclusions as reinforcement. The PPS method successfully consolidated NiAl-Al2O3 composite powders with limited grain growth in the NiAl matrix. The appropriate sintering temperature for composite powder was selected based on analysis of the grain growth and hardness of Al2O3 subjected to PPS consolidation at various temperatures. As a result of these tests, sintering of the NiAl-Al2O3 powders was carried out at temperatures of 1200 °C, 1300 °C, and 1400 °C. The microstructure and properties of the initial powders, composite powders, and consolidated bulk composite materials were characterized by SEM, EDS, XRD, density, and hardness measurements. The hardness of the ultrafine-grained NiAl-Al2O3 composites obtained via PPS depends on the Al2O3 content in the composite, as well as the sintering temperature applied. The highest values of the hardness of the composites were obtained after sintering at the lowest temperature (1200 °C), reaching 7.2 ± 0.29 GPa and 8.4 ± 0.07 GPa for 10 wt.% Al2O3 and 20 wt.% Al2O3, respectively, and exceeding the hardness values reported in the literature. From a technological point of view, the possibility to use sintering temperatures as low as 1200 °C is crucial for the production of fully dense, ultrafine-grained composites with high hardness.

Development of nanocrystalline Ni-Al coatings and its thermal stability

In this paper, Ni-Al compound coatings were electrodeposited from a nickel sulphate electrolyte containing metallic Al particles (1-3um). The deposition conditions adopted were: compound content 50 g/l, pH 4.0, and present density 1.6A/dm'. At electrode position, the particles were held in suspension by magnetic stirring. The deposition was carried out on a stainless steel substratum and the coating was mechanically Segregated out of the substratum as an electroform after completion of the deposition. The thermal sustainability of the Ni-Al coatings was observed by annealing electroforms at temperatures of 200°, 400°, 600°, and 800 °C for 6hrs in an electric furnace under normal conditions. To further enhance the thermal stability of Ni-Al coatings, the cobalt element has been code posited with the addition of cobalt sulphamate in Ni electrolytic bath to obtain Ni-Co alloy matrix containing Al particles. The electrodeposited coatings were characterized for their shape and the phases formed during annealing using the X-ray diffraction technique (XRD). The Al particle distribution was studied using optical microscopy (OM) and field emission scanning electron microscopy (FESEM). The Ni-Aland Ni-Co-Al coatings were also characterized by their cross-sectional micro hardness. As these Ni-Al coatings tend to form Al2O3; upon exposure to high temperatures, a comparative study of the structure, microhardness, thermal stability has also been made between Ni-Al. Ni-Co-Al and Ni-Al2O3; coatings. The Ni-Al2O3; (um) composite coating was obtained under the same conditions as that of the Ni-Al coating. The properties of the three coatings were compared and discussed.

NiAl-B composites with nanocrystalline intermetallic matrix produced by mechanical alloying and consolidation

Powder mixtures with equiatomic Ni–Al stoichiometry and with the addition of 5, 10, 20 and 30 vol% of boron were mechanically alloyed in a high-energy SPEX mill. Differential scanning calorimetry (DSC) was used for examination of the thermal behaviour of the milled powders. The mechanically alloyed powders and powders after DSC examinations were investigated by X-ray diffraction (XRD). For all the powder mixtures, a nanocrystalline NiAl intermetallic phase was formed during milling. With the increase of boron concentration in the mixtures, more intense refinement of the NiAl grain size during mechanical alloying was observed. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) examinations showed that the produced powders have composite structure, with boron particles uniformly distributed in the nanocrystalline NiAl intermetallic matrix. The density of the composite powders decreases with the increase of boron content, following the rule of mixture.The produced powders were subjected to consolidation by hot-pressing at 800 °C under the pressure of 7.7 GPa for 180 s. The produced bulk materials were investigated by XRD, SEM and EDS as well as characterised by hardness, density and open porosity measurements. It was found that during applied consolidation process the nanocrystalline structure of the NiAl matrix was maintained. The average hardness of the bulk composite samples is in the range of 10.58–12.6 GPa, depending on boron content, increases with the increase of boron content, and is higher than that of the NiAl intermetallic reference sample (9.53 GPa). The density of the bulk composite samples is the same as that of the corresponding powders after milling, decreases with the increase of boron content and is lower than that of the NiAl reference sample. To the best of our knowledge, the NiAl-B composites with nanocrystalline intermetallic matrix have been produced for the first time.

Shift of Creep Mechanism in Nanocrystalline NiAl Alloy.

We have examined the effects of temperature, stress, and grain size on the creep process including creep strain, crystal structure, dislocations and diffusions of nanocrystalline NiAl alloy through molecular dynamics simulations. A smaller grain size accelerates the creep process due to the large volume fraction of grain boundaries. Higher temperatures and stress levels also speed up this process in terms of dislocation changes and atom diffusion. In both primary creep and steady-state creep stages, atomic diffusion at the grain boundary could be seen and the dislocation density increased gradually, indicating that the creep mechanism at these stages is Coble creep controlled by grain boundary diffusion while accompanied by dislocation nucleation. When the model enters the tertiary creep stage, it can be observed that the diffusion of atoms in the grain boundary and in the crystal and the dislocation density gradually decreases, implying that the creep mechanisms at this stage are Coble creep, controlled by grain boundary diffusion, and Nabarro–Herring creep, controlled by lattice diffusion.

Nanocrystalline NiAl intermetallic alloy with high hardness produced by mechanical alloying and hot-pressing consolidation

An Ni-50% Al elemental powder mixture was mechanically alloyed in a SPEX ball mill. The powders after various milling times were investigated by X-ray diffraction and differential scanning calorimetry. A nanocrystalline NiAl intermetallic phase with the mean crystallite size of 13 nm was formed after 8 h of milling. The produced powder was consolidated by high-pressure hot-pressing at 800 °C under the pressure of 7.7 GPa. The consolidated material was characterised by structural investigations. Hardness, density and open porosity as well as heat resistance measurements were also conducted. The mean crystallite size of the NiAl intermetallic phase in the bulk material was 24 nm, which shows that the nanocrystalline structure was maintained during the consolidation process. The hardness and heat resistance of the produced nanocrystalline NiAl were compared with those of a reference microcrystalline NiAl. The hardness of the nanocrystalline NiAl intermetallic is 971 HV1 (9.53 GPa) and it significantly exceeds the hardness of the reference microcrystalline NiAl. The relative density of the consolidated sample is 100%. The produced nanocrystalline NiAl intermetallic exhibits very good oxidation resistance at 900 °C in air. For this material, the mass gain per area after 100 h of exposure is 1.4 × 10−4 g and is smaller than that for the reference microcrystalline NiAl sample. The quality of consolidation with preserving NiAl nanocrystalline structure is satisfactory and the hardness as well as the oxidation resistance of the produced material are relatively high.

Molecular dynamics studies on the grain growth of nanocrystalline Ni and Ni3Al

With molecular dynamics simulations, the growth of face-centered-cubic nanocrystalline materials Ni and Ni3Al has been studied. It is found that grain-rotation induced grain coalescence and curvature-driven grain-boundary migration are dominant mechanisms in the nanograin growth. A detailed comparison of the nanograin growth between the two systems is discussed in terms of grain rotation and grain sliding. We also study the temperature effect and the size effect in the nanograin growth. The tendency of twinning in the nanograin growth is discussed. It is found that in Ni3Al, it seems more possible for nanograins to grow into twin-like structures than single crystal unless at very high temperatures.

Mechanical and tribological properties of NiAl–NbC–Ag composites prepared by hot-pressing sintering

Abstract

Synthesis and microstructure characterization of nanocrystalline NiAl intermetallic compound with molybdenum additions obtained by mechanical alloying

Nanocrystalline Ni50 Al50−x Mox (X = 0 and 5 at.%) intermetallic powders were synthesized by mechanical alloying (MA) in a planetary ball mill. Microstructural characterization and structural changes of powder particles during mechanical alloying were studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results confirmed that the synthesis behavior of NiAl intermetallic depends on the milling time and Mo content. The mechanical alloyed powders have a microstructure consisting of nanometer size particles. Mo enhance has a considerable effect on the lowering of crystallite size. The TEM image showed that the Ni50 Al45 Mo5 nano-particles were less than 10 nm.

Enhancement of hardness and thermal stability of W-doped Ni3Al thin films at elevated temperature

Nanocrystalline (NC) Ni3Al thin films have poor mechanical properties at a high temperature and in order to improve the thermal performance, it is necessary to develop novel micro- or nanostructures that endow the coatings with both high thermal stability and hardness. In this work, Ni3Al thin films with different W concentrations up to 12.5at% are deposited on Si/SiO2 substrates by magnetron sputtering and then vacuum-annealed at 500°C and 700°C. Addition of W to the nanostructured Ni3Al thin films leads to grain refinement and high thermal stability. The hardness (H) of the film with 12.5at% W annealed at 700°C increases by 355% compared to pure Ni3Al. X-ray diffraction and transmission electron microscopy reveal non-equilibrium phases such as the amorphous phase and supersaturation solid solution in the films annealed at 500°C. After annealing at 700°C, the α-W phase precipitated in the fine nanocrystalline state plays a crucial role in the high thermal stability of the nano-grains as well as enhanced hardness. The results suggest that these durable Ni-based coatings are suitable for applications at high temperature.

Properties Evaluation and Studying Production Mechanism of Nanocrystalline NiAl Intermetallic Compound by Mechanical Alloying

Ni50Al50 intermetallic compound was synthesized by mechanical alloying (MA) of elemental mixtures of Ni and Al powders in a planetary ball mill. After 16 hours of milling and obtaining crystallites with a critical size, the initial NiAl compound was formed along with the combustive reaction after opening the vial lid. In the time interval of 16 to 128 hours, the reaction from combustive state reached the explosive state. Finally, after 128 hours of milling, the initial powders were wholly transformed into NiAl before completion of the milling time. Structural changes of powder particles during MA were studied by X-ray diffractometry and scanning electron microscopy. The crystallite size measurements revealed that the grain size of the NiAl phase decreased from 155 to 26 nm with increasing MA time from 8 to 128 hours. Microhardness for nanocrystalline Ni50Al50 intermetallic compound produced after 128 hours of milling was measured as about 350 Hv.

Nanocrystalline Ni3Al-based alloys obtained by recycling of aluminium scraps via mechanical alloying and consolidation

Bulk nanocrystalline Ni3Al-based alloys were produced by ball milling of powdered aluminium scrap alloys (two silumines with chemical compositions: Al87.5Si7.8Cu2.1Fe0.7Zn1.4Mn0.4Mg0.1 (R1 scrap) and Al88.4Si5,7Cu3,9Fe0,9Zn0,6Mn0.5 (R2 scrap)) with addition of Ni powder and subsequent hot-pressing consolidation. The final products of mechanical alloying were consolidated at 1000°C for 180s under the pressure of 7.7GPa. The powders and the compacted samples were examined by XRD method. The results obtained show that during the milling supersaturated solid solutions Ni(Al,Si…) are formed and during the consolidation the disordered solid solutions transform into ordered nanocrystalline Ni3Al intermetallic based phases with a mean crystallite size of ∼30nm. The microhardness of the produced Ni3Al-based alloys is 955 for R1 process and 928 HV0.1 for R2 process respectively. The density of the pellets is nearly 100% of theoretical value for the Ni3Al intermetallic phase and the open porosity of the compacts is negligibly small.

P8] Effects of alloying elements on the stacking fault energies of dilute Al-based alloys

Mechanical alloying (MA) of the ternary Ni–Al–Cr mixtures having the Ni50Al50, Ni25Cr25Al50 and Ni50Cr25Al25 compositions was investigated. The structural changes of powder particles during mechanical alloying were studied by X-ray diffractometry (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The results show that for all three compositions the gradual interdiffusion of elements during MA leads to the development of different phases. The final structures for the Ni50Al50 and Ni25Cr25Al50 compositions are nanocrystalline NiAl and (Ni, Cr)Al intermetallic compounds, respectively. However MA of the Ni50Cr25Al25 composition led to the formation of a Ni3Al intermetallic compound as well as Cr which transformed to the (Ni, Cr)3Al intermetallic compound after subsequent heat treatment. The structural transition upon annealing was investigated. Furthermore, thermodynamic analysis using the Miedema model for all three systems is carried out and discussed in detail.

Experimental and Finite Element Simulation of Wear in Nanostructured NiAl Coating

In this paper, the wear of nanostructured NiAl coating was studied both experimentally and numerically. First, the nanocrystalline NiAl intermetallic powder was synthesized by mechanical alloying (MA) of aluminum and Ni powders. The coatings were deposited onto the low carbon steel substrate using high velocity oxy-fuel (HVOF) technique. Nanoindentation test was conducted to find out the mechanical properties of the coating. The dry wear tests were then performed using a pin-on-block test rig under different operating conditions. Finally, finite element (FE) method was employed to model the wear characteristics of the prepared nanostructured material. A three-dimensional (3D) FE model was created and used to simulate the pin-on-block experiments. The results show that the volume losses predicted by the numerical analysis are in good agreement with the experimental data.

Effects of crystalline growth on corrosion behaviour of nanocrystalline NiAl coating

In the current work, the effect of crystalline growth on the corrosion behaviour of nanocrystalline NiAl coating was investigated. In this regard, NiAl coatings with different crystalline sizes in the range of 20–110 nm were produced by mechanical alloying, high-velocity oxy-fuel processing (HVOF) and isothermal annealing at 600°C for 0–30 h. The produced and annealed samples were characterized using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The corrosion behaviours of coatings were examined in NaCl 3/5% electrolyte by potentiostat analysis. The nanocrystalline NiAl coating with the average crystalline size of about 20 nm and the porosity content of about 2% was successfully produced by mechanical alloying and HVOF processing. By annealing the coating, the NiAl crystalline sizes increased sharply, approaching a constant value of about 110 nm. It was found that the corrosion resistance of nanocrystalline coating increased with the increase in the crystalline size.

Processing behavior of nanocrystalline NiAl during milling, sintering and mechanical loading and interpretation of its intergranular fracture

Processing behavior of Ni and Al powders during milling, sintering and compression loading was evaluated using SEM, TEM and XRD-Rietveld’s refinement analysis techniques. Nanocrystalline NiAl intermetallic along with microstrain and dislocations on its microstructure was obtained after milling. Sintering of nanocrystalline powders at 1180°C under a pressure of 48MPa for 1h time by hot press technique resulted in grain growths toward ultrafine along with decreasing of microstrain and dislocations on microstructure. Obtaining a microstructure of ultrafine grained NiAl resulted in improvement of mechanical properties. Intergranular fracture was characterized by the coupling of dislocations at grain boundaries that caused to formation of voids/cracks with progressive loading.

Contrasting magnetic behaviors of Ni3Al nanoparticles with and without NiO shell

In the nanocrystalline Ni3Al sample with the average crystallite size d=50nm but without NiO shell, magnetization does not saturate even at magnetic fields as high as H=90kOe, no long-range ferromagnetic order exists even at T=2.5K and the exchange bias (EB) effect is completely absent. By contrast, in the nanocrystalline sample with ferromagnetic Ni3Al core (diameter 5nm) and antiferromagnetic NiO shell (thickness ≃1nm), saturation in magnetization is reached at fields as low as 1kOe, saturation magnetization is ≃5 times higher, super-spin glass state coexists with inhomogeneous ferromagnetic ordering and a novel type of EB effect is observed.

Synthesis of nanocrystalline NiAl by mechanically activated self-propagating high-temperature synthesis and mechanically activated annealing process

The mechanically activated self-propagating high-temperature synthesis (MASHS) technique and the mechanically activated annealing process (M2AP) were used to produce NiAl intermetallic compound. Rietveld analysis of X-ray diffraction data was used to characterize the mechanically activated powders, MASHS and M2AP end-products. Two-phase (B2+L12) nickel aluminide intermetallic compounds were synthesized by the mechanically activated volume combustion synthesis (MASHS). A single phase B2 NiAl was formed when mechanically activated annealing process (M2AP) was performed. Starting from a mixture of elemental pure powders, both M2AP and MASHS milling processes lead to nanostructured powders. Structural analysis deduced from the Rietveld refinement of X-ray diffraction patterns of NiAl compounds synthesized by SHS after a milling of 6 h show the formation of β-NiAl phase with a cell parameter a = (0.2885 ± 1.2618 × 10−4) nm. The average crystallite size is D = (44.6421 ± 2.4263) nm and the microstrains values are close to those of metallic alloys τ = (0.2142 ± 1.6186 × 10−2)%. Finally, scanning electron microscopy (SEM) was carried out to characterize the microstructure of end-products.

Microstructural and wear characteristics of HVOF-sprayed nanocrystalline NiAl coating

In this research, the microstructure and dry sliding friction and wear behavior of nanocrystalline NiAl coatings were investigated. Toward this purpose, nanocrystalline NiAl powders were initially prepared by mechanical alloying (MA) and deposited on low carbon steel substrate by high velocity oxygen fuel (HVOF) thermal spray process with two sets of spray parameters. The coatings were characterized with respect to their microstructure and phase compositions using scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and X-ray diffractometry (XRD). In addition, the friction and wear behavior of coating were tested under three loads of 30, 60 and 90N. Microstructural observations showed that the as-sprayed coatings include mainly NiAl intermetallic compound with a crystallite size of ~20nm, although there exist some Al2O3 and Ni phases. HVOF spraying at higher fuel/oxygen ratio caused higher amounts of non-NiAl phases and lower hardness and wear resistance. The wear mechanisms and friction behavior of nanocrystalline NiAl coatings were evaluated and discussed in detail.