Incoherent Interface Research Articles

Continuous and Discontinuous Yielding Behaviors in Ferrite-Cementite Steels

The continuous and discontinuous yielding behaviors in ferrite-cementite steels were complementarily investigated via nano- and macro-scale deformation examinations. The continuous yielding behavior was observed in heat-treated specimens with a lamellar or cementite-spheroidal structure even after strain-aging treatment. However, the discontinuous yielding behavior accompanied by the Luders elongation appeared in ferrite-cementite steel that was recrystallized after cold rolling. The results obtained by electron microscopy, synchrotron X-ray, and neutron diffractions indicate that the ferrite-cementite interface of the heat-treated specimen is semi-coherent with a high internal stress field, whereas that of the recrystallized one is incoherent with a low internal stress field. Moreover, coherency strain, which depends on the total area of the ferrite-cementite interface, and thermal strain, which is governed by temperature, are the two factors that influence peak broadening. The nanoindentation tests revealed that the critical loads are significantly lower near the semi-coherent interface than those near the incoherent interface and the ferrite grain boundary; this suggests that dislocations are easily emitted from the semi-coherent interface.

The Effect of Strain Rate and Temperature on the Mechanical Behavior of Al/Fe Interface Under Compressive Loading

Molecular dynamics (MD) is employed to simulate the mechanical response of Al/Fe interface under compression at extreme conditions of seven temperatures and four strain rates ranging between 150 K and 900 K and 5.0 × 107 s−1 and 1.0 × 1010 s−1, respectively. Stress–strain histories show two distinct yield stress points for simulations at temperatures below 500 K, which tend to merge into one as the temperature increases. Microstructural simulations show nucleation of dislocations, which occur in the bulk of the aluminum region, is associated with the first yield point. In the iron region, dislocations nucleate at the Al/Fe interface and are associated with the second yield point. The incoherent interface employed in these simulations contributes to the heterogeneous nucleation in iron by creating a defected area favorable for this nucleation from the aluminum side. MD generated data show that the two yield stresses and the consequent flow stress decrease with increasing temperature for all strain rates and fit a thermally activated model function of strain rate. The competing mechanisms between dislocation motion and phonon drag driven deformation are also simulated and modeled. The flow stress of the interface was found to fall midway between Zerilli–Armstrong models of the two materials constituting it, whereas the relaxation in Fe followed similar trend to what is reported in the literature.

The role of incoherent interface in evading strength-ductility trade-off dilemma of Ti2AlN/TiAl composite: A combined in-situ TEM and atomistic simulations

The strength-ductility trade-off dilemma has inhibited the applications of many structural materials, TiAl alloys in particular. Here we report a new insight into the potential for increasing the ductility of Ti2AlN/TiAl composite without lowering its strength by tuning the interface with a unique incoherent atomic structure. The in-situ TEM nanoindentation tests indicate that the Ti2AlN(101‾3)//TiAl(111) incoherent interface micro-region possesses high compressive strength and good compressive ductility, because this incoherent interface can simultaneously play the role of softening and hardening in the process of compression due to the interface-dominated nucleation and annihilation of dislocations. The first-principles calculation and MD simulation results reveal that the reason why this incoherent interface displays a distinct compressive deformation behavior is that it has unique atomic structure, bonding character and interface-dislocation interactive mechanisms. By using first-principles calculations, it is found that this incoherent interface possesses a hierarchical atomic structure in the direction normal to the interface, the interface bonding characteristics are multiple and inhomogeneous depending on local atomic configurations, forming both the strong and weak interface interactive regions. The MD simulations indicate that the weakest interface interactive regions, i.e. incoherent regions in the Al2 atomic arrays of Ti2AlN(101‾3) plane at the interface, could provide the preferred nucleation source for primary dislocations, incepting the plastic deformation. After the deformation achieves a certain extent, the local disordered interface regions generated by internal stress shearing could act as sinks to annihilating the secondary dislocations propagated from Al1 atomic layer of Ti2AlN(0001) plane, resulting in strain hardening.

A New Elastic Theory of Nanocomposites With Incoherent Interface Effect Based on Interface Energy Density

Abstract A continuum theory of elasticity based on the concept of interface free energy density is proposed to account for the effect of incoherent interfaces in nano-phase reinforced composites. With the help of the lattice model, the corresponding interface energy density is formulated in terms of the surface free energy densities of two bulk materials forming interfaces, the lattice relaxation parameters due to the spontaneous surface relaxation and lattice misfit parameters yielded by interface incoherency, while the stress jump at interfaces is formulated with an interface-induced traction as a function of interface free energy density. Compared with existing theories, the interface elastic constants difficult to determine are no longer introduced, and all the parameters involved in the present theory have definite physical meanings and can be easily determined. The coupling effects of characteristic size and interface structure in nanoparticle-reinforced composites are further analyzed with the present theory. It is found that both the decrease of nanoparticle size and the increase of interface incoherence will lead to the decrease of interface fracture toughness and increase of effective bulk and shear moduli of nanocomposites. All these results predicted by the present theory are consistent well with those obtained by previous experiments and computations, which further indicate that the present theory can effectively predict the mechanical properties of nanomaterials with complex interfaces, such as nano-phase reinforced composites and nano-scale metal multilayer composites.

Fatigue Life of 7475-T7351 Aluminum After Local Severe Plastic Deformation Caused by Machining.

The fatigue properties of thermo-mechanically treated and machined aluminum alloy 7475-T7351 have been studied. The applied advanced machining strategy induced intensive plastic deformation on the machined surface under defined cutting conditions. Therefore, a detailed study of 3D surface topography was performed. Advanced characterization of the material structure and electron back scattered diffraction mapping of selected chemical phases were performed, as well as energy dispersive X-ray analysis of the surface. Advanced mechanical properties of the material were investigated in situ with a scanning electron microscope that was equipped with a unique tensile fixture. The fatigue results confirmed an evident dispersion of the data, but the mechanism of crack nucleation was established. Fracture surface analysis showed that the cracks nucleated at the brittle secondary particles dispersed in the material matrix. The surface topography of samples that had been machined in wide range of cutting/deformation conditions by milling has not proved to be a decisive factor in terms of the fatigue behavior. The incoherent interface and decohesion between the alumina matrix and the brittle secondary phases proved to significantly affect the ultimate strength of the material. Tool engagement also affected the fatigue resistance of the material.

Understanding the strength softening of Zr/Mo and Zr/Ti nanostructured multilayers

Zr/Mo and Zr/Ti nanostructured multilayers manifest a peak hardness at the critical layer thickness of ~10 and ~20 nm, respectively. The non-monotonical thickness-dependent hardness is understood in terms of the back stress, which is closely correlated with the dislocation absorption probability by the interfaces. It appears that strength softening occurs as gliding dislocations can be absorbed by interfaces at smaller layer thickness. Furthermore, incoherent interface in the Zr/Mo with discontinuous slip systems is more effective to block dislocation motion than the Zr/Ti with continuous ones, resulting in a higher dislocation storage ability.

Influence of interface with mismatch dislocations on mechanical properties of Ti/Al nanolaminate

As a representative boundary, interphase-interface may affect the strength or ductility of multilayered composites dramatically. However, the effect of the interface with mismatch dislocations on the mechanical behavior of multilayered composites is still not clear. In the present work, we performed molecular dynamics simulations to investigate the effect of interface structures and layer spacing on the mechanical properties of the Ti/Al nanolaminate. The results indicate that there are two transitions of the plastic deformation mechanism in the Ti layer with the increase of layer spacing in the sample with a coherent interface. The plastic deformation mechanism evolves from one that is dominated by dislocation to the phase transformation from the hcp-Ti to the fcc-Ti mode, which transfers to the dislocation slip deformation again. For the samples with an incoherent interface, the plastic deformation is dominated by the transformation from hcp-Ti to fcc-Ti, regardless of the variation of layer spacing, while the plastic deformations in the Al layers are mainly dislocations confined in the layer slip in the samples with both coherent and incoherent interfaces. When the layer spacing is larger than 6.6 nm, an obvious second hardening is observed due to the superior dislocation storage ability of the Ti/Al laminate with the incoherent interface. Meanwhile, extraordinary ductility is obtained when optimal layer spacing is employed in the Ti/Al laminate. Moreover, the phase transformation mechanism of hcp-Ti to bcc-Ti has also been explicated in the present work. The general conclusions derived from this work may provide a guideline for the design of high-performance Ti/Al multilayer and alloy devices.

Effect of I(Mg3YZn6)-, W(Mg3Y2Zn3)- and LPSO(Mg12ZnY)-phases on tensile work-hardening and fracture behaviors of rolled Mg–Y–Zn alloys

The effects of secondary phases (W, Mg3Y2Zn3), long-period stacking-ordered (LPSO, Mg12ZnY) and icosahedral (I, Mg3YZn6) phases on the tensile properties and work-hardening behavior of Mg–Y–Zn alloys (Mg–1.32Zn–1.79Y, ZW12; Mg–4.4Zn–2.2Y, ZW42; and Mg–5.2Zn–1.1Y, ZW51) prepared after application of differential speed rolling on their cast microstructures were studied at room temperature. The volume fractions of LPSO, W and I-phase in ZW12, ZW42 and ZW51 were 4.7, 6.3 and 4.8%, respectively. The rolled alloys exhibited a similar level of yield strength but different work hardening characteristics and tensile fracture behavior. The rolled ZW51 with I-phase particles exhibited the highest work-hardening rate and the largest tensile elongation (with ductile fracture). The ZW42 with W-phase particles exhibited a work-hardening rate slightly lower than that of the rolled ZW51, despite having the larger amount of secondary phase, and premature fracture occurred far before neck formation in ZW42, while fracture occured after neck formation in ZW51. The fractured surface showed that the coherent interface between the I-phase and Mg matrix phase is significantly more resistant to cracking than is the incoherent interface between the W-phase and Mg matrix phase. The rolled ZW12 with LPSO-phase, forming a coherent interface with the Mg matrix phase, exhibited a considerably lower work-hardening rate than the rolled ZW51, though they had the similar amounts of secondary phase. Thus, the uniform strain of the rolled ZW12 was quite small compared with that of the rolled ZW51, though fracture occurred after necking in both alloys. The same work-hardening and tensile elongation behaviors were retained after annealing treatment on the rolled alloys.

A new continuum model of the incoherent interface compared with growth of a spinel rim on an olivine grain

In a polymorphic change in which the phases differ only by a reversible difference in specific volume, kinematics requires a unit mass to suffer deviatoric strain in the instant it is transformed. Unlike the Eshelby stress–free strain, this strain is a property of the motion. Its existence must be considered when formulating the constitutive relation for the product of an incoherent transformation. To show this, two models are compared: in both, the (Nabarro) condition of vanishing shear stress is imposed at the incoherent interface; they differ only in the treatment of the deviatoric strain at issue. In the existing model, deviatoric stress within a unit mass of product is determined by total deviatoric strain from its initial state as parent phase. In the new model, lattice reconstruction is assumed to erase all memory within the unit mass of deviatoric strain suffered before, or during, its transformation. The existing model is not consistent with experiments on the olivine spinel–phase change in single crystals. It predicts that when the pressure applied exceeds a critical value, samples should transform completely at almost constant rate; instead, growth is seen to slow, and may even cease. The new model predicts this. Without adjustable constants, fair agreement is obtained with experiments on samples having 75–200 ppmw of water. Because elastic deformation by itself can explain those observations, the very thin rims seen on even drier samples suggest that water may be essential to lattice reconstruction in this phase change.

Molecular dynamics simulations of the tensile responses and fracture mechanisms of Ti2AlN/TiAl composite

In the present work, molecular dynamics simulations are carried out to investigate the tensile responses and fracture mechanisms of Ti2AlN/TiAl composite. It is found that the Ti2AlN/TiAl composite with different interface atomic structure shows significantly different tensile responses and fracture mechanisms. When the tensile loading is applied normal to the Ti2AlN(0 0 0 1)/TiAl(1 1 1) coherent interface, the micro-void would nucleate from the interface, rapidly propagate along the interface and evolve into the complete-crack, leading to the brittle fracture behavior of Ti2AlN/TiAl composite. The formation of stress concentration and Lomer-Cottrell lock in the misfit dislocations at the coherent interface is the major mechanism for the nucleation of micro-void. When the tensile loading is applied normal to the Ti2AlN(1 0 1¯ 3)/TiAl(1 1 1) incoherent interface, micro-voids can nucleate from the weak interactive regions at the incoherent interface, but they cannot propagate rapidly along the incoherent interface, which indicates that the Ti2AlN(1 0 1¯ 3)/TiAl(1 1 1) incoherent interface system shows ductile fracture behavior. The ductile fracture mechanism is twofold: firstly, the dislocations in Ti2AlN and TiAl slab can nucleate at the tip of micro-crack and effectively blunt the propagation of micro-crack along the incoherent interface; secondly, the strong NTi and NAl bonds at the incoherent interface can give a contribution to the overall stability of Ti2AlN(1 0 1¯ 3)/TiAl(1 1 1) incoherent interface.

The effect of interface atomic structure on the deformation mechanisms of Ti2AlN/TiAl composite under nanoindentation using MD simulations

In this study, we conduct molecular dynamics simulations to investigate the effect of interface atomic structure on the deformation mechanisms of Ti2AlN/TiAl composite under nanoindentation. It is found that the interface atomic structure has a remarkable effect on the deformation behavior of Ti2AlN/TiAl composite due to significantly different interface-dislocation interactive mechanisms. For the Ti2AlN(0 0 0 1)/TiAl(1 1 1) coherent interface system, although plenty of dislocations nucleate and propagate through the TiAl layer during the indentation process, there are no dislocations transmitting across the coherent interface. The formation of stair-rod dislocation and dislocation tangle is the major mechanism for blocking the slip motion of dislocations by the coherent interface. Thus, the Ti2AlN(0 0 0 1)/TiAl(1 1 1) coherent interface is beneficial for the strengthening effect of Ti2AlN/TiAl composite. For the Ti2AlN(1 0 3)/TiAl(1 1 1) incoherent interface system, the incoherent interface has some ability to annihilate the dislocations nucleated from the TiAl during the indentation process, but it cannot simultaneously annihilate plenty of dislocations. These dislocations accumulate in the incoherent interface and cause the stress concentration, providing the driving force for dislocation nucleation in Ti2AlN from the poor matching regions in the Al atomic arrays at the incoherent interface. Therefore, the incoherent interface can provide the access of dislocation transmission from TiAl to Ti2AlN, which benefits the ductility of Ti2AlN/TiAl composite.

Quantifying the role of interface atomic structure in the compressive response of Ti2AlN/TiAl composite using MD simulations

Unraveling the effects of interface atomic structures on the mechanical properties is a key step toward the elaborate design of Ti2AlN/TiAl composite with excellent performance. However, the impact of different interface atomic structures upon the mechanical properties of Ti2AlN/TiAl composite, which is extremely important from the perspective of material design, remains poorly understood and essentially unquantified so far. In this research work, molecular dynamics simulations of Ti2AlN(0001)/TiAl(111) coherent interface and $$ {\text{Ti}}_{2} {\text{AlN}}(10{\bar{\text{1}}}3)/{\text{TiAl}}\left( {111} \right) $$ incoherent interface system under parallel-to-interface compression are carried out. It is found that these two types of interface systems show different compressive deformation behaviors due to significantly different interface–dislocation interactive mechanisms. The compressive ultimate strengths of Ti2AlN(0001)/TiAl(111) coherent interface and $$ {\text{Ti}}_{2} {\text{AlN}}(10{\bar{\text{1}}}3)/{\text{TiAl}}\left( {111} \right) $$ incoherent interface systems are comparable, but the ductility of incoherent interface system is obviously higher than that of coherent interface system. This is because the incoherent interface can simultaneously serve as the source for dislocation nucleation and the barrier for dislocation motion, and thus plays a dual role of softening and hardening in the compressive deformation. Therefore, it can be expected that tuning the interface with $$ {\text{Ti}}_{2} {\text{AlN}}(10{\bar{\text{1}}}3)/{\text{TiAl}}\left( {111} \right) $$ incoherent atomic structure can contribute to increasing the compressive ductility of Ti2AlN/TiAl composite without lowering its strength.

Boosting Potassium Storage Capacity Based on Stress‐Induced Size‐Dependent Solid‐Solution Behavior

AbstractA prerequisite for successful development of a K‐ion battery anode based on solid‐solution behavior is to improve its potassium storage capacity. Increasing the solid‐solution domain by decreasing the particle size offers a promising strategy for enhancing the potassium storage capabilities of insertion anode materials. Extended solid‐solution composition range in nanostructured particles is mainly due to the existence of a coherency strain that suppresses phase separation upon intercalation. Here, the intercalation stress effect in size‐dependent solid‐solution behavior is explored by insertion of K+ into K2Ti6O13 nanowires with different diameters. K2Ti6O13 nanowires with small average diameter of ≈5.5 nm deliver a large initial reversible depotassiated capacity of ≈120 mAh g−1 (deinsertion of ≈2.5 K+) at 0.2 C. The remarkably high reversible depotassiated capacity is mainly ascribed to the decrease of the incoherent interface upon potassiation. The direct observation of enrichment of intragranular particles in potassiated K2Ti6O13 nanowires with average diameter of ≈38 nm provides evidence of strain‐accommodating misfits or dislocations in solid‐solution intercalation compounds. This work offers a promising route to utilize coherency strain energy for K‐ion batteries with improved specific capacity and alleviated irreversible capacity loss.

Effects of precipitates and its interface on thermal conductivity of Mg–12Gd alloy during aging treatment

How to simultaneously improve the thermal conductivity and mechanical properties of Mg alloys is an interesting and important issue topic. In the present study, the effect of aging treatment on the thermal conductivity and micro-hardness of Mg–12Gd alloy was investigated. Results indicated that the thermal conductivity and micro-hardness of the alloy could be significantly improved by aging treatment, and Mg–12Gd alloy exhibited a typical aging hardening response with the increasing aging time. At the early stage of aging (4 h), a large number of fine particles precipitated from α-Mg matrix, but thermal conductivity of alloy increased a little, which was attributed to the coherent interface hindering the increase of thermal conductivity. The thermal conductivity increased to the peak value of 75.7 W/(m·K) at the aging time of 300 h, which was about 2.17 times that of as-solutionized alloy. The huge increment in the thermal conductivity of as-aged Mg–12Gd alloy was related to the precipitation of particles and the incoherent interface between precipitates and α-Mg matrix.

Biological growth in bodies with incoherent interfaces

A general theory of thermodynamically consistent biomechanical–biochemical growth in a body, considering mass addition in the bulk and at an incoherent interface, is developed. The incoherency arises due to incompatibility of growth and elastic distortion tensors at the interface. The incoherent interface therefore acts as an additional source of internal stress besides allowing for rich growth kinematics. All the biochemicals in the model are essentially represented in terms of nutrient concentration fields, in the bulk and at the interface. A nutrient balance law is postulated which, combined with mechanical balances and kinetic laws, yields an initial-boundary-value problem coupling the evolution of bulk and interfacial growth, on the one hand, and the evolution of growth and nutrient concentration on the other. The problem is solved, and discussed in detail, for two distinct examples: annual ring formation during tree growth and healing of cutaneous wounds in animals.

The impact of annealing temperature on improving microstructure and toughness of electron beam welded duplex stainless steel

The influences of electron beam (EB) welding and subsequent post-weld heat treatment (PWHT) at different temperatures on the microstructure evolution and low-temperature toughness of duplex stainless steel were investigated. The results showed that the rapid cooling in EB welding led to excessive ferrite formation, abundant Cr2N precipitation, and significant dendritic segregation in the weld, ultimately resulting in deterioration of toughness. PWHT promoted austenite formation and Cr2N dissolution, and eliminated dendrite segregation, which consequently restored the toughness of the weld. The annealing temperature had a very significant influence on the type and content of austenite, as well as toughness. The toughness was not determined solely by the ferrite/austenite ratio. Spheroidal intragranular austenite particles played an important role in improving toughness due to incoherent interface boundaries with the ferrite matrix and low-energy special grain boundaries. The most favourable annealing temperature for the studied EB weld was found to be 1080 °C, at which the weld had the highest impact toughness.

Molecular dynamics simulation of effects of interface imperfections and modulation periods on Cu/Ta multilayers

Molecular dynamics (MD) simulations were performed to study the effects of the interface imperfections and modulation periods (λ) on deformation mechanisms of Cu/Ta nanoscale metallic multilayers (NMMs) under uniaxial tension. We found that the Cu(1 1 1)/Ta (1 1 0) interface can serves as the source for dislocation nucleation and the barrier impeding the motion of dislocations. The yield stress of Cu/Ta NMMs depends only on the material properties instead of the λ. With the increase of λ, more dislocations would store in the incoherent interface, resulting in larger local residual stress, and thus dislocations would be easier nucleate from the interface in Ta layer, so the maximum stress decreases with the increase of λ. There is a critical λ (about 11.18 nm), in which flow strength of the NMMs is maximized. In this stage, dislocations absorption in Ta layers and dislocations nucleation in Cu layers dominates the plastic deformation in the material, jointly.

On the role of weak interface in crack blunting process in nanoscale layered composites

Abstract Heterointerface in a nanoscale metallic layered composite could improve its crack resistance. However, the influence of metallic interface structures on crack propagation has not been well understood at atomic scale. By using the method of molecular dynamics (MD) simulation, the crack propagation behavior in Cu-Nb bilayer is compared with that in Cu-Ni bilayer. We find that the weak Cu-Nb interface plays an important role in hindering crack propagation in two ways: (i) dislocation nucleation at the interface releases stress concentration for the crack to propagate; (ii) the easily sheared weak incoherent interface blunts the crack tip. The results are helpful for understanding the interface structure dependent crack resistance of nanoscale bicrystal interfaces.

Development of Metallographic Etchants for the Microstructure Evolution of A6082-T6 BFSW Welds

BACKGROUND—The solid-phase joining of A6082-T6 plates by bobbin friction stir welding (BFSW) is problematic. Better methods are needed to evaluate the microstructural evolution of the weld. However, conventional Al reagents (e.g., Keller’s and Kroll’s) do not elucidate the microstructure satisfactorily, specifically regarding grain size and morphology within the weld region. APPROACH—We developed innovative etchants for metallographic observations for optical microscopy. RESULTS—The macrostructure and microstructure of A6082-T6 BFSW welds were clearly demonstrated by optical microscopy analysis. The microetching results demonstrated different microstructures of the Stir Zone (S.Z) distinct from the Base Metal (B.M) and Heat Affected Zone (HAZ) & Thermo-mechanical Affected Zone (TMAZ). The micrographs showed a significant decrease in grain size from 100 μm in B.M to ultrafine 4–10 μm grains for the S.Z. Also, the grain morphology changed from directional columnar in the B.M to equiaxed in the S.Z. Furthermore, thermomechanical recrystallization was observed by the morphological flow of the grain distortion in HAZ and TMAZ. The etchants also clearly show the polycrystalline structure, microflow patterns, and the incoherent interface around inclusion defects. ORIGINALITY—Chemical compositions are identified for a suite of etchant reagents for metallographic examination of the friction-stir welded A6082-T6 alloy. The reagents have made it possible to reveal microstructures not previously evident with optical microscopy.

Crystal Structure of Silicides in a Ti – 0.7 WT.% Si Nanostructured Alloy

Using warm, lengthwise grooved-roll processing of the Ti – 0.7 wt.% Si alloy, a nanostructured state is formed in it at 873 K with the grain-subgrain elements measuring about 280 nm. The titanium silicides precipitating during the dynamic strain-induced aging of the alloy are investigated by the methods of high-resolution scanning electron microscopy. It is found that the shape of the silicides is close to spherical, they have an FCClattice and an incoherent interface with the matrix.