Thermally Initiated Structural Transformations in the Temperature Range (624–643) ± 1 K for Amorphous Metal Alloy Al87Y4Gd1Ni8 and Influence on Mechanical Properties

Abstract

Highlights The activation energy of the nucleation, growth and stable crystallization processes calculated for AMA Al87Y4Gd1Ni8 equals to 193 ± 12, 199 ± 30, and 188 ± 18 kJ/mol and the frequency factor (k0) equals to 2.0 × 1012, 3.53 × 1013, and 4.41 × 1011 s −1, respectively. It has been established that during isothermal annealing in the temperature range of 624–643 K, a thermally stable compound Al19Ni5(Y,Gd)3 is formed. Mechanical properties are improved by five and nine times after annealing at the temperatures of nucleation (T1 = 624 ± 1 K) and crystal growth (T2 = 633 ± 1 K) compared to the initial state of the amorphous metal sample. Abstract Amorphous metal alloys (AMAs) are metastable materials that are characterized by good mechanical properties and corrosion properties. It is known that with certain thermal modifications, these properties improve or lose their value. The purpose of this research work is to investigate the optimal conditions of thermal modification that improve the mechanical properties of this alloy. The DSC method established the temperatures of phase transitions in the temperature range of 624–643 K, which correspond to the following processes: crystal nucleation (T1 = 624 ± 1 K), growth (T2 = 633 ± 1 K), and stable crystallization (T3 = 643 ± 1 K). The XRD and TEM/HREM methods revealed structural changes in the amorphous matrix as a result of thermal modification. As a result of isothermal annealing for 2 min. at temperatures T1, T2, T3, a solid solution based on aluminum and a thermally stable compound Al19Ni5(Y,Gd)3 were formed. The equation for the transformation of an amorphous matrix AMA Al87Y4Gd1Ni8 in the temperature range (624–643) ± 1 K during isothermal 2 min annealing is given by: Am → Am′resid+ solid solution Al(X) → Am′(enriched REE) + solid solution Al(X) + nano-Al19Ni5(Y,Gd)3 → Am′(enriched REE) + solid solution Al(X) + nano-Al19Ni5(Y,Gd)3. The Oliver–Pharr method established that AMAs annealed at temperatures T1 and T2 have microhardness indicators 5–9 times higher than amorphous samples; however, the material loses its elasticity under such thermal deformation conditions.