Christopher A. Schuh

A nanoindentation study of serrated flow in bulk metallic glasses

Abstract Plastic deformation of two Pd- and two Zr-based bulk metallic glasses (BMGs) is investigated through the use of nanoindentation, which probes mechanical properties at the length scale of shear bands, the carriers of plasticity in such alloys. These materials exhibit serrated flow during nanoindentation, manifested as a stepped load-displacement curve punctuated by discrete bursts of plasticity. These discrete “pop-in” events correspond to the activation of individual shear bands, and the character of serrations is strongly dependent on the indentation loading rate; slower indentation rates promote more conspicuous serrations, and rapid indentations suppress serrated flow. Analysis of the experimental data reveals a critical applied strain rate, above which serrated flow is completely suppressed. Furthermore, careful separation of the plastic and elastic contributions to deformation reveals that, at sufficiently low indentation rates, plastic deformation occurs entirely in discrete events of isolated shear banding, while at the highest rates, deformation is continuous, without any evidence of discrete events at any size scale. All of the present results are consistent with a kinetic limitation for shear bands, where at high rates, a single shear band cannot accommodate the imposed strain rapidly enough, and consequently multiple shear bands must operate simultaneously.

Nanoindentation studies of materials

Nanoindentation has become a commonplace tool for the measurement of mechanical properties at small scales, but may have even greater importance as a technique for experimental studies of fundamental materials physics. With high-resolution load-displacement data, discrete events including dislocation source activation, shear instability initiation, and phase transformations can be detected during a nanoindentation test. Recently-developed capabilities in, for example, high-temperature nanoindentation testing and in situ imaging of the indented volume, offer new quantitative details about these phenomena, and present many opportunities for future scientific inquiry.

The effect of solid solution W additions on the mechanical properties of nanocrystalline Ni

Abstract Although pure metals with grain sizes below about 10 nm are very difficult to prepare, alloying enables the realization of finer grain sizes, often down to the amorphous limit. In this work, the role of solid solution additions of ~13 at% W are considered with respect to the structure and mechanical properties of electrodeposited Ni alloys with grain sizes below 10 nm. Structure of the nanocrystalline alloys is analyzed by high-resolution transmission electron microscopy, and related to the mechanical properties assessed by instrumented nanoindentation and nano-scratch experiments. The Ni-W alloys exhibit higher hardness and scratch resistance as compared to the finest pure nanocrystalline Ni alloys, although the contribution of solid solution strengthening from W is expected to be essentially negligible. The improved properties are therefore most likely due to the finer length scale available in multicomponent nanocrystalline alloys, and suggest that alloying may suppress the breakdown of Hall-Petch strengthening to finer grain sizes. Finally, the present data are shown to smoothly bridge the hardness-grain size trend between nanocrystalline Ni (grain size>10 nm) and amorphous Ni-based alloys.

Hall–Petch breakdown manifested in abrasive wear resistance of nanocrystalline nickel

Abstract The abrasion resistance of electrodeposited nanocrystalline nickel is investigated using the nanoscratch technique with a ramping load. At the finest grain sizes studied (12–14 nm), a breakdown in Hall–Petch hardening is observed directly in hardness data, as well as indirectly in scratch resistance. The changes in abrasive wear behavior are quantitatively commensurate with the changes in hardness, despite the apparent transition in deformation mechanisms at the finest grain sizes.

Analysis of grain boundary networks and their evolution during grain boundary engineering

Abstract The goal of grain boundary engineering is to increase the fraction of so-called special grain boundaries, while decreasing the contiguity of the remaining random boundaries which are susceptible to intergranular degradation such as cracking, cavitation, corrosion and rapid self-diffusion. In the present work, we describe a technique for the quantitative experimental study of grain boundary network topology, with an emphasis on the connectivity of special and random grain boundaries. Interconnected grain boundary networks, or “clusters”, of either entirely random or entirely special boundaries are extracted from electron backscatter diffraction data on a Ni-base alloy, and characterized according to their total normalized length (their “mass”), as well as their characteristic linear dimensions. The process of grain boundary engineering, involving cycles of straining and annealing, is found to substantially reduce the mass and size of random boundary clusters. Furthermore, quantitative assessment of the boundary network topology shows that the special grain boundary fraction is a poor predictor of network topology, but that the higher-order correlation derived from triple junction distributions can successfully describe the length scales of random boundary clusters.

Yield surface of a simulated metallic glass

Abstract We perform molecular simulations of multiaxial deformation in a model metallic glass using a zero-Kelvin energy minimization technique. We find that there is a pronounced asymmetry between the magnitudes of the yield stresses in tension and compression, with the uniaxial compressive strength approximately 24% higher. By exploring a variety of biaxial stress states, we probe the plane-stress yield surface and find that it is not well-described by traditional yield criteria that depend only on the maximum shear stress. In contrast, the Mohr–Coulomb criterion, which includes an additional normal stress term, is found to describe the data quite well. Finally, the simulation results are shown to compare favorably with the available experimental data.

Nanoscale shape-memory alloys for ultrahigh mechanical damping

Shape memory alloys undergo reversible transformations between two distinct phases in response to changes in temperature or applied stress. The creation and motion of the internal interfaces between these phases during such transformations dissipates energy, making these alloys effective mechanical damping materials. Although it has been shown that reversible phase transformations can occur in nanoscale volumes, it is not known whether these transformations have a sample size dependence. Here, we demonstrate that the two phases responsible for shape memory in Cu-Al-Ni alloys are more stable in nanoscale pillars than they are in the bulk. As a result, the pillars show a damping figure of merit that is substantially higher than any previously reported value for a bulk material, making them attractive for damping applications in nanoscale and microscale devices.

Strength, plasticity and brittleness of bulk metallic glasses under compression: statistical and geometric effects

To investigate the flaw sensitivity and reliability of bulk metallic glasses (BMGs), compressive testing was performed on a statistically significant number of specimens. Despite the fact that BMGs exhibit little or no macroscopic plasticity before failure (similar to other brittle materials), we observe surprisingly high uniformity in compressive strength. Weibull analysis was employed to study the statistical dispersion in strength, giving very high Weibull moduli of about 25 for an intrinsically brittle glass, and near 75 for an intrinsically malleable one. This high uniformity is encouraging for the use of BMGs in structural applications. Furthermore, we illustrate that subtle imperfections in the test geometry (i.e. miscut or deviations from orthogonality) dramatically affect the compression response. In brittle glasses these act as failure-critical flaws, whereas in malleable glasses they constrain shear bands, lead to tilting and bending during testing, and give rise to misleading macroscopic measurements of plastic deformation.

Microstructural evolution during the heat treatment of nanocrystalline alloys

Nanocrystalline alloys often show exceptional thermal stability as a consequence of kinetic and thermodynamic impediments to grain growth. However, evaluating the various contributions to stability requires detailed investigation of the solute distribution, which is challenging within the fine structural-length-scales of nanocrystalline materials. In the present work, we use a variety of techniques to assess changes in the grain size, chemical ordering, grain-boundary segregation, and grain-boundary structure during the heat treatment of Ni–W specimens synthesized over a wide range of grain sizes from 3 to 70 nm. A schematic microstructural evolution map is also developed based on analytical models of the various processes activated during annealing, highlighting the effects of alloying in nanocrystalline materials.

Application of nucleation theory to the rate dependence of incipient plasticity during nanoindentation

We propose a nucleation theory-based analysis for incipient plasticity during nanoindentation and predict the statistical distribution of rate-dependent pop-in events for many nominally identical indentations on the same surface. In the framework of stress-assisted, thermally activated defect nucleation, we quantitatively rationalize new nanoindentation measurements on 4H SiC and extract the activation volume of the nucleation events that mark the onset of plastic flow. We also illustrate how this statistical approach can differentiate between unique nucleation events for different indenter tip geometries.