Talukder Alam

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.

Understanding the Origins of Intergranular Corrosion in Copper-Containing Al-Mg-Si Alloys

A definitive understanding of the mechanism of intergranular corrosion (IGC) in under-aged (UA) Cu-containing Al-Mg-Si alloys has not been clear to date. The grain boundary microstructure and chemistry in an UA Cu-containing Al-Mg-Si alloy were characterized by coupling atom probe tomography and scanning transmission electron microscopy. The rapid formation of an ultra-thin wetting Cu layer and discrete Q-phase (Al4Cu2Mg8Si7) precipitates along the grain boundaries, and a precipitate-free zone adjacent to the grain boundaries in the UA condition contribute to IGC.

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.

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

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].

On the Role of C Addition on α Precipitation in a β Titanium Alloy

The onset of α nucleation in a carbon containing β-titanium alloy has been investigated by coupling atom probe tomography (APT) with transmission electron microscopy. The analysis of the APT results indicates that in addition to ω precipitates that can act as potential α nucleation sites, carbon atoms tend to form clusters within the β-matrix, which in turn give rise to additional nucleation sites for α, resulting in finer scale α precipitates due to increased nucleation density.

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.

Investigation of Novel Phase Transformation Mechanisms in Titanium Alloys Using Atom Probe and Aberration-Corrected Scanning Transmission Electron Microscope

Metastable beta titanium alloys have attracted significant amount of attentions in recent years, because their microstructures can be manipulated by different phase transformation mechanisms and therefore the corresponding mechanical properties vary significantly by adopting various thermal/mechanical processes. In our recent studies, coupling atom probe and aberration-corrected scanning transmission electron microscope, several novel phase transformation mechanisms have been systematically explored in a beta titanium alloy, Ti-5Al-5Mo-5V-3Cr (Ti-5553, wt.%) [1-4]. It has been clearly shown that the size, morphology and number density of hcp structure alpha precipitates in Ti-5553 can be significantly influenced by the nano-scale structural and compositional instability, more specifically the metastable hexagonal structure isothermal omega phase, present in parent bcc structure beta phase. Computational simulation has shown that the compositional and/or stress field associated with such nano-scale instabilities may contribute to an increased driving force for alpha nucleation [3, 5, 6]. In order to provide accurate input into computational simulation, the compositional and structural insights of nanoscale instabilities in parent phase matrix are highly demanded in experiment. Such detailed characterization is relied on the recent technological improvement in atom probe and TEM that has the potential to provide atomic resolution information in titanium alloys.

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.