Thermal stability, crystallization kinetics and mechanical properties of Al81.8Ni9.2Co4.8Y3.1Zr1.1 amorphous alloy consolidated to a fully dense nanocrystalline matrix with some remaining amorphous phase

High-strength nanocrystalline-embedded-alloy was successfully fabricated by the consolidation of melt-spun amorphous ribbons of Al81.8Ni9.2Co4.8Y3.1Zr1.1. The as-melt amorphous spun, the annealed and the fully dense nanocrystalline extruded specimens were investigated using XRD, HRTEM, SAED, EDX line mapping, microhardness, compression tests, SEM at fractured surfaces and the crystallization's kinetics were determined from DSC. XRD, in good accordance with HRTEM, SAED and the associated EDX line mapping analysis, revealed the merely amorphous nature of the as-melt spun ribbons and confirmed bearing of nanocrystalline-embedded phases in the annealed and extruded specimens with the remaining of some amorphous phase. HRTEM and SAED revealed that annealing at 275 °C resulted in the formation of short-range ordered regions of fcc α-Al nanoparticles and intermetallic nanocrystalline-embedded grains of 5-10 nm that grown to 20-50 nm after extrusion. DSC showed that the as-melt spun ribbons crystallize at three stages, in contrast to one late stage shown by the extruded sample. Crystallization kinetics of the as-spun ribbons yielded crystallization activation energy (Ec) of 143.4 and 36 kJ/mol for the first and second crystallization peaks respectively. The extruded specimen showed improved strength of 550 at a load of 50 g while the as-melt spun ribbons indicated lower hardness of 367 at a similar load. The ultimate compressive stress of the extruded specimen reached 800 MPa with 0.66% strain and Young's modulus of 1.26 GPa. SEM micrographs of the fracture surface showed a transgranular failure with a dimpled surface reflecting the microplasticity of the final product. Improved mechanical properties and thermal stability of the nanocrystalline-embedded-specimen are due to the nanoparticle strengthening effect of the very hard and non-deformable nanosized α-aluminum and Al-based intermetallic nanograins that are densely formed during the annealing, extrusion and nano-indentation processes.