Prismatic Slip Systems Research Articles

Evolution of microstructure and texture of a near α titanium alloy during forging bar into disk

Abstract Ti6242S alloy bar with equiaxed microstructure and random textures was selected to investigate evolution of microstructure and texture during the process of forging bar into disk by isothermal constant speed hot compression test. Experimental investigation shows that αp grains experienced bend, rotation under prismatic slip system action and further separated into refined grains by dynamic recovery or dynamic recrystallization with the increase of strain. Sharp macrozones existed in Ti6242S alloy were completely eliminated when strain arrived at 50%. Two main texture components of 2 ¯ 0> and 2 ¯ 3>//compression direction were developed in all deformation strain. The former was mainly caused by the activated prismatic slip system motion of α phase during deformation and the precipitated αs phase neighboring αp phase after β→α transformation which hold the same orientation with αp phase. The latter was the results of αp phase dynamic recrystallization and polycrystalline coordinated deformation of αp/αp or αp/β phases, likewise, the same orientation αs laths with neighboring αp phase preferentially precipitated after β→α. It is also demonstrated that if the strain surpassed 50%, especially for multi-step deformation to 70% dynamic recrystallization lightly softened the texture density.

Universal relation for the orientation of dislocations from prismatic slip systems in hexagonal and rhombohedral strained heterostructures

We present universal formulas for the orientation of misfit dislocations from prismatic slip on a- and m-planes for growth of strained hexagonal and trigonal heterostructures on semipolar orientations. The formulas cover the present work on corundum-phase (Al,Ga)2O3/Al2O3 and reported results from wurtzite-phase nitride and oxide heterostructures.

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.

Plastic deformation mode and α/β slip transfer of Ti–5Al-2.5Cr-0.5Fe-4.5Mo–1Sn–2Zr–3Zn titanium alloy at room temperature

The plastic deformation mode and slip transfer between the α and β phases of Ti–5Al-2.5Cr-0.5Fe-4.5Mo–1Sn–2Zr–3Zn titanium alloy were investigated using a combination of ex-situ scanning electron microscopy tensile tests and electron backscatter diffraction characterization. With increasing tensile strain, local orientation changes (which lead to the sub-grain boundary generation and migration) occurred in the α phase, resulting in grain subdivision/coalescence. Afterward, the elongated α grains were subdivided (in general) into small equiaxed grains. Abrupt changes in the crystal orientation of the continuous β-matrix were, however, rare. Three primary slip systems (prismatic and basal slip: α phase and {110}<111> slip: β phase) accounted for more than 70% of the total slip system activity at each strain level. These systems could be activated with very low Schmid factors (<0.1) at high strains. The compatible deformation between the α and β phases was realized mainly through the slip transfer correlated with the three primary slip systems, and the prismatic slip system was the most active system. In general, the α phase underwent plastic deformation first and compelled plastic deformation of the separated β grains. Subsequently, the deformed β grains transmitted the strain between an adjacent α grain and other α grains, leading to deviations from the uniaxial stress state in the local region. This resulted in wide distributions of the activated slip systems with various Schmid factors, and considerable strain concentration and failure in the α phase near the phase boundaries.

An In-Situ Synchrotron Diffraction Study of Stress Relaxation in Titanium: Effect of Temperature and Oxygen on Cold Dwell Fatigue

There is a long-standing technological problem in which a stress dwell during cyclic loading at room temperature in Ti causes a drastic fatigue life reduction. To better understand the material characteristics that control or exacerbate this behaviour, evaluation of the time dependent plasticity of the main prismatic and basal slip systems is critical. Incorporating the influence of operating temperatures and common alloying elements on cold dwell fatigue will be beneficial for future alloy design to address this problem. In this work, characterisation of the time dependent plastic behaviour of two commercially pure titanium samples (grade 1 and grade 4) with different oxygen content at 4 different temperatures (room temperature, 75 oC, 145 oC and 250 oC) was performed during stress relaxation using synchrotron X-ray diffraction. Key parameters that govern the dislocation motion were determined for the major prismatic and basal slip systems as a function of temperature and oxygen content by calibrating a crystal plasticity finite element model with the measured lattice strain relaxation responses. From the temperatures assessed, 75 oC is found to be the worse-case scenario, where the macroscopic plastic strain accumulation was the significant during a relaxation cycle due to the greatest activity of both prism and basal slip systems. As temperature increases, the contribution of thermal energy becomes greater than mechanical energy for dislocation glide. Oxygen was found to have a stronger strengthening effect on prism slip over basal slip, through a significant change in their respective critical resolved shear stresses. This effect becomes more significant in higher oxygen content commercially pure Ti.

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.

Effect of crystallographic orientation and geometrical compatibility on fatigue crack initiation and propagation in rolled Ti-6Al-4V alloy

Fatigue crack initiation and short crack propagation was systematically investigated in a rolled α/β Ti-6Al-4V alloy through a combination of in situ optical microscopy observations during fully-reversed uniaxial fatigue experiments and post-mortem electron backscattering diffraction analyses. The effect of microstructural attributes (phase, crystal orientation, grain boundary) on fatigue crack formation and early growth was statistically quantified based on the analysis of a large dataset of grains. Crack initiation was mainly observed on prismatic slip systems which was related to the effect of the rolling texture. Crack propagation was found to be mostly transgranular and crystallographic on both basal and prismatic slip systems. An analysis of the effect of the β phase was proposed to explain the preferential slip systems for propagation in lamellar colonies. Crack deflection was quantified for a large set of grain boundaries. A combination of elastic, plastic and geometric compatibility criteria was identified to consistently predict the preferential slip systems for crack propagation across grain boundary. Finally, a discussion on the transition from microstructure-sensitive to microstructure-insensitive crack propagation was proposed.

Revealing deformation mechanisms in Mg–Y alloy by in situ deformation of nano-pillars with mediated lateral stiffness

Abstract

On microstructural homogenization and mechanical properties optimization of biomedical Co-Cr-Mo alloy additively manufactured by using electron beam melting

The electron beam melting (EBM), a layer-by-layer additive manufacturing (AM) technique, has been recently utilized for fabricating metallic components with complex shape and geometry. However, the inhomogeneity in microstructures and mechanical properties are the main drawbacks constraining the serviceability of the EBM-built parts. In the present study, we found remarkable microstructural inhomogeneity along build direction in the EBM-built Co-based alloy, owing to the competitive grain growth and subsequent isothermal γ-fcc → ε-hcp phase transformation, which affects the corresponding tensile properties significantly. Then, we succeeded in eliminating the inhomogeneities, modifying the phase structures and refining grain sizes via comprehensive post-production heat treatment regimes, which provides a valuable implication for improving the reliabilities of AM-built metals and alloys. The Co-based alloy can be selectively transformed into predominant ε or predominant γ phase by the regime, and the grains were refined to 1/10 of the initial sizes by repeated heat treatment. Finally, we investigated the tensile properties and fracture behaviors of the alloy before and after each heat treatment. The γ → ε strain-induced martensitic transformation is the major deformation mode of the γ phase, meanwhile the formation of stripped ε phase at {111}γ habit planes contributed to a good combination of strength and ductility. Nevertheless, the ε phase was deformed mainly by (0001)ε <11 2¯0 >ε basal and {1 1¯00}ε <11 2¯0 >ε prismatic slip systems, exhibiting very limited ductility and strength. In addition, the ε grains act as secondary hardening factor in the samples consisting of dual γ/ε phase, leading to a non-uniform deformation behavior.

Effect of yttrium addition on the hot deformation behaviors and microstructure development of magnesium alloy

Hot deformation behaviors accompanied with the development of new fine grains (NFGs) and microtexture were studied experimentally and numerically in a binary Mg-2.2 at% Y alloy by using compression test, SEM/OIM microscopy and full field crystal plasticity finite element modelling (CPFEM) method. NFGs are evolved along at original grain boundaries at low to medium strains and subsequently in grain interiors through grain fragmentation by frequent formation of kink bands in high strain. Such a series of strain-induced reactions occur in-situ during deformation. The addition of Y atoms to Mg results effectively in the appearance of much higher flow stress and also the slow development of NFGs even in high strain. Lattice orientations of NFGs are distributed almost randomly. The CPFEM results confirms that the dominant deformation mode changed from the combination of basal and prismatic slip system to the combination of pyramidal and basal slip system when deformation temperature increased from 673 K to 723 K, leading to a different microtexture between these two temperatures. Both mechanical and microstructural events are analyzed in detail and the operating mechanisms discussed.

Anisotropy of the Microstructure and Tensile Properties in Ti-5Al-5Mo-5V-1Cr-1Fe near β Titanium Alloy during Hot Rolling and Heat Treatment

Ti-55511 billet with the acicular α initial microstructure was hot rolled (HR sample) and then heat treated (HR+HT sample) at 750 °C. The effects of HR and HT on the anisotropy of microstructure, texture, and tensile properties were investigated. The tensile results show that there are obvious anisotropic tensile properties between RD and TD. The anisotropic elongation of HR sample is related to the morphology of α phase. After HR, the acicular α is parallel to RD. As for RD specimen, the transgranular propagation of microcrack passing through the acicular α phase leads to the ductile fracture, thus showing the higher ductility than TD specimen. While the intergranular propagation of microcrack passing by the equiaxed α phase in TD specimen causes the brittle fracture. The anisotropic strength of HR sample depends on the relationship among texture type of α phase, slip system, and loading direction. The maximum texture intensity at TD leads to the easy activation of basal slip system in RD and that of prismatic slip system in TD, and then causes the lower strength of RD specimen than TD specimen. After HT, the decreased anisotropy of elongation and strength can be attributed to the increased α size and the decreased texture intensity of α phase. These results demonstrate that anisotropic tensile properties mainly depend on the morphology and texture of α phase.

In-situ synchrotron X-ray diffraction deformation and EPSC modeling of AZ31 Mg alloy

We present a series of in-situ synchrotron X-ray diffraction deformation experiments conducted on polycrystalline AZ31 Mg alloy in a D-DIA apparatus at temperatures ranging from 154 °C to 286 °C, at ~3.1 GPa confining pressure and a strain rate of 5 × 10−6/s. For each experiment, we fit the X-ray diffraction data using elastic plastic self-consistent (EPSC) models that incorporate an isotropic deformation mechanism in addition to basal, prismatic and pyramidal slip systems. From the EPSC models, we report the sample strength and the critical resolved shear stresses (CRSS) for individual slip systems for each experiment and compared with previously reported data on this material.

Quantitative investigation of micro slip and localization in polycrystalline materials under uniaxial tension

Micro slip activation and localization in Ti-6Al-4V deformed in tension have been examined quantitatively using high-resolution (HR) digital image correlation (DIC), HR-electron backscatter diffraction (EBSD) and crystal plasticity finite element modelling. The measured polycrystal slip, strain, lattice rotation and geometrically necessary dislocation (GND) density distributions are generally well captured by the a priori crystal plasticity model based on the rate-sensitive properties of α-titanium. An overall slip trace analysis showed over 80% agreement between HR-DIC and crystal plasticity modelling of the primary slip activation. The texture beneath the characterised free-surface has been found to affect the local slip, stress distribution, lattice curvature and GND density and three texture variations have been considered. Grain-level slip trace analysis shows that the crystal plasticity modelling can capture single (straight) slip, multiple slip activation and complex wavy slip. The latter has been found to result from the interaction of independently activated basal and prismatic slip systems with common slip direction. Initial inter-granular misorientations greater than about 5° have been shown to influence the subsequent micromechanical grain behaviour including slip, lattice rotation and GND density. This work contributes to the understanding of slip localization and load shedding in dwell fatigue in polycrystalline hexagonal materials.

Mechanisms of WC plastic deformation in cemented carbide

The WC-Co cemented carbides with ultracoarse and ultrafine grain structures were tested by the bonded interface technique to investigate the deformation behaviour of WC. The electron backscattering diffraction assisted trace analysis and transmission electron microscopy were combined to examine the microstructural details in the deformation region. It was found that the plastic characteristics of the ultracoarse WC originate from both the prismatic (e.g. {101¯0} ⟨1¯21¯3⟩) and pyramidal (e.g. {01¯11} ⟨01¯10⟩) slip systems. The deformation mechanisms of the ultracoarse and ultrafine cemented carbides were compared. It was proposed that different from the ultrafine cemented carbides in which the plastic deformation is mainly attributed to the metal binder, the dislocations and stacking faults in WC make significant contributions to the plastic deformation hence the fracture toughness of the ultracoarse cemented carbide.

Effect of strain path on the evolution of microstructure, texture and tensile properties of WE43 alloy

Hot rolling of extruded and solutionized WE43 alloy was carried out at 400°C through different strain paths such as unidirectional rolling (UDR) and multistep cross rolling (MSCR). The effect of strain path on the evolution of microstructure and texture was investigated. The grain size was ∼ 7µm and 18µm after hot rolling of UDR and MSCR samples, respectively. A strong basal texture was observed during rolling which is independent of the strain path of the samples and the recrystallization mechanism was identified to be continuous dynamic recovery and recrystallization type. Further, the texture of the deformed and recrystallized grains was observed to be same. Hence, the texture was simulated successfully using visco-plastic self-consistent (VPSC) simulation. The VPSC simulation of UDR sample showed the dominant activity of basal and prismatic slip system up to a true strain of ∼ 0.5 and after that pyramidal < c + a > activity dominants, whereas MSCR sample showed the combined activity of basal and pyramidal < c + a > up to a true strain of ∼ 0.5 and later on basal activity dominants on further deformation. The room temperature tensile testing showed that the yield strength of MSCR sample was greater than UDR sample except along RD of UDR sample and the tensile strength was more for MSCR compared to UDR sample which suggests that higher volume fraction of metastable precipitates in MSCR than UDR sample during high temperature rolling. The higher volume fraction of metastable precipitates was attributed to lower dislocation activity on prismatic planes of MSCR compared to UDR sample during hot rolling. The current VPSC simulation showed that the prismatic slip activity was four times higher in UDR than MSCR sample. A decrease in mean free path of dislocations by the presence of metastable precipitates led to higher tensile strength of MSCR compared to UDR sample. The mean free path of dislocations increased from RD to TD and hence, results in a decrease in tensile strength from RD to TD of both the samples.

Relation between sample size and deformation mechanism in Mg-Zn-Y 18R-LPSO single crystals

With the long-period stacking ordered (LPSO) second phase, the promising mechanical properties of the Mg-Zn-Y alloys have been demonstrated. Because the LPSO phase plays an important role in strengthening mechanisms and it has the unique 18R crystal structure, it is interesting to clarify the deformation mechanism and size effect associated with this particular phase. In this study, uniaxial compression tests are conducted on LPSO along [112¯0] with different sample sizes. An apparent sample size effect is observed and the plastic deformation behavior of LPSO is found to change from kink to slip when sample size is reduced to a level about 1.8 μm in diameter. The non-basal prismatic slip system is found to operate within the smaller sized micro-pillars, different from the deformation kinking observed in larger sized samples. The probability of forming stair rod dislocations associated with the sample size effect is an important key to explain this change of plastic deformation behavior. It implies that the origin of deformation kink is non-basal prismatic slip.

Analysis of deformation inhomogeneity and slip mode of TA15 titanium alloy sheets during the hot tensile process based on crystal plasticity model

Understanding of the deformation inhomogeneity and slip mode of titanium alloys during the thermo-mechanical process is crucial for effectively guiding the forming process and controlling the property of final products. To this end, a crystal plasticity finite element model (CPFEM) was proposed to address these issues of polycrystalline TA15 titanium alloy sheets during the hot tensile process along the rolling direction (RD) at 750℃ in this paper. Based on the analysis of Schmid factors (SFs), the weighted averaged SF of basal and prismatic slip systems (WABPSF) was adopted as the criterion of ‘soft’ or’hard’ grains. In term of the simulated results, the localized stress concentration tends to spread at the parts of hard grains adjacent to soft grains, yet the distributions of strain manifest in a sequential fashion. As for the slip mode of TA15 rolling sheets during the hot tensile process along RD, multiple slip systems can be activated as long as the resolved shear stress is larger than the critical resolved shear stress (CRSS). And the prismatic slip systems act as the predominant slip mode, and exhibit an incremental role on the tensile deformation along RD. Meanwhile, the simulated results indicate that the activation of basal slip systems would result in the distinct reduction of basal SF.

Plastic deformation mechanisms of biomedical Co–Cr–Mo alloy single crystals with hexagonal close-packed structure

This is the first report of the successful fabrication of Co–Cr–Mo biomedical alloy single crystals with a hexagonal close-packed (hcp) structure and the resultant clarification of its deformation behavior. The (0001)〈112¯0〉 basal and {11¯00}〈112¯0〉 prismatic slip systems were found to be predominately operative. The critical resolved shear stresses for the basal and prismatic slip systems at ambient temperature are ~204 and ~272MPa, respectively, which are much higher than ~54MPa for {111}〈112¯〉 slip in the face-centered cubic (fcc) Co–Cr–Mo phase, quantitatively demonstrating that the hcp phase acts as an effective strengthening phase.

Measuring Ti-7Al slip system strengths at elevated temperature using high-energy X-ray diffraction

Full stress tensors of over four hundred individual grains in a titanium alloy specimen (Ti-7Al) are tracked during continuous uniaxial deformation through the elasto-plastic transition at 355°C. The experiment is facilitated by a unique radiative heating furnace and load frame designed for in-situ high-energy X-ray diffraction measurements. Average strengths of multiple slip system families are calculated directly from the ensemble grain stress data. Results show that elevated temperature lowers the strength of basal slip systems relative to prismatic slip systems, reduces the degree of flow softening due to precipitate shearing, and decreases the first-order pyramidal slip system work-hardening rate.