Slip Traces Research Articles

In situ EBSD/DIC-based investigation of deformation and fracture mechanism in FCC- and L12-structured FeCoNiV high-entropy alloys

This study investigated the plastic deformation behavior of a polycrystalline L12-structured FeCoNiV high-entropy alloy (HEA) using in situ electron backscatter diffraction (EBSD) and digital image correlation (DIC) methods. The different deformation mechanisms in two HEAs, which affected their mechanical performance, were explored using a face-centered cubic (FCC)-structured sample for comparison. Using slip traces and lattice rotation path analysis, {111}<110> slip systems were found to be activated in the L12-structured FeCoNiV HEA. In addition, a lower average lattice rotation rate was estimated for this sample compared to that of the FCC specimen; this macroscopically verified the existence of additional obstacles to dislocation slip caused by the ordered structure during plastic deformation, and was found to contribute to the high strength of the L12-structured FeCoNiV HEA. Furthermore, these additional obstacles blocked the formation of deformed substructures in the L12-structured sample and aggravated the intergranular incompatibility, which enabled crack initiation at the grain boundaries. These findings are important for understanding the deformation behavior and fracture mechanism in L12-structured HEAs and for designing new high-performance ordered HEAs.

Plastic Deformation of Copper and Its Alloys under the Action of Nanosecond UV Laser Pulse

The effect of nanosecond UV laser pulses on copper and low-alloy copper samples has been studied. Traces of high-temperature deformation have been found at the energy density of 0.1–1 J/cm2 in the below-threshold regime without obvious traces of melting. They manifest themselves as the results of slipping and cracking along grain boundaries, as well as traces of crystallographic slip inside grains. The surface of the metal in the irradiated zone exhibits an uplift. The damage increases with the number of pulses. The height of the resulting uplift can reach 1 μm, and even more in some cases. The results obtained are similar to the electroplastic and magnetoplastic effects. By analogy, we propose to call the detected effect opticoplastic.

Slip activity during low-stress cold creep deformation in a near-α titanium alloy

Near-α titanium alloys are known to be susceptible to cold dwell fatigue (CDF) debit, which has been linked to the occurrence of cold creep during high-load dwell times superimposed onto low cycle fatigue loading. In order to shed new light on the deformation mechanisms during cold dwell and to understand better the role of the microstructure, two different bimodal microstructures (fine and coarse transformation product) of TIMETAL®834 were investigated at stress levels below the 0.2% proof stress using a combination of grain orientation mapping and in-situ electron microscopy imaging. This enabled in-depth analysis of 2D slip patterns and slip system activity using High-Resolution Digital Image Correlation (HRDIC), showing that in both microstructures basal slip is initially the dominant slip mode before prismatic slip activity increases approaching the 0.2% proof stress. Comparing the two constituents in the bimodal microstructure, first slip bands are localised predominantly in primary α grains, indicating higher strength of secondary α colonies, particularly for finer transformation products. During 10 min load holds at stresses below 0.2% proof stress, more plastic strain and longer connected slip traces across several grains were observed in the sample with coarse transformation product, indicating higher susceptibility to cold creep deformation.Full-field crystal deformation modelling was used to determine local stresses in individual grains at the onset of plasticity and test the hypothesis that the dominance of basal slip at low-stress levels can be explained by the elastic anisotropy in Ti alloys. While consideration of elastic anisotropy increased resolved shear stress (RSS) values for basal slip relative to prismatic slip, it did not unambiguously explain the early activation of basal slip. Furthermore, thermal residual stresses at the crystal level, due to the anisotropy of coefficients of thermal expansion (CTE), were included in the simulation, which created a wider spread of the RSS data but did not preferentially promote high RSS values for grains well aligned for basal slip. In the absence of an unambiguous conclusion, it is hypothesised that basal slip might display lower critical resolved shear stress values than typically reported but high work hardening rates compared to prismatic slip.

A study of unstable fracture of a magnesium alloy caused by uneven microstructure

The influence of the uneven microstructure of the magnesium alloy sheet on the slip and twinning behavior during the room temperature uniaxial tension was investigated using slip trace analysis and EBSD-based misorientation analysis, and the reasons for the instability and fracture of the uneven microstructure are easily explored. The results show that the uneven microstructure makes the distribution of dislocation density and strain uneven, the stress is concentrated in small grains, and the intergranular fracture occurs at the boundary between the large and small grains. Large misorientation may cause slip to fail to be transferred between grains, and dislocations accumulate at the grain boundaries, leading to instability and fracture.

The role of grain boundary plane in slip transfer during deformation of magnesium alloys

Slip transfer across grain boundaries (GBs) is an important mechanism for HCP metals to accommodate intergranular deformation incompatibility. In this work, an in-situ SEM/EBSD study was conducted to study basal-slip-induced non-basal <a> slip behavior in a Mg–Y alloy. The indices of dislocation slip systems of slip traces at sample surface were determined by a lattice rotation analysis method based on EBSD images. It was shown that the disorientation angles of GBs have a strong influence on the slip transfer behavior. The experimentally determined preferential disorientation angle ranges for basal-slip-induced pyramidal I <a> slip and prismatic <a> slip events have been correlated to the geometrical compatibility between the source and triggered slip systems, in terms of m' criterion. However, a statistic showed that some slip transfer events do not obey the m' criterion. Instead, they are rather sensitive to the GB plane orientations. To reveal the roles played by the GB plane, sectional polishing and EBSD characterization to determine the orientation of GB plane have been conducted. It is revealed that, in addition to the GB disorientation angle, the activation of slip transfer in Mg is essentially dependent on the plane orientation of local GBs. It also demonstrated that the M criterion, based on the match of intersection lines between dislocation slip planes to GB planes and slip directions, was more rational to predict the activated slip systems by the m' criterion. This work emphasizes the importance of GB plane orientation when attempting to understand the slip transfer mechanism in deformed magnesium alloys.

Deformation and phase transformation in polycrystalline cementite (Fe3C) during single- and multi-pass sliding wear

Cementite (Fe3C) plays a major role in the tribological performance of rail and bearing steels. Nonetheless, the current understanding of its deformation behavior during wear is limited because it is conventionally embedded in a matrix. Here, we investigate the deformation and chemical evolution of bulk polycrystalline cementite during single-pass sliding at a contact pressure of 31 GPa and reciprocating multi-pass sliding at 3.3 GPa. The deformation behavior of cementite was studied by electron backscatter diffraction for slip trace analysis and transmission electron microscopy. Our results demonstrate activation of several deformation mechanisms below the contact surface: dislocation slip, shear band formation, fragmentation, grain boundary sliding, and grain rotation. During sliding wear, cementite ductility is enhanced due to the confined volume, shear/compression domination, and potentially frictional heating. The microstructural alterations during multi-pass wear increase the subsurface nanoindentation hardness by up to 2.7 GPa. In addition, we report Hägg carbide (Fe5C2) formation in the uppermost deformed regions after both sliding experiments. Based on the results of electron and X-ray diffraction, as well as atom probe tomography, we propose potential sources of excess carbon and mechanisms that promote the phase transformation.

Mechanism of quasi-viscous flow of zinc single crystals

Abstract This paper reports the results of mechanical investigations (tensile and relaxation tests), geometrical measurements as well as macro- and microscopic (slip traces) observations of zinc single crystals in the ‘hard’ orientation (from the line [1120] – [1010] of the standard triangle) tested in the temperature range 523– 673 K. Consideration of the experimental data lead to the conclusion that the quasi-viscous flow of crystals results from localized coarse slip in the basal system occurring in the structure of uniformly distributed slip in the 1st order prismatic (and/or the 1st order pyramidal) systems. The proposed mechanism explains the high strain-rate sensitivity and the ‘jerky’ character of the flow of the crystals.

The relation between ductility at high temperature and solid solution in Mg alloys

This work investigates the effect of solid solution on ductility and on the activation of individual deformation mechanisms at moderate temperatures and at quasi-static strain rates in Mg-Zn and Mg-Al alloys. With that aim, four solid solution Mg-Zn and Mg-Al binary alloy ingots containing 1 and 2 wt.% solute atoms were subjected to hot rolling and subsequent annealing to generate polycrystals with similar average grain size and basal-type texture for each composition. The activity of the different slip systems after tensile testing at 150°C and at 250°C was evaluated in pure Mg and in the alloys by EBSD-assisted slip trace analysis. In addition, segregation of Zn and Al atoms at grain boundaries during the thermo-mechanical processing was characterized by HAADF-STEM and EDX. It was found that while the addition of Al and Zn atoms to pure Mg does not lead to major changes in the mechanical strength at the investigated temperatures, it does enhance ductility significantly, especially at 250°C. Our results show that this increase in ductility cannot be attributed to a higher activation of non-basal systems in the alloys, as reported earlier, as the incidence of non-basal systems is indeed considerably higher in pure Mg. This work suggests, on the contrary, that the ductility increase may be attributed to the presence of a more homogenous basal activity in the alloys due to a lower degree of orientation clustering, to grain boundary solute segregation, and to a higher slip diffusivity at grain interiors.

Mesoscale deformation mechanisms in relation with slip and grain boundary sliding in TA15 titanium alloy during tensile deformation

• Mesoscale deformation mechanisms (slip activities, slip transfer and grain boundary sliding (GBS)) of Ti-alloy with tri-modal microstructure were investigated. • The slip mode, slip system and activation strain presents different characteristics in the equiaxed α (α p ) and lamellar α (α l ) grains. • Two types (straight and deflect) slip transfer occur at α p /α p and α l /β boundaries depending on the geometric alignment of two slip systems. • GBS along α l /β and α p /β boundaries were captured and found be dependent on the crystallographic and geometrical orientations. Revealing the mesoscale deformation mechanisms of titanium alloy with tri-modal microstructure is of great significance to improve its mechanical properties. In this work, the collective behavior and mechanisms of slip activities, slip transfer, and grain boundary sliding of tri-modal microstructure were investigated by the combination of quasi-in-situ tensile test, SEM, EBSD and quantitative slip trace analyses. It is found that the slip behavior presents different characteristics in the equiaxed α (α p ) and lamellar α (α l ) grains. Under a low level of deformation, almost all the slip deformation is governed by single basal and prismatic slips for both of α p and α l , despite small amount of < a >-pyramidal slip exists in α l grains. As deformation proceeds, < a >-pyramidal and < c + a >-pyramidal slip systems with high Schmid factors were activated in quantities. Specially, certain coarse prismatic slip bands were produced across both of single and colony α l grains whose major axes tilting about 40 °–70 ° from the tensile axis. Slip transfer occurs at the boundaries of α p /α p and α l /β under the condition that there exists perfect alignment between two slip systems and high Schmid factors of outgoing slip system. The slip transfer across α l /β boundary can be divided into two types: straight slip transfer and deflect slip transfer with a deviation angle of 5 °–12 °, depending on the alignment of slip planes of two slip systems. The grain boundary sliding along boundaries of α l / β and α p / β was captured by covering micro-grid on tensile sample. It is found that the crystallographic orientation and the geometrical orientation related to loading axis play great roles in the occurrence of grain boundary sliding.

Quasi-in-situ study of the twinning evolution of ZC61 alloy during dynamic ED- ERD compression process

In this study, the evolution of extension twinning in extrusion-shear deformed ZC61 alloy, including nucleation, growth, and detwinning, during two-step interrupted dynamic compression along the extrusion direction (ED) and extrusion radial direction (ERD) was investigated using quasi-in-situ electron backscatter diffraction (EBSD) observation. The results show that numerous extension twins, <a> dislocation slips and <c+a> dislocation slips are activated and some detwinning behavior is observed after the second indirect reverse loading step. Most of the activated extension twins satisfy Schmid's law; however, some low-Schmid factor twins form during the compression processes along both the ED and ERD due to strain accommodation among adjacent twins. The slip trace method can determine specific slip systems initiated during the compression process based on the best match between the Schmid factor calculated using the Euler angle and the observed slip trace.

Criteria for slip transfer across grain and twin boundaries in pure Ni

Slip transfer at grain boundaries and annealing twin boundaries was studied in polycrystalline Ni by means of slip trace analysis. Slip transfer or blocking was assessed in >200 boundaries and was related with geometrical criteria that establish the alignment between the active slip systems across the boundaries. It was found that slip transfer mainly occurs at low-angle regular grain boundaries and that a Luster–Morris parameter >0.8 stands for the best criterion to assess slip transfer. In the case of coherent or incoherent twin boundaries, slip transfer occurs when the residual Burgers vector is close to zero through dislocation cross-slip.

Tribologically induced crystal rotation kinematics revealed by electron backscatter diffraction

Tribological loading of metals induces microstructural changes by dislocation-mediated plastic deformation. During continued sliding, combined shear and lattice rotation result in the formation of crystallographic textures which influence friction and wear at the sliding interface. In order to elucidate the fundamental lattice rotation kinematics involved in this process during the early stages of sliding, we conducted unlubricated, linear single pass sliding experiments on a copper bicrystal using sapphire spheres. Electron backscatter diffraction (EBSD) performed directly on the bulk surface of the wear tracks in the vicinity of the grain boundary reveals crystal lattice rotations by approximately up to 35°. Predominantly, the tribologically induced crystal rotations appear to be kinematically constrained to rotations around the transverse direction (TD) and occur in both grains, irrespective of load (2 to 8 N). We demonstrate that inverting the sliding direction (SD) inverts the sense of crystal rotation, but does not change the principal nature of rotation for the majority of indexed EBSD data. A lower proportion of the crystal lattice rotates much farther around TD (roughly up to 90°), accompanied by a superimposed crystal rotation around ±SD. Analysis reveals that sliding direction and grain orientation exert a systematic influence of how crystal rotations are accommodated. This is rationalized in terms of geometry, anisotropic wear track profiles and slip traces. Under specific conditions, combined crystal rotation and twinning are observed. These detailed insights into the fundamental nature of tribologically induced lattice rotation kinematics provide important guidance for applied research targeting materials with superior tribological properties.

Micropillar compression of single crystal tungsten carbide, part 2: Lattice rotation axis to identify deformation slip mechanisms

The plastic deformation mechanisms of tungsten carbide at room and elevated temperatures influence the wear and fracture properties of WC-Co hardmetal composite materials. The relationship between residual defect structures, including glissile and sessile dislocations and stacking faults, and the slip deformation activity, which produce slip traces, is not clear. Part 1 of this study showed that 101¯0 was the primary slip plane at all measured temperatures and orientations, but secondary slip on the basal plane was activated at 600 °C, which suggests that 〈a〉 dislocations can cross-slip onto the basal plane at 600 °C. In the present work, Part 2, lattice rotation axis analysis of deformed WC micropillar mid-sections has been used to discriminate 〈a〉 prismatic slip from multiple 〈c + a〉 prismatic slip in WC, which has enabled the dislocation types contributing to plastic slip to be distinguished, independently of TEM residual defect analysis. Prismatic-oriented micropillars deformed primarily by multiple 〈c + a〉 prismatic slip at room temperature, but by 〈a〉 prismatic slip at 600 °C. Deformation in the near-basal oriented pillar at 600 °C can be modelled as prismatic slip along 〈c〉 constrained by the indenter face and pillar base. Secondary 〈a〉 basal slip, which was observed near the top of the pillar, was activated to maintain deformation compatibility with the indenter face. The experimentally observed lattice rotations, buckled pillar shape, mechanical data, and slip traces are all consistent with this model.

Micropillar compression of single crystal tungsten carbide, Part 1: Temperature and orientation dependence of deformation behaviour

Tungsten carbide cobalt hardmetals are commonly used as cutting tools subject to high operation temperature and pressures, where the mechanical performance of the tungsten carbide phase affects the wear and lifetime of the material. In this study, the mechanical behaviour of the isolated tungsten carbide (WC) phase was investigated using single crystal micropillar compression. Micropillars in two crystal orientations, 1-5 μm in diameter, were fabricated using focused ion beam (FIB) machining and subsequently compressed between room temperature and 600 °C. The activated plastic deformation mechanisms were strongly anisotropic and weakly temperature dependent. The flow stresses of basal-oriented pillars were about three times higher than the prismatic pillars, and pillars of both orientations soften slightly with increasing temperature. The basal pillars tended to deform by either unstable cracking or unstable yield, whereas the prismatic pillars deformed by slip-mediated cracking. However, the active deformation mechanisms were also sensitive to pillar size and shape. Slip trace analysis of the deformed pillars showed that 101¯0 prismatic planes were the dominant slip plane in WC. Basal slip was also activated as a secondary slip system at high temperatures.

A new approach to understand the deformation behavior and strengthening mechanism of molybdenum alloy: From single crystal to polycrystal

The deformation behavior and strengthening mechanism for Mo-3Nb single crystal with 〈111〉 orientation and polycrystal have been investigated and disclosed comprehensively in a wide temperature range by quasi-static compression with 7% plastic strain. The slip traces of single crystal and polycrystal at room temperature show different features, which long continuous slip traces appear on the surface of deformed Mo-3Nb single crystals while some branch-off slip lines can only be observed in large grains for polycrystals. The conjugate slip planes of single crystals are activated at the elevated temperature, while the slip lines of polycrystals are difficult to be observed because of the grain boundary sliding effect. The compressive stress of single crystals and polycrystals shows different results at different deformation temperatures, suggesting that grain boundaries in polycrystals play an important role in the strengthening mechanism. TEM analysis shows that dislocation entanglement is the main strengthening mechanism both in single crystal and polycrystal during the room temperature deformation. With the increase of temperature, the strengthening mechanism caused by dislocation entanglement becomes ineffective, while grain boundaries continue to hinder the dislocation movement and the strengthening effect greatly weakened due to the dynamic recovery.

Stacking fault energy and ductility in a new zirconium alloys: A combined experimental and first-principles study

A combined experimental and first-principles study was performed on a new zirconium alloy. Experimental results showed that alloy elements, such as Cu, Nb, etc., mainly dissolve into the matrix of zirconium alloy where a larger number of slip traces are detected except for dislocation after deformation. First-principles calculation showed that Nb significantly reduces unstable stacking fault energy (γus), leading to the increment of {101-0}〈112-0〉 slip systems activity, and the stable and unstable stacking fault energies are both dramatically decreased with the addition of Cu, which is beneficial to the activation of the {0001} <101-0> and {101-1} <112-3> slip systems. It is suggested that basal or pyramidal plane becoming the second primary slip system givesriseto excellent forming properties, especially ductility. A new zirconium alloy containing Nb, Fe, and Cu elements was tested through uniaxial tension tests with digital image correlation (DIC) equipment. Both experimental and theoretical results showed that adding Nb, Fe, and Cu can enhance the ductility of zirconium alloys. Electronic properties of various stacking faults were also analyzed to explore the origin of excellent forming properties. Added element and its matching amount can be considered from the aspects of electronegativity and concentration effect for the preparation of zirconium alloys with excellent forming properties.

Room-temperature plasticity and size-dependent mechanical responses in small-scale B1-NbC(001) single-crystals

We report on the mechanical responses of NbC(001) single-crystals subjected to nanoindentation and in situ scanning electron microscopy based uniaxial microcompression tests at room-temperature. Using X-ray diffraction and X-ray photoelectron spectroscopy, we determine that the as-purchased bulk NbC is a 001-oriented, B1-structured, single-crystal with lattice parameter of 0.4446 ± 0.0002 nm and C/Nb ratio of ∼0.99. From nanoindentation experiments performed on the bulk crystal using a Berkovich diamond indenter, we measure an elastic modulus of 495 ± 14 GPa and an indentation-depth (h) dependent hardness decreasing from 31.0 ± 1.2 GPa at h = 0.2 µm to 26.4 ± 0.4 GPa at h = 0.9 µm. During uniaxial compression of cylindrical pillars with top diameters D between ∼0.25 µm and ∼0.75 µm, we observe strain hardening, increasing yield strengths with decreasing D, and surprisingly extensive plastic deformation with strains exceeding 2% and up to 30% in larger-size pillars. Electron microscopy characterization of the compressed pillars reveal slip traces characteristic of {110}〈11¯0〉 and {111}〈11¯0〉 slip systems. We suggest that the unexpected activation of {111}〈11¯0〉 facilitates plasticity in NbC(001).

Inhibiting effect of I-phase formation on the plastic instability of the duplex structured Mg-8Li-6Zn-1.2Y (in wt.%) alloy

In this work, the tensile behaviors of Mg-8%Li and Mg-8%Li-6%Zn-1.2%Y alloys at ambient temperature were investigated and compared. It revealed that the plastic instability of Mg-8%Li alloy was quite remarkable and the variation range of serrated flowing stress can reach 5 MPa. For the Mg-8%Li-6%Zn-1.2%Y alloy, the formation of I-phase (Mg3Zn6Y) can simultaneously enhance the tensile strength and eliminate the plastic instability phenomenon, whilst its ductility was degraded. The in-situ tensile tests revealed that for the Mg-8%Li alloy, the severity and number of slip traces present in both α-Mg and β-Li matrix phases increased remarkably with the applied tensile strain. However, slip traces were quite fine in β-Li matrix phase and could be clearly observed when the applied tensile strain exceeded 18%. Due to the incompatibility of plastic deformation occurred in two matrix phases, the induced strain concentration at α-Mg/β-Li interfaces caused their subsequent cracking. For the Mg-8%-6%Zn-1.2%Y alloy, the I-phase distributed at α-Mg/β-Li interfaces suppressed the plastic deformation of α-Mg matrix phase and the tensile strain was dominated by the β-Li matrix phase, resulting in the disappearance of plastic instability. Moreover, the plastic strain would preferentially concentrate at I-phase/β-Li interfaces and subsequently induced the cracking of I-phase.

Dynamic mechanical properties and microstructure evolution of AlCoCrFeNi2.1 eutectic high-entropy alloy at different temperatures

The dynamic mechanical properties of the AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA) fabricated by drop-casting (DC) and suction-assisted casting (SC) are investigated under different strain rates and temperatures using Split Hopkinson Pressure Bar (SHPB) system. Microstructure analysis is conducted through SEM, EBSD and TEM to reveal the deformation and fracture mechanisms of the present EHEA. The yield strength and flow stresses for both the DC alloy and SC alloy increase with increasing strain rate, but decrease with increasing temperature. A mixture of ductile and brittle fracture morphologies exists in both the DC and SC EHEAs. Moreover, the SC alloy displays more shear dimples on the fracture surface, while brittle fracture features become dominant for the DC alloy. Abundant accumulation of dislocations in the quasi-static test produces slip traces in the FCC lamellar phase which further enhances the plasticity of the SC alloy. Meanwhile, the dislocation cells formed by high-density dislocation tangles at high strain rates, leading to apparent work hardening and strain rate sensitivity of the SC alloy. The adiabatic shear band (ASB) forms in the SC alloy before shear failure and the adjacent twisted and curved lamellar structures play a significant role in preventing shear localization. Overall, the SC alloy exhibits a stronger strain rate effect and better ductility than the DC alloy.

Influence of grain size and grain boundary misorientation on the fatigue crack initiation mechanisms of textured AZ31 Mg alloy

The deformation and crack initiation mechanisms were analyzed in a textured AZ31B-O Mg alloy subjected to fully-reversed, strain-controlled cyclic deformation along the rolling direction after 50 cycles (approximately 33% of the fatigue life). Distinct deformation bands corresponding to pyramidal slip or tensile twins were found in 538 grains out of 2100 grains. Slip trace analysis showed that 72.3% were pyramidal slip bands and 18.4% were twin boundaries. Both pyramidal slip and twinning was only found in 9.1% of the grains with deformation bands. Cracking was widespread after 50 cycles. Grain boundary cracks were found in ≈15% of the small grains (< 20 µm) and they were mainly associated with high angle grain boundaries (>40º). Cracking was also found to occur by transgranular cracks parallel to the pyramidal slip bands or twin boundaries in large grains (>45 µm). The majority (>60%) of these large grains presented transgranular cracks after 50 cycles.