Unraveling the Anomalous Grain Size Dependence of Cavitation

High-throughput quantification of quasistatic, dynamic and spall strength of materials across 10 orders of strain rates

The response of metals and their microstructures under extreme dynamic conditions can be markedly different from that under quasistatic conditions. Traditionally, high strain rates and shock stresses are achieved using cumbersome and expensive methods such as the Kolsky bar or large spall experiments. These methods are low throughput and do not facilitate high-fidelity microstructure-property linkages. In this work, we combine two powerful small-scale testing methods, custom nanoindentation, and laser-driven microflyer (LDMF) shock, to measure the dynamic and spall strength of metals. The nanoindentation system is configured to test samples from quasistatic to dynamic strain-rate regimes. The LDMF shock system can test samples through impact loading, triggering spall failure. The model material used for testing is magnesium alloys, which are lightweight, possess high-specific strengths, and have historically been challenging to design and strengthen due to their mechanical anisotropy. We adopt two distinct microstructures, solutionized (no precipitates) and peak-aged (with precipitates) to demonstrate interesting upticks in strain-rate sensitivity and evolution of dynamic strength. At high shock-loading rates, we unravel an interesting paradigm where the spall strength vs. strain rate of these materials converges, but the failure mechanisms are markedly different. Peak aging, considered to be a standard method to strengthen metallic alloys, causes catastrophic failure, faring much worse than solutionized alloys. Our high-throughput testing framework not only quantifies strength but also teases out unexplored failure mechanisms at extreme strain rates, providing valuable insights for the rapid design and improvement of materials for extreme environments.

Synergy of Spall Strength and Toughness in Nanograined Metals

Under shock loading, the spall strength of nanocrystals exhibits intricate grain-size effects due to the presence of abundant grain boundary and dislocation activities. However, the influence of size on spall toughness and void evolution has been largely overlooked. This study employs molecular dynamics simulations to investigate the damage accumulation characteristics of nanocrystalline aluminum across various grain sizes. Unlike the trade-off observed in quasi-static loading conditions, our study reveals a consistency in which grain size governs both nanovoid nucleation and coalescence, yielding a novel spall strength-toughness synergy. These insights highlight grain sizes that are particularly susceptible to spall fracture, offering a crucial understanding of nanocrystal failure mechanisms in extreme environments.

Deformation and failure of the CrCoNi medium-entropy alloy subjected to extreme shock loading

The extraordinary work hardening ability and fracture toughness of the face-centered cubic (fcc) high-entropy alloys render them ideal candidates for many structural applications. Here, the deformation and failure mechanisms of an equiatomic CrCoNi medium-entropyalloy (MEA) were investigated by powerful laser-driven shock experiments. Multiscale characterization demonstrates that profuse planar defects including stacking faults, nanotwins, and hexagonal nanolamella were generated during shock compression, forming a three-dimensional network. During shock release, the MEA fractured by strong tensile deformation and numerous voids was observed in the vicinity of the fracture plane. High defect populations, nanorecrystallization, and amorphization were found adjacent to these areas of localized deformation. Molecular dynamics simulations corroborate the experimental results and suggest that deformation-induced defects formed before void nucleation govern the geometry of void growth and delay their coalescence. Our results indicate that the CrCoNi-based alloys are impact resistant, damage tolerant, and potentially suitable in applications under extreme conditions.

The Quasi-Coarse-Grained Dynamics Method to Unravel the Mesoscale Evolution of Defects/Damage during Shock Loading and Spall Failure of Polycrystalline Al Microstructures

A long-standing problem in modeling of shock response of metals is the ability to model defect nucleation and evolution mechanisms during plastic deformation and failure at the mesoscales. This paper demonstrates the capability of the “quasi-coarse-grained dynamics” (QCGD) simulation method to unravel microstructural evolution of polycrystalline Al microstructures at the mesoscales. The various QCGD simulations discussed here investigate the shock response of Al microstructures comprising of grain sizes ranging from 50 nm to 3.20 µm and correspond to system sizes ranging from 150 nm to 9.6 µm, respectively. The QCGD simulations are validated by demonstrating the capability to retain atomistic characteristics of the wave propagation behavior, plastic deformation mechanisms (dislocation nucleation, dissociation/recombination behavior, dislocation interactions/reactions), evolution of damage (voids), and evolution of temperature during shock loading. The capability to unravel the mesoscale evolution of microstructure is demonstrated by investigating the effect of grain size, shock pulse and system size on the shock response and spall failure of the metal. The computed values of spall strengths predicted using the QCGD simulations agree very well with the trend predicted by MD simulations and a strain rate dependence of the spall strength is proposed that fits the experimentally available values in the literature.