High-Temperature Mechanical Properties and Structural Change in Amorphous Al–Ni–Fe–Nd Alloys

Abstract

Microstructures and tensile properties of the amorphous Al 88 Ni 10-x Fe x Nd 2 (X= 0, 3, 5 at%) alloys at elevated temperatures and their relations with thermal stability were investigated. It was found that the crystallization of the three alloys proceeded through two stages. One is the formation of an Al phase at the low temperature side and the other is the formation of intermetallic compounds (Al 3 Ni, Al 3 Fe, Al 11 Nd 3 ) at the high temperature side. The Al particles smaller than 15 nm and compounds larger than 50 nm in diameter were formed during uniaxial tension tests and found to be uniformly distributed in the amorphous matrix. The ultimate tensile strengths of the three alloys were above 800 MPa at 580 K, which were 3 - times stronger than conventional high-strength Al alloys with heat resistance. This is possibly due to both the dispersion strengthening effect by the defect-free Al particles and the enhanced thermal stability by the depletion of Al atoms in the amorphous matrix resulting from the formation of the Al particles. In contrast, a significant decrease in strength was observed at a temperature above 580 K. This is presumably because the concentration of the solute atoms in the amorphous matrix was greatly decreased below the nominal concentration by the formation of the compounds. Elongation increased significantly from 2-3% at room temperature to over 10-20% at the temperatures higher than the crystallization temperatures. The temperature at which a largest elongation in the three alloys was obtained corresponded to the exothermic peak temperature in the DSC curve. This fact indicated that such a large elongation originated from the precipitation of the crystalline phase. On the other hand, in a temperature range in which the Al phase is formed, deformation occurs through a shear deformation mode, whereas in a higher temperature range in which the compounds are formed, it occurs viscoelastically.