A. G. Aleksanyan

Specifics of the formation of alloys and their hydrides in Ti-Zr-H system

Studies of the combustion processes in the Ti-H, Zr-H, and Ti-Zr-H systems made it possible to develop a principally new method for producing refractory metal alloys through the compaction of a titanium hydride-zirconium hydride powder mixture followed by dehydrogenation. The procedure is briefly described. Experimental data on the formation of titanium-zirconium alloys with different structures, including an ω-phase alloy obtained at atmospheric pressure, are discussed. The experimental results clearly show that the structure of the alloy depends on the composition of the initial charge and the hydrogen content in the hydrides used. The interaction of the alloys obtained with hydrogen under conditions of self-propagating high-temperature synthesis yielded Ti0.9Zr0.1H1.87 (Ti9ZrH18.7), Ti0.67Zr0.33H1.81 (Ti2ZrH5.42), Ti0.5Zr0.5H1.37 (TiZrH2.74), and Ti0.3Zr0.7H1.96 (TiZr2.3H6.53) hydrides with face-centered cubic (fcc) and body-centered tetragonal (bct) structures. The removal of hydrogen from these hydrides (by annealing in vacuum at 700–1050°C) results in the recovery of the initial α and ω phases.

New Technique for Producing the Alloys Based on Transition Metals

In the present work, an essentially new approach to production of alloys of the refractory metals is presented. It consists in co-compacting and dehydrogenation-sintering of mixture of powders of two and more hydrides of transition metals of III, IV and V groups, or of mixture of powder of any of these hydrides with a transition metal of III–VIII groups. It was shown, that the heating of these mixtures after compacting in vacuum up to temperature higher than the temperature of a hydride dissociation, results in formation of strong, porousless, compact binary and triple alloys. The dependence of an alloys structure formation on the components ratio in initial charge is established. The thermodynamic and kinetic estimations of reaction of alloy formation from the hydrides are given. The interaction of some of received alloys with hydrogen in combustion mode is studied, and the hydrides of these alloys are synthesized.

The nanocrystalline forming by combustion synthesis of Ti (Zr) hydrides

Abstract The experimental data concerning nanoscale hydrides of transition metals, obtained by synthesis in combustion wave are presented. The systems Ti–H, Zr–H, Ti–N–H, Zr–N–H, Ti–Zr–N–H were studied. In all samples, the dimensions of grains were as low as 50 nm . The peculiarities of self-propagating high-temperature synthesis processing and structure causes of this phenomena are discussed. The relations between cubic structure of TiH2 hydride and hexagonal structure of Ti-hydridonitride were demonstrated.

Synthesis of transition metal hydrides and a new process for production of refractory metal alloys: An autoreview

Briefly overviewed are the SHS-based processes developed at our lab for synthesis of metal hydrides and their alloys. More than 200 binary and complex hydrides have been synthesized by SHS reactions of metals in hydrogen. Radiation-induced synthesis of transition metal hydrides, termed Thermal-Radiation Synthesis (TRS), was suggested and used to prepare more than 50 different binary and complex hydrides. A phenomenon of Cold Synthesis (CS) of hydrides at room temperature from irradiated metal powders has been observed. Advance in these studies has led to elaboration of a new technique, termed hydride-cycle process, for fabrication of compact blocks of hydrides and refractory metal alloys. A wide range of binary and multicomponent alloys and their hydrides have been synthesized by this route. The developed processes are offered for industrial-scale implementation.

Effect of hydrogen on the structure of alloys formed in the Ti-Zr-Hf-H system

Investigated was the crystal structure of the Ti-Zr, Ti-Hf, and Ti-Zr-Hf alloys prepared by the novel method of “hydride cycling” suggested by us recently. The XRD data showed formation of the ω-phase in alloys with hydrides ratios TiH2/ZrH2 (TiH2/HfH2) ≈ 2, 1, 0.44: Ti0.67Zr0.33 (Ti2Zr), Ti0.67Hf0.34 (Ti2Hf), etc. The ω-phase is unusual for binary Ti-Zr and Ti-Hf alloys at normal pressure. The SHS reaction of α- and ω-alloys with hydrogen was found to yield complex hydrogen-rich hydrides with a fcc or bct structure such as Ti0.9Zr0.1H1.9, Ti0.677Zr0.33H1.81, Ti0.5Zr0.5H1.37, and Ti0.3Zr0.7H1.96.

Synthesis of titanium aluminides by hydride cycle process

The hydride cycle process was successfully applied to synthesize single-phase α2-Ti3Al, γ-TiAl, and TiAl3 aluminides. The formation of aluminides was found to proceed by solid-state reaction mechanism, without melting. The process takes advantage of its relatively low temperatures (600–1000°C), short duration (2–3 h) of a single-step synthetic procedure, and lower energy requirements.

Formation of triple alloys and their hydrides in the Ti-Zr-Hf-H system

A novel technique is suggested for synthesis of refractory metal alloys by dehydrogenation of compacted mixtures of transition metal hydrides. This technique is applicable to preparation of not only binary but also multi-component alloys, intermetallics, and alloys of III, IV, V, and VIII Group metals with various additives. In this work, the results of detailed investigation on the conditions for formation of Ti-Zr-Hf triple alloys from the TiH2, ZrH2, and HfH2 are presented. The alloy structure was found to depend on the ratio of components in a green charge. The triple alloys containing the α- and ω-phases have been prepared. Demonstrated was the ability of Ti-Zr-Hf triple alloys to undergo reaction with hydrogen in a combustion mode yielding respective hydrides.

Omega-phase in Ti-Hf-Zr alloys produced by the hydride-cycle methodSpecial issue on Neutron Scattering in Canada.

We report the presence of large proportions of ω-phase in Ti-Hf-Zr alloys, prepared using the hydride cycle technique. We show that the ω-phase extends across the concentration triangle of Ti-Zr-Hf and report the partitioning of the three metals across the two sites in this structure from neutron and X-ray data. We examine the symmetry of the order parameter governing the β–ω phase transition and show that a two-step model for the phase transition involving site ordering followed by displacement is not likely to be correct. We suggest that an interstitial solid solution of oxygen and octahedral vacancies exists, and that the observation of any ω-phase diffraction pattern of alloys of these metals at ambient temperature and pressure should be viewed as a potential sign of the presence of oxygen in the octahedral interstitial sites.

The Peculiarities of Formation of Alloys Structure in the System Ti-Zr-Hf-H

In Ti-Zr, Ti-Hf and Ti-Zr-Hf systems, the alloys were received by developed in the IChPh of Armenian NAS principally new, “hydride cyclization”, method, and the alloys crystal structures were studied. The X-ray analyses proved that at hydride ratios in charge: TiH2/ZrH2; TiH2/HfH2 2; 1; 0.44; Ti0.67Zr0.33 (Ti2Zr), Ti0.67Hf0.34 (Ti2Hf), etc., the alloys of co-phase formed, unusual for the state diagram of the systems Ti-Zr and Ti-Hf at ambient pressure. The stability of w-phases of studied alloys under thermal action was defined. The interaction of α and w-phase alloys with hydrogen was studied in SHS regime. Complex hydrogen-rich hydrides of FCC and BCT structures — Ti0.9Zr0.1H1.87 (Ti9Zr1H18.7), Ti0.67Zr0.33H1.81 (Ti2ZrH5.42); Ti0.5Zr0.5H1.37 (TiZrH2.74) and Ti0.3Zr0.7H1.96 (TiZr2.3H6.53) were received.

Study of the formation of niobium aluminides in the hydride cycle

Current technology requires that newly created materials have not only a set of functional properties but also be solid, lightweight, plastic, and easily machined. Intermetallic compounds based on the Nb–Al system occupy a prominent place among the advanced materials due to their unique properties. High-temperature intermetallides based on Nb and Al (Nb3Al, Nb2Al, NbAl3) are of great interest to aerospace engineering, power industry, etc. Therefore, the studies aimed at developing new methods for producing these materials performed in the present work are highly relevant. It is shown that the hydride cycle (HC) method for synthesizing refractory metal alloys developed in our laboratory can be successfully used to produce niobium aluminides. The main regularities and mechanism of the formation of niobium aluminide in the HC are investigated. All the phases known from the Nb–Al phase diagram are obtained. The advantages of the HC method compared to the conventional are demonstrated. In addition, the combustion of the produced niobium aluminide in a hydrogen atmosphere in the SHS mode is studied. The samples are analyzed using chemical methods, differential thermal analysis, and X-ray phase analysis.