Metallurgical Behavior of Amorphous Alloy during Annealing at Different Temperatures via Molecular Dynamics Simulation

The mechanism of annealing-induced amorphization of metallic glass is investigated in this study via molecular dynamics simulation. Spherical nucleuses of Cu–Ni–Al alloy with a face-centered cubic structure are embedded to simulate nanograins in Cu–Ni–Al amorphous alloy; subsequently, the material is annealed at different temperatures. The results show that the critical radius for nucleation at temperatures above the glass transition temperature (Tg) affected the behavior, grain growth, and annihilation of nanograins in the Cu–Ni–Al amorphous alloy during annealing. When the temperature increased, the critical radius for nucleation increased as well. This causes the small nanograins to annihilate quickly and the large nanograins to develop rapidly. When the annealing temperature is higher than Tg, part of the crystal nuclei, which is smaller than the critical radius, can be eliminated. The crystallinity of the metallic glass decreased, and the minimum crystallinity is attained after a period of annealing simulation. Subsequently, as the residual effective nanograins began developing, the crystallinity of the amorphous metal increased again. Therefore, the annealing duration time is critical to the crystallinity of the amorphous alloy after annealing.

Langasite as Piezoelectric Substrate for Sensors in Harsh Environments: Investigation of Surface Degradation under High-Temperature Air Atmosphere

Langasite crystals (LGS) are known for their exceptional piezoelectric properties at high temperatures up to 1000 °C and more. In this respect, many studies have been conducted in order to achieve surface acoustic wave (SAW) sensors based on LGS crystals dedicated to high-temperature operations. Operating temperatures of more than 1000 °C and 600 °C for wired and wireless sensors, respectively, have been reached. These outstanding performances have been obtained under an air atmosphere since LGS crystals are not stable in high-temperature conditions under a low-oxygen atmosphere due to their oxide nature. However, if the stability of bulk LGS crystals under a high-temperature air atmosphere is well established, the surface deterioration under such conditions has been hardly investigated, as most of the papers dedicated to LGS-based SAW sensors are essentially focused on the development of thin film electrodes that are able to withstand very elevated temperatures to be combined with LGS crystals. Yet, any surface modification of the substrate can dramatically change the performance of SAW sensors. Consequently, the aim of this paper is to study the stability of the LGS surface under a high-temperature air environment. To do so, LGS substrates have been annealed in an air atmosphere at temperatures between 800 and 1200 °C and for durations between one week and one month. The morphology, microstructure, and chemical composition of the LGS surface was examined before and after annealing treatments by numerous and complementary methods, while the surface acoustic properties have been probed by SAW measurements. These investigations reveal that depending on both the temperature and the annealing duration, many defects with a corolla-like shape appear at the surface of LGS crystals in high-temperature prolonged exposure in an air atmosphere. These defects are related to the formation of a new phase, likely an oxiapatite ternary compound, the chemical formula of which is La14GaxSi9−xO39−x/2. These defects are located on the surface and penetrate into the depth of the sample by no more than 1–2 microns. However, SAW measurements show that the surface acoustic properties are modified by the high-temperature exposure at a larger deepness of at least several tens of microns. These perturbations of the LGS surface acoustic properties could induce, in the case of LGS-based SAW sensors operating in the 434 MHz ISM band, temperature measurement errors around 10 °C.

Shape Memory Behavior of Rapidly Quenched High-copper TiNiCu Alloys

Rapidly quenched quasibinary TiNi–TiCu system alloys with high copper contents (above 20 at.%) exhibit excellent shape memory effect and have considerably narrower hysteresis as compared with the TiNi binary alloy, this advantage being of special importance for cyclic load applications, e.g. for microelectromechanics (MEMS). The aim of this work is to study the effect of annealing parameters and copper content on the shape memory effect in TiNiCu alloys. Thin amorphous ribbons of TiNi-TiCu alloys with copper contents of 25 to 40 at.% were produced by planar flow casting at a melt cooling rate of about 106 K/s. The alloys were crystallized by isothermal annealing with variable duration and by exposing specimens to a short (10 ms) electric pulse. Increasing the copper content to above 30 at.% considerably reduces the plasticity and shape memory effect of the alloys. However, significant reduction of annealing duration greatly improves the shape memory performance due to prevention of the formation of brittle Ti-Cu phases in the alloys structure.

Effects of Thermal Annealing Duration on the Film Morphology of Methylamine Lead Triiodide (MAPbI3) Perovskite Thin Films in Ambient Air

In the present communication we have studied the effect of thermal annealing duration on morphology of methylamine lead triiodide (MAPbI3) perovskite (prepared using single step method) semiconductor that changes into lead iodide (PbI2). Furthermore, the effect of annealing duration on thin films is investigated and correlated with its potential photovoltaic application. Thin films characteristics study by X-ray diffraction and scanning electron microscopy results indicate MAPbI3 degraded strongly by annealing duration. However, thin films (about 1.25 micron-thick) annealed at 80 °C for 10 min in ambient conditions cause minimum degradation with smooth and uniform surface morphology. It also shows a higher absorption coefficient with the band gap of °1.5 eV rendering this perovskite suitable for practical applications.

Affect of prolonged thermal exposure on microstructure and mechanical properties of ultrafine-grained bioinert Zr - 1 wt. % Nb and Ti - 45 wt. % Nb alloys

The results for thermal stability of the microstructure and mechanical properties of ultrafine-grained alloys Zr-1 wt. % Nb and Ti-45 wt. % Nb alloys after long-term thermal annealing at temperature of 400°C are presented. It is shown that in the Zr-1 wt.%Nb alloy in the ultrafine-grained state, increase in the annealing duration from 5 to 360 hours leads to growth of the structural elements (grains, sub-grains, fragments) of the matrix phase a - Zr and the particles β-Nb. This fact is due to the processes of recrystallization. It leads to softening of the alloy and decrease of microhardness and yield stress. It was also found that annealing up to 360 hours does not affect the size of the structural elements of the matrix β-phase in the Ti-45 wt. % Nb alloy, but contributes to the noticeable growth of the α-phase and ω-phase grains. According to our results, softening of the Ti-45 wt. % Nb alloy and decrease in its mechanical characteristics are associated with the processes of return at the grain boundaries, as well as with growth of the nanoscale grains of α-phase and ω-phase and decrease in their contribution to dispersion hardening.