ACTA Vol. 195, 15 Aug. 2020, P109-122

Herein, we have revealed the phase transition path and magnetic properties of Ni–Mn–In alloys with excess Ni or Mn (Ni24+xMn12In12-x, Ni24+xMn12-xIn12 and Ni24Mn12+xIn12-x) by using ab initio calculations. It is demonstrated that excess Ni atoms prefer to be adjacent to each other for the Ni24+xMn12In12-x alloys, whereas excess Ni (Mn) atoms tend to keep away from each other for the Ni24+xMn12-xIn12 and Ni24Mn12+xIn12-x alloys. The variation of the lattice constants, which are dominated by the atomic radius, are basically the same for the three alloy systems. Excess-Ni decreases the stability of the ferromagnetic austenite (FA) as well as the 6M and NM martensites. Meanwhile, the Curie temperature (TC) is reduced but the martensitic transformation temperature (TM) is increased due to the presence of excess-Ni. In comparison, excess-Mn destabilizes the FA phase, but stabilizes the 6M and NM phases. Excess-Mn has a similar impact on the increase of TM as that of excess-Ni, but has a weak effect on TC, but increases TM. The critical composition of the martensitic transformation and martensitic transition path are determined. The highest total magnetic moment of the FA phase appears with Mn-excess compositions whilst the smallest total magnetic moment is found with Ni-excess compositions. The changes of the total magnetic moments for the 6M and NM phases demonstrate an opposite trend to that of FA phase. The above results are expected to provide information for the prediction of phase diagram and magnetic properties of Ni–Mn–In alloys.

ACTA Vol. 195, 15 Aug. 2020, P123-131

Ternary Al–Mg–Si alloys have been modelled based on a multi-scale approach that spans across atomistic and mesoscale models and uses theoretically determined parameters. First, a cluster expansion model for total energy has been trained for atomistic configurations (FCC lattice) based on the data from density functional simulations of electronic structure. Free energy curves as a function of solute (Mg, Si) concentrations and disorder have been obtained by using this parameterisation together with meta-dynamics Monte Carlo sampling. In addition, free energy data, surface tensions as well as strain energy using the linear elasticity theory have been collected to be combined for a mesoscale phase-field model. The application of this approach shows that the formation of a layered MgSi phase, with (100) planes, is a particularly stable solute aggregation motif within the Al host matrix. Moreover, the phase-field model demonstrates that the preferred shape of the MgSi precipitates is needle-like (in FCC), and they can act as precursors for the important and well-known β″-type precipitates which are formed by translating one Mg column by a 1/2 lattice vector. The results provide theoretical evidence that the solute aggregation into needle-like MgSi domains (precipitates) is an inherent property of Al-Mg-Si alloys, and that it takes place even without the presence of vacancies which is a precondition for the eventual formation β″ precipitates.

ACTA Vol. 195, 15 Aug. 2020, P132-140

This work explores the development of a heterogeneous nanostructured material through leveraging abnormal recrystallization, which is a prominent phenomenon in coarse-grained Ni-based superalloys. Through synthesis of a sputtered Inconel 725 film with a heterogeneous distribution of stored energy and subsequent aging treatments at 730°C, a unique combination of grain sizes and morphologies was observed throughout the thickness of the material. Three distinct domains are formed in the aged microstructure, where abnormally large grains are observed in-between a nanocrystalline and a nanotwinned region. In order to investigate the transitions towards a heterogeneous structure, crystallographic orientation and elemental mapping at interval aging times up to 8 h revealed the microstructural evolution and precipitation behavior. From the experimental observations and the detailed analysis of this study, the current methodology can be utilized to further expand the design space of current heterogeneous nanostructured materials.

ACTA Vol. 195, 15 Aug. 2020, P141-150

The grain size (GS) dependence of fatigue crack growth in nanocrystalline superelastic NiTi shape memory alloys (SMAs) with the average GS of 10, 30, 60 and 235 nm is investigated. Synchronized measurements of the fatigue crack growth, load-displacement response and temperature field of the compact tension specimens are performed. It is found that the fatigue crack growth rate (da/dN) is monotonically decreased by 10 times when the GS is increased from 10 to 235 nm. The significant GS effect on the fatigue crack growth is also confirmed by the direct observation of fatigue striation spacing on the postmortem fracture surface. It is shown that the increase of the GS significantly enhances the phase transition and plasticity of the material and therefore enhances the crack-tip shielding (Ksh) which reduces the effective driving force (ΔKeff) of the fatigue crack growth. Subtracting Ksh due to phase transition and plastic deformation from the external applied Kapp, the effective crack-tip fatigue driving force ΔKeff is obtained. The da/dN-ΔKeff curves for different GS specimens then almost collapse onto a single curve which may represent an intrinsic fatigue property of the NiTi in the absence of the shielding effects. Therefore the observed apparent GS dependence of the fatigue crack growth resistance in the superelastic NiTi SMAs is mainly caused by the shielding effect from the GS dependent phase transition and plastic deformation.

ACTA Vol. 195, 15 Aug. 2020, P151-162

Diffusional-displacive transformations are generally associated with mechanical properties of materials such as high strength. Understanding structure evolution of these transformations is of great interests from both physics and application perspectives. By combining atomic resolution electron microscopy, energy dispersive spectroscopy and first principles calculations, a continuous β→αʺ→α transformation process has been quantitatively characterized at the atomic level in a metastable β-Ti alloy during aging treatment. The transformation is revealed to develop by a novel mechanism involving continuous structural and compositional changes towards the equilibrium assisted by compositional fluctuation in the β matrix. Moreover, the product phase induces a precipitate-matrix lattice mismatch, thus produces a coherency strain field surrounding the precipitates. The coherent strain field contributes significantly to the increasing hardness of the alloy after aging. These results have great potential for tailoring thermomechanical treatment routes and improving mechanical properties of materials.

ACTA Vol. 195, 15 Aug. 2020, P109-122

The high-pressure torsion (HPT) of binary copper alloys with 3, 5, 8, 10 wt. % Ag and 14 wt. % Sn has been studied at room temperature THPT. Before HPT, the Cu–Ag alloys have been annealed at 12 different temperatures between 320 and 800 °C and Cu–14 wt. % Sn has been annealed at 9 different temperatures between 310 and 500 °C. Thus, before HPT the Cu–Ag alloys consisted of Ag-particles in the Cu-based matrix with silver content cinit from almost zero to 8 wt.%. The Cu–14 wt. % Sn samples had Cu-based matrix with tin concentration cinit from almost zero to 14 wt.% Sn and precipitates of ε or δ Hume-Rothery intermetallic phases. After about 1.5 plunger rotations a certain steady-state concentration css of the alloying element is reached in the matrix. The measured css values were 5.5±0.1 wt. % Ag and 13.1±0.1 wt. % Sn. If the initial concentration cinit in Cu matrix was below css (cinit < css), it increased towards css during HPT. If cinit > css it decreased towards css. We observed that css did not depend on cinit in broad interval of cinit and was, therefore, equifinal. The equifinal css values corresponded to the certain equilibrium solubilities of silver and tin in Cu matrix and allowed to estimate the (elevated) effective temperature as Teff (Ag) = 700±10 °C and Teff (Sn) = 400±10 °C, respectively. The observed phenomena are discussed using the ideas of non-equilibrium thermodynamics of open systems. During HPT the decomposition of a solid solution competed with dissolution of precipitates. As a result, a dynamic equilibrium established between precipitation and dissolution at steady-state deformation stage. In this dynamic equilibrium a certain steady-state concentration css of the alloying element is reached in the matrix. In Cu-based alloys, the obtained Teff is always higher than THPT and correlates with activation enthalpy of dopant diffusion in Cu. Other HPT-driven phenomena such as accelerated mass transfer, intermetallic phase formation, grain boundary faceting and grain boundary segregation are taken into account to evaluate the effective temperature Teff.

ACTA Vol. 195, 15 Aug. 2020, P199-208

G115 steel has gained a growing interesting recently for its use in next-generation ultra-supercritical power plant applications. Due to the high densities of dislocations and lath martensite boundaries in G115 steel, interactions between solutes and dislocations result in unique microstructural evolution during creep with the formation of dense Cu-rich precipitates (CRPs) and M23C6 carbides. Atom-probe tomography reveals that Mn is preferentially associated with CRPs, probably because the Mn atoms reduce the critical energy of nucleation. Solute-dragging and precipitate-pinning effects enhance the formation of dislocation network during earlier creep deformation. Compared with aged G115 steel, long-term creep deformation accelerates the coarsening of CRPs. The fast diffusion of solutes along dislocations, dislocation network walls, and lath boundaries significantly increases the CRP coarsening kinetics. Particle coarsening reduces the pinning strength, causing the dislocation density to decrease and the dislocation network to disappear during long creep stages. Our results enhance our understanding of CRP evolution in G115 steel during creep and provide a guide for the design of novel heat-resistant steels with excellent creep strength.

ACTA Vol. 195, 15 Aug. 2020, P240-251

Nanostructured metals and alloys are generally strong but lack strain hardening due to enhanced grain boundary diffusion and sliding. Here, equal nanograined (ENG) and gradient nanograined (GNG) layered Cu/Ta architectures were acquired by introducing a hard and stable “artificial” interphase boundary zone (IBZ). The ENG architecture produced uniform plastic strain reaching 70% and high yield strength exceeding 1 GPa, which is attributed to the constraint effect of the tough IBZ on dislocation slip mediated co-deformation. The GNG architecture exhibited a remarkable linear strain hardening with hardening exponent of 1 due to the strong strain partitioning between soft and hard layers. The dominant deformation mechanism of the GNG nanocomposite evolves from partial dislocation emission to dislocation accumulation at interface with increasing strain based on the results from experiments and molecular dynamics simulations. This finding demonstrates that heterostructure with “artificial” IBZ may offer an alternative approach to design strong and tough materials.

ACTA Vol. 195, 15 Aug. 2020, P263-273

It is commonly accepted that prismatic precipitates in Mg alloys are more potent strengtheners than the ones formed on basal planes. However, in most rare-earth (RE) free Mg alloys, precipitation commonly occurs on basal planes. Recent results showed that coupling {10-12} twinning, aging and detwinning (a process termed TAD) promotes prismatic precipitation in Mg-Al alloys by a mechanism that remains unclear. The present work aims to theoretically and experimentally evaluate possible crystallographic orientation relationships (ORs) between the prismatic precipitates and the detwinned matrix in TAD processed samples in order to understand their formation mechanism. A crystallography-based algorithm is proposed to predict such ORs and the predictions are subsequently validated through experimental observation in a transmission electron microscopy (TEM). Three new ORs between Mg17Al12 precipitates and the α-Mg-Al matrix are experimentally determined, which agree well with the predictions of the proposed algorithm after considering the 3.69° rotation of precipitates at twin boundaries. Additionally, the proposed algorithm is validated in other RE free Mg alloys produced via the TAD process, including Mg-Sn-Zn, Mg-Al-Ca, Mg-Zn and Mg-Ca-Zn systems. This work provides a clearer understanding of the ORs between prismatic precipitates and the matrices of Mg alloys that is deemed critical to fully exploit their potential for precipitation hardening.

ACTA Vol. 195, 15 Aug. 2020, P274-281

The glass transition is alternatively described as either a dynamic transition in which there is a dramatic slowing down of the kinetics, or as a thermodynamic phase transition. To examine the physical origin of the glass transition in fragile Cu-Ag liquids, we employed molecular dynamics (MD) simulations on systems in the range of 32,000 to 2,048,000 atoms. Surprisingly, we identified a 1st order freezing transition from liquid (L) to metastable heterogenous solid-like phase, denoted as the G-glass, when a supercooled liquid evolves isothermally below its melting temperature at deep undercooling. In contrast, a more homogenous liquid-like glass, denoted as the L-glass, is achieved when the liquid is quenched continuously to room temperature with a fast cooling rate of ~1011 K/sec. We report a thermodynamic description of the L-G transition and characterize the correlation length of the heterogenous structure in the G-glass. The shear modulus of the G-glass is significantly higher than the L-glass, suggesting that the first order G-G transition is linked fundamentally to long-range elasticity involving elementary configurational excitations in the G-glass.

ACTA Vol. 195, 15 Aug. 2020, P304-316

Grain boundary diffusion of the principal elements 57Co, 51Cr, 59Fe and 54Mn in a coarse-grained equiatomic CoCrFeMnNi high entropy alloy is measured in a wide temperature range of 643 to 1273 K in both C- and B-type kinetic regimes after Harrison’s classification. The results suggest that the product of the pertinent segregation factors, s, and the grain boundary width, δ, is about 0.5 nm for all elements at temperatures T > 800 K. Whereas one short-circuit contribution is observed at higher temperatures above 800 K, the penetration profiles in the C-type kinetic regime (643 – 703 K) reveal two distinct contributions that hint towards a phase decomposition at a fraction of high-angle grain boundaries at these temperatures and the existence of a structural multiplicity of high-angle grain boundaries. A correlative microscopy combining transmission Kikuchi diffraction and atom probe tomography manifests formation of neighboring Ni-Mn-rich and Cr-rich precipitates at high angle grain boundaries. Transmission electron microscopy revealed an increased dislocation density in the vicinity of such interfaces which is suggested to be a reason of the enhanced diffusion rates at low temperatures for such short circuits.

ACTA Vol. 195, 15 Aug. 2020, P317-326

The thermodynamic properties of α-Al and other phases (GP zones, θ'', θ' and θ) in the Al-rich part of the Al-Cu system have been obtained by means of the cluster expansion formalism in combination with statistical mechanics. This information was used to build the Al-rich part of the Al-Cu phase-diagram taking into account vibrational entropic contributions for θ', as those of the other phases were negligible. The simulation predictions of the phase boundaries between α-Al and either θ'', θ' or θ phases as a function of temperature are in good agreement with experimental data and extend the phase boundaries to a wider temperature range. The DFT calculations reveal the presence of a number of metastable Guinier-Preston-zone type configurations that may coexist with α-Al and θ'' at low temperatures. They also demonstrate that θ' is the stable phase below 550K but it is replaced by θ above this temperature due to the vibrational entropic contribution to the Gibbs energy of θ'. This work shows how the combination of cluster expansion and statistical mechanics can be used to expand our knowledge of the phase diagram of metallic alloys and to provide Gibbs free energies of different phases that can be used as input in mesoscale simulations of precipitation.

ACTA Vol. 195, 15 Aug. 2020, P327-340

Improving the properties of durable high-temperature alloys is based on the fundamental understanding of the link between microstructure and three-dimensional (3D) nanochemistry. Here we utilize a complementary approach of transmission electron microscopy and atom probe tomography to link microstructure and 3D nanochemistry of a ternary single crystal Ni83.9Si13Fe3.1 (at.%) model alloy. The formation of a γ/γ' microstructure is revealed, containing primary and secondary γ' precipitates analogous to Ni-based superalloys. Subsequently, microstructural hierarchy is created by the formation of γ particles inside primary γ' precipitates. The correlated supersaturation with γ forming elements (Ni, Fe) of primary γ' precipitates was identified as driving force for the formation of γ particles. The influence of aging on the mechanical properties is reported and peak hardness is achieved after 24 h of aging at 923 K. Thermo-Calc equilibrium phase concentrations based on the TTNi8 database where found to be closer to the APT data than the TCNi8 based values. Our results suggest that improved stability of γ particles can be achieved by tailoring the phase chemistry and the lattice misfit.

ACTA Vol. 195, 15 Aug. 2020, P341-357

Microstructure formation of a hypo-eutectic Al-11Cu (atom percent) alloy during solidification after laser melting has been studied by transmission electron microscopy (TEM). The evolution of the solid-liquid-interface velocity, VSL, during the solidification process has been determined from direct observation by in-situ TEM. This enabled correlating VSL with development of four distinct microstructure zones. Crystal growth mode transitions from planar to cellular, cellular to dendritic, dendritic to cellular, and cellular to planar, have been observed for the accelerating solid-liquid-interface. The transition from coupled two-phase growth to the single-phase growth occurred for VSL = Va = (0.80±0.05) m/s, where Va is the velocity of absolute stability, at the onset of banded morphology grain formation. The in-situ and post-mortem TEM uniquely permitted determination of the non-equilibrium solidus for the rapidly solidifying Al-11Cu alloy. Experimental evidence for solute clustering and chemical ordering tendencies at length scales on the order of ≤ 5nm has been detected in the single-phase regions of the banded grains. The structural features of the single-phase bands have been interpreted as signatures of ‘frozen in’ configurations present in the liquid boundary layer adjacent to the growing crystal, which has a width of about 3nm. The nano-scale spatiotemporal resolution experimental TEM studies performed here provided quantitative metrics, e.g. the solidification interface velocity dependent solute concentration of the α-Al phase and the near-atomic scale structure in the single-phase bands, that are uniquely suitable for comparison with theory and model predictions for solidification microstructure development in multicomponent alloys after laser melting.

ACTA Vol. 195, 15 Aug. 2020, P358-370

The present article focuses on interpreting mechanical interface energies/parameters which have been introduced within gradient plasticity and can capture the ability of grain boundaries to deform plastically. Molecular dynamics simulation compression tests were performed on body-centered cubic Fe bicrystals with tilt and twist grain boundaries of different misorientations. The simulations allowed for the formation of dislocation pile-ups, which give rise to gradients in the plastic strain, and therefore it was possible to employ interfacial gradient plasticity to interpret the stress-strain curves and obtain values for the mechanical interface parameter. The simulations showed that the atomic structure at the grain boundary changed during dislocation absorption, illustrating that the initial grain boundary static energy cannot characterize boundaries after the onset of plastic deformation. It was found that the value of the mechanical interface parameter (i) depended on the GB type and GB-dislocation interactions that led to slip transmission across the GB, and (ii) was positively related to the magnitude of residual Burgers vector, indicating that as the mechanical interface parameter increased the GB strength increased. Repeating the simulations by adding hydrogen atoms at the grain boundaries, showed that segregation increased the mechanical interface energy. Changing the sample size indicated that such mechanical energy terms are size-dependent, however their ratio with the internal length was size independent and depended only on the GB type. A detailed understanding of these mechanical grain boundary energies may pave a new way to engineering materials in the sub-micron scales.

ACTA Vol. 195, 15 Aug. 2020, P371-382

In this study, for the first time, the evolution of geometrically necessary dislocation (GND) and statistically stored dislocation (SSD) densities, as well as their roles in strain hardening during mechanical twinning, was experimentally investigated in a tensile-deformed Fe-22Mn-0.6C twinning-induced plasticity (TWIP) steel. GND and SSD densities were estimated via EBSD-acquired orientation data and a modified strain hardening model, respectively. The analysis demonstrates that the GND density increases non-linearly due to mechanical twinning. The SSD density increases much faster than the GND density, which shows that multiplication of the SSDs is heavily dependent on the imposed strain level. It is revealed that the GND density is higher at early strain stages (below 0.14 true strain), dominating dislocation hardening, but thereafter the SSD density contributes more. It is also found that the GND density is several times higher in this TWIP steel than in metals or alloys, which deform through dislocation slip only. We attribute this difference to the planar slip of dislocations and the occurrence of mechanical twinning, which leads to much more pile-ups of the GNDs at/near boundaries. Mechanical twinning directly contributes less than 100 MPa to flow stress increment in the studied true strain range of 0 to 0.34. Consequently, depending on dislocation types, dislocation multiplication governs strain hardening at all deformation ranges. The findings provide insight into the evolution behaviors of GNDs and SSDs in TWIP steels, which are particularly important for further understanding of the dynamic Hall-Petch effect and useful for TWIP alloy design efforts.

ACTA Vol. 195, 15 Aug. 2020, P416-424

Reliable data on self-diffusion of aluminium in α-Al2O3 are scarce and do not yet enable deriving quantitative conclusions about the impact of deliberate aliovalent doping. Therefore, the present study (1280 ≤ T/ °C ≤ 1500, pO2 = 200 mbar) was based on carefully grown high-purity single crystals doped with Ti. The experimentally determined 26Al tracer diffusivity increased with the Ti concentration and confirmed the incorporation of Ti on Al sites, TiAl•, with aluminium vacancies, VAl″′, formed for charge compensation. The observed relation between these two point defect concentrations as a function of the Ti concentration and the temperature can be quantitatively modelled by taking into account probable TiAl•:VAl″′ clusters. Using binding energy starting values from published computer simulations for the fit procedure cluster binding energies are obtained. The surprisingly high Al diffusivity in undoped samples can be tentatively rationalised by injection of Al interstitials at the liquid/solid interface because of the low oxygen potential during the crystal growth process.

ACTA Vol. 195, 15 Aug. 2020, P433-445

The microstructure and nanomechanical properties of three samples of the modified stainless steel (referred as the alloy D9) were systematically characterized after neutron irradiation in the Advanced Test Reactor. The samples were irradiated to 5.0, 8.2, and 9.2 displacements per atom at 448, 430, and 683°C, respectively. The evolutions of dislocation loops, cavities, and radiation-induced precipitates were quantitatively studied to reveal their dose and temperature dependencies. Nanohardness and nanoindentation creep tests were conducted at room temperature on the irradiated samples. Unexpected radiation hardening was observed in the highest-temperature-irradiated sample due to the formation of an unknown type of Ni- and Si-rich precipitates whose contributions to the radiation and mechanical performances of the alloy were discussed. We provide the radiation-microstructure-property correlations of alloy D9 with new insights, which can benefit the development and optimization of advanced austenitic alloys for future nuclear applications.

ACTA Vol. 195, 15 Aug. 2020, P454-467

Creep rupture life is a key material parameter for service life and mechanical properties of Ni-based single crystal superalloy materials. Therefore, it is of much practical significance to accurately and efficiently predict creep life. Here, we develop a divide-and-conquer self-adaptive (DCSA) learning method incorporating multiple material descriptors for rational and accelerated prediction of the creep rupture life. We characterize a high-quality creep dataset of 266 alloy samples with such features as alloy composition, test temperature, test stress, and heat treatment process. In addition, five microstructural parameters related to creep process, including stacking fault energy, lattice parameter, mole fraction of the γ' phase, diffusion coefficient and shear modulus, are calculated and introduced by the CALPHAD (CALculation of PHAse Diagrams) method and basic materials structure-property relationships, that enables us to reveal the effect of microstructure on creep properties. The machine learning explorations conducted on the creep dataset demonstrate the potential of the approach to achieve higher prediction accuracy with RMSE, MAPE and R2 of 0.3839, 0.0003 and 0.9176 than five alternative state-of-the-art machine learning models. On the newly collected 8 alloy samples, the error between the predicted creep life value and the experimental measured value is within the acceptable range (6.4486 h–40.7159 h), further confirming the validity of our DCSA model. Essentially, our method can establish accurate structure-property relationship mapping for the creep rupture life in a faster and cheaper manner than experiments and is expected to serve for inverse design of alloys.

ACTA Vol. 195, 15 Aug. 2020, P468-481

In the present study, we combine comprehensive site-specific microstructural and mechanical characterization studies to evaluate the toughening effect induced by applying a surface SPEX milling (SSM) approach to commercially-pure Mg. Our results show that the high frequency, multi-directional deformation strain induced by SSM generated gradients in the grain size and orientation with depth from the surface. More importantly, it also resulted in a gradient in the density of twin meshes, which are defined as two or more intersecting arrays of twins, along the sample thickness direction. Tensile tests at room temperature indicate that after SSM, the samples have higher ultimate tensile strengths and two-fold increases in ductility as compared to those of the untreated samples. The effect of the initial texture and SSM parameters on the microstructural evolution and texture randomization were analyzed by electron backscattered diffraction (EBSD). Furthermore, in order to correlate the variation in microstructure with the site-specific mechanical response, TEM and in-situ micropillar compression testing in SEM were performed at various distances from the SSM-treated surface. The results show that twin meshes may be responsible for the activation of more slip systems and higher strain hardening, which result in higher uniform plastic strain.

ACTA Vol. 195, 15 Aug. 2020, P501-518

Grain boundary structure-property relationship was studied in a Ni-base 602CA coarse-grained alloy using a novel correlative tracer diffusion-analytical microscopy approach. Co-existence of several short-circuit contributions to tracer diffusion was distinguished at higher temperatures. These contributions were related to different families of high-angle grain boundaries with distinct coverages by precipitates and segregation levels as revealed by HAADF-STEM combined with EDX measurements and a detailed atom probe tomography analysis. Annealing at such conditions resulted in Cr23C6-type carbides co-existing with an α-Cr-Mn-enriched phase in addition to sequential segregation layers of Al, Ni, and Fe around them. Curved and hackly grain boundaries showed a high density of plate-like carbides. In contrast, straight grain boundaries were composed of globular carbides with similar chemical composition variations and additionally with alternating layers of Cr and Ni in-between the carbides, similar to spinodal microstructures. At lower temperatures, hackly interfaces with Cr and Cr-carbide enrichment dominated and the alloy annealed at 403 K contained plate-like Cr23C6-type carbides surrounded by a Ni-rich layer around them. The Ni grain boundary diffusion rates at these relatively low temperatures showed an anomalous character being almost temperature independent. This specific diffusion behaviour corresponds to a metastable grain boundary transition which is explained by a concomitant relaxation of transformation-induced elastic strains occurring on a longer time scale in comparison to those for grain boundary diffusion. Thermodynamic insights into the probable mechanism of decomposition at grain boundaries are provided. The paper provides solid evidences towards the existence of grain boundary phase transitions in the Ni-base multi-component alloy. It underlines the capabilities of the correlated kinetic-structure measurements as a tool to probe such changes.

ACTA Vol. 195, 15 Aug. 2020, P541-554

A recurrent challenge with aluminum alloys is their longstanding trade-off between mechanical strength and formability. Recently recyclability has put further pressure on the development of single-alloy concepts for solving this challenge. This study addresses an AlMg-based system featuring additional elements to facilitate age-hardening but retaining a high Mg content for inherent pronounced strain hardening as a potential candidate. Age-hardening was enabled by T-phase based precipitation in the commercial alloy EN AW-5182 via the addition of 3.5 wt.% of Zn. The investigation shows that minor additions of Cu and Ag enhance and accelerate it. The study also compares single-step and double-step artificial aging. Hardness and tensile testing and scanning transmission electron microscopy methods were deployed to characterize the alloys investigated, mechanically and microstructurally. An alloy with added Zn, Cu and Ag showed improved strain hardening and reduced serrated flow in the soft state, while exhibiting an age-hardening response of up to 326 MPa in yield strength leading to an ultimate tensile strength of 550 MPa in peak-aged condition. The study discusses the evolution of the microstructure during artificial aging in the light of Zn, Cu and Ag additions and their effect on the precipitation process.