Pyramidal Slip Research Articles

<i>In Situ</i> Observation on the Deformation Behavior of Primary α-Ti in a Textured Ti-6Al-4V

The tensile deformation process and dislocation behavior of primary α-Ti of Ti-6Al-4V were studied by the in-situ tensile test combined with EBSD (electron backscatter diffraction). The initiation, evolution and distribution of dislocation slips at different strains were discussed. The results showed that the microtexture of the material had a significant influence on slip behavior. Typically, basal and prismatic <a> slips initiated first, but the dominant slip type was related to the local texture characteristics. Sometimes, the basal and prismatic <a> slips could still initiate when their Schmid factors were relatively low, while the pyramidal slips usually need a higher Schmid factor to initiate. With the increase of strain, the second slip system inside one grain was activated to accommodate the plastic deformation. When the deformation was localized in a specific microtextured region, basal <a> slips were dominant, but eventually the crack initiated from the <c+a> slip bands inside the grain.

The influence of solute atom ordering on the deformation behavior of hexagonal close packed Ti-Al alloys

A framework of compositionally graded Ti-Al alloys was proposed to elucidate the Al alloying dependent deformation mechanism at room temperature. Slip trace analysis demonstrated low-alloyed Ti-Al model materials were plastically deformed by prismatic slip, pyramidal slip, stacking faults, and deformation twins. Increasing Al concentration promoted chemical ordering, thus suppressing the dislocation motion and deformation twinning. The mechanism behind such a kind of composition-mediated deformation physics was discussed. Our findings are expected to be applicable in the design of next-generation high-performance structural materials through the tailoring of the chemical ordering of solute atoms in substitutional solid solution.

Effect of Dynamic Recrystallization on Grain Refinement during Interactive Alternating Forward Extrusion of AZ31B Magnesium Alloy

The dynamic recrystallization mechanisms leading to grain refinement in an extruded AZ31 rod during interactive alternating forward extrusion were investigated. Several dynamic recrystallization mechanisms were initiated due to extrusion deformation, including discontinuous dynamic recrystallization, continuous dynamic recrystallization (CDRX), and twinning-induced dynamic recrystallization. The microstructure of the AZ31 magnesium alloy formed at 350 °C exhibited the most uniform grain distribution, smallest grain size, and roundest grains compared to the samples formed at other temperatures. Furthermore, this sample exhibited the highest average Schmidt factor and hadsoftly orientated grains. The CDRX behavior is particularly significant, where a large number of regions exhibited continuous rotation of grains and the merging of sub-grains. This which led to uniform structure distribution and the improvement of the grain roundness. The prismatic slip system and the pyramidal slip system play an obvious role of the magnesium alloy grains in grain deflection. It provides scientific guidance for the formulation of interactive alternating forward extrusion process and the control of microstructures.

Deformation and Failure Mechanisms in a Magnesium Alloy Under Uniaxial Compressive Loading

We have investigated the deformation and failure mechanisms of a rolled AZ31B magnesium alloy under both quasi-static ( $$10^{-3}\,{\text {to}}\,10^{-2}\,{\text{s}}^{-1}$$ ) and dynamic ( $$10^{3}\,{\text{s}}^{-1}$$ ) compressive loading. The observed anisotropy in the plastic response originates from the different in-plane and out-of-plane deformation mechanisms activated when the loading orientation changes. For plasticity driven by extension twins, extension twin???twin boundaries were found to dominate in the late stage of deformation. Severe localization was observed after deformation mediated by pyramidal slip and contraction twinning compared to the deformation mainly caused by extension twinning. The fracture process was established as a function of both loading rate and loading orientation. Moreover, the intermetallic inclusions in the material, which appeared to be hard and brittle, might lead to macroscopic fracture under compressive loading.

Superplastic deformation behavior of fine-grained AZ80 magnesium alloy prepared by friction stir processing

Friction stir processing (FSP) is a new severe plastic deformation technique where microstructure refinement, homogenization, and densification occur simultaneously. In this study, the microstructure and superplastic deformation behavior of AZ80 magnesium alloy prepared by FSP were investigated. FSP led to remarkable grain refinement and modification of texture for AZ80 magnesium alloy. A maximum elongation of 606% was obtained at 3 × 10−4 s−1 and 350 ℃. When superplastic deformation was conducted at 3 × 10-3 s−1 and 300 ℃, or at 1 × 10-4 s−1 and 400 ℃, the superplasticity was relatively lower, thus exhibiting a lower strain rate sensitivity. The coarsening of both grains and β-Mg17Al12 precipitates deteriorated superplastic elongation. Applied deformation promoted the premature dissolution of β precipitates at 400 ℃ during superplastic deformation. Moreover, superplastic deformation weakened and spread the basal texture. Although the activation of prismatic and pyramidal slips is beneficial for superplastic deformation, grain boundary sliding is the main superplastic deformation mechanism.

Concentration-Dependent High-Temperature Deformation Mechanism of Hexagonal Close-Packed Ti-Al Alloys

The dependence of Al alloying on the high-temperature deformation physics of Ti-Al alloys was investigated in the present study. The mechanisms of planar dislocation slip, enhanced 〈c + a〉 activity and stacking faults were operative in low-alloyed Ti-Al model materials upon plastic deformation at 600 °C. Increasing Al content up to 10.2 at. pct triggered significant pyramidal slip and $$ \left\{ {10\bar{1}2} \right\} $$ twins, highlighting the role of Al alloying. Atomic-scale characterization revealed the nature of the topologically discontinuous “stepped” twin boundary, consisting of alternately stacked basal/prismatic interface couples without a traditional rigorous twin invariant plane. This is the first time significant twinning has been observed in severely alloyed polycrystalline hcp metallic alloys, especially plastically deforming at 600 °C with a low strain rate of 1 × 10−4 s−1. A lattice-reorientation-based twinning mechanism, as well as its influence on mechanical properties, was proposed and discussed.

Effects of Y on the Deformation Mechanisms of Extruded Mg-Y Sheets During Room-Temperature Compression

The effects of yttrium (Y) contents on the slip/twinning activity in extruded Mg-xY (x = 0-5, wt pct) sheets during room temperature (RT) compression were investigated quantitatively and statistically using a quasi-in-situ scanning electron microscope (SEM), electron backscatter diffraction (EBSD) combined with slip trace analysis and twin boundary misorientation analysis. No observable slip trace was found in the pure Mg, while the fraction of grains exhibiting slip traces increased up to ~ 24 pct with increasing Y content. Grain-by-grain slip trace analysis showed that the relative activity of pyramidal 〈c + a〉 slips increased up to ~ 16 pct with increasing Y. Schmid factor analysis implied that increasing Y could promote basal 〈a〉 slip, while it had little effect on pyramidal 〈c + a〉 slips. Considering the observed opposite trends of the slip activity, it is reasonable to conclude that Y addition could lower the critical resolved shear stress (CRSS) ratio of pyramidal 〈c + a〉 slip/basal 〈a〉 slip. The dramatic texture change and twin boundary misorientation analysis revealed that almost all the grains twinned heavily in pure Mg after failure, and twinning was remarkably suppressed with increasing Y content. The dominant twinning mode was the {10$$ \bar{1} $$2} tensile twinning for all the samples studied. The present study clearly showed that Y alloying could significantly depress twinning and promote slip, especially for pyramidal slip, even at RT deformation.

Defect nucleation from a pre-existing intrinsic I1 stacking fault in magnesium by molecular dynamics simulations

In order to clarify defect nucleation in the vicinity of a pre-existing intrinsic I1 stacking fault (SF) in magnesium (Mg), we carried out molecular dynamics (MD) simulations containing an I1 SF under several applied shear stresses, using a modified embedded atom method (MEAM) potential. Depending on stress states, different types of defects were generated from the I1 SF. Under shear stress on 0001 basal plane along 112¯0 direction, 112¯1 twin was formed from the side ends of the I1 SF. On the other hand, in the case of higher resolved shear stress (RSS) for c+a pyramidal slips, c+a partial dislocations were generated with SFs on the first-pyramidal planes as well as the second-pyramidal planes. The dissociation reactions of the dislocations at the side ends of the I1 SF were energetically evaluated, and it was implied that nucleation of the c+a dislocations on the first-pyramidal planes is more favorable than on the second-pyramidal planes. These results suggest that I1 SFs are attractive defects for nucleation of c+a dislocations.

In situ EBSD study of deformation behavior of primary α phase in a bimodal Ti-6Al-4V alloy during uniaxial tensile tests

Uniaxial tension experiments and electron backscatter diffraction were performed on a bimodal Ti-6Al-4V alloy to study in situ the deformation behavior of primary hcp-Ti (αp). It was found that the strain could be accommodated by the activation of slip systems and by grain rotations. The prismatic slip was the primary slip mode of the αp. From the analysis of kernel average misorientation and geometrically necessary dislocation, it was shown that the dislocations mainly distributed in the vicinity of grain and sub-grain boundaries, and part of the dislocations distributed around slip lines. It was the dislocation activities that led to the formation of the low angle grain boundary and its transformation to the high angle grain boundary. It's important that tracking of deformation heterogeneities with significance to performance. By analyzing the rotation angle, average rotation rate, and rotation path of grains, it was shown that grain rotation heterogeneity occurred during the deformation. From the observation of the loading direction, grain rotation paths kept with the texture evolution direction of all αp. The grains activated in the basal slip gradually rotated to the ⟨101¯1⟩ pole and enhanced the intensity of the ⟨101¯1⟩ texture. Meanwhile, the grains activated in prismatic or 1st-order pyramidal slip rotated to the ⟨101¯0⟩ pole and enhanced the intensity of the ⟨101¯0⟩ texture. Grain rotation and texture evolution are related to mechanical properties.

Deformation Characteristics in Micromachining of Single Crystal 6H-SiC: Insight into Slip Systems Activation

ABSTRACTSilicon carbide (SiC) is ideally suitable as a sensor material in harsh environments. Despite the brittleness in the macroscopic scale, plasticity in SiC is observed at small component length-scales. Previous nanoindentation based study combining experiment and numerical approaches of single-crystal 6H-SiC has shown that slip activation is rather complex, and that non-basal slip could potentially dominate the plastic deformation behaviour. In this study, we investigated the local deformation response evolution of shear strain directly under and in the vicinity of the indenter tip. The results show the pyramidal slip families contribute significantly to the deformation process.

Effect of basal precipitates on extension twinning and pyramidal slip: A micro-mechanical and electron microscopy study of a Mg–Al binary alloy

Understanding the effects of basal precipitates on plasticity is of scientific interest and practical importance in the development of high-strength magnesium alloys. In this study, compression experiments are carried out on solution-treated and aged Mg–9wt.% Al binary alloy single-crystal micropillars cut from large-grained polycrystalline samples. The samples are compressed in two different orientations (<10-10> and <0001>) to examine the effects of basal-precipitates on extension twinning and pyramidal <c+a> slip, respectively. In the solution-treated, precipitate-free, <10-10> samples, the propagation and thickening of single twins dominates, while in the aged <10-10> micropillars with Mg17Al12 basal-precipitates, multiple twins are observed suggesting a substantial increase in the stress required to propagate and thicken twins compared to nucleating twins. In addition, these basal precipitates are observed to be more effective than c-axis rod precipitates in Mg-Zn alloy in impeding twin growth for the studied volume fractions. In <0001> samples, the plate-like, Mg17Al12 precipitates are found to provide enormous strengthening under compression, compared to solution-treated samples, by hindering the motion of pyramidal <c+a> dislocations. Further still, our comparative analysis suggests that pyramidal-slip/precipitate interactions, whether via bowing between or cutting precipitates, differ from one Mg alloy to another depending on the geometry and alignment of their precipitates. Our findings offer insights into how manipulating precipitation in the design and processing of Mg alloys can lead to superior mechanical properties.

Crack propagation path selection and plastic deformation at a crack tip in zirconium

The crack propagation process of a center cracked tension specimen was studied in pure zirconium by quasi-in-situ microstructure characterization combined with finite element modeling (FEM). The microstructure characterization shows that as load increased, dislocations were emitted from crack-tip ledges and crystallographic planes were ‘thinned’ by accumulation of slip traces at the crack-tip. In the near-crack region, plastic deformation was achieved mainly through dislocation motion, rather than twinning. It was observed that the crack grew along grains with plastically ‘hard’ orientations (as determined by the angle between c-axis and loading axis) and that cleavage fracture planes were found to be non-basal. In addition, combined with the crack-tip stress field from a simulation, the Schmid factor was estimated for prismatic, basal and pyramidal slip systems at the crack tip; the results were compared with experimental determinations of geometrically necessary dislocation density. It was found that the existence of the crack-tip stress field slightly suppressed prismatic and pyramidal slip as compared to that determined by a far-field stress state Schmid factor analysis.

Enhancing hoop strength of titanium alloy tube by cross spinning

There is an increasing demand for thin-walled long tubular workpieces of titanium alloys with excellent mechanical properties, which can be produced by power spinning owing to its specific advantages over other forming processes. In this paper, a novel spinning forming process, named cross spinning (CS), was proposed to tune the hoop properties of as-spun tubular workpieces. Traditional unidirectional spinning (UDS) and CS experiments were conducted to form the tubular workpieces of Ti–6Al–4V alloy, and their mechanical properties and microstructure evolution were analyzed and compared. The tensile test indicated that both axial and tangential strengths of the CS specimens were higher than that of the UDS specimens, which was more remarkable for the tangential strength. To clarify this phenomenon, crystal plasticity finite element method (CPFEM) coupled with spinning texture information was used to simulate the tensile test for the CS and UDS specimens. The results indicated that a more homogeneous texture was formed during CS, inducing lower average Schmid factor for prismatic slip system and larger activities of the pyramidal slip systems, was the major cause for enhancing the hoop mechanical properties of the as-spun workpieces compared with that during UDS.

Study of basal < a > and pyramidal < c + a > slips in Mg-Y alloys using micro-pillar compression

ABSTRACT The effect of yttrium (Y) on the critical resolved shear stresses (CRSS) of < a > basal slip, tension twinning and < c + a > pyramidal slip has been studied using micro-pillar compression tests along selected orientations at room temperature, on two magnesium (Mg) alloys with different Y contents (0.4 and 4 wt.%). The CRSS of < a > basal slip increased slightly from 30 ± 1 MPa (for Mg-0.4Y) to 37 ± 3 MPa (for Mg-4Y). In Mg-0.4Y, the resolved shear stress to activate the tension twin was determined to be 45 ± 12 MPa, while in Mg-4Y it was 113 MPa. The compressed samples were studied using scanning electron microscopy (SEM), transmission Kikuchi diffraction (TKD) and transmission electron microscopy (TEM). It was found that < c + a > dislocations slip mainly on (Pyra II) planes with a CRSS of about 119 MPa in Mg-0.4Y and slip mainly on (Pyra I) planes in Mg-4Y with a CRSS of 106 MPa. There is a significantly lowered CRSS ratio between < c + a > slip and < a > basal slip in both alloys (2.8 and 4.8) compared with that reported in bulk pure Mg (∼100). The easy activation of < c + a > on Pyramidal I slip is expected to promote the frequent cross-slip in Mg-4Y alloy.

Effect of Al content on the critical resolved shear stress for twin nucleation and growth in Mg alloys

The effect of Al atoms in solid solution on the critical resolved shear stress for twin nucleation and growth was analyzed by means of the combination of diffusion couples with compression tests in micropillars oriented for twinning. The critical resolved shear stress for twin nucleation was higher than that for twin growth and both increased by the same amount with the Al content. Nevertheless, the increase was small (≈ 10 MPa) for 4 at%Al but large (up to 60–70 MPa) for 9 at%Al. These results were in agreement with Labusch-models based on first principles calculations in the dilute regime (< 5 at%Al). Comparison with recent data in the literature showed that Al atoms are more effective in increasing the critical resolved shear stresses for twin nucleation and growth than for basal slip. Finally, compression tests in micropillars oriented along [0001] showed the critical shear stress for pyramidal slip increased rapidly with the Al content from 98 MPa in pure Mg to 250 MPa in Mg-9 at%Al. Overall, the presence of Al atoms decreased slightly the plastic anisotropy of Mg (understood as the critical resolved shear stress ratios between non-basal and basal plastic deformation mechanisms).

Modelling of the Strain Hardening Behaviour of Die-Cast Magnesium-Aluminium-Rare Earth Alloy

The tensile deformation behaviour of magnesium alloy AE44 (Mg-4Al-4RE) under strain rates ranging from 10-6 to 10-1 s-1 has been investigated. Present study shows that the deformation mode begins with the activation of elastic (Stage 1), followed by &lt;a&gt; basal slip and twinning (Stage 2), &lt;a&gt; prismatic slip (Stage 3) and finally to &lt;c+a&gt; pyramidal slip (Stage 4). The commencement of these deformation mechanisms results in four distinct stages of strain hardening in the stress-strain curve. In this work, the four stages of deformation behaviour are modelled, and an empirical equation is proposed to predict the entire stress-strain curve. Overall, the model predictions are in good agreement with the experimental data. This study on the decomposition of stress-strain curve into four stages provides insights into the contribution of individual deformation mechanism to the overall deformation behaviour and opens a new way to assess mechanical properties of die-cast magnesium alloys.

Ex-situ EBSD analysis of hot deformation behavior and microstructural evolution of Mg–1Al–6Y alloy via uniaxial compression

The microstructure and texture evolution of as-extruded Mg-0.96 wt%Al-5.82 wt%Y (denoted as Mg–1Al–6Y) alloy with strain at a temperature of 350 °C and a strain rate of 0.1 s−1 were studied using ex-situ electron backscatter diffraction (EBSD). Moreover, the deformation mechanism was systematically analyzed. The results shown that weak <2-1-11> rare-earth texture and no twinning were observed in the as-extruded Mg–1Al–6Y alloy. The hot compression curve exhibited obvious work-hardening characteristics with a yield strength of 98 MPa. In the initial stage of deformation, {10–12} extension twinning was formed in some grains. As the strain increased, the number of {10–12} extension twinning and texture intensity gradually decreased, whereas the amount of low-angle grain boundaries increased. The Schmid factor (SF) of pyramidal slip remained at ~0.4. In addition, the TEM characterization revealed that as the strain increased, dislocations multiplied, and the pyramidal <c+a> dislocation was always observed. Therefore, the reduction of twinning and activation of pyramidal <c+a> slip lead to the dominance of slip during the deformation. Hence, work hardening appeared in the compression curve of Mg–1Al–6Y alloy.

Comparative Study of Optical and Electrical Properties of Grown-In and Freshly Introduced Dislocations in GaN by SEM Methods

Electrical and optical properties of grown-in and freshly introduced dislocations in GaN have been studied by the electron beam induced current and cathodoluminescence methods. It is observed that the recombination properties of grown-in and freshly introduced basal plane and threading dislocations are comparable. That allows to assume the intrinsic nature of dislocation recombination activity in GaN. It is demonstrated that the recombination properties of basal plane dislocations weakly depend on their type. The behavior of dislocation-related luminescence at 3.1 eV is more complex. It can be observed not in all GaN crystals even when dislocations are introduced in the similar conditions. Besides, it is not observed on basal plane and threading grown-in dislocations. This luminescence is not produced by freshly introduced basal plane dislocations. These observations can be explained assuming that the dislocation-related luminescence is associated with point defects generated by dislocations gliding in pyramidal or prismatic slip planes.

{10-12} Twinning Mechanism During &lt;i&gt;In Situ&lt;/i&gt; Micro-Tensile Loading of Pure Mg: Role of Basal Slip and Twin-Twin Boundary Interaction

An SEM in situ uniaxial tensile testing setup allowing HR-EBSD acquisition during deformation was used to study the extension twinning mechanism in magnesium (Mg) at the micron scale. Structures with strain gauge sizes of 20 × 5 × 8 µm were fabricated by Ga-FIB in pure Mg with two different crystal orientations, respectively perfectly aligned with, and at 5° to, the [0001] axis. Limited {10-12} twin formation was identified in the former case, while twinning was found to largely accommodate the plastic deformation in the latter case. These two different mechanisms are explained by the activation of basal slip when loading at 5° to the c-axis, which triggers {10-12} twin nucleation and strongly favors twin growth and propagation. The other orientation shows the activation of pyramidal slip together with only limited {10-12} twin growth. The critical resolved shear stress for {10-12} twinning has been determined to be ten times higher than in bulk material due to size effects. 3D HR-EBSD mapping enabled reconstruction of the three dimensional twin structure after deformation. From this, the interaction between twinning dislocations ahead of a twin tip and a pre-existing twin boundary was studied, where the GND distribution and the local shear stress in the active twin variant coordinate system were determined. The results show plastic accommodation up to ~11% of strain through transmission of twinning dislocations across a twin boundary, activation of several slip systems, and the formation of a third extension twin variant.

Investigating the deformation mechanisms of a highly metastable high entropy alloy using in-situ neutron diffraction

The present study correlates the effect of enhanced metastability on both the well-understood γ-f.c.c. stacking fault energy (SFE) and deformation mechanisms in the ε-h.c.p. phase of a metastable high entropy alloy (HEA). The SFE of a Fe40Mn20Cr15Co20Si5 alloy (CS-HEA) was experimentally determined to be ∼6.31 mJ m−2 using in-situ neutron diffraction. The relatively low-measured SFE of the CS-HEA results in a high fraction of the ε-h.c.p. phase (58 %) triggering significant stress partitioning to ε-h.c.p. and a marginal fraction of γ-f.c.c. → ε-h.c.p. transformation (∼25 %). The ε-h.c.p. phase accommodated a significant amount of strain marked by the large stress-induced decrease of c/a ratio (from ∼1.619 to 1.588), which was accompanied by activation of non-basal deformation modes, such as deformation twinning and pyramidal slip. Using in-situ neutron diffraction, we show by decreasing SFE and stabilization of large fractions of ε-h.c.p., activation of non-basal deformation modes are responsible for high work hardenability in absence of significant γ-f.c.c. → ε-h.c.p. transformation.