Pyramidal Slip Research Articles

Rare earth improves strength and creep resistance of additively manufactured Zn implants

Poor mechanical strength and creep resistance limit the orthopedic application of biodegradable Zinc (Zn). In present work, cerium (Ce) was alloyed with Zn using laser additive manufacturing technique. As one kind of rare earth element, Ce possessed high surface activity, which effectively interrupted the grain growth and caused the formation of stable intermetallics, thus contributing to grain refinement strengthening and precipitate strengthening. More significantly, Ce alloying activated more pyramidal slip by means of reducing the critical resolved shear stress during plastic deformation, and resultantly formed the sessile dislocations, which caused the accumulated strain hardening and improved the creep resistance. As a result, Zn-Ce alloy exhibited a considerably improved ultimate tensile strength of 247.4 ± 7.2 MPa, and a reduced creep rate of 1.68 × 10−7 s−1. Moreover, it exhibited strong antibacterial activity, as well as favorable cytocompatibility and hemocompatibility. All these results demonstrated the great potential of Zn-Ce alloy as a candidate for bone repair application.

Bauschinger-Like Effect of AZ31 Magnesium Alloy Wide Sheet During the Straightening Process

The Bauschinger-like effect, which showed up in the straightening process of the AZ31 magnesium alloy wide sheet, was investigated through the multi-pass discrete three-point bending–unloading–bending (BUB) experiments. The stress variable, anisotropy, and asymmetry of tension and compression on the straightening process were corresponding to the Bauschinger-like effect. The Bauschinger-like effect in the initial, intermediate, and end of the straightening process was dominated by the twinning, detwinning, and dislocation induced by pyramidal slip, respectively. Also, they were responsible for the transformation of the Bauschinger stress parameter (BSP) and Bauschinger energy parameter (BEP). Later, the anisotropic nonlinear kinematic hardening model (ANK model) to describe the Bauschinger-like effect was optimized and simplified.

Enhanced plasticity at cryogenic temperature in a magnesium alloy

The mechanical behaviours of magnesium alloy (WE43) at cryogenic temperatures are investigated. The significant improvement in both fracture strength (28%) and strain-to-failure (24%) at −196 °C is observed when compression direction is normal to the extrusion direction, as benchmarked against room-temperature properties. This considerable improvement in strain-to-failure and strength at cryogenic temperature is attributed to unusual enhanced pyramidal slip.

Twinning characteristics in rolled AZ31B magnesium alloy under three stress states

Stress-strain responses and twinning characteristics are studied for a rolled AZ31B magnesium alloy under three different stress states: tension along the normal direction (NDT), compression along the rolled direction (RDC), and torsion about the normal direction (NDTOR) using companion specimens interrupted at incremental strain levels. Tension twinning is extensively induced in twinning-favorable NDT and RDC. All the six variants of tension twin are activated under NDT, whereas a maximum of four variants is activated under RDC. Under NDTOR, both tension twins and compression twins are activated at relatively large strains and twinning occurs in a small fraction of favored grains rather than in the majority of grains. Secondary and tertiary twins are observed in the favorably-orientated grains at high strain levels. Deformation under each stress state shows three stages of strain hardening rate: fast decrease (Stage I), sequential increase (Stage II), and progressive decrease (Stage III). The increase in the hardening rate, which is more significant under NDT and RDC as compared to NDTOR, is attributed to the hardening effect of twin boundaries and twinning texture-induced slip activities. The hardening effect of twin boundaries include the dynamic Hall-Petch hardening induced by the multiplication of twin boundaries (TBs) and twin-twin boundaries (TTBs) as well as the hardening effect associated with the energetically unfavorable TTB formation. When the applied plastic strain is larger than 0.05 under NDT and RDC, the tension twin volume fraction is higher than 50%. The twinning-induced texture leads to the activation of non-basal slips mainly in the twinned volume, i.e. prismatic slips under NDT and pyramidal slips under RDC. The low work hardening under NDTOR is due to the prevailing basal slips with reduced twinning activities under NDTOR.

Influence of strain rate and crystallographic orientation on dynamic recrystallization of pure Zn during room-temperature compression

This work investigates the strain rate dependence of dynamic recrystallization behaviour of high-purity zinc in room temperature compression under strain rates of 10−4 s-1, 10-2 s-1 and 0.5 s-1. Results from electron backscatter diffraction provide insight into the deformation and dynamic recrystallization mechanisms operative. Continuous dynamic recrystallization, twin-induced dynamic recrystallization, and discontinuous dynamic recrystallization are all active under compressive deformation at room temperature. Due to the high stacking fault energy of Zn, continuous dynamic recrystallization is the dominant mechanism while discontinuous dynamic recrystallization only operates in the early stages of compression at 10−4 s-1. Dynamic recrystallization kinetics are enhanced at higher strain rates (10-2 s-1 and 0.5 s-1) due to an increased contribution from twin-induced dynamic recrystallization. The present study reveals that the controlling mechanisms for continuous dynamic recrystallization are basal < a> slip and 2nd order pyramidal < c + a> slip activity. Because the activation of slip systems is mainly determined by crystallographic orientation, continuous dynamic recrystallization behaviour varies with grain orientation according to their propensity for basal and 2nd order pyramidal slip.

Critical resolved shear stress of activated slips measured by micropillar compression tests for single-crystals of Cr-based Laves phases

Abstract Single-crystal micropillar compression tests were performed in this study to investigate the mechanical properties and deformation mechanisms of Cr-based Laves phases. The results showed that, the activated slip system in the C15–Cr2Nb was exclusively the basal slip {111}⟨110⟩, transiting from the pyramidal slip {11 2 ¯ 2}⟨ 1 ¯ 1 ¯ 23⟩ to the prismatic slips {10 1 ¯ 0}⟨0001⟩ and {11 2 ¯ 0}⟨1 1 ¯ 00⟩ in the C14–Cr2Ta with the increased inclination angle. The average compressive strength of the C15–Cr2Nb changed between 10.0 GPa and 11.7 GPa, and that of the C14–Cr2Ta ranged within 11.6–12.4 GPa. The compressive strength changed due to the evolutions of activated slip systems in both Laves phases. The critical resolved shear stress (CRSS) of the C15–Cr2Nb was estimated as 4.4–4.6 GPa, which was lower than 5.1–6.1 GPa in the C14–Cr2Ta, and the higher CRSS of the C14–Cr2Ta resulted from higher shear modulus. The mechanical properties and deformation mechanisms obtained were helpful for better understanding and designing the Cr-based Laves phase alloys.

Microstructure evolution and constitutive relation establishment of Mg–7Gd–5Y–1.2Nd–0.5Zr alloy under high strain rate after severe multi-directional deformation

At room temperature, dynamic mechanical response of Mg–7Gd–5Y–1.2Nd–0.5Zr alloy under high strain rate after severe multi-directional deformation were measured by a Split-Hopkinson pressure bar equipped with a set of limited strain rings under the strain rates of 0.001, 1800, and 2300 s−1 along the extrusion direction (ED). An electron back-scattering diffraction and a transmission electron microscopy were used to characterize the microstructure evolution of the alloy. When compressed along ED direction at the strain rate of 2300 s-1, prismatic and pyramidal < a > slip mechanisms first occurred in the ultra-fine grain band, and then tensile twins, basal slip, prismatic slip, pyramidal slip, dynamic recovery, and recrystallization occurred in the equiaxed fine-grained region. At the same strain rate (7% strain), various deformation mechanisms in the ultra-fine grain region and equiaxed fine-grained region, as well as the dynamic recovery and recrystallization mechanisms in the equiaxed fine-grained region were strengthened with the increase of strain rate. Based on the phenomenological Johnson–Cook relationship and the microstructure evolution analysis during deformation, a new constitutive relation was constructed by inserting a softening term with strain into the formula to calculate the stress–strain curve, which was in good agreement with the experimental data.

Precipitate strengthening of pyramidal slip in Mg–Zn alloys

The mechanical properties of Mg-4wt.% Zn alloy single crystals along the [0001] orientation were measured through micropillar compression at 23 °C and 100 °C. Basal slip was dominant in the solution treated alloy, while pyramidal slip occurred in the precipitation hardened alloy. Pyramidal dislocations pass the precipitates by forming Orowan loops, leading to homogeneous deformation and to a strong hardening. The predictions of the yield stress based on the Orowan model were in reasonable agreement with the experimental data. The presence of rod-shape precipitates perpendicular to the basal plane leads to a strong reduction in the plastic anisotropy of Mg.

Effects of precipitate on the slip activity and plastic heterogeneity of Mg-11Y-5Gd-2Zn-0.5Zr (wt. %) during room temperature compression

To understand the effects of precipitate on the deformation mechanisms and plastic heterogeneity of a cast Mg–11Y-5Gd-2Zn-0.5Zr (wt.%) alloy, quantitative comparison studies were performed on the solid solution (T4) and peak-aged (T6) samples during room temperature compression based on slip trace analysis and electron backscatter diffraction (EBSD) analysis. At 10% plastic strain: no twins were observed in both T4 and T6 samples; the dominant slip mode transitioned from 2 nd order pyramidal <c+a> slip (34.1%) to basal <a> slip (50.5%) after the T6 treatment, which indicated that the precipitate-hardening effect for pyramidal slip was stronger than that for basal slip. Substantial active slip systems (up to 40%) exhibited low normalized Schmid factors (<0.1), implying that local stress deviated from the globally applied stress. Compared to the T4 sample, a larger magnitude of geometrically necessary dislocation (GND) density was found in the T6 sample, which indicated that the precipitates enhanced the plastic heterogeneity in terms of GND density. Most of the high GND density areas were near grain boundaries (GBs), but they also appeared in grain interiors. Almost every near-GB area with larger GND density tended to follow the conditions: (1) large orientation discrepancy between neighborhood grains; (2) at least one grain exhibiting slip traces; (3) the GB with the X phase. The GBs where slip transfer occurs tended to exhibit a homogeneous GND density distribution.

The Effects of Free Surfaces on Deformation Twinning in HCP Metals

Deformation twinning is a predominant mode of plastic deformation for hexagonal close packed metals, like Mg and Ti. The heterogenous microstructure and the local stresses associated with twinning play a key role in their mechanical response and fracture. Surface analyses, like electron microscopy, are frequently employed to spatially map microstructure and micromechanical fields in order to study twinning behavior. However, these measurements are inherently influenced by the vicinity of the free surface. Here, an elasto-visco-plastic fast-Fourier-transform (EVP-FFT) polycrystal modeling approach is employed to investigate the effects of free surfaces on twin development before and after loading. We compare calculated micromechanical fields on free surfaces with those calculated inside the bulk and in some cases, experimental surface measurements. The results indicate that the creation of free surfaces can promote twin propagation and growth and can influence twin morphology by causing a twin lamella to become larger, more blunted and skewed. The structure along the twin boundaries are also affected, due to the higher driving stresses that extend prismatic-basal and basal-prismatic facets. Furthermore, free surfaces invoke different slip activities in the twin and the surrounding parent crystal by enhancing basal, prismatic and pyramidal slip in some localized regions, while reducing slip in others. We demonstrate that the simulated free-surface effects lead to better qualitative and quantitative agreement with experimental measurements from scanning electron microscopy and digital image correlation.

FTMP-Based Kink Deformation and Strengthening Mechanisms for Mille-Feuille Structures

Kink-strengthening for mille-feuille structures has attracted many attentions in recent years. This study aims at identifying the kink formation/strengthening mechanisms via numerical reproductions of emerging kink-like morphologies based on FTMP (Field Theory of Multiscale Plasticity)-incorporated FE simulations, considering the Rank1 connection, where the incompatibility-based relevant underlying microscopic degrees of freedom for kinking are introduced. The targeted phenomena here include an experimentally-observed unique feature recently reported based on the combined ND–AE (neutron diffraction - acoustic emission) technique, i.e., scale-free-like energy release before (precursor) and during kink formations. This study uses a Mg single crystal model with alternatingly aligned soft and hard layers in parallel to the basal plane under c-axis plane-strain compression, where the soft/hard regions are controlled by the values of the hardening ratio. Also, we assume that the kink mode is only active, while the basal, prismatic and pyramidal slip and the twin systems are not operative associated with the layered structure. From the simulated results, we confirm kink-like morphologies and the attendant significant misorientation in the basal plane angles. Also, the simulated results are demonstrated to exhibit power-law type distributions in the strain energy fluctuation from the early stage of deformation even before the massive emergence of kink-like regions, which are analogous to the above-mentioned ND–AE observations.

Tensile Deformation Behavior of Magnesium Single Crystals with Different Yttrium Concentrations

Tensile tests of Mg-Y single crystals with different yttrium concentrations: 0.07 and 0.3 at.% were carried out to investigate effects of yttrium on pyramidal &lt;c+a&gt; slip system. In Mg-0.07at%Y alloy single crystals, {11 2}&lt; 23&gt; second order pyramidal &lt;c+a&gt; slip was activated and yield stress increased, compared to pure Mg single crystals. On other hand, in Mg-0.3at%Y alloy single crystals, {10 1}&lt; 23&gt; first order pyramidal &lt;c+a&gt; slip was activated and yield stress increased, compared to Mg-0.07at%Y alloy single crystals. The change in slip system by yttrium addition would be caused by increasing critical resolved shear stress (CRSS) for second order pyramidal slip.

Crystallographic and Experimental Disproof of Pyramidal &amp;lt; c+a &amp;gt; Slip in Magnesium

The activation of non-basal pyramidal slip has been perceived as key to enhance the ductility of magnesium and its alloys. However, there has never been convincing evidence to show the physical existence of dislocations and their involvement in deformation has been a core issue in magnesium research. In the present work, the impossibility of slip is analyzed based on fundamental concepts of dislocation and atomic interactions. The atomic configurations and crystallographic features in association with dislocations are unambiguously revealed for the first time, demonstrating that any possible dislocation core structures would involve too many atoms on multiple lattice planes and are physically impossible. Experiments of magnesium single crystal compression along its c-axis were conducted at temperatures from 20°C to 500°C and the results showed no evidence of the involvement of dislocations in any form as a mechanism of deformation during either plastic flow or fracture. von Mises criterion for compatible deformation, which drives the pursuit of pyramidal slip, is critically discussed.

On the activation energy for plastic flow during tension test at room temperature and the anisotropy behaviour of Ti-6Al-4V alloy

The movement of dislocations inside of crystals during the tension test at constant crosshead velocity is related to activation energy for plastic flow in spatially extended polycrystalline systems. In this work, the main objectives are to establish the influence of the slip systems for α hexagonal compact packaging (hcp) titanium phase on the activation energy for plastic flow and the anisotropy influence on the plastic flow behaviour of Ti-6Al-4V alloy in sheet form under tension test at constant crosshead velocity of 1 mm/min in three in-plane orientations, namely in the rolling 0° (RD), 45° (DD) and transverse (TD) directions, respectively. By applying the quantum mechanics and relativistic model (QM-RM) proposed by Muñoz-Andrade, it is shown that for a <c+a> pyramidal type slip with a Burgers vector b=0.553 nm, the activation energy barrier for plastic flow at maximum stress in uniaxial tension in the sheet rolling direction, Q=115.115 kJ/mol is lower in comparison with Q=116,629 kJ/mol obtained by the <a> basal type slip with a Burgers vector b=0.295nm. These results are in accordance with the reduced cross-slip activation energy barrier for cross-slip onto pyramidal planes in an a hexagonal crystalline structure with compact packaging (hcp) titanium phase. Additionally, it was shown that transverse direction (TD) of Ti-6Al-4V alloy in sheet form has developed the maximum stress in uniaxial tension test and ductility.

High strain rate deformation mechanism of Mg–2.5Zn–4Y magnesium alloy containing LPSO phase

ABSTRACT Dynamic compression experiments were carried out on Mg–2.5Zn–4Y magnesium alloy containing long period stacking ordered phase (LPSO) to understand its deformation mechanism using ‘Split Hopkinson Pressure Bar’. High strain rates ranging from 700 to 2100 s−1 were applied along Extrusion Direction. The results demonstrate that maximum compression stress increases with increasing strain rate. In the mean the alloy shows a positive strain rate strengthening effect. The deformation mechanism of Mg–2.5Zn–4Y magnesium alloy with LPSO phase is mainly 〈c + a〉 pyramidal slip supplemented with deformation by extension twin in association with kinking of LPSO phase. The impact fracture surface of the investigated magnesium alloy suggested quasi-cleavage fracture.

Compressive/tensile deformation behavior and the correlated microstructure evolution of Ti–6Al–4V titanium alloy at warm temperatures

The present study deals with the warm temperature deformation behavior of the Ti–6Al–4V alloy throughout investigating the flow behavior and the correlated microstructure evolutions. Toward this end, the tension and compression tests were conducted at temperatures of 100, 300, 500, and 600 °C under the strain rate of 0.001s−1. The low compressive formability of the material and premature fracture at 100 and 300 °C was attributed to the insufficient number of operative slip systems at this relatively low temperature regime. In contrast, in tensile mode the work hardening capacity was increased and the portion uniform elongation was considerable. In fact, the higher activity of pyramidal slip systems and higher dislocation maneuverability in tensile mode provided a proper condition for substructure development, which resulted in higher strain accommodation. Interestingly, the tensile deformed microstructures at 100 and 300 °C were strongly texturized and most of α-grains were oriented around the midline of [0001]-[-1100] or even toward the [-1100]. This was attributed to the lower critical resolved shear stress of pyramidal slip in tension mode, which caused the rotation of the <c> axis toward extension direction. The significant flow softening in compressive mode and appreciable post-necking deformation (due to diffused necking) in tensile mode were the main characteristics of the flow curves at 500, and 600 °C. This was attributed to the extended dynamic recovery and continuous recrystallization, spheroidization, and geometric softening. It was believed that the intensified substructure development facilitated the fragmentation and boundary splitting mechanism, therefore, trigger the spheroidization of α phase.

Effect of uniaxial loading direction on mechanical responses and texture evolution in cold pilgered Zircaloy-4 tube: Experiments and modeling

Mechanical responses of cold pilgered Zircaloy-4 (Zr-4) tubes are measured under different uniaxial (tension/compression) loading directions. Tensile tests were carried out along three directions: rolling direction (RD), transverse direction (TD) and 45° to the rolling direction (45RD) at quasi static strain rate. Compression test was also performed in the RD. Texture was characterized at intermediate strains with electron backscattered diffraction method. For different uniaxial loading conditions, anisotropic yield stress and strain hardening behavior can be observed. Such complex mechanical behavior is well explained with the variations in activation of deformation mechanisms predicted by the Schmid factor distribution and visco-plastic self-consistent (VPSC) model. In order to calibrate the VPSC model’s parameters more accurately, both mechanical responses and texture changes for four different directional tests are compared with VPSC simulations. The calculated Lankford coefficients are also calibrated with anisotropic tensile results. Furthermore, a deepening understanding of the texture evolution under different loading directions were disclosed based on VPSC method. The results indicate that the activity of deformation mechanisms changes significantly with different uniaxial loading directions. Non-prismatic slips with higher critical resolved shear stress, e.g. basal and pyramidal slip, make larger contributions to the yield and strengthening stress resulting in the highest stress in TD compared with RD and 45RD. Compression test along RD prefers the activation for tensile twinning (TTW). Meanwhile, TTW can be detected in 45RD and TD tension owing to the spread distribution of initial texture towards TD.

Effect of Y content and equal channel angular pressing on the microstructure, texture and mechanical property of extruded Mg-Y alloys

The microstructure, texture and mechanical property evolution of the extruded Mg- x Y ( x = 1, 5 wt.%) alloys during equal channel angular pressing (ECAP) were systematically investigated using an optical microscope, electron backscatter diffraction (EBSD) and uniaxial tensile test. The Mg-Y alloys exhibited a weakened basal texture before the ECAP, and the texture was further weakened with the max basal poles dispersed along ∼45° between the extrusion direction and the transverse direction after the ECAP. The Mg-5Y alloys always exhibited a finer grain size comparing to that of Mg-1Y for the same ECAP process. With a proper ECAP process, both the strength and elongation of Mg-5Y alloy could be improved simultaneously after the ECAP, i.e., the yield strength (273.9 ± 1.2 MPa), ultimate strength (306.4 ± 3.0 MPa), and elongation (23.9 ± 1.0%) were increased by 10%, 6%, and 72%, respectively, comparing to that before the ECAP. This was considered to be arose from the combined effects of grain refinement, significant improved microstructure homogeneity and solid solution hardening. In addition, it was found that Mg-Y alloy with better comprehensive properties could be obtained by the decreasing-temperature ECAP processes. The yield strength-grain size relationship could be well described by the Hall-Petch relation for all the ECAPed Mg-Y alloys, which was consistent with that the texture changes did not significantly affect the average Schmid factors of basal, prismatic and pyramidal slips for both Mg-Y alloys.

Deformation behavior of Ti-6Al-4V microstructures under uniaxial loading: Equiaxed Vs. transformed-β microstructures

The dual-phase titanium alloy Ti-6Al-4V can be thermomechanically treated to produce a variety of microstructures to obtain desired mechanical properties. The extreme microstructure morphologies were developed by heat treatment of mill annealed microstructure to equiaxed, and transformed-β microstructures (lamellar) of coarser α-lath and α′-laths (martensite). The uniaxial tensile test shows the highest elongation in the equiaxed microstructure followed by coarse α-lath lamellar, while the α′-lath morphology has the least elongation. The higher ductility in the equiaxed microstructure is due to smaller slip length compared to coarse α-lath lamellar. On the other hand, the poor ductility of the α′-lath is due to premature crack initiation. Both equiaxed and coarse α-lath lamellar microstructures mostly show prismatic and pyramidal slip <a>/ <c + a>. In addition to this, though less prevalent, these microstructures exhibit twinning as the other deformation mechanism, which is uncommon in Ti-6Al-4V. The twin boundary interaction with the grain boundary led to the damage nucleation, causing the intra-grain crack. In the coarse lath lamellar (furnace cooled) morphology, the crack was mostly observed at the junction of α-colonies as well at the α-layer grain boundary/α-colony interface due to strain localization. However, in the lamellar (water quenched), the primary α′-lath shows the void nucleation at the junction of the primary and secondary-α′ interface, which coalesce to form microcrack and further grow instantly to fracture. The void nucleation is generally observed in a basal orientation along the primary α′-lath, oriented 45° to the loading axis, and having dominant pyramidal slip (<a>/<c + a>). Thus, the deformation mechanisms slip, twin, and fracture depend on the microstructure morphology in Ti-6Al-4V.

Modeling the role of local crystallographic correlations in microstructures of Ti-6Al-4V using a correlated structure visco-plastic self-consistent polycrystal plasticity formulation

This paper presents a multi-level crystal plasticity-based simulation framework for modeling mechanical response and microstructure evolution of Ti-6Al-4V with α-lath/lamellar microstructures. The model is a correlated structure visco-plastic self-consistent (CS-VPSC) formulation linking three scales: a single crystals micro-scale, a lath/lamellar colony meso-scale, and a lath/lamellar aggregate macro-scale. A selected hardening law for the evolution of critical resolved shear stress per slip system used in CS-VPSC is phenomenological. However, it adjusts the resistances of basal and prismatic slip systems based on the geometry of slip transfer between adjacent lamellae. Consistent with experimental evidences, the resolved shear stress on the pyramidal slip planes is dependent not only on the stress in the direction of slip but also on the two orthogonal shear stress components and the three normal stress components (non-Schmid effects). Electron backscatter diffraction (EBSD) data in conjunction with a procedure relying on α→β phase transformation is used to construct paired variants of α-laths/lamellae satisfying their local crystallographic correlations. The procedure fits volume fractions of individual laths/lamellae with the experimental EBSD data and selects a distribution of habit planes between adjacent variants with respect to the loading direction. The simulation framework is applied to interpret the deformation behavior in tension and compression along two sample directions of Ti-6Al-4V fabricated via laser powder bed fusion. Moreover, the model is used to simulate texture evolution during rolling of the material to large strains. It is demonstrated that the model is capable of predicting plastic anisotropy/asymmetry and the concomitant texture evolution. While the model reveals a significant effect of habit plane inclination with respect to the loading direction on yield stress, the comparison of the data and model predictions shows that a random distribution of habit planes fits the flow response. It is further inferred that the tension-compression asymmetry arises from the non-Schmid effects.