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The slip behavior and mechanism of large-size Mo-3Nb single crystal have been investigated and disclosed comprehensively at room temperature by quasi-static compression with various strains. With the increase of deformation, the slip traces change from shallow non-uniform slip lines to dense and uniform slip bands. Different slip traces in the same deformation condition were observed, suggesting that the slip traces in the single crystal are controlled by different types and arrangement mechanisms of mobile dislocation. To clarify the relationship between slip behavior and dislocation arrangement, TEM and AFM analyses were performed. Significant discrepancy between the mobility of screw segments and edge segments caused by double cross-slip multiplication mechanism is the reason why different slip features were witnessed. During the whole slip deformation process, screw dislocations play a dominant role and they are inclined to form wall-substructures by interaction and entanglement. With the development of dislocation accumulation, the entangled dislocation walls evolve into dislocation cells with higher stability.

Ti-6Al, Ti-6Al-2Mo and Ti-6Al-3Nb alloys were prepared to investigate the toughening effects of β stabilizers Mo and Nb on impact toughness and crack resistance of titanium alloys. Instrumented Charpy impact tests showed that the total impact absorbed energy of Ti-6Al-2Mo and Ti-6Al-3Nb (∼64 J) were two times higher than that of Ti-6Al (∼30 J), indicating the higher impact toughness of Ti-6Al-2Mo and Ti-6Al-3Nb alloys. Analysis of load-displacement curves revealed the similar crack initiation energy of Ti-6Al, Ti-6Al-2Mo and Ti-6Al-3Nb (15.4 J, 16.1 J and 15.0 J, respectively). However, the higher crack propagation energy of Ti-6Al-2Mo and Ti-6Al-3Nb (46.7 J and 48.3 J, respectively) were about three times higher than that of Ti-6Al (14.4 J), indicating the stronger resistance to crack propagation in Ti-6Al-2Mo and Ti-6Al-3Nb. Post-mortem analysis of impact samples demonstrated that the increased dislocation density and deformation twinning were mainly responsible for the stronger resistance to crack propagation in Ti-6Al-2Mo and Ti-6Al-3Nb. Due to the invisibility of dislocation activation and deformation twinning during the Charpy impact process, a mathematical model has been proposed to evaluate the effects of Al, Mo and Nb elements on dislocation mobility based on the Yu Rui-huang electron theory. Addition of Mo and Nb elements significantly improved the dislocation mobility in Ti-6Al-2Mo and Ti-6Al-3Nb compared to that in Ti-6Al alloy. Therefore, more dislocations were activated in Ti-6Al-2Mo and Ti-6Al-3Nb which supplied the larger plastic deformation under impact loading. A dislocation-based model also has been proposed to interpret the nucleation and propagation of deformation twinning under the impact loading. Dislocation pileup at α/β interfaces provided potential sites for nucleation of deformation twinning in Ti-6Al-2Mo and Ti-6Al-3Nb. Furthermore, deformation twinning facilitated the dislocation motion in α grains with hard orientations. The increased dislocation mobility and deformation twinning were responsible for the stronger crack resistance as well as the higher impact toughness of Ti-6Al-2Mo and Ti-6Al-3Nb alloys.

Cu-55 vol.%diamond-1 vol.%Ti composites were fabricated by hot-forging of powder mixtures (referred to as M-Cu/Dia) and cold-pressed powder preforms (referred to as P-Cu/Dia). Dispersion of diamond particles in the size range of 70–80 µm at 55% volume fraction was attained in the P-Cu/Dia composites while diamond agglomerations were observed in the M-Cu/Dia composites. TiC was observed at the copper/diamond interface in both composites, in which most diamond particle surfaces (> 95%) were enveloped by TiC. It was found that TiC formed preferentially on the diamond-{100} facets compared with the diamond-{111} facets, which can be attributed to the fact that the C atoms are bonded by two C–C bonds on the diamond-{100} facets (more active) while three C–C bonds on the diamond-{111} facets (less active). The dispersion of diamond particles and formation of a TiC interface layer contributed to the thermal conductivity (388 W/mK) of the P-Cu/Dia composites, which is nearly twice that of pure copper hot-pressed from powder.

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.

Ti-5Al-5V-5Mo-3Cr (Ti-5553) is a metastable beta titanium alloy and has excellent combined properties for extensive applications, but the hot workability and deformation mechanisms of powder metallurgy (PM) metastable beta titanium alloys are seldom reported. This study prepared the Ti-5553 alloy from powder using thermomechanical powder consolidation approach and investigated its hot deformation behaviour at 800 °C to 1150 °C and 0.001 s−1 to 10 s−1 through thermal physical simulation using the Gleeble®-3800 simulator. The hot processing map of the Ti-5553 alloy was successfully established based on the dynamic materials model. Various material characterization techniques, including optical microscopy (OM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD), were used to reveal the related deformation mechanisms. The calculated apparent activation energies for the Ti-5553 alloy are 323.8 kJ/mol in (α + β) region and 227.0 kJ/mol in β region, and four primary optimum efficiency domains and one instability domain are identified in the hot processing map. The deformation mechanisms for the Ti-5553 alloy processed at the different optimum efficiency domains are governed by various deformation mechanisms including α phase globalization, superplastic deformation, discontinuous dynamic recrystallization (DDRX) and dynamic recovery (DRV), while the deformation mechanism in the instability domain is mainly dominated by flow localization. Also, the Ti-5553 alloy in this study exhibits a wider processing window and lower deformation resistance than its counterpart and/or other similar metastable beta titanium alloys produced by ingot metallurgy (IM) approach, suggesting that the Ti-5553 alloy we produced from powder has better hot workability.

Short-time processing route has been designed to manufacture cost-affordable and high-quality powder metallurgy (PM) metastable β titanium alloy, containing rapid powder consolidation (modified thermomechanical pressing), one-step thermomechanical processing (simple open die uniaxial hot forging by industrial press) and fast heat treatment (one-step annealing at various temperatures for only one hour). Based on comprehensive microstructure characterizations and mechanical property examinations, underlying microstructural evolution mechanism and microstructure-property relationship of the produced alloys were uncovered and elucidated thoroughly. Homogeneous macrostructure and fine-grain microstructure without undissolved particles and large pores are obtained for the alloy after thermomechanical powder consolidation as a result of the concurrent effect of external deformation and high-temperature diffusion. One-step open-die forging is verified to produce full-dense and sound PM alloy pancake with large-scale and high strength. Attributed to the harmonious concurrence of hierarchical α precipitation and heterogeneous grain structure, synergistic strength-ductility combinations are achieved for the alloy after specific processing and heat treatment with the tensile strength and strain at failure values of 1386.5 MPa/7.3% and 1252.3 MPa/9.0%, respectively. These strength-ductility combinations are comparable and/or even better than other metastable β titanium alloys prepared by some PM and ingot metallurgy approaches with relatively high cost and time consumption.

Coarse macrograins have been observed in aerospace components for a high temperature near-α titanium alloy Ti60. The present work was inspired to understand the origin of coarse macrograin and examine how strain affects the macro/microstructure. Thus, thermomechanical processing was conducted in α + β phase field to simulate industrial production. This alloy with a bimodal microstructure was isothermally pre-deformed at 1019 °C and a constant strain rate of 0.1 s−1 to height reductions of 15, 35, 55 and 75%. Heat treatment was conducted following deformation at 1024 °C for 30 min. The results showed that coarse macrograins originated from inadequate recrystallization of prior β grains of initial billet. Deformation in the α + β phase field does not form many new orientation β grains but slightly scatters β grain orientations around the orientations of big prior β grains in small strain samples. Each of different coarse macrograins corresponds to a family of prior β grains with similar crystallographic orientations with about 20° of spread, which is outlined by secondary α (αs) plates having nearly parallel basal planes with similar crystallographic orientations. The size of macrograin decreases with increasing strain, which is consistent with the extent of recrystallization of prior β grain. These understandings can be applied to optimize processing routes for the control of final macro/microstructure and properties in near-α titanium alloys.

Ti-6Al-4V (Ti-64) swarf is produced every day in commercial operations but its direct use for the fabrication of strong and ductile Ti-6Al-4V alloy is seldom reported. This study consolidated as-received Ti-64 swarf into near fully dense Ti-6Al-4V alloy by hot pressing. The microstructures and tensile mechanical properties of the hot-pressed Ti-64 alloy and the effect of subsequent solution treatment and ageing (STA) were studied. The hot-pressed Ti-64 achieved tensile yield strength (TYS) of 928–998 MPa, ultimate tensile strength (UTS) of 1076–1139 MPa and strain to fracture of 6.92–7.80%. The STA increased the TYS to 1103–1143 MPa and UTS to 1188–1223 MPa, accompanied by a decline in the strain to fracture to 5.32–6.48%. These tensile property data suggest that it is possible to directly consolidate Ti-6Al-4V swarf into a strong and ductile Ti-6Al-4V alloy by hot pressing for a variety of potential applications.

The "titanium fire" as produced during high pressure and friction is the major failure scenario for aero-engines. To alleviate this issue, Ti-V-Cr and Ti-Cu-Al series burn resistant titanium alloys have been developed. However, which burn resistant alloy exhibit better property with reasonable cost needs to be evaluated. This work unveils the burning mechanisms of these alloys and discusses whether burn resistance of Cr and V can be replaced by Cu, on which thorough exploration is lacking. Two representative burn resistant alloys are considered, including Ti14(Ti-13Cu-1Al-0.2Si) and Ti40(Ti-25V-15Cr-0.2Si)alloys. Compared with the commercial non-burn resistant titanium alloy, i.e., TC4(Ti-6Al-4V)alloy, it has been found that both Ti14 and Ti40 alloys form "protective" shields during the burning process. Specifically, for Ti14 alloy, a clear Cu-rich layer is formed at the interface between burning product zone and heat affected zone, which consumes oxygen by producing Cu-O compounds and impedes the reaction with Ti-matrix. This work has established a fundamental understanding of burning resistant mechanisms for titanium alloys. Importantly, it is found that Cu could endow titanium alloys with similar burn resistant capability as that of V or Cr, which opens a cost-effective avenue to design burn resistant titanium alloys., 6 figures

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.

Powder metallurgy is a very attractive method for producing titanium alloys, which can be near-net-shape formed and have freedom in composition selection. However, applications are still limited due to product affordability. In this paper, we will discuss a possible cost-effective route, combining fast heating and hot processing, to produce titanium alloys with similar or even better mechanical properties than that of ingot metallurgy titanium alloys. Two titanium alloys, Ti-5Al-5V-5Mo-3Cr (Ti-5553) and Ti-5Fe, were successfully produced from HDH titanium powder and other master alloy powders using the proposed processing route. The effect of the processing route on microstructural variation and mechanical properties have been discussed.

V-4Cr-4Ti alloy, one of the promising candidate structural materials for fusion first wall/blanket applications, was annealed in different temperatures (800 °C and 1000 °C) followed by hydrogen and subsequent helium ions irradiation. It is designed for ion irradiations by forming different size of precipitates, intending to investigate the effect of interfacial coherency characteristic on the irradiation damage behavior at an atomic scale. The microstructure was studied by transmission electron microscopy and the effect of irradiation hardening was evaluated by nanoindentation technique. The results show that TiC precipitate with different size and density was formed during the heat treatment. The lattice matching and the resulting strain state at the interface between the precipitate and the matrix were also investigated quantitatively. The samples with large precipitate exhibited a relatively lower irradiation hardening rate than the one with smaller precipitate, which attributes to the distinct aggregation of He atoms at the interface between the larger precipitate and vanadium alloy matrix. It suggests that the strain state at the interface between the precipitate and the matrix plays a key role in determining the distribution state of irradiation defects thus affect the irradiation damage behavior significantly.

The static coarsening behavior of a Ti-14Cu alloy relative to the Ti2Cu phase after long-term isothermal thermal exposure was comprehensively investigated. The growth of the precipitated phase is shown to be controlled by the diffusion mechanism characterized by morphological variation during thermal exposure. The static coarsening process is composed of a rapid coarsening stage and stable coarsening stage, and it is primarily governed by volume diffusion of the Lifshitz-Slyozov-Wagner (LSW) model. The rapid coarsening stage is mainly dominated by the terminal migration mechanism, while the stable coarsening stage is governed by the Ostwald ripening mechanism. The rapid coarsening of the Ti2Cu phase enhances the effects of precipitation strengthening significantly and simultaneously improves the plasticity of the alloy. However, with stable coarsening, the presence of the high-volume Ti2Cu phase in a Ti-14Cu alloy increases the effective slip length during deformation and reduces plasticity.

Graphene nanoplates (GNPs) have been adopted in titanium matrix composite manufacturing and design due to its superior mechanical properties. In this study, Graphene nanoplates reinforced Ti-6Al-2Sn-2Zr-3Mo-1Cr-2Nb-Si alloys (named as GNPs/TC21 composites) were prepared by using spark plasma sintering (SPS), and the effects of sintering temperature on the microstructure, mechanical and wear properties of GNPs/TC21 composites were systematically investigated. The results indicate that the sintering temperatures show a great influence on the interfacial structure. With the increase of sintering temperature from 850 °C to 1050 °C, the interfacial microstructures exhibit three morphographies: (1) The original TC21 particles coated with GNPs (850 °C); (2) A special TiC@GNPs band (900 °C−1000 °C); (3) A discontinuous network structure wrapped by TiC particles/plates (1050 °C). Mechanical and tribological properties of the GNPs/TC21 composites were investigated at room temperature. The optimal SPS temperature was 1000 °C, at which the GNPs/TC21 composites show the best comprehensive mechanical properties i.e. the ultimate tensile strength and yield strength are 1167 MPa and 1041 MPa, the friction coefficient is 0.16275 and the wear loss is 0.0179 g. The improvement of mechanical properties can be attributed to the synergistic strengthening of the load transfer effect of TiC@GNPs bands and the pinning effect of TiC particles/plates. The excellent wear resistance is mainly attributed to the pinning of TiC particles on the surrounding metal matrix and the strengthening and lubrication of GNPs.

Fatigue crack propagation behaviors of a novel high strength Ti-5Al-3Mo-3V-2Zr-2Cr-1Nb-1Fe alloy with equiaxed and lamellar microstructures were systematically investigated. Fatigue crack propagation tests at both increasing and decreasing stress intensity factor range values were carried out with a frequency of 15 HZ and stress radio R of 0.1 by using compact tension samples. The sample with lamellar microstructure displayed the faster fatigue crack propagation rates in near-threshold regime compared to the sample with equiaxed microstructure. The fatigue threshold (ΔKth) was 2.7 MPa·m1/2 for lamellar microstructure, and 3.2 MPa·m1/2 for equiaxed microstructure. The steady state crack growth rate in lamellar microstructure was slightly slower than that in equiaxed microstructure in the Paris regime, while much faster in the rapid growth regime. The interactions between crack path and microstructures as well as the microscopic fracture surface morphology in both microstructures were detected and analyzed by scanning electron microscope (SEM), revealing the different crack propagation behaviors in three regimes. The results showed that the crack propagation behaviors were a result of two contributing factors: interface obstacles and crack front profile. The increased fatigue crack propagation resistance of equiaxed microstructure in near-threshold regime can be attributed to the interface obstacles which dominated over the rougher crack front profile. While the higher fatigue crack growth resistance for lamellar microstructure in Paris regime was attributed to the more positive effect of rougher crack front profile. In the rapid growth regime, the crack in lamellar microstructure was found to be extended along the grain boundary α, which was regarded as a low energy crack path and led to the faster crack propagation rate.

Binary Ti-4(Nb, V, Fe) alloys were prepared to investigate the effects of β-stabilizers (Nb, V and Fe) on microstructure formation and mechanical properties of titanium alloys by using experimental and theoretical methods. Microstructure observations revealed that addition of Nb, V or Fe significantly tailored the microstructure under different heat-treated conditions, i.e., furnace-cooled (FC), air-cooled (FC) and water-quenched (WQ). The FC Ti–4(Nb, V, Fe) alloys exhibited the Widmanstatten microstructure and the AC samples showed the spheroidized microstructure. The WQ Ti–4Fe exhibited the microstructure composed of equiaxed β grains and nanoscale ω particles while the WQ Ti–4Nb and Ti–4V showed the martensite microstructure. Differentials of microstructure can be attributed to the effects of Nb, V and Fe elements on the phase stability and energy barrier of phase transformation according to the empirical electron theory (EET) of solids and molecules. Results of tensile tests showed that Nb, V and Fe significantly influenced the tensile properties under the different heat treatments. Both the WQ Ti–4Nb and Ti–4V showed the larger tensile strength than that of the FC and AC sample due to the stronger solid-solution and phase-interface strengthening in martensite structure. The WQ Ti–4Fe sample exhibited the largest tensile strength (1104 MPa) compared to that of the FC (754 MPa) and AC (981 MPa) samples due to the strongest solid-solution and phase-interface strengthening from the ω phase. By executing the methods of the empirical electron theory (EET) of solids and molecules, effects of β-stabilizers (Nb, V and Fe) on microstructure formation and mechanical properties of titanium alloys were revealed on the view of valence electron.

Graphene nanoplates (GNPs) reinforced TC21(Ti-6Al-2Sn-2Zr-3Mo-1Cr-2Nb-Si) (GNPs/TC21) composites were prepared by using spark plasma sintering (SPS). Microstructure, mechanical and tribological properties of the sintered GNPs/TC21 composites were systematically investigated. Microstructural observations demonstrated the formation of a special sandwich interface product is in-situ TiC coated GNPs (TiC@GNPs) bands and TiC particles/plates in the microstructure of the sintered GNPs/TC21 composites. Results of mechanical testing indicated the excellent combination of strength and ductility of GNPs/TC21 composites owing to the dispersion strengthening effects of TiC particles/plates as well as the load transfer between TiC@GNPs bands and TC21 matrix. GNPs/TC21 composite with 0.05 wt% GNPs addition exhibits the relatively excellent mechanical properties, i.e., yield strength of 1017.2 MPa, ultimate tensile strength of 1161.99 MPa and an elongation about 13.2%. Among them, the dispersion strengthening effect of TiC particles/plate was the dominant factor in 0.05 wt% GNPs /TC21 composites. When the GNPs content exceeds 0.1 wt%, TiC particles/plates and TiC@GNPs bands formed at grain boundaries. The excellent combination of strength and ductility of GNPs/TC21 composites owing to the dispersion strengthening effects of TiC particles/plates as well as the load transfer between TiC@GNPs bands and TC21 matrix. Furthermore, additions of GNPs have significantly improved the tribological properties of the sintered GNPs/TC21 composites. The wear loss of 0.05 wt% GNPs/TC21 composite decreased by 21% compared to that of the sintered TC21 alloy. The superior mechanical and tribological performances of GNPs/TC21 composites can be attributed to the unique strengthening effect and the lubricating efficiency of GNPs. This demonstrates that GNPs is an ideal filler for TC21 matrix composites, as the effective lubricant and favorable reinforcement.

Tensile properties and fracture toughness of Ti–5Al–3Mo–3V–2Zr–2Cr–1Nb–1Fe (Ti-5321) alloy with bi-modal (BM), basket-weave (BW) and heterogeneous lamellar microstructure (HL) were investigated. The results showed that HL specimen with hierarchical features (e.g., mixtures of elongated βt matrix, discontinuous grain boundary α, intragranular α colonies and α plates) exhibited the best combination of strength, ductility and fracture toughness. The fracture toughness was up to 102 MPa m1/2 with a yield strength of 1200 MPa and elongation of 14%. The toughening mechanisms and fracture behaviors in three microstructures were analyzed. The toughening mechanisms in HL primarily involved heterogeneous microstructure that acted to reduce local stress/strain distribution at crack tip and enlarged crack tip plastic zone (CTPZ), which lead to more energy consumption during plastic deformation. As for fracture behaviors, the fracture mode in three microstructures were predominantly dimple ductile fracture although the crack path were both transgranular and intergranular. Interactions between microstructure constituents and crack path were studied by EBSD to discuss the crystallographic crack propagation mechanisms in BW and HL.

Ti-6Al-3Mo-2Sn-2Zr-2Nb-1.5Cr-0.1Si (TC21) alloy exhibits high specific strength and superior fracture toughness, making it attractive in the aerospace field. However, its fracture resistance under impact loading was not fully understood. Therefore, the impact toughness of TC21 alloy with various microstructures was studied using instrumented Charpy impact testing. Combining microstructure characterization, crystallographic orientation analysis, nanoindentation test, fractography observations with analysis of kernel average misorientation (KAM) distribution near crack path, the heterogeneity of deformation at small length scales, as well as crack initiation and propagation mechanisms were revealed. Analysis of load-displacement curves indicated that the crack initiation energy accounts for most of the total impact energy. The crack initiation energy is proportional to the degree of plastic deformation of the microstructures in the crack initiation region. The thick lamellar structure either in bimodal or fully lamellar structure can generate large plastic deformation, thus promote the crack initiation energy. In contrast, the KAM value in crack propagation region was much lower than that in crack initiation region, suggesting that impact energy consumed by plastic deformation during crack propagation was lower than during crack initiation. The crack propagation energy increased with the increasing degree of the tortuosity of propagation path. Especially in fully lamellar microstructure, the high angle α colony boundaries deflected the crack propagation direction and thus increased the crack propagation energy

Charpy impact properties and crack resistance of commercial pure (CP) Ti and Ti–6Al alloy with different oxygen contents were systematically investigated by using the instrumented Charpy impact testing machine. The scanning electron microscope (SEM) equipped with electron backscatter diffraction (EBSD) detector and transmission electron microscope were employed to detect the deformation mechanisms. The experimental results showed that decreasing the oxygen content significantly improved the impact toughness of CP Ti and Ti–6Al alloy. Furthermore, decreasing the oxygen content simultaneously strengthened the resistance to crack initiation and propagation of CP Ti while only improved the resistance to crack propagation of Ti–6Al alloy. EBSD analysis demonstrated the significant effects of oxygen content on dislocation activities and deformation twinning behaviors in CP Ti and Ti–6Al. Decreasing the oxygen content significantly increased the dislocation activities and deformation twinning behaviors in CP Ti and Ti–6Al alloy which were mainly responsible for the improved impact properties and the strong crack resistance of CP Ti and Ti–6Al alloy. A mathematical model was introduced for the first time to uncover the interactions between oxygen atoms and dislocation as well as the deformation twinning on the view of valence electron by employing the Yu Rui-huang electron theory. By conducting the systematically experimental and theoretical investigations, we are aiming to provide deep insights into the relationships between interstitial oxygen and impact toughness as well as the crack resistance of titanium alloys and promote the production of titanium alloys with excellent impact properties.

This work elaborates detailed strain partitioning mechanisms of the weld metals containing both equiaxed and columnar grains under tensile stress by multiple analysis methods. The results show that weld metals exhibit the multi-level strain partitioning behaviors, among which the columnar grains form the typical strain gradient, which appears as the alternating structure of strain-free and strained bands. The formation of the strain-free bands is due to the plastic flow of individual columnar grains being restricted by the back stress. More importantly, the micro-strain gradient with alternating distribution of high-density and low-density dislocation zone is formed in the strained bands. This gradient structure further promotes the accumulation of masses of GNDs at the boundaries and twin boundaries, thereby enhancing work hardening. Compared to equiaxed grains, this novel multi-level strain gradients structures seriously hinder the slip and proliferation of the SSD-type dislocations in the columnar grains. The strengthening structure formed thereby makes the strain values of the columnar grains always stay at a very low level.

To achieve preferable impact performance of a newly designed high strength Ti-5321 alloy, samples with three kinds of microstructures, i.e., bimodal microstructure (BM), tri-modal microstructure (TM) and lamellar microstructure (LM) were prepared for instrument Charpy tests at room temperature. Results of impact tests indicated that LM presented the higher impact toughness of 37.5 J/cm2 than that of BM (11.25 J/cm2) and TM (25 J/cm2). Moreover, both crack initiation energy and crack propagation energy of LM were obviously raised. Further analysis containing fracture surfaces and the cross-sectional microstructures of the impact samples by the usage of SEM and EBSD provided insights into the influence of microstructure on crack initiation and propagation during impact process. The largely plastic deformation of primary α near the V-notch tip impeded crack initiation and enhanced the crack initiation energy. The interweaved α colonies with high angle boundaries deflected the crack propagation direction and thus increased the crack propagation energy. Besides, TEM analysis showed that the typical deformation characteristic under impact test with high strain rate. Except for dislocation lines, the pile-up of highly dense dislocations, twinning, dislocation cells, sub-grains, shear bands and fragmentation of α phase can be found. These approaches presented in this study could provide certain insights into fabricating typical microstructure in the high strength titanium alloys to ameliorate impact properties.

"Titanium fire" can cause catastrophic consequences for machinery components made of titanium alloys, especially in the case of aero-engines. As such, there is continuing interests in developing burn resistant titanium alloys. The burn resistant functionality and mechanical strength of titanium alloys rely on their microstructural characteristics that are significantly determined by the alloying components. This work investigated the burn resistant performance of Ti-Cu alloy and analyzed the role of alloying Cu. With sufficient content of Cu in the system, the formation of Cu rich layer cuts off the oxygen transfer pathway and hinders the burning process. Meanwhile, the addition of Cu results in the reduction of chemical kinetics. For the Ti-Cu alloy with insufficient Cu content, the burn resistance is sacrificed by the discontinuous Cu-rich clusters, among which oxygen penetration prolongs the burning process. Apart from the Cu-rich protective layer, various burning processes accompanied with oxidation products are detected via reduced area X-ray diffraction along the burnt cross section. This work provides a comprehensive understanding of the impacts of Cu content on the burn resistant characteristics of Ti-Cu alloy, which offers theoretical foundation of burn resistant mechanism to the design and fabrication of high-performance titanium alloys.

The fracture behavior of Ti17 billet processed by the β forging was examined. The results demonstrate that the specimen orientation has a pronounce effect on the fracture toughness value as well as on the crack propagation path. The fracture anisotropy can be attributed to the elongated and aligned β grain. The crack preferentially nucleates and propagates along the precipitate-free zone nearby the grain boundary. It causes either crack deflection or delamination, enhancing the fracture resistance as compared to one weak specimen orientation. The exceptionally good combination of strength and fracture toughness can be achieved by activating delamination toughening. The delamination transforms the fracture under plane-strain condition into a series of fracture processes in plane-stress condition through the thickness, and the fracture toughness is enhanced. Furthermore, a fracture toughness prediction model considering both the intrinsic and extrinsic contributions is constructed. The results show that the extrinsic toughness varies greatly than the intrinsic toughness in different orientations, indicating that the extrinsic toughness plays a significant role in the fracture toughness anisotropy of the Ti17 billet.

Tensile behaviors of Ti–6Al with extra low interstitial (ELI) have been systematically investigated using the in-situ tensile testing monitored by the scanning electron microscopy (SEM). The electron backscatter diffraction (EBSD) technique was employed to reveal the detailed deformation mechanisms during the in-situ tensile process. The results showed that multiple slip activities occurred during the in-situ tensile process. Moreover, prismatic and basal slips dominated the slip activities. To accommodate the increased macrostrain under the tensile loading, coordinated deformation mechanisms were involved in the tensile process, including crystalline orientation rotation, slip transmission and deformation twinning. α grains were rotated about 9.8 (±4)° for convenience of slip activation to accommodate the macrostrain when the tensile displacement reached to 0.4 mm. Slip transmission supplied the intergranular coordinated deformation which avoided the localized stress concentration at grain boundaries. Deformation twinning accommodated the intragranular strain in α grains with hard orientations. Furthermore, the mechanisms for microcrack formation was thoroughly investigated by combining the in-situ SEM observations with the EBSD technique. Localized stress concentration at the grain boundaries derived from the geometric incompatibility between neighboring α grains were mainly responsible for the microcrack formation in Ti–6Al ELI alloy.

The high cycle fatigue properties of Ti-6.5Al-2.2Mo-2.2Zr-1.8Sn-0.7W-0.2Si with three typical microstructures (equiaxed, bimodal and full lamellar) prepared by carefully controlled isothermal forging and heat treatment procedures are investigated by comparison. Equiaxed and bimodal microstructures exhibit a higher HCF resistance than the full lamellar one and a mismatching of HCF strength with yield strength for equiaxed and bimodal microstructures is observed. Fractography analysis shows crack initiates primarily at the subsurface of ∼30 μm–300 μm for the three microstructures. Interior initiation occurs in 5 specimens of the 19 specimens that were measured. For equiaxed and bi-modal microstructures, the interior initiation samples have significant higher fatigue life than those initiated at (sub)surface under the same fatigue stress, while in lamellar structure, the initiation location shows no such effect on fatigue life. It is found that two factors, i. e. the effective slip length and the intrinsic strength, jointly decide the fatigue crack initiation in the present alloy. The dependence of crack initiation on microstructure is responsible for the microstructural sensitivity of HCF properties.

The microstructure and mechanical properties of ultrahigh strength Aermet 100 steel tempered at five different temperatures in the 472–492 °C range have been studied. The results show that the mechanical properties of Aermet 100 steel are sensitive to the tempering temperature; especially at the temperature of 482 °C, in which significant changes can be observed. At 482 °C, the strength of Aermet 100 steel sharply decreases by 70–100 MPa when the tempering temperature increases by only 5 °C. This is mainly due to the growth of M2C carbide at relatively high tempering temperature that can greatly decrease its interface strengthening effect. In addition to strength, the plasticity and fracture toughness of Aermet 100 steel obviously increase at 482 °C when compared with lower tempering temperatures. The rising amplitudes of elongation and fracture toughness can reach 1.7% and 30 MPa m1/2 respectively. This is attributed to the reverted austenite film precipitated at 482 °C. For one respect, the existence of austenite film between lath martensite can reduce the local stress concentration and the chance of microcrack formation in Aermet 100 steel during deformation, which is responsible for the obvious increase of plasticity. For another respect, the excellent softness, toughness and stability of reverted austenite prompt the advancing crack to divert, branch and blunt or appear “zigzag” route, which results in an increase in the fracture toughness. Aermet 100 steel can achieve the optimum combination of strength, plasticity and toughness at the tempering temperature of 482 °C, which has the yield strength of 1845 MPa, the elongation of 12.9% and the fracture toughness of 113 MPa m1/2.

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.

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 (αp) and soft β transformation microstructure (βt). During crack propagation, large plastic deformation occurs within soft βt region, and the deformation of βt was constrained by surrounding hard αp, resulting in kink deformation of βt and local plastic deformation of αp. In addition, {10-12} tensile twins with both high and low Schmid factor were activated in αp by the sever kink deformation of βt and impact energy. Furthermore, some αp 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 αp's grain boundary in EM. Moreover, the coordinate deformation ability between αp and βt was weak due to the high content of αp, resulting in a significant reduction of impact toughness. For LM1 and LM2, although {10-12} tensile twins and {10-11} 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.

In this work, the effects of strain rates (10−4 s−1~10−1 s−1) on ferrite hardening of dual-phase steel were investigated through pre-straining tests. The strain localization was analyzed by the evolution of Kernel average misorientation (KAM). The micromechanism of ferrite hardening was elaborated by the evolution analysis of geometrically necessary dislocations (GNDs), low angle grain boundaries (LAGB) and nano-indentation. The results showed that GNDs and LAGBs in ferrite increased with increasing the strain rate, the average KAM values and nanohardness increased accordingly. Strain localization was observed in the ferrite grains at lower strain rates (10−4 s−1~10−3 s−1), and the considerable strain localization occurred at ferrite/bainite (F/B) interface at higher strain rates. Due to the nearly saturated GNDs and LAGBs density at the strain rate of 10−2 s−1, the completely ferrite hardening occurred and its nanohardness was close to dual-phase interface. During the tensile deformation after pre-straining, the yield ratio increased while the uniform elongation decreased with strain rates of pre-straining. It was worth noting that the completely ferrite hardening led to the early plastic deformation of tensile tests was interface deformation. This work provides a theoretical basis for the ferrite hardening behavior after pre-straining at different strain rates, and then contributes to the safe service of dual-phase pipeline steel.

The mechanical performance of Ti-13Cu-Al alloy is highly determined by its microstructure, which is significantly affected by the cooling process from semi-solid state as it induces peritectic solidification and the subsequent eutectoid transformation of the alloy. This work investigated the effects of cooling rates on the morphology of Ti2Cu phase and mechanical properties of Ti-13Cu-Al alloy after cooling from semi-solid state (β+L phase). Different cooling mediums have been considered, including furnace cooling (0.5 °C/s), air cooling (50 °C/s) and water cooling (150 °C/s), corresponding to a slow, normal and rapid cooling process, respectively. It is found that α+Ti2Cu phases are uniformly distributed in the alloy after different cooling processes, and the morphology of Ti2Cu phase is controlled by the peritectic solidification and eutectoid transformation. Specifically, the thickness and the volume fraction of primary Ti2Cu phase are larger than the sample cooling from β phase field when the cooling rate is the same. The morphology of Ti2Cu phase varies from “laths + blocky + sheaves” to “short laths + blocky + rodlike” as the cooling rate gradually increases. It is further revealed that the β/L interface provides a fast channel for solute atom diffusion, and the peritectic Ti2Cu phase provides a favorable nucleation site for the precipitation of eutectoid Ti2Cu. Based on the tensile property and hardness examinations, the mechanical properties of the Ti-13Cu-Al alloy show a clear dependency with the precipitation of Ti2Cu phase after cooling from semi-solid state, indicating the importance of cooling mode from semi-solid state on the microstructure and mechanical properties of the alloy.

The hot deformation behavior of powder metallurgy Ti-5Al-5V-5Mo-3Cr (Ti-5553) alloy was investigated by isothermal compression test at the wide temperature range of [Formula: see text]–[Formula: see text]C, under an intermediate strain rate of 0.1 s[Formula: see text], and with the sample deformation degree of 30%, 50% and 70%. Results suggested that the flow stress was very sensitive to the deformation variables and it decreased with increasing the deformation temperature. Flow localization occurred when the temperature was lower than [Formula: see text]C, accompanied by the dynamic morphology changing of [Formula: see text] phase. Dynamic recovery (DRV) features appeared when the temperature was over [Formula: see text]C, while dynamic recrystallization (DRX) took a more important role than DRV for the alloy compressed at [Formula: see text]–[Formula: see text]C.

To realize the systematic comparison of the hot workability and guide the further hot-processing of powder metallurgy (PM) and ingot metallurgy (IM) Ti-5Al-5V-5Mo-3Cr (Ti-5553) alloys, the hot deformation behaviour and microstructural evolution of the two alloys were investigated at a wide temperature range of 700 °C–1100 °C and strain rate of 0.001 s−1-10 s−1. The activation energy maps and processing maps for both PM and IM alloys were constructed, as well as the specific deformation mechanisms were identified for each processing region. The results showed that PM alloy has lower deformation resistance, smaller activation energy and larger optimal processing windows than those of IM alloy. The dynamic α precipitation mechanisms in PM alloy were diffusional globularization and coarsening, rather than diffusionless shearing and fracturing in IM alloy. The extensive dynamic recrystallization (DRX) happened at 900 °C–1050 °C for PM alloy and at 1000 °C–1100 °C for IM alloy. The DRX process was dominated by discontinuous dynamic recrystallization (DDRX) for PM alloy while continuous dynamic recrystallization (CDRX) for IM alloy. Furthermore, PM alloy had smaller flow instability region than IM counterpart in the hot processing map. The schematic deformation mechanism maps were eventually developed for both PM and IM Ti-5553 alloys. Keywords: Ti-5553 alloy, Deformation mechanism differences, Powder metallurgy, Ingot metallurgy, Hot deformation behaviour, Microstructural evolution