Prismatic Slip Systems Research Articles

Modifying Microstructure and Improving Mechanical Properties of New Ti-Al-V Titanium Alloys via Fe Addition.

A comprehensive study was carried out to investigate the effects of Fe addition (0-0.9 wt.%) on the microstructure evolution and mechanical properties of Ti-6Al-4V alloys. The results indicate that Fe addition has a significant refinement effect on the microstructure of titanium alloys; specifically, 0.9 wt.% Fe addition can lead to a 47.37% decrease in the width of lamellar α. The modulus also decreases by 18.89% with the increase in the Fe content, being 91.40 GPa in Ti-6Al-4V-0.9Fe. And the microhardness and wear resistance are improved due to Fe addition. In addition, the constitutive equation of the Fe content and the elastic compliance coefficient were calculated, which can better describe the relationship between Fe addition and the elastic-plastic properties of titanium alloys. The slip systems' activity during the deformation process was also discussed using the Schmid factor. It shows that Fe addition is beneficial for the activity of prismatic and pyramidal slip systems, especially in the {101¯0} <112¯0>, {101¯1} <112¯3>, and {112¯2} <112¯3> slip systems.

In-plane biaxial low-cycle dwell fatigue behavior of CP-Ti based on DIC method

This study investigates the biaxial dwell fatigue behavior of commercial pure titanium (CP-Ti) under different load ratios and dwell times. The evolution of mean strain, ratcheting strain, and creep strain was systematically analyzed. The findings reveal that with the decrease of load ratio or the increase of dwell time, both ratcheting and creep strain exhibit an upward trend. The interaction between ratcheting strain and creep strain promotes crack initiation and propagation. The statistical analysis of crack paths by the digital image correlation (DIC) method indicates that crack initiation occurs between 0.7Nf and 0.8Nf, and fatigue failure is governed by the maximum principal strain. Moreover, the electron backscatter diffraction (EBSD) analysis results indicate that as the load ratio increases, the activity of prismatic slip systems decreases, while the activities of other slip systems and twinning modes increases. To enhance predictive accuracy, a modified SWT model by incorporating time-dependent creep damage and anisotropy is proposed for life prediction under biaxial dwell fatigue conditions.

Influences of grain orientation on high-cycle fatigue performance of laser powder bed fused Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy at room temperature and 500℃

The fatigue performance of titanium (Ti) alloys fabricated by laser powder bed fusion (LPBF) is a crucial factor influencing the structural safety in service. The high-cycle fatigue (HCF) of LPBF Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy at the room temperature and 500 °C was tested. Compared with the room-temperature samples, the fatigue strength of fatigue samples tested at 500 °C is reduced by about 55.8 % after 106 fatigue cycles. Crack initiation on the surface is mainly related to fatigue after fewer cycles under high stress, while interior cracks initiate after a longer fatigue life under low stress. Electron back scattering diffraction (EBSD) was adopted to characterize the orientations of grains on the crack propagation paths and the activated slip systems. All prismatic oriented grains were found to activate prismatic slip systems, the activation of which achieves symmetric slips surrounding crack tips, forms ductile striations, and increases energy absorption during crack propagation. These characteristics are conducive to improved fatigue performance. About half of the basal oriented grains activate prismatic slip systems. These activated prismatic slip systems not only have a high Schmid factor (SF) but also need to have a high geometrical compatibility factor with slip systems activated by adjacent grains. The residual β phase and α phase are aligned according to the Burgers orientation relationship (BOR), so the β phase does not alter the direction of crack propagation. The results were used to predict the fatigue crack propagation paths in LPBF Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy during loading along building and transverse directions were predicted. This, to some extent, provides a basis for comparing fatigue performance of LPBF Ti alloys in different directions.

On the Temperature and Composition Dependence of Non‐basal Stacking Faults in C14 Laves Phases

Activation of non‐basal slip is essential in improving the deformability of hexagonal crystals. However, for complex intermetallics such as Laves phase, the mechanism of non‐basal slip remains largely unknown. In this work, the prismatic slip systems of C14 Laves phase crystals and possible metastable states along the slip paths are assessed using atomistic simulations. Multiple prismatic stacking fault states with the same projected positions of atoms but different site occupancies and chemical distributions are identified at different inter‐atomic layers, temperatures and chemical compositions. The formation of energetically favorable prismatic stacking faults involves short‐range diffusion which implies the thermally activated nature of prismatic slip. The outcomes of this work advance the understanding of temperature and composition‐dependent non‐basal slip in Laves phases.

Characterization of the strain rate sensitivity of basal, prismatic and pyramidal slip in Zircaloy-4 using micropillar compression

The slip strength of individual slip systems at different strain rates will control the mechanical response and strongly influence the anisotropy of plastic deformation. In this work, the slip activity and strain rate sensitivity of the <a> basal, <a> prismatic, and <c+a> pyramidal slip systems are explored by testing at variable strain rates (from 10−4 s−1 to 125 s−1) using single crystal micropillar compression tests. These systematic experiments enable the direct fitting of the strain rate sensitivities of the different slips using a simple analytical model and this model reveals that deformation in polycrystals will be accommodated using different slip systems depending on the strain rate of deformation in addition to the stress state (i.e. Schmid's law). It was found that the engineering yield stress increases with strain rate, and this varied by slip systems. Activation of the prismatic slip system results in a high density of parallel, clearly discrete slip planes, while the activation of the <c+a> pyramidal slip leads to the plastic collapse of the pillar, leading to a ‘mushroom’ morphology of the deformed pillar. This characterization and model provide insight that helps inform metal forming and understanding of the mechanical performance of these engineering alloys in the extremes of service conditions.

Basal and prismatic slip in hafnium

Metals with hexagonal crystal structure are found in many safety-critical applications, from titanium alloy fan blades in jet engines to magnesium alloys in the automotive industry. They also exist in nuclear reactors, for example hafnium alloy control rods whose structural integrity is critical to nuclear safety. However, mechanistic understanding of the micromechanical properties and deformation behaviour of Hf remains elusive. To aid in this understanding, we performed in situ scanning electron microscopy single crystal micropillar compression tests of a commercial Hf alloy, where the samples were aligned to activate 〈a〉 prismatic and 〈a〉 basal slip systems. We then employed transmission electron microscopy to examine the dislocation structure in the deformed pillars. These two slip systems exhibited distinctly different behaviour, where prismatic slip is planar and basal slip is wavy. Prismatic slip is the easiest deformation mode, while basal slip is at least twice as difficult. Generally, the characters of prismatic and basal slip in Hf show similarities to those in some other hexagonal metals such as Ti and Zr, except for the higher relative difficulty of basal slip. Our results provide fundamental knowledge and parameters that can be used for modelling degradation and predicting performance of Hf, and shed light on the general behaviour of basal and prismatic slip systems in low-c/a-ratio hexagonal metals.

Strengthening and failure mechanisms during tension of a Ti–6Al–4V alloy-based nanocomposite processed by laser powder bed fusion

The present work aims to clarify the tensile behavior of a novel metallic matrix composite of Ti–6Al–4V alloy reinforced with 1 wt% nano-yttria-stabilized zirconia, processed by laser powder bed fusion technology. The as-processed samples were post-processed using notably hot isostatic pressing in order to investigate the most promising material for actual structural applications. The cracking behavior is governed by the activation of prismatic slip systems in primary α-Ti grains. Under horizontal loading configuration (i.e., perpendicular to building direction), average yield stress and ultimate tensile strength of 999 and 1195 MPa have been found, respectively. Strengthening mechanisms caused by (i) solid solution due to raising oxygen concentration and (ii) dispersion hardening have resulted in similar yield stress increases of 108 and 99 MPa, correspondingly. Furthermore, the effect of the texture on the mechanical properties of the investigated material was assessed via a statistical approach, which forecasts a strong strength anisotropy depending on the loading direction. In addition, a particular attention has been paid to the effect of nano-yttria-stabilized zirconia addition on strain-hardening ability, underlining that the enhanced amount of solute oxygen yielded a strong strain hardening in stage II of plastic deformation.

Micro-mechanisms of anisotropic deformation in the presence of notch in commercially pure Titanium: An in-situ study with CPFEM simulations

This study investigates the impact of notch severity and initial texture on the micro-mechanisms during low strain deformation in commercially pure Titanium, exploiting in-situ electron back scatter diffraction (EBSD) experiments and crystal plasticity finite element model (CPFEM). In-situ tests were performed at different strain steps of un-notched and notched samples of transverse direction (TD) and rolling direction (RD) orientations. CPFEM, based on initial EBSD microstructures, predicted profuse prismatic slip traces and early activation of prismatic slip in notched sample, with RD orientation exhibiting higher activity. Further, CPFEM results revealed early activation of high CRSS slip systems as well as evidence of early twin activity at notch tip due to severely localized plastic deformation and steep strain gradient, as observed by GND maps causing higher stress at grain boundaries. At the notch tip, digital images correlation (DIC) at microscale indicated strain localization at 45∘ and 90∘ to the tensile axis for TD and RD orientations, respectively. Furthermore, 2.5D and 3D CPFEM confirmed distinct strain patterns at notch tip: TD orientation exhibited combined basal and prismatic slip influences, while RD orientation displayed dominant prismatic slip systems at localized strain. The model also successfully predicted anisotropic surface roughness, contributing to early necking in RD orientation.

Flow behavior and dynamic recrystallization mechanism of a new near-alpha titanium alloy Ti-0.3Mo-0.8Ni-2Al-1.5Zr

A new near-α titanium alloy Ti-0.3Mo-0.8Ni-2Al-1.5Zr was designed based on Ti-0.3Mo-0.8Ni (TA10). The hot compression tests were carried out at temperatures of 800–970 °C and strain rates of 0.01–5 s−1. The results show that the flow softening phenomenon in the α + β phase region is more distinct than that in the single β phase region. The Arrhenius constitutive models based on strain compensation in the α+β phase region and β phase region were established respectively, and the correlation coefficient between the predicted flow peak stress values and the experimental values reached 0.99365. The hot processing map of the alloy was established, the processing parameters of the peak efficiency: 800 ≤ T ≤ 818 °C, 0.01 ≤ ε˙ ≤ 0.0235 s−1, and 960 ≤ T ≤ 970 °C, 0.01 ≤ ε˙ ≤ 0.0213 s−1. The microstructure evolution and deformation mechanism of the compressed specimens were researched by electron backscatter diffraction (EBSD) technique. There were two nucleation mechanisms for dynamic recrystallization (DRX): sub-grains growth nucleation and high angle grain boundaries (HAGBs) bulging nucleation, the former was continuous dynamic recrystallization (CDRX), and the latter was discontinuous dynamic recrystallization (DDRX). The flow stress in the α+β phase region was softened by CDRX and dynamic recovery (DRV), while the principal deformation mechanisms in the β phase region are DRV and DDRX. The dislocation strain energy and recrystallization ability at different temperatures were quantitatively analyzed by the kernel average misorientation (KAM) and the geometrically necessary dislocation (GND), and the results showed that more complete recrystallization occurred at lower temperature in the α + β phase region. The HCP slip was principally completed by prismatic ⟨a⟩ and pyramidal ⟨c+a⟩ slip systems during the hot compression, while BCC was a mixed mode of {110}, {112} and {113} multiple slips.

Vibration fatigue properties and deterioration mechanism of diffusion bonded TC4 titanium alloy

To analyze the effect of diffusion bonding process on vibration fatigue properties, a series of fatigue tests and microstructure characterization of as-received and diffusion bonded TC4 titanium alloy were conducted. The results show that the thermal cycling of diffusion bonding process results in microscopic change and deteriorates vibration fatigue lifetime of TC4 alloy. For bonded TC4 alloy, the increased grain size inhomogeneity stimulates incompatible deformation of differently oriented α grains and leads to micro-void formation at boundaries of small/large α grain pairs or inside large α grains. Meanwhile, the orientation variation of α grains promotes activation of prismatic slip systems for earlier crack initiation. In addition, the increased fraction of β phase provides more nucleation locations for micro-voids. Thus, the increase of the grain size inhomogeneity, grain orientation change, and the increase of β phase fraction due to the thermal cycling result in fatigue performance deterioration of diffusion bonded TC4 titanium alloy.

Tailoring lamellar orientation and tensile properties of TNM alloy via extrusion

The rectangular billet of TNM alloy with oriented lamellar microstructure was obtained by extrusion above its α transus temperature. The as-extruded TNM alloy exhibits a nearly lamellar microstructure with an average colony size of approximately 60 μm, and the proportion of lamellar colonies with an angle between the interface and the extrusion direction less than 30° exceeds 70 %. EBSD results show that strong basal texture of α phase with basal planes parallel to the extrusion direction was formed during the extrusion process in the (α+β) two-phase field. During the subsequent cooling process, the γ lamellae precipitates along the (0001) plane of α phase according to the Blackburn orientation relationship, allowing the lamellar structure to inherit the orientation of the parent phase. Tensile test results indicate that the strength and fracture elongation of the as-extruded TNM alloy along the extrusion direction (ED) increased simultaneously, which were 1100 MPa and 1.26 %, respectively. The hindrance effect of interfaces on dislocations and stacking faults in lamellar colonies parallel to the extrusion direction helps to improve strength, while the activation of the prismatic slip and pyramidal slip systems in the α2 lamellae is beneficial to the plasticity. This study shows that it is feasible to control the lamellar orientation and improve the mechanical properties of TNM alloy by extrusion above its α transus temperature.

Understanding phase stability and deformation mechanisms of zirconium under high pressures from generalized stacking fault energy curves: A first-principles study

The mechanical properties of metals are strongly depend on their phase structures, in this work, to understand the phase stability and deformation mechanisms of zirconium(Zr) under high pressures, the generalized planar fault energy(GPFE) curves based on first-principles calculations are utilized to illustrate the deformation processes of α-Zr at high pressures. Firstly, it is found the prismatic slip system is more prominent toward α-Zr at zero pressure, while the slips based on basal plane undertake the main participant during the deformation processes of α-Zr at high pressures. Meanwhile, based on the basal slip, α-Zr phase containing the pre-existing stacking fault is found more favorable to generate twin fault instead of FCC fault at zero pressure; However, the deformation twinning is overwhelmed by the HCP-to-FCC transformation for the α-Zr when the external pressure is higher than 10.4 GPa. It is thus speculated that the slips of partial dislocations for forming FCC fault based on the basal plane (0001) act as a contributing participant during the phase transition processes of Zr under high-pressure. Additionally, the computed elastic constants and the HCP-to-FCC allotropic transformation under specified conditions have revealed the metastable nature of FCC Zr in comparison with HCP Zr.

Effects of lamellar α orientation on the mechanical behavior of Ti–6Al–4V alloy manufactured by electron beam directed energy deposition

Crystallographic texture of alloy can trigger anisotropic properties, which have been studied extensively. However, the effect of spatial orientation of grains on the mechanical properties, especially for additive manufactured metals with lamellar morphology has remained elusive. Herein, this work theoretically calculated the spatial and crystallographic orientations of α lath from the <100>β//Z-axis (build direction) fiber texture, and investigate the relationship between the orientations of α lath and the deformation behaviors and mechanical properties of the Ti–6Al–4V alloy built via electron beam directed energy deposition (EB DED). Deformation features display the prismatic slip system of α lath that exists a smaller angle to the broad face of α lath showing a lower effective critical resolved shear stress (CRSS). This is because this prismatic slip system has a greater slip range when compares with other slip systems. Such deformation features, combined with the spatial orientation distribution characteristic of α lath that is induced by the <100>β//Z-axis fiber texture, lead to strongly tensile anisotropy, specifically, the samples oriented 45° and 67.5° angles to z-axis present greater strengths but lower ductility, whereas the sample in the z-axis direction generally displays a minimum strength but a superb ductility. This finding is vital for providing an important theoretical basis for optimizing the consistency of mechanical properties of the additive manufactured α+β dual phase titanium alloy.

Enhanced deformability of WE43 magnesium alloy by activation of non-basal slip and twinning deformation modes in the equal channel angular pressing at room temperature

Activation of the prismatic and pyramidal slip systems, along with the twinning as the secondary deformation mechanism, assists in the accommodation of a larger plastic strain during the deformation of magnesium alloys. In this paper, deformation mode activity and texture evolution of the WE43 magnesium alloy were investigated during core-sheath equal channel angular pressing (ECAP) at room temperature. Texture studies and transmission electron microscopy inspections suggest that the mechanisms behind the ECAP at room temperature assisted by back pressure were activation of (101¯0) prismatic slip (as the primary mechanism), as well as extension and contraction twinning.

Numerical study on the ductility improvement mechanism of dilute Mg-Ca alloys

In recent years, magnesium (Mg) alloys have attracted extensive attention because of their potential use as alternatives to dense metals and Al alloys. Therefore, the low ductility of Mg alloys should be improved. Previous studies indicated that the addition of low concentrations of Ca improves the ductility of Mg alloys; however, the associated mechanisms are not well understood. The present numerical study demonstrates that the ratio of the critical resolved shear stresses associated with prismatic and basal < a > slip systems is significantly reduced (to a value of approximately 1) in a Mg-0.1 at.% Ca alloy. The incorporation of Ca was also found to harden the basal slip system while softening the prismatic slip system. The addition of Ca to Mg evidently provides a thermal-like effect that promotes the nucleation of kinks and enhances the dislocation motion. This effect is significantly more pronounced in the case of a prismatic slip system. The difference in importance of the thermal effect between the basal and prismatic slip systems and the Ca-promoted nucleation of kinks are the two main factors responsible for softening the prismatic slip system and improving the ductility of dilute Mg-Ca alloys.

Modeling of dynamic recrystallization texture in hot extruded Mg

Because of the strong effect of texture on the properties of alloys, the evolution of texture during hot extrusion of Mg has been studied through experiments and simulations. For a better understanding of the texture evolution at high processing temperatures, the model based on viscoplastic formulation was extended to a cellular automaton procedure to consider the recrystallization mechanisms. Texture evolution was simulated for the basal, prismatic and pyramidal 2nd order slip systems. Twinning was not considered because no twins were observed in the microstructural investigation. In the experimental investigations, the samples were extruded to a strain of ε = 2.7 at different temperatures, namely 180 °C, 280 °C, and 380 °C. The microstructure, texture, and hardness were evaluated by optical and electron microscopy using EBSD, X-ray diffraction, and Vickers tests, respectively. The main textural components were identified for each deformation temperature. Based on the simulations, a mechanism for the formation of a specific component was proposed. It was found that the change in the main texture component depends not only on the critical resolved shear stress for a specific slip system but also on the recrystallization parameters. The vpsc model has been shown to predict only the deformation texture components because the temperature is accounted for in accordance with the critical resolved shear stress and rate sensitivity. These two parameters affect the slip system activity. The new model using the cellular automaton procedure, which covers recrystallization processes, produces textures that fall in line with the experimental results.

High temperature strengthening mechanism of pure Zn with trace Mn addition

This work reveals how trace Mn significantly improves high temperature strength of pure Zn. The peak stress of pure Zn increases from 46 MPa to 84 MPa at 300 °C/0.1 s−1 after 0.8 wt.% Mn addition. The as-compressed Zn-0.8Mn alloy has a bimodal grain structure with fine grains surrounding a coarse grain. Transmission electron microscopy results show that Mn addition promotes activation of non-basal <c+a> slip and pile-up of dislocations near coarse/fine grain boundary or MnZn13/Zn interface or on the MnZn13 particles. First-principles calculations indicate that Mn addition can reduce stacking fault energy values of basal and prismatic slip systems of Zn, therefore activating <c+a> slip. Piled dislocations generate forward and back stresses, resulting in hetero-deformation induced strengthening. It serves as a suggestion for designing bimodal grain structures to improve strength of Zn alloys.

The formation mechanism of modulation structures along three different crystallographic directions in pure hafnium

The microstructure of pure hafnium subjected to micro-hardness test was studied in detail by using transmission electron microscopy (TEM). Results indicated that the {101‾0}<112‾0> prismatic slip system was commonly activated and modulation structures along three modulation directions can be frequently observed. Based on the crystallographic characteristics of these modulation structures and the electronic structure theory, the formation mechanism of modulation structures in Hf was discussed. It was confirmed that the original Brillouin zone of α-Hf was shifted along three different directions, g1, g2 and g3, under the action of external load, which changed the relative locations of the Brillouin zone and the Fermi sphere of the α-Hf and stabilized the whole system, thereby causing the formation of the modulation structure along three different directions.

Determination of critical resolved shear stresses associated with slips in pure Zn and Zn-Ag alloys via micro-pillar compression

The room-temperature plastic deformation behavior of pure Zn and Zn-Ag (0–2.2 at.%) biodegradable alloys has been investigated via uniaxial in situ micro-pillar compression tests performed within a scanning electron microscope. The critical resolved shear stresses (CRSS) were quantified as a function of micro-pillar diameter. The compression of single crystal micro-pillars was performed at various strain rates in carefully selected grains, the crystallographic orientation of which facilitates deformation either via basal 0001〈112¯0〉 or prismatic 101¯0〈112¯0〉 slip. The CRSS values increased with decreased micro-pillar diameter, revealing a more pronounced size effect in pure Zn deformed via basal slip. The observed solid solution strengthening effect in Zn-Ag alloys with increasing Ag content was associated with a CRSS increase from 26.6 MPa to 37.0 MPa (by ∼ 40 %) in the basal slip system and from 99.1 MPa to 188.4 MPa (by ∼ 104 %) in the prismatic slip system. The extraordinarily high CRSS values for basal slip activated in pure Zn and Zn-0.14Ag alloy compared to the solid solution strengthening model was attributed to the critically low dislocation density. In the Zn-Ag alloy with Ag content > 0.5 at.% higher dislocation densities are expected, which result in a more predictable plastic deformation behavior.

First‐Principles Calculations and a Theoretical Model for Predicting Stacking Fault Energies in Binary Magnesium Alloys

Comprehensive first‐principles density functional theory (DFT) calculations are conducted to study the effects of solutes on the stacking fault energies (SFEs) for basal, prismatic, and pyramidal slip systems in binary magnesium alloys. The two main aspects prescribing the solute effect on SFE are elucidated to be mechanical and electronic in nature, respectively, and critically assessed. On the basis of the misfit volume and d‐electrons, these two aspects are then quantified and subsequently a predictive model of the solute effect on the SFE is developed, with good agreement achieved between the model prediction and DFT results. Herein, important mechanistic insights and predictivity critical for rational design of Mg alloys of enhanced mechanical properties are provided.