M. A. Bhatia

Extreme creep resistance in a microstructurally stable nanocrystalline alloy

Nanocrystalline metals, with a mean grain size of less than 100 nanometres, have greater room-temperature strength than their coarse-grained equivalents, in part owing to a large reduction in grain size. However, this high strength generally comes with substantial losses in other mechanical properties, such as creep resistance, which limits their practical utility; for example, creep rates in nanocrystalline copper are about four orders of magnitude higher than those in typical coarse-grained copper. The degradation of creep resistance in nanocrystalline materials is in part due to an increase in the volume fraction of grain boundaries, which lack long-range crystalline order and lead to processes such as diffusional creep, sliding and rotation. Here we show that nanocrystalline copper–tantalum alloys possess an unprecedented combination of properties: high strength combined with extremely high-temperature creep resistance, while maintaining mechanical and thermal stability. Precursory work on this family of immiscible alloys has previously highlighted their thermo-mechanical stability and strength, which has motivated their study under more extreme conditions, such as creep. We find a steady-state creep rate of less than 10−6 per second—six to eight orders of magnitude lower than most nanocrystalline metals—at various temperatures between 0.5 and 0.64 times the melting temperature of the matrix (1,356 kelvin) under an applied stress ranging from 0.85 per cent to 1.2 per cent of the shear modulus. The unusual combination of properties in our nanocrystalline alloy is achieved via a processing route that creates distinct nanoclusters of atoms that pin grain boundaries within the alloy. This pinning improves the kinetic stability of the grains by increasing the energy barrier for grain-boundary sliding and rotation and by inhibiting grain coarsening, under extremely long-term creep conditions. Our processing approach should enable the development of microstructurally stable structural alloys with high strength and creep resistance for various high-temperature applications, including in the aerospace, naval, civilian infrastructure and energy sectors.

Atomic scale investigation of grain boundary structure role on intergranular deformation in aluminium

The role that grain boundary (GB) structure plays on the directional asymmetry of an intergranular crack (i.e. cleavage behaviour is favoured along one direction, while ductile behaviour along the other direction of the interface) was investigated using atomistic simulations for aluminium 〈1 1 0〉 symmetric tilt GBs. Middle-tension (M(T)) and Mode-I crack propagation specimens were used to evaluate the predictive capability of the Rice criterion. The stress–strain response of the GBs for the M(T) specimens highlighted the importance of the GB structure. The observed crack tip behaviour for certain GBs (Σ9 (2 2 1), Σ11 (3 3 2) and Σ33 (4 4 1)) with the M(T) specimen displayed an absence of directional asymmetry which is in disagreement with the Rice criterion. Moreover, in these GBs with the M(T) specimen, the dislocation emission from a GB source at a finite distance ahead of the crack tip was observed rather than from the crack tip, as suggested by the Rice criterion. In an attempt to understand discrepancy between the theoretical predictions and atomistic observations, the effect of boundary conditions (M(T), Mode-I and the edge crack) on the crack tip events was examined and it was concluded that the incipient plastic events observed were strongly influenced by the boundary conditions (i.e. activation of dislocation sources along the GB, in contrast to dislocation nucleation directly from the crack tip). In summary, these findings provide new insights into crack growth behaviour along GB interfaces and provide a physical basis for examining the role of the GB character on incipient event ahead of a crack tip and interface properties, as an input to higher scale models.

The role of Ta on twinnability in nanocrystalline Cu–Ta alloys

ABSTRACT Nanostructured Cu–Ta alloys show promise as high-strength materials in part due to their limited grain growth. In the present study, we elucidate the role of Ta on the transition from deformation twinning to dislocation-mediated slip mechanisms in nanocrystalline Cu through atomistic simulations and transmission electron microscopy characterization. In particular, computed generalized stacking fault energy curves show that as Ta content increases there is a shift from twinning to slip-dominated deformation mechanisms. Furthermore, heterogeneous twinnability from microstructural defects decreases with an increase in Ta. The computed effect of Ta on plasticity is consistent with the HRTEM observations. GRAPHICAL ABSTRACT IMPACT STATEMENT We show for the first time using atomistic simulations and TEM that, similar to grain size, the Tanano-particles can be used to tailor the governing deformation mechanisms in NC-alloys.

Investigating occupant safety through simulating the interaction between side curtain airbag deployment and an out-of-position occupant.

The objective of this research is to elucidate the effect of side curtain airbag deployment on occupant injuries and safety when the occupant is either in-position or out-of-position (OOP). We used side impact vehicle collision simulations with a 1996 Dodge Neon model, which was further modified to include a side curtain airbag, a seatbelt, and a 50th percentile Hybrid III dummy. The airbag used in the study was inflated using both the uniform pressure (UP) and smooth particle hydrodynamics (SPH) methods. In-position and OOP simulations were performed to assess and establish guidelines for airbag aggressivity thresholds and occupant position versus risk of injury. Three different OOP scenarios (OOP1, OOP2, OOP3) were initially setup following the work of Lund (2003), then modified such that the dummys head was closer to the airbag, increasing the chance of injury caused by the airbag. The resultant head acceleration as a function of time for in-position and OOP simulations shows that both UP and SPH methods produce similar peak accelerations in cases where the airbag is fully inflated prior to impact. In all cases, the head peak accelerations and the head injury criteria for simulations with an airbag were significantly lower when compared with the no airbag case, which would typically indicate that the use of an airbag results in improved occupant protection during side impact. However, in the case of OOP2 and OOP3, the neck flexion forces actually increase significantly when compared with the no airbag case. This finding indicates that the HIC and neck flexion forces criterion are in conflict and that there may be a tradeoff in terms of occupant injury/safety with a side curtain airbag that is strongly correlated to the occupant position. Consequently, this study shows that safety devices result in a significant effect on occupant injury/safety when the occupant is in OOP conditions. Moreover, in some cases, simulation results show that the side curtain airbag may not make the occupant safer. This study requires further investigation of the vehicle-specific airbag and its interaction with an occupant in various OOP conditions.

Solute Effect on Strength and Formability of Mg: A First-Principle Study

In wrought magnesium alloys, room temperature plasticity is largely controlled by limited slip systems such as basal slip and tension/compression twins. The insufficient number of active slip systems limits strength and ductility preventing broader structural applicability of Mg-alloys. Hence, we employ first-principle calculations to investigate the effects of Y and Al alloying elements on shearability and dislocation motion on various slip systems through ideal shear resistance and generalized stacking fault energy calculations. Yttrium is seen to lower the ideal shear resistance and dislocation motion energetics on all the slip systems. On the other hand, aluminum increases the ideal shear resistance but decreases the energy barrier for dislocation motion on various slip systems. The profound effects of solute addition result from the charge transfer between the solute atom and surrounding magnesium atoms.

Dislocation core properties of β-tin: a first-principles study

Dislocation core properties of tin (β-Sn) were investigated using the semi-discrete variational Peierls–Nabarro (SVPN) model. The SVPN model, which connects the continuum elasticity treatment of the long-range strain field around a dislocation with an approximate treatment of the dislocation core, was employed to calculate various core properties, including the core energetics, widths, and Peierls stresses for different dislocation structures. The role of core energetics and properties on dislocation character and subsequent slip behavior in β-Sn was investigated. For instance, this work shows that a widely spread dislocation core on the {110} plane as compared to dislocations on the {100} and {101} planes. Physically, the narrowing or widening of the core will significantly affect the mobility of dislocations as the Peierls stress is exponentially related to the dislocation core width in β-Sn. In general, the Peierls stress for the screw dislocation was found to be orders of magnitude higher than the edge dislocation, i.e., the more the edge component of a mixed dislocation, the greater the dislocation mobility (lower the Peierls stress). The largest Peierls stress observed was 365 MPa for the dislocation on the {101} plane. Furthermore, from the density plot, we see a double peak for the 0° (screw) and 30° dislocations which suggests the dissociation of dislocations along these planes. Thus, for the {101} slip system, we observed dislocation dissociation into three partials with metastable states. Overall, this work provides qualitative insights that aid in understanding the plastic deformation in β-Sn.

Influence of Yttrium on Creep Behavior in Nano‐Crystalline Magnesium Using Molecular Dynamics Simulation

Due to their light weight, magnesium (Mg) and its alloys have great potential for reducing vehicular mass and energy consumption. However, the use of Mg alloys is currently restricted to low-temperature automotive components application. This work strives to gain a better understanding of the effect of yttrium (Y) (up to 3at%) on creep behavior of columnar nanocrystalline Mg with a grain size of 5nm and 10nm. Using molecular dynamics (MI)) simulations, nanocrystalline Mg with various local concentrations of Y was subjected to a constant-stress loading, ranging from 0–500MPa, at different initial temperatures, ranging from 473–723K. Our simulations reveal that the secondary stage creep rate \((\dot \varepsilon )\) decreases by 71% with the addition of only 1at% Y at 500MPa and 623K. With the addition of alloying elements such as Y, the creep rate in the secondary region decreases and the creep deformation mechanism is changed from the void nucleation, growth, and coalescence to GB rotation/sliding. The analyses of the diffusion coefficient and energy barrier reveal a stronger contribution to the overall deformation by the grain boundary diffusion at the low-temperature (423K) and by the lattice diffusion at the higher-temperature (723K).