Papers by Keyword: Metastability

Abstract: Optical changes caused by ⁶⁰Co γ-irradiation are studied in glassy-like As40S60. The observed long-wave shift in the range of fundamental optical absorption edge accompanied by increase in transmittance is explained as a manifestation of complicated nature of radiation-induced structural transformations associated with coordination topological defects and additional shrinkage input from natural physical ageing.

Abstract: It is well known that microstructure evolution during rapid solidification depends on the rate between undercooling and solidification velocity. Microstructural evolution in a nonequilibrium solidification process depends critically on the melt undercooling and growth (solidification) velocities of competing phases. One way of obtaining metastable structures is by Thermal Spray Technology. Metastable coatings can be produced starting from microstructured powders through Atmospheric Plasma Spray (APS) technique, followed by a quenching route. The initial powders are melted during the spraying and deposited over a substrate that is quenched with nitrogen feeders, producing metastable coatings. The obtained coatings were characterized using XRD, SEM, FESEM and TEM in the Thermal Spray Centre (CPT) of the University of Barcelona. The properties of such coatings were studied by hardness, obtaining promising results.

Abstract: Recent experiment [1] revealed many new characteristics of the domain patterns in a superelastic polycrystalline NiTi tube during tensile loading, such as domain wall instability and branching, dynamic topology transition of domain patterns. In this paper, we use the continuum mechanics approach and model the polycrystal as a phase-transformable continuum described by non-local nonlinear elasticity [2]. We simulate the equilibrium macroscopic domain patterns and their evolution in the tubes under tensile loading by the nonlocal Finite Element Method (FEM). It is revealed that the loading path dependence and dynamic topology transition of domain patterns are mainly due to thermodynamic metastability of the tube system. Our simulations capture all the key features of the domain patterns observed in the NiTi polycrystalline tubes.

Abstract: The aim of this work is the achievement of nanostructured TiC–TiB2 powders through a metastability approach based on the Self–propagating High–temperature Synthesis (SHS) process followed by quench to obtain highly metastable powder agglomerates. An optimisation of the reaction stoichiometry was carried out in order to obtain products with approximately eutectic composition (i.e. 67%mol TiC – 33%mol TiB2). An optimized amount of sodium borate was used as gasifying additive to produce dispersed nanostructured powder agglomerates. The metastability of the nanocomposite powders obtained through the (SHS+quench) route was evaluated by annealing. The morphological evolution of the powders after thermal treatment yielded a recrystallisation with limited grain growth of the nanostructured TiC–TiB2 phases and demonstrated the metastability of the products obtained by the (SHS+quench) route.

Abstract: Non-oxide ceramic nanostructured powders are synthesized through metastable transformation processing based on the Self-propagating High-temperature Synthesis (SHS) process followed by quenching. Binary systems like the investigated TiC–TiB2, when quenched from the liquid state give rise to metastable structures capable of being converted into a stable, fine-grained (nanocomposite) microstructure upon recrystallization by medium temperature treatments. A necessary condition is that the combustion temperature of the SHS reaction is higher than the eutectic temperature. A previous optimisation of the reaction stoichiometry was carried out to obtain SHS products with composition approximately equal to the eutectic (i.e. 67%mol TiC0,7 – 33%mol TiB2), according to the reaction: 6Ti + B4C + 1.8C → 4TiC0.7 + 2TiB2. In this work, different amounts of sodium borate (borax) were used in order to determine the optimum amount of additive to produce nanostructured TiC0.7–TiB2 composites. The morphological evolution of the powders after thermal treatment yielding re–crystallized structures demonstrates the metastability of the SHS–quench products. Therefore, the metastability process based on SHS– quench represents an extremely attractive route suitable for the achievement of nanocomposites.