Slip Traces Research Articles

Activated slip systems in bimodal Ti–6Al–4V plastically deformed at low and moderately high temperatures

Activated slip systems were investigated by analyzing slip traces and measuring the distribution of the Schmid factor to understand the controlling mechanism behind the change in effective stress seen in bimodal Ti–6Al–4V. It was ascertained that both basal and prismatic slips are dominant at 77 K and 225 K. It was suggested that the critical resolved shear stress (CRSS) for basal and prismatic slips are close to each other at 77 K and 225 K. The activation of basal alongside prismatic slips was unexpected, owing to previous reports which determined that the CRSS of prismatic slips is lower than that of basal slips in single-phase Ti-6.6Al. It is presumed that the difference arises between single phase Ti-6.6Al and bimodal Ti–6Al–4V from the boundary of α/β interfaces in bimodal Ti–6Al–4V, which acts as obstacles for prismatic and pyramidal slips. In addition to those, it was found that pyramidal slips are dominant at 550 K in this study. The reasons for the higher activation of pyramidal slips in comparison with basal and prismatic slips at 550 K are also discussed. Present study evidenced that the change in the dominant active slip systems is responsible for the change in the trend of temperature dependence interpreted in the effective stress.

The effect of cubic-texture on fatigue cracking in a metastable β titanium alloy subjected to high-cycle fatigue

In this work, stress-controlled high-cycle fatigue tests were performed on Ti-7Mo-3Nb-3Cr-3Al alloy with β-cubic texture to investigate fatigue crack-initiation mechanisms and microstructurally small fatigue crack growth behaviors. The S-N curve shows a general feature of monotonic increase in the mean lifetime with decreasing stress level. Fatigue crack-initiation sites revealed the contiguous β facets. The fracture surface observations and the slip trace analysis show that most faceting occurred in {110} 〈111〉 slip system with the highest Schmid factor. Besides, a higher Kernel average misorientation (KAM) was found in fatigued microstructure. This strain localization would contribute to the fatigue crack-initiation process.

Slip initiation in interlayered β and corresponding slip transfer during compression of a lamellar-structure titanium alloy

The scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) were used for a quasi in-situ observation on the slip trace and crystallographic orientation at four strains during the compression of a widmannstätten-structure α/β titanium alloy. The previous investigations suggested that the slip always initiated in lamellar α prior to interlayered β due to the lower yield strength and higher Young's modulus of HCP α phase. Interestingly, it is found that the slip initiates in the interlayered β prior to lamellar α within the α colony in this work. Meanwhile, the preferential slip initiation in interlayered β does not occur at the highest Schmid factor (SF) which is calculated by the applied stress. The preferential slip in interlayered β is related to the heterogeneous deformation among different α colonies, which is related to the distribution of deflection angle θ between loading direction and c-axis of α phase. The slip transfers from interlayered β to lamellar α indirectly, which not only requires a good collaboration between SF and geometric compatability parameter (m'), but also needs a large accumulation of deformation in interlayered β.

Fatigue Fracture Mechanism of a Nickel-Based Single Crystal Superalloy with Partially Recrystallized Grains at 550 °C by In Situ SEM Studies

The fatigue fracture mechanism of a nickel-based single crystal (NBSC) superalloy with recrystallized grains was studied at 550 °C by in situ observation with a scanning electron microscope (SEM) for the first time. Multiple crack initiations associated with recrystallized grain boundaries and carbides were observed. By analysis of the slip traces and crack propagation planes, the operated slip systems were identified to be octahedral for both single crystal substrate and recrystallized grains. Distinct crystallographic fractures dominated, accompanied by recrystallized grain boundary associated crack initiations. This is different from the widely reported solely intergranular cracking at high temperature. Fatigue crack growth rate curves showed evident fluctuation, due to the interaction of fatigue cracks with local microstructures and the crack coalescence mechanism. Both the recrystallized grains and the competition between different slip systems were responsible for the deceleration and acceleration of fatigue microstructurally small crack behavior.

Automated Quantitative Analyses of Fatigue-Induced Surface Damage by Deep Learning.

The digitization of materials is the prerequisite for accelerating product development. However, technologically, this is only beneficial when reliability is maintained. This requires comprehension of the microstructure-driven fatigue damage mechanisms across scales. A substantial fraction of the lifetime for high performance materials is attributed to surface damage accumulation at the microstructural scale (e.g., extrusions and micro crack formation). Although, its modeling is impeded by a lack of comprehensive understanding of the related mechanisms. This makes statistical validation at the same scale by micromechanical experimentation a fundamental requirement. Hence, a large quantity of processed experimental data, which can only be acquired by automated experiments and data analyses, is crucial. Surface damage evolution is often accessed by imaging and subsequent image post-processing. In this work, we evaluated deep learning (DL) methodologies for semantic segmentation and different image processing approaches for quantitative slip trace characterization. Due to limited annotated data, a U-Net architecture was utilized. Three data sets of damage locations observed in scanning electron microscope (SEM) images of ferritic steel, martensitic steel, and copper specimens were prepared. In order to allow the developed models to cope with material-specific damage morphology and imaging-induced variance, a customized augmentation pipeline for the input images was developed. Material domain generalizability of ferritic steel and conjunct material trained models were tested successfully. Multiple image processing routines to detect slip trace orientation (STO) from the DL segmented extrusion areas were implemented and assessed. In conclusion, generalization to multiple materials has been achieved for the DL methodology, suggesting that extending it well beyond fatigue damage is feasible.

Mechanical behavior and thermal activation analysis of HfNbTaTiZr body-centered cubic high-entropy alloy during tensile deformation at 77 K

Plastic deformation of HfNbTaTiZr body-centered cubic high-entropy alloy (BCCHEA) was investigated at 77 K. Tensile stress-strain curves showed strong temperature dependence of yield strength. Characteristics of dislocation slip at 77 K was analyzed using slip traces which confirmed active slip on {112} planes. The apparent activation volume (V*) suggested thermally activated kink/kink-pair nucleation at 77 K (V*~7b3). V* values of BCCHEA and pure Nb were nearly same at 77 K. Low-temperature plasticity of the BCCHEA showed characteristics of pure BCC metals, suggesting chemical-complexity of HEAs may not alter the Peierls potential of a/2<111> screw dislocations.

Ex-situ study on mechanical properties and deformation mechanism of three typical microstructures in TA19 titanium alloy

The near-α titanium alloy TA19 with three typical microstructures (lamellar, equaixed and bimodal microstructure) were compressed, accompanied by ex-situ observation on the slip trace and crack nucleation, which aimed to reveal the dependence of mechanical properties on microstructure based on analyzing the deformation mechanism. The results revealed that the lamellar microstructure had the lowest yield strength because of very large colony size. Meanwhile, although the equiaxed α grain size in equiaxed microstructure was similar to the size of equiaxed α grain and α colony in the bimodal microstructure, the strengthening effect of α/β interface in α colony made the bimodal microstructure possess the highest yield strength. The fracture strain was strongly influenced by the crack nucleation and propagation in different microstructures. It was found that the largest α colony size induced an earliest crack nucleation and easy crack propagation in the lamellar microstructure, and then resulted in the smallest fracture strain. Conversely, the small size of α grain and colony induced a higher fracture strain in the equiaxed and bimodal microstructure. Furthermore, the intergranular β phase could coordinate the deformation between the two adjacent α grains and benefit the fracture strain in the equiaxed microstructure.

Quantitative study on slip/twinning activity and theoretical critical shear strength of Mg alloy with Y addition

Slip/twinning activity and theoretical critical shear strengths (τmax) during room-temperature tensile deformation of as-extruded pure Mg and Mg-1wt.%Y sheets were investigated quantitatively via electron back-scattered diffraction (EBSD), scanning electron microscope (SEM) and first-principles calculation. Slip trace analysis results indicated the relative frequency of the active slip modes transformed from basal slip (100%) for pure Mg, to basal slip (84.8%), prismatic slip (9.7%) and pyramidal slip (5.5%) for the alloy. The proportion of grains containing twins reduced significantly from 20.5% to 5.92% with Y addition. To explain the mechanism for the enhancement of the non-basal slip activity in Mg–Y alloy, the critical resolved shear stress of basal slip and pyramidal II slip were assessed by calculating τmax. The calculated results showed that the τmax ratio of the pyramidal II slip to basal slip decreased from 13.7 to 7.5, which may be responsible for the observed enhanced activity of non-basal slip.

Extraordinary room temperature tensile ductility of laminated Ti/Al composite: Roles of anisotropy and strain rate sensitivity

Al/Ti/Al laminated metal composites (LMCs), with layer thicknesses ≥800 μm, were fabricated via hot rolling bonding and annealing. The fabricated Al/Ti/Al LMCs displayed improved tensile ductility compared with both the single Ti and single Al layers. Combinational analyses based on digital image correlation and finite element modeling revealed that the width of the interface affected zones (IAZ) could exceed 150 μm, which is more than one order of magnitude larger than previously reported values. The mismatch of plastic anisotropy between the Ti layer and the Al layer may play a crucial role in realizing such a wide IAZ. Slip trace analysis indicated that more pyramidal and basal slips were activated at the Ti/Al interface than in the central part of the Ti layer, which may be attributed to the complex stress/strain state in the IAZ. Although a wide IAZ was observed, the strain hardening capability of the Al/Ti/Al LMCs did not obviously improve. In contrast, the strain rate sensitivity improved, which is assumed to play a key role in the improved ductility of the Al/Ti/Al LMCs. This study can provide a direction for the design of heterostructured materials with high strength and high ductility.

In-situ study of creep in Sn-3Ag-0.5Cu solder

The creep behaviour and microstructural evolution of a Sn-3Ag-0.5Cu wt% sample with a columnar microstructure have been investigated through in-situ creep testing under constant stress of 30 MPa at ~298 K. This is important, as 298 K is high temperature within the solder system and in-situ observations of microstructure evolution confirm the mechanisms involved in deformation and ultimately failure of the material. The sample has been observed in-situ using repeat and automatic forescatter diode and auto electron backscatter diffraction imaging. During deformation, polygonisation and recrystallisation are observed heterogeneously with increasing strain, and these correlate with local lattice rotations near matrix-intermetallic compound interfaces. Recrystallised grains have either twin or special boundary relationships to their parent grains. The combination of these two imaging methods reveal grain 1 (loading direction, LD, 10.4° from [100]) deforms less than the neighbour grain 2 (LD 18.8° from [110]), with slip traces in the strain localised regions. In grain 2, (11¯0)[001] slip system is observed and in grain 1 (11¯0)[1¯1¯1]/2 and (110)[1¯11]/2 slip systems are observed. Lattice orientation gradients build up with increasing plastic strain and near fracture recrystallisation is observed concurrent with fracture.

The significant size effect on nucleation and propagation of crack during tensile deformation of copper foil: Free surface roughening and crystallography study

Tensile tests were conducted on copper foils with 80 μm in thickness to elucidate the effect of grain size on nucleation and growth of crack. Experimental results indicated that samples with larger grain size had smaller fracture stress and strain when the ratio of sample thickness to grain size (T/D) was below 10. This is attributed to strain localization during the early stage of tensile deformation. Confocal laser scanning microscopy (CLSM) indicated an increase in free surface roughening with increase in grain size during tensile straining. The local depression trough and thinning of sample caused by surface roughening were precursors to crack propagation. (EBSD) studies of crack in sample with T/D value of 1.8, it was observed that in the vicinity of the crack tip, a high density of dense and deep slip traces were present. The slip process gradually changed from single slip to multiple slip and then to cross slip. Both grain boundary and annealed twin boundary acted as obstacles to the movement of dislocations and led to strain localization. Furthermore, during tensile deformation, the crack propagation direction was not always perpendicular to the loading direction. A change in the direction of crack propagation occurred at the junction of grain boundaries and entered into the adjacent grain. Large angle grain boundaries provided greater resistance to the propagation of crack.

Dislocation-particle interactions in magnesium alloys

The aim of this work is to investigate the fundamentals of dislocation-particle interactions in a Mg-1%Mn-1%Nd (wt.%) (MN11) alloy in order to better understand the limited precipitation strengthening exhibited by this material. A combined approach including micromechanical testing, slip trace analysis, and high resolution transmission electron microscopy (TEM) is put in place to analyze the interaction between basal and non-basal dislocations with the Mg-Nd plate precipitates that are characteristic of the investigated alloy. It is shown that basal dislocations can easily shear the precipitates, both when their long axis lays parallel and perpendicular to the matrix c axis. This is attributed to the high coherency between the matrix and the particles, which is characterized in detail. It is additionally shown that pyramidal ⟨c + a⟩ dislocations, on the contrary, interact with precipitates both in the form of shearing along the precipitate pyramidal planes, as well as by Orowan looping. This study reveals that pyramidal <c+a> dislocations are able to shear the plate precipitates along pyramidal planes when there is a good alignment between the slip plane of the incoming matrix dislocation and the outgoing precipitate pyramidal plane. Otherwise pyramidal <c+a> dislocations sliding on matrix planes with small geometric compatibility with the precipitate pyramidal planes bypass these particles by an Orowan looping mechanism.

Deformation of µm- and mm-sized Fe2.4wt%Si single- and bi-crystals with a high angle grain boundary at room temperature

Plasticity in body-centred cubic (BCC) metals, including dislocation interactions at grain boundaries, is much less understood than in face-centred cubic (FCC) metals. At low temperatures additional resistance to dislocation motion due to the Peierls barrier becomes important, which increases the complexity of plasticity. Iron-silicon steel is an interesting, model BCC material since the evolution of the dislocation structure in specifically-oriented grains and at particular grain boundaries have far-reaching effects not only on the deformation behaviour but also on the magnetic properties, which are important in its final application as electrical steel. In this study, two different orientations of micropillars (1, 2, 4 µm in diameter) and macropillars (2500 µm) and their corresponding bi-crystals are analysed after compression experiments with respect to the effect of size on strength and dislocation structures. Using different experimental methods, such as slip trace analysis, plane tilt analysis and cross-sectional EBSD, we show that direct slip transmission occurs, and different slip systems are active in the bi-crystals compared to their single-crystal counterparts. However, in spite of direct transmission and a very high transmission factor, dislocation pile-up at the grain boundary is also observed at early stages of deformation. Moreover, an effect of size scaling with the pillar size in single-crystals and the grain size in bi-crystals is found, which is consistent with investigations elsewhere in FCC metals.

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.

Crystal plasticity finite‐element modelling of cyclic deformation and crack initiation in a nickel‐based single‐crystal superalloy under low‐cycle fatigue

AbstractNickel‐based single‐crystal superalloys are predominantly used for turbine blades in aircraft engines and land‐based gas turbines. Understanding and predicting the fatigue failure of Ni‐based single‐crystal superalloys are critical to ensure the safety of these components during operation. In this paper, low‐cycle fatigue experiments were carried out to investigate cyclic deformation of a nickel‐based single‐crystal superalloy MD2, recently developed by GE Power, with different crystallographic orientations. Specialty in situ scanning electron microscope (SEM) tests were also conducted to study the slip‐controlled initiation of short cracks under low‐cycle fatigue. In particular, the stress–strain response for both [001] and [111] orientations was used to calibrate a crystal plasticity model, which allowed us to simulate the activation of crystallographic slip systems and predict the initiation of short fatigue crack. Using the accumulated shear strain as a criterion, the simulations confirmed that the slip system with the maximum accumulated shear strain appeared to control the crack initiation. The location and direction of slip traces and short cracks, captured by the crystal plasticity finite‐element simulations, agreed with the in situ SEM observations. The modelling tool will be valuable for assessing the structural integrity of critical gas turbine blades.

Low temperature atomic-scale observations of slip traces in niobium

Slip traces in body-centred cubic single crystals usually correspond to various slip planes and even, in the case of wavy slip traces, to none identifiable crystallographic slip planes. We recently developed an experimental scanning tunnelling microscope that allows characterizing slip traces at the atomic scale. Unique experimental results obtained at 200 K and 293 K have already been published. We report here new relevant experimental results obtained at 90 K on niobium single crystals, that provide additional information in the understanding of the motion of screw dislocations in bcc metals.

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.

Comparison of sub-grain scale digital image correlation calculated using commercial and open-source software packages

High-resolution strain mapping is an increasingly prominent technique for characterising the deformation behaviour of metals. In this study, HRDIC analysis was performed on materials exhibiting either more or less intense planar slip, using different software packages to highlight any issues that might arise depending on the algorithm that is used to calculate the displacements. The differences between the algorithms were investigated using frequency distributions, strain profiling and Burgers vector direction analysis to determine their significance in terms of any subsequent interpretation of the data. A dilute zirconium alloy showed maximum strain concentrations of <4.5 and similar to previous comparison studies of different software packages on optical images, little difference was observed in the resulting strain maps. However, in a more highly alloyed two-phase titanium alloy with significant planar slip and strain values of up to 20 times the applied strain, one of the algorithms had difficulty tracking the features particularly at low strains when the difference between the strain in a slip trace is markedly higher than that in the neighbouring region. The consequences of inaccurate displacement data around a slip trace region were highlighted by an incorrect prediction of the Burgers vector when using the relative displacement ratio for slip system identification.

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.

Deformation of Μm- and Mm-Sized Fe2.4wt.%Si Single- and Bi-Crystals with a High Angle Grain Boundary at Room Temperature

Plasticity in body-centred cubic (BCC) metals, including dislocation interactions at grain boundaries, is much less understood than in face-centred cubic (FCC) metals. At low temperatures additional resistance to dislocation motion due to the Peierls barrier becomes important, which increases the complexity of plasticity. Iron-silicon steel is an interesting, model BCC material since the evolution of the dislocation structure in specifically-oriented grains and at particular grain boundaries have far-reaching effects not only on the deformation behaviour but also on the magnetic properties, which are important in its final application as electrical steel.In this study, two different orientations of micropillars (1, 2, 4 µm in diameter) and macropillars (2500 µm) and their corresponding bi-crystals are analysed after compression experiments with respect to the effect of size on strength and dislocation structures. Using different experimental methods, such as slip trace analysis, plane tilt analysis and cross-sectional EBSD, we show that direct slip transmission occurs, and different slip systems are active in the bi-crystals compared to their single-crystal counterparts. However, in spite of direct transmission and a very high transmission factor, dislocation pile-up at the grain boundary is also observed at early stages of deformation. Moreover, an effect of size scaling with the pillar size in single crystals and the grain size in bi-crystals is found, which is consistent with investigations elsewhere in FCC metals.