D. Choudhuri

Evolution of a honeycomb network of precipitates in a hot-rolled commercial Mg–Y–Nd–Zr alloy

Coupled processes of dynamic recovery and precipitation, occurring during hot-rolling and subsequent aging, lead to the formation of a unique honeycomb network of precipitates in commercial Mg–Y–Nd–Zr or WE43 alloy. The honeycomb network is developed on the (0 0 0 1)Mg basal planes and consisted of fine Nd-rich β 1 platelets lying on all three planes presumably decorating recovery-generated dislocation subcell boundaries. Three variants of β 1 platelets are connected by Y-rich precipitates at the nodes of the hexagonal honeycomb network.

The effect of boron on the grain size and texture in additively manufactured β-Ti alloys

AbstractOne of the critical microstructural attributes affecting the properties of additively manufactured (AM) alloys is the growth of large columnar grains along the build direction. While most of the work in the reported literature is focused on Ti–6Al–4V and other α/β alloys, there are rather limited investigations on grain growth and texture development in AM β-Ti alloys. The addition of trace amounts of boron to these AM β-Ti alloys resulted in significant changes in the microstructure. Depending on the alloy system, a grain refinement of 50–100 times was noted. The change in the grain size has been attributed to a combined effect of constitutional supercooling, caused by boron rejection from the growing β grains, and the growth restriction factor (Q) of the grains caused by the solute elements. The addition of boron also changed the morphology of the grains from being columnar to more equiaxed, a much more pronounced change than observed in traditional α/β alloys such as Ti–6Al–4V. A change in texture of the β grains along the build direction was also noted, wherein the addition of boron randomized the texture from the typically observed strong (001)β oriented grains in AM Ti alloys. Finally, the addition of boron changed the morphology of the α precipitates in the Ti–Mo system from lath-like to more equiaxed, while preserving the Burgers orientation relationship between the α and β phases.

Precipitation in Uniaxially Stressed Mg-Nd Alloys During Creep Testing

The role of uniaxial stress on the precipitation behavior in a Mg-2.6wt pct Nd alloy has been examined by comparing creep-tested and isothermally aged samples using transmission electron microscopy. Both types of samples exhibited precipitation of the β′(orthorhombic) and β1(fcc) phases. During creep testing, dynamically formed β1 precipitates are aligned along favorable slip directions within the α-Mg matrix, presumably influencing the creep response of the Mg-Nd alloy.

Strengthening strategy for a ductile metastable β-titanium alloy using low-temperature aging

ABSTRACT While ω precipitates in metastable β titanium alloys are typically considered embrittling and consequently deleterious to the mechanical properties, this paper demonstrates that low-temperature aging (LTA) treatments for short time periods can in fact enhance the yield strength while preserving substantial elongation-to-failure in ω containing β titanium alloys. LTA treatments, carried out on a ductile β metastable Ti–12Mo alloy, significantly improve the yield strength of the material (∼55% increase as compared to solution-treated samples) while keeping both TRIP/TWIP effects and a large elongation-to-failure (ϵ = 0.4 in true strain), resulting in a balance of mechanical properties. IMPACT STATEMENT Employing a novel low-temperature annealing strategy, omega precipitates, typically considered embrittling in beta titanium alloys, can effectively increase the yield strength by 50% while preserving 40% tensile ductility. GRAPHICAL ABSTRACT

Crystallographically degenerate B2 precipitation in a plastically deformed fcc-based complex concentrated alloy

ABSTRACT Bcc-ordered B2 and fcc phases manifest three different orientation relationships (ORs) in the same microstructure: Kurdjumov–Sachs, Nishiyama–Wasserman and Pitsch. This unique microstructure was developed via conventional cold-rolling and subsequent annealing of an fcc-based Al0.3CoCrFeNi complex concentrated alloy (CCA). The degeneracy in crystallographic ORs was caused by {111}⟨112⟩twins, on multiple {111}, from the prior cold-rolling step. Annealing produced B2 precipitates on all the major fcc slip-systems by heterogeneously nucleating B2 at twin-matrix interfaces and twin–twin intersections. Such a precipitation-hardenable microstructure is expected to increase the strength of fcc-based CCAs by effectively blocking 1/2⟨110⟩and 1/6⟨112⟩mobile dislocations. Impact statement Three different fcc-B2 orientation relationships (ORs) were observed for the first time in complex concentrated alloys. Such degenerate ORs in B2 precipitation can potentially block dislocation on multiple slip planes. GRAPHICAL ABSTRACT

Exceptional increase in the creep life of magnesium rare-earth alloys due to localized bond stiffening

Several recent papers report spectacular, and unexpected, order of magnitude improvement in creep life of alloys upon adding small amounts of elements like zinc. This microalloying effect raises fundamental questions regarding creep deformation mechanisms. Here, using atomic-scale characterization and first principles calculations, we attribute the 600% increase in creep life in a prototypical Mg–rare earth (RE)–Zn alloy to multiple mechanisms caused by RE–Zn bonding—stabilization of a large volume fraction of strengthening precipitates on slip planes, increase in vacancy diffusion barrier, reduction in activated cross-slip, and enhancement of covalent character and bond strength around Zn solutes along the c-axis of Mg. We report that increased vacancy diffusion barrier, which correlates with the observed 25% increase in interplanar bond stiffness, primarily enhances the high-temperature creep life. Thus, we demonstrate that an approach of local, randomized tailoring of bond stiffness via microalloying enhances creep performance of alloys.Adding very small amounts of zinc to magnesium alloys containing rare earth elements dramatically improves their creep life. Here, the authors use first principles calculations and atomic-scale characterization to show that this is due to stiff covalent bonding of zinc and rare earth elements such as neodymium.

Hierarchical features infused heterogeneous grain structure for extraordinary strength-ductility synergy

ABSTRACT A synergistic balance of strength and ductility was achieved in a prototypical fcc-based Al0.3CoCrFeNi complex concentrated alloy by incorporating hierarchical microstructural features into heterogeneous grain structure. Microstructural hierarchy was composed of different morphologies and size-scales of B2 precipitates and nano-twins that were incorporated in parent fcc matrix, which, additionally, was comprised of domains of fine and coarse grains. Strain partitioning between refined and coarse grains produced geometrically necessary dislocations during plastic deformation. This facilitated long-range back stresses during further deformation leading to simultaneous enhancement of strength and ductility. Furthermore, B2 precipitates complemented back stress and increased inherent matrix strength. GRAPHICAL ABSTRACT IMPACT STATEMENT Architecting hierarchical microstructural features into a heterogeneous grain structured complex concentrated alloy avoided the classic strength-ductility trade-off paradigm.

Bonding Environments in a Creep–Resistant Mg–RE–Zn Alloy

Density functional theory (DFT) based first principle calculations was used to examine the effect of transitional element Zn addition on the bonding environment of Mg–Gd solid solutions. Our calculations reveal that Zn strongly interacts with Gd, while simultaneously disperses the p-orbital valence electron density of Zn along the [0001]Mg and \( \left\langle { 1 1 {\bar{\text{2}}}0} \right\rangle_{\text{Mg}} \) directions of hcp-Mg lattice. These results suggest that Zn addition stiffens the Mg–Mg bonds between the {0002}Mg-basal planes, and along \( \left\langle { 1 1 {\bar{\text{2}}}0} \right\rangle_{\text{Mg}} \). The enhanced bonding between the Mg basal planes may potentially drives basal precipitation in Mg–Gd–Zn alloys seen in experiments. On the other hand, bond stiffening along \( \left\langle { 1 1 {\bar{\text{2}}}0} \right\rangle_{\text{Mg}} \) noticeably increased the vacancy migration barrier for basal plane diffusion in Mg. This increase has bearing on the high temperature creep properties, because vacancy diffusion is a dominant creep deformation mechanism at operation temperatures of 150–300 °C for Mg-alloy parts. Thus, our calculations predict that Zn addition to Mg–Gd alloy will strongly influence its microstructure and creep response.

Creep Response of a Zn Containing Mg-Nd-La Alloy

A significant improvement in creep resistance was achieved by adding Zn to a Mg-Nd-La alloy. Such an improvement is indicated by an order of magnitude reduction in minimum creep rate by and maintaining remarkably low creep-strain for prolonged duration compared to the non-Zn containing alloy. Addition of Zn resulted in the formation of high number density of fine scale γ” precipitates which presumably enhanced the load bearing capacity of Mg-Nd-La-Zn alloy. Observation of dislocation substructures further revealed that both intragranular precipitates as well as the interdendritic phase retarded dislocation motion.

Compositional Analysis of as-cast and Crystallized Pd 43 Cu 27 Ni 10 P 20 Bulk Metallic Glass

Noble metal-based bulk metallic glasses (BMG) composed of Pd-Cu-Ni-P are viewed as potential materials for use in thermoplastic nanofabrication/transfer mold lithography [1] and electrocatalytic applications [2]. The stability, formability and physical properties of these glasses are related to the kinetic pathways taken towards their decomposition and crystallization, which can be probed by analyzing on the nanometer scale the local composition and structure. Atom probe tomography (APT) is capable of atomic resolution elemental mapping and with the use of laser pulsing has proven to be capable of efficiently detecting nanometer scale compositional fluctuations indicative of phase separation in BMGs [3].