Zener Pinning Research Articles

A coupled model for phase mixing, grain damage and shear localization in the lithosphere: comparison to lab experiments

SUMMARYThe occurrence of plate tectonics on Earth is rooted in the physics of lithospheric ductile weakening and shear-localization. The pervasiveness of mylonites at lithospheric shear zones is a key piece of evidence that localization correlates with reduction in mineral grain size. Most lithospheric mylonites are polymineralic and the interaction between mineral phases, such as olivine and pyroxene, especially through Zener pinning, impedes normal grain growth while possibly enhancing grain damage, both of which facilitate grain size reduction and weakening, as evident in lab experiments and field observations. The efficacy of pinning, however, relies on the mineral phases being mixed and dispersed at the grain scale, where well-mixed states lead to greater mylonitization. To model grain mixing between different phases at the continuum scale, we previously developed a theory treating grain-scale processes as diffusion between phases, but driven by imposed compressive stresses acting on the boundary between phases. Here we present a new model for shearing rock that combines our theory for diffusive grain mixing, 2-D non-Newtonian flow and two-phase grain damage. The model geometry is designed specifically for comparison to torsional shear-deformation experiments. Deformation is either forced by constant velocity or constant stress boundary conditions. As the layer is deformed, mixing zones between different mineralogical units undergo enhanced grain size reduction and weakening, especially at high strains. For constant velocity boundary experiments, stress drops towards an initial piezometric plateau by a strain of around 4; this is also typical of monophase experiments for which this initial plateau is the final steady state stress. However, polyphase experiments can undergo a second large stress drop at strains of 10–20, and which is associated with enhanced phase mixing and resultant grain size reduction and weakening. Model calculations for polyphase media with grain mixing and damage capture the experimental behaviour when damage to the interface between phases is moderately slower or less efficient than damage to the grain boundaries. Other factors such as distribution and bulk fraction of the secondary phase, as well as grain-mixing diffusivity also influence the timing of the second stress drop. For constant stress boundary conditions, the strain rate increases during weakening and localization. For a monophase medium, there is theoretically one increase in strain rate to a piezometric steady state. But for the polyphase model, the strain rate undergoes a second abrupt increase, the timing for which is again controlled by interface damage and grain mixing. The evolution of heterogeneity through mixing and deformation, and that of grain size distributions also compare well to experimental observations. In total, the comparison of theory to deformation experiments provides a framework for guiding future experiments, scaling microstructural physics to geodynamic applications and demonstrates the importance of grain mixing and damage for the formation of plate tectonic boundaries.

Microstructure and mechanical properties of additive manufactured Inconel 718 alloy strengthened by oxide dispersion with 0.3 wt% Sc addition

Additively manufactured superalloys have attracted increasing attention due to their inherent hierarchical microstructure, but have been impeded by anisotropic properties originating from columnar grains. In this work, we present a strategy for inhibiting the formation of columnar grains in which 0.3 wt% scandium (Sc) was introduced as an oxide dispersion strengthener (ODS) into the Inconel 718 (IN718) alloy fabricated via laser powder bed fusion (LPBF). The effects of Sc on the microstructure and mechanical properties of the IN718 were investigated, revealing the in-situ formation of Sc 2 O 3 nanoparticles as well as the cellular sub-grains microstructure with high-density of dislocations into the as-prepared IN718-Sc alloy. The addition of Sc disrupted the typical columnar grains generated by the LPBF process, resulting in, the strong (100) texture in pure IN718 alloy changing to the irregular orientation in IN718-Sc alloy. In addition, the average grain size and the aspect ratio also show decreases when Sc introduction. As a result, the as-designed IN718-Sc alloy achieved a higher yield strength of 1030 MPa at room temperature and 837 MPa at 650 ºC, respectively, which can be attributed to Zener pinning of grain boundaries by dispersed Sc 2 O 3 nanoparticles. Importantly, the insight into dispersing oxide particles to regulate microstructure and improve properties in this study could provide guidance for other alloy fabrication through powder additive manufacturing. • Generating in situ rare earth oxides as nucleating agent, the columnar grains trend was suppressed. • With Sc addition, the grain size was refined and Sc 2 O 3 was generated to pin dislocation. • IN718-Sc exhibited a 20% increase in yield strength at room temperature and high temperature.

Grain Growth in High-Entropy Alloys (HEAs): A Review

Grain growth is an unavoidable phenomenon occurring during processing of multicomponent high-entropy alloys (HEAs) and medium-entropy alloys (MEAs), which significantly affects the properties and performance of these compositionally complex alloys during service. Accordingly, the present overview article is dedicated to summarize the state of the art of the grain coarsening behavior and its kinetics for HEAs and propose opportunities and scope for future research, with special attention to the Cantor-based compositions. Firstly, the primary recrystallization and grain growth of equiatomic and non-equiatomic CrMnFeCoNi alloys with face-centered cubic (FCC) crystal structure in the single-phase regime and with the presence of intermetallics such as the Cr-rich sigma (σ) phase is considered, which was accompanied by the powerful utilization of the solute drag effect, Zener pinning, and grain boundary segregation engineering via introducing carbon, nitrogen, boron, titanium, and niobium. Subsequently, the quaternary and ternary alloys based on Cr, Mn, Fe, Ni, and Co are taken into account. In these systems, the novel concept of grain boundary high-entropy effect (via the addition of molybdenum, niobium, and zirconium), the formation of pinning insoluble oxide and thermally stable carbide particles during mechanical alloying, the simultaneous solute drag and Zener pinning effects via Mo and C addition, and the formation of Cu-rich phase and the strong segregation effect of Cu atoms to grain boundaries in the FeCoNiCu HEA are critically discussed. Afterward, the thermal stability of AlxFeCoCrNi(Mn) HEAs is treated, where the formation of the Ni–Al-rich B2 phase and the corresponding Zener pinning effect, the influence of aluminum addition on the temperature range for the stability of the σ phase, and cluster drag mechanism of Al atoms are explained. What is more, the grain growth of refractory HEAs with body-centered cubic (BCC) crystal structure is also summarized, where the high grain growth activation energy is attributed to the presence of elements with very low self-diffusivity such as tantalum and niobium. Moreover, the possible effects of short-range ordering on the mobility of grain boundaries are discussed. Finally, the prospects for future research work are presented, including the development of precipitation-temperature-time (PTT) diagrams, systematic investigation of cold rolling and annealing for prior grain refinement, studying abnormal grain growth (secondary recrystallization), and assessment of grain growth behavior of HEAs manufactured by advanced methods such as additive manufacturing (3D printing).

Stability of immiscible nanocrystalline alloys in compositional and thermal fields

Alloying is often employed to stabilize nanocrystalline materials against microstructural coarsening. The stabilization process results from the combined effects of thermodynamically reducing the curvature-dominated driving force of grain-boundary motion via solute segregation and kinetically pinning these same grain boundaries by solute drag and Zener pinning. The competition between these stabilization mechanisms depends not only on the grain-boundary character but can also be affected by imposed compositional and thermal fields that further promote or inhibit grain growth. In this work, we study the origin of the stability of immiscible nanocrystalline alloys in both homogeneous and heterogeneous compositional and thermal fields by using a multi-phase-field formulation for anisotropic grain growth with grain-boundary character-dependent segregation properties. This generalized formulation allows us to model the distribution of mobilities of segregated grain boundaries and the role of grain-boundary heterogeneity on solute-induced stabilization. As an illustration, we compare our model predictions to experimental results of microstructures in platinum-gold nanocrystalline alloys. Our results reveal that increasing the initial concentration of available solute progressively slows the rate of grain growth via both heterogeneous grain-boundary segregation and Zener pinning, while increasing the temperature generally weakens thermodynamic stabilization effects due to entropic contributions. Finally, we demonstrate as a proof-of-concept that spatially-varying compositional and thermal fields can be used to construct dynamically-stable, graded, nanostructured materials. We discuss the implications of using such concepts as alternatives to conventional plastic deformation methods.

Microstructure tailoring to enhance mechanical properties in CoCrFeNiMn high-entropy alloy by Ti addition and thermomechanical treatment

CoCrFeNiMnTix (x = 0.1 and 0.3) high-entropy alloys (HEAs) were processed by thermomechanical treatment including heavy cold rolling followed by post-deformation annealing (PDA) at 773 to 1173 K for up to 60 min. Microstructural evolutions by adding titanium and thermomechanical treatment revealed significant enhancement in the mechanical properties with combination of high strength and ductility in CoCrFeNiMn HEA. The initial microstructures consisted of a single fcc phase while the PDA led to formation of complicated microstructure due to precipitation and recrystallization. The results indicated that adding titanium promoted thermal stability of precipitates and postponed coarsening by solute drag and Zener pinning. The CoCrFeNiMnTi0.3 HEA showed high strength even after PDA at 1173 K due to stability of precipitates at this temperature. However, similar procedure resulted in dissolution of precipitates which facilitated coarsening and reduce the strength significantly in the CoCrFeNiMnTi0.1 HEA. The latter alloy showed remarkable mechanical properties with an ultimate tensile strength (UTS) and elongation to failure of ~900 MPa and ~ 30%, respectively, after PDA at 973 K.

On the Formation Mechanism of Banded Microstructures in Electron Beam Melted Ti–48Al–2Cr–2Nb and the Design of Heat Treatments as Remedial Action

The formation mechanism of banded microstructures of an electron beam melted engineering intermetallic Ti–48Al–2Cr–2Nb alloy, the solidification behavior, and the heat treatment response are investigated via a process parameter study. Scanning electron microscopy, hardness testing, X‐ray diffraction, electron probe microanalysis, thermomechanical analysis, electron backscatter diffraction, heat treatments, as well as thermodynamic equilibrium calculation, and numerical simulation were performed. All specimens show near‐γ microstructures with low amounts of α2 and traces of βo. Fabrication with an increased energy input leads to an increased Al loss due to evaporation, a lower α‐transus temperature, and to a higher hardness. Banded microstructures form due to abnormal grain growth toward the bottom of original melt pools, whereas α2 in Al‐depleted zones enables a Zener pinning of the γ‐grain boundaries, leading to fine‐grained areas. Via numerical simulation, it is shown that increasing the energy input leads to larger maximum temperatures and melt pool sizes, longer times in the liquid state, and more remelting events. Solidification happens via the α‐phase and increasing the energy input leads to an alignment of (111)γ in building direction. Furthermore, banded microstructures respond heterogeneously to heat treatments. Heat treatment is introduced based on homogenization via phase transformation to obtain isotropic microstructures.

The origin of abnormal grain growth upon thermomechanical processing of laser powder-bed fusion alloys

Parts produced by laser powder-bed fusion (LPBF) show unique microstructures consisting of dislocation structures and an oxide nanoparticle dispersion usually embedded in epitaxially-grown grains. Thermomechanical processing is an alternative to enhance the microstructure of such materials. However, the deformation mechanisms and the resulting microstructures following annealing are not yet well understood, hindering further microstructure control. We apply cold rolling and subsequent annealing in AISI 316L stainless steel processed by LPBF and perform an in-depth microstructural characterization to understand the origin of abnormal growth and how to avoid it. Upon deformation, mechanical twinning occurs. Early plastic instabilities arise due to the fine substructure with high defect density, resulting in profuse shear banding. Such shear bands carry most of the subsequent deformation, reducing the volume fraction of oxide particles along these regions due to enhanced particle dissolution via cracking/fragmentation. Upon annealing, the cold-rolled specimens show abnormal <110> || ND grains nucleating at shear bands. The earlier recrystallization onset and fragmented particle dissolution in shear bands result in a local lower Zener pinning and generate a size advantage for <110> || ND grains. Based on this investigation, abnormal growth may be triggered by shear bands in cold-rolled and annealed LPBF alloys for grain boundary engineering. Our results suggest that avoiding shear banding (and the consequent particle fragmentation) inhibits abnormal grain growth, thus yielding a more uniform and fine-grained microstructure.

Microstructural investigation of WC reinforced Ti-6Al-4V matrix surface composite fabricated via additive friction stir processing

Additive friction stir processing (AFSP) was effectively utilized to fabricate defect-free WC reinforced surface composite on Ti-6Al-4V substrate. Uniform distribution of fragmented WC particles was achieved in the composite layer. Completely β transformed microstructure, containing lamellar α/β structure coupled with ultrafine martensite α′ was observed in stir zone of FSPed Ti-6Al-4V. The addition of WC reinforcement during AFSP has led to additional microstructure refinement; as a consequence of Zener pinning and particle stimulated nucleation caused by fragmented WC particles. More than two times improvement in hardness and wear resistance was noted in WC reinforced Ti-6Al-4V (surface composite) fabricated by AFSP compared to base metal Ti-6Al-4V. Despite a little drop in hardness value, post FSP heat treatment of WC reinforced surface composite has brought about a significant increase in impact toughness.

Defect Engineering in Solution-Processed Polycrystalline SnSe Leads to High Thermoelectric Performance.

SnSe has emerged as one of the most promising materials for thermoelectric energy conversion due to its extraordinary performance in its single-crystal form and its low-cost constituent elements. However, to achieve an economic impact, the polycrystalline counterpart needs to replicate the performance of the single crystal. Herein, we optimize the thermoelectric performance of polycrystalline SnSe produced by consolidating solution-processed and surface-engineered SnSe particles. In particular, the SnSe particles are coated with CdSe molecular complexes that crystallize during the sintering process, forming CdSe nanoparticles. The presence of CdSe nanoparticles inhibits SnSe grain growth during the consolidation step due to Zener pinning, yielding a material with a high density of grain boundaries. Moreover, the resulting SnSe–CdSe nanocomposites present a large number of defects at different length scales, which significantly reduce the thermal conductivity. The produced SnSe–CdSe nanocomposites exhibit thermoelectric figures of merit up to 2.2 at 786 K, which is among the highest reported for solution-processed SnSe.

Nanostructured Oxide‐Dispersion‐Strengthened CoCrFeMnNi High‐Entropy Alloys with High Thermal Stability

A nanostructured oxide‐dispersion‐strengthened (ODS) CoCrFeMnNi high‐entropy alloy (HEA) is synthesized by a powder metallurgy process. The thermal stability, including the grain size and crystal structure of the HEA matrix and oxide dispersions, is carefully investigated by X‐ray diffraction (XRD) and electron microscopy characterizations after annealing at 900 °C. The limited grain growth may be attributed to Zener pinning of yttria dispersions that impede the grain boundary mobility and diffusivity. The high hardness is caused by both the fine grain size and yttria dispersions, which are also retained after annealing at 900 °C. Herein, it is implied that the combination of ODS and HEA concepts may provide a new design strategy for the development of thermally stable nanostructured alloys for extreme environments.

Microstructure and mechanical properties of casting Al-3Li-2Mg-1Zn-0.1Zr alloys modified by Sc additions

In the present study, microstructure and mechanical properties of casting Al-3Li-2Mg-1Zn-0.1Zr alloys modified by Sc additions have been investigated. As-aged Al-3Li-2Mg-1Zn-0.2Sc-0.1Zr alloy possesses a desirable comprehensive mechanical performance. The YS, UTS and El reach 253 MPa, 406 MPa and 7.4% respectively. With the addition of Sc, a more than 50% reduction in the grain size was achieved by the promoted heterogeneous nucleation on primary Al3(Sc, Zr) phases and the grain growth resistance was enhanced by the Zener pinning. After the heat treatment, core-shell composite particles precipitated from the matrix. The shell consists entirely of Al3Li phases. In contrast, both Al3(Sc, Zr) and Al3Sc cores can be observed in the Sc-containing alloys. Increasing the Sc content from 0.1 wt% to 0.2 wt%, the mean radius of cores increases, while the mean width of shells and the number density of particles seem not to be affected. With the passage of ageing time, the coarsening of composite particles occurred, though the dimension of the core remained almost constant. The kinetics follows the KV model and the exponent (n−1) is about 0.11, which is lower than the expected value (0.33) probably due to the unreached quasi-steady-state at the present low temperature.

Ultrastrong nanocrystalline oxide-dispersion-strengthened ferritic alloy with exceptional thermal stability

As structural materials candidates of advanced fission and future fusion reactors, oxide-dispersion-strengthened (ODS) alloys, which are often ultrafine-grained (with a grain size between 100 and 1000 nm), have attracted widespread interest in past decades. However, fabrication of fully dense bulk nanocrystalline ODS alloys has rarely been reported. In this work, a novel Zr-doped nanocrystalline 14YWTZ ferritic ODS alloy (Fe–14Cr–3W-0.4Ti-0.8Zr-0.3Y2O3) was prepared by mechanical alloying and high-pressure consolidation techniques. The high consolidation pressure (4 GPa) endows the bulk nanocrystalline 14YWTZ alloy with a nearly full relative density (~99.5%) and a reasonable tensile elongation. The consolidated nanocrystalline 14YWTZ alloy with an average grain size of ~50 nm shows an ultra-high compressive yield strength of ~2641 MPa. Besides, the alloy exhibits an exceptional thermal stability, i.e., limited grain growth occurs after annealing at a high temperature of 1000 °C. After hot rolling, the alloy retains nanocrystalline microstructures and exhibits an unprecedented tensile yield strength of 2730 MPa, far exceeding those of previous reported ferritic ODS alloys. The high thermal stability of the annealed nanocrystalline 14YWTZ alloys can be ascribed to the combined effects of the thermodynamic multicomponent co-segregation to grain boundaries and strong kinetic Zener pinning by high-density Zr–Ti–Y–O-enriched nanoparticles with an average particle size of ~3.18 nm and a high number density of ~3 × 1024 m−3. The outstanding strength of the present nanocrystalline 14YWTZ alloys primarily originates from the Hall-Petch strengthening of the nanocrystalline matrix, distinguishing them from conventional ODS alloys strengthened by dislocation forest and/or dispersed oxide nanoparticles.

Influence of Fe and Mn on the Microstructure Formation in 5xxx Alloys-Part II: Evolution of Grain Size and Texture.

In recent decades, microstructure and texture engineering has become an indispensable factor in meeting the rising demands in mechanical properties and forming behavior of aluminum alloys. Alloying elements, such as Fe and Mn in AlMg(Mn) alloys, affect the number density, size and morphology of both the primary and secondary phases, thus altering the grain size and orientation of the final annealed sheet by Zener pinning and particle stimulated nucleation (PSN). The present study investigates the grain size and texture of four laboratory processed AlMg(Mn) alloys with various Fe and Mn levels (see Part I). Common models for deriving the Zener-limit grain size are discussed in the light of the experimental data. The results underline the significant grain refinement by dispersoids in high Mn alloys and show a good correlation with the Smith–Zener equation, when weighting the volume fraction of the dispersoids with an exponent of 0.33. Moreover, for high Fe alloys a certain reduction in the average grain size is obtained due to pinning effects and PSN of coarse primary phases. The texture analysis focuses on characteristic texture transformations occurring with pinning effects and PSN. However, the discussion of the texture and typical PSN components is only possible in terms of trends, as all alloys exhibit an almost random distribution of orientations.

Influence of Fe and Mn on the Microstructure Formation in 5xxx Alloys-Part I: Evolution of Primary and Secondary Phases.

The increasing demands for Al sheets with superior mechanical properties and excellent formability require a profound knowledge of the microstructure and texture evolution in the course of their production. The present study gives a comprehensive overview on the primary- and secondary phase formation in AlMg(Mn) alloys with varying Fe and Mn additions, including variations in processing parameters such as solidification conditions, homogenization temperature, and degree of cold rolling. Higher Fe alloying levels increase the primary phase fraction and favor the needle-shaped morphology of the constituent phases. Increasing Mn additions alter both the shape and composition of the primary phase particles, but also promote the formation of dispersoids as secondary phases. The size, morphology, and composition of primary and secondary phases is further affected by the processing parameters. The average dispersoid size increases significantly with higher homogenization temperature and large primary particles tend to fragment during cold rolling. The microstructures of the final soft annealed states reflect the important effects of the primary and secondary phase particles on their evolution. The results presented in this paper regarding the relevant secondary phases provide the basis for an in-depth discussion of the mechanisms underlying the microstructure formation, such as Zener pinning, particle stimulated nucleation, and texture evolution, which is presented in Part II of this study.

Molecular Dynamics Simulation of Zener Pinning by Differently Shaped and Oriented Particles

Zener pinning between a curved Cu grain boundary (GB) and a differently shaped and oriented Ag particle has been simulated via molecular dynamics. The computed magnitudes of the maximum pinning force agreed with theoretical predictions only when the force was small. As the force increased, discrepancy became obvious. Through careful inspection of the structures of the Cu–Ag interfaces, detailed interaction processes, and variation of the Cu GB during the interaction, the discrepancy is found to correlate with GB faceting, which very likely reduces the maximum pinning force and facilitates boundary passage. GB anisotropy and/or interface characteristics are also found to slightly contribute to the discrepancy. These findings suggest that the assumption of an isotropic GB with constant energy utilized in previous theoretical studies for deriving the maximum pinning force might be inappropriate and that an accurate maximum pinning force could not be predicted without knowing the effects of GB evolution together with detailed properties of both GBs and interfaces.

Precipitation Criterion for Inhibiting Austenite Grain Coarsening during Carburization of Al-Containing 20Cr Gear Steels

AlN precipitates are frequently adopted to pin the austenite grain boundaries for the high-temperature carburization of special gear steels. For these steels, the grain coarsening criterion in the carburizing process is required when encountering the composition optimization for the crack-sensitive steels. In this work, the quantitative influence of the Al and N content on the grain size after carburization is studied through pseudocarburizing experiments based on 20Cr steel. According to the grain structure feature and the kinetic theory, the abnormal grain growth is demonstrated as the mode of austenite grain coarsening in carburization. The AlN precipitate, which provides the dominant pinning force, is ripened in this process and the particle size can be estimated by the Lifshitz−Slyosov−Wagner theory. Both the mass fraction and the pinning strength of AlN precipitate show significant influence on the grain growth behavior with the critical values indicating the grain coarsening. These criteria correspond to the conditions of abnormal grain growth when bearing the Zener pinning, which has been analyzed by the multiple phase-field simulation. Accordingly, the models to predict the austenite grain coarsening in carburization were constructed. The prediction is validated by the additional experiments, resulting in accuracies of 92% and 75% for the two models, respectively. Finally, one of the models is applied to optimize the Al and N contents of commercial steel.

A level set approach to simulate grain growth with an evolving population of second phase particles

In numerous polycrystalline materials, grain size is controlled by second phase particles (SPPs) that hinder the grain boundaries (GBs) by pinning mechanisms. The Smith–Zener pinning (SZP) model describes the physical interaction between SPPs and GBs. Both of them can evolve when applying a heat treatment to the material. As industrial forging processes involve hot deformation steps near the solvus temperature, it is thus of prime importance to characterize the evolution of the SPPs due to their impact on the final microstructure, notably on the grain size. The level set (LS) method is classically used to describe the influence of SPPs on grain growth (GG) by considering the simulated particles as inert and represented by static holes in the used finite element (FE) mesh. A new formalism to model GG mechanism under the influence of the SZP phenomenon, able to take into account evolving particles is proposed. It involves the representation of SPPs by a LS function and a particular numerical treatment around the grain interfaces encountering SPP, making possible the modelling of SPPs evolution without altering the undergoing pinning pressure. Validation and comparison of the new method regarding previous FE-LS formulation in 2D and 3D simulations and an application on GG under the influence of dissolving particles are described.

Microstructural evolution and strengthening mechanisms in Al7075/ graphene nano-plates/ carbon nano-tubes composite processed through accumulative roll bonding

Abstract In the present research, Al7075 matrix composites containing carbon nano-tubes (CNTs) and graphene nano-plates (GNPs) processed through accumulative roll bonding (ARB). Accumulative roll bonding carried out up to 3 passes at 400 °C with 50% thickness reduction per pass. The effect of CNTs and GNPs on the microstructural evolution and strengthening mechanisms were investigated. Microstructural evolutions in processed composites were characterized by using field emission scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD). The grain orientation spread (GOS) analysis shows that the volume fraction of recrystallization in deformed grains depends on the level of particle stimulated nucleation (PSN) in the interface between matrix and reinforcement. The occurrence of PSN is affected by the type of reinforcement and the number of ARB passes. The level of PSN depends directly on the extent of the plastic deformation zone (PDZ) in the vicinity of the matrix/reinforcements. In the hybrid composite, the PSN mechanism was restrained due to Zener pinning of grain boundaries. Texture evolution is mainly carried out through β fiber for the composites subjected to the ARB process. In all ARBed samples, the main texture components include one or two of S, Brass, and Copper. The presence of CNTs and GNPs in the composite led to a change in both intensity and type of the main texture components formed during the ARB. In the composite containing GNPs, with increasing of ARB passes the volume fraction of PSN decreased remarkably due to strain hardening and consequently recrystallization was limited. It was found that the most significant strengthening mechanism which has the key role in the increase yield strength is the load transfer mechanism. The role of coefficient of thermal expansion mismatch and Orowan mechanisms in the increase yield strength and hardness of processed composites is less and the effect of the Hall–Petch mechanism is negligible.

Manipulation of solute partitioning mechanisms for nanocrystalline stability

We describe an approach that enables the activation of two distinct nanocrystalline stability mechanisms in the same Ni(P) alloy by controlling the annealing temperature and chemical composition. Using a two-step heating process, it is shown that a low temperature first stage anneal (350 °C/1 h) drives the solute in a Ni-1at%P alloy to decorate the grain boundaries (GBs) in achieving thermodynamic stabilization at higher temperatures (550 °C). Atom probe tomography reveals a very low contamination level of species (< 0.2 at.%) that concretely demonstrates this stability mechanism. If the annealing avoids this initial, low temperature step and directly reaches the higher temperatures (550 °C), the solute precipitates a stable Ni3P phase pinning the GBs. Until this secondary phase forms, the grains coarsen significantly. On changing the alloy chemistry to Ni-4at.%P, in either the two-step (350 °C/1 h then 550 °C) or single-step (directly to 550 °C) anneal, the solute precipitates the Ni3P phase and engages the Zener pinning mechanism. Comparing the two stabilization mechanisms, thermodynamic versus Zener pinning, the thermodynamic mechanism yields a more refined, stable grain size with annealing temperature even though the pinning strength from a uniform distribution of solute is found to be lower than that of the Zener mechanism. These results are discussed using complementary atomistic simulations.

Thermal conductivity of porous polycrystalline PbTe

PbTe is a leading thermoelectric material at intermediate temperatures, largely thanks to its low lattice thermal conductivity. However, its efficiency is too low to compete with other forms of power generation. This efficiency can be effectively enhanced by designing nanostructures capable of scattering phonons over a wide range of length scales to reduce the lattice thermal conductivity. The presence of grain boundaries can reduce the thermal conductivity to $\sim 0.5$ Wm$^{-1}$K$^{-1}$ for small vacancy concentrations and grain sizes. However, grains anneal at finite temperature, and equilibrium and metastable grain size distributions determine the extent of the reduction in thermal conductivity. In the present work, we propose a phase-field model informed by molecular dynamics simulations to study the annealing process in PbTe and how it is affected by the presence of grain boundaries and voids. We find that the thermal conductivity of PbTe is reduced by up to 35\% in the porous material at low temperatures. We observe that a phase transition at a finite density of voids governs the kinetics of impeding grain growth by Zener pinning.