ACTA Vol. 200, Nov. 2020, P735-744

The present article advances the idea of compositional flexibility. The intent is to provide an avenue for value creation from waste alloys in cases where costs of separating co-mingled alloy flows are prohibitively high. A compositionally flexible alloy has the potential to be produced solely from varying and intermingled flows of end-of-life alloys. While the alloy composition may vary from day-to-day or from heat-to-heat, the intent is that it will still satisfy base performance targets. Using FeNiCr based high entropy alloys as inspiration, we exploited Bayesian Optimization to define a number of example compositionally complex alloys that display strengths within specified limits (150-250 MPa, 200-350 MPa and 300-350 MPa). The concept was motivated by the surprising number of high entropy alloys reported in the literature that display a similarity of properties despite their wide dispersion of composition. We propose a simple measure of compositional flexibility by combining the ranges over which each elemental addition is permitted to vary and suggest methods of alloy specification. A series of 68 prototype compositions were produced and characterized to demonstrate the concept and approach. We show that our model alloys could conceivably be produced using multiple different combinations of existing Ni alloys in combination with 316 stainless steel. Our hope is that the concept will find application to facilitate alloy recovery where it might not otherwise be feasible.

ACTA Vol. 200, Nov. 2020, P793-802

The correlation between the thermal transport properties and microstructural evolution of amorphous alloys is crucial for their application in thermal insulation. Herein, Fe₄₈Cr₁₅Mo₁₄C₁₅B₆Y₂ amorphous alloy with low thermal conductivity of 7.74 W/mK was investigated to reveal this relationship. Isochronal annealing experiment demonstrates a limited increase in thermal conductivity at temperatures below the crystallization temperature (T_x1= 620.7°C), despite the occurrence of structural relaxation or partial crystallization, which is ascribed to the conflicting variations of electron and phonon contributions with increasing temperature. At annealing temperature above T_x1, the two contributors start to cooperate, leading to abrupt enhancement of thermal conductivity. On the other hand, isothermal annealing experiments reveal that at temperatures below T_x1, the thermal conductivity is independent of annealing time. Although full crystallization can be induced slowly by annealing at 600 °C, the thermal conductivity keeps nearly constant at 8.35 W/mK, which is attributed to additional scattering by a newly introduced phase interface. Moreover, grain growth upon prolonged annealing at 850 °C results in a slow increase in thermal conductivity, which asymptotically saturates at 12.44 W/mK. The obtained results demonstrate the potential of the Fe-based amorphous alloy as thermal insulator and form a basis for future works aiming to shed further light on the evolution of amorphous alloy and the sluggish effect of transformation kinetics.

ACTA Vol. 200, Nov. 2020, P803-810

Optimizing two conflicting properties such as mechanical strength and toughness or dielectric constant and breakdown strength of a material has always been a challenge. Here we propose a machine learning approach to dramatically enhancing the combined ultimate tensile strength (UTS) and electric conductivity (EC) of alloys by identifying a set of key features through correlation screening, recursive elimination and exhaustive screening of existing datasets. We demonstrate that the key features responsible for solid solution strengthened conductive Copper alloys are absolute electronegativity, core electron distance, and atomic radius, based on which, we discovered a series of new alloying elements that can significantly improve the combined UTS and EC. The predictions are then validated by experimentally fabricating four new Cu-In alloys which could potentially replace the more expensive Cu-Ag alloys currently used in railway wiring. We show that the same set of key features can be generally applicable to designing a broad range of conductive alloys.

ACTA Vol. 200, Nov. 2020, P821-834

A phase-field model for describing dynamic interactions between deformation twins and grain boundaries in a hexagonal close packed (HCP) metal is established. It is applied to simulating the coupled evolution mechanisms of grains and twins in magnesium (Mg) by probing the transmission of deformation twins across grain boundaries. We analyze the effect of the strain relaxation near a grain boundary, which is related to the geometric compatibility arising from the misorientation angle between adjoining grains. Phase-field simulations demonstrate that twin transmission across a grain boundary into a neighboring grain leads to grain boundary migration towards the neighboring grain with a reduced grain boundary width. The preferred nucleation site of new twin variant in the neighboring grain is related to the elastic interaction energy distribution. These predicted transmission behaviors agree well with existing experimental observations and molecular dynamics simulations. By analyzing set of systematic phase-field simulation results, we establish a rotation-angle-related twin variant selection rule for analyzing the transmission of deformation twins across grain boundaries in Mg and its alloys.

ACTA Vol. 200, Nov. 2020, P857-858

During the annealing of Ni and Ni-Mo films, a {110} texture, usually considered to be both of high surface energy and high strain energy, was found to develop. Quantitative thermodynamic model calculations showed that the anomalous formation of this {110} texture originates fundamentally from the grain yielding anisotropy of Ni. Based on extensive grain yielding anisotropy model calculations, a “tex- ture map”based on {111}, {100}, and {110} textures was constructed to predict the texture transition temperatures for different film thicknesses. A kinetic analysis of the texture evolution in the films is further presented, revealing that the texture evolution is controlled by the self-diffusion of atoms at grain boundaries. These findings pave the way for the achievement of unusual surface orientations through the quantitative texture design of thin films.

ACTA Vol. 200, Nov. 2020, P869-882

This paper discusses the effect of stress on the solubility and diffusivity of vacancies using the elasticity theory of point defects. To support the discussion, results are compared with DFT calculations to verify model accuracy. The particular case of vacancies in aluminum is discussed in detail (DFT-elasticity), while three other metallic fcc systems – Ni, Cu and Pd – are discussed through the elasticity approach only. Different types of loading were considered: hydrostatic, multi-axial and shear stresses. In the case of a uni-axial loading, two different directions were investigated: the first along a main crystallographic direction, i.e. [001], and the second perpendicular to the dense plane (111). In order to quantify the effect of stress on diffusivity, the diffusion coefficient of each configuration was expressed for further calculations. By analyzing the symmetry break during the loading process, non-equivalent atomic jumps were identified and diffusion equations obtained. A multi-physic approach was carried out by combining first-principles calculations, to study atomic-scale processes, and a multi-state formalism, to obtain exact diffusion equations. Results show that elasticity accurately captures the effects of stress on vacancy diffusion and solubility and an application method is presented.

ACTA Vol. 200, Nov. 2020, P893-907

This paper presents a novel multiscale approach for analyzing multi-axial stress-strain evolution in Ti-7Al cruciform specimens under dwell loading, through the use of an uncertainty-quantified, parametrically homogenized constitutive model (UQ-PHCM). The thermodynamically-consistent UQ-PHCM is built from rigorous upscaling of crystal plasticity FE models (CPFEM) using machine learning and uncertainty quantification. They explicitly incorporate microstructural information in the form of representative aggregated microstructural parameters (RAMPs). Uncertainty quantification accounts for uncertainty in model reduction, data sparsity and microstructural descriptors. This paper integrates advanced multiscale, multi-axial experiments with computational modeling at multiple scales to establish the UQ-PHCM as an effective tool for bridging the gap between laboratory specimen-scale experimental observations and micro-scale stresses and strains using CPFEM. The CPFEM is calibrated and validated by experimental data from surface strain measurements using digital image correlation (DIC) and grain-by-grain lattice strain measurements with in-situ far field high energy diffraction microscopy (ff-HEDM). A computational method is developed in CPFEM, to incorporate initial residual stresses consistent with measured lattice strains. The UQ-PHCM is validated with biaxial tensile dwell test results performed on the cruciform specimen with satisfactory prediction of gauge strain evolution in DIC measurements. Uncertainty in the strain field due to microstructural variability is also corroborated by the DIC measurements.

ACTA Vol. 200, Nov. 2020, P908-921

Homogeneous face-centered cubic (fcc) polycrystalline CoCrFeNi films were deposited at room temperature on (0001) α-Al₂O₃ (c-sapphire). Phase and morphological stability of 200 to 670 nm thick films were investigated between 973 K and 1423 K.The fcc-phase persists while the original texture of 30-100 nm wide columnar grains evolves into ~10 or ~1000 μm wide grains upon annealing. Only the metallic M grains having two specific orientation relationships (ORs) to the c-sapphire grow. These ORs are OR1 (M(111)[110]//α-Al₂O₃(0001)[1-100]) and OR2 (M(111)[110]//α-Al₂O₃(0001)[11-20]) and their twin-related variants (OR1t and OR2t). They are identical to those reported for several pure fcc metal (M) films. Thus, the ORs in these fcc/c-sapphire systems appear not to be controlled by the fcc phase chemistry or its lattice parameter as usually assumed in literature. Upon annealing, the films either retain their integrity or break-up depending on the competing kinetics of grain growth and grain boundary grooving. Triple junctions of the grain boundaries, the major actors in film stability, were tracked. Thinner films and higher temperatures favor film break-up by dewetting from the holes grooved at the triple junctions down to the substrate. Below 1000 K, the film microstructure stabilizes into 10 μm wide OR1 and OR1t twin grains independent of film thickness. Above 1000 K, the OR2 and OR2t grains expand to sizes exceeding more than a 1000 times the film thickness. The grain boundaries of the OR2 and OR2t grains migrate fast enough to overcome the nucleation of holes from which break-up could initiate. The growth of the OR2 and OR2t grains in this complex alloy is faster than in pure fcc metals at equivalent homologous annealing temperatures.

ACTA Vol. 200, Nov. 2020, P922-931

Nano-oxide precipitates in a modern nanostructured ferritic alloy were investigated after extreme thermomechanical processing into a thin-walled tube geometry. It was found that the morphology of the precipitates changed from spherical to rod-shaped, with some increasing to aspect ratios of up to 9, despite the precipitate volume fraction (0.3%) and number density (>10²³m⁻³) of precipitates remaining unchanged. High-resolution electron microscopy showed that the precipitates likely remained coherent with the Fe-matrix, while atom probe tomography confirmed that the precipitate compositions remained unaffected by the transformation. The morphological change was attributed to the shearable nature of the (Y,Ti,O)-rich precipitates, indicating they should be considered as “soft” obstacles to dislocation motion. The elongation was most pronounced in larger (>5 nm) precipitates, which may be caused by preferential dissolution of the smallest (1–3 nm) precipitates followed by the competition between re-precipitation and solute diffusion to larger precipitates during recovery heat treatments.

ACTA Vol. 200, Nov. 2020, P932-942

Hydrogen (H) embrittlement in multicomponent austenitic alloys is a serious limitation to their application in many environments. Recent experiments show that the High-Entropy Alloy (HEA) CoCrFeMnNi absorbs more H than 304 Stainless Steel but is less prone to embrittlement while the HEA CoCrFeNi is not embrittled under comparable conditions. As a first step toward understanding H embrittlement, here a comprehensive first-principles study of H absorption, surface, and fracture energies in the presence of H is presented for 304 Stainless Steel, 316 Stainless Steel, CoCrFeNi, and CoCrFeMnNi. A collinear paramagnetic model of the magnetic state is used, which is likely more realistic than previous proposed magnetic states. All alloys have a statistical distribution of H absorption sites. Hence, at low concentrations, H is effectively trapped in the lattice making it more difficult for H to segregate to defects or interfaces. Agreement with experimental H solubilities across a range of chemical potentials can be achieved with minor fitting of the average H absorption energy. The (111) surface energies for 0, 50, and 100% H surface coverage are very similar across all alloys. The fracture energies for two representative thermodynamic conditions are then determined. SS304 and CoCrFeNi are found to have the lowest fracture energies, but experiments suggest rather different embrittlement tendencies. These results indicate that differences in H embrittlement across these austenitic alloys are not due solely to differences in H absorption or H-reduced fracture energy, thus requiring more sophisticated concepts than those recently found successful for fcc Ni.

ACTA Vol. 200, Nov. 2020, P943-958

High manganese steels are promising candidates for applications in cryogenic environments. In this study, we investigate the mechanical and microstructural responses of a high manganese twinning induced plasticity (TWIP) steel at a low-temperature range (from 373 to 77 K) via in situ neutron diffraction qualification and correlative microscopy characterization. During plastic deformation, stacking fault probability and dislocation density increased at a faster rate at a lower temperature, hence, higher dislocation density and denser mechanical twins were observed, confirmed by microscopic observation. Stacking fault energy was estimated, dropping linearly from 34.8 mJm⁻² at 373 K to 17.2 mJm ⁻² at 77 K. A small amount of austenite transferred to martensite when deforming at 77 K. The contributions to flow stress from solutes, grain boundary, dislocation, and twinning were determined at different temperatures, which shows that the high work strain hardening capacity of the TWIP steel originates from the synergetic strengthening effects of dislocations and twin-twin networks. These findings reveal the relationship among stacking fault energy, microstructure, and deformation mechanisms at the low-temperature range, paving a way in designing TWIP steels with the superb mechanical performance for cryogenic applications.

ACTA Vol. 200, Nov. 2020, P959-970

Contemporary powder-based polycrystalline nickel-base superalloys inherit microstructures and properties that are heavily determined by their thermo-mechanical treatments during processing. Here, the influence of a thermal exposure to an alloy powder is studied to elucidate the controlling formation mechanisms of the strengthening precipitates using a combination of atom probe tomography and insitu neutron diffraction. The initial powder comprised a single-phase supersaturated γ only; from this, the evolution of γ’ volume fraction and lattice misfit was assessed. The initial powder notably possessed elemental segregation of Cr and Co and elemental repulsion between Ni, Al and Ti with Cr; here proposed to be a precursor for subsequent γ to γ’ phase transformations. Subsolvus heat treatments yielded a unimodal γ’ distribution, formed during heating, with evidence supporting its formation to be via spinodal decomposition. A supersolvus heat treatment led to the formation of this same γ’ population during heating, but dissolves as the temperature increases further. The γ’ then reprecipitates as a multimodal population during cooling, here forming by classical nucleation and growth. Atom probe characterisation provided intriguing precipitate characteristics, including clear differences in chemistry and microstructure, depending on whether the γ’ formed during heating or cooling.

ACTA Vol. 200, Nov. 2020, P980-991

Serrated plastic deformation at temperatures close to 0 K has been previously reported in some metals and alloys, and is associated with two possible origins: (i) thermomechanical instability or (ii) mechanical instability. While some recent results indicate that serrations are a mechanical dislocation-based phenomenon, a comprehensive model does not exist. CoCrFeMnNi, an expectedly ideal candidate, exhibits severe serrated plastic deformation with large stress drops in excess of 100 MPa. Furthermore, it also shows cryogenic serrated plastic deformation at a higher temperature (35 K) than previously reported for any other alloy. The exacerbated nature of serrated plastic deformation in CoCrFeMnNi led to the following inferences: (i) temperature and dislocation density are indisputable controlling parameters for cryogenic serrated plastic deformation and they cannot supersede each another; (ii) a phenomenological model is elucidated based on the increasing difficulty for cross-slip with decreasing temperature, leading to sudden massive dislocation proliferation event; (iii) the model establishes a gradual transition from completely non-serrated to completely serrated deformation, mediated by cross-slip, as opposed to the conventional model which proposed a discrete transition; (iv) solute dislocation interaction and associated Stacking Fault Energy (SFE) during deformation plays a key role in controlling dislocation constriction and cross-slip and correspondingly serrated plastic deformation; (v) the need/direct influence of deformation twinning, transformation induced plasticity and especially thermomechanical factors on serrated plastic deformation is invalidated. Some of these points were further clarified through comparisons with CoCrNi and CoNi, also presented in the present article.

ACTA Vol. 200, Nov. 2020, P992-1007

Microscopic crystalline defects are of fundamental importance in unraveling the plastic deformation response and thereby tailoring the macroscopic load-bearing performances of metallic alloys. Especially in the microstructural metastability engineering context of complex concentrated alloys (CCAs), while profuse interest has been focused on phase and twin boundaries as well as their interactions with glissile dislocations, stacking faults, as another essential planar defects, have remained comparatively lessexplored and unutilized. In the present work, by investigating a metastable CoCrNiW CCA via the combination of in-situ synchrotron X-ray diffraction and in-situ electron channeling contrast imaging (ECCI), we show that stacking faults formation can also operate as the predominant deformation micro-mechanism, accommodating plastic strain while enabling macroscopic strain hardening. Through the examination of relative phase stability by thermodynamic modeling, we reveal that this sort of deformation faulting response is largely correlated with a negative intrinsic stacking fault energy. The corresponding physical revelation is explored in greater details regarding thermodynamics, structure, and mechanics, followed by in-situ experimental verification of the stacking fault activities.

ACTA Vol. 200, Nov. 2020, P1022-1037

Additive manufacturing (AM) is a new and promising production methodology adept at producing complex geometries, which can be optimized for lower weight and enhanced capabilities. The material properties of these additive components are dictated by the microstructures developed during processing, with a high sensitivity to grain structure and associated anisotropy. With this new processing modality comes the added difficulty of understanding the thermodynamics and kinetic mechanisms that dictate the evolution of microstructure. This research addresses the unique thermal conditions present in AM and the pathways for grain refinement in nanofunctionalized aluminum alloys. The Al-Ta system, in which Al₃Ta intermetallic compounds are demonstrated to have substantial grain refining capacity, are the focus of this study. The grain size is shown to be reduced relative to pure aluminum by 1000X when tantalum is added at 1 vol%. The effectiveness of the Al₃Ta intermetallic is dictated by the crystallography and availability of the inoculant phase under AM conditions.