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A prominent strengthened Y2O3 stabilized t-ZrO2 (YSTZ)/MgO nanocomposite coating is achieved by the plasma electrolytic oxidation (PEO) process and in-situ synthesized YSTZ reinforced phase with a quantitative control approach. The idea of activating double slip systemic semi-coherent interface dislocations in YSTZ/MgO nanocomposite coating to realize crack self-healing is proposed. High dislocation densities are associated with {101} < 101> YSTZ slip and {111} < 101> MgO slip system to coordinate interfacial deformation to stop crack initiation and propagation. This crack propagation path can absorb more fracture energy, providing more opportunities for crack deflection and bridge, which closes crack and realizes crack self-healing. This paper reveals semi-coherent interface dislocation of double slip systems in nanocomposite coating stop crack initiation and propagation is proposed to realize crack self-healing. [ABSTRACT FROM AUTHOR]

Quasi-static tensile deformation behaviors at room temperature (RT) and 77 K of Ti-3Al-2Zr-1.5Mo alloy with lamellar microstructure were systematically investigated in this study. The experimental results indicated the excellent cryogenic mechanical properties of Ti-3Al-2Zr-1.5Mo alloy with lamellar microstructure. The strength and elongation of the Ti-3Al-2Zr-1.5Mo alloy at 77 K are much larger than those of RT specimen. It demonstrated that the strength, plasticity and strain-hardening rate were improved simultaneously at cryogenic temperature. Microstructural analysis indicated that strong cryogenic twinning induced plasticity (TWIP) effect occurs in the Ti-3Al-2Zr-1.5Mo alloy with lamellar microstructure. The intersection between the substructure and the α/β interface is one of factors to the nucleation of twins at cryogenic temperature. The optimization of the properties of Ti-3Al-2Zr-1.5Mo alloy should be attributed to the significant TWIP effect at cryogenic temperature. In addition, the cryogenic environment weakens the hindering effect of the interfaces on crack propagation. [Display omitted] [ABSTRACT FROM AUTHOR]

The impact toughness of titanium alloys is one of the key factors determining their safe service in cryogenic temperature environments. In this work, the temperature-related impact properties of grade 2 pure titanium and Ti-2.5Al–3Zr–1Mo alloys are investigated. It is found that the impact properties and deformation mechanisms of the above two materials are different with decreasing temperature (20 °C, 0 °C, −50 °C, −100 °C and −196 °C). Speaking of pure titanium: the impact toughness doesn't change obviously, and crack initiation energy (W i) gradually increases while crack propagation energy (W p) decreases (W p of −196 °C is 40% lower than that of 20 °C while W i is 90% higher). The slip-dominated deformation mechanism at 20 °C gradually transforms into twinning-dominated deformation mechanism under cryogenic temperature. High-density twins formed at −100 °C and −196 °C can effectively relieve stress concentration and lead to better cryogenic impact toughness. By contrast, W i and W p of Ti-2.5Al–3Zr–1Mo alloy decrease with decreasing temperature. The impact toughness of Ti-2.5Al–3Zr–1Mo alloy is higher than that of pure titanium when the temperature is above −100 °C. At 20 °C, the synergistic effect of the tortuous crack path and deformation twins of Ti-2.5Al–3Zr–1Mo alloy promotes its impact toughness higher than that of pure titanium. However, the twinning density of Ti-2.5Al–3Zr–1Mo at −196 °C is much lower than that of pure titanium, which cannot release stress effectively and cause a brittle fracture. The research provides theoretical and experimental basis for the application of titanium alloys in cryogenic environment, and provides guidance for the development of titanium alloys with better cryogenic impact toughness. [ABSTRACT FROM AUTHOR]

The impact characteristic and crack propagation mechanism for large-size Mo 3Nb single crystal with a diameter of about 30 mm have been investigated and disclosed comprehensively by Charpy impact test. The fracture analysis shows that the single crystal crack propagates radially, resulting in a multi-step distribution on the fracture surface. The fracture of Mo 3Nb polycrystal is composed of cleavage and quasi-cleavage, exhibiting a trans-granular fracture feature. The impact toughness of single crystal shows orientation dependence, which is attributed to the crack propagation path. Compared with the straight path, the waved path can delay the crack propagation and absorb higher impact energy. The impact toughness of Mo 3Nb single crystals is lower than that of Mo 3Nb polycrystal at room temperature because the single crystal lacks the coordinated deformation of grain boundary. The analysis of dislocation arrangement for Mo 3Nb single crystal shows the different crack propagation paths is caused by the mobility of screw segments, which are controlled by the double cross-slip multiplication mechanism. However, the waved fracture morphology of Mo 3Nb polycrystal is caused by the effect of grain boundaries instead of the cross-slip of screw dislocation. [Display omitted] • The impact toughness of molybdenum alloy single crystal shows orientation dependence • The single crystal crack propagates radially and exhibits a multi-step distribution feature. • Different crack propagation paths are caused by the mobility of screw segments [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 impact toughness of a high-strength metastable β titanium alloy (Ti-5Cr-4Al-4Zr-3Mo-2W-0.8Fe) with two typical microstructures is studied by Charpy impact tests. The bimodal microstructure (BM) and the lamellar microstructure (LM) are obtained by the solution and aging treatments and the β annealing, slow cooling and aging treatments, respectively. In the impact crack initiation process, the deformation capacities of the primary α (αp) phase, secondary α (αs) phase and transformed β (βt) matrix in the BM are very different, and the stress gradient at the interface of the three causes the crack initiation. The lamellar α (αl) phase and βt in the LM satisfy the BOR relationship, and the effective slip transfer between α and β phases slows down the crack initiation. Meanwhile, the appearance of deformation twins in the LM improves the crack initiation energy. In the crack propagation process, the lack of coordinated deformation between the α and β phases in the BM leads to rapid crack propagation. In the LM, the deformation of αl and βt is relatively more coordinated, so the severe plastic deformation is only concentrated near the crack and at the interface. The secondary crack initiation and the crack propagation along the twin boundary reduce the stress concentration at the crack tip. The deformation twins and zigzag propagation path can improve the crack propagation energy. To summarize, the alloy with LM exhibits better impact toughness than the alloy with BM. [ABSTRACT FROM AUTHOR]

• The differences of the FCP rates in rapid growth regime between EM and LM are owing to the varied fracture modes. • The FCP behaviors in near-threshold and Paris regimes are a result of two contributing factors: interface obstacles and crack front profile. 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 (ΔK th) 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. [ABSTRACT FROM AUTHOR]

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 β t 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. [ABSTRACT FROM AUTHOR]

Ionic crystal ceramics are known to be susceptible to brittle cracks along the intergranular due to the strong ionic bond forces. Accordingly, an outstanding toughened coating consisting of yttria-stabilized tetragonal zirconia (YSTZ) and MgO, with a reinforced heterointerface, has been successfully achieved through the plasma electrolytic oxidation process. The results demonstrate that the enhanced Zr/Y–O bond strength improves the interfacial strength of the ionic crystal, while the lattice distortion activates continuous dislocation pinning at the (101) YSTZ//(111) MgO semicoherent heterointerface effectively hinders crack propagation of the coating. With the YSTZ content approaching 65 %, the toughness (K IC) value surpasses that of PEO coating by 42 %. These findings not only elucidate the scientific foundation for the improved toughness of the YSTZ/MgO nanocoating but also provide valuable insights into design strategies for improving ceramic toughness. [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]

In this work, different morphologies and structures of lamellar α are obtained by controlling the cooling rates, i.e., water cooling (WC), air cooling (AC) and furnace cooling (FC), and their effects on tensile properties and impact toughness of a new near α CT20 alloy (Ti-3Al-2Zr-1Mo) are investigated. Results showed that a heterogeneous three-dimensional network lamellar martensite α structure was formed after WC, a mixed structure composed of fine lamellar α/β and a small amount of martensite was formed after AC, and a coarse lamellar α structure was formed after FC. The tensile strength gradually decreased with decreasing cooling rate, and AC had the superior strength-plasticity match (yield strength was 13% higher than FC while their plasticity is equivalent). However, the impact toughness of AC (comparable to that of WC) was about 16% lower than that of FC (90 J/cm2). During tensile deformation, the synergistic effect of the good plasticity of the α colony, the obstruction of dislocations by the colony interface and the plastic deformation of the martensite of the AC contribute to the best strength-plasticity matching. During impact, crack initiates and propagates rapidly at the V-notch root, and the finite element simulation results showed that the plastic deformation was concentrated in a tiny region near the crack tip, resulting in the coordination deformation among the microstructures in AC cannot be fully realized. By contrast, the coordinated deformation of multi-level martensite and the generation of nano twins in WC make its impact toughness comparable to that of AC. The highest impact toughness of FC was attributed to the superior plasticity of coarse lamellae, slip transfer between coarse α and β lamellae, the generation of twins and the formation of a small jagged and large wave-like composite crack propagation path. • Cooling rate has a significant effect on lamellar α morphology and structure of CT20 alloy. • The multi-level lamellar martensitic α structure exhibits better strength-ductility-impact toughness synergy. • Coordinated deformation of multi-level martensite and generation of nano-twins result in high impact toughness. • The overall microstructure's deformation determines tensile properties, while impact toughness depends on the micro-deformation near crack tips. [ABSTRACT FROM AUTHOR]

In this work, crack propagation behaviors and fracture mechanism of CT20 alloy with lamellar α at the temperatures ranging from 20 to − 90 °C were investigated and revealed. The results showed the fracture toughness of CT20 alloy was decreased from 430.89 KJ/m2 to 171.67 KJ/m2, and the fracture mechanism was gradually changed from typical ductile fracture to quasi-cleavage fracture with the decreasing testing temperature. The reduction of fracture toughness was mainly influenced by the internal factor (the crack tip plastic zone) and external factor (the tortuosity of the crack propagation path), in which the contribution of the internal factor increased from 87.06% to 95.68% as the temperature decreased. Therefore, the high contribution of the internal factor means the plastic zone at the crack tip could mainly affect the crack propagation behaviors. The lamellar α was found to be deformed by the plastic zone, and then inhibited the crack propagation and deflected its propagation direction at 20 °C, which consumed the additional deformation energy and increased the fracture toughness. With the decline of the plastic zone from 0.526 mm to 0.107 mm, the lamellar α deformation and the crack propagation resistance were significantly reduced at − 90 °C, which accelerated crack propagation and decreased the fracture toughness. • The crack propagation behaviors of CT20 alloy are first investigated from 20 to − 90 °C. • The effect of internal and external contributions on fracture toughness are revealed. • The collaborative effect of both the plastic zone and microstructure are revealed. [ABSTRACT FROM AUTHOR]

By adjusting the particle size of Ti6Al4V(TC4) powders and the distribution of boron particles, a high-performance heterogeneous network structure titanium matrix composites were fabricated through the spark plasma sintering (SPS) technique. In the present work, TiB whisker (TiBw) are in situ synthesized around the boundaries of the small size TC4 (STC4) particles, and formed into a TiBw network structure. The STC4 particles of the network surround the larger size TC4 (LTC4) particles to form a new heterogeneous network structure similar to the shape of a sunflower. The heterogeneous network structure consisted of coarse grain zones (LTC4) and fine grain zones (TiB/STC4) is formed in the microstructure. In light of tensile results, the tensile strength and ductility of TiB/TC4 composites with heterogeneous network structure are about 987.5 MPa and 11.5 %, respectively, which are 16.6 % and 35.3 % higher than those of TC4 alloy. The improvement of mechanical properties is mainly due to the interface strengthening mechanism of heterogeneous network structure and the hindrance and pinning mechanism of reinforcements to dislocation movement. The improvement of ductility depends on the inhibition of crack propagation by the heterogeneous network structure and the existence of high ductility coarse grain zone. [Display omitted] • A titanium matrix composite with heterogeneous network structure was prepared successfully. • Heterogeneous network structure composite achieves the matching of strength and ductility. • The improvement of the strength and ductility is attributed to the fracture mechanism of the heterogeneous network structure. [ABSTRACT FROM AUTHOR]

Boron (B) and graphene oxide nanoplates (GOs) reinforced Ti6Al4V (Ti64) titanium matrix composite ((B + GOs)/Ti64) with a heterogeneous network structure was prepared via spark plasma sintering (SPS). The results show that the heterogeneous network structure composites consist of fine grain zones with a network structure and coarse grain zones with good ductility. Compared with Ti64 alloy, heterogeneous network structure (B + GOs)/Ti64 composites has higher strength and ductility, both of which are superior to homogeneous network structure (B + GOs)/Ti64composite. The increase in strength is attributed to the grain refinement effect within the fine grain zone and the load transfer effect of the reinforcement at the interface. The ductility is mainly dependent on the existence of large size matrix and the improvement of deformation coordination and cracks propagation by the heterogeneous network structure. Finally, the conflict between the strength and plasticity of the composites is effectively reconciled. This provides a new idea for the design of high-performance titanium matrix composites. [ABSTRACT FROM AUTHOR]

[Display omitted] • The stress concentration and evolution at crack tip are simulate by the XFEM. • The dislocation behaviors of crack propagation are analyzed by TEM and EBSD. • Crack propagation behavior is explained by Peierls-Nabarro stress theory. In this work, the crack propagation behavior and mechanism of dual-phase steel were studied under low temperature environment. The results show that the strength of dual-phase steel increases from −20 °C to −60 °C, and the fracture toughness presents the opposite trend. When the crack began to extend at low temperature, the size of plastic zone at the crack-tip is reduced, inducing the opening displacement rate, strain rate and temperature-strain parameters are reduced, which shows the extreme sensitivity to ambient temperature. More importantly, the fracture of dual-phase steel presents the transgranular fracture along {1 0 1} and {1 1 1} crystal planes, due to the accumulation of dislocation density. Therefore, the crack propagation of dual-phase steel at low temperature is caused by the dislocation accumulation and movement behaviors. [ABSTRACT FROM AUTHOR]