Tomasz Czujko

Fracture toughness of intermetallic compacts consolidated from nanocrystalline powders

Abstract Ball-milled and fully disordered intermetallic powders of Fe–45at.%Al (iron aluminide) and titanium trialuminide (Al 3 Ti) stabilized to cubic (L1 2 ) structure by alloying with 9 at.% Mn, with nanocrystalline (nanophase) grain size in the range of ∼10 and ∼3 nm (from X-ray diffraction, XRD), respectively, were successfully consolidated into nearly pore-free bulk compacts. Fe–45at.%Al powders were consolidated only by explosive shock wave compaction and titanium trialuminide powders were consolidated by hot pressing and explosive shock wave compaction. After shock consolidation a microcrystalline structure appeared in larger powder grains of the Fe–45Al compacts. Compacts were re-ordered after hot or shock consolidation. Vickers indentation fracture toughness of compacts was investigated. Fe–45Al compacts did not develop any corner cracks up to 2000 g indentation load, indicating some intrinsic fracture resistance. Cubic titanium trialuminide compacts developed corner cracks under the indentation load and their average measured fracture toughness was barely ∼2 MPa m 0.5 , i.e. even lower than the fracture toughness of bulk specimens of coarse-grained cubic titanium trialuminides (∼4–5 MPa m 0.5 ). The results demonstrate that refining the grain size towards the nanolevel is not sufficient to beneficially modify toughness of brittle intermetallics.

Microstructural evolution during controlled ball milling of (Mg2Ni+MgNi2) intermetallic alloy

Nearly dual-phase Mg–Ni alloy fabricated by ingot metallurgy (IM) and comprising ∼30 vol% Mg2Ni and ∼61 vol% MgNi2 intermetallic compounds (remaining ∼9 vol% of unreacted Mg) was mechanically (ball) milled under controlled shearing for 10, 30, 70 and 100 h. The majority of the medium- and small-sized powder particles exhibited a relatively homogeneous microstructure of milled Mg2Ni and MgNi2. A fraction of large-sized particles developed the ‘core and mantel’ microstructure after milling for 70 and 100 h. The ‘core’ contains poorly milled MgNi2 particles and the ‘mantel’ is a thoroughly milled mixture of Mg2Ni, MgNi2 and, possibly, residual Mg. X-ray diffraction provides evidence of nanostructurization and eventual amorphization of a fraction of a heavily ball milled Mg2Ni phase. The remnant Mg2Ni developed a nanocrystalline/submicrocrystalline structure. The co-existing MgNi2 phase developed a submicrocrystalline structure within the powder particles. The results are rationalized in terms of enthalpy effects by the application of Miedema’s semi-empirical model to the phase changes in ball milled intermetallics.

Characterization of Low-Symmetry Structures from Phase Equilibrium of Fe-Al System—Microstructures and Mechanical Properties

Fe-Al intermetallic alloys with aluminum content over 60 at% are in the area of the phase equilibrium diagram that is considerably less investigated in comparison to the high-symmetry Fe3Al and FeAl phases. Ambiguous crystallographic information and incoherent data referring to the phase equilibrium diagrams placed in a high-aluminum range have caused confusions and misinformation. Nowadays unequivocal material properties description of FeAl2, Fe2Al5 and FeAl3 intermetallic alloys is still incomplete. In this paper, the influence of aluminum content and processing parameters on phase composition is presented. The occurrence of low-symmetry FeAl2, Fe2Al5 and FeAl3 structures determined by chemical composition and phase transformations was defined by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) examinations. These results served to verify diffraction investigations (XRD) and to explain the mechanical properties of cast materials such as: hardness, Young’s modulus and fracture toughness evaluated using the nano-indentation technique.

The effect of MgNi2 intermetallic compound on nanostructurization and amorphization of Mg–Ni alloys processed by controlled mechanical milling

Abstract Two Mg–Ni alloys with 27.9±11.1 and 57.5±0.8 at% Ni, fabricated by ingot metallurgy (IM) and containing ∼9 and ∼79 vol% of the MgNi2 phase, respectively, were ball (mechanically) milled in a magnetic Uni-Ball-Mill 5 under controlled shearing mode for 10, 30, 70 and 100 h. The evolution of the microstructure of milled powders is presented. It is observed that the Mg2Ni phase undergoes a partial amorphization in the Mg–Ni alloys containing ∼79 vol% of the MgNi2 phase while no amorphization of Mg2Ni is observed in the alloy containing only ∼9 vol% of MgNi2. The results are rationalized in terms of the enthalpy effects based on the application of Miedema’s semi-empirical model to the phase changes in ball-milled intermetallics and the critical nanograin size required to be formed in the Mg2Ni phase before triggering its amorphization, which is enhanced by the presence of hard MgNi2 phase during ball milling. The milled powders of 27.9±11.1 at% Ni alloy, after long-term milling for 100 h, did not absorb hydrogen.

Overview of processing of nanocrystalline hydrogen storage intermetallics by mechanical alloying/milling

The objective of this article is to overview processes of mechanical alloying/milling (MA/MM), and their modifications applied to produce nanostructured single- and multi-phase intermetallics, and their composites, for hydrogen storage. In the most typical processing, MA is used as a preliminary step in synthesizing a nanostructured intermetallic powder starting from elemental metal powders. In a subsequent step, the intermetallic powder is hydrogenised under high pressure of hydrogen to produce nanostructured intermetallic hydride. A modified processing variant combines the synthesis of nanostructured intermetallic and its subsequent hydrogenising in one step by MA of elemental metal powders directly under hydrogen atmosphere to form nanostructured intermetallic hydrides (so-called Reactive Mechanical Alloying—RMA). The MM can be applied to produce nanostructured intermetallic powders from pre-alloyed intermetallic cast ingots or to manufacture nanocomposites, by mixing with dissimilar material before milling, which could be hydrogenised in a separate process. In addition, pre-alloyed bulk intermetallics can be mechanically milled directly under hydrogen atmosphere (Reactive Mechanical Milling—RMM) in order to obtain nanostructured intermetallic hydrides as a final product. All the above processes are critically discussed in the present article. The effect of nanostructurization on the hydrogen sorption/desorption characteristics of intermetallics and/or their hydrides is also discussed.

Cold-work induced phenomena in B2 FeAl intermetallics

For the last several years we have been conducting systematic studies of microstructural changes and mechanical behavior of B2 FeAl alloys, cold-worked by ball-milling and shock-wave loading. Fundamental physical phenomena induced in iron aluminides such as an accelerated chemical disordering, increase in the lattice parameter, non-magnetic/magnetic transformation and formation of nanocrystals in ball-milled powder particles are presented and discussed.

Fabrication and geometric characterization of highly-ordered hexagonally arranged arrays of nanoporous anodic alumina

Abstract Anodic aluminum oxide (AAO) has been fabricated in the 0.3 M oxalic acid at voltage range 20-60 V and temperature range of 35-50oC. The resulting nanoporous alumina surfaces were characterized by high resolution scanning electron microscopy, and the images were quantitatively analysed by means of an innovative approach based on fast Fourier transform. The influence of operating anodization voltage and electrolyte temperature on nanopores geometry (pore diameter, interpore distance, porosity, pores density) and arrangement has been studied in details and compared to literature data and theoretical calculations. It was found that independently from the temperature, the best arrangement of the nanopores is for anodic aluminum oxide formed at voltages ranging from 40 to 50 V. Moreover, it was found that pore diameter and interpore distance increase linearly with voltage, what is in line with the literature data.

Nanomaterials for Hydrogen Storage Produced by Ball Milling

Abstract Three methods of hydrogen desorption temperature reduction and desorption kinetics improvement of nanostructured hydrides processed by mechanical (ball) milling are discussed. The first method is based on a simultaneous particle size refinement of MgH2 hydride and the formation of an unstable γ-MgH2 phase. The second method utilizes catalytic effects of nanometric Ni (n-Ni) additives. The third method is based on the compositing of nanohydride mixtures such as NaBH4+MgH2 and MgH2+LiAlH4 where the first hydride in a pair has higher decomposition temperature than the second one. The low decomposition temperature hydride results in the destabilization of the high temperature constituent hydride.

The Effect of the Traverse Feed Rate on the Microstructure and Mechanical Properties of Laser Deposited Fe3Al (Zr,B) Intermetallic Alloy

The results of the fabrication of components made with Fe-30%Al-0.35%Zr-0.1%B alloy powder using the Laser Engineered Net Shaping (LENSTM) system operated at different traverse feed rates are described in this paper. The temperature of the molten metal pool was recorded during this process. Depending on the assumed feed rate, the formation of Zr–based precipitates with various morphologies and distributions was observed in the structure of the investigated material. It was found that as the traverse speed increased, spheroidization, refinement, and a more homogeneous distribution of these precipitates occurred.

The role of Mg2FeH6 formation on the hydrogenation properties of MgH2-FeFx composites

AbstractThe hydrogenation properties of magnesium hydride mechanically milled with iron fluorides (FeF2 and FeF3), were investigated by Temperature Programmed Desorption (TPD) and volumetric methods using a Sieverts-type apparatus, as prepared upon dehydrogenation and finally upon subsequent hydrogenation. The activation energy of hydrogen desorption (Ea), calculated from the Kissinger formula using TPD measurements obtained with different heating rates, showed significant decreases of Ea in comparison to that of milled MgH2 without any dopants. Moreover, the influence of these metal fluorides on the thermodynamics of the decomposition process was also examined. In the case of the FeF2 dopant, rehydrogenation following desorption caused the complete decomposition of the iron fluoride to BCC iron and the formation of a predominant MgH2 phase. In contrast to FeF2, the addition of FeF3 led to the formation of β-MgH2 as a major phase coexisting with Mg2FeH6 and MgF2 compounds. The presence of pure Fe in the MgH2+FeF2 composite, as opposed to MgH2+FeF3 containing Mg2FeH6 and MgF2, did not cause any significant influence on the sorption properties of MgH2. Moreover, the original material doped with FeF3 predominantly showed iron in the Mg2FeH6 compound, while the FeF2 dopant iron mostly showed the nearly pure BCC metallic phase