Favorable Slip System Research Articles

New route for fabricating low-carbon ductile martensite to optimize the stretch-flangeability of dual-phase steel

Traditional cold-rolling dual-phase steel (CR sheet), limited by poor stretch-flangeability as indicated by the hole expansion rate (HER), faces challenges in manufacturing complex automobile parts. To address this, a new type of hot-rolling dual-phase steel with an 800 MPa grade (manufactured by tyipical thin slab continuous casting and rolling process, TSCCR) was designed to replace the traditional one. The TSCCR sheet, with a lower carbon content of 0.047wt.%, compared to 0.093wt.% in the CR sheets, features 40.6% martensite significantly higher than 28.2% in the CR sheet. This increased martensite content, despite being softer and more ductile due to its low hardness, compensates for the tensile strength in the TSCCR sheet. Furthermore, the TSCCR sheet exhibits a unique heterostructure characterized by uneven carbon distribution from the ferrite/austenite interface to other boundaries/interfaces, estimated by the negligible partition local equilibrium (NPLE) model of ferrite transformation. This unique heterostructure diminishes the mechanical disparity between ferrite and martensite, resulting in a higher yield strength of 568 MPa, compared to 456 MPa in the CR sheet. During uniaxial tension, the ferrite and ductile martensite in the TSCCR sheet initiate cooperative plastic deformation earlier than in the CR sheet, despite initially having fewer favorable slip systems in the TSCCR martensite. The low working hardening rate of the TSCCR sheet facilitates the forming layered structure of ferrite and martensite during necking compared to the CR sheet. Consequently, the HER of the TSCCR sheet sharply increased by 144% compared to the CR sheet. This enhancement in HER can be attributed to the crack initiation area, longer crack propagation path, and higher crack propagation resistance facilitated by the ductile martensite, culminating in superior HER.

Unveiling the kinetics of interphase and random quaternary Nb[sbnd]Ti precipitations, and strengthening mechanism of HSLA steel

Traditional methods for analyzing multi-element precipitation are often time-consuming, labor-intensive, and expensive. Particularly, the initial nucleation stages of such precipitations remain poorly understood. A revised precipitation theory was employed to elucidate the nucleation and growth kinetic mechanisms of quaternary (Ti, Nb)(C, N) precipitation with two modes: interphase precipitation (IP) along the migrating α/γ interface and random precipitation (RP) along dislocations. The findings indicated that TiN preferentially nucleates at the ferrite side compared to that at the austenite side. The nucleation occurring at α/γ interface leads to a precipitation particle angle of approximately 5.3° within a single IP sheet. The interface provided more favorable nucleation sites than dislocations, significantly reducing the relative nucleation time. Additionally, the initial yielding behavior and strengthening mechanism were analyzed based on the two factors of the hot-rolling microstructure and precipitation. A favorable slip system was along the RD-ND plane as the texture index of α-{110} approximately 3.74, compared to 2.20 along the TD-ND plane, resulting in the lower upper yield strength. Once the number density of mobile dislocations increased during plastic deformation, the tensile stress decreased, leading to the local plastic instability. Finally, (Ti, Nb)(C, N) contributed to a strengthening increment of about 154 MPa. This study provides critical insights into the precipitation behavior in HSLA steels, offering significant implications for the design and development of advanced materials with enhanced mechanical properties.

Modeling of the intrinsic softening of γ-carbides in cemented carbides

Cemented carbides are widely used materials in industrial applications due to their remarkable combination of hardness and toughness. However, they are exposed to high temperatures during service leading to a reduction of their hardness. A common practice to damp this softening is to incorporate transition metal carbides in cemented carbide compositions, which keeps the hardness relatively higher when temperature increases. Understanding the underlying mechanisms of this softening is crucial for the development of cemented carbides with optimal properties. In this work, atomic-scale mechanisms taking place during plastic deformation are analyzed and linked to the effect that they have on the intrinsic macro-scale softening of the most common TMC used in cemented carbides grades (TiC, ZrC, HfC, VC, NbC and TaC). The proposed model uses the generalized stacking fault energy obtained from density functional theory calculations as an input to Peierls-Nabarro analytical models to obtain the critically resolved shear stress needed for deformation to occur in different slip systems. Subsequently, this information is used to predict the hardness variation across the temperature service range experienced by cemented carbides in wear applications.In addition to the prediction of hot-hardness for TMC, the obtained results also offer valuable insights into the intrinsic mechanisms governing TMCs deformation. The results facilitate the identification of dominant dislocation types influencing plasticity within distinct temperature regimes, define energetically favorable slip systems, and predict the brittle-ductile transition temperature in these materials. For instance, for group IV carbides at low temperatures, the slip system with a lower GSFE is {110}<11̅0> and around 30% of their melting temperature, the GSFE of partial slip in {111}<12̅1> becomes lower, changing the dominant slip mechanism and characterizing the Brittle-Ductile transition.

In-situ EBSD study of 409 L ferritic stainless steel during tensile testing

Two ferritic stainless steel (FSS) specimens, denoted as loading axis along the rolling direction(LR) and the transverse direction(LT) respectively, were produced to elucidate the mechanical anisotropyof409L FSS at grain scale. This approach was realized by the combination of in situ tensile test and field emission scanning electron microscopy (SEM) with electron backscattering diffraction (EBSD) at room temperature. Microstructure evolution, grain orientation rotation, and crystallographic slip were investigated in the tensile test. During tensile deformation, the tensile axis of LR specimens rotated towards the 〈101〉 direction, which is the stable end orientation of body-centered cubic (BCC) metals. However, the rotation of tensile axis towards 〈101〉 was restrained in LT specimens due to the operation of less favorable slip systems. {110}〈111〉 was the most favorable slip system in both specimens. The mechanical anisotropy in grain scale is due to different slip behaviors of LR and LT specimens.

Characterization and statistical modeling of texture and microstructure evolution in dynamically fractured electron beam melted Ti-6Al-4V

Two different Ti-6Al-4V cylindrical rods, horizontally and vertically built, fabricated through the electron beam melting technique, were underwent compression loadings to failure at the strain rates of 2350s−1 and 1750s−1, respectively. Low-angle grain boundary formation, dislocation array, and dislocation pinning were observed and attributed to the stress-induced dislocation formation in the as-built microstructure. Superior strength at each strain rate in vertically built samples was concluded to be a consequence of its finer microstructure and the presence of martensite α˙. Hardness measurements revealed higher values at the areas close to the fracture surface. Electron microscopic characterization revealed parallel-twin formation resulting from adiabatic temperature rise, increasing short-ranged clustering, and the high stacking fault energy. Dynamic compressive deformation led to the appearance of dislocation structure, cell blocks, and extended dislocation walls formation. Texture analysis showed <c+a> type pyramidal slip systems and contraction twins as the most favorable slip systems. Texture evolution interpretations from the region far from the area close to the fractured surface indicated that mean grain size decreased, and higher dislocation densities were obtained. The more preferred texture tended to rotate by approaching the fracture surface so that the basal plane became parallel to the fracture surface, which is directly related to the facilitation of crack propagation. Moreover, the generalized spherical harmonics were used to apply 2-point statistics on the texture and then statistically compare the texture changes. The results were in good agreement statistically, where principal components (PC) were utilized to explain variances in the database.

VHCF damage in duplex stainless steel revealed by microbeam energy-dispersive X-ray Laue diffraction

Microstructure of austenitic-ferritic duplex stainless steel loaded in the Very High Cycle Fatigue regime was investigated using microbeam energy-dispersive X-ray Laue diffraction. Scanning electron microscopy analysis of the surface shows that damage in the form of fatigue cracks is initiated at grain boundaries assisted by slip bands observed in austenite grains. Energy-dispersive X-ray Laue diffraction was then used to scan a damaged area containing both a fatigue crack and slip bands, in order to measure the changes in microstructure. Results from the X-ray data from the austenite grain indicates slip activation of the most favored slip system tilted by 45° with respect to the external loading direction, dividing the grain into two regions on either side of the slip band. In the ferrite phase, in front of the crack, variations in the angle and energy spectra of the diffraction peaks indicate the presence of lattice curvature and a strain gradient. In regions around the crack, diffraction peaks spatially split into several sub-peaks indicating the presence of fine granular areas separated by polarized dislocation walls. Possible reasons for the observed structural evolution are discussed and the advantages of using energy-dispersive X-ray Laue diffraction in fatigue damage analysis are illustrated.

Effect of lamellar α on the stress corrosion cracking of Ti-6Al-4 V alloy in simulated oilfield brine

The stress corrosion cracking (SCC) behavior of Ti-6Al-4 V alloy with different lamellar α microstructures in simulated oilfield brine was investigated in this study. The SCC susceptibility of the Widmanstätten sample was 11.0%, lower than that of the bimodal and trimodal samples (19.9%, and 18.0%, respectively). In the Widmanstätten sample, the crack growth path transitioned from a zigzag to a linear path when the crack moved from one colony to another because of the sliding variations in the slip bands with different orientations. The decrease in a SCC resistance of bimodal and trimodal samples resulted from the decrease in α/β colony size. Owing to the larger thickness and different orientation of lamellar α, the trimodal sample exhibited a slightly higher SCC resistance and more circuitous crack propagation than that in the bimodal sample. The deflection angle of the crack increased with an increase in the angle between the slip system, and the favorable slip system was prismatic in three samples. It can be concluded that one of the strategies to improve the SCC and crack growth resistance of Ti-6Al-4 V alloy in oil field applications is to tailor the lamellar α microstructure with different orientations and larger α colonies.

Atomistic modeling of anisotropic mechanical properties of lanthanum zirconate nanocystal

Lanthanum zirconate (La2Zr2O7, or LZ) has been widely recognized as a promising candidate material for thermal barrier coating (TBC) applications since it has low thermal conductivity, high-temperature phase stability, and low sintering activity. However, the mechanical properties of LZ crystal have not been fully understood, which hinders it from future applications. In this work, atomistic simulations were performed to study the anisotropic mechanical properties of LZ crystal. Using both the first principles and molecular dynamics (MD) calculations, uniaxial tensile tests of LZ crystal in [001], [011], and [111] directions were simulated. The stress-strain curves of the tensile tests were calculated, and the ultimate tensile strength and toughness were derived. The Young's moduli in [001], [011], and [111] directions were calculated using both the stress-strain curves and an analytical method for small deformation. Additionally, shear stress-strain curves in {111}<110> and {111}<112‾> directions were investigated using both the first principles calculations and the MD method. Results show that Young's modulus of LZ crystal is highly anisotropic. The crystal has the highest Young's modulus in [111] direction, and {111}<112‾> direction is the favorable slip system during shear deformations.

Molecular Interpretation of the Compaction Performance and Mechanical Properties of Caffeine Cocrystals: A Polymorphic Study.

The 1:1 caffeine (CAF) and 3-nitrobenzoic acid (NBA) cocrystal (CAF:NBA) displays polymorphism. Each polymorph shares the same docking synthon that connects individual CAF and NBA molecules within the asymmetric unit; however, the extended intermolecular interactions are significantly different between the two polymorphic modifications. These alternative interaction topologies translate to distinct structural motifs, mechanical properties, and compaction performance. To assist our molecular interpretation of the structure-mechanics-performance relationships for these cocrystal polymorphs, we combine powder Brillouin light scattering (p-BLS) to determine the mechanical properties with energy frameworks calculations to identify potentially available slip systems that may facilitate plastic deformation. The previously reported Form 1 for CAF:NBA adopts a 2D-layered crystal structure with a conventional 3.4 Å layer-to-layer separation distance. For Form 2, a columnar structure of 1D-tapes is displayed with CAF:NBA dimers running parallel to the (110) crystallographic direction. Consistent with the layered crystal structure, the shear modulus for Form 1 is significantly reduced relative to Form 2, and moreover, our p-BLS spectra for Form 1 clearly display the presence of low-velocity shear modes, which support the expectation of a low-energy slip system available for facile plastic deformation. Our energy frameworks calculations confirm that Form 1 displays a favorable slip system for plastic deformation. Combining our experimental and computational data indicates that the structural organization in Form 1 of CAF:NBA improves the compressibility and plasticity of the material, and from our tabletability studies, each of these contributions confers superior tableting performance to that of Form 1. Overall, mechanical and energy framework data permit a clear interpretation of the functional performance of polymorphic solids. This could serve as a robust screening approach for early pharmaceutical solid form selection and development.

Effect of slip transmission at grain boundaries in Al bicrystals

The effect of slip transfer on the deformation mechanisms of Al bicrystals was explored using a rate-dependent dislocation-based crystal plasticity model. Three different types of grain boundaries (GBs) were included in the model by modifying the rate of dislocation accumulation near the GB in the Kocks-Mecking law, leading to fully-opaque (dislocation blocking), fully-transparent and partially-transparent GBs. In the latter, slip transmission is only allowed in pairs of slip systems (SS) in neighbour grains that are suitably oriented for slip transfer according to the Luster-Morris parameter. Modifications of the GB character led to important changes in the deformation mechanisms at the GB. In general, bicrystals with fully-opaque (dislocation blocking) boundaries showed an increase in the dislocation density near the GB, which was associated with an increase in the Von Mises stress. In contrast, the dislocation pile-ups and the stress concentration were less pronounced in the case of partially-transparent boundaries as the slip in one grain can progress into the next grain with some degree of continuity. No stress concentrations were found at these boundaries for fully-transparent boundaries, and there was continuity of strain across the boundary, which is not typical of most experimentally observed GBs (Hémery et al., 2018; Bieler et al., 2019). Simulations of ideal bicrystals oriented for favorable slip transfer on the most highly favored slip system in grains with high Schmid factors for slip transfer depends on the number of active SS in operation in the neighborhood indicating that most boundaries will lead to nearly opaque conditions while some boundaries will be transparent. Finally, the model was applied to a particular experimentally observed GB in which slip transfer was clearly operating indicating that the model predicted a nearly transparent GB.

Creep deformation related to γʹ phase cutting at high temperature of a [111] oriented nickel-base single crystal superalloy

The configurations of superdislocations in a [111]-oriented nickel-base single crystal superalloy have been investigated after creep deformation at 1100 °C/150 MPa. The superdislocation arrays and hexagonal superdislocation networks are formed in irregular rafted γʹ phase, which are rarely observed in [001] crept specimens. It is believed that the superdislocation networks are formed by reactions of superdislocation arrays and another kind of superdislocations. Careful diffraction contrast analysis shows that the Burgers vector of the superdislocation arrays is a[011¯], which is not the favorable slip system when loaded along [111]. The formation of the arrays is supposed to be related to the matrix dislocations cross slipping in two {001} channels and the stress concentration caused by inhomogeneous rafted structures. The networks in γʹ phase may prevent dislocation movement and slash the increase of creep rate to some extent.

Stronger and more failure-resistant with three-dimensional serrated bimetal interfaces

Low-energy structures of bimetal interfaces commonly occur in nature, yet higher energy forms, made by deviations in the interface plane, are also likely. While these variants may occur less frequently, they can still play an important role in the response of the material under deformation, by acting as preferential sites for defect formation and boundary motion. Here, using atomic-scale simulation and interface defect theory and considering two bimetal systems, Cu/Ag and Cu/Nb, we show that high-energy interfaces can achieve a local, low-energy state by forming atomic-scale serrations and that the predicted size and location of the serrations are consistent with experimental observation. For several distinct strain states, we reveal that interfaces with atomic-scale serrations bear both higher barriers for dislocation nucleation and higher resistances to interfacial shear compared to their planar interface variants. This desirable combination is not a characteristic of the more commonly studied low-energy interfaces, which typically possess a high nucleation barrier and low sliding resistance. We explain, through an analysis of the misfit dislocation structure, that the serrations alter the number of dislocations emitted, change the favorable slip systems, and alleviate the stress concentrations generated by misfit dislocations. An interface engineering strategy is then proposed for designing atomically serrated interfaces to improve mechanical strength and hinder localization and creep of metallic nanomaterials.

In Situ Neutron Diffraction Analyzing Stress-Induced Phase Transformation and Martensite Elasticity in [001]-Oriented Co49Ni21Ga30 Shape Memory Alloy Single Crystals

Recent studies demonstrated excellent pseudoelastic behavior and cyclic stability under compressive loads in [001]-oriented Co–Ni–Ga high-temperature shape memory alloys (HT-SMAs). A narrow stress hysteresis was related to suppression of detwinning at RT and low defect formation during phase transformation due to the absence of a favorable slip system. Eventually, this behavior makes Co–Ni–Ga HT-SMAs promising candidates for several industrial applications. However, deformation behavior of Co–Ni–Ga has only been studied in the range of theoretical transformation strain in depth so far. Thus, the current study focuses not only on the activity of elementary deformation mechanisms in the pseudoelastic regime up to maximum theoretical transformation strains but far beyond. It is shown that the martensite phase is able to withstand about 5% elastic strain, which significantly increases the overall deformation capability of this alloy system. In situ neutron diffraction experiments were carried out using a newly installed testing setup on Co–Ni–Ga single crystals in order to reveal the nature of the stress–strain response seen in the deformation curves up to 10% macroscopic strain.

Ab-initio calculation of the [formula omitted]-surface and cleavage energy in the B2 FeCo intermetallic compound

The plastic-deformation behavior of the FeCo intermetallic compound with CsCl-type B2 structure is investigated using ab initio techniques. For that purpose, stacking-fault energies (intrinsic, extrinsic, and twinning), the entire γ-surface, the shear strength, the cleavage energy, and the cleavage strength are calculated in {11¯2} and {110} planes of the compound single crystal. Analyzing γ-surfaces reveals the existence of stable anti-phase boundaries in both planes. Moreover, the favored slip system in this compound is found to be {110}[1¯11]. The brittle/ductile response of the material is investigated within the Rice model. It is found that both planes are equally important when investigating the brittle/ductile behavior of this compound. In agreement with observed experiments, the model successfully predicts the brittle behavior of this compound against an applied stress.

Deformation behavior of Mo5SiB2

First-principles calculations were employed to analyze the possible slip systems in Mo5SiB2. A striking result was obtained that the three most favorable slip systems, <100>(001), <110>(001) and [001]{010}, have close stacking fault energies, and the preference among them cannot be established. This finding explains a large variety of experimentally observed slip systems in Mo5SiB2. The dislocations associated with these slip systems may dissociate into partials joined with stacking faults and separated by the large splitting width of 5–6 nm.

Texture development during progressive deformation of hematite aggregates: Constraints from VPSC models and naturally deformed iron oxides from Minas Gerais, Brazil

We show that naturally-deformed hematite from the Quadrilátero Ferrífero Province, Minas Gerais, Brazil, develops CPOs by dislocation creep, strongly influenced by basal plane parallel glide, even when this is not the favored slip system. Characterization of microstructure and texture, particularly intragranular misorientations, of naturally deformed hematite aggregates by EBSD allowed us to determine the importance of different slip systems, and confirm dislocation creep as the dominant deformation mechanism. Viscoplastic self-consistent (VPSC) models were constructed to constrain the slip systems required to operate for the observed CPO to develop, and its rheological implications. Changes in the CRSS ratio of hematite prism and basal slip systems and deformation regime lead to the development of distinct patterns of hematite crystallographic orientations. The basal slip-dominated simple shear model is the only one that can develop quasi-single-crystal CPO of the kind observed in highly deformed rocks from Quadrilátero Ferrífero. Comparison between naturally deformed hematite aggregates and VPSC models shows that CPO development of hematite is strongly influenced by a highly viscoplastic anisotropy through dislocation creep on hematite basal plane. Nonetheless, our results demonstrate that even the unfavorable slip systems should be regarded when the bulk rheology of mineral aggregates is evaluated.

Grain Boundary Responses to Heterogeneous Deformation in Tantalum Polycrystals

The evolution of heterogeneous deformation in a tantalum polycrystal was examined during a three-point bending experiment using electron backscatter pattern mapping. Slip bands formed at strains as low as 1%, and they became more intense with strain. Heterogeneous deformation was evident as intragranular orientation gradients as large as 30° were observed after a strain of about 8%. Nonmonotonic changes in the local average misorientation distribution were observed, implying that dislocation substructure developed in a complex manner. Slip bands were analyzed using plane traces computed from local orientation information. With the assumption of uniaxial stress, Schmid factors for favorable slip systems were identified for each grain and compared with observations, showing evidence for macroscopic activity on both {110} and {112} slip systems. Reconstructed boundary data were used to estimate the geometric potential for slip transfer at grain boundaries. The correlations indicated that when active slip systems were favorably oriented for slip transfer across the boundary, it was often observed in the form of continuous slip bands aligned across the boundary. In boundaries where geometrical alignment and Schmid factors were not favorable for slip transfer, there was a higher likelihood to form ledges (topographic discontinuities) along the grain boundaries. Dislocation pileups at grain boundaries were also correlated with a low potential for slip transfer.

Nanoscale anisotropic plastic deformation in single crystal GaN

Elasto-plastic mechanical deformation behaviors of c-plane (0001) and nonpolar GaN single crystals are studied using nanoindentation, cathodoluminescence, and transmission electron microscopy. Nanoindentation tests show that c-plane GaN is less susceptible to plastic deformation and has higher hardness and Young's modulus than the nonpolar GaN. Cathodoluminescence and transmission electron microscopy characterizations of indent-induced plastic deformation reveal that there are two primary slip systems for the c-plane GaN, while there is only one most favorable slip system for the nonplane GaN. We suggest that the anisotropic elasto-plastic mechanical properties of GaN are relative to its anisotropic plastic deformation behavior.PACS: 62.20.fq; 81.05.Ea; 61.72.Lk.

Zinc Single-Crystal Deformation Experiments Using a “6 Degrees of Freedom” Apparatus

A new experimental technique to study crystallographic slip system activity in metallic single crystals deformed under a condition of uniaxial stress is applied to study the behavior of Zn single crystals. The experimental apparatus promotes unconstrained shape change of inherently anisotropic materials under a condition of uniaxial stress by allowing three translational and three rotational degrees of freedom during compression; hence, we have named the experiment “6 degrees of freedom” (6DOF). The experiments also use a three-dimensional (3-D) digital image correlation (IC) system to measure full-field displacements, which are used to calculate strain and make direct observations of slip system activity. We show that the experimental results associated with a pristine zinc single crystal are precisely consistent with the theoretical predicted shape change (sample distortion) assuming that the most favored slip system on the basal plane is the only one that is active. Another experiment was performed on a processed and annealed Zn single crystal to investigate slip that is inconsistent with the critical resolved shear stress (CRSS) theory. These experiments on zinc illustrate the ability of the 6DOF experiment, together with image correlation (IC) data, to measure slip system activity with a high degree of fidelity.

Micro and Macro Deformation of Single Crystal NiTi

Abstract We present experimental results on the instrumented Vickers micro-indentation and compression of solutionized Ni-rich NiTi single crystals. The tests are conducted at room temperature where the solutionized Ti-50.9 at percent Ni material is 18 degrees above Af and the solutionized Ti-51.5 at percent Ni material is more than 100 degrees above Af. Aside from elastic deformation, it is discovered that dislocation motion and a reversible stress-induced martensitic transformation influence the micro-indentation response of Ti-50.9 at percent Ni, while the micro-indentation of Ti-51.5 at percent Ni only induces irreversible dislocation motion. The effect of the surface normal orientation on material hardness was negligible in the Ti-51.5 at percent Ni and followed trends anticipated by the activation of favorable slip systems in the Ti-50.9 at percent Ni. Compression tests on the identical Ti-50.9 at percent Ni samples revealed deformation by coupled stress-induced martensite and plastic flow, depending on the crystallographic orientation. The trends in hardness with surface normal orientation were not commensurate with the orientation dependence of the uniaxial compressive transformation or “yield” strength. The ramifications of the results in terms of comparing micro-indentation and macro-compression and the interactions between plasticity and the stress-induced martensitic transformation are discussed.