ICMCTF 2022 Session H3-TuA: Characterization of Coatings and Small Volumes in Harsh Environments

Nanocrystalline thin films can be alloyed to promote thermal stability via solute segregation to grain boundaries.The segregation can impede boundary migration kinetically via solute drag or Zener pinning, or thermodynamically by reducing the boundary’s energetic cost.In this study we examine a Pt-Au alloy where the Au has been shown to promote boundary stability upon annealing.Going beyond the thermal contribution, we explore the Pt-Au alloys performance under monotonic tension, fatigue loading, wear loading, and ion irradiation.In each of these cases, the presence of solute can provide synergistic benefits on material properties.However, there are cases where the solute is also detrimental; for example, an over abundance of Au at the grain boundaries can lead to an overall embrittlement and reduced ductility.Through a series of in-situ TEM, in-situ SEM, and ex-situ experiments, we explore the complex role of solute segregation on the thermal, mechanical, and radiation properties.Finally, we propose a chemical pathway to achieve gradient nanostructured metals via compositional tailoring.Unlike the existing plastic deformation methods, like surface mechanical attrition, the compositional tailoring can lead to a heterogeneous grain structure that is ‘antifragile’: naturally stabilized against further evolution.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525.

Transition metal nitride (TMN) based thin films have a hugely diverse range of applications due to their unique properties including high hardness, thermal stability, high-temperature oxidation resistance and low coefficient of friction. Recently, high entropy transition metal nitrides (HENs) have proven to be among the most promising ways to advance TMN-based thin films. HENs have gained considerable interest in the scientific community, demonstrating a blend of properties often highly enhanced in comparison to their conventional TMN counterparts. Although the outstanding properties of HENs have been well demonstrated at room temperature, their thermal stability and related high temperature mechanical properties are rarely investigated: the question of their potential as high temperature structural materials remains open.

Here, we investigated the mechanical properties of different HENs at temperatures up to 1000 °C by micromechanical testing. Equimolar (TiHfNbVZr)N and (TiHfNbVZrTa)N thin films alloyed with different Al content were deposited using a novel hybrid method combining high-power impulse magnetron sputtering (HiPIMS) from an Al target with DC magnetron sputtering (DCMS) from high entropy alloy targets, i.e. equiatomic TiHfNbVZr and TiHfNbVZrTa targets, in which a negative substrate bias is synchronized with the Al-rich portion of the HiPIMS pulse. The elevated temperature performance of the HENs was evaluated using a state-of-the-art high-temperature micromechanical testing system to carry out micropillar compression. The tests were performed in situ in an SEM at 25, 500, 700, 800, 900 and 1000 °C under vacuum, allowing observation of the deformation mechanism changes at elevated temperature, and avoiding oxidation. This direct measurement of mechanical properties (strength, plasticity limit, stress-strain behaviour, brittle-to-ductile transition) of HENs at elevated temperatures was interpreted in relation to the microstructure, phase composition, lattice distortion and valance electron concentration of the HENs, as well as phase changes upon annealing, e.g. spinodal decomposition. Further high temperature notched microcantilever fracture toughness measurements are currently underway.

Understanding the changes in mechanical behavior at high deformation speeds and the influence of the microstructure are crucial steps to increase the damage tolerance of components that are exposed to impacts during their lifetime, e.g. safety-relevant components in cars and airplanes. Currently, there is a deficit in experimental techniques for probing the mechanical behavior of coatings and small volumes at high strain rates. In this presentation, we will introduce a nanoindenter with enhanced electronic components, which was used to overcome the indentation strain rate limitation of conventional nanoindentation (ca. 0.1 s^-1). An intrinsically displacement-controlled piezo-actuator is operated in combination with a piezo-based load cell and a 1 MHz data acquisition system. Novel testing methods allow measurements with constant indentation strain rates up to ca. 10⁴ s^-1. This presentation will focus on challenges with the experimental procedures and show first applications to superplastic alloys over several orders of magnitude of indentation strain rate.