Bharat Gwalani

On the role of Ag in enhanced age hardening kinetics of Mg–Gd–Ag–Zr alloys

Abstract The addition of Ag to the age hardenable Mg–Gd–Zr alloy system dramatically enhances early stage age hardening kinetics. Using atom probe tomography (APT), Ag-rich clusters were detected in a Ag-containing Mg–Gd–Zr alloy immediately after solution treatment and water quenching. During subsequent isothermal ageing at 200 °C, a high density of basal precipitates was observed during the early stages of ageing. These basal precipitates were enriched with Ag and Gd, as confirmed by APT. It is posited that Ag-rich clusters in the context of quenched-in vacancies can attract Gd atoms, increasing diffusion kinetics to facilitate the formation of the Ag + Gd-rich basal precipitates. The rapid formation of Ag + Gd-rich precipitates was responsible for accelerated ageing.

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

Designing and characterizing a complex concentrated gamma/gamma prime ‘superalloy’

Complex concentrated alloys (CCAs), a more general class of high entropy alloys (HEAs) are often near equi-atomic alloys with at-least five different principal components [1,2]. The recently exploding interest in HEAs/CCAs has led to various complex alloy compositions being prepared, characterized using different techniques, and mechanically tested. Though HEAs were initially proposed to have only a single random solid solution phase, in many cases these were found to have ordered intermetallic phases that could in fact be potentially useful for high temperature applications of these complex systems [3]. Hence the motivation of this study is to design HEAs/CCAs containing strengthening ordered L12 precipitates in an fcc matrix (the architecture of nickel or cobalt base super-alloys) [4]. This study focuses on a detailed atom probe tomography (APT) using Imago LEAP 3000X HR Atom Probe Microscope, coupled with high resolution transmission electron microscopy (FEI Tecnai G2 F20 HRTEM) investigation of fcc/L12 HEA systems. Two systems will be discussed, Al0.3CoCrFeNi and Al0.3CuCrFeNi2. These alloys have been melt-processed and subsequently heat-treated to develop an appropriate gamma + gamma prime (fcc+L12) microstructure [4].

Surface degradation mechanisms in precipitation-hardened high-entropy alloys

Design of high-entropy alloys with complex microstructures presents a unique opportunity to combine the best of solid solution strengthening and precipitation hardening. These alloys are potentially disruptive for several high-performance applications including aerospace, oil and gas, nuclear industry, and next-generation tribology. This study presents the thermo-mechanical treatment, microstructural evolution, and surface degradation resistance in precipitation hardenable Al0.3Cu0.3Ti0.2CoCrFeNi high-entropy alloy. The heat-treated alloy consisted of fine precipitates of L12 and L21, strengthening the face-centered cubic solid solution matrix. Potentiodynamic polarization showed nano-galvanic coupling between the intermetallic particles and the matrix. Phase-specific nano-indentation showed that hard intermetallic particles caused three-body wear and higher friction values during sliding wear. The microstructure was correlated with processing conditions and resulting surface degradation resistance using transmission electron microscopy imaging and analysis.High-entropy alloys: effect of hardening on corrosion resistanceHardening high-entropy alloys by introducing precipitates negatively effects corrosion resistance. A team led by Rajarshi Banerjee at the University of North Texas designed a new high entropy alloy, Al0.3Cu0.3Ti0.2CoCrFeNi, whose composition promoted precipitation strengthening of nanometer-scale L12 intermetallics as well as copper clusters. By using cold rolling and annealing at different times and temperatures, the alloys ranged from a solid solution to an alloy with two different types of intermetallics. This caused two different types of corrosion patterns: the more traditional large pits, and basket-weave elongated pits resulting from the galvanic coupling between the two intermetallic phases. These hard intermetallic phases were also dislodged during sliding, contributing to increased wear. Better understanding the corrosion and wear of high-entropy alloys may hasten their industrial implementation.

Influence of Cr Substitution and Temperature on Hierarchical Phase Decomposition in the AlCoFeNi High Entropy Alloy

While the AlCoFeNi high entropy alloy exhibits a single ordered B2 phase at high temperature, both the substitution of ferromagnetic Co with antiferromagnetic Cr, and lower annealing temperatures lead to a tendency for this system to decompose into a two-phase mixture of ordered B2 and disordered BCC solid solution. The length scale of this decomposition is determined by the combination of composition and annealing temperature, as demonstrated in this investigation by comparing and contrasting AlCoFeNi with the AlCo0.5Cr0.5FeNi alloy. The resulting phase stability has been rationalized based on solution thermodynamic predictions. Additionally, it is shown that replacement of Co by Cr in the AlCoFeNi alloy resulted in a substantial reduction in saturation magnetization and increase in coercivity. The microhardness is also strongly influenced by the composition and the length scale of B2 + BCC decomposition in these high entropy alloys.

Microstructural Design for Improving Ductility of An Initially Brittle Refractory High Entropy Alloy

Typically, refractory high-entropy alloys (RHEAs), comprising a two-phase ordered B2 + BCC microstructure, exhibit extraordinarily high yield strengths, but poor ductility at room temperature, limiting their engineering application. The poor ductility is attributed to the continuous matrix being the ordered B2 phase in these alloys. This paper presents a novel approach to microstructural engineering of RHEAs to form an “inverted” BCC + B2 microstructure with discrete B2 precipitates dispersed within a continuous BCC matrix, resulting in improved room temperature compressive ductility, while maintaining high yield strength at both room and elevated temperature.

High-entropy alloy strengthened by in situ formation of entropy-stabilized nano-dispersoids

A significant increase in compressive yield strength of the Al0.3CoCrFeMnNi high-entropy alloy (HEA) from 979 MPa to 1759 MPa was observed upon the introduction of 3 vol.% Y2O3. The HEAs were processed using spark plasma sintering of mechanically alloyed powders. Transmission electron microscopy and atom probe tomography confirmed the presence of compositionally complex nano-dispersoids in the Y2O3-added HEA. The significant increase in strength can be attributed to the nano-dispersoid strengthening coupled with grain refinement. Therefore, the in-situ formation of the compositionally complex nanoscale dispersoids during the alloy processing could be a novel approach to create entropy-stabilized oxide particles in strengthening of HEAs.

Investigation of Clusters and Their Effect on Grain Growth in Single Phase AlxCoCrFeNi High Entropy Alloys

Solid solution strengthening, within random solid solutions, is a fundamental strengthening mechanism in most alloy systems. While traditionally this strengthening mechanism was largely developed and applied to alloys with one principal component, this notion can be easily extended to high entropy alloys (HEAs). This new class of alloys typically consists of five or more principal components in near equi-atomic proportion. The high configurational entropy in these alloys, often results in the stabilization of single concentrated solid solution phases.

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.

Grain Boundary Precipitation in Ni Based Superalloy 690 Investigated via Site-specific Atom Probe Microscopy

Alloy 690 has been proposed to be an alternative to Alloy 600 for use in tubing materials that carry the super-heated steam onto the turbine blades of land-based nuclear power generators. Alloy 690 is known to be more resistant to primary water stress corrosion cracking than alloy 600, consequently aiding in the life extension process of nuclear reactors [1]. A thorough study of the long term aging response of alloy 690 is yet to be done in great detail.