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Several physical processes can conspire to generate avalanches in materials. Such processes include avalanche mechanisms like dislocation movements, friction processes by pinning magnetic domain walls, moving dislocation tangles, hole collapse in porous materials, collisions of ferroelectric and ferroelastic domain boundaries, kinks in interfaces, and many more. Known methods to distinguish between these species which allow the physical identification of multiavalanche processes are reviewed. A new approach where the scaling relationship between the avalanche energies E and amplitudes A is considered is then described. Avalanches with single mechanisms scale experimentally as E = SiAi2. The energy E reflects the duration D of the avalanche and A(t), the temporal amplitude. The scaling prefactor S depends explicitly on the duration of the avalanche and on details of the avalanche profiles. It is reported that S is not a universal constant but assumes different values depending on the avalanche mechanism. If avalanches coincide, they can still show multivalued scaling between E and A with different S‐values for each branch. Examples for this multibranching effect in low‐Ni 316L stainless steel, 316L stainless steel, polycrystalline Ni, TC21 titanium alloy, and a Fe40Mn40Co10Cr10 high‐entropy alloy are shown. [ABSTRACT FROM AUTHOR]

The ω-phase in metastable β-titanium (Ti) alloys attracts great research interests owing to the metastability and the involved microstructural complexity upon straining, while the fundamental understanding on its formation mechanism is still insufficient. In this study, deformation-induced ω-plates have been systematically investigated using Ti-10 wt% Cr metastable β-Ti alloy containing athermal ω-precipitates. It is found that single-variant ω-plates form either along {332}β twin interfaces or in the twin interiors, which closely depends on twin morphologies. For the acicular β-twin, the ω-plate appears in the twin interiors accompanied by high density of parallel straight dislocations, while for the lenticular twin, the ω-plate attaches to the twin interface without the surrounding dense parallel straight dislocations. These unique formation characteristics result from local stress field induced by the twinning itself and the passive transformation of athermal ω-precipitates. It is further revealed that the passive transformation of athermal ω-precipitates dominates the formation of ω-plates inside the acicular twins, while it is the locally preferential twin thickening that promotes the ω-plates to form along the lenticular twin interfaces. These findings provide new insight into deformation-induced ω-phase transformation and the intricate deformation microstructures in metastable β-Ti alloys. [ABSTRACT FROM AUTHOR]

Background: In current clinical practice, the most commonly used fusion cage materials are titanium (Ti) alloys. However, titanium alloys are non-degradable and may cause stress shielding. ZK60 is a bio-absorbable implant that can effectively avoid long-term complications, such as stress shielding effects, implant displacement, and foreign body reactions. In this study, we aimed at investigating the biomechanical behavior of the cervical spine after implanting different interbody fusion cages.Methods: The finite element (FE) models of anterior cervical disc removal and bone graft fusion (ACDF) with a ZK60 cage and a Ti cage were constructed, respectively. Simulations were performed to evaluate their properties of flexion, extension, lateral bending, and axial rotation of the cervical spine. Moreover, a side-by-side comparison was conducted on the range of motion (ROM), the deformation of cages, the stress in the cages, bone grafts, and cage-end plate interface. Simultaneously, according to the biomechanical analysis results, the microporous structure of the ZK60 cage was improved by the lattice topology optimization technology and validation using static structure.Results: The ROMs in the current study were comparable with the results reported in the literature. There was no significant difference in the deformation of the two cages under various conditions. Moreover, the maximum stress occurred at the rear of the cage in all cases. The cage's and endplate-cage interface's stress of the ZK60 group was reduced compared with the Ti cage, while the bone graft stress in the ZK60 fusion cage was significantly greater than that in the Ti fusion cage (average 27.70%). We further optimized the cage by filling it with lattice structures, the volume was decreased by 40%, and validation showed more significant biomechanical properties than ZK60 and Ti cages.Conclusion: The application of the ZK60 cage can significantly increase the stress stimulation to the bone graft by reducing the stress shielding effect between the two instrumented bodies. We also observed that the stress of the endplate-cage interface decreased as the reduction of the cage's stiffness, indicating that subsidence is less likely to occur in the cage with lower stiffness. Moreover, we successfully designed a porous cage based on the biomechanical load by lattice optimization. [ABSTRACT FROM AUTHOR]

The operation of 〈c + a〉 slip is explored using in situ mechanical testing in a transmission electron microscope by compressing a single-crystal Ti alloy oriented along the [0001] direction. In this direction 〈c + a〉 slip is the preferred slip mode. We observed the operation of a 〈c + a〉 dislocation source while simultaneously measuring the mechanical response. Our results show that 〈c + a〉 dislocations could operate as single-arm sources. A model for the source mechanism of 〈c + a〉 slip is proposed based on the experimental observations. [ABSTRACT FROM AUTHOR]

Through ex situ and in situ compression and tension tests of micrometer- and submicrometer-sized single crystal hexagonal close packed (HCP) Ti alloy pillars oriented for prismatic slip, we have observed that 'smaller is stronger' and the larger samples exhibit obvious strain bursts. However, for extremely small samples, the plastic flow becomes much more stable both in compression and tension, mainly due to the emergence of a high density of basal stacking faults (SFs) driven by extremely high stress, which rarely appear in larger samples and bulk counterpart. This work demonstrates a recipe towards ultra-high strength (GPa level) nanoscale samples with continous plastic flow. [ABSTRACT FROM AUTHOR]

Deformation twinning in crystals is a highly coherent inelastic shearing process that controls the mechanical behaviour of many materials, but its origin and spatio-temporal features are shrouded in mystery. Using micro-compression and in situ nano-compression experiments, here we find that the stress required for deformation twinning increases drastically with decreasing sample size of a titanium alloy single crystal, until the sample size is reduced to one micrometre, below which the deformation twinning is entirely replaced by less correlated, ordinary dislocation plasticity. Accompanying the transition in deformation mechanism, the maximum flow stress of the submicrometre-sized pillars was observed to saturate at a value close to titanium’s ideal strength. We develop a ‘stimulated slip’ model to explain the strong size dependence of deformation twinning. The sample size in transition is relatively large and easily accessible in experiments, making our understanding of size dependence relevant for applications. [ABSTRACT FROM AUTHOR]

[Display omitted] High specific-strength lightweight titanium (Ti) alloys, in the absence of interstitial strengthening of oxygen (O) atoms to avoid O-embrittlement, are mainly strengthened via densely semi-coherent nanoprecipitates in the β-matrix that act as dislocation obstacles and often result in high-stress concentrations, contributing to their strength-ductility trade-off. Here, using a low cost Ti-2.8Cr-4.5Zr-5.2Al duplex alloy as a model material, we present a counterintuitive O-doping strategy to create topologically coherent, interstitial-O α′ nanotwinned nanomartensites (NTNMs) with good interfacial strain compatibilities. The interstitial atoms tailor the stress field of edge dislocation cores from planar to non-planar, facilitating multiple variants nucleate simultaneously along O-rich edge dislocations to construct interstitial-O NTNMs. The interstitial-O NTNMs endow our duplex Ti alloys with superior strength of 1.64 gigapascals and large uniform elongation of 11.5%, surpassing all previously reported bulk Ti alloys. This unprecedented combination of mechanical properties is conferred mainly by the interstitial NTNMs, which serve as a sustainable ductility source via a self-hardening deformation mechanism and utilize the pronounced interstitial strengthening of concentrated O atoms. As such, the coherent interstitial NTNMs engineering strategy efficiently combines interstitial solid solution strengthening, and coherent interface strengthening mechanisms, that provides new insights into designing high-strength and large ductility O-tolerant alloys for cost-effective and lightweight applications. [ABSTRACT FROM AUTHOR]

With the increase of time, the shrinkage of materials at fixed temperature could enhance the failure of fasteners. We report a potential way to alter the volume/length of alloy automatically through isothermal aging due to pseudospinodal decomposition mechanism. The volume of Ti-10V-2Fe-3Al alloy first shrunk and then expanded during isothermal aging at 550 °C. It can fit tightly and make up for volume loss. Transmission electron microscopy observation exhibits no obvious coarsening of intragranular α phase with the increasing time. However, composition evolution with time shows a gradual change through energy dispersive spectrometer analysis. The result shows that β stabilizers, V and Fe, are prone to diffuse to the β matrix, while α stabilizers, Al, prefer to segregate to the α phase. First principle calculations suggest that the structure transition for β to α cause the first decrease of volume, and the diffusion of V, Fe and Al is the origin of the later abnormal increase of volume. [ABSTRACT FROM AUTHOR]

Titanium (Ti) alloys, which are highly promising as structural materials in critical industrial applications, generally require high strength and ductility, particularly high fracture toughness. However, the conventional approach of enhancing strength through mechanical processing to induce dislocations often leads to a compromised ductility known as the strength-ductility trade-off. Here, we develop a new strategy of the nanocrystallized kinks to overcome this issue in Ti-11V metastable β -Ti alloys via combining industrially used cold forging (CF) and cold rolling (CR) processes. Deformation kinks are firstly activated by CF, and subsequently they are fragmented into nanograins during CR, architecting nanocrystallized kinks in the coarse-grained matrix. This unique microstructure effectively balances the strength-ductility conflict, endowing this Ti-V binary alloy with high yield strength ∼1200 MPa, appreciable ductility ∼17 % and high fracture toughness ∼ 52.0 MPa·m½, which is superior to numerous multielement engineering Ti alloys. This design strategy of nanocrystallized kinks can be extended to other engineering materials, e.g. , Mg and Zr alloys, for advanced performance at large industrial scales. [Display omitted] [ABSTRACT FROM AUTHOR]

Metastable β -titanium (Ti) alloys have always attracted great attention due to the available ultra-high strength via precipitation strengthening, while they often succumb to a strength-ductility trade-off known as improving strength but degrading ductility. In this study, we exploit a novel strategy to address the issue in Ti–6Cr–5Mo–5V–4Al (Ti6554) high-strength metastable β -Ti alloy through the trace boron addition. It is found that an ultra-high strength ∼1600 MPa with elongation of ∼9 % is synchronously achieved with the addition of minor boron, corresponding to a ∼7 % increase in strength and ∼70 % improvement in ductility compared to the boron-free counterpart. Microstructural characterizations manifest that boron addition promotes almost complete transformation from abnormal β -grain region to normal β -grain region, which possesses better deformation accommodation capacity at the aged state. Meanwhile, TiB whiskers are produced and adhered by a layer of α s -film, creating a sandwiched structure architected by TiB whisker/ α s -film/the aged β -matrix. This unique structure not only effectively retards the deteriorative development of highly localized deformation propagating from the aged β -matrix, but also sufficiently activates dislocation slip within TiB whiskers. These findings demonstrate a counterintuitive result that TiB intermetallic compound well-known as the susceptive brittleness can be harnessed in ductilizing materials, and also provide a practical strategy for enhancing plasticity of high-strength Ti alloys by microalloying design. [ABSTRACT FROM AUTHOR]

A metastable β-titanium Ti–Al–Mo–V–Cr–Nb alloy exhibits a synergistic increasing in strength and ductility via coupling thermomechanical processing and heat treatment in present work. Synergistic effect of the thermomechanical processing and heat treatment on microstructure evolution and mechanical properties of the metastable β-Ti alloy was systematically investigated. α+β rolled sample sheet consumes less time to finish recrystallization, and leads to more uniformly distribution and smaller size of β grains than β rolled sample after same solution treatment. Primary α phase in α+β rolled sample hinders the growth of β grains during solution treatment and the spacing distance of α lamella in α+β rolled sample is smaller than β rolled sample after aging treatment, enhancing the strength significantly. The small spacing distance of α lamella is due to an increase in precipitation density. The small size and homogeneous distribution of β grains provides moderate ductility. After solution treatment at 770 °C for 2 h and aging treatment at 525 °C for 6 h, α+β rolled samples exhibit higher ultimate tensile strength (1550 MPa) and better ductility (6.5 %) than β rolled sample. The relationship between the average grain size of β grains, the average spacing distance of α lamella, and the yield strength of the alloy is quantitatively calculated using mathematical methods. The mechanisms of synergistic enhancement of strength and ductility in the alloy is qualitatively analyzed. These findings facilitate manufacturing high-performance metastable β-Ti alloys at an industry scale using commercially available thermomechanical processing. [ABSTRACT FROM AUTHOR]

Precipitates in alloys are traditionally considered as dislocation obstacles that often lead to high stress concentration and even microcracks owing to the strain incompatibility of semi-coherent interfaces, a cause of progressive strain localization and the origin of the strength-toughness conflict in engineering materials, such as titanium (Ti) alloys. Based on the metastability engineering, here we architect hierarchically ordered coherent interfaces for strength-toughness optimization through densely dispersed nanomartensites in a ductile Ti-Cr-Zr-Al alloy with ultrahigh specific-strength and superior fracture toughness. It is unveiled that these ordered coherent interfaces simultaneously serve as dislocation obstacles and sources, leading to a sustainable and self-hardening deformation mechanism via hierarchical nanomartensite-dislocation interactions for ultra-high strength and toughness of Ti alloys. These nanomartensites are thermally stable at elevated temperature less than 400 °C, above which tempering-induced ductile-to-brittle transition occurs due to the decomposition of hierarchically ordered nanomartensites and the spheroidization of prior β lamellae. The design strategy of hierarchically ordered coherent interfaces confers our cost-effective nanomartensite Ti alloys an unprecedented combination of strength, ductility and toughness, which provides a new pathway in the microstructural design for strong and ductile structural materials with superior fracture resistance. [Display omitted] [ABSTRACT FROM AUTHOR]

α-titanium alloys are extensively applied under cryogenic environment and thereby their cryogenic mechanical properties and service failures deserve to be deeply studied. Here, tensile behaviors and damage-fracture characteristics of Ti-2.5Cu α-titanium alloy were systematically investigated at liquid-nitrogen temperature (LT) plus room temperature (RT) for comparison. It is found that the LT samples possess superior strength and ductility concurrently. Both yield strength (σ y) and ultimate tensile strength (σ b) improve more than 70%, whilst total elongation at fracture (A) adds more than 33% in comparison with the RT counterparts. Meanwhile, the LT samples are stretched mainly by uniform strain without noticeable necking. Fracture analysis indicates that being different from typical cup-and-cone fracture surface in the RT samples, the 'radial' fracture surface appears in the LT samples. Such specific fracture surface is constructed by two characteristic morphologies of crack-initiation zone at sample surface and the ensuing crack-propagation zone. Furthermore, it is seen that a large number of micro-voids are dispersed underneath fracture surface in the RT samples, whereas the number of micro-voids is sharply reduced at LT. Nevertheless, Vickers hardness at the position is always much higher in the LT samples. These novel mechanical properties and damage-fracture characteristics in the LT samples are closely associated with the prevalent deformation twinning in microstructure. Deformation twinning itself and plentiful twins-induced dynamic Hall-Petch effect steadily accommodate plastic deformation, contributing to the simultaneous enhancement of strength and ductility. This process also results in high stress state in samples, increasing the sensitivity of surface cracking. Hence, the 'radial' fracture morphology is finally formed. These findings enrich our fundamental understanding on deformation-damage-fracture process of α-titanium alloys, and also provide implicit guidance for the design of titanium alloys with high performance. • Ti-2.5Cu alloy achieves superior tensile strength and ductility synergy at LT. • 'Radial' rather than cup-and-cone fracture surface appears in the LT samples. • Few micro-voids but higher hardness are displayed underneath fracture surface. • Twinning and plentiful twins steadily accommodate strain without localization. • Cracking sensitivity at sample surface caused by high stress makes radial fracture. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ti6554 β -titanium alloy hot-rolled wires were modified by the additions of trace boron (B). • Microstructural features and mechanical properties of B-modified Ti6554 hot-rolled wires were investigated. • B-addition promotes microstructural homogenization of wires which relieves vast mechanical fluctuation. • A good plastic accommodation resulting from NGRs and strengthening by TiB laths endow Ti6554 wires superior strength-ductility synergy. One of the long-term concerns regarding titanium alloy wires involves microstructural inhomogeneity, mechanical instability and product unreliability in service. In this study, a strategy of micro-alloying was utilized to address the issue in Ti-6Cr-5Mo-5V-4Al (Ti6554) β -titanium alloy wires. The microstructures and mechanical properties of Ti6554 hot-rolled wires were optimized through boron (B) additions. The results manifest that B-addition promotes microstructural homogenization of wires, wherein the microstructure nearly completely consists of normal grain region (NGR) without abnormal grain region (AGR) like that in the B-free alloys. This microstructural homogenization relieves mechanical fluctuation amongst different samples and renders plastic flow highly repeatable upon stretching. Meanwhile, B-addition strengthens wires whilst maintains decent ductility and thereby achieves the better combination of strength and ductility in comparison with that in the B-free wires. The analyses indicate that the addition of B facilitates the universal occurrence of dynamic recrystallization (DRX) to form NGRs at the expense of AGRs, which is beneficial to microstructural homogenization of hot-rolled wires. A good plastic accommodation resulting from NGRs and strengthening caused by TiB laths endow Ti6554 wires superior strength-ductility synergy. These findings provide helpful guidance for microstructural and performance optimization of titanium alloy wires. [ABSTRACT FROM AUTHOR]

In conventional duplex titanium (Ti) alloys, the soft primary α-precipitates (α p) are beneficial to ductility with the loss of high yield strength, while the continuous α-precipitates at grain boundaries (GBs, α GBs) often accompanied with α precipitate-free zones (α-PFZs) renders the strain incompatibility for limited ductility. Here, to overcome this longstanding issue, we utilize interstitial (O and N) atoms to notably strengthen the α-phase, and simultaneously architect the multi-morphologic secondary α-precipitates (α s) so as to improve the strain compatibility nearby GBs, thus develop a Ti-4.1Al-2.5Zr-2.5Cr-6.8Mo (wt.%) alloy with ultra-high yield strength of ∼1580 MPa and good ductility of ∼8.2%. The phase boundaries between the hard α-precipitates and the soft β-matrix not only act as dislocation barriers for strengthening, but also as sustainable dislocation sources for ductilizing to achieve a good combination of strength and ductility in our Ti alloys. This strategy not only sheds light on the understanding of the strength-ductility synergy of duplex Ti alloys, but also offers an available pathway to design ultra-strong and ductile alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

The ductile-to-brittle transition is commonly observed in metastable β -titanium (Ti) alloys containing ω -precipitates, while the fundamental understanding on ω -embrittlement hitherto remains elusive. In this work, the prototypical Ti-20 wt% Mo metastable β -Ti alloy has been systematically investigated by coupling experiments and first-principles calculation to eliminate this puzzle. It is shown that the structural evolution of ω -phase controls the deformation mechanism transition of twinning-to-slip in Ti Mo alloy, being the origin of ductile-to-brittle transition of this alloy. The initial trigonal ω -structure continuously collapses to hexagonal ω -structure (structural collapse) whilst Mo-atoms are rejected out concurrently (stoichiometric varieties), both leading to hardening of ω -precipitates. This self-hardening of ω -precipitates was further rationalized in terms of the enhanced propensity for a covalent character of the atomic bond demonstrated by the electronic density of states (DOS) from first-principles calculation. Specifically, the self-hardening behavior of ω -precipitates promotes dislocation slip on isolated planes in lieu of correlative slip on successive planes inside ω1 -variant, while dislocations are completely blocked ahead ω2/ω3/ω4- variants. This in turn renders the transition from deformation twinning that contributes to great macro-plasticity to ordinary dislocation slip that contributes to localized deformation bands in the present Ti Mo alloy. Image 1 [ABSTRACT FROM AUTHOR]

Abstract The effects of ω precipitates and orthorhombic (α″) martensitic transformation on stress-strain behavior were studied in metastable β Ti-10V-2Fe-3Al alloy single-crystal in submicron scales. We successfully distinguished the size effects induced by ω precipitates and α″ phase transformation through compressing the metastable β Ti-10V-2Fe-3Al along the orientation of <011> β , which is unfavorable for α″ phase transformation. Two individual exponential curves express well the relationship between the strengthening exponents, which were regressed using the flow stresses at different ranges of plastic strains of 0.2–13% and the pillar widths, and the corresponding plastic strain. It is worth noting that the strengthening exponent jumps from 0.256 to 0.293 at 8% plastic strain. Double size effects are uncovered in metastable β Ti-10V-2Fe-3Al alloy single-crystal micropillars. The first (ω precipitate-related) size effect has been predicted with the proposed models, and the second (martensitic transformation-induced) size effect is interpreted with the modified Liu's equation. [ABSTRACT FROM AUTHOR]

Abstract Double size effects are uncovered in metastable β Ti-10V-2Fe-3Al alloy single-crystal micropillars subjected to compress along <011> β in the Part I of this paper. In this Part II, a series of samples at different deformation stages were systematically analyzed by transmission electron microscopy (TEM) to understand the deformation mechanism for double size effects. Two distinct phase transformations were observed to take place at different stages of plastic deformation. In the initial stage, dislocation interaction with ω precipitates and deformation-induced ω variants transformation from one to another are predominant plastic deformation mechanisms when the total strain is less than 10%. As a result, high density of ω precipitates contributes to the weak size effect and the continuous stable strain-hardening behavior in the metastable β Ti-10V-2Fe-3Al alloy. In comparison, the martensitic transformation from β to α″ is induced when the critical strain reaches to 10%. The strain-induced martensitic transformation becomes primary deformation modes, which result in higher strain-hardening exponent and stronger size effect, in the second deformation stage. These results provide a new perspective of designing micro-electromechanical materials with an excellent combination of the enhanced yield strength and stable plasticity. [ABSTRACT FROM AUTHOR]

The damping and transformation behaviors of Ti 50 (Pd 50− x Cr x ) shape memory alloys with x ranging from 4.0 to 5.0 are systematically investigated. The damping capacity (Q −1 ) at the martensitic transformation is found to be inversely proportional to the square root of frequency, i.e., Q −1 ∝ ω −0.5 . A relaxation peak or shoulder is observed slightly below the martensitic transformation damping peak for compositions within the compositional crossover region (4.5 ⩽ x ⩽ 4.8). Furthermore, the damping capacity at the martensitic transformation is smaller within the compositional crossover region (4.5 ⩽ x ⩽ 4.8), compared with that of compositions at both sides ( x = 4.0 and x = 5.0 ). These observations can be ascribed to the hysteretic motion of interfaces between different phases near the compositional crossover region. [ABSTRACT FROM AUTHOR]

In present work, the localized deformation, crack initiation and propagation behavior of Ti-6Al-2Sn-2Zr-3Mo-1Cr-2Nb-0.1Si (TC21) titanium alloy was investigated through the in-situ scanning electron microscope (SEM) test and electron backscatter diffraction (EBSD). The results indicate that deformation in α phase is mostly accommodated by the prismatic slip. The localized deformation is concentrated at the interface of primary α lath. Some parallel slip bands with prismatic and pyramidal slip are prevalent in primary α lath, while the multiple slip bands with prismatic slip are dominant in equiaxed primary α phase. Higher ability to coordinate local plastic strain (nearly to 79.5%) is induced in equiaxed α than the primary α lath (7.9%). Most of the cracks easily initiate at the shear bands in primary α lath, which is a fatal site for the tensile deformation. The main crack propagates quickly along the shear bands or interfaces of α lath and then penetrates into the β trans matrix and also transfers across the equiaxed α phase along the slip bands. The results indicate that the high compatibility of deformation in equiaxed primary α phase delays the initial crack nucleation. Whereas, it does not show satisfied resistance against crack propagation in front of the main crack tip due to the abundant slip bands. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations were performed with a Finnis-Sinclair many-body potential to investigate the mechanical properties and deformation mechanisms under applied uniaxial tensile and compressive loads in α-titanium (Ti) single-crystal nanopillars oriented along the 〈 1 1  2 ¯  0 〉 direction. The results indicate that the mechanical properties and plastic deformation mechanism display tension–compression asymmetry. The non-linear elastic behavior is attributed to the difference in friction between the neighboring atomic planes at the elastic deformation stage under push and pull loading conditions. Increasing the friction leads to hardening with compression. Decreasing the friction leads to softening with tension. Increasing the friction may also lead to higher yield stress with compression compared with tension. Perfect nanopillars are yielded via the nucleation and propagation of { 1 ¯  0 1 0} 〈1  2 ¯  1 0 〉 dislocations on the surface and corners of the nanopillars. Prismatic slip is the dominant mode of plastic deformation in the Ti nanopillars under compressive loading. However, {1 0  1 ¯  1} 〈1 0  1 ¯ 2 ¯ 〉 twinning is the dominant plastic deformation mechanism together with prismatic slip under tensile loading. Two typical intrinsic stacking faults (SFs) with different propensities exist in the nanopillars. The microstructural evolution of the SFs was also simulated. [ABSTRACT FROM AUTHOR]

Slip transmission behavior across α/β interface has been investigated in an α/β titanium alloy, TC21 alloy (Ti-6Al-2Sn-2Zr-3Mo-1Cr-2Nb-0.1Si). Scanning electron microscopy (SEM) was used to observe slip lines morphology and transmission across the interface in samples which undergo a certain tensile strain. Details of slip traces in α and β lamellae have been observed in detail by Transmission electron microscope (TEM). Atomic Force Microscopy (AFM) technique was used to observe and measure height of a single slip step at the surface of α lamellae. Two processes of slip transmission, direct transmission and a deviation of 10° angle from α to β lamellae, are observed at α/β interface corresponding to basal slip and prismatic slip in α phase, respectively. Slip transmission is not observed across α/β trans interface when β phase is strengthened by fine α laths. A modified rule of mixtures is developed to predict yield strength of TC21 alloy by introducing an interaction term. Based on experimental data and slip transmission behavior from α to β lamellae, interaction term is calculated by multiplication of barrier stress and a coefficient. The coefficient is closely related to slip transmission behavior. Easy slip transmission across the interface leads to high absolute value of the coefficient, which indicates that the interaction term has an apparent effect on prediction of strength. The low absolute value means that interaction term from interface has less effect on prediction of strength than the high one. [ABSTRACT FROM AUTHOR]

Unlike the general belief that either dislocations cut or by-pass the ω -precipitates contributes to the high strength of Ti alloys, here we uncovered a new strengthening mechanism of lattice disordering of ω -precipitates by investigating the deformation behavior of body-centered-cubic (bcc) single crystalline Ti-10V-2Fe-3Al (Ti1023) alloy micropillars. This unique ω -precipitate disordering strengthening mechanism emerges in-between the precipitate cutting and by-passing mechanisms, because the dislocations pile-up stress reaches the ideal strength of ω -phase. The ω -precipitates with four variants manifest the strong deformation anisotropies, as verified by both the crystallographic orientation analysis and energy landscapes from the first principle calculations. The combination of precipitates cutting of ω 1-variant and precipitates disordering of ω 2/ ω 3/ ω 4-variants leads to the formation of precipitate-free channel in bcc (146) β -oriented Ti1023 alloy micropillars. Due to the presence of ω -precipitates, the bcc Ti1023 alloy micropillars exhibit stable plastic flow, associated with weak-size dependent superior strength. These findings provide fundamental understanding of the deformation characteristic of precipitate-hardening alloys, and have important implications for the design of small-scale materials. [ABSTRACT FROM AUTHOR]

Interstitial solutes (e.g. O, N) often enhance strength; however, their poisoning effect leads to markedly decreased ductility and even embrittlement in titanium (Ti) alloys. Thus, using unavoidably interstitial O and N atoms to achieve low-cost, ductile Ti alloys with ultrahigh-specific-strength is significant for industrial applications. Here, taking the Ti-4.1Al-2.5Zr-2.5Cr-6.8Mo-0.17O-0.10N (wt.%) alloy as a model material, we successfully achieved an ultra-high yield strength of ∼1800 MPa in this low-cost Ti alloy by a hierarchically heterogeneous microstructure consisting of micron-scaled primary α, nano-scaled secondary α and ultrafine α-Widmanstätten nano-precipitates in the β-matrix. In particular, utilizing grain boundary engineering (GBE), the percolative nano-precipitates network directly precipitated from β-GBs, which not only strengthens GB cohesion, but also effectively blunts the crack tip and hinders crack propagation, rendering enhanced ductility. This strategy combining GBE and interstitial solutes opens an avenue to design ultra-strong and ductile Ti alloys with increased tolerance to interstitial impurities. [Display omitted] [ABSTRACT FROM AUTHOR]

A long-standing conflict concerning metastable β -titanium (Ti) alloys is that high strength can be achieved through precipitation strengthening of α -phase while plasticity is largely compromised and even lost completely. Such strength-plasticity trade-off severely limits their structural applications. Here, an attempt to address the issue has been made in Ti–6Cr–5Mo–5V–4Al (Ti6554) high-strength metastable β -Ti alloy. Heterogeneous β -structure is architected by partial static recrystallization (SRX) through properly manipulating solution treatment in the hot-rolled bars. The resulting microstructure is composed of the recrystallized 'new' equiaxed β -grains (β e) and the remnant elongated β -fibers (β f). This β -morphology dramatically affects the subsequent α s -precipitation behavior during the aging process. α s -precipitation is rather uniform throughout β f -grains because the pre-existing dislocations left by hot rolling provide copious nucleation sites. However, α GB -films are preferentially formed along β e -grain boundaries, which leads to a boundary-affect zone (BAZ) and thereby renders α s -precipitation in a size gradient from β -grain boundaries to grain interiors. As a result, an ultra-high strength of ∼1700 MPa is achieved whilst the ductility can be up to be 5% in the β -hetero aged samples, which is superior to the β -homo aged siblings that have the same strength level but fully succumb to macroscopic brittleness. The enhanced plasticity in the β -hetero aged samples originates from much better deformation compatibility conferred by the aged β f -grains. However, the soft feature of BAZ around β e -grain boundaries in the β -homo aged samples causes strain localization, which eventually induces intergranular brittle fracture. These findings provide an effective strategy for enhancing plasticity of high-strength metastable β -Ti alloys by structural design, where homogeneous α s -precipitates can be easily constructed by combining conventional thermomechanical processing and heat treatment. • Strength-ductility trade-off is evaded by a new structural design in Ti6554 alloy. • Enhanced ductility of ∼5% together with high strength of ∼1700 MPa is achieved. • Heterogeneous β -structure is architected by partial static recrystallization. • α -precipitation is rather uniform whilst α GB -film is effectively suppressed. • The improved ductility is conferred by superior deformation compatibility. [ABSTRACT FROM AUTHOR]

[Display omitted] Large-scale applications of elastocaloric cooling demand bulk materials showing both large adiabatic temperature change (Δ T a d ) and low-fatigue characteristics at room temperature. Using cold-rolling and aging treatment, we synthesize a bulk Ti 49.2 Ni 40.8 Cu 10 polycrystalline alloy that has microstructural features of nanocrystallinity and epitaxially related Ti(Ni,Cu) 2 nanoprecipitates. It exhibits a large Δ T a d of 13.8 K and a coefficient of performance of 13 at room temperature. Moreover, the degradation of Δ T a d is only 0.3 K after 450 tensile cycles. We attribute the favorable properties to the enhanced reversibility of martensitic transformation during stress cycling, aided by the internal epitaxy-generated stress at the interface between the Ti(Ni,Cu) 2 nanoprecipitates and matrix, together with grain refinement. The results indicate that the alloy offers a good balance of multiple objectives, holding promise for solid-state refrigeration applications. [ABSTRACT FROM AUTHOR]

Shape memory alloys (SMAs) have attracted considerable attention for their high damping capacities. Here we investigate the damping behavior of Ti 50 (Pd 50− x D x ) SMAs (D=Fe, Co, Mn, V) by dynamic mechanical analysis. We find that these alloys show remarkably similar damping behavior. There exists a sharp damping peak associated with the B2–B19 martensitic transformation and a high damping plateau ( Q −1 ~0.02–0.05) over a wide ambient-temperature range (220–420 K) due to the hysteretic twin boundary motion. After doping hydrogen into the above alloys, a new relaxation-type damping peak appears in the martensite phase over 270–360 K. Such a peak is considered to originate from the interaction of hydrogen atoms with twin boundaries and the corresponding damping capacity ( Q −1 ~0.05–0.09) is enhanced by roughly twice that of the damping plateau for each alloy. Moreover, the relaxation peaks are at higher temperatures for the TiPd-based alloys (270–370 K) than for the TiNi-based alloys (190–260 K). We discuss the influence of hydrogen diffusion, mobility of twin boundaries and hydrogen–twin boundary interaction on the temperature range of the relaxation peak. Our results suggest that a martensite, with appropriate values for twinning shear and hydrogen doping level, provides a route towards developing high damping SMAs for applications in desired temperature ranges. [ABSTRACT FROM AUTHOR]

Abstract: Effect of the initial grain size on plastic deformation behavior and grain refinement mechanism has been investigated in Ti–2Al–2.5Zr alloy subjected to multi-impact process. The results show that grain refinement is dominated by dislocation slip in the fine-grained sample. In contrast, deformation twinning becomes prevalent in the coarse-grained one, and contributes to grain refinement by means of twin subdivision and fragmentation. Dynamic recrystallization (DRX) takes place in both samples. But, it is delayed in the coarse-grained one due to relative low fraction of grain boundaries and energy dissipation of deformation twinning. Nevertheless, the final grain size in the refined area does not show an obviously difference between the two samples at a large strain, i.e., 1.61. Moreover, grain refinement is inhomogeneous in both samples. The heterogeneity in the coarse-grained sample is much stronger than that in the fine-grained one. It was ascribed to the superimposed effect of large initial grain size and subsequent deformation twinning. A criterion for achieving an optimum grain refinement was finally suggested. [Copyright &y& Elsevier]

Abstract: Thermal stability of the nanocrystalline Ti–10V–2Fe–3Al alloy produced by cold-forging and corresponding hardness variations were investigated. When the annealing temperature was lower than 400°C, the primary features were the decline of lattice defect density and the increase of lattice ordering level in the vicinity of grain boundaries, named as grain boundary relaxation. The nanocrystalline grains grew slowly. In comparison, when the annealing temperature was higher than 400°C, the decomposition of the deformation-induced α″ martensites and the subsequent rapid grain growth were observed. Vickers hardness measurement shows an initial hardening followed by a softening with the increase of annealing temperature. The hardening peak appeared at 400°C. The critical grain growth temperature, i.e., T cgg, in the nanocrytalline Ti–10V–2Fe–3Al alloy, was slightly lower than those in other nanocrystalline alloys. That could be attributed to the martensitic decomposition-induced grain growth in this alloy. [Copyright &y& Elsevier]

Abstract: Strain glass is a new glassy state discovered recently in Ni-rich Ti–Ni ferroelastic alloys. However, it remains unclear whether or not strain glass can be found in a wide range of ferroelastic systems. Here, we investigated the transition behavior of three different defect-doped systems Ti50(Ni50− x D x ) (D=Co, Cr, Mn) and found a striking similarity in their transition behavior as a function of defect concentration x. In all these systems, there exists a critical doping level x c at which the transition behavior shows an interesting crossover. At x < x c, these materials undergo a martensitic transition and the transition temperature decreases with increasing x. However, at x > x c, the martensitic transition is suppressed and a strain glass transition occurs. These results imply that strain glass may be quite general in defect-containing ferroelastic systems. An analysis with a modified Landau-type free energy landscape suggests that although point defects always favor the formation of strain glass, to actually form a strain glass they need to destabilize the martensite of the system. [ABSTRACT FROM AUTHOR]

• Deformation kinking is found in Ti-V metastable β -Ti alloys for the first time. • Kink bands co-appear with slip bands in the deformed β -grains. • Deformation-induced ω -precipitates are produced in kink bands due to local strain. • Kink bands are suggested to follow a dislocation-based formation mechanism. [Display omitted] Unlike stress-induced martensitic transformation, deformation twinning as well as dislocation gliding, deformation kinking is an uncommon deformation mechanism in metastable β -titanium (Ti) alloys. In this study, the unique deformation mechanism was reported in Ti-V binary β -Ti alloys for the first time. It was found that a large number of kink bands were distributed in β -grains after cold forging at a strain rate of ~103 s−1. Microstructural characterization manifests that the appearance of kink bands is frequently accompanied by slip bands, whilst dense dislocations are accumulated around the kink bands on a microscopic scale. Such local plastic deformation is quite strong so that the pre-existing athermal ω -precipitates in the initial microstructure is completely destroyed within the kink bands and instead fresh deformation-induced single variant of ω -precipitates is produced. This deformation localization mainly originates from the impediment of dislocation activity caused by both pre-existing athermal ω -precipitates and the intrinsic slip retardation in BCC crystal structures. The deformation kinking is thus suggested to follow a dislocation-based formation mechanism, whilst the resulting Taylor axis of lattice rotation was deduced to be<011> β on basis of extensive crystallographic analysis. These findings enrich fundamental understanding on deformation kinking, and provide helpful information for utilizing the unique deformation mechanism to tune mechanical properties of β -Ti alloys. [ABSTRACT FROM AUTHOR]

Abstract: After aging at intermediate temperatures (400–500°C), Ni-rich Ti–Ni alloys undergo an abnormal 3-stage martensitic transformation behavior (1-stage R and 2-stage B19′), which stems from a preferential Ti3Ni4 precipitation around grain boundary. On the other hand, if aged at low-temperatures (250–300°C), they undergo 2-stage R-phase transformation, but the origin of this strange phenomenon is unclear. In the present study, we made a systematic study of this phenomenon by considering the grain boundary effect and composition effect. We found that all single crystals undergo 1-stage R-phase transformation; in contrast, the transformation behavior of polycrystals is dependent on Ni content: low-Ni (50.6Ni, 51Ni) polycrystals undergo 2-stage R-phase transformation while high-Ni (52Ni) polycrystals undergo 1-stage R-phase transformation. The abnormal 2-stage R-phase transformation is attributed to a large-scale compositional heterogeneity in B2 matrix between grain boundary region and grain interior, due to the heterogeneity in precipitate density between the grain boundary and grain interior. But for high-Ni polycrystals, precipitates are essentially homogeneously distributed across the whole grain and this leads to normal 1-stage R-phase transformation. The different transformation behavior of low-Ni and high-Ni polycrystals stems from a competition between two opposing tendencies: (1) for preferential precipitation in the grain boundary; (2) for homogeneous precipitation across the whole grain with high-Ni content. The difference between the effect of intermediate-temperature and low-temperature aging lies in the difference in the ability for long-range diffusion of Ni (from the grain interior to the grain boundary), which results in whether or not Ti3Ni4 precipitates can form in the grain interior. Our results lead to a unified explanation for different transformation behaviors of both low-temperature and intermediate-temperature aged alloys in terms of the kinetics of precipitation in supersaturated polycrystals. [Copyright &y& Elsevier]

[Display omitted] • The ω -embrittlement of high misfit metastable β -Ti alloys was investigated. • Mechanically-driven ω -mediated amorphization is the underlying mechanism. • Covalent character of atomic bonding of ω -precipitates causes ω -lattice disordering. ω -embrittlement is ubiquitous in metastable β -titanium (Ti) alloys, while the fundamental understanding on the damage-fracture mechanism hitherto remains elusive. In this study, we systematically investigate ω -embrittlement of high misfit Ti-10Cr (wt.%) alloys by coupling experiments and first-principles calculation. It is found that brittle cleavage-like fracture prevails in tensile samples, irrespective of the quenching or subsequent aging states. Microscopically, cracks nucleation and propagation proceed along slip bands, inside which ω -lattices are first disordered and then the localized (β + ω)-amorphous-like structures are developed in the shape of white patches. The underlying mechanism of mechanically-driven localized amorphization is that due to the remarkable covalent character of atomic bonding of ω -precipitates caused by composition partitioning of the Cr element, ω -precipitates impart extremely high energy barrier opposed to dislocation gliding and render dislocations pile-up ahead of ω -precipitates, thus leading to their lattice disordering. It is unveiled that the hydrostatic pressure, serving as the driving force for dislocations pile-up, plays a critical role in this unusual cleavage-like fracture of Ti-10Cr alloys caused by mechanically-driven ω -mediated localized amorphization. Accompanied by the transition from the co-operation of deformation twining and ordinary dislocation slip in the quenched Ti-10Cr alloys to the exclusive ordinary dislocation slip in the long-time aged Ti-10Cr samples, it is unexpected that the resulting tensile fracture strength monotonically decreases to a stress level of ~ 100 MPa. These findings provide new insights into the damage and fracture behavior of high misfit β -titanium alloys, such as Ti-Cr alloys. [ABSTRACT FROM AUTHOR]

Metastable β -titanium (Ti) alloys have been extensively used in aerospace industry owing to the outstanding mechanical properties. Temperature arising sometimes becomes unavoidable in service and thus it is necessary to evaluate plastic behavior of the alloys at elevated temperatures prior to applications. Here, plastic stability of metastable β -titanium alloys containing the β- spinodal decomposition structures at different temperatures is investigated for the first time using Ti–6Cr–5Mo–5V–4Al alloy as prototypical material. It is found that a serrated plastic flow with little frequency accompanied by overall strain softening appears in the tensile stress-strain curves at 298 K, while the serrated plastic flow occurs much more frequently and overall strain hardening capability is recovered appropriately when the tensile temperature is increased to 573 K. Microstructural characterizations reveal that ordinary dislocation slip dominates the alloy plasticity at all the testing temperatures. Dislocations strongly interact with the β- striations and dislocation debris are left around the striation interfaces. The resulting local stress accumulation destroys the β- striations in the form of shear bands so that sudden release of an avalanche of dislocations in the soft channels leads to stress drop. Subsequent dynamic reconstruction of the β- striations with the assistance of thermal activation especially at elevated temperature hardens the prior soft channels, which results in the following stress rise. The striking difference on deformation characteristics at 298 K and 573 K is that more homogeneous plasticity conferred by dense deformation bands at the latter, which is the direct reason on high serrated frequency and overall strain hardening. These findings offer valuable insights into plastic behavior of metastable β -Ti alloys, which will have important implications for the applications. • Plastic stability in β- Ti alloys containing spinodal structures is firstly studied. • A serrated plastic flow appears at the tensile temperatures of 298 K and 573 K. • Dislocations-spinodal structure interactions result in serrated plastic flow. • A new mechanism regarding plastic flow fluctuation is proposed. [ABSTRACT FROM AUTHOR]

The microstructure evolution, mechanical property and deformation behavior of α lamella structure in a novel metastable β alloy Ti–5Al–4Zr–8Mo–7V were studied in this paper. Via different heat treatments, large differences in size, morphology and volume fraction of α lamella were obtained. Tensile tests show that high strength with low ductility is represented after low temperature aging. Tensile brittleness at low temperature aging is originated from precipitation of high fraction nano-scaled α lamella, which induced intensive stress concentration along α/β interface where local shear and plastic instability takes place. The alloy exhibits improved strength-ductility combination after BASCA (β annealing slow cooling plus aging) treatment and high temperature aging. High resolution-TEM results suggest that highly defected substructure and local distortion of (10 1 ‾ 0) planes are exhibited in α lamella. Both the highly defected substructure and finely dispersed α lamella could effectively strengthen the alloy. Improvement of ductility is on account of the increase of free slip length in β matrix where a large amount of dislocation accumulated and compatible deformation between coarse α lamella and β matrix. In addition, deformation twining is generated in coarse α lamella which will increase the ductility as well. • The α lamella spacing length determines the slip in β phase which in turn dominant the mechanical properties of the alloy. • The tensile brittleness at low temperature aging (490–550 °C) is attributed to the intense strain localization in shear bands of β grains. • Improved ductility of 680°C aging and BASCA samples is attributed to compatible deformation between β matrix and α lamella. [ABSTRACT FROM AUTHOR]

The ω -phase transformation in metastable β -titanium (Ti) alloys has attracted great attention in the past decade due to its intrinsic complexity and modifying mechanical properties. Interfacial twin boundary (ITB) ω -phase was reported to appear along {332} β or {112} β twin boundaries in metastable β -Ti alloys. The formation of such ITB ω -phase was proposed to arise from the reverse α″ to β martensitic transformation and subsequent stress relaxation along the twin boundaries. In this study, a new formation mechanism is revealed in Ti-10wt.%Cr alloy containing ω -precipitates in the initial microstructure. It is experimentally found that the formation of ITB ω -phase is closely correlated with the favored pre-existing one ω -variant at the expense of the other three-siblings, i.e. , reorientation of ω -variants. The ω -reorientation mechanism is further rationalized by first-principles calculation in terms of the energy barrier of transformation pathway between ω -variants. These findings advance our fundamental understanding to the ω -phase transformation and further plastic deformation behavior of Ti alloys. Unlabelled Image • A new formation mechanism of interfacial twin boundary (ITB) ω -phase has been revealed in metastable β -Ti alloys. • The formation of ITB ω -phase is experimentally ascribed to the reorientation of ω -variants. • The ω -reorientation mechanism is further rationalized in terms of the encountered energy barrier by first-principles calculation. [ABSTRACT FROM AUTHOR]

A novel metastable β-Ti alloy based on Ti–Al–Zr–Mo–Cr system was successfully designed and fabricated. Superior combination of high strength (UTS = 1534-1198 MPa) and ductility (El = 7.0%–20.9%) is achieved in this alloy. After high reduction β transus forging and subsequent heat treatment, a multi-scale distribution of α phase is displayed which contains micro-scale elongated primary α (α p), sub-micro α rod (α r) and nano-sized α platelets (α s). Detailed selected area electron diffraction (SAED) pattern and high resolution-TEM analysis show the production of FCC substructure inside α p and α r with an orientation relationship of <0001> hcp //<001> fcc , {11 2 ‾ 0} hcp //{ 2 ‾ 20} fcc , {10 1 ‾ 0} hcp //{220} fcc. Two variants of FCC lamella are discovered which is related by a 120° rotation around <001> axis. The effect of microstructure on the improved strength-ductility combination is briefly discussed. Both FCC lamella and multi-scale distribution of α phase could create an effective strain partition during plastic deformation and increase the ductility. High strength originates from the distribution of high fraction nano-scaled α s which could effectively hinder dislocation slip in β matrix. [ABSTRACT FROM AUTHOR]

As a novel high strength metastable β alloy, Ti–5Al–4Zr–8Mo–7V, it is essential to establish the correlation between its microstructure and mechanical property. In this paper, the microstructure evolution, mechanical properties and work hardening behavior with Bi-modal structure were studied in detail. With the increase of solution treatment (ST) temperature, the volume fraction of primary α phase (α p) decreases, and α platelets (α s) increases during subsequent aging treatment. Aging temperature directly determines the size and volume fraction of α s which significantly influences the strength of the alloy. Low temperature aging (510 °C) treatment produces a high density nano-scale distribution of α s precipitates which induces an extremely high ultimate tensile strength (UTS~1630 MPa) together with considerable total elongation (El~6%). As the ageing temperature increases to 630 °C, the UTS reduces to ~1208 MPa but the El dramatically improves to ~13%. The alloy exhibits a good strength-ductility combination with UTS~1390 MPa and El~10% after ST at 800 °C followed by aging at 570 °C. A Hall-Petch like equation is used to calculate the strengthening effect of α s , which shows a good agreement with the experimental results. It's concluded that the α s spacing length (λ) dominates the free slip length in β matrix, and determines the alloy strength. The ductility evolution is explained by work hardening behavior. Limited elongation of samples aged at 510 °C is attributed to the rapid decrease of work hardening rate after yielding. A slow decrease of working hardening rate of the samples aged at 630 °C results from the slightly dynamic recovery which could effectively postpone the occurrence of necking and increase the alloy's ductility. [ABSTRACT FROM AUTHOR]

Abstract A novel high-strength metastable β -Ti alloy Ti-5Al-4Zr-8Mo-7V has been successfully designed utilizing the " d -electron theory" combined with the semi-empirical element-hardening criteria. The excellent combination of strength σ b ~ 1460 MPa and ductility ɛ f ~ 10% is achieved by tailoring hierarchical distribution of α -phase in β -matrix. This hierarchical microstructure consisting of elongated primary α -phase (α p), sub-micro α -rods (α r) and nano-scale α -platelets (α s) was produced by β -transus forging together with heat treatment. Detailed TEM analysis shows that both the α p and α r could deform plastically, resulting in dense tangled dislocations. Furthermore, dislocation features of α p show that the α p /β interface is effectively hardened whilst the entire α p presents prominent deformation capability. Meanwhile, the plastic strain is partitioned among α p , α r and β -matrix compatibly, which is beneficial to improve the ductility. As for the α s , HRTEM observation shows highly defected substructure and local lattice rotation. This highly defected substructure and nano-scale α -distribution effectively block dislocation motion and ultimately strengthen the alloy. The approach presented in this study provides a guideline for the design and fabrication of other β -Ti alloys with hierarchical structure and superior mechanical properties. Graphical abstract Unlabelled Image Highlights • A novel β -titanium alloy, Ti-5Al-4Zr-8Mo-7V, was successfully designed using the 'd-electron theory'. • Hierarchical distribution α-phase in β-matrix was developed by proper β -transus forging and heat treatment. • The novel alloy possesses superior combination of strength and ductility. [ABSTRACT FROM AUTHOR]