W.J. Botta

Mg alloy for hydrogen storage processed by SPD

Abstract Mg-based nanocomposites are promising candidates for hydrogen storage applications exhibiting fast H-sorption kinetics at reasonably low temperatures when processed by high-energy ball milling techniques. However, since compaction of the highly reactive nanometric powder is desirable before application, the search for other effective processing routes for the preparation of Mg-based nanocomposites is relevant. In this work, we have used a combination of equal channel angular pressing, cold rolling and high-energy ball milling in the processing of the commercial AZ31 extruded alloy to evaluate its use as a hydrogen storage material. Severe plastic deformation carried out at different temperatures, combined with further mechanical processing resulted in a controlled texture and signifiant grain refinement, which are desirable microstructural characteristics for hydrogen storage applications.

Topological instability and electronegativity effects on the glass-forming ability of metallic alloys

The glass-forming ability (GFA) of metallic alloys is associated with a topological instability criterion combined with a new parameter based on the average electronegativity difference of an element and its surrounding neighbours. In this model, we assume that during solidification the glassy phase competes directly with the supersaturated solid solution having the lowest topological instability factor for a given composition. This criterion is combined with the average electronegativity difference among the elements in the alloy, which reflects the strength of the liquid. The GFA is successfully correlated with this combined criterion in several binary glass-forming systems.

Reduced electronegativity difference as a factor leading to the formation of Al-based glassy alloys with a large supercooled liquid region of 50K

The influence of the electronegativity difference among the constituent elements on the stability of the supercooled liquid in two Al-based glassy alloys is studied. A record-large value of the supercooled liquid region of about 50K is obtained based on the electronegativity difference concept within a certain composition range.

Severe Plastic Deformation of Mg-Fe Powders to Produce Bulk Hydrides

We describe in the present work the production of bulk Mg hydrides by hydrogenation treatment of samples processed by severe plastic deformation. The compact bulk samples of Mg-Fe have been obtained by high pressure torsion. The ternary complex Mg2FeH6 and the binary MgH2hydrides have been synthesized by hydrogenation treatment at 350°C, at 3 MPa during 24 h. The average grain refinement after HPT was modest as estimated by XRD. A comparison between the XRD patterns of the powders and of the HPT samples showed the formation, as expected, of a preferred orientation in the latter. The XRD of the hydride HPT samples (H-HPT) showed the presence of Mg, Fe, MgH2 and Mg2FeH6. The first de-hydrating reaction of the alloys (after H-HPT) was studied by differential scanning calorimetry (DSC). These results showed a reduction in the hydrogen dessorption temperature in comparison with commercial MgH2, indicating thermodynamic destabilization of the hydrides as a result of the high density of dislocations in the H-HPT samples.

Shaping of Bulk Metallic Glasses by Simultaneous Application of Electrical Current and Low Stress

Using the intrinsic materials properties of bulk metallic glasses (BMG), namely electrical resistivities two orders of magnitude higher than good conductors and a Newtonian viscous-flow regime of deformability, a new electromechanical process has been developed for shaping, joining and engraving of BMGs. The wider the liquid supercooled region between the glass transition temperature T g and the crystallisation temperature T x of the bulk metallic glass, the easier the application of the new process. In this range, the undercooled liquid deforms in a quasi-Newtonian way, allowing thermomechanical shaping in the low viscosity range as for oxide glasses. The new electromechanical processing technology has been used for economical and rapid shaping at low applied stresses by eliminating the thermal mass of the furnace and the need to heat the deformation dies. The process parameters are adaptable for the full maintenance of the glassy state or when desired, for appropriate compositions, for nanocrystallisation during the joining or shaping operation.

Phase transformation in Nb—16 at.% Si processed by high-energy ball milling

Abstract Evolution of phases during high-energy ball-milling of elemental powders of Nb and Si of composition Nb—16 at.% Si were studied as a function of milling time and milling power. Powders obtained were characterized by X-ray diffraction and calorimetry. Milling can result either in NbSi supersaturated solidsolution or in a very fine partially amorphized mixture. Further milling of the amorphous mixture can result in mechanical crystallization and transformation that leads to formation of Nb, and Nb 5 Si 3 and Nb 3 Si intermetallic compounds.

Synthesis of MgH2 and Mg2FeH6 by reactive milling of Mg-based mixtures containing fluorine and iron

A good method to store hydrogen is in it atomic form in crystalline structure of metals at low pressure. Thanks to magnesium’s high hydrogen storage capacity, its low weight and its high natural abundance, it is an attractive material to develop hydrogen solid state storage. The production of Mg-based nanocomposites can enhance the kinetics of H-sorption of magnesium and the temperature of release of hydrogen. Transition metals as iron, which have important catalytic activity in hydrogen reactions with Mg, and the surface protective compound MgF2, are interesting additions for magnesium mixtures for hydrogen storage. In this work, Mg-based nanocomposites containing Fe and MgF2 were produced by reactive milling under hydrogen using the addition of FeF3, or directly MgF2 and Fe. The efficiency of centrifugal and planetary mill in MgH2 synthesis was compared. The phase evolution during different milling times (from 1 to 96 h) using the planetary was investigated. The different H-desorption behavior of selected milled mixtures was studied and associated with the different present phases in each case.

Anelastic behaviour in NbTi alloys containing interstitial elements

Abstract Internal friction measurements as a function of temperature were performed between 250 and 700 K in an Nb48wt.%Ti alloy heat treated under various conditions, using a torsion pendulum of the inverted Ke type with an oscillation frequency of about 3.5 Hz. The internal friction data were used to obtain spectra of multiple anelastic relaxation as a function of temperature, which were resolved into elemental interactions. The following metal—interstitial interactions were identified: NbO (430 K), NbN (541 K), TiO (467 K), TiOO (495 K), NbOO (443 K) and TiOOO (523 K). The anelastic relaxation parameters, height of the internal friction peak, peak temperatures and activation energy were calculated for each of the processes using Debyes elemental peaks.

Atomization and Selective Laser Melting of a Cu-Al-Ni-Mn Shape Memory Alloy

Shape memory alloys (SMAs) are a class of material that undergoes a reversible shape change after a plastic deformation. The recovery of the original shape is possible due to a structural transformation upon heating to a critical temperature. The shape memory effect is related to a martensitic-austenitic transformation from a phase with a low symmetry (martensite) to a high-temperature phase (parent phase) [1]. Cu-based shape memory alloys have the advantage of large thermal and electrical conductivities and the system Cu-Al-Ni alloys are quite attractive due to better stabilisation against aging phenomena [2].

High-Yield Direct Synthesis of Mg2FeH6 from the Elements by Reactive Milling

Magnesium complex hydrides as Mg2FeH6 are interesting phases for hydrogen storage in the solid state, mainly due to its high gravimetric and volumetric densities of H2. However, the synthesis of this hydride is not trivial because the intermetallic phase Mg2Fe does not exist and Mg and Fe are virtually immiscible under equilibrium conditions. In this study, we have systematically studied the influence of the most important processing parameters in reactive milling under hydrogen (RM) for Mg2FeH6 synthesis: milling time, ball-to-powder weight ratio (BPR), hydrogen pressure and type of mill. Low cost 2Mg-Fe mixtures were used as raw materials. An important control of the Mg2FeH6 direct synthesis by RM was attained. In optimized combinations of the processing parameters, very high proportions of the complex hydride could be obtained.