Multiple Slip Systems Research Articles

In-situ observation of microstructure and orientation evolution of the Cu–Cr–Zr-Hf alloys

The roles of Hf element in the mechanical properties, deformation mode and orientation evolution of the Cu-0.4Cr-0.2Zr-0.2Hf (wt.%) alloys were investigated using both ex-situ and in-situ electron backscatter diffraction (EBSD). The results indicate that the trace Hf element (0.2 wt%) addition into the Cu–Cr–Zr alloys motivates the multiple slip activities. The activation of multiple slip systems indirectly or directly influences the tensile strength, strain distribution and lattice rotation. Firstly, it promotes the generation of dislocations, and thus enhances the work hardening rate, contributing to the improvement of the ultimate tensile strength. Secondly, it inhibits the movement of dislocations to the grain boundaries, and thus hinders the strain localization at the grain boundaries, promoting the homogeneous deformation. Thirdly, the interaction between dislocations on different slip systems restricts the intragranular lattice rotation, leading to the high stability of the grains with Goss orientation in the Cu–Cr–Zr-Hf alloys. The mutual transform between the R-CubeRD orientation and Goss orientation is also observed. This study deepens the understanding of deformation mechanisms and texture evolution of the Cu–Cr–Zr-Hf alloys.

Experiment and atomistic simulation of uniaxial compression of Ni–W single-crystal alloy

The present work investigates the deformation behavior and identify the deformation relief patterns in Ni-28 W (28 wt. %W) single crystal under uniaxial compression using experimental and atomistic simulations. EAM (Embedded Atomic Method) potential is used for modelling the atomic interaction between Ni and W atoms. The compression test is carried at a constant temperature of 300 K and at a strain rate of 10-3 s−1 in experiment and 108 s−1 in atomistic simulation. Both experimental and simulation stress–strain curves show linear elastic region followed by strain hardening region till 20% strain and are in qualitative agreement. The observed slip lines can be seen inclined at an angle of 45° to the loading direction formed as a result of plastic deformation by slip on multiple slip systems in both experimental and simulated Ni–W single crystal alloy.

In-situ investigation into the deformation behavior of Ti-6Al-4V processed by laser powder bed fusion

In this work, we presented an investigation into the deformation behavior of laser powder bed fusion (LPBF) Ti-6Al-4 V (Ti64), particularly how microstructure characteristics determined the grain deformation behavior by using in-situ SEM tensile characterization. Results demonstrated that α laths with their elongation directions 45° to the tensile loading axis and α laths with the larger thickness could induce severe strain accumulation during the uniaxial tensile process. Crystallographic orientations and grain morphologies of α lath were considered the key factors that could influence the deformation behavior. The strain localization was observed in the α laths dominated by multiple slip systems. Furthermore, the α laths with their elongation directions 45° to the tensile loading axis had the longest effective slip length, which could make α laths favorable for deformation. The dislocations multiplication of coarse α laths was much greater than in fine α laths, which could also result in strain localization. These findings could shed light on the deformation behavior related to lamellar structure in LPBF Ti64 and could further help the mechanical property manipulation. • Strain distribution of LPBF Ti64 during the tensile process was characterized by using in-situ SEM tensile testing • Crystallographic orientations and grain morphologies of α lath were considered the key factors that could determine the deformation behavior • The strain localization was observed in the α laths dominated by both prismatic <a > and pyramidal <c + a > slip systems • The α laths with elongation directions 45° to the tensile loading axis have the longest effective slip length, which could make them favorable for deformation • The dislocations multiplication of coarser α laths is more rapid than in fine α laths, which could also result in strain localization

Nano-twinning induced high strain hardening behavior in a metastable as-cast Co30Cr30Fe20Ni20 high entropy alloy at room temperature

High entropy alloys (HEAs) utilize multiple deformation mechanisms to overcome the strength-ductility trade-off by deformation treatment and low temperature conditions. To develop original engineering materials with strength-ductility synergy properties, we induced multiple deformation strengthening mechanisms into the as-cast face-centered cubic (FCC) HEAs at room temperature. A metastable as-cast Co 30 Cr 30 Fe 20 Ni 20 HEA was designed by adjusting composition and prepared by melting. The tensile properties and deformation microstructure evolution were investigated. Co 30 Cr 30 Fe 20 Ni 20 HEA has larger lattice distortion, stronger atomic binding force, and lower stacking fault energy (SFE) than equiatomic CoCrFeNi HEA. The low SFE and large grain size promote multiple slip systems (single slip, crossing slip, and blocked slip) activation and the formation of deformation twins. The as-cast Co 30 Cr 30 Fe 20 Ni 20 shows excellent properties at room temperature, including yield strength of 267 MPa, tensile strength of 613 MPa, and fracture strain of 96.2%, which increases by 92.2%, 47.9%, and 10.50% than that of equiatomic CoCrFeNi HEA. The enhancement of yield strength results from solid solution strengthening and strong atomic binding forces. The tensile strength-ductility synergy is attributed to a high strain hardening rate by utilizing multiple slip interactions of dislocations and twinning-induced plasticity (TWIP). • A metastable as-cast Co 30 Cr 30 Fe 20 Ni 20 HEA was designed by adjusting Co and Cr composition. • Co 30 Cr 30 Fe 20 Ni 20 HEA has large lattice distortion, strong atomic binding force, and low stacking fault energy. • As-cast Co 30 Cr 30 Fe 20 Ni 20 shows excellent strength-ductility synergy at room temperature, including tensile strength of 613 MPa and elongation of 96.2%. • Properties improvement is attributed to high strain hardening rate by utilizing multiple slip interactions of dislocations and twinning-induced plasticity.

Automated identification of slip system activity fields from digital image correlation data

Crystallographic slip system identification methods are widely employed to characterize the fine scale deformation of metals. While powerful, they usually rely on the occurrence of discrete slip bands with clear slip traces and can struggle when complex mechanisms such as cross-slip, curved slip, diffuse slip and/or intersecting slip occur. This paper proposes a novel slip system identification framework, termed SSLIP (for Slip Systems based Local Identification of Plasticity), in which the measured displacement gradient fields (from Digital Image Correlation) are locally matched to the kinematics of one or multiple combined theoretical slip systems, based on the measured crystal orientations. To identify the amount of slip that conforms to the measured kinematics, an optimization problem is solved for every datapoint individually, resulting in a slip activity field for every considered slip system. The identification framework is demonstrated and validated on an HCP virtual experiment, for discrete and diffuse slip, incorporating 24 slip systems. Experimental case studies on FCC and BCC metals show how full-field identification of discrete slip, diffuse slip and cross-slip becomes feasible, even when considering 48 slip systems for BCC. Moreover, the methodology is extended into a dedicated cross-slip identification method, which directly yields the orientation of the local slip plane trace orientation, purely based on the measured kinematics and on one or two chosen slip directions. For even more challenging cases revealing a persistent uncertainty in the slip identification, a two-step identification approach can be employed, as is demonstrated on a highly challenging HCP virtual experiment.

Experimental and Crystal Plasticity Finite Element Study of the Deformation Behavior of High‐Mn Steel Micropillars

Experimental and crystal plasticity finite element modeling approaches are used to predict the deformation behavior of micropillar in the 16Mn steel. The intrinsic (crystallographic orientation) and extrinsic (friction coefficient on the micropillar) factors are considered for both experiment and modeling on the deformation behavior of micropillars. When the friction coefficient is low, buckling is occurred by slipping on the top surface of the micropillar, while barreling is accommodated in the high friction coefficient condition. In the multiple slip system, the influence of the friction coefficient is negligible on the deformation behavior of the micropillar because slip initiation can occur at multiple points.

Microstructure and mechanical properties of gradient ultrafine-grained Mg-Gd-Zr alloy

Fabricating gradient structure (GS) is increasingly adopted in structural engineering materials due to the unique combinations of mechanical properties. In the present work, a gradient ultrafine-grained structure with a change in the grain size up to three orders of magnitude from nano to micrometer is prepared on Mg-2Gd-0.4Zr (wt.%) alloy sheet with coarse grains (CG) by sliding friction treatment (SFT). Compared with homogeneous CG structure, the yield strength of GS + CG + GS is improved from 126 MPa to 243 MPa, acquiring an increase of 93%, and maintains an excellent ductility of 25%, showing an extraordinary strength-plasticity combination among the reported GS-containing Mg alloys. Two-beam condition TEM and EBSD analyses reveal that the refined grain structure and grain orientation modification can promote the activation of multiple dislocation slip systems such as <a> and <c+a> slipping in the GS area, which improves the coordinate ability of plastic deformation. In addition, GS + CG + GS owns higher strength, ductility and better strain hardening ability than that of the GS + CG sample. This is because the symmetrical sandwich structure can alleviate the strain localization so exhibits a better work hardening effect and resistance for failure. This work sheds light on the deformation behavior of heterostructured material and provides a new strategy to improve the mechanical properties of wrought Mg alloys.

Fracture toughness and martensitic transformation in type 316LN austenitic stainless steel extra-thick plates at 4.2 K

The plane strain fracture toughness of type 316LN nitrogen-strengthened austenitic stainless steels depends on their stability at 4.2 K, which is characterized by the Md30 index. Deformation microstructures and martensite that developed at the crack tip of type 316LN steel extra-thick (535 mm) plates at 4.2 K were evaluated. Two kinds of type 316LN steel, LF and HF, represented low and high fracture toughness, respectively. The Md30 of the plates correlated well with their respective fracture toughness values, where a higher Md30 indicated lower fracture toughness. The plastic strains developed in austenite grains at the crack tip for the HF were higher and more homogeneous and extensive than those of the LF. The volume of α′-martensite detected in the HF was lower than that of the LF. Planar slip bands with the primary slip system and α′-martensite were dominant in the LF. In the HF, however, microslip bands with multiple slip systems, deformation twins, and α′-martensite formed. For the LF, extensive martensite leaves with multiple variants were formed at the crack tip. The martensite leaves grew along the high stress field at crack tip developed by the crack extension. For the HF, α′-martensite platelets were dispersed along {111} slip bands. Shear bands containing martensite were favored by low plastic strains and constituted a smaller fraction of their total density. Therefore, low plastic strains were responsible for an increase of α′-martensite formation at the crack tip. An alloy design having a higher Ni, Mn and Mo, and lower N content in its chemical composition would be favorable for type 316LN steel to provide a higher fracture toughness owing to higher stacking fault energy.

Superior tensile properties of a novel as-cast non-equimolar Zr45Ti15Nb20Ta20 complex concentrated alloy

A high specific strength (∼137 MPa⋅g−1⋅cm3) Zr45Ti15Nb20Ta20 refractory complex concentrated alloy (RCCA) with fine grain size (∼20.42 μm) was investigated. The as-cast alloy exhibits a stable BCC structure with a yield strength is ∼1087 MPa and elongation of ∼11.8 %. The activation of multiple slip systems and the deformation dominated by cross-slip make the alloy have excellent plastic deformability. The dislocation-dominated deformation provides strain-hardening capability on the room temperature (RT) tensile deformation. The discovery of this novel alloy provides a new RCCAs design strategy to meet the actual needs of different scenarios.

Crystallographic orientation-dependent deformation characteristics of additive manufactured interstitial-strengthened high entropy alloys

In this study, laser powder bed fusion (LPBF) was used for the fabrication of an interstitial-strengthened high entropy alloy (iHEA), Fe49.5Mn30Co10Cr10C0.5 (at.%). The as-fabricated iHEA possesses excellent strength-ductility synergy during tensile loading, with fracture strength reaching up to 1109 MPa at 37% engineering strain. Electron backscatter diffraction (EBSD) and high energy synchrotron X-ray diffraction were used to evaluate the microstructural characteristics of the material. In-situ EBSD analysis during uniaxial tensile testing was performed to unveil the deformation mechanisms. Moreover, crystallographic orientation-specific micropillar compression tests were conducted to determine how the grain deformation characteristics differ between orientations. Due to the activation of multiple slip systems and the homogeneous plastic flow, the [111] orientation demonstrated a higher yield strength and continuous work hardening rate. This research helps in clarifying grain orientation-specific contributions to the bulk mechanical response of additively manufactured HEA.

Avoiding oxygen-induced early fracture in titanium with high strength via entangled grains through laser powder bed fusion

Titanium (Ti) samples with oxygen contents of 0.13% (weight %) (0.13%O-Ti), 0.18% (0.18%O-Ti) and 0.24% (0.24%O-Ti) are printed through laser powder bed fusion (L-PBF) process. With increasing oxygen content, yield strength of L-PBF Ti under tensile testing increases without losing ductility, and becomes larger than that of conventionally produced Ti. Probably this is not resulted from even oxygen distribution, because nano-scale oxygen segregation is observed in 0.24%O-Ti through high-resolution scanning transmission electron microscopy (STEM). In order to get insight into fundamental mechanism of the oxygen-induced early fracture avoidance and high strength, tensile testing of L-PBF Ti is followed by quasi-in-situ electron backscatter diffraction (EBSD)/backscattered electron microscopy (BSEM). It is found that avoidance of the oxygen-induced early fracture and high strength are probably attributed to extensive entangled grains, which promotes formation of multiple slip systems and prevents the propagation of intergranular crack.

TitaniQ temperatures and textural analysis as a record of the deformation history in a major continental shear zone system, Borborema Province, Brazil

We combine quartz textural analysis with Titanium-in-quartz (TitaniQ) geothermometry to investigate the late stages of deformation in mylonitic rocks from the Sucuru dike swarm in the Borborema Province. The wide range in TitaniQ temperatures (>340 to 740 °C) closely related to grain sizes are attributed to inefficient Ti resetting during bulging (BLG) and subgrain rotation (SGR) recrystallization. The highest temperature values (>500–740 °C) occur in the core of quartz ribbons and porphyroclasts. They are interpreted as the record of early-stage plastic deformation, likely during the intrusion of magmatic bodies, and are progressively overprinted during a lower-temperature stage (>340–500 °C) associated with intense dynamic recrystallization and production of a large amount of fine-grained matrix. The dominantly weak crystallographic preferred orientations (CPOs) can be attributed to a large variety in crystallographic orientations of parent grains, which control to a large extent the CPO evolution of daughter grains, and to activation of multiple slip systems in the <a > direction (basal, rhomb and prism) due to synkinematic temperature decrease. The CPOs are consistent with strain localized by operation of dislocation creep-accommodated grain boundary sliding (DisGBS) during the low-temperature stage. DisGBS seems to be less important in coarse pure quartz domains, where moderate to strong CPOs indicate the dominance of dislocation creep. Several evidence of intracrystalline deformation in quartz ribbons and porphyroclasts point to dominant activation of dislocation creep at the early high-temperature stage of deformation, with activation of prism <c>. The dike orientations, geometry and evidence of solid-state deformation in their margins, fit well with progressive E-W dextral shearing, where they intrude and crystallize in a NW-SE direction and rotate clockwise during shearing. This implies that the dextral shear regime that dominated the Borborema province from ca. 600 Ma was still in effect into the Cambrian some 60–70 million years later.

The effect of deformation temperature on recrystallization in a Ni-based single crystal superalloys

A series of experiments have been carried out to investigate the effect of deformation temperature on the recrystallization of nickel-based single crystal superalloys using isothermal (compression) and non-isothermal (directional solidification) deformation. The recrystallization features induced by accumulated plastic strain in two temperature ranges (1300–1150 °C and 600–400 °C) are distinctly different from that induced by deformation in one temperature range (1300–1150 °C). The different recrystallized features (area and number of grains) induced by isothermal deformation at temperatures ranging from room temperature to solidus can be attributed to different dislocation features, such as slip band, KW-lock, γ/γ' interfacial dislocations and Low Angle Boundary. Microscopic observations disclose that interactions of multiple slip systems have higher tendency of recrystallization. A close view between deformed and recrystallized samples of different deformation temperatures reveals that the critical kernel average misorientation (KAM) to induce recrystallization is about 0.4°. Furthermore, the number of recrystallized grains is proportional with the average KAM value based on the experimental measurements in a wide temperature range. Isolated recrystallized grains are observed at the stress concentration locations in the low plastic strain zone due to local high misorientation.

Path Dependency of Plastic Deformation in Crystals: Work Hardening, Crystallographic Rotation and Dislocation Structure Evolution

This paper reviewed the research progress of studies on the crystal rotation of single crystals that were deformed by tension and shear and the influences of crystal rotation and dislocation evolution on strain hardening behavior in crystals that were deformed with different initial orientations. The crystal rotation is entirely different depending on whether the single crystal was deformed by tension or shear. A three-stage work hardening behavior, which is not one of the intrinsic properties of materials, is generated when FCC metallic single crystals are deformed by tension along unstable oriFigurFigurentations, but single crystals do not exhibit this three-stage hardening behavior when they are deformed by simple shear at room temperature. Under tension, crystal rotation causes the transition from work hardening stage I to stage II, while the transition from work hardening stage II to III is caused by dislocation evolution. The evolution of the dislocation structure is related to deformation loading and can be classified into three types when a crystal is deformed by tension. Different from tension, shear stress can directly act on one of the 12 slip systems when a crystal is deformed by simple shear. When FCC single crystals are deformed by shear along the (11¯1)[110], (111)[112¯] and (001)[110] orientations, the single slip system, co-planar slip systems and co-directional slip systems are activated, respectively, and the crystals hardly rotate under the shear conditions. The slip direction of [110] forces the crystal to rotate toward the shear direction under simple shear. The dislocation tangles tend to form the dislocation cells and wall structures when multiple slip systems are activated under simple shear.

Dynamic recrystallization and deformation constitutive analysis of Mg–Zn-Nd-Zr alloys during hot rolling

Due to limited basal slips, the recrystallization of Mg–Zn-Nd-Zr alloys originated at grain boundaries with large orientation gradient based on strain-induced-boundary-migration nucleation mechanism. The primary driving force of the recrystallization was the distortion energy difference around grain boundaries. However, more non-basal slips occurred near the grain boundary at larger strain deformation, so that some dislocations of adjacent subgrain boundaries with less orientation difference transferred to other subgrain boundaries by means of dissociation and disassembly, which is called the subgrain boundary merging. Meanwhile, some ductile shear zones appeared surrounding the primary grain, which held multiple slip systems moving around the grain boundary, gave rise to larger orientation gradients and facilitated recrystallization grains growing.

Life prediction and damage analysis of creep-fatigue combined with high-low cycle loading by using a crystal plasticity-based approach

Many high-temperature rotating components are always subjected to complex loading waveforms, arising the concerns for multi-damage driving crack initiation. In this paper, a series of strain-controlled low cycle fatigue (LCF) and creep-fatigue interaction (CFI) tests as well as the novel creep-fatigue combined with high-low cycle (CF-HL) tests were performed in a nickel-based superalloy at 650 ℃. Then, post-test microstructure observations were carried out to reveal the damage mechanisms under the multi-damaged CF-HL loading based on the EBSD-TEM combinative characterizations. In this aspect, the single-slip-dominated deformation mechanism under CFI loadings transformed to double-slip-dominated one under CF-HL loading was revealed. Computationally, a numerical procedure with a combination of crystal plasticity theory and finite element implementation was constructed for predicting the CF-HL crack initiation life and quantifying the crack initiation mechanisms. With the help of the developed fatigue and creep indicator parameters represented by accumulated energy dissipation, good agreements between experimental data and simulated results were achieved within a scatter band of ± 2 on life prediction. In addition, the simulation results indicate that the combined effects of grain orientation and multiple slip system activation showed great influence on the CF-HL crack initiation.

Enhanced plastic deformability of copper single crystals under low-frequency vibrational loading

Tensile tests on copper single crystal ([110]) foils using low-frequency vibrational loading of frequencies ranging from 0 to 50 Hz were performed at room temperature to explore the mechanisms of vibration-facilitated plastic deformation in metallic materials. It was found that the total elongation of copper single crystal foils under vibrational loading increased gradually with increasing vibration frequency, indicating enhanced plastic deformability. This behavior could be rationalized with the formation of equiaxial dislocation cells (EDC) in copper single crystal ([110]) foils under vibrational loading, which reflects the activation of multiple codirectional slip systems induced by the approximate biaxial tensile stress state during the unloading stage of vibrational loading. Additionally, the formation of Lüders bands under vibrational loading at high frequencies validates their dislocation-proliferation-mediated nucleation mechanism. This work would help to understand the vibration-lowered forming resistance in metallic materials of FCC lattice structures.

Potential TRIP/TWIP coupled effects in equiatomic CrCoNi medium-entropy alloy

High-entropy alloys (HEA) and medium-entropy alloys (MEA), containing multiple principal elements typically with equiatomic or near-equiatomic ratios, have drawn considerable attention due to their unique and promising mechanical profiles, such as high tensile strength, superior ductility and exceptional fracture toughness. However, the elementary mechanisms controlling the deformation and fracture of this new class of alloys still need complementary analyses. In the present study, the nucleation and growth mechanisms of deformation bands in the medium-entropy CrCoNi alloy were investigated by atomic-resolution scanning transmission electron microscopy (STEM). It was revealed that planar dislocation slip is the dominant deformation mode in the early stages of deformation. With increasing strain, both deformation twins and hexagonal close packed (HCP) lamella simultaneously appear. Careful analysis of the dislocations involved in these processes confirms that two different mechanisms are responsible for the nucleation of deformation bands in the CrCoNi alloy: the three-layer mechanism proposed by Mahajan et al. and the transformation from HCP phase to twin. Activation of multiple slip systems at larger deformation levels leads to the activation of deviation-based mechanisms, which contribute to the twin growth. It was also observed that many HCP bands remain and overlap with nanoscale twins, leading to short range HCP-twin stackings, which contribute to the high work hardening rate of this alloy.

Quasi-in-situ study on the crystallographic lattice rotation of tantalum during compression deformation

By employing a quasi-in-situ method, we investigated the crystallographic texture evolution process of high purity tantalum during compression deformation. The influence of grain size, aspect ratio, and initial orientations on the crystallographic lattice rotation process was analyzed in-depth using the electron backscatter diffraction technique. It is found that the randomly distributed grain orientations mainly evolve toward {111} and {100} orientations during compression, forming the two typical deformation textures in body-centered cubic metals. Furthermore, the crystallographic rotation direction and degree have no apparent linear relationship with grain size or grain aspect ratio but are mainly related to the grain orientation. Especially, the slight crystallographic rotation angle for {100} grain is primarily associated with the activation of multiple slip systems. Multiple slip systems mean different slip planes and different slip directions, and it is difficult for {100} grains to be dominated by a certain slip system and rotate to a large extent like {111} grains. In comparison, the large crystallographic rotation angle for {110} grain is mainly related to the strain coordination in local regions. {110} grains are easier to active slip systems than grains with other crystal plane orientations inferred from the critical shear stress, thereby the rotation of grains and inhomogeneous deformation occurred during the compression process.

Mechanism of notch sensitivity elimination via precipitating Laves phases at grain boundaries in a low expansion superalloy

The effect of Laves phase on the notch sensitivity of Thermo-Span alloy has been systematically investigated in the study via scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). The results show that amounts of secondary Laves phase increase and γ′ phase decrease with the dissolution of primary Laves phase. The γ′ phase depletion zones are formed at the vicinity of secondary Laves phases, which are the critical factor eliminating the notch sensitivity. The γ′ phase depletion zones around secondary Laves phases at grain boundaries (GBs) resulted in a decrease in GBs strength along with improvement in plasticity. Therefore, GBs can accommodate high strain localization and produce Geometrically Necessary Dislocation (GND) to coordinate incompatible deformation among neighboring grains. Thus, the resistance to crack initiation and propagation at GBs is improved. Subsequently, the accumulated strain at GBs activates multiple slip systems to further coordinate deformation between GBs and intragranular regions.