Christopher R. Hutchinson

On the origin of the high coarsening resistance of Ω plates in Al–Cu–Mg–Ag Alloys

The thickening kinetics of Ω plates in an Al–4Cu–0.3Mg–0.2Ag (wt. %) alloy have been measured at 200, 250 and 300°C using conventional transmission electron microscopy techniques. At all temperatures examined the thickening showed a linear dependence on time. At 200°C the plates remained less than 6 nm in thickness after 1000 h exposure. At temperatures above 200°C the thickening kinetics are greatly increased. Atomic resolution Z-contrast microscopy has been used to examine the structure and chemistry of the (001)Ω‖(111)α interphase boundary in samples treated at each temperature. In all cases, two atomic layers of Ag and Mg segregation were found at the broad face of the plate. The risers of the thickening ledges and the ends of the plates were free of Ag segregation. The necessary redistribution of Ag and Mg accompanying a migrating thickening ledge occurs at all temperatures and is not considered to play a decisive role in the excellent coarsening resistance exhibited by the Ω plates at temperatures up to 200°C. Plates transformed at 200°C rarely contained ledges and usually exhibited a strong vacancy misfit normal to the plate. A large increase in ledge density was observed on plates transformed at 300°C, concomitant with accelerated plate thickening kinetics. The high resistance to plate coarsening exhibited by Ω plates at temperatures up to 200°C, is due to a prohibitively high barrier to ledge nucleation in the strong vacancy field normal to the broad face of the plate. Results also suggest that accommodation of the large misfit that exists normal to the broad face of the plate is unlikely to provide the driving force for Ag and Mg segregation.

Modeling recrystallization of microalloyed austenite: effect of coupling recovery, precipitation and recrystallization

Abstract In this contribution, existing models for precipitation, recovery and recrystallization have been coupled, with their interdependencies explained, to describe the microstructural evolution in a supersaturated alloy after hot deformation. Microalloyed austenite has been used as an example system and the time evolution of the precipitate diameter and the recrystallization and softening fractions are compared with the available experimental data. The model predictions are in excellent quantitative agreement with the experimental observations. Particular attention is paid to the occurrence of ‘plateaus’ or ‘humps’ in the softening and recrystallization fraction plots. In both cases, the incorporation of recovery is an essential ingredient for a quantitative description of the microstructural evolution in the hot-worked structure.

Refinement of precipitate distributions in an age-hardenable Mg–Sn alloy through microalloying

It has been known for some time that trace additions (<∼0.1 at.%) of selected elements to some age-hardenable alloys can have a disproportionate effect on the hardening response. This is now known to be related to the formation of heterogeneities (clusters or precipitates) on the matrix lattice at which the nucleation of hardening precipitates may be enhanced leading to a refinement in the particle distribution. In this work, qualitative thermo-kinetic criteria for choosing microalloying additions for precipitation-hardenable alloys are outlined and applied to a model Mg–Sn alloy. Additions of Na and In + Li are chosen, and an acceleration and enhancement of the ageing process is observed for both sets of additions at 200°C. The microstructures are examined using transmission electron microscopy and the microalloying additions are shown not to affect the identity or crystallography of the Mg2Sn phase formed, although a substantial refinement of the distributions is observed. In the case of Na additions, the number density of particles is increased by two orders of magnitude, resulting in hardening increment increases of 270%. The In + Li additions lead to increases of approximately one order of magnitude in number density and 150% increases in the hardening increment. The generality of the observed effects and the resulting hardening responses are discussed.

Precipitation processes in Al-Cu-Mg alloys microalloyed with Si

Microalloying additions of Si are known to increase significantly the response to age hardening of 2xxx series Al-Cu-Mg alloys, and commercial alloys such as 2618 are based on this effect. Previous work has attributed this effect to a refined dispersion of S′ or S phase (Al2CuMg) precipitates. This work reports the results of a detailed microstructural characterization, employing transmission electron microscopy-based techniques, on the effects of Si additions to a base Al-2.5Cu-1.5Mg (wt pct) alloy. It was found that the peak hardness microstructure contains a fine and uniform dispersion of Si-modified Guinier-Preston-Bagaratsky (GPB) zones. These zones are lath shaped, possessing {100}α facets, elongated along 〈100〉α directions and contain Si. The S phase was also observed at peak hardness, although it is concluded that these precipitates do not contribute significantly to hardening due to their coarse dispersion, which arises from their heterogeneous nucleation on the quenched-in defect structure. Overaging was associated with the replacement of the zones by the S phase through a process involving dissolution and reprecipitation together with heterogeneous nucleation of S at the zones. The precipitation of ϑ′ (Al2Cu) and σ(Al5Cu6Mg2) phase was also observed in alloys containing ≥0.5 wt pct Si. It is demonstrated that the total solute content of the alloy has a major influence on the precipitation reactions during aging.

Precipitation processes in an Al-2.5Cu-1.5Mg (wt. %) alloy microalloyed with Ag and Si

Abstract We have produced precipitation on and along both the α and the {111}α crystal systems by microalloying an Al-2.5Cu-1.5Mg (wt. %) base alloy with Ag and Si. Using electron microscopy and atom probe field ion microscopy, we have demonstrated that Ag+Si additions produce multi-component clustering reactions. When compared to comparable quaternary compositions, the quinary alloy exhibits a higher hardness. The first precipitates observed were GPB zones, rich in Cu and Mg and containing traces of both Si and Ag. Following ageing at 200 °C, the peak hardness microstructure consisted of rod-shaped GPB zones along α together with X′ and Ω precipitation, both as plates on the {111}α planes. The Ω phase was less stable than the X′ phase and dissolved soon after ageing to peak hardness. This was associated with a rapid drop in hardness. The overaged microstructure was dominated by S phase.

Self-consistent model for planar ferrite growth in Fe-C-X alloys

A self-consistent model for non-partitioning planar ferrite growth from alloyed austenite is presented. The model captures the evolution with time of interfacial contact conditions for substitutional and interstitial solutes. Substitutional element solute drag is evaluated in terms of the dissipation of free energy within the interface, and an estimate is provided for the rate of buildup of the alloying element “spike” in austenite. The transport of the alloying elements within the interface region is modeled using a discrete-jump model, while the bulk diffusion of C is treated using a standard continuum treatment. The model is validated against ferrite precipitation and decarburization kinetics in the Fe-Ni-C, Fe-Mn-C, and Fe-Mo-C systems.

A comparison of EDS microanalysis in FIB-prepared and electropolished TEM thin foils

This paper reports the results of a fine-probe EDS microanalytical study of cellular precipitation in a Cu-Ti binary alloy. Compositional profiles across the solute depleted Cu-rich FCC lamellae and the Cu4Ti lamellae within isothermally formed cellular colonies were measured in a FEG-TEM from thin-foil specimens prepared by conventional electropolishing and by a technique using a Ga+ focused ion-beam (FIB). The Cliff-Lorimer ratio method, with an absorption correction, was employed to quantify the compositions. Two FIB samples were prepared with different orientations of the lamellae with respect to the ion-milling direction. The compositional profiles across the Cu-rich FCC lamellae and the Cu4Ti compound lamellae in both the FIB-prepared samples and the electropolished sample were, within experimental error, numerically equivalent. The composition of the Cu4Ti compound phase lamellae was very close to the ideal stoichiometric composition of 20 at % Ti. It is concluded that for this system, and for the specimen preparation procedures used in this study, the Ga+ ion-milling process has had no detectable effect on the chemistry changes across the interlamellar interface at the scale studied. These results indicate that the possible sources of chemical artifacts which include redeposition, preferential sputtering and ion-induced atomic migration can be minimized if several precautions are taken during milling in the FIB. Consistent with previous investigators, it was also found that the ion-milling process does introduce significant structural artifacts (e.g., dislocations) into the softer FCC Cu-rich phase compared with a specimen produced by conventional electropolishing.

Quantifying the Solute Drag Effect on Ferrite Growth in Fe-C-X Alloys Using Controlled Decarburization Experiments

The kinetics of ferrite growth in the Fe-C-Co and Fe-C-Si systems has been quantified using controlled decarburization experiments. The Fe-C-Co system is a particularly interesting system since a large range of Co contents can be considered providing a suitable data set for examination of the composition dependence of the solute drag effect. Six Fe-C-Co alloys containing Co from 0.5 to 20 pct have been considered. Three Fe-C-Si alloys have also been considered and each has been transformed at three temperatures proving a suitable data set for examining the temperature dependence of the solute drag effect. This data set, along with ferrite growth data from decarburization experiments on an Fe-C-2Cr alloy has been used to test the ferrite growth model proposed in the companion article by Zurob et al. It is shown that this model for ferrite growth, that includes diffusional dissipation due to interaction between the solute and the migrating boundary, quantitatively captures both the temperature and composition dependence of the deviation of experimental ferrite growth kinetics from the PE and/or LENP models.

Additive manufacturing of metals: a brief review of the characteristic microstructures and properties of steels, Ti-6Al-4V and high-entropy alloys

Abstract We present a brief review of the microstructures and mechanical properties of selected metallic alloys processed by additive manufacturing (AM). Three different alloys, covering a large range of technology readiness levels, are selected to illustrate particular microstructural features developed by AM and clarify the engineering paradigm relating process–microstructure–property. With Ti-6Al-4V the emphasis is placed on the formation of metallurgical defects and microstructures induced by AM and their role on mechanical properties. The effects of the large in-built dislocation density, surface roughness and build atmosphere on mechanical and damage properties are discussed using steels. The impact of rapid solidification inherent to AM on phase selection is highlighted for high-entropy alloys. Using property maps, published mechanical properties of additive manufactured alloys are graphically summarized and compared to conventionally processed counterparts.

Vacancy behavior and solute cluster growth during natural aging of an Al-Mg-Si alloy

The natural aging behavior of an Al-0.46Mg-1.05Si-0.14Fe (wt pct) alloy was studied with positron annihilation lifetime spectroscopy at four temperatures and with 25Mg solid-state nuclear magnetic resonance. The evolution of positron lifetime is shown to consist of three distinct stages. In an effort to understand the physical processes occurring during natural aging, a phenomenological model was developed for the first two stages and is shown to describe the experimental observations well. The description accounts for the decay in positron lifetime due to diffusion of vacancies to sinks and an increase in positron lifetime attributed to growing solute clusters. NMR measurements of 25Mg solute partitioning during natural aging support the model.