Basketweave Microstructure Research Articles

Microstructural evolution and mechanisms in additively manufactured AlCrCuFeNix complex concentrated alloys via selective laser melting

This article discusses the influence of the Ni content on the microstructures of AlCrCuFeNix complex concentrated alloys prepared by selective laser melting. The microstructural evolution mechanisms were also uncovered. It is found that the as-built Ni1.0 alloy possesses equiaxed-columnar bimodal grain structures and basket-weave microstructure consisting of striped body-centered cubic (bcc) nano-precipitates and ordered bcc (B2) phase. Adding Ni triggers the lattice transition from B2 to face-centered cubic (fcc), leading to the change of the primary solidification phase from B2 to fcc beyond the critical content of Ni. Meanwhile, the striped bcc nano-precipitates inside the B2 lamellae gradually transform into spherical morphologies due to the consumption of Fe and Cr solutes in eutectic reaction. Furthermore, the microstructures change from hypo-eutectic (Ni2.0, Ni2.5) to near-eutectic (Ni3.0) to hyper-eutectic (Ni3.5), accompanied by a columnar-to-equiaxed transformation due to the large constitutional supercooling induced by the eutectic reaction.

Characterization of Microstructure and Mechanical Properties of Ti-6Al-4V Alloy after Cyclic Heat Treatment

A cost-effective approach which consists of cyclic heat treatment (CHT) and annealing is used to refine the coarse lamellar structure of as-cast Ti-6Al-4V alloy. The coarse lamellar microstructure was significantly refined after the CHT process, α lamellae were broken up while the original orientations of α lamellae were maintained. After annealing treatment, a basketweave microstructure was achieved in Ti-6Al-4V alloy, and the formation of globular α grains was observed. The microstructure refinement mechanisms during CHT and annealing are mainly considered to be break-up of lamellar structure and static recrystallization. The room temperature mechanical properties of Ti-6Al-4V alloy were significantly improved. The microhardness of Ti-6Al-4V alloy increased from 306 HV to 402 HV. The values of yield strength (YS) and ultimate tensile strength (UTS) increased from 734 MPa and 740 MPa to 911 MPa and 1010 MPa, respectively. The value of ductility increased from 1.1 % to 3.3 %.

Zoning study on the fatigue crack propagation behaviors of a double-sided electron beam welded joint of TC4 titanium alloy with the thickness of 140 mm

This study investigated fatigue crack propagation rates in the base metal (BM), heat affected zone (HAZ) and weld metal (WM) of a double-sided electron beam welded (EBW) joint of Ti alloy with a thickness of 140 mm in different layers. Firstly, this research measured fatigue crack propagation rates at different zones in each layer of the joint and drew curves thereof. The results demonstrate that the WM in the second layer of the double-sided EBW joint shows poor resistance to fatigue crack propagation. Secondly, the metallographic microstructures of different zones in different layers of the joint were observed. It is found that there are a lot of needle-shaped martensites growing in a specific direction at the HAZs in different layers of the joint. Finally, precipitates in different zones in each layer of the joint were studied by using a high-resolution scanning electron microscope (SEM). The results show that needle-shaped martensites in the WM of the joint are interlaced and present basketweave microstructures, while there are needle-shaped, lamellar martensites at the HAZs in each layer of the joint.

The effect of bonding temperature on the bending behaviors and toughening mechanism of W/(Ti/Ta/Ti) multilayer composites prepared by field activated sintering technique

The gradient layered structure is beneficial to improve both strength and fracture toughness of tungsten. In this study, a series of W/(Ti/Ta/Ti) multilayer composites with gradient layered structure are prepared by field activated sintering technique (FAST) with bonding temperatures ranging from 1000 °C to 1400 °C. The results show that both the microstructure and mechanical properties vary with bonding temperatures. Both W and Ta diffuse into Ti layer to stabilize β phase, resulting in the Ti layer with α/β phase. The W/(Ti/Ta/Ti) composites bonded at 1200 °C exhibited the highest flexural strength (1700 MPa) attributed to excellent microstructure combination in the different layers, which are elongated grains and fine recrystallized grains in the W layer, and the martensite basketweave microstructure in Ti layer. Shear bands can be seen in the Ti layer with planar slip-bands and Ta layer with wavy slip-bands. These plastic deformation behaviors cannot be observed in the W layer because it is inherently brittle, similar to ceramic. The toughening mechanism of the W/(Ti/Ta/Ti) composites is as follows: crack deflection and delamination between the interface, multi-crack propagation in the W layer, and local shear deformation in the toughened layer. The bending properties of W/(Ti/Ta/Ti) multilayer composites are related to the interface and the gradient structure of the toughened layer (Ti/Ta/Ti), which can guide the design of the microstructure of the composites and improve its mechanical properties.

Microstructure and Mechanical Properties of Ti-6Al-4V Fabricated by Electron Beam Melting

Electron beam melting technique is a kind of near-net shaping, environmentally friendly metal additive manufacturing technique, which can form high-performance metal parts with complex shapes. It has been widely applied in the aerospace industry, biomedical application, automobile manufacturing, and other fields. Ti-6Al-4V is the most widely researched and applied alloy in the additive manufacturing field, but its microstructure is diverse, and its mechanical properties vary greatly. In this study, the effect of process parameters on the microstructure and the resulting mechanical properties of Ti-6Al-4V alloy was researched. The results show that the yield strength of Ti-6Al-4V alloy with a bimodal microstructure is higher than those with a basketweave microstructure. Energy dispersion spectrum (EDS) line scan and area scan results show that there is no element enrichment for the specimens with the highest yield strength. A speed factor of less than 40 is a must for obtaining relatively dense Ti-6Al-4V parts built with electron beam melting. We have done basic research for the subsequent manufacturing of complex shape parts, which is helpful to promote the application of electron beam melting technology in the aerospace field.

Five-parameter characterization of intervariant boundaries in additively manufactured Ti-6Al-4V

Additive manufacturing has emerged as a promising route to fabricate complex-shaped Ti-6Al-4V parts. The microstructural evolution and variant selection across builds in response to different printing strategies processed by electron beam powder bed fusion has been previously clarified. However, a detailed knowledge of the grain boundary plane characteristics of the α-α intervariant interfaces is still missing. The aim of this study was to reveal the full ‘five-parameter’ crystallographic characteristics of the intervariant boundaries. The most common α-α intervariant for colony and basketweave microstructures was 60°/[1 1 2¯ 0], while in the acicular microstructure, the maximum was at 63.26°/[10¯ 5 5 3¯]. This is discussed in terms of self-accommodation during the β to α phase transformation, and the degree of coherence of the α laths in the as-deposited condition and during further growth. The grain boundary plane distributions reveal a high tendency for intervariant boundaries to terminate on prismatic and pyramidal planes rather than on low-energy basal planes. This suggests that, during additive manufacturing of Ti-6Al-4V and irrespective of the α morphology, the crystallographic constraints imposed by the Burgers orientation relationship determine the boundary plane distribution characteristics.

Crack initiation mechanism and fatigue life of titanium alloy Ti–6Al–2Sn–2Zr–3Mo-X: Effects of stress ratio and loading frequency

In this paper, the fatigue behavior of Ti–6Al–2Sn–2Zr–3Mo-X alloy with basketweave microstructure is investigated at stress ratios R = −1, −0.5, 0.1 and 0.3 and loading frequencies f = 0.4 Hz, 4 Hz and 35 Hz. It is shown that the Ti–6Al–2Sn–2Zr–3Mo-X alloy presents two different crack initiation mechanisms. One is due to the cleavage of α grains and the other is due to the microstructure inhomogeneity and deformation incompatibility. For the former case, the fracture surface exhibits facet feature in the crack initiation region. For the latter case, the crack nucleates and initiates at α grains or interfaces and the fracture surface presents no facet feature in the crack initiation region. The higher stress ratio and the lower frequency tend to induce the higher probability for the occurrence of multiple crack initiation mode. Moreover, the paper indicates that the effect of stress ratio on fatigue life could be expressed as Nf = A [(1–R)/2]lσma,R, where A, l and m are material parameters. The scatter of fatigue life is related to both the stress amplitude and the loading frequency. For relative higher stress amplitude, the fatigue life and the frequency at R = −1 could be correlated by a linear relation in log-log scale.

Formation and evolution of new α grain boundary and its influence on globularization of α lamellae in TC17 alloy

Formation and evolution of new α grain boundary and its influence on globularization behavior of α lamellae in TC17 alloy with a basketweave microstructure during elevated temperature deformation and subsequent heat treatment are quantitatively investigated using electron backscatter diffraction (EBSD) technique. The results show that the volume fraction of recrystallized α grains firstly increases and then decreases with increasing strain rate and height reduction at a deformation temperature of 740 °C. The increasing annealing temperature is beneficial for fragmenting α lamellae into nearly equiaxed grains due to enhanced termination migration. Based on the analysis of the distribution of in-grain misorientation axe and Schmid factor, it is concluded that the operating slip systems in the α lamellar interiors are dependent on the strain rate, height reduction and heat treatment conditions. The formation and evolution of new α grain boundary during the globularization process are described as follows: (i) the loss of coherency of α/β interphase and the process of dislocation accumulation; (ii) the rotation of different parts in α lamellae caused by activated slip system; (iii) the increase of misorientation between α grains and the formation of LABs; (iv) the formation of HABs; (v) separation of grain boundary; (vi) completion of globularization process via the termination migration.

Improved tensile properties of a novel near-α titanium alloy via tailoring microstructure by hot-rolling

Novel near-α titanium alloy sheets with improved tensile properties at room temperature and 650 °C were fabricated by hot-rolling. When rolled at 1000 °C, the sheet exhibits a duplex microstructure. When the rolling temperature is above 1025 °C, the basket-weave microstructure is obtained. Compared with the as-forged alloy, the intragranular microstructures of sheets are obviously refined. With increasing the rolling temperature, both volume fraction and mean size of silicieds decrease, and these silicides disappear completely when rolled at 1100 °C. The tensile strengths at both room temperature and 650 °C are significantly improved by hot-rolling. The UTS of all sheets at room temperature exceeds 1300 MPa. When rolled at 1100 °C, the UTS and elongation at 650 °C are 711 MPa and 24.4%, respectively. The strengthening mechanism at both room temperature and 650 °C is mainly attributable to the refined intragranular α platelets. In addition, the micro-voids are mainly induced by decohesion of the silicide/matrix interface at room temperature but by matrix damage at 650 °C.

Investigation of cracks during electron beam welding of γ-TiAl based alloy

Abstract Electron beam welding (EBW) of γ-TiAl intermetallic with duplex microstructure has been conducted with various welding parameters. Experiments and numerical simulation were investigated to reveal the cracking mechanism of the joint. The weld undergoes the martensitic transformation via β→α evolution, resulting from the ultra-fast cooling rate, which eventually results in the formation of α2-Ti3Al dominated basket weave microstructure with poor ductility at low temperatures. The brittle microstructure can hardly deform during the cooling process. The simulation results indicate high-level longitudinal residual tensile stress distributed within the weld zone, which accumulates gradually and even exceeding the tensile strength of the joint. The synthetic effect of brittle microstructure and high residual stress promotes the initiation of cracks. Based on this, an original criterion, initial crack generation time of EB welded γ-TiAl joint, is proposed to evaluate the crack sensitivity of the joint and provide guidance for the improvement of welding process. The EB reciprocating scanning process is adopted to extend the residence time of the joint above the initial crack generation temperature. Crack-free joints with the microstructure of bulk γ phase and the α2/γ lamella colonies are obtained, which dramatically ameliorates the ductility of γ-TiAl joints.

Effects of triple heat treatment on microstructure and the dry sliding wear of TC21 titanium alloy

Triple heat treatment was applied to TC21 titanium (Ti) alloy, and microstructural changes therein were analysed by using an optical microscope (OM). The dry sliding wear properties of TC21 Ti alloy were assessed by conducting “pin-on-disk” friction wear tests under a load of 200 N, at a sliding speed of 0.230 m s-1, for a sliding time of 600 s, at room temperature (25 °C). Using scanning electron microscopy (SEM), morphologies of the worn surface and wear mechanism were analysed. Results show that, by performing solid solution treatment to the β phase region of the TC21Ti alloy, a single-phase β micro structure was obtained. On this condition, the wear rate is the highest, appearing as abrasive wear and adhesive wear. Through one-time high-temperature aging and low-temperature aging in the α+β phase region, the basket-weave microstructure of TC21 Ti alloy was obtained, with a low wear rate, appearing as abrasive wear and slight oxidation wear in terms of prevailing wear mechanisms.

Quantitative analysis of interleaved degree in lamellar microstructure of titanium alloys

To establish the relationship between mechanical properties and microstructural features, it is necessary to comprehensively describe microstructural characterizations. In present study, an efficient and innovative quantitative method of interleaved degree in lamellar microstructure was developed. Firstly, four types of microstructural variables were defined as key factors that confirmed the microstructural features. Lath length (L) and variation coefficient for distribution of lath angle (CV) were measured to obtain dimension information of laths, and number of lath intersecting points per area (N) and uniformity of lath intersecting points (U) were defined to describe the lath distribution and the lath intersection complexity. Then, the microstructural variables were normalized based on multiple attribute decision-making. Finally, the equation of interleaved degree in lamellar microstructure was established based on entropy weight method. Present quantitative method was applied to analyze the interleaved degree of TC17 alloy containing a basketweave microstructure. It was shown that interleaved degree was higher at a deformation temperature of 930 °C, a height reduction of 40% and a strain rate of 0.1 s−1 and decreased with increasing cooling time. Besides, a model that can theoretically calculate fracture toughness based on tensile properties was utilized to prove the prediction ability of interleaved degree.

Effect of strain rate on α-lath thickness of TC17 alloy after deformation and subsequent heat treatment

Effect of strain rate on α-lath thickness of TC17 alloy with a basketweave microstructure was studied in the present work. For this purpose, this alloy was deformed in the β phase region and subsequently soluted and aged in α+β phase region. Moreover, optical micrograph (OM) and electron backscatter diffraction (EBSD) were applied to analyze the change of lath thickness at different strain rates. The result showed that α-lath thickness increased with increasing strain rate. This phenomenon was possibly attributed to the higher degree of variant selection (DVS) at higher strain rate (0.1 s-1). The higher DVS was beneficial for the formation of parallel α-lath colonies during cooling after deformation. And, these parallel α-lath colonies would more easily grow up and coarsen during subsequent heat treatment. Therefore, α-lath at higher strain rate is more thick.

Texture of electron beam melted Ti-6Al-4V measured with neutron diffraction

Texture in materials is important as it contributes to anisotropy in the bulk mechanical properties. Ti-6Al-4V built with the additive manufacturing process (AM) electron beam melting (EBM) has been found to have anisotropic mechanical properties. Therefore, this work has been performed to investigate the texture variations of EBM built Ti-6Al-4V with neutron time of flight (TOF). For the work, samples were produced with different build geometries off-set by 90 degrees. A cast sample was additionally analyzed to investigate the bulk texture of conventionally manufactured material. Microstructural characterization was performed and the cast material was found to have a coarse colony α microstructure, whereas the EBM built material had a finer basket weave microstructure. Overall, the texture of the EBM built material was found to be weak having an multiple of random distribution (MRD) index of ~1 for the α phase, whilst the cast material possessed more than twice the amount of preferred orientation i.e. MRD 2.51 for the α phase.

The high-temperature deformation behavior of a novel near-α titanium alloy and hot-forging based on the processing map

Limited by the composition, the service temperature of high-temperature titanium alloys have been restricted to 600 °C for many years. In this study, a self-designed Ti–6Al–3Sn–10Zr-(Mo, Nb, W, Si) near-α titanium alloy that can be used at 650–700 °C was fabricated by vacuum arc remelting (name as DsTi700). Isothermal compression tests were performed at 975–1100 °C and in the strain rate range of 0.01–1 s−1. The results indicate that the flow behaviors of DsTi700 alloy under high strain rate are different from that of some traditional near-α titanium alloys and the peak stress values are generally higher than those of some common near-α titanium alloys at the same/similar deformation conditions. The deformation activation energy Q in β phase field is 240.2 kJ/mol. Moreover, the processing map were developed to determine the optimal processing window. Finally, DsTi700 alloy pancake with basket-weave microstructure was successfully prepared by multi-pass hot-forging. Tensile tests exhibit that the elongation of DsTi700 alloy at room temperature is nearly 13%, the UTS exceeds 600 MPa at 650 °C, which is almost equal to the UTS of some traditional near-α titanium alloys at 600 °C. Moreover, the UTS and elongation of DsTi700 pancake are 511 MPa and 24.1% at 700 °C.

Microstructure design and mechanical properties of annealed TiZrAlB alloys

The microstructure evolution mechanisms and the effects of microstructure on the mechanical properties of a TiZrAlB alloy following different annealing processes were investigated. Globularization occurred, which was promoted by the annealing time (ATI) when annealed at a temperature below the α-transus temperature due to boundary splitting and termination migration mechanisms. Due to the diffusion behavior of the elements, the globular structure coarsened with the ATI. As the annealing temperature (ATE) increased, a duplex microstructure with grain boundaries α (αGB) + acicular secondary α phase appeared, while, when the ATE exceeded the β-transus temperature, a basket-weave microstructure appeared. The fine duplex microstructure and the globular structure exhibited the highest strength and the best combination of strength and ductility, respectively. In addition, the globularization, nucleation, and growth mechanisms of annealed α-type Ti alloys were also discussed in detail.

Distribution of Al Element of Ti–6Al–4V Joints by Fiber Laser Welding

In the process of laser welding, the uneven distribution of solute elements caused by element burning loss and flow of molten pool affects the quality of joints. In this paper, butt welding experiments were conducted on the 3 mm thick Ti–6Al–4V specimens with different preset ratios of Al and Si powders by using 4 kW fiber laser. The distribution of Al solute element and its influence on the microstructure and mechanical properties of the final weld joint were investigated. The results showed that the self-diffusion of Al element and the flow of molten pool affects the alloy elements distribution in laser welding. And the microhardness of the welded joint with Ti–6Al–4V and 90% Al + 10% Si powders was significantly higher than that with only Ti–6Al–4V, with the difference of about 130HV. At the same time, in the joint with 90% Al and 10% Si powders, the acicular α’ size was finer, and basketweave microstructure was present as well. This research is helpful to better understand the distribution of Al solute element and its influence on the joint quality during laser welding of Ti–6Al–4V alloy, which provides a certain reference for improving the weld or surface properties of Ti–6Al–4V alloy during laser processing.

Effect of Rise and Fall Time on Dwell Fatigue Behavior of a High Strength Titanium Alloy

Frequency is an important factor influencing the fatigue behavior. Regarding to the dwell fatigue, it corresponds to the effect of rise and fall time, which is also an important issue especially for the safety evaluation of structure parts under dwell fatigue loading, such as the engines of aircrafts and the pressure hulls of deep-sea submersibles. In this paper, the effect of rise and fall time (2 s, 20 s, 110 s, and 200 s) on the dwell fatigue behavior is investigated for a high strength titanium alloy Ti-6Al-2Sn-2Zr-3Mo-X with basket-weave microstructure. It is shown that the dwell fatigue life decreases with increasing the rise and fall time, which could be correlated by a linear relation in log–log scale for both the specimen with circular cross section and the specimen with square cross section. The rise and fall time has no influence on the crack initiation mechanism by the scanning electron microscope observation. The cracks initiate from the specimen surface and all the fracture surfaces present multiple crack initiation sites. Moreover, the facet characteristic is observed at some crack initiation sites for both the conventional fatigue and dwell fatigue tests. The paper also indicates that the dwell period of the peak stress reduces the fatigue life and the dwell fatigue life seems to be longer for the specimen with circular cross section than that of the specimen with square cross section.

Hot compressive deformation behavior and microstructure evolution of a Ti-55511 alloy with basket-weave microstructures

The hot deformation behavior and microstructural evolution of a Ti-55511 alloy in α+β region are investigated by uniaxial compressive tests at elevated temperatures. Transmission Electron Microscope (TEM) and Electron Backscattered Diffraction (EBSD) are employed to observe the microstructures. The spheroidization of lamellate α phases and the dynamic recrystallization (DRX) of β grains are analyzed. It is observed that the flow characteristics are prominently affected by deformation parameters, i.e., the flow stress, peak stress and flow softening stress decrease with the reduced strain rate or the increased deformation temperature. When the deformation temperature is slightly raised or the strain rate is properly reduced, the fraction of spheroidized α phase greatly increases. The substructures are formed in the lamellate α phase, and the β phase wedges into the lamellate α phase. Then, the lamellate α phase is fragmentized and spheroidized. The DRX of β grains is promoted with the raised strain rate or the reduced deformation temperature. Additionally, the higher dislocation density and obvious dislocation pile-ups can be noticed at the α/β interface, which induces the DRX nucleation of β grains at the α/β interface.

Weld Quality Assessment in Fiber Laser Weldments of Ti-6Al-4V Alloy

Laser welding experiments are performed on Ti-6Al-4V alloy sheets by adopting fiber laser. A special kind of workpiece fixture is designed and fabricated for providing shielding gas. After experiments, penetration depth, fusion zone width and heat-affected zone size at different locations within weld bead are measured and discussed in detail. Influence of line energies on the formation of non-uniform microstructure within weld bead is explored by conducting microstructural analysis. Various kinds of microstructural morphology of martensitic structure such as α′ martensite, blocky α, massive α and basket-weave microstructure are found in fusion zone. Experimentally, it is found that beam power is the key parameter for controlling penetration depth. Higher hardness is noticed within fusion zone due to the existence of large volume of α′ martensite. Tensile strength and hardness of welded specimens are increased with decreasing line energy. Small amount of micropores are also found in solidified weld bead. However, its sizes are in acceptable range as per BSEN:4678 standard. Most favorable welding condition is identified as a combination of beam power of 800 W and welding speed of 1000 mm/min which yields full penetration, narrower bead width, small heat-affected zone, minimal defects and acceptable mechanical properties.