Suveen N. Mathaudhu

Microstructure and mechanical properties of tantalum after equal channel angular extrusion (ECAE)

We have investigated the microstructure and mechanical properties of equal channel angular extruded (ECAE) Ta. Mechanical properties were measured both under quasi-static loading and dynamic loading (in the latter case, the compression Kolsky bar technique was employed to attain strain rates of ∼10 3 s −1 ). It is shown that four passes of ECAE with route C at room temperature, which results in an equivalent strain of ∼4.64, increases the strength of Ta by a factor of 2–3 under quasi-static loading, and by a factor of more than 1.5 under dynamic loading. Under quasi-static loading, the ECAE processed samples exhibit almost elastic-perfect plastic behavior; under dynamic loading, slight softening is observed, presumably due to adiabatic heating. It is found that ECAE decreases the strain rate sensitivity. Comparison of the X-ray diffraction (XRD) between the un-processed and ECAE processed Ta indicates significant broadening of the XRD peaks in the ECAE processed sample. Transmission electron microscopy reveals textured, elongated substructures with an average size of about 200 nm, and the substructures are separated by small angle grain boundaries. This work shows the potential for the production of ultra-fine grained or even nano-structured refractory metals with high melting points by using severe plastic deformation. Signs indicating increased shear localization tendancy were observed at high strain rates.

Twinning partial multiplication at grain boundary in nanocrystalline fcc metals

Most deformation twins in nanocrystalline face-centered cubic (fcc) metals have been observed to form from grain boundaries. The growth of such twins requires the emission of Shockley partials from the grain boundary on successive slip planes. However, it is statistically improbable for a partial to exist on every slip plane. Here we propose a dislocation reaction and cross-slip mechanism on the grain boundary that would supply a partial on every successive slip plane for twin growth. This mechanism can also produce a twin with macrostrain smaller than that caused by a conventional twin.

Effects of Alloying Elements on Stacking Fault Energies and Electronic Structures of Binary Mg Alloys: A First-Principles Study

The growth, deformation, and extrinsic faults in binary Mg–X alloys are investigated via first-principles calculations. Here, the alloying elements X include Al, Ca, Cu, Fe, K, La, Li, Mn, Na, Nd, Pr, Si, Sn, Sr, Y, Zn, and Zr. In addition to stacking fault energies, the effect of the elements on the bond structure of Mg are studied in term of electron localization morphology. It is observed that rod-like directional bonds in non-fault planes transform into tetrahedral morphologies in fault planes and are strengthened by addition of Zn and Al, but weakened by Na.

In-situ atomic-scale observation of irradiation-induced void formation

The formation of voids in an irradiated material significantly degrades its physical and mechanical properties. Void nucleation and growth involve discrete atomic-scale processes that, unfortunately, are not yet well understood due to the lack of direct experimental examination. Here we report an in-situ atomic-scale observation of the nucleation and growth of voids in hexagonal close-packed magnesium under electron irradiation. The voids are found to first grow into a plate-like shape, followed by a gradual transition to a nearly equiaxial geometry. Using atomistic simulations, we show that the initial growth in length is controlled by slow nucleation kinetics of vacancy layers on basal facets and anisotropic vacancy diffusivity. The subsequent thickness growth is driven by thermodynamics to reduce surface energy. These experiments represent unprecedented resolution and characterization of void nucleation and growth under irradiation, and might help with understanding the irradiation damage of other hexagonal close-packed materials.

Physics and model of strengthening by parallel stacking faults

We have recently reported that parallel stacking faults (SFs) can tremendously increase the strength of a magnesium alloy. The strengthening is found to increase linearly with the reciprocal of the mean SF spacing, d. In this study we analyze dislocation interactions with SFs, and then propose a physics-based model to explain the observed relationship between yield strength and SFs spacing. Similar to the empirical Hall-Petch relationship for grain size, it is expected that this strengthening mechanism will hold true for a variety of materials engineered with parallel spaced stacking faults over a wide range of fault spacing.

Nanotwins in nanocrystalline Mg–Al alloys: an insight from high-resolution TEM and molecular dynamics simulation

Twinning in hexagonal close-packed Mg alloys has been reported to be unfavorable when the grain size is reduced below a couple of microns and suppressed at the nanoscale. Using high-resolution transmission electron microscopy, we present evidence of nanotwins (<1 nm) in nanocrystalline Mg–Al alloys processed by cryomilling. The commonly observed twinning modes for coarse-grained Mg are identified and supported with atomistic molecular dynamics simulations. The specific thermomechanical conditions offered by cryomilling facilitate the generation of deformation twins that are not observed with conventional deformation processing methods.

Ultrafine and Nanostructured Refractory Metals Processed by SPD: Microstructure and Mechanical Properties

Severe plastic deformation (SPD) has been demonstrated to be the most efficient method to produce bulk metals with ultrafine grained (UFG, 100 nm < grain size d < 500 nm) and nanocrystalline (NC, d<100 nm) microstructures. Such metals exhibit some unique properties owing to their unusual microstructures such as high-energy, non-equilibrium grain boundaries. Efforts in the past two decades have focused on metals with face-centered cubic (fcc) structures. Recent experimental results have shown that UFG/NC metals with body-centered cubic (bcc) structures have some properties that are distinct from their fcc counterparts. Further, the majority of the fcc metals are very ductile and have relatively low melting points, making them easier to process using SPD. On the contrary, many bcc metals are refractory, and are very sensitive to interstitial impurities, rendering them difficult to work via SPD. In this article, we attempt to summarize the state-of-the-art of UFG/NC refractory metals processed by SPD, with focus on the microstructure and mechanical properties. Comparisons with UFG/NC fcc metals are made where appropriate. Outstanding issues and future directions are also addressed.

Magnesium Alloys in U.S. Military Applications: Past, Current and Future Solutions

Since the 1940’s Mg-alloys have been used for military applications, from aircraft components to ground vehicles. The drive for usage was primarily availability and lightweighting of military systems. But the promise of widespread military usage was not met largely based on corrosion and flammability concerns, poor mechanical behavior and inferior ballistic response. This review paper will cover historical, current and potential future applications with a focus on scientific, engineering and social barriers relevant to integration of Mg-alloy. It will also present mechanical and physical property improvements solutions which are currently being developed to address these issues.

Severe plastic deformation of bulk Nb for Nb/sub 3/Sn superconductors

Cast pure Nb with very large grains was processed by multipass equal channel angular extrusion (ECAE) to refine the microstructure. Extrusions were performed on 25 and 50 mm square cross section bars in a right angle die at room temperature following different extrusion routes to strains above nine. The hardness of Nb reaches a saturation level after eight extrusion passes where it is /spl sim/20% above that of fully worked Cu. Recrystallization bands appear to be absent in material processed by a new multipass route (E) which imparts shear on three intersecting planes and gives high processing yields. Hardness and optical microscopy measurements on recrystallized specimens are similar for 25 and 50 mm square bars which indicates a favorable scale-up response. Comparisons to commercially processed Nb and a cost estimate indicate that ECAE may be a viable method for manufacturing fine-grained, homogenous Nb for Nb/sub 3/Sn multifilamentary superconductor applications.

Twinning in cryomilled nanocrystalline Mg powder

Nanocrystalline (nc) Mg powder was synthesized via cryomilling. Extension twins were identified with high-resolution transmission electron microscopy in the cryomilled powders and the study presents the first evidence of twinning in unalloyed nc Mg. The formation of twins in the nc Mg is attributed to a high strain rate, the low (cryogenic) temperature and high local shear stresses present around the grain boundaries during deformation by cryomilling.