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This paper reviews the dislocation slip behavior of α phase in α, near α and α + β titanium alloys dominated by α-Ti deformation under quasi-static loading. The relation of slip activity, slip transfer, slip blocking, twinning and crack initiation is discussed, mainly combined with in situ tensile technology. The slip behavior in Ti-alloys is analyzed in detail from the aspects of critical resolved shear stress (CRSS), grain orientation distribution and geometric compatibility factor m′. In addition, slip blocking is an important factor of the formation of twins and micro-cracks. The interaction of slip behavior and interfaces is clarified systematically. Finally, the insufficiency of current research, future research directions and key difficulties of study are also discussed. [ABSTRACT FROM AUTHOR]

Abstract The plastic deformation processes and fracture mechanisms of as-consolidated powder metallurgy (PM) and as-cast ingot metallurgy (IM) metastable beta titanium alloys (Ti-5Al-5V-5Mo-3Cr) were thoroughly investigated by room temperature tensile tests with in-situ scanning electron microscopy (SEM) observations and further microstructure characterizations including transmission electron microscopy (TEM) and atomic force microscopy (AFM). The results suggested that IM alloy has a better compatible deformation capability than PM alloy characterized by varying slip deformation mechanisms and the constantly changing slip modes. The microcracks in PM alloy were observed to be originated at the interfaces of α″/β and grain boundary α/β. Whereas, the microcracks in IM alloy were observed on the grain boundaries and the intracrystalline α precipitation areas, where the slip steps were generated. Particularly, the common defects in PM (residual pores and microvoids) and IM (casting cavity) alloys did not exhibit significant negative effects, contrary to previous awareness. Finally, the coalescence of the grain boundary α and the widened α″/β microcracks led to the sudden brittle transgranular-intergranular mixed fracture of PM alloy. For IM alloy, the serious cracking initiated the V-shape notch made its way through the microcracks at the α/β slip steps, resulting in the gradual transgranular fracture with better ductility. [ABSTRACT FROM AUTHOR]

Understanding the mechanisms of deformation and fracture of metastable β titanium alloys is of great significance for improving formability and service life. By combining the in-situ tensile test, TEM characterization and EBSD analysis, the tensile deformation behavior, activation of slip systems, crack initiation, and propagation of a high strength metastable β titanium alloy (Ti-5Cr-4Al-4Zr-3Mo-2W-0.8Fe) with equiaxed microstructure are investigated. The equiaxed microstructure is composed of primary α (αp) phase, transformed β (βt) matrix phase, and secondary α (αs) phase. In contrast to the hexagonal αp grain with limited slip systems, the body-centered βt matrix has more slip systems, however the hindering effect of αs phases on dislocation slip leads to the different deformability of the αp phase and βt matrix. The equiaxed αp grains are more prone to deformation and rotation to coordinate the overall deformation. The shear band leads to the formation of sub-grain boundary and even the fragmentation of αp grains. As a result, the microvoids tend to nucleate at the grain boundary, phase interface, slip band, and shear band. The inhomogeneous deformation in the plastic deformation zone around the crack tip is the primary cause of damage. The crack propagation caused by microvoids coalescence advances along the grain boundaries and phase interfaces in the form of intergranular, and along the activated slip systems and shear bands in the form of transgranular. Pinpointing the situation in the equiaxed microstructure and combining that in other typical microstructures will help to summarize the universal deformation and fracture mechanisms of metastable β titanium alloy, and provide a basis for alloy design and microstructure tailoring. [ABSTRACT FROM AUTHOR]

The deformation behavior and strengthening mechanism for Mo-3Nb single crystal with 〈111〉 orientation and polycrystal have been investigated and disclosed comprehensively in a wide temperature range by quasi-static compression with 7% plastic strain. The slip traces of single crystal and polycrystal at room temperature show different features, which long continuous slip traces appear on the surface of deformed Mo-3Nb single crystals while some branch-off slip lines can only be observed in large grains for polycrystals. The conjugate slip planes of single crystals are activated at the elevated temperature, while the slip lines of polycrystals are difficult to be observed because of the grain boundary sliding effect. The compressive stress of single crystals and polycrystals shows different results at different deformation temperatures, suggesting that grain boundaries in polycrystals play an important role in the strengthening mechanism. TEM analysis shows that dislocation entanglement is the main strengthening mechanism both in single crystal and polycrystal during the room temperature deformation. With the increase of temperature, the strengthening mechanism caused by dislocation entanglement becomes ineffective, while grain boundaries continue to hinder the dislocation movement and the strengthening effect greatly weakened due to the dynamic recovery. [Display omitted] • <111> oriented Mo-3Nb single crystal and polycrystal have been firstly designed • Different slip features are observed between single crystal and polycrystal • The grain boundary strengthening effect weakens when the deformation temperature reaches to 1500 °C [ABSTRACT FROM AUTHOR]

Gradient structure metals have good comprehensive properties of strength and toughness and are expected to improve the dynamic mechanical properties of materials. However, there are few studies on the dynamic mechanical properties of gradient structured materials, especially titanium alloys. Therefore, in this study, ultrasonic surface rolling is used to prepare a gradient structure layer on the surface of Ti-6Al-4V, and the quasi-static and dynamic compressive properties of coarse-grained Ti-6Al-4V (CG Ti64) and gradient-structured Ti-6Al-4V (GS Ti64) are investigated. The results show that a GS with a thickness of 293 µm is formed. The quasi-static compressive strength of GS Ti64 is higher than that of CG Ti64. Both CG Ti64 and GS Ti64 exhibit weak strain hardening effects and strain rate insensitivity during dynamic compression, and the compressive strength is not significantly improved. The lateral expansion of CG Ti64 is more obvious, while the lateral side of GS Ti64 is relatively straight, indicating that uniform deformation occurs in GS Ti64. The α phase in the GS produces dislocation cells and local deformation bands, and the lamellar structure is transformed into ultrafine crystals after dynamic compression. Both of them produce an adiabatic shear band under 2700 s−1, a large crack forms in CG Ti64, while GS Ti64 forms a small crack, indicating that GS Ti64 has better resistance to damage. The synergistic deformation of GS and CG promotes Ti-6Al-4V to obtain good dynamic mechanical properties. [ABSTRACT FROM AUTHOR]

High-purity titanium exhibited ultrahigh impact toughness about 312.5 J/cm2 in this study. To uncover the underlying deformation mechanisms for the ultrahigh impact toughness, postmortem analysis of the deformation microstructure was performed using the electron back-scatter diffraction (EBSD) and transmission electron microscope (TEM) techniques. The results indicate that multiple deformation behaviors occurred in high-purity titanium. Dense dislocation activities were activated in the regions near and away from the crack tip. The average GND densities of the deformation microstructure (1.71 × 1015/m2 and 1.21×1015/m2, respectively) were much larger than that (0.06×1015/m2) of the initial specimen. Moreover, plentiful deformation twinning and detwinning behaviors were observed in the deformation microstructure which significantly contributed to plastic deformation and high impact toughness. Moreover, deformation induced α-Ti → FCC Ti transformation was also activated which played as an additional deformation mode in the high-purity titanium. Multiple deformation behaviors including dislocation activities, deformation twinning/detwinning behaviors and deformation induced α-Ti → FCC Ti transformation, were responsible for the ultrahigh impact toughness of high-purity titanium. • The high-purity titanium exhibited the ultrahigh impact toughness about 312.5 J/cm2. • Dense dislocation activities were activated in high-purity titanium. • Plentiful twinning and detwinning behaviors were activated in high-purity titanium. • Deformation induced α-Ti → FCC Ti transformation was also activated under the impact loading. • Multiple deformation behaviors induced the ultrahigh impact toughness of high-purity titanium. [ABSTRACT FROM AUTHOR]

The hot workability of metallic materials is significantly dependent on its ability to form plastic without cracking and fracturing. In this work, the cracking behavior of powder metallurgy (PM) Ti-5Al-5Mo-5V-3Cr (Ti-5553) alloy, consolidated from powder mixture, at a deformation temperature range of 600 °C–850 °C and strain rate of 0.1 s−1–10 s−1, was investigated through isothermal compression tests. The cracking behavior of the as-cast ingot metallurgy (IM) Ti-5553 alloy, at a deformation temperature of 700 °C was also investigated for comparison. Results suggested that the PM Ti-5553 alloy had a better hot workability, with a larger cracking-free processing window, and a lower deformation resistance than the IM counterpart. 45° shear fracture occurred in the PM alloy, compressed at the condition of 600 °C/10 s−1, and edge cracking was observed at the 700 °C/10 s−1. 45° shear fracture was also significant in the IM alloy specimen tested at 700 °C/10 s−1, and all the other IM alloy specimens compressed at 700 °C displayed longitudinal cracking. Moreover, the microscopic cracking observation showed that ductile dimple cracking can be found in the IM alloy, but brittle cleavage fracture was dominant in the cracking surface of PM alloy with a relatively low cracking ductility. [ABSTRACT FROM AUTHOR]

Deformation and crack initiation of industrial Ti-6Al-4V titanium alloy sheet were investigated using in-situ scanning electron microscopy (SEM) at room temperature. The results showed that slip band originated from α/α and α/β interfaces when the tension reaches the yield stage, and the slip mode develops from single slip to multi slip and finally the cross-slip occurs. Microcracks initiate primarily at the phase boundary or along the slip band within α phase, cracks are mainly connected along the 45° line. The fracture surface is full of dimples. Intergranular cracking is the main fracture mechanism. [ABSTRACT FROM AUTHOR]

The impact toughness and deformation mechanism of near-α-type low-temperature titanium alloy CT20 at different temperatures (20 °C, 0 °C, −50 °C, −100 °C and − 196 °C) are studied. The results show that the impact toughness is as high as 90 J/cm2 at 20 °C, and does not decrease obviously at 0 °C, −50 °C and − 100 °C, but brittle fracture occurs at −196 °C (29 J/cm2). The tortuosity of the crack path decreases with decreasing temperature. At 20 °C, the grain boundary and α colony interface with high-angle grain boundary deflect the crack propagation direction effectively, forming a tortuous crack path, while crack tends to cut through α lamellae, forming a relatively straight path at cryogenic temperature. The lamellar α undergoes kink deformation at 20 °C, showing severe plastic deformation characteristics. In addition, deformation twins and geometrically necessary dislocations (GNDs) are observed near and far from the fracture surface, indicating that the plastic zone is large. As the temperature decreases, the twin density near the crack path tends to increase. However, few twins are seen in the region far from the fracture surface, and GND densities are low at −196 °C, implying that the plastic zone is small. In a word, the synergistic effect of tortuous crack path, dislocation slip and deformation twinning enable CT20 to achieve high impact toughness at 20 °C. At −196 °C, the plastic deformation capacity of the microstructure decreased significantly, resulting in the formation of a small plastic zone and brittle fracture. • α colony interface deflects crack propagation direction at 20 °C, while crack tends to cut through α lamellae at cryogenic temperature. • The synergistic effect of tortuous crack path, dislocation slip and twinning enables CT20 to achieve the highest impact toughness at 20 °C. • The coordinated deformation of dislocation slip and twins keeps the impact toughness at a high level at 0 °C ∼ -100 °C. • The plastic deformation ability decreases significantly, resulting in the formation of a small plastic zone and brittle fracture at −196 °C. [ABSTRACT FROM AUTHOR]

The impact toughness and deformation modes of Ti–6Al–4V alloy with different microstructures, i.e., equiaxed microstructure (EM), bimodal microstructure (BM), fine lamellar microstructure (LM1) and coarse lamellar microstructure (LM2), were studied and revealed in detail. The impact toughness of BM was determined as 85 J/cm2, which was about 70%, 42% and 35% higher than that of EM, LM1 and LM2, respectively. BM possesses a composite-like microstructure containing hard primary α grain (α p) and soft β transformation microstructure (β t). During crack propagation, large plastic deformation occurs within soft β t region, and the deformation of β t was constrained by surrounding hard α p , resulting in kink deformation of β t and local plastic deformation of α p. In addition, {10-12}<-1011> tensile twins with both high and low Schmid factor were activated in α p by the sever kink deformation of β t and impact energy. Furthermore, some α p grains near the fracture surface were crushed significantly by impact loading. As a result, the synergistic effect of these aforementioned factors can effectively dissipate impact energy, correspondingly enhancing the impact toughness of Ti–6Al–4V alloy. By contrast, the crack mainly propagates along the α p 's grain boundary in EM. Moreover, the coordinate deformation ability between α p and β t was weak due to the high content of α p , resulting in a significant reduction of impact toughness. For LM1 and LM2, although {10-12}<-1011> tensile twins and {10-11}<-1012> compression twins were activated, the impact toughness was still lower than that of BM due to little plastic deformation in LM1 and heterogeneous deformation in LM2. These results indicated that the coordinated deformation ability of the microstructure plays an important role in impact toughness of the alloy. [ABSTRACT FROM AUTHOR]

A special recrystallization mechanism called ‘particle stimulated nucleation’ (PSN) is found responsible for the pervasive discontinuous dynamic recrystallization (DDRX) in the burn resistant β-stablized Ti–35V–15Cr–0.3Si–0.1C alloy. This might be the first time that PSN is found in titanium alloys. PSN mechanism is studied by transmission electron microscopy (TEM) and Electron Back Scattered Diffraction (EBSD) techniques. Local strain incompatibility between the matrix and the titanium carbides leads to high density of dislocation in the particle deformation zone (PDZ), which provides the ideal sites for PSN. Recrystallized grains produced by PSN is randomly oriented and the microtexture gets weaker as the fraction of DRX increases. [ABSTRACT FROM AUTHOR]

Abstract: Effects of the processing parameters on the flow stress behavior and the microstructural revolution are investigated in the hot compression of as-cast Ti60 alloy. The flow stress behavior reveals greater flow softening in the two-phase field compared with that of single-phase field. In the two-phase field, flow softening is caused by break-up of lamellar α, deformation heating, flow localization and free-surface cracking. While in the single-phase field, flow softening is associated with dynamic recovery and recrystallization. In addition, the flow stress curves display discontinuous yielding, which is attributed to rapid dislocation generation and multiplication from the grain boundary. Moreover, the magnitude of yield drop is inversely proportional to the average grain size. In the α+β phase field, the distorted degree of lamellar α within the prior β grains increases with increasing strain rate and reduction, while flow instability including low localization and oxidation cracking occurs in higher strain rate (>1s−1) and larger reduction (>60%), a small amount of recrystallized grains are observed at the prior β grain boundaries in lower strain rates (≤0.1s−1) with reduction of 60%. In the β phase field, however, the concentration of recrystallized β grains at prior β grain boundaries is much higher than that observed during deformation in the α+β phase field. The fraction of recrystallized β grains increases with increasing temperature, reduction and decreasing strain rate. It indicates that the processing parameters have a significant influence on the deformation behavior and microstructural revolution of as-cast Ti60 titanium alloy. It is helpful for the controlling of the microstructure and the optimization of processing parameters. [Copyright &y& Elsevier]

Tensile deformation and damage evolution of a metastable β titanium alloy (Ti–5Cr–4Al–4Zr–3Mo–2W–0.8Fe) with lamellar microstructure are studied by in-situ tensile test under scanning electron microscopy. In the tensile process, the main deformation modes include the dislocation slip, phase interface shear and grain boundary shear, and the geometrical orientation of α lamellae determines the activation of different slip systems and whether the interface shear is involved in deformation. The α lamellae may kink and even fragment under severe deformation. Slip transfer is prone to occur between α lamella and β interlayer that satisfy the Burgers orientation relationship, and the slip lines will deflect and bifurcate as more slip systems are activated and grains rotate. Due to the localized stress concentration and inhomogeneous deformation, the grain boundaries, phase interfaces, tips of β interlayers, junctions of colonies composed of α lamellae and shear bands are all the positions where are easy to generate microvoids. The crack propagates along a zigzag path on account of the synergistic reaction mechanism of critical resolved shear stress, activated slip system, shear band and grain boundary shear, resulting in an intergranular and transgranular mixed fracture mode. [ABSTRACT FROM AUTHOR]