Interlamellar Fracture Research Articles

High Temperature Fatigue Behavior and Failure Mechanism of Ti-45Al-4Nb-1Mo-0.15B Alloy

Strain-controlled low cycle fatigue experiments were carried out on the TiAl alloy Ti-45Al-4Nb-1Mo-0.15B at 400 °C and 750 °C to reveal the cyclic mechanical behavior and failure mechanism. The TiAl alloy presents stable cyclic characteristics under fatigue loading at elevated temperatures. No obvious cyclic softening or cyclic hardening was manifested during experiments. The cyclic stress–strain relationship is well described by the Ramberg–Osgood equation. The fatigue lifetime at different temperatures has a log-linear relationship with the total strain ranges. The fracture morphology indicates the main fracture mode of fatigue specimens at 400 °C is a brittle fracture, while there is a ductile fracture at 750 °C. Meanwhile, the trans-lamellar fracture is dominant for the lamellar microstructure and the percentages of the inter-lamellar fracture decreases with the strain amplitude.

Intuitive Analysis of the Microtribological Behavior of Copper-Coated Graphite-Graphite/Cu Composites with High Graphite Contents.

To improve the combination of graphite and copper, the friction and wear of a graphite/copper composite with a high content of graphite (50 wt %), copper-coated graphite were used to modify it. To observe the distribution law of each phase in the material and the change of composite surface structure after the friction and wear experiment, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the micro-structure, friction film, debris, and friction cross section of the composites. The results show that the large particle size of copper-coated graphite is anisotropic in the material, which helps to form a friction film with a high graphite content on the contact surface. TEM images of the friction film and debris directly reflect the structure changes of graphite and copper during friction; under normal load and shear force, interlamellar detachment and interlamellar fracture of graphite occur, and its edge is folded and crimped, resulting in the loss of an ordered state in some regions, which results in the instability of crystal lattice and the transition from an ordered to disordered state of graphite, resulting in the (002) halo ring in FFT results. Severe plastic deformation and oxidation reactions occur in copper, and copper oxides are formed, forming a high-strength and smooth oxide film in the metal-rich area and improving the wear resistance of the material. TEM images of the friction section directly show that an inverted triangular deformation zone is formed on the surface of the sample after friction and wear experiments. The edge of the deformation zone is stepped, consisting of a drag zone and an accumulation zone, and the surface of the contact zone is covered by a carbon film.

Effect of nitrogen addition and aging treatment on microstructure and high temperature mechanical properties of Ti‐48Al‐2Cr‐2Nb (at%) intermetallic alloy

Effects of nitrogen (N) addition (0.5, 1, and 2 at%) on microstructure formation and high temperature mechanical properties of the age-hardened Ti‐48Al‐2Cr‐2Nb (4822, at%) intermetallic alloy were investigated. Alloys were fabricated by vacuum arc re-melting followed by hot isostatic pressing, solution annealing, and finally aging at two different conditions of 700 °C/10 h or 900 °C/1 h. Mechanical properties of the age-hardened samples were investigated via small punch testing at 800 °C. Results showed that N addition resulted in microstructure refinement and increased uniformity of microstructures and mechanical properties. Lamella coarsening and fragmentation was occurred during both aging conditions; however, a more stable lamellar microstructure was achieved by N addition. Colonies with various lamellar orientation showed different fragmentation behavior. Better mechanical properties was achieved in the alloy containing 1 at%N after aging at 900 °C/1 h. Inter-lamellar fracture was the predominant fracture mechanism in all intermetallic alloys.

Effects of γ/γ lamellar interfaces on interlamellar crack propagation behaviors of TiAl alloys

Interlamellar fracture is one of the failure modes of fully lamellar (FL) TiAl alloys. Here, the effects of three types of γ/γ lamellar interfaces, which are termed true twin (TT), 120° rotational order fault (ROF), and pseudo-twin (PT), subjected to uniaxial tensile loading on the interlamellar crack propagation behaviors of an FL γ-TiAl alloy have been investigated by molecular dynamics method. The results show that the introduction of the PT and ROF interface weakens the tensile strength of single crystal TiAl, while the introduction of the TT interface can effectively improve the tensile strength of the material. Moreover, the cracks in the three interfaces exhibit different propagation behaviors and propagation rates. Specifically, the ROF and PT interfaces exhibit the most desirable and least desirable crack resistances, respectively, which are related to the geometric compatibilities of the interfaces and plastic deformation of crack tips. We also found that the sequence of fracture toughness (ROF > TT > PT) is consistent with that of the flow stresses (ROF > TT > PT). In addition, in the early stage of crack propagation, deformation twins serve as a toughening mechanism that reduces crack tip stress and retards crack extension.

Transient liquid phase bonding of cast Al0.3CoCrFeNi high-entropy alloy using Ni/Zr/Ni laminated foils

Cast Al0.3CoCrFeNi high-entropy alloy was successfully bonded by Ni/Zr/Ni laminated foils using a transient liquid phase bonding method. When the bonding temperature exceeded 1140 °C, the active Zr atoms from Zr-Ni liquid alloy diffused into the grain boundaries and interdendritic region of the Al0.3CoCrFeNi base material and triggered a eutectic reaction with Al0.3CoCrFeNi in this region, which acted as a pseudo element. The formed liquid phase accelerated the dissolution of cast Al0.3CoCrFeNi base metal and a composite microstructure with the soft FCC Al0.3CoCrFeNi phase embedded into the brittle Ni7Zr2 intermetallic compound formed in the seam upon cooling. The mechanical performance of the joint was significantly affected by the bonding parameters. When the bonding was carried out at 1160 °C for 30 min, a joint with balanced ductility (face-centered cubic phase) and strength (Ni7Zr2) was obtained. The maximum shear strength of approximately 247 MPa was obtained. This value was approximately six times higher than that of the joint bonded at 1120 °C. The joints fractured in ductile interlamellar fracture mode.

Microstructure and elevated temperature tensile property of Ti–46Al–7Nb-(W,Cr,B) alloy compared with binary and ternary TiAl alloy

The newly developed high-Nb TiAl with a small amount of W, Cr, and B addition prepared by electromagnetic cold crucible has been investigated by comparing with the microstructure and elevated temperature tensile properties of binary and ternary high-Nb TiAl alloy. Results indicate that microstructures of the three alloys are fully lamellar (FL), near fully lamellar (NFL) and near fully lamellar alloy with TiB (NFL + TiB). The dispersed TiB particles in the alloy have the B27 structure and are highly enriched with Nb and W but depleted with Al by the characterization of transmission electron microscopy. The elevated temperature tensile test results show that the high-Nb TiAl alloy with NFL + TiB microstructure exhibits excellent mechanical properties compared with the other two alloys, and the tensile strength and elongation at 800 °C are 648 MPa and 5.73%, respectively. Fracture morphology shows the alloy has a mixed mode of trans-lamellar and inter-lamellar fracture. As the dispersed second phase particles, rod-like TiB can act as the obstacle to crack propagation and dislocation movement. The excellent mechanical properties of the alloy at elevated temperatures are attributed to the combined effects of dislocation block and nanotwins formation during deformation within the γ lamellae. Moreover, the probable generation and propagation mechanism of dislocations and twins in the γ lamellae are further discussed.

Reverse effect of hot isostatic pressing on high-speed selective laser melted Ti–6Al–4V alloy

Abstract Despite recent progress in achieving high mechanical properties of 3D printed metal products, the low productivity still remains a major limitation for their cost-effective feasibility in practical applications. To achieve high-speed printing with affordable mechanical properties, we increased the scanning speed of selective laser melting process with Ti–6Al–4V up to 1800 mm/s and applied a hot isostatic pressing (HIP) process to compensate for the porosity. In these high-speed printed specimens, the HIP process led to a microstructural change from αʹ-lath martensite to a Widmanstӓtten α-lamellar structure, which deteriorated their tensile properties due to the segregation of β-stabilizing atoms and caused inter-lamellar fracture. The deterioration phenomenon of high-speed printed Ti–6Al–4V specimens after the HIP process was found to be critically affected by the surface roughness of as-built state, which can be efficiently controlled with a build angle set-up.

Fractographic Study on Naturally Initiated Short Fatigue Cracks in a Near-Lamellar TiAl Alloy at Room Temperature

Short crack phenomena are considered important for lamellar structures in γ-TiAl alloys and have been thoroughly investigated in the past. However, the short cracks in the previous studies were nearly all introduced artificially. No particular investigations have looked into the initiation of fatigue short cracks. Therefore, naturally initiated short fatigue cracks at room temperature under two different stress ratios (0.1 and 0.5) were investigated in a near-lamellar γ-TiAl alloy (Ti-45Al-2Mn-2Nb) in this study. The observations show that the fatigue crack initiation behaved differently at low and high stress ratios. At low stress ratio, the specimens failed at lower ultimate stress levels (σmax = 450 and 475 MPa), and the crack initiated from the cluster of interlamellar fracture near mode-I orientation or stress concentration areas. At the higher stress ratio, the specimens failed at higher but consistent stress levels (σmax = 560 and 570 MPa), and in the specimen crack initiation areas, the interlamellar fractures were still the primary fracture mode, whereas some were found at tilted angles due to shear deformation. The results suggest that short fatigue cracks can naturally initiate in lamellar γ-TiAl alloys, thus attention should be paid to their microstructure design, surface finishing and cleanliness.

Formation of Ti2AlN and TiB and its effect on mechanical properties of Ti46Al4Nb1Mo alloy by adding BN particles

The synergistic effects of B and N on the microstructure evolution, mechanical properties and fracture behavior of Ti46Al4Nb1Mo alloy were investigated by adding BN particles. Ti46Al4Nb1Mo-xB-xN (x = 0, 0.4%, 0.8%, 1.2%, 1.6%, atomic percent, hereafter in at.%) alloys were prepared by vacuum arc melting. Compared with Ti46Al4Nb1Mo alloy, B2 phase disappeared and curvy TiB phase appeared at the interdendritic region in the Ti46Al4Nb1Mo-0.4B-0.4 N alloy. When 0.8% B and 0.8% N were added, Ti2AlN particles appeared in α2/γ lamellar colonies, tiny TiB particles and a few curvy TiB phases formed both at lamellar boundaries and lamellar colonies. With further increasing B and N content, the volume fraction of Ti2AlN and tiny TiB phases increased, and some long TiB phases appeared in the Ti46Al4Nb1Mo-1.6B-1.6 N alloy. Ti2AlN and TiB phases formed by the reaction between Ti46Al4Nb1Mo alloy and BN particles. The different formation mechanism between Ti2AlN and TiB was related to the different solid solubility of B and N in the Ti46Al4Nb1Mo alloy and solute redistribution during solidification process. When the content of B and N increased from 0 to 1.2%, the average lamellar colony size decreased from 358.4 μm to 48.7 μm, which resulted from synergistic effect of Ti2AlN and TiB. Compression results showed that the strength was improved from 1961 MPa to 2493 MPa without decreasing compressive strain. The fracture morphology showed inter-granular and inter-lamellar fracture mode in the Ti46Al4Nb1Mo alloy, while the fracture morphology showed trans-lamellar mode in the reinforced composites. The second phase strengthening of Ti2AlN and TiB, grain boundary strengthening by refined α2/γ lamellae, and elimination of B2 phase contributed to the improvement of strength.

Microstructure and phase evolution in γ-TiAl/Ti2AlNb dual alloy fabricated by direct metal deposition

As a potential material used in aero-engine, a γ-TiAl/Ti2AlNb dual alloy was fabricated by depositing γ-TiAl alloy on a Ti2AlNb alloy plate using direct metal deposition (DMD) technique. Due to re-melting of previous layer during DMD, a transition zone with gradual change of compositions and microstructures was generated between γ-TiAl and Ti2AlNb alloy. In this paper, formation mechanism of the transition zone was analyzed after considering solidification route and compositional variations. Results reveal that the transition zone mainly consists of three layers, and compositions are comparatively uniform in layers but change stepwise between each other. Microstructures of layers are dominantly controlled by their compositions and cooling schedules, and the first layer possesses columnar grains with α2 phase and γ phase dispreading in B2 matrix. While for the second layer, a mixed microstructure of (α2+γ) lamella, B2 phase and γ phase is formed. The third layer exhibits equiaxial (α2+γ) colony with fully lamellar microstructure, which is similar to microstructure of γ-TiAl alloy but without elemental segregations. Results of room temperature tensile test indicate that fracture takes place at γ-TiAl alloy side, and the fracture morphology is a mixture of interlamellar fracture and translamellar fracture.

Microstructure and mechanical properties of CoCrFeNiZrx eutectic high-entropy alloys

The emergence of eutectic high-entropy alloys containing Laves phase provides a new and exciting research direction towards developing advanced structural alloys, in light of the inherent advantages of Laves phase at elevated temperatures. In this work, CoCrFeNiZrx alloys were prepared with varying zirconium contents by vacuum arc-melting method. Typical eutectic microstructure was identified in the as-cast alloy having x=0.5. The alloys consist of face-centered cubic solid solution and C15 Laves phase in the form of lamellae. Crystallographic orientation relationship between these two phases was determined. With the increase of the volume fraction of hard C15 Laves phase, the resulting alloys showed an increase in strength, but became more brittle at room temperature; the fracture process changed from ductile inter-lamellar fracture to brittle trans‑lamellar fracture. With the increase of test temperature, however, the fracture mode converted to be ductile fracture. As such, the eutectic microstructure can accommodate considerable plastic strain and has potential for engineering applications involving elevated temperatures.

Comparison of microstructures and mechanical properties of as-cast and directionally solidified Ti-47Al-1W-0.5Si alloy

The as-cast Ti-47Al-1W-0.5Si alloys were prepared by induction skull melting (ISM) technology under argon atmosphere. Some bars were cut from as-cast ingot longitudinally and then were directionally solidified by Bridgman solidification technique. The microstructures and mechanical properties of the as-cast and directionally solidified Ti-47Al-1W-0.5Si alloy ingots with different lamellar orientations were investigated. After directional solidification, the grain boundaries were eliminated and the lamellar orientation was nearly parallel to the growth direction. The interlamellar spacing of directionally solidified samples decreased from average 1500 nm to less than 1200 nm and the interlamellar spacing was more homogeneous. In addition, the shape of Ti5Si3 transformed from short rod to strip along the growth direction, and the B2 phase disappears after directional solidification. At room temperature, the compressive yield strength of Ti-47Al-1W-0.5Si alloy increases from 573 in as-cast condition to 651 Mpa after directional solidification. The fracture mode transforms from inter-granular fracture to inter-lamellar fracture after directional solidification.

Microstructure determined fracture behavior of a high Nb containing TiAl alloy

Static tensile and fatigue fracture behavior of a high Nb-containing TiAl alloy with the composition of Ti-45Al-8.5Nb-(W, B, Y) fabricated by the plasma arc melting (PAM) have been investigated and correlated with microstructure in this work. The results show that the as-obtained alloy preserves near lamellar microstructure with homogeneous and relatively fine lamellar colonies. Meanwhile, minor β phase is observed on colony boundaries or within lamellar colonies. The existence of the random distributed borides is one of the main reasons for the refinement of the experimental alloy. The ultimate tensile strength of the experimental alloy is 710MPa and the fatigue limit with the 50% survival rate is 309.6MPa. The fracture mechanism has been proposed and correlated with the microstructure of the experimental TiAl alloy. It is found that fine lamellar microstructure accounts for the high fracture strength and moderate fatigue performance of the experimental high Nb-containing TiAl alloy. Fractography analysis indicates the alloy exhibits a predominant transgranular and occasional interlamellar fracture mode. The deformation incompatibilities occur at the boride/matrix and β phase/matrix interfaces lead to the crack initiation and propagation in these positions, which are detrimental to the ductility of the alloy.

Mechanical properties of high niobium TiAl alloys doped with Mo and C

Effects of alloying elements (i.e., variable content of Mo and C) on the microstructure and mechanical properties of five types of experimental material based on TiAl alloy containing 7at.% Nb were investigated. Mechanical properties (tensile, compression and fatigue) at room and at elevated temperatures were evaluated. Addition of Mo and/or C resulted in different phase compositions and three typical microstructures. To judge the effect of alloying on the microstructure the comparison with reference TiAl alloy with 7at.% Nb without C and Mo addition was used. Evaluation of mechanical properties of individual alloys revealed two most promising variants: (i) alloy with 2at.% Mo containing β phase, having mixed microstructure and exhibiting the highest ductility, (ii) alloy with 0.5at.% C having nearly lamellar microstructure, enhanced amount of α2 phase, without β phase and exhibiting the highest strength. No positive effect of combined Mo and C addition on mechanical properties was identified. Fractographic observations revealed that the major fracture mode in all alloys was trans-lamellar. Inter-lamellar fracture was observed occasionally and mainly in C containing alloys.

Positive influence of hydrogen on the hot workability and dynamic recrystallization of a γ-TiAl based alloy

High temperature deformation behaviors of the unhydrogenated and hydrogenated Ti-46Al-2V-1Cr-0.3Ni (at%) alloys were investigated in the temperature range 1050–1200°C and strain rate range 0.001–1s−1. The flow stress of the hydrogenated alloy was lower than that of the unhydrogenated alloy, which was mainly attributed to hydrogen-promoted lamellar decomposition, twinning and dynamic recrystallization. Processing maps of the unhydrogenated and hydrogenated alloys were constructed. The stability domain of the hydrogenated alloy was enlarged in comparison with that of the unhydrogenated alloy, meaning hydrogen enhanced the hot workability of the Ti-46Al-2V-1Cr-0.3Ni alloy. Hydrogen activated slip systems so that lamellar bending more easily occurred in the hydrogenated alloy, which made inter-lamellar fracture convert into trans-lamellar fracture. Therefore, crack propagation was restrained in the hydrogenated alloy during hot deformation. The critical strain of the hydrogenated alloy was less than those of the unhydrogenated alloy, meaning the dynamic recrystallization occurred earlier in the hydrogenated alloy. γ-phase discontinuous dynamic recrystallization was promoted due to hydrogen addition, leading to a consumption of plentiful dislocations, which restrained continuous dynamic recrystallization to some extent. Hydrogen-promoted discontinuous dynamic recrystallization was mainly attributed to hydrogen-promoted lamellar decomposition, twinning, and hydrogen-decreased stacking fault energy.

Microstructure and room temperature tensile property of as-cast Ti44Al6Nb1.0Cr2.0V alloy

The nominal Ti44Al6Nb1.0Cr2.0V alloy was newly designed and prepared by vacuum consumable melting technique with the ingot sizes of d225 mm×320 mm. The results show that the average lamella colony size is 780–1830 μm. This as-cast alloy has a modified near lamellar (M-NL) structure that is composed of mainly larger (α2+γ) lamella colonies and smaller (B2+equiaxed γ) blocky morphology. It exhibits the moderate tensile properties at room temperature, in which the Region (5) yields the ultimate tensile strength (UTS) about 499 MPa and the elongation about 0.53%. The obvious brittle fracture characteristics and trans-granular interlamellar fracture are the predominant modes. After room temperature tensile testing, there are some <101] and a few ½<112] superdislocations in the γ phase. The as-cast microcrack is the main factor to deteriorate the tensile property, which results in the premature fracture, poor ductility and few dislocations. The addition of Nb, Cr and V can decrease stacking fault energy (SFE) obviously, which is helpful to enhancing the ductility of the alloy.

The Effects of Cr and Mo on the Microstructure and Mechanical Properties of As-Cast TiAl Alloys

The effect of the alloying elements of Cr and Mo on the microstructure and mechanical properties of as-cast TiAl alloys produced by a locally made arc-melting furnace was studied. X-ray diffraction (XRD) was used to identify the phases present in the samples. The microstructure of the TiAl samples was characterized using scanning electron microscopy combined with energy dispersive spectroscopy (EDS). Compression tests were carried out at room temperature using an Instron servohydraulic testing machine. The results show that the Ti-48Al alloy exhibited a duplex microstructure, whereas with the addition of Cr a nearly lamellar microstructure was observed in Ti-48Al-2Cr and with the addition of both Cr and Mo also in Ti-48Al-2Cr-2Mo. The hardness values increased slightly as compared to the Ti-48Al alloy with the addition of the alloying elements. The presence of Cr in Ti-48Al-2Cr resulted in a slight increase in compressive fracture strain. The as-cast Ti-48Al-2Cr-2Mo alloy produced a higher yield strength and fracture strain in compression as compared to the other as-cast TiAl alloys. On the fracture surfaces of the as-cast TiAl alloys, mixed brittle transgranular and interlamellar fracture modes were predominantly observed.

Microstructural characterisation of fatigue crack growth fracture surfaces of lamellar Ti45Al2Mn2Nb1B

Investment cast Ti45Al2Mn2Nb1B with fine lamellar microstructures was subject to fatigue crack propagation testing at 650 °C and a stress ratio of R = 0.1. The fracture surfaces were examined under SEM and the observed features are correlated with both stress intensity range (ΔK) and lamellar orientation. Translamellar fracture is primary fracture mode for most of the lamellar orientations. Interlamellar fracture is influenced by a combination of the ΔK and lamellar orientation. At low ΔK only the lamellar colonies with their lamellar interfaces almost perpendicular to the stress axis fractured via interlamellar fracture mode. At high ΔK interlamellar fracture can occur in lamellar colonies with any orientations.

Microstructure and Mechanical Properties of a Rapid Directionally Solidified Ti47Al Alloy

The ingot of Ti47Al alloy was prepared by a newly developed rapid directional solidification, the microstructure and compressive properties of the ingot was observed and tested. The results show that the macrostructure consisted mainly of coarse columnar grains parallel to the axial direction, with the size of 0.5mm wide and 10mm in length. The direction of lamellar is almost perpendicular to the growth direction in the longitudinal section and no dendritic core is found. The average ultimate compressive strength of the specimens with grain growth parallel/perpendicular to the compressive direction is 1233.3 and 861.7 MPa respectively. The fracture mode for specimens with grain growth parallel to the compressive direction exhibits predominantly translamellar fracture, however, that for specimens with grain growth perpendicular to the compressive direction exhibits predominantly delamenation or interlamellar fracture.

Fracture process and fracture mechanism in fully lamellar TiAl alloys with tensile tests

By means of in-situ tensile tests of fully lamellar TiAl based alloys, the analysis of experiment data, scanning electron microscope (SEM) S-520 observation of fracture surface, the tensile fracture process and fracture mechanism of TiAl alloys are investigated. The result of in-situ tensile experiment reveals that cracks are initiated directly from the notch root for the straight notch specimens, and the fracture mechanism is that the main crack is initiated, propagated and connected, when the main crack extends to a length, which acts as a Griffiths crack and matches the loading stress which the crack propagates catastrophically through entire specimen. Cracks initiate at interfaces between lamellae and lamellar interface is the most weakest part. Fracture surface is composed of the trans-lamellar fracture and inter-lamellar fracture, trans-lamellar fracture is dominant. For flat in-situ tensile specimen or those with a big circle arc gauge at a higher applied load, a number of large cracks are produced along lamellar interfaces within the gauge-limited volume of tensile specimens. With increasing applied load, microcracks propagate and connect, the final crack develops at a most heavily damaged section within the gauge-limited volume by passing through the remaining ligaments among the interlamellar cracks. For most specimens, fracture process is the crack direct initiation process. Once a crack is produced, the specimen will fracture. It is also to say the strength of the material is very higher, and crack initiation and damage are more difficult.