Low Critical Resolved Shear Stress Research Articles

Micro-mechanical behavior of lamellar structured Ti6Al4V alloy upon compression via experimental and crystal plasticity study

The micro-mechanical behavior of lamellar structured Ti6Al4V alloy to a small compressional strain of 4.5% was studied. Strain and stress intends to localize along grain boundaries (GBs), triple points (TPs) as well as grain interiors (GIs). The maximum strain and stress is found at some GB-TPs with soft and grain clustering mostly owing to their strong strain incompatibility. Their heterogeneous distribution is highly related to individual crystallographic orientation, their size, shape and grain-grain interactions. The local dislocation behavior forms ledges along with Geometrically necessary dislocations (GNDs) to accomodate strain incompatibility. EBSD and crystal plasticity (CP) simulation disclosed some long range meso-bands (about 45° to loading direction) which crosse multi-grains and some mini-bands within grain interiors (GIs) reflecting a concentrated strain behavior and ductile nature of the Ti6Al4V material. Prismatic slip dominates Ti6Al4V lattice (mainly from surface dislocation activation) determined by its low critical resolved shear stress (CRSS) which is away from some findings of literature. Dislocation is found commenced from some high angle GBs (HAGBs) and extened into GI which can pass through GBs of both low (LA) and high angles given their slip plane coherency. The substructures (α-lamellae) separated by LAGBs within α-colonies, have no significant effect to dislocation movement. Fatigue failure can be inferred from the early stage micro-mechanical behavior through accumulated plastic strain and strong strain incompatability. Fractography reaveals fatigue failure from surface, sub-surface, shearing, cleavage and intergranular modes are presumed highly related to surface defects, compressional residual stress gradient, easy prismatic slip and soft and hard GBs. Multiple crack origins from such combinations are deemed for reduced fatigue life of shot peened samples regardless of the surface compressional residual stress being generated.

Analytical model for estimating calcium solution strengthening in single-crystal Mg-Ca alloys at finite temperatures

Mg is the lightest structural metal and a suitable replacement for heavy steel and Al alloys. However, Mg exhibits poor plastic formability at room temperature, and the strength of conventional Mg alloys is less than that of most Al alloys. Its low formability at room temperature is attributed to the lack of slip systems because of the extremely large gap in the critical resolved shear stress (CRSS) between the lowest basal system and the other slip and twin systems. This low CRSS of the basal slip usually limits the strength of Mg alloys. Therefore, strengthening the basal slip is important to improve the strength and deformability of Mg alloys and enhance their ductility. In this study, we construct an analytical model for predicting the CRSS in single-crystal Mg–Ca alloys at finite temperatures by quantitatively investigating the CRSS for the 〈a〉 dislocation gliding on the basal plane at temperatures from 1 to 500 K through molecular-dynamics computations. The prediction results agree well with our simulation results and the micropillar compression test results of Mg–0.3at.%Ca and Mg–0.6at.%Ca alloys at room temperature.

A unified non-empirical strength model

Strength, as an important indicator of structural materials, has always been an important research topic in materials science. Theoretically, building a strength model is a rewarding method to understand the relationship between the mechanical properties and microstructure of materials. Although many strength models can reduplicate experimental values very well, they are empirical models, and their applicability is limited to materials for which empirical parameters have been obtained. Here, a non-empirical strength model is proposed based on the two-dimensional (2D) displacement potential of dislocation slipping, which can be applied to different chemically bonded crystals. Owing to the large electron localization function (ELF), covalent and ionic crystals have a high 2D displacement potential of dislocation slipping, and their dislocation slip mode prefers the kink-pair mode, further exhibiting a high critical resolved shear stress (CRSS). In contrast, metallic crystals with a small ELF have a low 2D displacement potential of dislocation slipping, and their dislocation slip mode is more inclined to the string mode, showing a low CRSS. This work provides new insights into dislocation-slipping configurations that will be useful for the development of new high-performance structural materials.

The role of biaxial stress ratio on the mechanical behavior and deformation mechanisms in HCP α-Ti

The mechanical behavior of hexagonal close-packed (HCP) Ti alloy is closely related to the stress state. Renewing the mechanical behavior and associated mechanisms of HCP α-Ti alloy under biaxial stress states is critical for their manufacturability and serviceability. In this work, biaxial tensile tests with different stress ratios closing to actual service conditions were conducted to investigate the mechanical behavior of commercially pure Ti (CP-Ti). Then, comprehensive microstructural characterizations were performed using electron back-scattering diffraction (EBSD) and transmission electron microscopy (TEM) techniques. The effects of biaxial stress ratio on the evolution of dislocation and deformation twinning throughout plastic deformation were clarified. It was found that the activity of prismatic slip was suppressed while pyramidal slip was promoted, which was attributed to the decrease of external stress component on the prismatic planes and the increase of that on the pyramidal planes with increasing biaxiality. Moreover, the increased stress component in the c-axis direction also encouraged the occurrence of extension twinning at high biaxiality. The decrease of prismatic slip with low critical resolved shear stress (CRSS) and the increase of twinning and pyramidal slip with high CRSS contributed to an increase in deformation resistance and work hardening rate with increasing biaxiality.

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.

Microstructures and mechanical properties of α‐SiC ceramics after high‐temperature laser shock peening

AbstractA novel high‐temperature laser shock peening (HT‐LSP) process was applied to polycrystalline α‐SiC to improve the mechanical performance. HT‐LSP prevents microcrack formation on the surface while induces plastic deformation in the form of dislocation slip on the basal planes, which may be caused by the combination of high shock pressure and a lower critical resolved shear stress at 1000℃. A maximum compressive residual stress of 650 MPa, measured with Raman spectroscopy, was introduced into the surface of α‐SiC by HT‐LSP, which can increase the nanohardness and in‐plane fracture toughness of α‐SiC by 8% and 36%, respectively. This work presents a fundamental base for the promising applications of HT‐LSP to brittle ceramics to increase their plasticity and mechanical properties.

Intermediate stacking fault and twinning induced cooperative strain evolution of dual phase in lean duplex stainless steels with excellent cryogenic strength-ductility combinations

The effects of intermediate substructures on the strain evolution in dual phase and the correlated mechanical properties at different temperatures were investigated in two lean duplex stainless steels (LDSSs) with different stacking fault energy (SFE). At room temperature (RT), the 1#-LDSS shows a lower yield strength (YS) but high tensile elongation (TEL) values than those of the 2#-LDSS due to a more significant TRIP effect, resulting from a lower SFE and mechanical stability of austenite (γ). Meanwhile, the plasticity of the ferrite (α) strip is also accommodated by the grain rotation process and texture transformation from component ⟨123‾⟩{111} to ⟨112‾⟩{111}, preventing a drastic dislocation generation in the α/γ interfaces. However at liquid nitrogen temperature (LNT), although a strong dislocation hardening of dual phase accompanied with a two-stage γ→ε→α’ transformation sequence occurs at low strains of 1#-LDSS, the severe dislocation accumulation in ferrite becomes the potential site of crack nucleation, leading to a high ductility loss compared to the RT. In contrast, a simultaneous enhancement of strength and ductility can be obtained in the 2#-LDSS at LNT. The introduction of intermediate stacking faults (SFs) and twins due to a lower critical resolved shear stress (CRSS) can not only provide additional work hardening capacity in the early deformation stage, but also lead to a gentle dislocation increment in ferrite. The cooperative co-deformation contributes to a sustainable high strain hardening until large strains. Therefore, a high YS over 1.3 GPa and a large TEL of about 50% can be obtained in the 2#-LDSS

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.

Cold creep of titanium: Analysis of stress relaxation using synchrotron diffraction and crystal plasticity simulations

It is well known that titanium and some titanium alloys creep at ambient temperature, resulting in a significant fatigue life reduction when a stress dwell is included in the fatigue cycle. It is thought that localised time dependent plasticity in ‘soft’ grains oriented for easy plastic slip leads to load shedding and an increase in stress within a neighbouring ‘hard’ grain that is poorly oriented for easy slip. Quantifying this time dependent plasticity process is key to successfully predicting the complex cold dwell fatigue problem. In this work, synchrotron X-ray diffraction during stress relaxation experiments was performed to characterise the time dependent plastic behaviour of commercially pure titanium (grade 4). Lattice strains were measured by tracking the diffraction peak shifts from multiple plane families (21 diffraction rings) as a function of their orientation with respect to the loading direction. The critical resolved shear stress, activation energy and activation volume were established for both prismatic and basal slip modes by fitting a crystal plasticity finite element model to the lattice strain relaxation responses measured along the loading axis for three strong reflections. Prismatic slip was the easier mode having both a lower critical resolved shear stress (τcbasal = 252 MPa and τcprism = 154 MPa) and activation energy (ΔFbasal= 10.5×10−20J = 0.65 eV andΔFprism = 9.0×10−20J = 0.56 eV). The prism slip parameters correspond to a stronger strain rate sensitivity compared to basal slip. This slip system dependence on strain rate has a significant effect on stress redistribution to ‘hard’ grain orientations during cold dwell fatigue.

Design of Nickel-Cobalt-Ruthenium multi-principal element alloys

The possibility of designing multi-principal element alloys in the Ni-Co-Ru system is investigated by first-principles and experimental means. A high-throughput first-principles approach is employed to probe the effect of composition on planar fault energies. Of most interest, a FCC alloy, Ni40Co40Ru20, exhibits a high room temperature strength compared to existing nickel-containing solid solution FCC high entropy alloys. The presence of Ru imparts the alloy with unique properties. First, the low stacking fault energy provides a low critical resolved shear stress for twinning, allowing a TWIP effect. Second, the high lattice friction and the associated high yield strength enables the critical resolved shear stress for twinning to be reached at low strains at room temperature, resulting in the formation of a dense network of planar defects for increased work hardening. A TRIP effect is also observed at higher strains at room temperature.

Cold Creep of Titanium: Analysis of Stress Relaxation Using Synchrotron Diffraction and Crystal Plasticity Simulations

It is well known that titanium and some titanium alloys creep at ambient temperature, resulting in a significant fatigue life reduction when a stress dwell is included in the fatigue cycle. It is thought that localised time dependent plasticity in 'soft' grains oriented for easy plastic slip leads to load shedding and an increase in stress within a neighbouring 'hard' grain poorly oriented for easy slip. Quantifying this time dependent plasticity process is key to successfully predicting the complex cold dwell fatigue problem. In this work, synchrotron X-ray diffraction during stress relaxation experiments was used to characterise the time dependent plastic behaviour of commercially pure titanium (grade 4). Lattice strains were measured by tracking the diffraction peak shift from multiple plane families (21 diffraction rings) as a function of their orientation with respect to the loading direction. Critical resolved shear stress, activation energy and activation volume were established for both prismatic and basal slips modes by fitting a crystal plasticity finite element model to the lattice strain relaxation responses measured along the loading axis for three strong reflections. Prismatic slip was the easier mode having both a lower critical resolved shear stress (τc basal=252MPa and τcprism=154MPa) and activation energy (∆F basal=10.5×1020 J=0.65eV and ∆F prism=9.0×1020 J=0.56eV). The prism slip parameters correspond to stronger strain rate sensitivity compared to basal slip. This slip system dependence on strain rate will have a significant effect on stress redistribution to hard grain orientations during cold dwell fatigue.

Role of grain size and crystallographic texture on tensile behavior induced by sliding mechanism in Ti-6Al-4V alloy

The aim of this paper is to investigate the coupled effects of grain size and crystallographic texture on the mechanical behavior induced by gliding mechanism in Ti-6Al-4V alloy. Thus, four microstructures of Ti-6Al-4V alloy whose grain size and crystallographic texture are different were examined by tensile tests along the rolling direction at room temperature. In this study, the contribution of gliding on basal a, prismatic a and pyramidal c+a plans in the accommodation of plastic strain was estimated by means of a slip trace analysis. The role of the individual grain size and the crystallographic texture was then statistically evaluated. Based on the results of slip trace analysis, numerical optimizations of the Critical Resolved Shear Stress (CRSS) of basal a, prismatic a and pyramidal c+a in the four microstructures were then carried out, using transition scale rules and a local behavior model. The results suggest that the mechanical behavior of Ti-6Al-4V is controlled by the activation of slip systems that depend not only on their CRSS but also the initial orientation and size of each individual grain. The low CRSS of prismatic a slip systems can lead to early activation of these systems in favorably oriented coarse grains. Therefore, a local plastic deformation can be shown. At high levels of loading, increasing the grain size can minimize the crystallographic texture effects by deforming the unfavorably oriented coarse grains. Moreover, based on the results of the numerical optimization, it can be also suggested that the CRSS can decrease with the increase in grain size according to the local Hall–Petch relationship.

Anisotropy of strength and plasticity in single-colony lamellar structure of Ti‒6Al‒4V alloy

Micro-tensile tests were performed on single-colony specimens to elucidate the anisotropic slip behaviour in Ti‒6Al‒4 V alloy with a lamellar microstructure. The critical resolved shear stress (CRSS) of 〈c + a〉 pyramidal slip was more than 1.5 times higher than that of 〈a〉 slip. The CRSSes of 〈a〉 slips increased with increasing length fraction of β phase along the slip direction, revealing that the β phase acts as a hard phase for soft orientations. The 〈c + a〉 pyramidal slip with β laths exhibited a low CRSS value compared with that of a Ti–5Al solid solution, suggesting that the β phase facilitates the 〈c + a〉 slip.

Effects of strain state and slip mode on the texture evolution of a near-α TA15 titanium alloy during hot deformation based on crystal plasticity method

A thorough understanding of the texture evolution of near-α titanium alloys during the hot metal forming can help obtain an optimal crystallographic texture and material performance. The strain state has an obvious effect on the texture evolution of near-α titanium alloys during the hot metal forming. In this paper, the texture evolution of a near-α TA15 titanium alloy during the hot metal forming under different strain states were discussed based on the crystal plasticity finite element method. It is found that the basal and prismatic slip systems are regarded as the dominant slip modes due to the similar low critical resolved shear stress during the hot metal forming of the TA15 sheet rotating the lattice around the [101¯0] and 〈0001〉 axis, respectively. Once both of them cannot be activated, the pyramidal-2 slipping occurs rotating the lattice around the [101¯0] axis. The relationship between the texture evolution and strain state is established. All the (0001) orientations form a band perpendicular to the direction of the first principal strain. The width of the band along the direction of the second principal strain depends on the ratio of the compressive effect to the tensile effect of the second principal strain. This relationship can help control the crystallographic texture and mechanical properties of the titanium alloys component during the hot metal forming.

Anisotropy of Strength and Plasticity in Single-Colony Lamellar Structure of Ti‒6Al‒4V Alloy

Micro-tensile tests were performed on single-colony specimens to elucidate the anisotropy of slip behaviour in Ti‒6Al‒4V alloy with a lamellar microstructure. The critical resolved shear stress (CRSS) of pyramidal slip was more than 1.5 times higher than that of slip. The CRSS values of slips increased with increasing length fraction of β phase along the slip direction, revealing that the β phase acts as the hard phase. The pyramidal slip with β laths exhibited a low CRSS value compared with that of a Ti–5Al solid solution, suggesting that the β phase facilitates the slip.

Springback Suppression of AZ31 Magnesium Alloy Sheet in Draw Bending at Elevated Temperature

In recent years, warm forming of magnesium sheets has been investigated by many researchers since the ductility of the sheets becomes considerably higher due to low CRSS (critical resolved shear stress) at high temperatures (e.g., [1]-[3]). In the present research, the springback of AZ31 magnesium sheet was investigated by performing a draw-bending experiment at several temperatures from 20 °C (room temperature) to 200 °C at drawing speeds ranging from 0.01 to 1.0 mm/s. From the experiment, it was found that the springback was remarkably reduced at 200 °C, especially at a low forming speed, since the flow stress was very low under such a forming condition, and furthermore, the stress relaxation effect was dominant. The effects of temperature and forming speed on springback were discussed.

Coupling multi-scale mechanical testing techniques reveals the existence of a trans-granular channel fracture deformation mechanism in high dose Inconel X-750

Recent testing has shown that Inconel® X-750 materials have lost strength and ductility following irradiation, which may raise concerns over the structural integrity of the material. Bulk component compression testing shows irradiation temperature has a clear influence on mechanical behaviour; material irradiated at higher temperatures exhibits less ductility compared to material irradiated at lower temperatures. This has been re-affirmed with the most recent large scale compression testing of material irradiated to doses between 70 dpa and 80 dpa. Measuring material yield strength from this data has proven to be challenging, due to the complex geometry of the material and the testing methodology. This work outlines the use of micro-indentation, and novel small-scale micro-mechanical testing to measure the mechanical properties of neutron irradiated Inconel X-750 material. The results and conclusions provide valuable insights into the mechanical property evolution, and embrittlement of the material. Microhardness and micro-mechanical testing have confirmed that material irradiated at lower temperatures has a lower yield strength, lower critical resolved shear stress (CRSS), and higher plasticity. The current results confirm the existence of an additional mechanism of crack initiation not previously considered for Inconel X-750 material. In addition to intergranular fracture, re-examination of component-scale test fracture surfaces also reveals that some portion of the component fails via transgranular, faceted pathways. Transmission electron microscopy (TEM) of deformed micro-tensile specimens has confirmed channel fracture. When subject to a slip band or dislocation channel, helium bubbles within the grain interior shear along the slip direction and coalesce prior to shearing the ligaments between the bubbles. The two deformation mechanisms are expected to interact in the bulk material. When discrete flow localization events (dislocation channels) intersect compromised grain boundaries decorated with helium bubbles, they will be expected to produce significant stresses which drives fracture propagation along the grain boundary.

Development and Characterization of a Novel Ti-Modified High-Si Medium-Mn Steel Possessing Ultra-High Strength and Reasonable Ductility After Hot Rolling

The present work systematically correlates the strength–ductility relationship of a Ti-Si-modified Fe-Mn-Al-C steel with its microstructural characteristics such as grain size, type and morphology of precipitates, and the transformation-induced plasticity (TRIP) effect during hot deformation. The Ti-Si-modified multicomponent steel developed through melting and casting route is subjected to thermo-mechanical processing such as hot forging (at 1373 K) and subsequent hot rolling (at 1173 K). An excellent combination of ultra-high tensile strength (UTS ~ 1700 ± 20 MPa), reasonable ductility (elongation ~ 11%) and high work hardening behavior (n ~ 0.89) is achieved in the hot-rolled specimen as compared to the hot-forged one (UTS ~ 824 ± 9 MPa, n ~ 0.07) with negligible change in the elongation. The better tensile properties of the hot-rolled specimen in contrast to the hot-forged one are due to the combined effects of grain refinement during rolling, twin–twin interactions, precipitation strengthening by mixture of hexagonal structured Ti3(Al,Si)C2 and Mn-Al-Si-rich carbide precipitates and most significantly the enhanced TRIP effect. The lower critical resolved shear stress value of martensitic transformation instigates the TRIP effect during tensile testing which resulted in the increased hardness and strength of the hot-rolled specimen. The above observation offers a strong support to the proposition that TRIP effect is the dominant plasticity-enhancing mechanism activated during the deformation of the low-stacking fault energy (~ 12.2 mJ/m2) Si-Ti-modified, medium-Mn multicomponent steel employed in the present investigation.

On Shear Testing of Single Crystal Ni-Base Superalloys

Shear testing can contribute to a better understanding of the plastic deformation of Ni-base superalloy single crystals. In the present study, shear testing is discussed with special emphasis placed on its strengths and weaknesses. Key mechanical and microstructural results which were obtained for the high-temperature (T ≈ 1000 °C) and low-stress (τ ≈ 200 MPa) creep regime are briefly reviewed. New 3D stereo STEM images of dislocation substructures which form during shear creep deformation in this regime are presented. It is then shown which new aspects need to be considered when performing double shear creep testing at lower temperatures (T < 800 °C) and higher stresses (τ > 600 MPa). In this creep regime, the macroscopic crystallographic [11−2](111) shear system deforms significantly faster than the [01−1](111) system. This represents direct mechanical evidence for a new planar fault nucleation scenario, which was recently suggested (Wu et al. in Acta Mater 144:642–655, 2018). The double shear creep specimen geometry inspired a micro-mechanical in-situ shear test specimen. Moreover, the in-situ SEM shear specimen can be FIB micro-machined from prior dendritic and interdendritic regions. Dendritic regions, which have a lower γ′ volume fraction, show a lower critical resolved shear stress.

Dislocation nucleation from symmetric tilt grain boundaries in body-centered cubic vanadium

We perform molecular dynamics (MD) simulations with two interatomic potentials to study dislocation nucleation from six symmetric tilt grain boundaries (GB) using bicrystal models in body-centered cubic vanadium. The influences of the misorientation angle are explored in the context of activated slip systems, critical resolved shear stress (CRSS), and GB energy. It is found that for four GBs, the activated slip systems are not those with the highest Schmid factor, i.e., the Schmid law breaks down. For all misorientation angles, the bicrystal is associated with a lower CRSS than their single crystalline counterparts. Moreover, the GB energy decreases in compressive loading at the yield point with respect to the undeformed configuration, in contrast to tensile loading.