Αp Grains Research Articles

Compressive creep damage of Ti-6Al-4V ELI alloy with bimodal structure at room temperature

The compressive creep performance of Ti alloy is critical for the application of submersibles due to the pressure hull of deep-diving equipment mainly subjected to the seawater pressure during its service. The compressive creep behaviors and the corresponding deformation mechanism of Ti-6Al-4V ELI alloy under different applied stress amplitudes were investigated. It was found that the stress threshold for Ti-6Al-4V ELI alloy produced significant compressive creep damage was 0.8 times yield strength (Rp0.2), and the primary mechanism was prismatic slip. Specifically, a large number of prismatic slips were preferentially activated in the αp grains with a relative high Schmid factor when the applied stress amplitude was greater than 0.8Rp0.2. As the applied stress amplitude increases, the prismatic slips were also activated in the αlath. Meanwhile, the Burgers relationship between αlath and βlamellae gradually destroyed due to the dislocation pile-up at the αlath/βlamellae interface. This investigation provides data support for the compressive creep performance evaluation of Ti-6Al-4V ELI pressure hull.

Transformation behavior of ω/α″ precipitates during aging and their influences on tensile properties in Ti-3Al-5Mo-4.5 V alloy

The metastable ω and α″ in titanium alloys have a significant impact on the subsequent α precipitates and the final mechanical properties. Herein, the microstructure transformation of coexisting ω/α″ during aging and their effects on tensile properties have been systematically investigated in Ti-3Al-5Mo-4.5 V (wt%) α/β titanium alloy. The results demonstrate that the secondary phase within the β-transformed (βtrans) grain evolves from embryonic ω + α″ to isothermal ω + α″, and ultimately transforms to secondary α (αs) with the aging temperature ascending from 300 °C to 550 °C. As embryonic ω transforms to isothermal ω, the yield strength increases from 1130.0 ± 14 MPa to 1259.5 ± 5.5 MPa with weak elongation due to the excellent strengthen effect of undeformed isothermal ω. After aging at 550 °C, coexisting micron-scale αp and nanoscale αs microstructures lead to enhanced elongation (increase by 93.7 %), sacrificing only 21.7 % yield strength. Both <a> and <c + a> dislocations are simultaneously presented in micron-scale αp grain, while nanoscale αs plates act as effective obstacles for dislocation motion resulting in accumulation of dense dislocations near the β/αs interfaces. This work provides new insights on strengthening strategy and developing new Ti-alloys.

Preferential fatigue cracking at basal twist grain boundary (BTGB) in bimodal Ti-5Al-4V alloy: Dislocation activities and crack initiation

In recent years, (0001) twist grain boundaries (BTGBs) located in primary α grain clusters were identified as fatigue crack nucleation sites in different Ti alloys. In the present study, crack initiation was investigated in a bimodal Ti-5Al-4V alloy subjected to low-cycle fatigue and dwell-fatigue loadings at room temperature. The low fraction of primary α grains was not associated with a lack of sensitivity to BTGB cracking. Transmission electron microscopy and electron back-scattered diffraction were used to characterize BTGBs in the initial microstructure. The fatigue mechanisms were then analyzed with a focus on dislocation activity. αp grains adjacent to cracked BTGBs contained a high dislocation density. It was primarily composed of planar slip bands of <a> dislocations. In addition, <c+a> dislocations were noticed in the vicinity of cracked BTGBs. They supposedly pertain to crack tip plasticity during growth, and no evidence of a role of an incoming slip event in crack nucleation was obtained. Also, basal slip bands extending across adjacent grains were found to emerge from BTGBs. This feature provides an easier path for crack extension when growth along the grain boundary becomes difficult owing to a deviation from the basal plane. Atom probe tomography analyses evidenced V and Fe segregation at a grain boundary with a significant deviation from the BTGB configuration. This suggests a possible contribution of local solute segregation to the high cracking resistance of general αp / αp grain boundaries. This work provides new insights into the mechanisms involved in cracking of BTGB in Ti alloys subjected to cyclic loadings.

Fracture mechanism of Ti60 alloy at high temperature in very high cycle fatigue regime and fatigue life prediction

AbstractThis study aimed to investigate the failure mechanism of Ti60 titanium alloy in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) at 500°C. Ti60 specimens were characterized before and after the 500°C VHCF test, and the fracture surfaces were observed. The results show that there are three different fatigue failure mechanisms at 500°C: (i) equiaxed primary α phase (αp) cleavage, forming small unsmooth facets and rough fracture area (RA), and fusion of non‐adjacent facets to grow into the main crack. (ii) αp grains gather into large grain with triple junction, and large grain slip to form large fusion facet. (iii) Oxide shedding and then cracks form. A dynamic recurrent neural network model is used to predict the fatigue life of Ti60 alloy, 91% of the overall predictions were within the scatter band of 3.0, and 100% were within the scatter band of 5.0.

Evolution of Microstructure and Mechanical Properties of Ti-6Al-4V Alloy under Heat Treatment and Multi-Axial Forging.

The mechanical properties of various Ti-6Al-4V alloys are influenced by their respective microstructures. This study generated an ultrafine-grain (UFG) Ti-6Al-4V alloy featuring bimodal grain distribution characteristics achieved through initial heat treatment, multi-axial forging (MF), and annealing. The study also extensively examined the evolution process of the alloy's microstructure. By subjecting the materials to heat treatments at 900 °C with air cooling and 950 °C with air cooling, both materials were found to be consisted of primary α (αp) and transformed β (αs+β) regions with different proportions. Following MF, the sample treated at 900 °C displays a microstructure featuring UFGs of α+β surrounding larger micron-sized αp grains. On the other hand, the sample treated at 950 °C displays a microstructure distinguished by twisted αs lamellar and fragmented β grains surrounding larger micron-sized αp grains. Following annealing, no significant grain growth was observed in the sample. The geometrically necessary dislocations (GNDs) within the UFGs were eliminated, though some GNDs persisted within the αp grains. The samples undergoing the 900 °C heat treatment, MF, and subsequent annealing exhibited elevated strength (1280 MPa) and total elongation (10.7%). This investigation introduces a novel method for designing the microstructure of the Ti-6Al-4V alloy to achieve superior performance.

Heterogeneous dynamic recrystallization in the isothermal compression of Ti55-TiBw composite in α + β region

Dynamic recrystallization (DRX) mechanism of the primary α phase (αp) and β phases in the isothermal compression of TiBw reinforced Ti55 composite in the α + β region was investigated by characterizing the grain boundaries misorientation, local strain, subgrain/grain and dislocation morphologies. The results showed the significant heterogenous DRX in both αp and β phases due to the existence of TiBw. The αp recrystallized grains derived from the deformed αp grains either near or away from TiBw were attributed to the continuous dynamic recrystallization (CDRX) mechanism, in which the TiBw accelerated the recrystallization process of the neighboring αp grains. The evolution of β phase away from TiBw displayed the typical discontinuous dynamic recrystallization (DDRX) features. While the β phase near TiBw underwent the CDRX, where the TiBw reinforcements accelerated the accumulation of dislocations and the occurrence of rapid dynamic recovery (DRV). In particular, the effect of TiBw on the DRX was discussed in detail by considering the special crystal structure of TiBw. It was shown that the DRX grains tended to appear next to the incoherent interfaces (101)TiB, (101¯)TiB or the sharp curvature edges, such as tips and growth steps of the whiskers due to the stronger hindering effect for dislocation movement in these regions.

Effect of thermomechanical processing on microstructure evolution and mechanical properties of metastable β Ti–5Al–5V–5Mo–3Cr alloy

Two distinct thermomechanical processing (TMP) schedules are applied to as-cast Ti-5553 alloy pancakes, to optimize its microstructure and mechanical performance. While TMP-I involves 40 % thickness reduction via forging, TMP-II employs rolling with 20 % thickness reduction. Typical bimodal microstructures with nearly similar sizes, distributions, and morphologies for the primary αp grains and colonies of secondary αs and β are noted to generate for both TMP-I and TMP-II. Certainly, rolling for only half the extent of that used for forging, appears ideal and economical for obtaining the targeted microstructure. Interestingly, similar and improved localized hardness values with respect to as-cast alloy, are obtained for bimodal microstructure, generated through both the TMP schedules. However, indentation size effect is noted to be nominally lower for the rolled alloy, thereby proving its suitability for large-scale application. Global behavior assessed through tensile tests also shows that bimodal microstructures obtained through forging or rolling leads to higher strengths, toughness and ductility, as compared to as-cast counterparts. This is primarily owing to non-uniform strain distribution in the as-cast alloy, resulting to crack initiation and failure, as revealed through digital image correlation-based study. Overall, TMP employing rolling with reduced deformation in particular appears beneficial for the mechanical property improvement of the alloy.

Effect of α+β phase rolling and aging treatment on laminated bimodal structure in high temperature titanium alloy

In this work, we have prepared a novel laminated bimodal structure with layer-by-layer distribution of αp and αs phase in high temperature titanium alloy by α+β phase rolling and aging treatment. The lamellar bimodal microstructure is comprised of long-primary αp and secondary αs phase in the Ti-6.0Al-3Sn-5Zr-0.5Mo-1.0Nb-1.0Ta-0.4Si-0.2Er alloy. During α+β phase rolling at 990 °C, the primary αp grain deformed to elongated layer structure, while the lamella secondary αs deformed to kinked and broken chaos morphology. After 800 °C for 1h stabilization and 700 °C for 5h aging treatment, the thickness of the elongated αp layer slightly grew, while the kinked and broken αs were recovered and partially recrystallized to form a layered fine grain structure. The lamellar bimodal microstructure, i.e., elongated primary αp with layered bimodal αs phase has enhanced strength and ductility of the high temperature titanium alloy. This enhanced strength and toughness is mainly attributed to the lamellae αp structure and layered fine αs grain formed by hot rolling and aging treatment.

Effect of β recrystallization on the precipitation evolution of primary α phase and the phase transformation inheritance during thermomechanical processing in Ti-55511 alloy

The crystallographic defects introduced after thermal deformation of near-β Ti-55511 alloy can have an important impact on the precipitation of primary α phase (αp), β grain evolution and the texture evolution during the subsequent heat treatment. The investigation indicates that the intragranular short-rod αp precipitated by strain-induced nucleation deviates from Burgers orientation relationship (BOR) by boundary splitting in 0.27–0.64, accompanied by β→αp transformation, which strictly obeys BOR during ST. In addition, the strain and strain rate can modify the αp-type after deformation. Three main αp types are Type I: 50–60°/<-12-10>, Type II: 40–50° or 80–90°/<01–10> and Type III: 90–100°/<-27-53>. The phase transformation inheritance occurs after ST based on the variant selection effect, and the αp varieties are mainly 80–90°/<-12-10> and 80–90°/<01–10>. Furthermore, the density of the deformation texture (000–1) [11–20] is weakened by the new αp transformation texture (0001) [-1-780] and the recrystallization texture (23-5-3) [11–22], but it is inherited at 1.23. However, the αp recrystallization texture significantly restrains the occurrence of the deformation texture inheritance phenomenon at 0.001 s−1. Moreover, the adjacent αp grains are mainly 60°/<11–20> to each other, while the β grains around αp with a relatively high degree of recrystallization are mainly 30–50°/<34x> to each other. The first one of the static sphericalized αp is partly generated by coarsening, which is derived from the dispersed nucleus αp precipitated at the trigonal/tetragonal β sub-boundaries. The second one is by the GB separation mechanism triggered by the formation of α/α boundaries within the short-bar αp grain.

Uncovering the hierarchical clusters in the heat-affected zone of an electron beam welded α/β titanium alloy joint

Using high-resolution transmission Kikuchi diffraction (TKD) and transmission electron microscopy (TEM), we examined the hierarchical clusters that form in situ in the heat-affected zone (HAZ), which are commonly referred to as “ghost” structures, of bimodal titanium alloy Ti-5Al-2Sn-2Zr-4Mo-4Cr (wt%, TC17). The ghost structures are enriched with Al elements but poor in Mo and Cr compared to the surrounding β matrix. TKD results show that the ghost structure in middle-HAZ mainly consists of αL laths with a high-angle grain boundary, which exhibits the classic Burgers orientation relationship (BOR) with the host matrix, while it encircles the αP grains in far-HAZ. And the ghost structure is evidenced to form via incomplete martensitic transformation. TEM results further confirm that the ghost structure is composed of αL and tiny βL laths with BOR, with the former being enriched with Al and poor with Cr and Mo, while the latter is the opposite. Interestingly, two αL variant clusters with a check-mark morphology are frequently observed viewed along [0001]αL//[110]βL directions, which are dominated by the crystallographic and geometrical relationships between α and β phases. Based on the microstructural characterization, it is hypothesized that the ghost structure is transformed from the initial αP phase, due to the coupling effect of high thermal stress (which induces the formation of a large number of dislocations) and element diffusion caused by sudden temperature increase and plunge cooling in the HAZ during the welding process.

Effects of rapid heating on non-equilibrium microstructure evolution and strengthening mechanisms of titanium alloy

In this paper, the effects of heating parameters, including temperature ranging from 900 °C to 1000 °C, heating rates ranging from 2 °C∙s−1 to 100 °C∙s−1, and 120 s soaking on the non-equilibrium microstructure evolution of Ti-6Al-4V alloy were studied. Microstructures after heating were characterized to reveal the mechanism of non-equilibrium phase transformation. Uniaxial tensile tests at room temperature were carried out to evaluate the effects of non-equilibrium microstructure on the mechanical properties. Results show that higher heating rate and lower temperature lead to lower β phase volume fraction and finer β grains. A transition region with element gradient forms in the αp grain near the αp/β phase boundary and transfers into β phase gradually during the heating. Rapid heating could confine the movement of the transition region, and therefore reduce the α→β transition and growth of the β phase. When the Ti-6Al-4V alloy was heated to 1000 °C at a rate of 50 °C/s and then quenched immediately, the tensile strength was improved by 19.5% and reached up to 1263.0 MPa with the elongation only decreasing from 13.6% to 9.6% compared with the initial material. The main strengthening mechanism is that the rapid heating in the single-phase region avoids the rapid growth of the β phase, which leads to fully fine martensite formation after water quenching.

Micro-Deformation and Fracture Features of Ti834 Titanium Alloy under Fatigue Loading

A sustained load holding period imposed during fatigue loading is detrimental to material performances, causing a sharp decline in the fatigue life of near-α titanium alloys. Therefore, the deformation discrepancies of dwell fatigue (DF) and low cycle fatigue (LCF) were studied for Ti834 titanium alloy with bimodal structures in this work. The fractographies after dwell fatigue and low cycle fatigue testing were characterized using scanning electron microcopy (SEM), and the crack propagation paths at the subsurface were investigated using an optical microscope (OM). In order to reveal the mechanism of fatigue damage, detailed dislocation structures were observed using transmission electron microcopy (TEM). The crack propagation paths in microscales and the dislocation distributions were observed in the LCF and DF. The reasons for the discrepancies are also discussed in this work, which effectively enhances the understanding of the dwell failure procedures. The results show that the near basal cracks are formed under dwell fatigue, and the deformation is highly localized at the boundary of αp grains under dwell fatigue. In contrast, during low cycle fatigue, the sample tends to deform homogenously. An intergranular fracture along the primary αp grains is formed due to the localized deformation during dwell fatigue. However, a transgranular fracture is formed in the primary αp grains under low cycle fatigue.

Excellent dynamic mechanical properties of a newly developed titanium alloy with bimodal structure

In this work, a new dual-phase Ti-6Al-2Zr-2Mo-2 Nb-2Sn-1 V alloy was designed based on the classical Al-equivalent and Mo-equivalent design criteria, combined with the role of alloying elements in titanium alloys. The microstructure, microtexture and dynamic compressive properties of the alloy were studied systematically. A bimodal microstructure with 85 vol% α and 15 vol% β phase was obtained after annealing treatment. The equiaxed α phase (αp) and β phase conforms to Burgers orientation relationship (BOR), and part of lamellar α (αs) had similar orientations to the surrounding αp grains, resulting in the overlap of αp and αs texture components. The newly developed titanium alloy exhibited excellent dynamic compressive strength-plasticity-impact absorption energy matching, attributed to the coordinated deformation between α and β phases as well as αp and transformed β microstructure. The flow stress, plastic strain and impact energy at the critical failure strain rate of 4000 s−1 were 1586 MPa, 0.31 and 490 J/cm3, respectively. The dynamic mechanical properties of the new alloy are superior to those of typical titanium alloys and have the potential to be applied in high-strain rate environments.

Internal tensile properties of characteristic zones in the electron beam weldment of TC17 titanium alloy

The internal tensile deformation of the individual base metal zone (BM), heat-affected zone (HAZ), and fusion zone (FZ) in a titanium alloy (Ti–5Al–2Sn–2r–4Mo–4Cr, wt.%) electron beam welding (EBW) joint is investigated. The ultimate strength of BM with a bimodal microstructure is 1047.5 MPa, and the elongation to fracture is 9.8%. HAZ is distinguished by the formation of a ghost structure with a hierarchical cluster structure within the initial αP grains, while the embedded αs laths dissolve in the matrix and relatively coarsening β grains are generated. During tension, the ghost structures strongly inhibit and deflect slip initiation and extension in the relatively coarsening β matrix. The competitive balance between strengthening from the ghost structure and weakening from the coarsening matrix dominates the mechanical properties of the HAZ, which then induces a hybrid fracture. The needle-shaped thermal martensite that formed during EBW causes the FZ to fail prematurely. The microstructural evolution of HAZ and FZ decreases their mechanical properties. This research may shed some light on the mechanical enhancement of titanium alloys by utilizing the multiple αL laths precipitated within the initial αP structure associated with the bimodal matrix.

Microstructural effects on fatigue crack initiation mechanisms in a near-alpha titanium alloy

The role of different microstructural constituents on crack initiation in two-phase titanium alloys is still an area of great controversy. The present study investigates the effects of primary α volume fraction and concomitant macrozones on two different fatigue crack initiation modes concurrently observed in a near-α titanium alloy. Statistically representative regions were monitored in quasi-in-situ studies by interrupting fatigue testing to detect slip trace formation leading to crack initiation. In addition, high-resolution 2D strain maps were generated to quantify in-plane shear strains of slip traces related to microstructural features. The detailed analysis demonstrates that at 90% of the proof stress basal 〈a〉 slip plays a crucial role in crack initiation, regardless of whether cracking is transgranular or intergranular. In addition, a distinct shift from transgranular to intergranular crack initiation was observed with increasing αp fraction driven by the increased number of αp/αp grain boundaries. The high-resolution 2D strain mapping suggest that these intergranular cracks initiated from a burst of basal 〈a〉 slip starting from (0001) twist grain boundaries but penetrating one side of the grain pair in case of a partial (0001) twist grain boundary. From these observations, a new criterion for intergranular cracking is proposed based on a geometrical grain boundary parameter and the preference of neighbouring αp grain pair well aligned for basal 〈a〉 slip. It was found that while (0001) twist grain boundaries increase with increasing αp fraction, macrozones do not further enhance their frequency. However, macrozones of hard orientated grains did enhance transgranular crack formation by potentially higher local stress and increasing slip length over clustered αp grains with low misorientation reducing the requirement for out-of-plane shear.

Crack initiation mechanism in a high-strength Ti-5Al-7.5V alloy subjected to high cycle fatigue loading

Fatigue crack initiation mechanism at high cycle regime in a bimodal Ti-5Al-7.5V alloy are investigated by fracture observations, focused-ion-beam cross-section and electron back-scattered diffraction characterizations. The aged Ti-5Al-7.5V exhibits superior tensile and fatigue properties, alongside a wide range of fatigue life when tested at the stress levels of ∼ 700–800 MPa. Fatigue failure was dominated by surface and subsurface crack initiation at facets matching basal planes. These faceted primary α (αp) grains generally have a medium or high Schmid factor (SF) for basal < a > slip. Moreover, a basal faceting process also occurred in the “hard” grains, in which the c-axis of the HCP-α grains was approximately parallel to the loading axis so that the < a > type slips were thought to be suppressed. The observed internal microcracks were formed along the grain boundaries located in clusters of αp grains, especially along (0001) twist boundaries. Furthermore, transmission electron microscope observations evidenced the absence of intense slip bands within faceted grain and fatigued αp grains, which indicates that the facet growth should not along the pre-existing slip bands resulting from high cycle fatigue loadings. The pre-existing structural defects including dislocation networks and subgrain boundaries within αp grains are relatively stable configurations under fatigue loadings. The observed fine (∼500 nm in diameter) silicides were widely dispersed, whether the silicide can act as the crack nucleation site still needs further more detailed studies and this remains an open question.

Effect of cutting speed on shear band formation and chip morphology of Ti–6Al–4V alloy using nanoindentation and EBSD mapping

The hardness and orientations of the primary α grains (αp) and the transformed β (βt) grains in segmented chips obtained by turning an as-received solution treated-and-aged bar of Ti–6Al–4V (wt.%) at cutting speeds of 1, 1.5 and 2 m/s (61, 91, and 122 m/min) were analyzed using nanoindentation mapping and electron backscattered diffraction (EBSD) maps. The hardness of each αp grain in the as-received Ti–6Al–4V bar highly depends on the crystal orientation, varying from 4.5 GPa to 6.7 GPa, and the hardness values of βt were about 20% lower. In the machined chips, αp grains showed hardness values similar to the as-received material while βt became slightly harder. The homogeneous shear strain within the segments (between shear bands) was estimated to be around 0.4–0.7 at all three cutting speeds. The width of the shear bands in the 1 and 1.5 m/s chips were ∼1.3 μm and ∼2 μm in the 2 m/s chips, which are 10–40% of thicknesses predicted by Molinari's isotropic continuum model, respectively. A smaller homogeneous shear strain was correlated with larger shear cracks at each cutting speed. Grain orientations favored by prism ⟨a⟩ slip in αp when cutting at 1 m/s made the localized shear band less “adiabatic” since both the shear stress and strain were lower when these orientations were dominant. In contrast, in orientations that favored pyramidal <c+a> slip, the αp grains experienced much higher shear stress and more localized shear strain when cutting at 2 m/s, leading to higher temperatures that were much more likely to induce the α to β phase transformation. A transformation to soft β phase during shearing accelerates plastic instabilities. These observations and comparisons with finite element simulations indicate that temperatures reached the α+β phase field locally, and higher cutting speeds led to more β phase that is detrimental to the tool life.

High-strength and low-dwell-sensitivity titanium alloy showing high tolerance to microcracking under dwell fatigue condition

Dwell fatigue failure of titanium alloy components has been one of the major threats to aircraft safety for the last 50 years. Numerous studies have focused on the identification of critical microstructural configurations, such as microtextured regions, rogue grain pairs and (0001) twist grain boundaries. In this work, a microstructure with a low primary α phase (αp) content was designed to avoid the known weaknesses in the Ti-5Al-7.5 V alloy to achieve high-strength and low-sensitivity to 2-min dwell loading. Conventional and dwell-fatigue tests confirmed the efficiency of such an approach. The deformation occurred primarily by planar slip in the αp grains, resulting in possible cracking along preexisting slip bands in the cycling process. Fatigue and dwell-fatigue failure was dominated by surface crack initiation at facet matching basal planes. Moreover, numerous internal microcracks were formed along basal slip bands and occasionally along basal twist grain boundaries. Load holds were found to facilitate cracking along prismatic planes, which highlights a mechanism switch between basal-dominant cracking mode and collective basal-prismatic mode. The mechanism switch suggests that the prismatic dislocation activity considerably increases due to the low temperature creep induced by load holding. Thanks to the low αp phase microstructure, the alloy shows remarkable tolerance to microcrack growth under dwell condition. Limited growth of the internal microcracks across transformed β regions was identified as a key feature of high dwell-fatigue resistance of the investigated material.

Growth mechanism of αp and interface relationships between αp and αs during cooling of a near α titanium alloy

This paper focused on the interface relationships between primary α (αp) and the precipitation secondary α (αs) phase at different cooling rates from 965 °C. The rapid cooling rate favors the precipitation of the secondary αs. And the low cooling rate is conducive to the epitaxial growth of the primary αp. One key finding is that once the grain boundary (αGB) nucleated on the αp could grow forward without maintaining the Burgers orientation relationship (BOR) with adjacent β grains. What’s more, HADDF and Fourier-filtered HRTEM images elucidated that two adjacent αp particles share a common plane but not the same atomic arrangement direction in crystal spatial. Three precipitation types of the αs were discussed through EBSD and 3D spatial crystalline orientation. Specifically, the αs shares common {1 1 2¯ 0} plane with the αp grains respectively in Type-I and Type-II, but displaying different c axis angle (60° in Type-I and 80° in Type-II). For the Type-III, the secondary αs shares common {1 0 1¯ 0} plane with the primary αp, and the angle of c axis between them is 60°. Moreover, the primary αp do not maintain the BOR with the pre-existing β, except for Type-I.

Heterogeneous deformation and damage mechanisms in multi-phase TA15 Ti-alloy: Insights from experiments informed damage-crystal plasticity modelling

Understanding the mesoscale heterogeneous deformation and its induced material damage is significant to enhance the mechanical properties of polycrystalline aggregates. In this work, a damage-coupled crystal plasticity finite element (DC-CPFE) model based on high-fidelity microstructure was developed to investigate the heterogeneous deformation and damage behavior of TA15 Ti-alloy with tri-modal microstructure composed of globular alpha (αp), lamellar alpha (αl) and transformed beta (βt). A three-step procedure combining the nanoindentation test, stress relaxation test, and uniaxial tensile test was proposed to determine the material parameters in the crystal plasticity constitutive and damage models of the three constituent phases. Based on the developed DC-CPFE model, the heterogeneous slip modes and traces of tri-modal microstructure were predicted and analyzed. The heterogeneous deformation of tri-modal microstructure was found to be mainly caused by the heterogeneous deformation in α phases, which was featured by the deformation bands and local shear bands. Deformation bands occurred in both of the deformation-preferential and -lagged αp grains and the hard-to-deform αl grains. Local shear bands only produced in αl grains, which were related to both of the soft geometry orientation and the activation of prismatic slip systems. Meanwhile, the formation of deformation bands and local shear bands could be promoted by the local lattice rotation. The above heterogeneous deformation led to four types of damage, which were caused by the slip band intersection, the slip transfer impedance, the local shear band, and the property and orientation mismatches among the adjacent grains, respectively.