Atomic Shuffling Research Articles

Formation of I1 stacking fault by deformation defect evolution from grain boundaries in Mg

I 1 stacking faults (SFs) in Mg alloys are regarded as the nucleation sites of 〈 c+a 〉 dislocations that are critical for these alloys to achieve high ductility. Previously it was proposed that the formation of I 1 SFs requires the accumulations of a large number of vacancies, which are difficult to achieve at low temperatures. In this study, molecular dynamics (MD) and molecular statics (MS) simulations based on empirical interatomic potentials were applied to investigate the deformation defect evolutions from the symmetric tilt grain boundaries (GBs) in Mg and Mg-Y alloys under external loading along 〈 c 〉 -axis. The results show the planar faults (PFs) on Pyramidal I planes first appear due to the nucleation and glide of 〈 1 2 c + p 〉 partial dislocations from GBs, where 〈 p 〉 = 1 3 〈 10 1 ¯ 0 〉 . These partial dislocations with pyramidal PFs interact with other defects, including pyramidal PFs themselves, GBs, and 〈 p 〉 partial dislocations, generating a large amount of I 1 SFs. Detailed analyses show the nucleation and growth of I 1 SFs are achieved by atomic shuffle events and deformation defect reactions without the requirements of vacancy diffusion. Our simulations also suggest the Y clusters at GBs can reduce the critical stress for the formation of pyramidal PFs and I 1 SFs, which provide a possible reason for the experimental observations that Y promotes the 〈 c+a 〉 dislocation activities.

In situ observation of atomic-scale processes accomplishing grain rotation at mixed grain boundaries

A detailed monitoring of atomic-scale processes accomplishing grain rotation is important for understanding the deformation mechanisms in nanocrystalline metals. However, direct observations have been rare thus far, such that our knowledge about grain rotation has to rely heavily on hypothetical models and simulations. Here, we present in situ atomic-resolution evidence, indicating that the atomic processes accomplishing grain rotation in nanocrystalline Pt depend on the type of grain boundary (GB) separating the rotating grains. The grains around general (mixed) GBs rotate via the Frank–Bilby dislocation activities together with atomic shuffling and disconnection activities, the former being the generation, climb, glide and reaction of GB dislocations, whereas the latter leading to GB migration accompanying the grain rotation. While for the grains with a tilt GB in between: their rotation is accomplished almost entirely via the Frank–Bilby dislocation activities. We also discover that the GB dislocation climb, glide, and reaction often involve the formation and destruction of Lomer-like dislocations.

Nucleation and growth of [formula omitted] compression twin in Mg single crystals

The {101¯1} compression twin is more commonly observed under a-axis tension rather than c-axis compression in hexagonal close-packed (hcp) Mg single crystals both in experiments and atomistic simulations. In this work, molecular dynamics simulations are applied to investigate the nucleation and growth of {101¯1} twin in Mg single crystal, which is crucial for a comprehensive understanding of the plasticity of hcp metals. A {101¯1} twin embryo with 4-layer height is found to be naturally nucleated at the free surface via 〈a〉 dislocation slip in Mg single crystal. The nucleation of the {101¯1} twin embryo involves the cross-slip of the nucleated prismatic 〈a〉 dislocation onto pyramidal {101¯1} plane, which is achieved by slip of 〈a〉 dislocation and shear along the 〈101¯2〉 direction. Basal stacking faults (SFs) forms in twin embryo and grows with the migration of {101¯1} twin boundary. The formation of basal SFs in {101¯1} twin embryo can greatly reduce the required atomic shuffling in 〈12¯10〉 direction, thus facilitate the development of a {101¯1} twin embryo. The basal SFs inside {101¯1} twin are anomalous SFs without dislocation activities involved, thus the {101¯1} twinning shear is not influenced by the existence of basal SFs, which is consistent with the irrational shear of 2-layer twinning dislocation.

Bcc → hcp phase transition significantly enhancing the wear resistance of metastable refractory high-entropy alloy

In this paper, cold rolling (55% reduction in thickness) plus different annealing temperatures were performed on the as-cast TiZrHfTa0.5 metastable refractory high-entropy alloy. The most bcc → hcp phase transition was found in the cold-rolled plus 870 °C-annealed specimens (average grain size of ∼ 30 μm), which exhibit the lowest coefficient of frictions (0.12–0.15) and wear rates ((4.08–9.68) × 10−5 mm3/N m) under the dry-sliding loads of 16 N to 64 N at room temperature, relative to the as-cast and cold-rolled specimens with lower annealing temperatures. Atomic-scale observations revealed that composition-segregated bcc → hcp phase transition is further activated in the self-organized gradient worn subsurface, where the dual-phase structure with increased hcp phase fraction continues accommodating the repeated sliding-caused plasticity. Accordingly, two kinds of atomic movement mechanisms of bcc → hcp phase transition were dissected to be mainly executed by the cooperation of atom shuffling or/and partial dislocation dipoles gliding.

Polytypic omega/omega-like transformation in a refractory high-entropy alloy

Understanding the atomistic details on microstructures and phase transformation mechanisms is essential to tailor the performances of materials. Refractory body-centered cubic (bcc) high-entropy alloys (HEAs) are considered as promising candidates for future applications in the high-temperature, superelasticity, and superconductivity fields. The classical omega (ω) transformation occurs in such systems is hardly expected due to the high configurational entropy and sluggish diffusion. Here, we confirmed unusual ω and ω-like transformations occurred in a configurational entropy stabilized refractory bcc TiZrNbTa model HEA facilitating the formation of colony hierarchical microstructures. These chemically ordered ω and polytyped ω-like superstructures nucleate from one of two phase-separated bcc products. Self-adapted atomic shuffling can convert the simple bcc lattice to the non-close-packed hexagonal ω superstructure, and a certain strain induced collective displacement of the (0001) atomic-layers along one of the <211¯0>ω directions can transition the traditional orthogonal ω to the non-orthogonal polytyped ω-like superstructures. Two ordered ω and/or ω-like superstructures with Ti, Zr, Nb, and Ta atoms occupied specific sites have thus been confirmed. Moreover, the chemistry, orientations, and interface relationships of the ω and polytyped ω-like superstructures were fully revealed by using aberration-corrected scanning transmission electron microscopy combined with first-principles calculations. These atomistic insights into the ω and polytyped ω-like superstructures transformations would provide theoretical guidance for the design of advanced refractory HEAs, especially for designing alloys with specific properties based on the control of transformed microstructures and interfacial modification.

An alternative formation mechanism of {332}BCC twinning in metastable body-centered-cubic high entropy alloy

In this work, the origin of {332}BCC twinning was investigated in a body-centered-cubic (BCC) high entropy alloy (TiZrHf)87Ta13 (at.%). Detailed transmission electron microscopy analysis of the deformed microstructure revealed that a special ‘‘{351}’’<2¯11>α" type II twinning generates in the early deformation stage, and with further straining, the ‘‘{351}’’<2¯11>α" type II twin variant accommodates into a {351}α" type I twin variant. The resultant {351}α" type I twinning exhibits good lattice correspondence with the {332}BCC twinning and further propagates into the retained BCC grain with orientation relationship of {351}α”//(332)BCC, structuring into a {332}BCC twin-like configuration. Based on the experimental results and twinning analysis through evaluating twinning shear and atomic shuffle mechanism of this specific feature, our findings provide an alternative mechanism for the formation of {332}BCC twinning in which a special martensitic type II to type I twinning transformation mechanism took effect.

Spinodal decomposition coupled with a continuous crystal ordering in a titanium alloy

Spinodal decomposition mechanism is well-known for producing nanoscale modulated microstructure which is either sponge-like or plate-like. Our recent investigations of a Ti-Nb based titanium alloy have found two kinds of decomposition-induced transitions triggered simultaneously, a microstructural transition from the spongy-like to the plate-like and a crystal structure transition from bcc to hcp via the coupled atomic shear and shuffle mechanism along the Burgers pathway. Here focuses on thermal evolution kinetics of both transitions and their quantitative relations. Small angle neutron scattering profiles reveal that the aging treatments lead to a gradual change from an oval pattern to a butterfly pattern due to the spongy-to-plate microstructural transition. To characterize the crystal transition and its induced habit plane change, we define an ordering parameter from the shear component of the bcc-hcp transition. The results reveal that both follow the well-known power-law approximation with an activation energy of ∼209 kJ/mol, which is identical with diffusion energy of Nb in binary Ti-Nb alloy. However, their time exponential factors are different, about 1/5 for the microstructure growth and 1/16 for the crystal ordering and its induced habit plane change. The former is less than the common 1/3 law of the decomposition mechanism and the nucleation and growth mechanism. Aided by these kinetic equations, the microstructural and chemical parameters can be modeled by the crystal ordering. This provides a new strategy to tune the nanoscale microstructure by this novel spinodal decomposition coupled with a continuous crystal ordering.

Asymmetric [formula omitted] twin boundary and migration mechanism in hexagonal close-packed titanium

Classical twinning theory predicted that a simple shear was able to create a perfect, twinned structure without the need of atomic shuffles for (112¯1)[112¯6¯] twinning mode in hexagonal close-packed (HCP) metals, and the elementary twinning dislocation should be a two-layer zonal. However, it was revealed in the literature that this particular twinning mode could not have mirror symmetry between the parent and twin, and shuffles should be involved during twin boundary (TB) migration. These conflicting reports indicate that the (112¯1)[112¯6¯] twinning mechanism has not been completely resolved, and what configuration of twinning dislocation mediates twin growth is not well understood either. In this work, atomistic simulation is conducted to further understand the twinning mechanism in titanium. Novel structural analyses are performed to reveal how the parent lattice is transformed to the twin lattice. The results show that the mirror symmetry of a perfect, unrelaxed TB breaks down after relaxation, because of the very high repulsive force between the atom pairs on mirrored positions with a spacing less than half of the lattice constant. As a result, the stacking sequence of the twin differs from that of the parent after relaxation. During twin growth, each {11¯00} prismatic plane of the parent splits into two layers and reorganize into the prismatic planes of the twin, such that the twin prismatic are no longer aligned with the parent prismatic and the mirror symmetry breaks down. This process is accomplished by atoms shuffling in the opposite direction that is perpendicular to the twinning shear. The atomic shuffles spread over multiple consecutive twinning planes to minimize the shuffling magnitude on each twinning plane. Thus, the actual TB is no longer a single twinning plane, but a multi-layer interface zone. Careful examination shows that no well-defined twinning dislocation core can be identified, although single-layer steps can still be observed.

Passive Oxide Film Growth Observed On the Atomic Scale

AbstractDespite the ubiquitous presence of passivation on most metal surfaces, the microscopic‐level picture of how surface passivation occurs has been hitherto unclear. Using the canonical example of the surface passivation of aluminum, here in situ atomistic transmission electron microscopy observations and computational modeling are employed to disentangle entangled microscopic processes and identify the atomic processes leading to the surface passivation. Based on atomic‐scale observations of the layer‐by‐layer expansion of the metal lattice and its subsequent transformation into the amorphous oxide, it is shown that the surface passivation occurs via a two‐stage oxidation process, in which the first stage is dominated by intralayer atomic shuffling whereas the second stage is governed by interlayer atomic disordering upon the progressive oxygen uptake. The first stage can be bypassed by increasing surface defects to promote the interlayer atomic migration that results in direct amorphization of multiple atomic layers of the metal lattice. The identified two‐stage reaction mechanism and the effect of surface defects in promoting interlayer atomic shuffling can find broader applicability in utilizing surface defects to tune the mass transport and passivation kinetics, as well as the composition, structure, and transport properties of the passivation films.

Direct observation of zonal dislocation in complex materials by atomic-resolution scanning transmission electron microscopy

Dislocation glide to carry plastic deformation in simple metals and alloys is a well-understood process, but the process in materials with complex crystal structures is not yet understood completely as it can be very complicated often involving multiple atomic planes during dislocation glide. The zonal dislocation is one of the examples predicted to operate in complex materials, and during glide it involves multiple atomic planes called shear zone, in which non-uniform atom shuffling occurs. We report direct observation made by Z-contrast atomic-resolution microscopy of the zonal dislocation in the σ phase FeCr. The result confirms the zonal dislocation indeed operates in this material. Knowledge gained on the dislocation core structure will lead to improved understanding of deformation mechanisms in this and other complex crystal structures and provide ways to improve the brittleness of these complex materials.

Omega phase transformation – morphologies and mechanisms

Abstract Various morphologies of the ω phase are observed in the α and the β phases of group-IV elements and their alloys where formation of this phase is induced by various means, namely, quenching from the β-phase field, isothermal annealing, under high static as well as dynamic pressure, and irradiation. The presence of various morphologies, such as ellipsoidal, cuboidal, granular or plate shapes, under different conditions is demonstrated. It is shown that various factors, including shuffling of atoms, shearing of atomic planes, long-range diffusion of solute atoms, change in volume during transformation, etc., influence the formation of different morphologies of the ω phase. The underlying mechanisms in each case are discussed. Ordered ω phases, which exist as equilibrium phases in various alloy systems, are shown as a case of superimposition of the concentration wave on the displacement wave.

Formation mechanism of nano-sized η and ω structures in β phase in ECP treated Cu-40Zn alloy

Two nano-scaled hexagonal distortions in high temperature BCC phase during cooling is commonly observed in many alloys systems. Although efforts have been made on studying the lattice softening of the BCC structure during cooling, the distortion mechanisms are less addressed. Thus, the formation of two hexagonal structures in the β precipitates in a Cu-40Zn alloy treated by ECP was thoroughly investigated. Results show that the β precipitates contain two kinds of nano-sized and diffuse atomic clusters one with a η structure obeying the Burgers OR and the other a ω structure obeying the Blackburn OR. They were each formed through a two-stepped atomic displacement. For the η structure, the first step is the atomic shuffle of each second 110β plane in the <11¯0>β direction and the second is a structure change mainly by a {1 1¯ 2} < 1¯11>β shear. For the ω structure, the first is an atomic shuffle on each second and third {112¯}β plane in the ± [111]β directions and then normal strains in three mutually perpendicular directions. The concomitant formation of the two structures minimizes the lattice distortion of single formation. The present results provide new information on the formation mechanism of the two hexagonal structures in a BCC matrix.

Twin evolution in cast Mg-Gd-Y alloys and its dependence on aging heat treatment

Effects of rare-earth (RE) and precipitates on twin evolution in cast Mg-10Gd-3Y-0.5Zr (wt.%) (GW103) alloys of solid solution (T4) and aged (T6) states are investigated performing quasi-static room temperature compression tests and microstructural characterization. It is found that both {10–12} and {11–21} extension twins (ET1 and ET2) can appear in the T4 and T6 states but with different emergence sequences. As the aging heat treatment leads to consumption of RE solutes which could inhibit atomic shuffling required for nucleation of ET1 but not ET2, ET2 occurs prior to ET1 in the T4 state, and ET1 emerges before ET2 in the T6 state. The extension twins here mainly coordinate the plastic deformation through the non-Schmid effect. Our results shed light on the influence of RE elements on twin evolution in magnesium alloys and have implications in developing high-performance Mg-RE alloys.

Twin boundary migration mechanisms in quasi-statically compressed and plate-impacted Mg single crystals.

Twinning is a prominent deformation mode that accommodates plasticity in many materials. This study elucidates the role of deformation rate on the atomic-scale mechanisms that govern twin boundary migration. Examination of Mg single crystals deformed under quasi-static compression was compared with crystals deformed via plate impact. Evidence of two mechanisms was uncovered. Atomic-level observations using high-resolution transmission electron microscopy revealed that twin boundaries in the <a>-axis quasi-statically compressed single crystals are relatively smooth. At these modest stresses and rates, the twin boundaries were found to migrate predominantly via shear (i.e., disconnection nucleation and propagation). By contrast, in the plate-impacted crystals, which are subjected to higher stresses and rates, twin boundary migration was facilitated by local atomic shuffling and rearrangement, resulting in rumpled twin boundaries. This rate dependency also leads to marked variations in twin variant, size, and number density in Mg. Analogous effects are anticipated in other hexagonal closed-packed crystals.

Effects of Al on crack propagation in titanium alloys and the governing toughening mechanism

In this work, molecular dynamics simulation is performed to investigate the effect of Al element on the crack propagation along the {101‾1} twin boundary in Ti alloys with different contents and distribution of Al element during the uniaxial tensile deformation. The results show that increasing Al content obviously promotes the percentage of face-centered cubic (FCC) structure during the deformation process, which hinders the micro-crack propagation and enhances the plasticity and toughness of the Ti–Al alloys. The FCC bands transform from the hexagonal close-packed (HCP) matrix satisfying the basal-type (B-type) orientation relationship (OR) represented by (0001)HCP||(111)FCC and [12‾10]HCP||[11‾0]FCC. The increase of Al element reduces the basal stacking fault (BSF) energy of the HCP structure and thus contributes to the formation of BSFs, which grow into FCC bands with the B-type orientation by the slip of Shockley partial dislocations. The {101‾1} pyramidal SF activated at the crack tip evolves into FCC grains in the Ti–Al alloys following the prismatic-type (P-type) OR with the HCP matrix represented by (101‾0)HCP||(110)FCC and [12‾10]HCP||[11‾0]FCC, during which atomic shear and shuffle is involved.

Two-Dimensional Frank−Kasper Z Phase with One Unit-Cell Thickness

Z phase is one of the three basic units by which the Frank-Kasper (F-K) phases are generally assembled. Compared to the other two basic units, that is, A15 and C15 structures, the Z structure is rarely experimentally observed because of a relatively large volume ratio among the constituents to inhibit its formation. Moreover, the discovered Z structures are generally the three-dimensional ordered Gibbs bulk phases to conform to their thermodynamic stability. Here, we confirmed the existence of a metastable two-dimensional F-K Z phase that has only one unit-cell height in the crystallography in a model Mg-Sm-Zn system, using atomic-scale scanning transmission electron microscopy combined with the first-principles calculations. Self-adapted atomic shuffling can convert the simple hexagonal close-packed structure to the topologically close-packed F-K Z phase. This finding provides new insight into understanding the formation mechanism and clustering behavior of the F-K phases and even quasicrystals in general condensed matters.

Watching grain boundaries

Ceramics Grain boundaries play an important role in determining the properties of ceramics and other materials. However, tracking the migration of grain boundaries is difficult. Wei et al. used atomic-resolution scanning transmission electron microscopy to trigger and probe grain boundary migration in alumina. The authors found that cooperative shuffling of atoms at the grain boundary ledge is how migration proceeds, which leads to structural transitions. This new method for determining the fine details of these sorts of processes will help us better understand how microstructures develop in a range of materials. Nat. Mater. 20 , 951 (2021).

A novel atomic movement mechanism of intersection-induced bct-α → bcc-α′ martensitic phase transformation

The atomic movements for completing the fcc-γ → hcp-ε → bct-α Plastic Deformation-Induced Martensitic Transformation (PDIMT) has been revealed to be confined purely on one specific {111}γ plane. However, so far nothing has been known for the possible intersection of two fcc-γ → hcp-ε → bct-α PDIMTs pre-developed from two individual {111}γ slip systems. For the first time, here we capture the nucleation of bcc-α′ in the intersection of two bct-α phases in 304 stainless steel, following a novel polymorphic fcc-γ → hcp-ε → bct-α → bcc-α′ PDIMT. High-resolution transmission electron microscopy observations were mainly performed to unveil the underlying atomic process in the final-step bct-α → bcc-α′ transition, which is executed by the cooperation of partial dislocation dipoles gliding on every second {110}α planes of two bct-α phases (equivalent to {110}α′ and {011}α′ planes), and atom shuffling along the [11¯0]α1/[1¯12]α′ and [11¯0]α2/[211¯]α′ directions.

Atomic-scale mechanism of rhombohedral twinning in sapphire

Deformation twinning is a fundamental plastic deformation mode in crystals. Upon twinning, individual atoms must move in different directions to satisfy the twin symmetry, which is called atomic shuffling. However, actual atomic motions during shuffling are still unknown, especially for ionic compounds. Here, we report the dynamic twinning behavior dominated by the atomic shuffling in sapphire (α-Al2O3). The propagation and annihilation of twins are revealed to be mediated by migration of step structures on the matrix/twin interfaces. The step migration is driven by cooperative motions of a group of five atoms with relatively few recombination of Al–O bonds. Our findings imply that the atomic shuffling associated with twinning is determined by a collective property of a group of several atoms.

Lattice transformation in grain boundary migration via shear coupling and transition to sliding in face-centered-cubic copper

Migration of symmetric tilt grain boundaries (GBs) via shear coupling has been studied extensively in experiments and simulations. It was reported that shear coupling transitioned to GB sliding at high temperatures, but how such transition occurs at low temperatures has not been investigated. Lattice transformation on the atomic scale during shear coupling has not been fully understood. In this work, mode of motion of symmetric tilt GBs with [001] tilting axis in face centered cubic copper under a shear strain parallel to the boundary plane at 100 K was carefully characterized by tracking the positions of the corresponding planes in atomistic simulations and new features of GB motion were observed. The results show that the angles between the two low-index planes, (110) and (100), and the boundary plane can be used to define a nominal magnitude of shear s. Approximately, if one of these two planes has a value of s < 0.5, shear coupling occurs with this plane being the active invariant plane; if 0.5 < s < 0.6, GB moves by shear coupling + sliding, i.e. a hybrid mode by which shear coupling transitions to sliding; if both planes have a value of s> 0.6, only GB sliding occurs. Careful structural analyses show that, for all the GBs that undergo shear coupling, some GB atomic planes remain invariant, very similar to the first invariant plane in deformation twinning, whereas the other GB atomic planes swap their positions in the GB normal direction through highly coordinated and complex atomic shuffles. This behavior allows identification of transformation units that are reoriented toward the neighboring grain. Rate-limiting factors are identified for lattice transformation and can be used to infer a kinetics model for shear coupling.