17 - Mechanical properties of L12 nanostructures

Abstract

Ni3Al and Fe3Al nanosized specimens were tested to understand their mechanical properties. According to the Hall–Petch relation decreasing grain size should increase stress. Atomic force microscope (AFM)-based testing was used to characterize Ni3Al particles (γ′) by nanoindentation. Isolated, freestanding γ′ particles were extracted from a binary alloy. Load–depth curves of these nanoparticles were used to evaluate the hardness and the Young’s modulus. The measured hardness increases slightly with increasing load. In all of the tests, no work hardening was observed and the maximum strength of the sample corresponds to the yield strength at which the plastic instability first sets in. The absence of work hardening is due to dislocations annihilation at the free surface at a high rate. The scanning electron microscope (SEM) fracture surfaces of Fe3Al reveal that the fracture mode was not sensitive to the test environment. Specimens exhibited transgranular cleavage fracture and river pattern even though the tensile ductility was quite different. Increasing Al changes fracture mode from transgranular cleavage to grain boundary fracture. A high ductility of 12% is obtained in Fe3A1 when tested in vacuum, but environmental embrittlement at ambient temperatures is induced by water vapor.