Dislocation Twin Research Articles

Microstructural Evolution and Mechanical Properties of Extruded AZ80 Magnesium Alloy during Room Temperature Multidirectional Forging Based on Twin Deformation Mode.

This study investigates the preparation of ultrahigh-strength AZ80 magnesium alloy bulks using room temperature multidirectional forging (MDF) at different strain rates. The focus is on elucidating the effects of multidirectional loading and strain rates on grain refinement and the subsequent impact on the mechanical properties of the AZ80 alloy. Unlike hot deformation, the alloy subjected to room temperature MDF exhibits a lamellar twinned structure with multi-scale interactions. The key to achieving effective room temperature MDF of the alloy lies in combining multidirectional loading with small forging strains per pass (6%). This approach not only maximizes the activation of twinning to accommodate deformation but ensures sufficient grain refinement. Microstructural analysis reveals that the evolution of the grain structure in the alloy during deformation results from the competition between {101¯2} twinning or twinning variant interactions and detwinning. Increasing the forging rate effectively activates more twin variants, and additional deformation passes significantly enhance twin interaction levels and dislocation density. Furthermore, at a higher strain rate, more pronounced dislocation accumulation facilitates the transformation of twin structures into high-angle grain boundaries, promoting texture dispersion and suppressing detwinning. The primary strengthening mechanisms in room temperature MDF samples are grain refinement and dislocation strengthening. While increased dislocation density raises yield strength, it reduces post-yield work hardening capacity. After two passes of MDF at a higher strain rate, the alloy achieves an optimal balance of strength and ductility, with a tensile strength of 462 MPa and an elongation of 5.1%, significantly outperforming hot-deformed magnesium alloys.

Exploring the effect of Cr and Mn on the intrinsic strength of the tensile properties of FeCoNi, FeCoNiMn, FeCoNiCr, and FeCoNiCrMn multi-principal element alloys using in-situ EBSD

As the composition elements in multi-principal element alloys increase, it can bring excellent mechanical properties. However, the strengthening mechanism of the additional element is still unclear. In this work, we establish a method based on the in-situ EBSD technology to explore the possible effect of additional elements on the intrinsic strength of tensile properties. We prepared four different multi-principal element alloys, including FeCoNi, FeCoNiMn, FeCoNiCr, and FeCoNiCrMn with similar initial status. We systematically investigated the evolution of the microstructure, dislocation density, twin boundary, grain size, and element distribution during the tensile process by in-situ EBSD and EDS. By carefully analyzing the results of four different multi-principal element alloys, the strength effects of the solid-solution hardening, grain-boundary hardening, twin boundary hardening, precipitate hardening, and dislocation hardening were peeled. The effect of the Cr and Mn element addition on the intrinsic strength can be explored. It is found that the element addition indeed increases the intrinsic strength from quaternary to quinary but not very clear from ternary to quaternary no matter Cr or Mn, which indicated that the intrinsic strength was more related to the number of elements in the alloy than to which element was present. This can be explained using the mixing entropy theory, which states that the intrinsic strength is enhanced when the mixing entropy is over a threshold between the MEA and HEA. This paper presents a method to study the individual factors affecting the tensile properties, which can help other researchers to better investigate HEA.

Influences of symmetric tilt grain boundary on the stretching deformation behaviors of Cu bicrystals

The microstructure of grain boundary (GB) becomes an important factor affecting the strength of nanomaterials during plastic deformation. In this work, Cu bicrystals with different coincidence site lattice (CSL) interfaces are constructed, and the influences of the symmetric tilt interface on the mechanical properties and plastic deformation behaviors during uniaxial tension are studied by molecular dynamics simulation. The results indicate that the peak stress of tensile stress-strain curves for most bicrystals is lower than its monocrystalline samples, except CSL3, CSL11 and CSL57 bicrystals with a continuous and single structural unit. The CSL3 bicrystal has the highest strength, while CSL33 bicrystal has the lowest strength among all the tested samples. The CSL11 and CSL57 bicrystals with a lower GB energy have a higher stability of interface, which delays the yielding. Moreover, the generation of twin and stair-rod dislocation at the interfaces strengthens the CSL11 and CSL57 bicrystals. The CSL3 almost has the same parameters as its monocrystalline sample S3, massive research has confirmed that the sample with a higher density twin (CSL3 interface) exhibits higher combined properties of strength and plasticity. This work may provide a guideline for GB engineering to improve material properties by controlling the GB structure.

Study on the plasticity improvement mechanism and grain refinement of AZ80 Mg alloy under cryogenic multi-directional forging

The introduction of twin lamellae and dislocation interaction through Multi-directional forging (MDF) at room temperature (RT) has been reported to significantly improve the mechanical properties of Mg alloys. However, the Mg alloys typically exhibited poor plasticity at RT, making it highly susceptible to cracking. This study further enhanced the deformation capacity of AZ80 Mg alloy by adding a pre-cryogenic step before MDF. Compared to direct MDF at RT, MDF at cryogenic temperature (CT, −196 °C) further increased the alloy's deformation capacity from 0.72 (4P) to 1.08 (6P), with the tensile yield strength (TYS) and ultimate tensile strength (UTS) increasing to ∼420 MPa and ∼512 MPa. The alloy subjected to cryogenic MDF exhibited denser twin lamellae and a higher proportion of {10–12}-{10–12} twin interactions. The more frequent twin interactions under CT inhibited excessive twin growth and further increased the dislocation density of the matrix, facilitating the nucleation of new twins. Additionally, with increasing deformation passes, the texture of the RT specimens became more single, while the CT specimen showed a trend toward texture dispersion. The study elucidated that poor forgeability of the alloy under MDF at RT was hindered by unfavorable texture and limited twin activation. However, this issue was mitigated under CT, where the activation of more atypical twins with low Schmid factors (SFs) significantly enhanced the deformation coordination ability. Furthermore, the activation of atypical twins and extensive twin interactions promoted texture dispersion in CT specimens, which facilitated the activation of various slips/twins and suppressed excessive twinning-induced hardening, thereby improving forgeability.

Interactions between basal dislocation and [formula omitted] twin boundary in HCP metal: A molecular dynamics study

For hexagonal close-packed (HCP) metals having a c/a<3, basal <a> slip is the most common slip system, and {112̅2} twinning is one of the primary plastic deformation modes when compressive stress is applied along the c-axis. In this work, molecular dynamics simulations have been applied to reveal the interaction mechanism between different basal <a> dislocations and {112̅2} twin boundaries (TBs) in Mg, Zr and α-Ti. When the 000113[112̅0] dislocation interacts with the {112̅2} TB, it is transmuted into a 112̅2-{112̅1} double twin (DT). With increasing shear stress, the growth of the 112̅2-{112̅1} DT is impeded by the other side of the {112̅2} TB, it transmutes into (0001)13<101̅0> dislocations and twin dislocations (TDs). Furthermore, the {112̅1} TB is capable of transforming into a {112̅2} TB and a new co-zone {112̅1} TB, which then forms a twin-twin junction (TTJ) to release stress concentration. When the 000113[21̅1̅0] dislocation interacts with {112̅2} TBs, it transmuted into a I2 SFs in the twin and a TD with one-layer height on the {112̅2} TB. But the 000113[1̅21̅0] dislocation would be impeded by the {112̅2} TB and couldn’t be transmuted further.

Molecular Dynamics Study on the Mechanical Behaviors of Nanotwinned Titanium

Titanium and titanium alloys have been widely applied in the manufacture of aircraft engines and aircraft skins, the mechanical properties of which have a crucial influence on the safety and lifespan of aircrafts. Based on nanotwinned titanium models with different twin boundary spacings, the impacts of different loadings and twin boundary spacings on the plastic deformation of titanium were studied in this paper. It was found that due to the different contained twin boundaries, the different types of nanotwinned titanium possessed different dislocation nucleation abilities on the twin boundaries, different types of dislocation–twin interactions occurred, and significant differences were observed in the mechanical properties and plastic deformation mechanisms. For the {101-2} twin, basal plane dislocations were likely to nucleate on the twin boundary. The plastic deformation mechanism of the material under tensile loading was dominated by partial dislocation slip on the basal plane and face-centered cubic phase transitions, and the yield strength of the titanium increased with decreasing twin boundary spacing. However, under compression loading, the plastic deformation mechanism of the material was dominated by a combination of partial dislocation slip on the basal plane and twin boundary migration. For the {101-1} twin under tensile loading, the plastic deformation mechanism of the material was dominated by partial dislocation slip on the basal plane and crack nucleation and propagation, while under compression loading, the plastic deformation mechanism of the material was dominated by partial dislocation slip on the basal plane and twin boundary migration. For the {1124} twin, the interaction of its twin boundary and dislocation could produce secondary twins. Under tensile loading, the plastic deformation mechanism of the material was dominated by dislocation–twin and twin–twin interactions, while under compression loading, the plastic deformation mechanism of the material was dominated by partial dislocation slip on the basal plane, and the product of the dislocation–twin interactions was basal dislocation. All these results are of guiding value for the optimal design of microstructures in titanium, which should be helpful for achieving strong and tough metallic materials for aircraft manufacturing.

Oxidation of additive manufactured 699XA Ni-based alloy under different atmospheres

The oxidation behaviour of an additively manufactured (AM) NiCrAl alloy (VDM® Powder 699XA) was compared with the hot rolled counterpart in different atmospheres (dry air, Ar-10 %H2 and Ar-10 %H2-10 %H2O) for 48 h at 950 and 1100°C. Internal oxidation occurred through the twin boundaries and dislocations in the AM material. Such defects promoted the outward diffusion of Al and the growth of a protective Al2O3 scale at reduced PO2 (Ar-10 %H2 and Ar-10 %H2-H2O), hence restraining the internal oxidation. It is thus demonstrated that this high Cr-containing alloy is a marginal alumina former under controlled conditions like the hot rolled alloy.

The influence of annealing temperature on the microstructure and mechanical properties of Fe-0.52 C-11Mn-5.14Al-1Cr lightweight steel

Steel used in the automotive industry must strike a delicate balance between strength and ductility to maintain the structural integrity of vehicles and achieve intricate designs. Traditional methods of reducing weight by using high-strength low-carbon steels have limitations. As a result, a new dual-phase lightweight steel with an austenite-ferrite structure and the composition Fe-0.52 C-11Mn-5.14Al-1Cr was developed. The relationship between the microstructure and mechanical properties of this lightweight steel, which underwent various annealing processes (700°C-1000°C) after cold rolling, was examined using specialized equipment like a universal testing machine, electron microscopy, and X-ray diffractometer. The findings reveal that the cold-rolled test steel (CR) exhibits high tensile strength (1500 MPa) but low elongation at break (3.9 %). The performance of the experimental steel post-annealing treatment has shown significant improvement, with the average grain size increasing with higher annealing temperatures. The γ phase generally shows an increasing trend, with finer grains post-annealing, and the distribution of the α phase displays a consistent pattern along γ boundaries. Additionally, a substantial number of annealing twins appear in the matrix. These twin boundaries absorb some dislocations, and slip leads to the movement of twin boundaries, effectively reducing stress concentration from deformation and enhancing the ability of twin boundaries to accommodate plastic strain, creating twin dislocation interaction. The interplay among dislocations, stacking faults, and twins regulates the balance between strength and plasticity. Following treatment at 850°C × 1 min + 500°C × 3 min, the steel displays excellent mechanical properties, including ultimate tensile strength (836 MPa), elongation (53 %), and strength-ductility product (43,723 MPa·%). This study identifies a specific heat treatment parameter that achieves a harmonious equilibrium between outstanding strength and plasticity (strength-ductility product) within the designed composition.

Atomistic modeling of idealized equal channel angular pressing process

Severe plastic deformation (SPD) processes are forming techniques that impose large plastic strains to achieve ultra-fine grained and nanocrystalline microstructures. The refined microstructure results in a significant increase in strength and in many cases without a considerable loss in ductility. Despite their long existence, a comprehensive understanding of the deformation and grain refinement mechanisms during SPD is still lacking. Atomistic simulations that are regularly used to obtain insights into material behavior have rarely been used to study SPD processes. In this work, we use large scale atomistic simulations of the molecular statics/dynamics kind to model the equal channel angular pressing (ECAP) process, a specific SPD process. Three different materials—Al, Ni and Cu—are used to model high, medium and low stacking fault energy materials. Large simulation cells are used to facilitate grain refinement at the nanoscale. The simulations are carefully analyzed in terms of stress–strain behavior, dislocation activity and grain refinement. All samples show the formation of deformation twins at the nanoscale, which generally detwin at increased strains via dislocation twin interactions. A significant number of stair-rod dislocations form via dislocation–dislocation interactions and are present at low angle grain boundaries in the early stages of deformation. Such stair-rod dislocations also result in the formation of stacking fault tetrahedra which are present in significant numbers in all samples. Finally, difficulties in studying grain refinement at very large strains in atomistic simulations are discussed.

Uneven Strain Distribution Induces Consecutive Dislocation Slipping, Plane Gliding, and Subsequent Detwinning of Penta-Twinned Nanoparticles.

Twin structures possess distinct physical and chemical properties by virtue of their specific twin configuration. However, twinning and detwinning processes are not fully understood on the atomic scale. Integrating in situ high resolution transmission electron microscopy and molecular dynamic simulations, we find tensile strain in the asymmetrical 5-fold twins of Au nanoparticles leads to twin boundary migration through dislocation sliding (slipping of an atomic layer) along twin boundaries and dislocation reactions at the 5-fold axis under an electron beam. Migration of one or two layers of twin planes is governed by energy barriers, but overall, the total energy, including surface, lattice strain, and twin boundary energy, is relaxed after consecutive twin boundary migration, leading to a detwinning process. In addition, surface rearrangement of 5-fold twinned nanoparticles can aid in the detwinning process.

Enhancing the thermoelectric properties of TiO2-based ceramics through addition of carbon black and graphene oxide

Donor-doped TiO2 ceramics are attractive thermoelectrics. Herein, two types of Nb-doped TiO2/carbon black (CB) and Nb-doped TiO2/graphene oxide (GO) composites were prepared by spark plasma sintering (SPS) at 1473 K. Both composites exhibit the same rutile structure. The addition of CB and GO helps to enhance thermoelectric performance through different mechanisms. GO flakes and CB clusters are distributed at grain boundaries, but grain-boundary segregation occurs in high CB content samples. Assisted by the high stress and high temperature during sintering, the GO flakes can induce significant lattice distortion near grain boundaries and reduce the concentration of twin boundaries, whilst CB has minimal impact on the grain boundary characteristics. However, all composites contain high densities of twin boundaries, stacking faults and dislocations; the crystallographic details have been resolved at the atomic scale. GO flakes can significantly reduce thermal conductivity but have minimal effect on carrier transport behavior, leading to a ∼17 % improvement in ZT values. CB is believed to promote the carrier concentration, which strongly boosts the power factor to ∼700 μW m−1 K−2 but marginally increases thermal conductivity, resulting in 25 % improvement in ZT values to 0.19 at 873 K. This investigation provides new understandings of microstructural modification of TiO2/carbon nanostructure systems, which could guide the future development of hybrid systems for thermoelectric applications.

Deformation microstructure dependence of low cycle tensile fatigue life of NiTi wires

This study prepared near-equiatomic NiTi wires with excellent cyclic tensile fatigue life and proposed the mechanism by which deformation microstructure impacts fatigue life. The fatigue life of Ni50.2Ti49.8 was discovered to be 1.8–2.1 times greater than Ni50.9Ti49.1 by the low cycle fatigue test with various test conditions (loading stress is 700 MPa/900 MPa, frequency is 5 Hz/2.5 Hz, and stress ratio is 0.1/0.2) at room temperature. Ni50.2Ti49.8′s matrix phase is R + B19′ with fewer slip systems. It tends to coordinate deformation with twinning, stacking faults, and wave dislocations during cyclic tensile. Cross-grain boundary twinning strengthens grain boundaries. Also, twinning can expand available space for dislocation movement, thereby reducing stress concentration and extending the fatigue life of Ni50.2Ti49.8. Ni50.9Ti49.1′s matrix phase is B2 + R and easy to form planar dislocation. This reduces the space for dislocation movement and causes premature fatigue fracture. Moreover, the second phase Ni4Ti3 in Ni50.2Ti49.8 is distributed uniformly throughout the matrix and exerts a strong pinning influence on the dislocation movement. This also facilitates the formation of multiple deformation microstructures in Ni50.2Ti49.8.

Molecular dynamic simulations of the martensitic transformation for the dual-phase structure and dislocation activities in Ti80 alloys

Titanium alloy (Ti alloy) has been widely used in various industry areas and attracted extensive attentions due to their excellent properties. In this work, the martensitic transformation (MT) and complex dislocation activities are thoroughly simulated and analyzed for the commercial Ti80 alloy in molecular dynamic (MD) simulations. One specific interatomic potential is developed for Ti80 alloy within the framework of embedded atom method (EAM), which is capable of describing the related physical properties and the plastic deformation mechanism for this alloy. The stabilities of hexagonal close-packed (hcp) and body-centered cubic (bcc) phase boundary and the martensitic transformation mechanism is fully investigated in MD simulations. The orientation relationship between hcp and bcc phases obeys the Burgers relationship. Complex dislocation activities are reproduced in the simulation of the compression of the polycrystalline configuration, including the formation of dislocation wall, (112‾2) compression twin boundaries and dislocation cross slip mechanisms. The MD simulation results are well agreement with our experimental observations and provides an in-depth understanding of the deformation mechanisms of the Ti80 alloy and related Ti alloys. The revealed deformation mechanism and the newly-developed interatomic potential should be a useful guide to the development of the new series of high-performance Ti alloys in the future.

Elucidating friction properties and strengthening mechanisms of Ni-based surfacing layer containing 9R phase

This study analyzed the strengthening mechanisms and friction properties of Inconel 625 surfacing layers fabricated by plasma transfer arc surfacing (PTA-surfacing) in ball-on-disc friction tests at room temperature and 650 °C. Results showed that Inconel 625 exhibited poor tribological performance at room temperature, but excellent at 650 °C. The average Vickers hardness of the high-temperature wear area (1.6826 ± 0.4059 GPa) was higher than that of the room temperature wear area (1.3900 ± 0.1875 GPa). The coefficient of friction for Inconel 625 decreased from 0.5362 at room temperature to 0.2846 at 650 °C. The wear rates of Inconel 625 at 650 °C and room temperature were 1.2063 × 10−5 mm3/N·m and 20.9740 × 10−5 mm3/N·m, respectively. The wear mechanisms at 650 °C were mainly abrasive, adhesion, oxidation wear, and some degree of delamination wear. The microstructural characterizations revealed that the wear subsurface at 650 °C comprised polycrystalline-mixed oxide layers (Ni(Cr,Fe)2O4, (Cr,Fe)2O3, and NiO), gradient grain refinement, nanoscale precipitations (M23C6, M6C, and laves), 9R structural phases, I-type twinning, and high-density dislocations, which synergistically enhance the outstanding high-temperature friction properties and strength.

A New Structure with Localized sp2 Bonding for Fivefold Twinning in Diamond.

Changes in atomic bonding configuration in carbon from sp3 to sp2 are known to exist in certain structural defects in diamond, such as twin boundaries, grain boundaries, and dislocations, which have a significant impact on many properties of diamond. In this work, the atomic structure of fivefold twinning in detonation synthesized ultra-dispersed diamonds is investigated using a combination of techniques, including spherical aberration-corrected high-resolution electron microscopy (HREM), HREM image simulations, and molecular mechanics (MM) calculations. The experimental HREM images reveal clearly that the fivefold twinning in diamond has two distinct structures. In addition to the concentric fivefold twins, where the core structure is the intersection of five {111} twinning boundaries, a new extended core structure with co-hybridization of bonding is identified and analyzed in fivefold twinning. The atomic structure forming these fivefold twinning boundaries and their respective core structures is proposed to involve both the tetrahedral sp3 and planar graphitic sp2 bonding configurations, in which a co-hybridized planar hexagon of carbon serves as a fundamental structural unit. The presence of this sp2 -bonded planar unit of hexagonal carbon rings in general grain boundaries is also discussed.

Influence of grain boundary modification on color transition behavior of Cu-Al-Zn-Sn alloys with low stacking fault energy

In this study, we investigated the color transition mechanism of Cu-5Al-5Zn-1Sn (in wt%) alloy during cold-rolling and annealing processing by analyzing the correlation between microstructural characteristics and optical transition behavior. Single-phase Cu alloys have limited solubility which restrict on controlling their optical properties. Therefore, we propose a novel strategy for modulating the optical properties of single-phase Cu alloys. Cu alloys with low stacking fault energy (SFE) exhibit highly dense twin defects and dislocations when subjected to severe plastic deformation. In addition, annealed low SFE samples contain unique grain boundary structure, with different energies according to atomic structure and misorientation angle, which influences the density of states and electronic structure of the alloys. Consequently, the optical transition energy of the samples is affected by the grain boundary energy, which this influence to the chromaticity of the samples. These findings provide a new approach for adjusting the color of the alloys.

Multi-twinned deformation and fracture characteristics of directional solidified Ti-45.5Al–5Nb-0.5Ta alloys during high-temperature rotary-bending fatigue process

Ф 16.5 mm directional solidified Ti-45.5Al–5Nb-0.5Ta alloy was obtained through electromagnetic confinement methods. Relative rotary-bending fatigue experiment with different loads were also performed at 760 °C. The rotary-bending fatigue limit of the alloy at 760 °C is 269 MPa, and the fracture mode is a composite fracture mode of cleavage fracture and lamellar tearing. Based on the combination of EBSD and PED technology, crack bridging characteristics and microstructure deformation during the rotary-bending fatigue process were analyzed. Crack bridging is induced by banded dislocation pileups at the tips of adjacent cracks, which releases the stress concentration in the local microstructure and enhances the scale of crack extension. Moreover, the orientation of γ phase is deflected by 60°–70° during the crack bridging process. Furthermore, combined with the analysis of the mechanical load characteristics during the rotary-bending fatigue process, the initiation and evolution mechanism of the multi-twinned deformation and its influence on the deformation capacity of the alloy were obtained. The initiation of one type γ-twins is the stacking fault caused by dislocation slip from the α/γ phase boundary, and the initiation of the other γ-twins is atomic plane slips caused by twin misalignments or dislocation slips.

Effects of structure and strain rate on deformation mechanism of twin lamellar Al0.3CoCrFeNi alloys

Lamellar twined materials display simultaneous ultrahigh strength and high ductility, which is attributed to the interaction between twin boundaries and dislocation. However, deformation at the atomic level is rarely seen in experiments. To study the deformation properties and mechanical behavior of the lamellar twined Al0.3CoCrFeNi high-entropy alloys (HEA) sample, we applied the uniaxial tension by employing molecular dynamics (MD) simulations. The impact of various twin inclination angles, twin boundary spacing (TBS), and strain rates are investigated. The mechanical softening as the increasing TBS from 4a to 12a was observed, which corresponds to Hall – Petch relationship. The yield strength of the medium inclination angle (450, 600) records the minimum value with all various TBS due to the elastic energy being easier released. The strain-hardening phenomenon appears at these angles. The sample with an inclination angle perpendicular to the tension loading axis (900) displays the highest stress value. The shrinking migration twin has been observed in 00, 150, and 300 twin inclination angles. The migration and disappearance of initial TBs lead to grains being reoriented into the same orientation at 450 and 600. With twin orientations of 750 and 900, the TB is no longer as crisp and straight as the initial TB under the tension process. An extensive dislocation process occurs at higher strain values, and their interaction with TB leads to establishing the shear band. After that, twin boundaries within the shear band are almost destroyed. Besides, the results also indicate that the flow stress, ultimate strength, and Young's modulus rise with the rising strain rate and decreasing temperature.

High-Throughput Investigation of Multiscale Deformation Mechanism in Additively Manufactured Ni Superalloy

In this paper, Inconel 718 (IN718) superalloy was processed by laser powder-bed fusion additive manufacturing (L-PBFAM), followed by heat treatment. High-resolution nanoindentation was used to investigate the complex deformation mechanisms that occurred at various length scales in both conditions. The nanoindentation elastoplastic maps show a strong crystal orientation dependency of modulus and hardness, which is attributed to the high mechanical anisotropy of IN718. The hardness map effectively resolves complex microscale strength variation imparted due to the hierarchical heat distribution associated with the thermal cycles of L-PBFAM. The disproportionately high hardening effect of Nb, Mo-rich chemical segregations and Laves phases in dendritic structures is also observed. The heat treatment resulted in a 67% increase in yield strength (from 731 MPa in the L-PBFAM condition to 1217 MPa in the heat-treated condition) due to the activation of multiple precipitation-strengthening mechanisms. The nanoindentation mapping of a heat-treated sample delineates the orientation-dependent hardness distribution, which apart from high mechanical anisotropy of the alloy, is also contributed to by a high degree of coherency strengthening of the D022 γ″-precipitates oriented parallel to the &lt;001&gt; crystal plane of the γ-matrix. The mean hardness of the sample increased from 13.3 GPa to 14.8 GPa after heat treatment. Evidence of extensive deformation of twin networks and dislocation cells was revealed by transmission electron microscopy of the deformed region under the nanoindentation tip.

Hardening effects of sheared precipitates on {112¯1} twinning in magnesium alloys

Abstract The interactions between a plate-like precipitate and two twin boundaries (TBs) ( { 10 1 ¯ 2 } , { 11 2 ¯ 1 } ) in magnesium alloys are studied using molecular dynamics (MD) simulations. The precipitate is not sheared by { 10 1 ¯ 2 } TB, but sheared by { 11 2 ¯ 1 } TB. Shearing on the (110) plane is the predominant deformation mode in the sheared precipitate. Then, the blocking effects of precipitates with different sizes are studied for { 11 2 ¯ 1 } twinning. All the precipitates show a blocking effect on { 11 2 ¯ 1 } twinning although they are sheared, while the blocking effects of precipitates with different sizes are different. The blocking effect increases significantly with the increasing precipitate length (in-plane size along TB) and thickness, whereas changes weakly as the precipitate width changes. Based on the revealed interaction mechanisms, a critical twin shear is calculated theoretically by the Eshelby solutions to determine which TB is able to shear the precipitate. In addition, an analytical hardening model of sheared precipitates is proposed by analyzing the force equilibrium during TB-precipitate interactions. This model indicates that the blocking effect depends solely on the area fraction of the precipitate cross-section, and shows good agreement with the current MD simulations. Finally, the blocking effects of plate-like precipitates on the { 10 1 ¯ 2 } twinning (non-sheared precipitate), { 11 2 ¯ 1 } twinning (sheared precipitate) and basal dislocations (non-sheared precipitate) are compared together. Results show that the blocking effect on { 11 2 ¯ 1 } twinning is stronger than that on { 10 1 ¯ 2 } twinning, while the effect on basal dislocations is weakest. The precipitate-TB interaction mechanisms and precipitation hardening models revealed in this work are of great significance for improving the mechanical property of magnesium alloys by designing microstructure.