Zhaoxuan Wu

The origins of high hardening and low ductility in magnesium

Magnesium is a lightweight structural metal but it exhibits low ductility—connected with unusual, mechanistically unexplained, dislocation and plasticity phenomena—which makes it difficult to form and use in energy-saving lightweight structures. We employ long-time molecular dynamics simulations utilizing a density-functional-theory-validated interatomic potential, and reveal the fundamental origins of the previously unexplained phenomena. Here we show that the key 〈c + a〉 dislocation (where 〈c + a〉 indicates the magnitude and direction of slip) is metastable on easy-glide pyramidal II planes; we find that it undergoes a thermally activated, stress-dependent transition to one of three lower-energy, basal-dissociated immobile dislocation structures, which cannot contribute to plastic straining and that serve as strong obstacles to the motion of all other dislocations. This transition is intrinsic to magnesium, driven by reduction in dislocation energy and predicted to occur at very high frequency at room temperature, thus eliminating all major dislocation slip systems able to contribute to c-axis strain and leading to the high hardening and low ductility of magnesium. Enhanced ductility can thus be achieved by increasing the time and temperature at which the transition from the easy-glide metastable dislocation to the immobile basal-dissociated structures occurs. Our results provide the underlying insights needed to guide the design of ductile magnesium alloys.

Nanowire Failure: Long = Brittle and Short = Ductile

Experimental studies of the tensile behavior of metallic nanowires show a wide range of failure modes, ranging from ductile necking to brittle/localized shear failure-often in the same diameter wires. We performed large-scale molecular dynamics simulations of copper nanowires with a range of nanowire lengths and provide unequivocal evidence for a transition in nanowire failure mode with change in nanowire length. Short nanowires fail via a ductile mode with serrated stress-strain curves, while long wires exhibit extreme shear localization and abrupt failure. We developed a simple model for predicting the critical nanowire length for this failure mode transition and showed that it is in excellent agreement with both the simulation results and the extant experimental data. The present results provide a new paradigm for the design of nanoscale mechanical systems that demarcates graceful and catastrophic failure.

Mechanisms of Failure in Nanoscale Metallic Glass

The emergence of size-dependent mechanical strength in nanosized materials is now well-established, but no fundamental understanding of fracture toughness or flaw sensitivity in nanostructures exists. We report the fabrication and in situ fracture testing of ∼70 nm diameter Ni-P metallic glass samples with a structural flaw. Failure occurs at the structural flaw in all cases, and the failure strength of flawed samples was reduced by 40% compared to unflawed samples. We explore deformation and failure mechanisms in a similar nanometallic glass via molecular dynamics simulations, which corroborate sensitivity to flaws and reveal that the structural flaw shifts the failure mechanism from shear banding to cavitation. We find that failure strength and deformation in amorphous nanosolids depend critically on the presence of flaws.

Magnesium interatomic potential for simulating plasticity and fracture phenomena

Magnesium has multiple dislocation and twinning systems with starkly different properties, which make its plastic deformation strongly anisotropic and highly complex. Existing empirical interatomic potentials fail to capture the full scope of these properties, making current molecular statics and dynamics simulation results of limited quantitative and predictive use. Here, based on the work by Kim et al, a new modified embedded-atom method potential for magnesium is introduced and rigorously validated against existing ab initio, continuum theory and experimental results. The new potential satisfactorily reproduces all the necessary mechanical properties for plastic deformation, including the various generalized stacking fault energy surfaces, dislocations core structures, Peierls stresses, surface energies and basal plane cohesive strength. The capability of this potential to accurately describe all the important slip systems and fracture behavior makes it valuable for future realistic atomistic studies of general magnesium deformation and failure problems.

Mechanistic origin and prediction of enhanced ductility in magnesium alloys

A framework for more ductile magnesium Development of ductile magnesium alloys is key to their use in reducing the weight of vehicles and other applications. Wu et al. tackle this issue by determining the underlying mechanisms in unprocessed magnesium alloys. Dilute amounts of solutes enhanced certain ductility-improving mechanisms over ones that cause brittle fracture. From this, the authors developed a theory that may be helpful for screening the large number of potential magnesium alloy compositions. Science, this issue p. 447 Specific solutes promote ductility-improving slip mechanisms in magnesium alloys. Pure magnesium exhibits poor ductility owing to pyramidal 〈 c+a 〉 dislocation transformations to immobile structures, making this lowest-density structural metal unusable for many applications where it could enhance energy efficiency. We show why magnesium can be made ductile by specific dilute solute additions, which increase the 〈 c+a 〉 cross-slip and multiplication rates to levels much faster than the deleterious 〈 c+a 〉 transformation, enabling both favorable texture during processing and continued plastic straining during deformation. A quantitative theory establishes the conditions for ductility as a function of alloy composition in very good agreement with experiments on many existing magnesium alloys, and the solute-enhanced cross-slip mechanism is confirmed by transmission electron microscopy observations in magnesium-yttrium. The mechanistic theory can quickly screen for alloy compositions favoring conditions for high ductility and may help in the development of high-formability magnesium alloys.

Dislocation junctions as barriers to threading dislocation migration

Level set simulations of dislocation dynamics in biaxially strained, heteroepitaxial films reveal interesting kinetic and thermodynamic mechanisms for blocking the migration of threading dislocations. Two dislocations on the same or on intersecting slip planes may react to form a threading dislocation segment that does not glide under the influence of the misfit strain. In the coplanar case, a kinetic barrier exists that slows down dislocation migration. For the reaction involving dislocations on intersecting planes, an energetic barrier impedes other advancing dislocations. These barriers create significant and frequent impediment to threading dislocation flow, resulting in pileups and high threading dislocation densities.