Ch. Chiu

Feasibility study of the direct mechano-chemical synthesis of nanostructured magnesium tetrahydroaluminate (alanate) [Mg(AlH4)2] complex hydride

The present work reports a feasibility study of the direct mechano-chemical synthesis by controlled reactive mechanical alloying (CRMA) in a magneto-ball mill of the nanostructured magnesium tetrahydroaluminate (magnesium alanate) Mg(AlH(4))(2) complex hydride. Three stoichiometric Mg-2Al mixtures, (a) elemental Mg and Al powders, (b) elemental Al powder and commercial AZ91 alloy (Mg-Al-Zn alloy) and (c) powder of as-cast Mg-2Al alloy, have been used. No successful synthesis of Mg(AlH(4))(2) has been achieved. The only nanocrystalline hydride formed up to 270 h of CRMA is beta-MgH(2), and it does not react with Al and H(2) to form Mg(AlH(4))(2). It has been found that there is strong competition between formation of Al(Mg) solid solution and the beta-MgH(2) hydride occurring to a various extent up to approximately 10 h of CRMA in all three Mg-2Al mixtures. It is hypothesized that the presence of Al(Mg) solid solution inhibits the reaction of beta-MgH(2), Al and H(2) to form Mg(AlH(4))(2). Furthermore, despite the fact that after prolonged milling the Al(Mg) solution eventually decomposes into secondary Al(s) (derived from solid solution), the latter retains its physico-chemical characteristics of the former solid solution which still inhibits the reaction to form Mg(AlH(4))(2). Experimental evidence from DSC measurements shows increasing ranges of the melting enthalpy with increasing amounts of Al(Mg) solid solution and consequently the secondary Al(s) for all the three Mg-2Al mixtures. This strongly supports the hypothesis about the different nature of Al(Mg) and the secondary Al(s) as compared to the primary elemental Al powder.

Observations of crack tip process zones in cubic titanium trialuminide intermetallics

Despite the fact that the single-phase L12-ordered titanium trialuminides, derived from D022-ordered Al3Ti by alloying with fourth-period transition elements such as Cr, Mn, Fe, Co, Ni, Cu, and Zn have a cubic lattice structure, their room temperature fracture toughness remains quite low (4–5 MPa m1/2). In general, process zones developed at the crack tips determine the fracture toughness of a material. In this work the results of the crack tip fracture studies of cubic (L12) Al3Ti alloys stabilized with Mn are presented. The process zones at the crack tip in nearly stoichiometric single-phase L12 9Mn–25Ti (at.%) titanium trialuminides were not observed in most of the specimens studied. Occasionally, two types of process zones were observed: either small, heavily localized process-plastic zones accompanied by a crack tip “collapse”, or “pseudo-bifurcated” ones, reminiscent of those in brittle ceramics. Observations of the crack tip process zones in multiphase, high Ti (up to ∼33 at.%), B-doped trialuminides, exhibiting increased fracture toughness (∼7 MPa m1/2), show the presence of secondary microcracks in the zone ahead of the crack tip and adjacent to the propagating crack, and more plasticity at the crack tip.

The Effects of Oxidized and Oxide-Free Boron on the Mg-B-H Nanohydrides Transformation in the Nearly Nanosized Powders

In this work oxidized and oxide-free amorphous boron (a-B) powder and elemental Mg were used in an attempt to directly synthesize the Mg(BH4)2 complex hydride by controlled reactive mechanical alloying (CRMA) under hydrogen in a magneto-mill up to 200h. The particle size was refined to the 100-200nm range. Nanocrystalline MgH2 (~6nm crystallite size) was formed within the particles when an oxidized a-B is used. In contrast, a mixture of MgB2 and an amorphous hydride MgHx was formed when an oxide-free a-B was used. Differential scanning calorimetry (DSC) test up to 500°C produced a single endothermic heat event at 357.7°C due to hydrogen desorption. In addition, desorption conducted in a Sieverts-type apparatus revealed ~1.4wt.% of hydrogen release. The X-ray diffraction pattern after DSC test of the 200h milled sample made with oxide-free boron showed the presence of MgB2.

Mechanosynthesis of Nanocrystalline MgB2 Ceramic Powders in Hydrogen Alloying Mills via Amorphous Hydride Intermediate

In the present work we report on the synthesis of nanocrystalline MgB2 by mechanochemical reaction (mechanosynthesis) conducted in a high-energy mechanical alloying mill filled with hydrogen. The solid-state reaction of mechanochemical alloying between Mg and B with H (hydrogen alloying) leads to formation of an intermediate amorphous (Mg,B)Hx hydride. This amorphous intermediate is subsequently annealed (devitrified) to nucleate and grow nanocrystalline boride. The first stage of synthesis was carried out at room temperature from elemental Mg and B powders in a high-energy ball mill under sequential supply of hydrogen. The subsequent annealing of the amorphous product led to nearly single-phase MgB2, with only small fraction of MgO impurity. The easy room-temperature synthesis renders the method promising for production of MgB2, which recently gained attention as a new 39K ceramic superconductor. The amorphous intermediate itself can be studied further for its capacity to store ca. 2 wt% H in a metastable hydride phase. The effort was undertaken to predict formation of amorphous hydride phase through analysis of atomic volume mismatch between atoms of Mg, B, and H.