Micromechanical characterization of wire-arc additive manufactured and cast nickel aluminum bronze: Ambient and intermediate temperatures

Abstract

Instrumented indentation is a semi-destructive and robust technique to assess micromechanical characteristics (e.g., local properties) of metallic materials at ambient and elevated temperatures. In the present study, the instrumented indentation is employed to assess and compare the micromechanical response and fundamental mechanisms of plastic deformation (e.g., rate-controlling plasticity) in wire-arc additively manufactured (WAAM) and cast nickel aluminum bronze (Cu–9Al–4Fe–4Ni–1Mn) at ambient and intermediate temperatures. Two separate sets of indentation-based test schedules are employed to investigate these mechanisms. In the first set of experiments, load-controlled (peak load of 500 mN) indentation tests are performed under various loading rates (5–50 mN/s) at a constant (25 °C) temperature. In the second set of experiments, load-controlled indentation tests (peak load of 200 mN) at various temperatures of 25, 200, and 300 °C are performed under a constant loading rate. Indentation load versus indentation depth and time data are analyzed to collect a wealth of information including indentation stress, indentation strain rate, size effect, dislocation activation energy, and activation volume to assess fundamentals of governing mechanisms of plastic deformation. Optical, scanning, and transmission electron microscopy are utilized to provide microstructural evidence for the proposed micromechanical mechanisms of plastic deformation based on dislocation activities and interactions. The nanoindentation hardness results show that the strength of the WAAM alloy is higher than the cast counterpart. However, neither cast nor WAAM-NAB materials are rate-sensitive at ambient (room) temperature. When the temperature is raised, both alloys become softer, and the indentation size effect becomes less pronounced. The activation volume and the activation energy data show that a steady-state microstructure does not necessarily exist around the indentations, and the nature of rate-limiting obstacles (against dislocation activities) may vary in the course of the indentation.