Glide Of Shockley Partial Dislocations Research Articles

Atomic-scale investigation of the mechanisms of deformation-induced martensitic transformation at ultra-cryogenic temperatures

Liquefied natural gas storage and transportation as well as space propulsion systems have sparked interest in the martensitic transformation and behaviours of 316 L stainless steels (SS) under ultra-cryogenic deformation. In this study, high-resolution transmission electron microscopy (HRTEM) and molecular dynamics (MD) simulations were used to investigate the atomic arrangements and crystalline defects of deformation-induced γ-austenite → ε-martensite → α′-martensite and γ → α′ martensitic transformations in 316 L SS at 15 and 173 K. The γ → ε transformation involves the glide of Shockley partial dislocations on (111)γ planes without a change in atomic spacing. The formation of an α′ inclusion in a single ε-band is achieved by a continuous lattice distortion, accompanied by the formation of a transition zone of α′ and the expansion of the average atomic spacings due to dislocation shuffling. As α′ grows further into γ, the orientation relationship (OR) of the α′ changes by lattice bending. This process follows the Bogers-Burgers-Olson-Cohen model despite it not occurring on intersecting shear bands. Stacking faults and twins can also serve as nucleation sites for α′ at 173 K. We also found that direct transformation of γ → α′ occurs by the glide of 6aγ[112¯]/12 dislocations on every (111)γ plane with misfit dislocations. Overall, this study provides, for the first time, insights into the atomic-scale mechanisms of various two-step and one-step martensitic transformations induced by cryogenic deformation and corresponding local strain, enhancing our understanding of the role of martensitic transformation under ultra-cryogenic-temperature deformation in controlling the properties.

Bidirectional phase transformation facilitated by ε-martensite bands interaction in metastable Fe50Mn30Co10Cr10 dual-phase high entropy alloys

The phase transformation-induced plasticity (TRIP) metastable Fe50Mn30Co10Cr10 dual-phase high entropy alloys exhibit an excellent combination of strength and ductility. The interaction between ε-martensite (HCP phase) bands plays a fundamental role in determining the mechanical properties of Fe50Mn30Cr10Co10 alloys. In this work, the phase transition mechanism of Fe50Mn30Co10Cr10 dual-phase high entropy alloy was studied by using electron backscatter diffraction (EBSD), transmission electron microscope (TEM) observation and molecular dynamic (MD) simulations. The results indicated that the orientation relationship and character of the ε-martensite band play a crucial role in the phase transition and growth of ε-martensite bands. As an expanding ε-martensite band is obstructed by another ε-martensite band, the cross slip or dissociation of the Shockley partial dislocations at the ε-martensite junction lead to the growth of the ε-martensite bands. When two ε-martensite bands form a cross structure and the phase transition partial dislocations of the incident ε-martensite band are edge type, a new FCC-structured grain will be formed in the domain where the ε-martensite bands intersect. While the dissociation and gliding of Shockley partial dislocations at the phase boundaries will lead to the growth of the ε-martensite bands when the phase transition dislocations of the incident ε-martensite band are mixed type.

Molecular dynamics simulation of tension and compression deformation behavior in CoCrCuFeNi high-entropy alloy: Effects of temperature and orientation

This study utilizes molecular dynamics (MD) simulations to investigate the uniaxial tensile and compressive deformation behavior of the face-centered cubic (FCC) CoCrCuFeNi high-entropy alloy (HEA) with varying orientations at different temperatures. The results indicate that the material experiences temperature-induced softening in all simulation cases. In addition, it has been observed that the mechanical properties of the CoCrCuFeNi HEA exhibit orientation dependence and tension-compression asymmetry. Analysis of the Schmid factor of the leading and trailing partial dislocations suggests that the yield asymmetry is contingent upon the leading Schmid factor asymmetry. Microstructure evolution analysis and quantitative statistics of deformation defects indicate that the plastic deformation mechanisms of uniaxial tensile deformation along the [001], [111], and [112] orientations and compressive deformation along the [110], [111], and [112] orientations can be characterized by the formation and annihilation of intrinsic stacking fault (ISF), extrinsic stacking fault (ESF), and a few twins resulting from the gliding of Shockley partial dislocations. Twins play a crucial role in compression along the [001] direction and tension along the [110] direction due to the proliferation of twin boundaries (TBs)-assisted dislocations. The interactions between stacking faults (SF) and abundant TBs cause an increase in dislocation density in the later stage of plastic deformation in the aforementioned models. A comparison with low-entropy materials and theoretical calculations reveals that alloying reduces the stacking fault energy (SFE) of the CoCrCuFeNi crystal and enhances its twinnability.

Novel deformation mechanism of nanolamellar microstructure and its effect on mechanical properties of TiAl intermetallics

Generally, dislocation slipping and mechanical twinning govern the plastic deformation of lamellar microstructure (LM) in TiAl intermetallics. However, when the interlamellar spacing is reduced to the nanometer scale, the deformation mechanism switches to another mode, triggering different mechanical responses. This work reveals that the development of long-period stacking-ordered (LPSO) structure along the lamellar interface, such as the 9R and 6H structures, dominates the tensile plastic deformation of nano-LM. The formation of two LPSO structures is due to the gliding of the 1/6 <11 2‾ > Shockley partial dislocations on the (111) close-packed plane. In particular, the periodic alternating gliding of Shockley partial dislocations on every third layer of atoms and continuous gliding of Shockley partial dislocations on three neighboring planes result in 9R and 6H structures, respectively. LPSO formation during deformation can induce greater high-temperature yield strength compared to LMs deformed via dislocation gliding or mechanical twinning. These findings advance the understanding of the deformation mechanism of nano-LM and pave a new route for designing TiAl alloys with better high-temperature mechanical performance.

Characterization of face-centered cubic structure and deformation mechanisms in high energy shot peening process of TC17

• The B-type face-centered cubic titanium (fcc-Ti) occurred in the severely deformed TC17. • The deformation twinning acted as a primary deformation mechanism for fcc-Ti. • The gliding of Shockley partials was responsible for different deformation twins in fcc-Ti. • The occurrence of fcc-Ti effectively promoted the grain refinement of TC17. The face-centered cubic structure (fcc) and its deformation behaviors, as well as the distinctive role of fcc-Ti in nanocrystallization in TC17 subjected to high energy shot peening (HESP), were investigated by using comprehensive high-resolution transmission electron microscopy (HRTEM). The results showed that there was a stress-induced fcc-Ti in TC17 with a lattice constant of 0.420–0.433 nm and the B-type orientation relationship between the hcp-Ti and the fcc-Ti as [2-1-10] hcp //[-110] fcc and (0001) hcp //(111) fcc , which was accomplished by the gliding of Shockley partial dislocations with Burgers vector of 1/3[01-10] on the basal plane. The deformation twinning dominated the subsequent deformation of fcc-Ti, producing two types of {111}<11-2> twins with different characteristics. Among them, the I-type twin with complete structure was generated by successive gliding of Shockley partial dislocations with the same Burgers vector of 1/6[11-2]. In contrast, the cooperative slip of three Shockley partials, whose sum of Burgers vectors was equal to zero, produced the II-type twin with zero net macroscopic strain. And then, the emission of Shockley partial with the Burgers vector of 1/6[11-2] on every three (111) fcc planes resulted in the formation of a 9R structure. Due to the dissociation effect of lamellar fcc-Ti and the superior deformation ability of fcc structure, the occurrence of fcc-Ti effectively promoted surface nanocrystallization of TC17.

Microstructure and hardness of solution treated and cold-rolled Co75Cr25 alloy

In this study, the effects of solution treatment and plastic deformation on the microstructure and hardness of a Co75Cr25 alloy were investigated. The microstructures of the solution-treated and deformed specimens were investigated through X-ray diffraction (XRD), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). The results indicated that the Co75Cr25 alloy cooled by air (AC) during solution treatment has a single hexagonal close-packed (HCP) phase structure, while the alloy quenched by water (WQ) has a dual phase structure with the mixture of HCP phase and face-centered cubic (FCC) phase. Fast cooling rate inhibited the FCC to HCP martensitic phase transformation and led to more FCC phase retained at room temperature. During subsequent cold rolling (CR), stress-induced HCP to FCC martensitic transformation further increased the fraction of FCC phase in the WQ-CR specimen, through gliding of Shockley partial dislocations. Besides, stress-induced 101¯2 and 101¯1 twins were also widely observed in the WQ-CR specimen. Both martensitic transformation and twinning refined the microstructure of the WQ-CR specimen. However, for the AC-CR specimen, the deformation did not introduce obvious martensitic phase transformation. Some 101¯2and 112¯1 twins were activated to accommodate deformation. The hardness values of specimens after cold rolling were higher than those of the specimens before cold rolling. Deformation induced grain refinement played a very important role for improving the hardness.

Atomic scale observation of FCC twin, FCC → 9R and 9R → 12R’ transformations in cold-rolled Hafnium

The face-centered cubic (FCC) twin, FCC →9R and 9R → 12R’ transformations were observed simultaneously in cold-rolled pure Hf. The formation of FCC twin was due to the Shockley partial dislocations extended in the different basal planes. For FCC →9R transformation, the collective gliding of Shockley partial dislocations along (111) planes with every three atomic layers eventually lead to the stacking sequence of close-packed structure, i.e. ACBACBACBA changed to ABACACBCBA. The 9R → 12R’ transformation was related to the formation of the full dislocations at every three atomic layers.

High pressure induced the polymorphism phase transition in the Fe40Mn40Co10Cr10 multi-principal element alloy

In this study, we systematically studied the high pressure polymorphism phase transition process in the Fe40Mn40Co10Cr10 multi-principal element alloy through the hexahedron pressure equipment and the high pressure synchrotron angle-dispersive x-ray diffraction technique. A mixture of face-center cubic (fcc) + hexagonal close packed (hcp) nanolaminate structure was observed for the samples after high pressure treatment at 5 GPa, through the hexahedron pressure equipment. Especially, the hcp lath interaction structure with a new secondary hcp phase at the intersection zone was discovered in the sample. Transmission Electron Microscopy (TEM) analysis revealed that the transition from fcc to hcp was accomplished by the glide of Shockley partial dislocations. In addition, the in-situ synchrotron angle-dispersive x-ray diffraction technique was carried out to detect the phase transition process of the Fe40Mn40Co10Cr10 alloy up to 40.4 GPa. The lattice constant, atomic volumes, bulk modulus, and volume fraction of the fcc and hcp phase were obtained during compression and decompression. These results can expand our understanding of the structure evolution and properties of multi-principal element alloys under extreme conditions such as high pressure, which is contribute to the development of new metastable structural materials.

FCC phase transformation of Zr alloy during air cooling and aging

• Face-centered cubic (FCC) phase was firstly reported in air cooling and aging Zr alloy. • The orientation relationship between FCC-Zr and matrix was characterized at the atomic scale. • The mechanism of HCP→FCC phase transformation is thoroughly explained. Face-centered cubic (FCC) phase in air-cooled and aged Zr-1.0Sn-1.0Nb-0.3Fe alloy was characterized by transmission electron microscopy (TEM) and high angle annular dark field scanning TEM (HAADF-STEM). The gliding of Shockley partial dislocations on basal planes of hexagonal close-packed (HCP) matrix results in the observed < 1 2 ¯ 10 > HCP / / < 110 > FCC and { 0001 } HCP / / { 1 1 ¯ 1 } FCC orientation relationship. In addition, FCC-Zr phase can continue to grow during the aging process at 580°C instead of being replaced by HCP matrix.

The Plastic Deformation Mechanisms of hcp Single Crystals with Different Orientations: Molecular Dynamics Simulations

The deformation mechanisms of Mg, Zr, and Ti single crystals with different orientations are systematically studied by using molecular dynamics simulations. The affecting factors for the plasticity of hexagonal close-packed (hcp) metals are investigated. The results show that the basal <a> dislocation, prismatic <a> dislocation, and pyramidal <c + a> dislocation are activated in Mg, Zr, and Ti single crystals. The prior slip system is determined by the combined effect of the Schmid factor and the critical resolved shear stresses (CRSS). Twinning plays a crucial role during plastic deformation since basal and prismatic slips are limited. The twinning is popularly observed in Mg, Zr, and Ti due to its low CRSS. The twin appears in Mg and Ti, but not in Zr because of the high CRSS. The stress-induced hcp-fcc phase transformation occurs in Ti, which is achieved by successive glide of Shockley partial dislocations on basal planes. More types of plastic deformation mechanisms (including the cross-slip, double twins, and hcp-fcc phase transformation) are activated in Ti than in Mg and Zr. Multiple deformation mechanisms coordinate with each other, resulting in the higher strength and good ductility of Ti. The simulation results agree well with the related experimental observation.

Radiation response of nanotwinned Cu under multiple-collision cascades

In this paper, multiple collision cascades (MCC) of nanotwinned (nt) Cu with three different twin spacings are performed to model the response of nt Cu upon a radiation dose of 1 displacements per atom (dpa). Considering the defects developed with high randomness in the material during a MCC process, each MCC in a nt Cu is conducted for eight times. This enables us to analyze some average properties of defect clusters in the radiated nt Cu with different twin spacings at the different radiation doses. We also analyze the microstructural evolution in the nt Cu during the MCC. Smaller size of defect clusters and lower defect density are seen in the nt Cu with smaller twin spacing. In addition, a number of defect clusters could be removed via their frequent interactions with the coherent twin boundaries (CTBs) during the MCC. This induces either the migration of CTBs or the healing of CTBs. Moreover, the potential formation and elimination mechanisms of stacking fault are found to be due to the climb of Frank partial dislocation and glide of Shockley partial dislocations. This study provides further evidence on the irradiation tolerance of CTBs and the self-healing capability of CTBs in response to radiation.

Strain induced phase transformation in zirconium thin films

While high temperature is known to transform zirconium from the hexagonal closed pack (hcp) to the body centered cubic (bcc) phase, there is little or no evidence of mechanical strain induced transformation to the face centered cubic (fcc) phase in the literature. We performed molecular dynamics simulation to show irreversible hcp to fcc phase transformation in zirconium, triggered at about 14.6% tensile strain. The transformation mechanism depends on the crystallographic direction of loading. Gliding of Shockley partial dislocations on prism plane {101¯0} is suggested facilitate this transformation. Nudged elastic band (NEB) theory was used to estimate the transformation energy barrier, which was observed to decrease with any increase in temperature.

Atomistic modeling of Mg/Nb interfaces: shear strength and interaction with lattice glide dislocations

Using a newly developed embedded-atom-method potential for Mg–Nb, the semi-coherent Mg/Nb interface with the Kurdjumov–Sachs orientation relationship is studied. Atomistic simulations have been carried out to understand the shear strength of the interface, as well as the interaction between lattice glide dislocations and the interface. The interface shear mechanisms are dependent on the shear loading directions, through either interface sliding between Mg and Nb atomic layers or nucleation and gliding of Shockley partial dislocations in between the first two atomic planes in Mg at the interface. The shear strength for the Mg/Nb interface is found to be generally high, in the range of 0.9–1.3 GPa depending on the shear direction. As a consequence, the extents of dislocation core spread into the interface are considerably small, especially when compared to the case of other “weak” interfaces such as the Cu/Nb interface.

On the deformation twinning mechanisms in twinning-induced plasticity steel

Deformation twinning in coarse-grained fcc metals results from the highly coordinated glide of Shockley partial dislocations with the same Burgers vector on successive {111}-type twinning planes. The main issue of the formation mechanism of deformation twinning is how the arrangement of Shockley partials required for twinning evolves. Here, we present the deformation twinning mechanism that operates in Fe-17Mn-0.45C-1.5Al-1Si TWIP steel by means of in-situ TEM deformation tests and post-mortem TEM characterization. The in-situ TEM observation shows the formation of a thin twin by Shockley partial dislocations generated in a grain boundary, which indicates the nucleation of deformation twins at grain boundary defect sites. The observed twinning process is similar to deformation twinning in nano-crystalline materials through successive emission of partial dislocations from grain boundaries. In addition, a high density of Frank partial dislocations is observed inside deformations twins. These defects affect the growth of deformation twins and could contribute to the high work hardening of TWIP steel.

Mechanisms for deformation induced hexagonal close-packed structure to face-centered cubic structure transformation in zirconium

Two distinctive orientation relations between hexagonal close-packed (HCP) phase and face-centered cubic (FCC) phase were observed simultaneously in cold-rolled pure Zr, which were denoted as B-type and P-type orientation relations according to whether the phase interface is parallel to the basal plane or prism plane of the HCP matrix. The mechanisms for the phase transformation generating the two different HCP-FCC orientation relations were thoroughly analyzed. B-type orientation relation was achieved when HCP to FCC phase transformation occurred via collective gliding of Shockley partial dislocations on basal planes, while P-type orientation relation was attained through pure-shuffle and shear-shuffle mechanisms.

Strain-induced structural defects and their effects on the electrochemical performances of silicon core/germanium shell nanowire heterostructures.

We report on strain-induced structural defect formation in core Si nanowires of a Si/Ge core/shell nanowire heterostructure and the influence of the structural defects on the electrochemical performances in lithium-ion battery anodes based on Si/Ge core/shell nanowire heterostructures. The induced structural defects consisting of stacking faults and dislocations in the core Si nanowire were observed for the first time. The generation of stacking faults in the Si/Ge core/shell nanowire heterostructure is observed to prefer settling in either only the Ge shell region or in both the Ge shell and Si core regions and is associated with the increase of the shell volume fraction. The relaxation of the misfit strain in the [112] oriented core/shell nanowire heterostructure leads to subsequent gliding of Shockley partial dislocations, preferentially forming the twins. The observation of crossover of defect formation is of great importance for understanding heteroepitaxy in radial heterostructures at the nanoscale and for building three dimensional heterostructures for the various applications. Furthermore, the effect of the defect formation on the nanomaterial's functionality is investigated using electrochemical performance tests. The Si/Ge core/shell nanowire heterostructures enhance the gravimetric capacity of lithium ion battery anodes under fast charging/discharging rates compared to Si nanowires. However, the induced structural defects hamper lithiation of the Si/Ge core/shell nanowire heterostructure.

Low-energy, Mobile Grain Boundaries in Magnesium

The strong basal texture that is commonly developed during the rolling of magnesium alloy and can even increase during annealing motivates atomic-level study of dislocation structures of both <0001> tilt and twist grain boundaries (GBs) in Magnesium. Both symmetrical tilt and twist GBs over the entire range of rotation angles θ between 0° and 60° are found to have an ordered atomic structure and can be described with grain boundary dislocation models. In particular, 30° tilt and twist GBs are corresponding to energy minima. The 30° tilt GB is characterized with an array of Shockley partial dislocations bp:-bp on every basal plane and the 30° twist GB is characterized with a stacking faulted structure. More interesting, molecular dynamics simulations explored that both 30° tilt and twist GBs are highly mobile associated with collective glide of Shockley partial dislocations. This could be responsible for the formation of the strong basal texture and a significant number of 30° misorientation GBs in Mg alloy during grain growth.

Growth and Stress-induced Transformation of Zinc blende AlN Layers in Al-AlN-TiN Multilayers.

AlN nanolayers in sputter deposited {111}Al/AlN/TiN multilayers exhibit the metastable zinc-blende-structure (z-AlN). Based on density function theory calculations, the growth of the z-AlN is ascribed to the kinetically and energetically favored nitridation of the deposited aluminium layer. In situ nanoindentation of the as-deposited {111}Al/AlN/TiN multilayers in a high-resolution transmission electron microscope revealed the z-AlN to wurzite AlN phase transformation through collective glide of Shockley partial dislocations on every two {111} planes of the z-AlN.

Role of strain-induced martensitic transformation on extrusion and intrusion formation during fatigue deformation of biomedical Co–Cr–Mo–N alloys

The mechanism of extrusion and intrusion formation in Co–Cr–Mo–N alloys during fatigue deformation was investigated. In particular, we focused on the role of the strain-induced martensitic transformation (SIMT), which is the transformation of the metastable γ face-centered cubic (fcc) phase into a stable ε hexagonal close-packed (hcp) phase at room temperature because of the gliding of Shockley partial dislocations in the γ-phase matrix. We found that the SIMT also plays a crucial role in the formation of extrusions and intrusions. Further, the morphology of the extrusions and intrusions formed in the Co–Cr–Mo–N alloy specimens was very different from that seen in other fcc alloys. The extrusions and intrusions were formed by the gliding of perfect dislocations with a Burgers vector of perfect <a> dislocation on the basal plane of the ε-hcp phase. This suggests that the ε-phases introduced by the SIMT can deform readily.

Stress-induced hexagonal close-packed to face-centered cubic phase transformation in commercial-purity titanium under cryogenic plane-strain compression

A rare transformation from a hexagonal close-packed (hcp) phase to a face-centered cubic (fcc) phase was found in pure Ti deformed by cryogenic channel-die compression indented for microstructural refinement. While grain refinement occurred upon recurrent intersection of the persistent slip bands, nucleation of the platelet fcc phase was identified at the areas of intersecting slip bands. The stress-induced phase transformation was attributed to the gliding of Shockley partial dislocations, resulting in 〈 0 0 0 1 〉 hcp / / 〈 0 0 1 〉 fcc , 〈 1 0 1 ¯ 0 〉 hcp / / 〈 1 1 ¯ 0 〉 fcc and 〈 1 1 2 ¯ 0 〉 hcp / / 〈 1 1 0 〉 fcc orientation relationships.