ACTA Vol. 200，Nov. 2020, P378-388

Nanoprecipitates or grain refinement can effectively enhance the mechanical strength of Al alloys, but the room-temperature strengths of precipitation hardened and nanocrystalline Al alloys often fall below 1 GPa. Furthermore, they are largely plagued by precipitous mechanical softening at elevated temperature below 300°C, mostly due to degraded microstructural stability. Here, we report a mechanism of coupled solute effect in nanotwinned Al-Fe-Ti alloys that enables stability of nanograins up to 400°C and an unprecedented high-temperature flow stress of ~ 1.7 GPa at 300°C. The supersaturated Fe solutes in Al act as effective grain refiner, forming superstrong solid solution alloys. More importantly, empirical evidence combined with first principle calculations indicate that the Ti solutes delay the agglomeration of Fe solutes, thereby remarkably extending the temperature window for the stability of nanograins in nanotwinned Al alloys. This study highlights the opportunity to design ultrastrong and stable nanostructured alloys for potential high temperature applications via a coupled solute effect.

ACTA Vol. 200, Nov. 2020, P389-403

The combination of different phase constituents to realize a mechanical composite effect for superior strength-ductility synergy has become an important strategy in microstructure design in advanced high-strength steels. Introducing multiple phases in the microstructure essentially produces a large number of phase boundaries. Such hetero-interfaces affect the materials in various aspects such as dislocation activity and damage formation. However, it remains a question whether the characteristics of phase boundaries, such as their chemical decoration states, would also have an impact on the mechanical behavior in multiphase steels. Here we reveal a phase boundary segregation-induced strengthening effect in ultrafine-grained duplex medium-Mn steels. We found that the carbon segregation at ferrite-austenite phase boundaries can be manipulated by adjusting the cooling conditions after intercritical annealing. Such phase boundary segregation in the investigated steels resulted in a yield strength enhancement by 100–120 MPa and simultaneously promoted discontinuous yielding. The sharp carbon segregation at the phase boundaries impeded interfacial dislocation emission, thus increasing the stress required to activate such dislocation nucleation process and initiate plastic deformation. This observation suggests that the enrichment of carbon at the phase boundaries can enhance the energy barrier for dislocation emission, which provides a favorable condition for plastic flow avalanches and thus discontinuous yielding. These findings extend the current understanding of the yielding behavior in medium-Mn steels, and more importantly, shed light on utilizing and manipulating phase boundary segregation to improve the mechanical performance of multiphase metallic materials.

ACTA Vol. 200, Nov. 2020, P404-416

Mechanically induced migration of grain boundaries and triple junctions not only determines the size and shape of nanostructures created by severe plastic deformation, it substantially affects their mechanical properties as well. Grain growth during deformation of nanostructures has been widely observed. As the nanostructures processed by severe plastic deformation are a consequence of a dynamic equilibrium between refinement and local mechanically induced coarsening, it is widely accepted that such nanostructures would remain stable upon further loading. However, pronounced grain coarsening can be observed when pure UFG copper prepared by high pressure torsion (HPT) is additionally cold rolled. The coarsening continues up to rolling strains, ε = 1, i.e. until favourable grain orientations for rolling are developed. For larger strains subsequent refinement to minimum boundary spacing identical to the HPT microstructure is observed. Interestingly, less grain growth is observed for two other sample orientations of the HPT microstructure which are rolled along different directions with respect to the sample coordinate frame. Crystallographic texture of these samples were favourable with respect to the new strain path, highlighting its role for mechanically induced growth and suggesting a distinct influence of grain orientation on the mechanically induced coarsening process.

ACTA Vol. 200, Nov. 2020, P417-431

Primary spacing is a key phenomenon during solidification of single crystal alloys. It determines microsegregation, defect formation, the time required to solutionise the solidified structure, and the final performance of single crystal components. A novel stereological characterisation algorithm: Shape-Limited Primary Spacing (SLPS) is developed and applied to study the formation of cellular and dendritic packing patterns in single crystal alloys solidified under different casting conditions. The results reveal the tendency for single crystals to form hexagonally packed structures under steady state conditions, while all other packing arrangements constitute a metastable state. Using the SLPS algorithm, it has been demonstrated that packing pattern formation and local primary spacing can be related to tip growth kinetics. Further, the role of curved isotherms that occur in directional solidification has been identified. Isotherm curvature results in non-uniform liquid compositional gradients developing parallel to a growing solid interface, leading to the formation of metastable packing, low-angle grain boundaries, and porosity formation within the microstructure.

ACTA Vol. 200, Nov. 2020, P432-441

Wear and friction of nanocrystalline (NC) aluminum were investigated via molecular dynamics simulations and the effects of dopants were considered. Zr-doped NC Al was found to have a better wear resistance and a smaller friction force, which is consistent with a higher hardness and a higher strength of the doped sample. The underlying mechanisms are suppressed emission of dislocations from grain boundaries (GBs), suppressed GB migration, and suppressed GB sliding. After multiple sliding cycles, the trend in mechanical response was reversed, with the pure NC Al showing a better wear resistance and a lower friction force than the doped sample. One reason is that the higher dislocation density introduced during wear into the pure sample leads to more strain hardening. Another reason is that the pure NC Al has undergone more significant grain growth than the doped sample. Since the grain size of our samples is in the inverse Hall-Petch regime, here grain growth leads to strengthening of the pure sample. Mechanisms of grain growth in the pure NC Al and its suppression in the doped NC Al are analyzed and discussed.

ACTA Vol. 200, Nov. 2020, P442-454

Intermetallic γ-TiAl based alloys have found applications in the low-pressure turbine of aircraft engines as well as in the turbocharger unit of automotive engines. However, these light-weight alloys must still be improved, through micro-alloying and tailoring the microstructure, to increase their creep resistance and consequently their maximum working temperature. In this work, a fully nano-lamellar advanced γ-TiAl based alloy doped with small amounts of C and Si is investigated in order to gain a deeper understanding of the atomic mobility mechanisms taking place at high temperature, thus controlling the creep properties. The study was approached through internal friction measurements up to 1223 K. We demonstrate that C has a notable influence on Ti diffusion in α2 phase, leading to an increase of the activation energy for Ti diffusion, which is assessed at ΔETi(α2)=0.32 eV per at% C. An atomic model for the relaxation process is proposed capable to explain this phenomenon. An additional internal friction peak, which, up to now, remained hidden by the high temperature background, was observed in this nano-lamellar TiAl alloy and analyzed through a careful de-convolution of the internal friction spectra. This new relaxation process, with activation energy of 3.70 eV, is attributed to the short distance diffusion of Al atoms in the γ-TiAl lattice. A novel concept of stress-induced cell-lattice reorientation is proposed to explain this relaxation. Finally, a new experimental method to analyze the high temperature internal friction background, which is closely related to the creep behavior, was developed to study the fully nano-lamellar microstructure, whose high temperature background exhibits the highest activation energy ever measured in a γ-TiAl based alloy.

ACTA Vol. 200, Nov. 2020, P455-462

We present a detailed study of solid state dewetting choosing epitaxial bismuth films on silicon as a model system. Exploiting both diffraction and imaging methods, we determine atomistic parameters like unit cell coverage, lattice spacings and gradients thereof through the analysis of x-ray diffraction crystal truncation rods. A different dewetting behavior of samples with and without strain gradient is revealed. Additionally, we discuss possible reasons for the impeding influence of a strain gradient, such as a reduced interface energy or increased step edge diffusion energy barrier, which we quantify by using a Johnson-Mehl-Avrami-Kolmogorov model. A model for the step edge self diffusion of Bismuth is presented.

ACTA Vol. 200, Nov. 2020, P471-480

Ductile alloys fail in corrosive environments by intergranular stress corrosion cracking, through interactions between mechanical and chemical processes that are not yet understood. We investigate formation and mechanical effects of metal defects produced by grain boundary corrosion of low-alloy pipeline steel, at conditions of high susceptibility to stress corrosion cracking in the absence of hydrogen evolution. Nanoindentation measurements show local softening near corroded grain boundaries, indicated by significantly reduced critical loads for dislocation nucleation. Molecular dynamics simulations of nanoindentation of bulk iron showed that metal vacancies and not interstitial hydrogen atoms explain the observed critical load reduction. Both the dislocation activation volume and dislocation activation energy for vacancy-charged samples are found to be nearly one-half of that for a hydrogen charged samples. Quantitative agreement with experimentally measured indentation response was found for vacancy concentrations equivalent to the bulk silicon concentration in the steel, suggesting that vacancies originate from oxidation of reactive silicon solute atoms at grain boundaries. The results help explain the chemical mechanism of formation of vacancy defects that may participate in grain boundary degradation in the absence of hydrogen embrittlement environment.

ACTA Vol. 200, Nov. 2020, P481-489

The interaction between carbon and screw dislocations in tungsten is investigated using ab initio calculations. The presence of carbon atoms in the vicinity of the dislocation induces a reconstruction, with the dislocation relaxing to a configuration, the hard core structure, which is unstable in pure tungsten. The reconstruction corresponds to a strong binding of carbon in the prismatic sites created by the dislocation which is perfect for high concentrations of carbon segregated on the dislocation line. However, the reconstruction is only partial for lower atomic fractions, with the dislocation tending to fall back in its easy core ground state. This pinning by carbon atoms of the dislocation in an unstable position is well described by a simple line tension model. A strong carbon-dislocation attraction is also evidenced at larger separation distances, when the solute is in the fourth nearest neighbour octahedral sites of the reconstructed core. The equilibrium concentrations of carbon in these different segregation sites are modelled with an Ising model and using a mean-field approximation. This thermodynamic model evidences that screw dislocations remain fully saturated by carbon atoms and pinned in their hard core configuration up to about 2500 K.

ACTA Vol. 200, Nov. 2020, P510-525

Two-step nucleation, in which a metastable intermediate phase acts as a precursor for nucleating a thermodynamically stable phase, has been widely observed in many materials systems and solid-state reactions. Among the advantages of two-step nucleation is that the stable phase may nucleate heterogeneously at the hetero-phase interface between the original and the precursory phases. Although heterogeneous nucleation (HN) theories for homo-phase grain boundaries and inert surfaces are well established, our understanding of HN at reactive hetero-phase interfaces remains incomplete. This deficiency stems from the discontinuity of the chemical potential driving force across the hetero-phase interface, which profoundly affects the fundamental properties of the nucleus in a way that is not properly accounted for in existing models. Herein, we incorporate these effects to extend the classical nucleation theory to HN at hetero-phase interfaces. Our extended model demonstrates that the nucleus shape along the minimum energy path is strongly size-dependent, and this additional degree of freedom can result in the reduction of the critical nucleus volume and associated activation energy barrier by orders of magnitude relative to conventional predictions. The simulation results are used to construct a sensitivity map in the parameter space of interfacial energy and bulk driving force ratios, which quantifies the difference in nucleation barriers predicted by different models.

ACTA Vol. 200, Nov. 2020, P537-550

The interactions between δ-hydrides and plastic slip in a commercial zirconium alloy, Zircaloy-4, under load were studied using in situ secondary electron microscope (SEM) micropillar compression tests of single crystal samples and ex situ digital image correlation (DIC) macroscale tensile tests of polycrystalline samples. The hydrides decorate near basal planes in orientation, and for micropillars orientated for <a> basal slip localised shear at the hydride–matrix interface is favoured over slip in α-Zr matrix due to a lower shear stress required. In contrast, for pillars oriented for <a> prismatic slip the shear stress needed to trigger plastic slip within the hydride is slightly higher than the critical resolved shear stress (CRSS) for the <a> prismatic slip system. In this case, slip in the hydride is likely achieved through <110>-type shear which is parallel to the activated <a>-type shear in the parent matrix. At a longer lengthscale, these results are used to inform polycrystalline samples analysed using high spatial resolution DIC. Here localised interface shear remains to be a significant deformation path which can both cause and be caused by matrix slip on planes closely-oriented to the phase boundaries. Matrix slip on planes nearly perpendicular to the adjacent hydride–matrix interfaces can either result in plastic slip within the hydrides or get arrested at the interfaces, generating local stress concentration. Through these mechanisms, the presence of δ-hydrides leads to enhanced strain localisation in Zircaloy-4 early in the plastic regime.

ACTA Vol. 200, Nov. 2020, P570-580

The influence of magnetism on the properties of screw dislocations in body-centered cubic chromium is investigated by means of ab initio calculations. Screw dislocations having Burgers vectors 1/2 <111> and <100> are considered, following experimental observations showing activity for both slip systems. At low temperature, chromium has a magnetic order close to antiferromagnetism along <100> directions, for which 1/2<111> is not a periodicity vector. Hence, dislocations with Burgers vectors 1/2 <111> generate magnetic faults when shearing the crystal, which constrain them to coexist and move pairwise, leading to dissociated <111> super-dislocations. On the other side, <100> is a periodicity vector of the magnetic order of chromium, and no such magnetic fault are generated when <100> dislocations glide. Dislocation properties are computed in the magnetically ordered and non magnetic phases of chromium for comparison purposes. We report a marginal impact of magnetism on the structural properties and energies of dislocations for both slip systems. The Peierls energy barrier opposing dislocation glide in {110} planes is comparable for both 1/2 <111> {110} and <100> {110} slip systems, with lower Peierls stresses in the magnetically ordered phase of chromium.

ACTA Vol. 200, Nov. 2020, P581-596

Hydrogen diffusion through the oxide grown on Zr alloys by aqueous corrosion processes plays a critical role in determining the rate of hydrogen pickup (HPU) which can result in embrittlement of fuel cladding and limit the burnup of the nuclear fuel it encapsulates. Mapping the hydrogen/deuterium distributions in these oxide layers, especially in the barrier layer close to the metal/oxide interface, is a powerful way to understand the mechanism of both oxidation and hydrogen pickup. Here we have characterised by high-resolution SIMS analysis the deuterium distribution in oxide layers on a series of Zr alloys, including autoclave-oxidised Zircaloy-4, Zr-1Nb and Zr-2.5Nb alloys, and in-flux and out-of-flux corroded Zr-2.5Nb samples. Pre-transition Zircaloy-4 samples show a high deuterium trapping ratio in the oxide and a higher diffusion coefficient than in oxides on the Nb-containing samples. Neutron irradiation increases the deuterium diffusion coefficient, the deuterium concentration in the oxide and the pickup fraction in Zr-2.5 Nb samples. Comparative NanoSIMS and EDX/SEM analysis demonstrates that the deuterium is not preferentially trapped at second phase particles in the oxides on any of the alloys studied, but there is direct evidence for trapping at the surfaces of small oxide cracks especially in Zircaloy-4 samples. The high resolution mapping of these hot-spots in 3D can provide unique information on the mechanisms of hydrogen uptake, and suggests that the development of interconnected porosity in the oxide may be the critical rate-determining mechanism that controls HPU in the aqueous corrosion of zirconium alloys in water-cooled reactors.

ACTA Vol. 200, Nov. 2020, P597-607

Carbon trapping at dislocations and carbide precipitation in martensite could significantly reduce the amount of carbon partitioning into austenite, e.g. incomplete carbon partitioning phenomenon, which would alter austenite decomposition behavior and austenite stability during the Quenching and Partitioning (Q&P) process. In this study, an integrated model is developed to clarify the mechanism of incomplete carbon partitioning and quantify its effects on austenite stabilization in the low-alloy medium-carbon Fe–C–Mn–Si steels. The fraction of carbon consumed by Cottrell atmospheres around dislocations is described using a semi-empiric equation. Then, the kinetic competition among carbide precipitation, carbon partitioning and austenite decomposition during the partitioning step is simulated by coupling the Deschamps–Bréchet model and quenching and partitioning-local equilibrium (QP-LE) model. It is found that transition carbide precipitation in martensite and carbon partitioning into austenite are kinetically coupled at the very early stage of the partitioning step and subsequently promotes austenite decomposition. Taking the synergy effects of incomplete carbon partitioning and austenite decomposition into account, our model is capable of predicting the evolution of volume fraction of austenite and its carbon content during partitioning.

ACTA Vol. 200, Nov. 2020, P619-631

The present study employs in situ X-ray microtomography to characterize the impact of grain size on void nucleation, growth and linkage during tensile loading of magnesium alloy AZ31. It was found that the tensile strain to failure increased almost threefold when the grain size was reduced from 60 to 3 μm. Grain refinement led to reduced twin formation and reduced void growth rates but did not impact markedly on the relationship between strain and the detected void number density. Because the finer grained samples experienced higher strains to failure, greater void number densities were thus detected at failure in these samples. The void volume fraction at failure remained constant despite changing grain size, within error. Final failure occurs via a shear localization and there appears to be a role of void formation in triggering the final shear instability. We thus favour ascribing failure to a void-sheeting type mechanism. Failure is seen to follow rapidly after a critical void volume fraction is attained and this is broadly consistent with predictions made via the application of a simple McClintock model. The higher strains to failure in the present fine-grained samples are thus ascribed chiefly to the lower rates of void growth. The suppression of void growth by grain refinement seen here may explain why finer grain magnesium alloys often display higher tensile ductility.

ACTA Vol. 200, Nov. 2020, P659-673

The yield strength of random metal alloys, i.e. alloys with random occupation of the crystalline lattice sites by the elemental constituent atoms all considered as solutes, is primarily understood as controlled by solute/dislocation interactions. Solute-solute interactions exist and provide the energetic driving force for both short-range and long-range order but can then also affect yield strength even in the random alloy. Here, a recent theory for random alloys is extended to include solute-solute interactions described by pair-wise interactions. The new theory involves the standard deviation in total solute-solute interaction energies as a dislocation segment glides through the material, which changes specific solute-solute pairs across the glide plane at every pair distance. An analytic expression is derived for the above standard deviation and validated against numerical simulations on a wide range of model random alloys consisting of 2–5 elements interacting via Lennard–Jones pair potentials. The theory is applied to the bcc MoNbTaW high entropy alloy, using solute-solute interactions computed via first-principles, and a model NbTaV alloy, described by EAM potentials, where the strength increases negligibly by 2% and 0.45%, respectively. Application to random dilute fcc Ni-Al, where the first-neighbor Al-Al interaction is very strongly repulsive, shows significant strengthening of 60–100% at 10% Al, depending on the origin of the inputs. Some connections to literature atomistic simulations on Ni–Al are also presented. Overall, the present theory provides a quantitative framework for assessing the relative roles of solute-dislocation and solute-solute interactions on strengthening in random alloys.

ACTA Vol. 200, Nov. 2020, P686-698

The precipitation of niobium carbide (NbC) is a superior approach to mitigating hydrogen embrit tlement (HE). The role of the semi-coherent interface between NbC and α-Fe on hydrogen trapping and HE resistance in high-strength tempered martensitic steel was investigated in this study. Highresolution transmission electron microscopy observations are performed to reveal the atomic-scale crystallographic orientation relationship, atomic arrangements, and associated crystalline defects in the NbC/α-Fe semi-coherent interface. We observed the Kurdjumov–Sachs orientation relationship with (1-1-1)NbC // (101)α−Fe and [0-11]NbC // [-111]α−Fe between the NbC and α-Fe phases. Noticeably, two sets of misfifit dislocations with Burgers vectors of b(¹) =a_b/2[111] on (01-1) α-Fe planes and b(²) = a_b/2[111] on (110) α-Fe planes (ab is the lattice constant of α-Fe), which would be the deep hydrogen trapping sites, were characterized in the NbC/α-Fe semi-coherent diffuse interface. In addition, density functional theory-based first-principles calculations revealed that the deep binding energy between the NbC/α-Fe semi-coherent interface and hydrogen is 0.80 eV, which well matches the hydrogen desorption activation energy of 81.8 kJ/mol determined via thermal desorption spectroscopy experiments. These demonstrate that the nature of the deep hydrogen trapping sites of the NbC/α-Fe semi-coherent interface is the misfit dislocation core. Distinguished HE resistance was obtained and ascribed to the deep hydrogen trapping of uniformly dispersed NbC nanoprecipitates with an average diameter of 10.0 ± 3.3 nm. The strategy of deep hydrogen trapping in the NbC/α-Fe semi-coherent interface is beneficial for designing HE-resistant steels.