Full Lamellar Structure Research Articles

The newly discovered competitive growth behavior of γ and α2 phases

This study reports a new phenomenon of competitive growth between TiAl alloy structures. By annealing the β-solidified Ti-44Al-8Nb-0.1B alloy for a short time, a new phenomenon of repeated competitive growth between α2 and γ phases is found. Finally, the (α2 + γ) lamellar structure engulfs other structures and grows, transforming into a full lamellar structure.

Synergistic control of microstructures and properties in eutectic high-entropy alloys via directional solidification and strong magnetic field

The AlCoCrFeNi2.1 eutectic high-entropy alloy (Ni2.1 EHEA), as an exemplary representative of the high-entropy alloy family, has garnered significant research attention owing to its exceptional comprehensive properties. In this study, we investigated the influence of various growth velocities on the microstructure, lamellar spacing, and mechanical properties of the Ni2.1 EHEA. We observed that at lower growth velocities, the structure consisted of an alternating face-centered-cubic (FCC) phase and B2 phase lamellae aligned in a single direction, with the lamellae orientation parallel to the direction of the heat flow. The yield strength increased with the growth rate, while the ultimate tensile strength decreased with increasing the growth velocity. Ductility remained relatively consistent, and a double yield phenomenon was observed in the elastic-plastic deformation region. Under conditions of high growth velocities, the microstructure transitioned from a single-directional full lamellar structure to a multi-stage lamellar arrangement. The most favorable comprehensive mechanical properties were achieved at a growth rate of 200 μm/s, resulting in a yield strength of 450 MPa, an ultimate tensile strength of 1092 MPa, and a remarkable ductility of ∼32% in the directionally solidified samples—double that of the arc-melted sample. The evolution law of directional solidification structures under the coupling effect of different magnetic fields and different growth rates was studied. The interaction of the thermoelectric-magnetic force and thermoelectric-magnetic convection and the potential mechanism of microstructure evolution under the effect of magnetic field were deeply analyzed. The results reveal that at a growth rate of 2 μm/s, the spacing between eutectic layers decreases as the magnetic induction intensity increases, leading to the transformation of some regular layers into spherical layers. Similarly, at a growth velocity of 10 μm/s, the eutectic structure exhibited a Columnar-to-equiaxed transition (CET). However, as the growth rate further increases, the limited exposure time to the magnetic field prevented significant structural changes.

Mechanism of impact toughness enhancement obtained by globularization of αGB phase for selective laser melted Ti–6Al–4V alloy

The low impact toughness of selective laser melted (SLM) Ti–6Al–4V (Ti64) is a main factor limiting its widespread application in engineering. In this work, we've greatly improved its toughness by modifying the morphology of the α phase via annealing. Specifically, the as-built sample had a toughness of 14.87J/cm2. Annealing at 800 °C for 4 h boosted the toughness to 25J/cm2. Notably, annealing at 950 °C for 4 h increased the toughness by a remarkable 90 %, reaching an impressive 36.88J/cm2, which is comparable to its wrought counterpart. Particularly, 950 °C annealing induces the globularization of αGB phase (α phase along prior β grain boundary) while maintains the lamellar morphology of the α phase within the prior β grain. This mixed structure significantly outperforms the full lamellar structure in the 800 °C annealed sample in terms of toughness. The mechanism of toughness enhancement differs from the traditional one where thick lamellar α phase improves the toughness by increasing the crack path's tortuosity. Our study indicates that more deformation twins are activated in 950°C-annealed samples in impact test, which effectively absorb additional impact energy and improve toughness. In the 800°C-annealed sample, only a limited number of grains with specific orientations can activate deformation twins. Whereas in the 950°C-annealed sample, a broader range of grain orientations demonstrated the capability to activate deformation twins. The mechanism for twin activation in the 950°C-annealed sample is proposed. It is associated with equiaxed αGB, inducing significant interface deformation accompanied by stress concentration. This ultimately leads to the formation of the deformation twins.

Thermomechanical characterization and conventional rolling of a novel β-solidifying γ-TiAl alloy with excellent comprehensive properties

The development of the β-solidifying γ-TiAl alloys with good oxidation resistance and thermal deformation ability has always been crucial for the practical application. However, increasing the content of the antioxidant elements, such as Nb, Al, etc., tends to be detrimental to the deformability of the alloys. In this work, a novel Nb, Mn containing β-solidifying TiAl alloy with the nominal composition of Ti-43Al-1.5Mn-3Nb-0.2C-0.2Si-0.1B (at. %) (named as NbMn-TiAl) is invented, which goes through the single β phase as well as the α phase during solidification, resulting in the final as-cast microstructure of nearly full lamellar structure (NFL). Subsequently, the isothermal compression tests were performed within a temperature range of 1100 °C–1300 °C and a strain rate range of 0.001 s−1-10 s−1, the hot processing maps and constitutive equation were established. Moreover, the deformation mechanisms were clarified, including the bending/kinking of the α2/γ lamellar colonies, the dynamic recrystallization (DRX) of γ phase and α phase at different temperatures. Rolled bars with a diameter of 12 mm were then directly fabricated from the ingot by conventional hot rolling process. The as-rolled bars exhibited excellent oxidation resistance and mechanical properties that outperform many available TiAl alloys manufactured using various methods.

Preparation of Ti-46Al-8Nb Alloy Ingots beyond Laboratory Scale Based on BaZrO3 Refractory Crucible

The high Nb-containing TiAl-based alloy ingot beyond laboratory scale with a composition of Ti-46Al-8Nb (at.%) was prepared by a vacuum induction melting process based on a BaZrO3 refractory crucible. A round bar ingot with a diameter of 85 mm and a length of 430 mm was finally obtained, and the chemical composition, solidification pathway, microstructure and tensile properties of the ingot were investigated. The results show that the deviations of Al and Nb content along a 430 mm long central part of the ingot are approximately ±0.39 at.% and ±0.14 at.%, and the oxygen content in the ingot can be controlled at around 1000 ppm. The structure of the alloy ingot is a full lamellar structure composed of γ and α2 phases, and the thickness of the lamellae is approximately 0.53 μm. In case of the α2 phase, the surface content of the ingot is higher than the middle region and the centrical region; also, it indicated a decreasing trend. During cooling, the alloy solidified from a peritectic reaction (L + β→α) rather than the solidified via β phase (β→α). In addition to Al segregation and Nb segregation, β-phase particles associated with γ phase at the triple junction of the colonies were observed. Moreover, the tensile properties of the longitudinal-cut sample in the ingot is significantly better than those of the transverse-cut sample, with a tensile strength of up to as high as 700 MPa and a corresponding fracture elongation of 1.1%. However, the tensile strength of the transverse-cut sample is only 375 MPa, and the fracture elongation is 0.52%.

Three-dimensional lamellar orientation and stress condition induced sudden fracture of directional solidified γ-TiAl alloys produced by electromagnetic confinement

Ф 16.5 mm directional solidified Ti-45Al-5Nb alloys with full-lamellar microstructure were successfully obtained through electromagnetic confinement and seed crystal methods. Tensile experiments at ambient temperature were performed to evaluate the industrial processability of the alloy. Although the angle between the full-lamellar structure and the applied stress is approximately 0°, the elongation is still extremely low. The fracture morphology and internal microstructure were observed from multiple directions. Combined with external mechanical stress condition, the reason for sudden fracture was discovered and the lamella orientation control plan is proposed based on different specimen conditions. The cognition of the relationship between the lamellar orientation and mechanical performance was supplemented and expanded on the three-dimensional scale.

Phase selection and mechanical properties of directionally solidified AlCoCrFeNi2.1 eutectic high-entropy alloy

Recent studies report that eutectic high entropy alloys (EHEA) which possess both high strength and high ductility have potential industrial applications. In the present work, the solidification behaviors and mechanical properties of directionally solidified AlCoCrFeNi2.1 EHEA obtained at different growth velocities are investigated. The microstructure of the as-cast AlCoCrFeNi2.1 EHEA is composed of bulky dendrites (NiAl phase) and lamellar eutectic structures which consists of the CoCrFeNi (FCC) phase and the NiAl (BCC) phase. Although the actual composition of the alloy is shown to slightly deviate from the eutectic point, it is interesting to observe that the full lamellar structure of this alloy is obtained through directional solidification. In order to explain this contradiction, the maximum interface temperature criterion and the interface response function ( IRF) theory are applied to calculate the velocity range of the transition from the primary phase to the eutectic, which is 1.2 µm/s-2 × 104 μm/s. Furthermore, the tensile test indicates that the directionally solidified AlCoCrFeNi2.1 EHEA possesses a good combination of strength (1340 Mpa) and ductility (30.5%) at 100 µm/s, which can be attributed to the full lamellar eutectic structure after directional solidification. In addition, the compression tests demonstrated that the compressive strength of the axial specimen is greater than the radial specimen at the same growth velocity. Thus the directional solidification can effectively adjust the alignment of the eutectic structure and achieve better mechanical properties along the growth direction.

Grain Refinement of Co-Cr-Mo-C Through Plastic Deformation Followed by Reversion of Lamellar Eutectoid Structure

Small plastic straining was combined with a two-step heat treatment to obtain grain refinement of the Co-28Cr-6Mo-0.33C alloy. The solution-treated specimens were furnace cooled to room temperature, obtaining partially transformed pearlite at grain boundaries. Afterward, the specimens were compressed up to 10% of engineering strain, followed by aging at 850 °C that contributed to the decomposition of the austenite phase into a lamellar eutectoid structure (α + M23C6). The full lamellar structure was then reverse-treated at temperatures from 1000 to 1237 °C, where the austenite phase was stable. The reversion treatment led to the nucleation of a fine-grained austenitic structure (with average size of 48.23 ± 21.30 μm—i.e., about 1/10 of its initial average size) at the lamellar eutectoid structure. Compression tests carried out on reverse-transformed samples showed better mechanical properties compared to those of the samples tested before reversing transformation.

Low temperature heat treatments on Ti-15Zr-xMo alloys

This paper regards the study of the effect of heat treatments on the microstructure and microhardness of Ti-15Zr-xMo alloys aimed to biomedical applications. The alloys were melted in an arc-furnace, with four different compositions in order to obtain samples with α′, α′+β and β phase microstructures. The as-cast alloys exhibited a full lamellar structure for the Ti-15Zr, a bimodal structure composed by β phase grains along with an α phase acicular structure at the grain boundaries for the Ti-15Zr-5Mo, predominant β phase for the Ti-15Zr-10Mo and only β phase for the Ti-15Zr-15Mo. Results showed that both microstructure and mechanical properties of the studied alloys can be tailored by relatively low temperature heat treatments, as they resulted in the growth of the α′ phase in all alloys. The formation of α′ phase resulted in a decrease in the microhardness of the alloys.

Effect of vapor grown carbon fiber content on microstructure and tensile properties of Ti64/TiC composite fabricated by powder metallurgy method

Ti-6%Al-4%V (Ti64) alloy powder coated with vapor grown carbon fibers was consolidated to fabricate titanium matrix composites by spark plasma sintering, followed by hot extrusion process. Four compositions of the additive vapor grown carbon fibers were 0.1, 0.2, 0.3 and 0.4 wt. %. The microstructure was changed from the full lamellar of monolithic Ti64 alloy to the bimodal structure by addition of vapor grown carbon fibers to Ti64 alloy. The changing of microstructure was attributed by an α-stabilize effect of carbon (vapor grown carbon fiber). Almost all the vapor grown carbon fibers reacted with Ti and result in formation of Ti6C3.75 carbide phases detected by x-ray diffractometer and energy dispersive spectrometer. Hardness was improved by effect of solid solution strengthening of carbon in the Ti matrix and reached the maximum value of 495.8 Hv. 0.2%YS and UTS were significantly increased by addition of 0.1 wt. % vapor grown carbon fiber. However, they were not much improved when more vapor grown carbon fiber content was applied. Ductility of the sample was affected by a change in microstructure when 0.1 wt. % of vapor grown carbon fibers was added because the bimodal structure shows higher ductility compared to the full lamellar structure. The carbide precipitation in TMCs was not much contributed in improving of strength and decreased a ductility of material.

Reaction-Diffusion Bonding of High Nb Containing TiAl Alloy

Abstract In this study, high Nb containing TiAl alloy (TAN) was welded by reaction-diffusion bonding using Al foil as interlayer at 1200 °C for 2 h. The desired full lamellar microstructure ( γ +α 2 ) was obtained across the bonding seam when the joint was heat-treated according to the heat treatment process of TAN alloy. The present paper also investigated microstructure and bonding mechanism in details. The results show that liquid Al can react with TAN to form TiAl 3 intermetallic layer, and then the TiAl 3 layer will transform to γ -TiAl by solid-state diffusion at high temperature, finally a full lamellar structure is formed after heat treatment. Furthermore, sound joints with full lamellar structure can be also obtained when TAN alloy is directly bonded in accordance with the heat treatment cycle of TAN alloy.

The effect of a small addition of nickel on the sintering, sintered microstructure, and mechanical properties of Ti–45Al–5Nb–0.2C–0.2B alloy

• A small addition of Ni (⩽1.25 at.%) transforms the alloy from essentially unsinterable to fully sinterable at 1375 °C. • A τ 3 phase (Al 3 NiTi 2 ) forms during heating before reaching 1150 °C which enables subsequent liquid formation. • The as-sintered Ni-doped alloy shows excellent compression properties. Titanium aluminide based pre-alloyed powder is difficult to consolidate by pressureless sintering. The effect of a small addition of nickel (Ni) on the sintering, sintered microstructure, and mechanical properties of a Ti–45Al–5Nb–0.2C–0.2B alloy, compacted from pre-alloyed powder, has been investigated. The relative density of the alloy, sintered at 1375 °C for 60 min, increased progressively from 74% to >99% with increasing Ni content from 0% to 1.25 at.%. The addition of Ni leads to the formation of an intermediate hexagonal-structured Al 3 NiTi 2 phase (known as the τ 3 phase) during heating prior to reaching 1150 °C. The presence of the τ 3 phase enables the formation of a Ni-containing sintering liquid through three reactions between 1250 °C and 1350 °C: τ 3 + γ → Liquid, τ 3 + γ → Liquid + α and τ 3 + α → Liquid. The sintering shrinkage of the powder blend Ti–45Al–5Nb–0.2C–0.2B–1.25Ni occurs mostly before reaching the isothermal sintering temperature (1375 °C). Consequently, the sintered density increases only marginally during the subsequent isothermal sintering at 1375 °C from 15 min to 60 min and remains unchanged afterwards. However, achieving a near full lamellar structure requires a longer isothermal hold (120 min) at the single α phase region. The as-sintered Ti–45Al–5Nb–0.2C–0.2B–1.25Ni alloy shows compression strength of 2200 ± 50 MPa, yield strength of 750 ± 20 MPa, and strain to fracture of 28 ± 0.5%.

Transgranular shearing introduced brittlement of Ti–5Al–5V–5Mo–3Cr alloy with full lamellar structure at room temperature

β titanium alloys with full lamellar structure often exhibit the low elongation, and the brittle fracture mechanism of Ti–5Al–5V–5Mo–3Cr β titanium alloy with full lamellar microstructure is investigated in this paper. After tensile test, the fracture surface characterization and the cross sectional observation show that the transgranular dimple fracture, the intergranular fracture and the transgranular shearing fracture are the fracture mode of the alloy. It is discovered that the cracks form by the coalescence of the microvoid at the α/β interfaces, and the shearing is triggered by the gradual increment of the crack dimension inside the prior β grain at a high stress level. The first two fracture progresses contribute to the plastic deformation of the alloy, and the transgranular shearing occurs in the large scale β grain and is the main brittle fracture mechanism of Ti-5553 alloy.

Effect of heat treatment on microstructures and mechanical properties in a full lamellar PM TiAl alloy

The effect of heat treatment on the microstructures and tensile properties of a powder metallurgical (PM) TiAl based alloy has been investigated in this paper. The near gamma (NG) microstructure is transformed to a full lamellar (FL) microstructure with an average grain size of 100 μm by heat treatments. The lamellar spacing of FL structure decreases with the increase of cooling rate. For cooling rates of 5, 10 and 50 °C/min, the lamellar spacing is 1.9, 1.0 and 0.8 μm respectively. The room temperature tensile properties exhibit an increasing trend with decrease of lamellar spacing.

Microstructural characteristics and mechanical properties of Ti–6Al–4V friction stir welds

Friction stir welding (FSW) was applied to 3 mm-thick Ti–6Al–4V plates under different rotational speeds. Defect-free welds were successfully produced at rotational speeds of 400 and 500 rpm. The base material (BM) had a deformed α/β lamellar microstructure. FSW produced a full lamellar structure with refined prior β grains in the SZ, while the HAZ contained a bimodal microstructure consisting of the equiaxed primary α and α/β lamellar structure within the prior β structure. An increase in rotational speed increased the sizes of α colonies and prior β grains. The SZ exhibited higher hardness than the BM, with the lowest hardness found in the HAZ. Results of the transverse tensile test showed that all welds fractured in the HAZ and that they exhibited lower strength and elongation than the BM. The tensile test for only the SZ showed it to be characterized by higher strength and elongation than the BM.

410 Ti-Al基金属間化合物の組織と被削性(OS-13 先端材料・難削材の加工)

Machinability of Ti-47.7at%Al intermetallic compound having TiAl-Ti_3Al full lamellar structure is investigated by orthogonal cutting test using cemented carbide tool K10. Defects such as cracks, peelings due to brittle fracture along with the direction of lamellar were observed on the machined surface. The larger brittle cracks were occurred in cutting by tool with the rake angle of +5°. For machining in the rake angle of -5°, the depth of crack occurrence was independent on the direction of lamellar. On the other hand, for machining in the rake angle of +5°, it increased rapidly at the lower angle between lamellar direction and cutting direction than 90°.

The Control of Microstructures in (Mo<sub>1-x</sub>, Nb<sub>x</sub>) Si<sub>2</sub> Intermetallics

The intermetallics of (Mo1-xNbx) Si2 consisted of C11b phase and C40 phase when x is equal to 0.05, 0.10 and 0.15, and consisted of only C40 phase when x is equal to 0.20. Systemic heat treatments were conducted on the alloys. The evolution of microstructure was investigated with varying annealing temperature and time. Through the appropriate heat-treated processing, the microstructures of the alloys can be controlled to form full lamellar structure

The effect of lamellar structure in Mo 5Si 3–MoSi 2 alloy refined by annealing on the Vickers hardness at room temperature

Mo 5Si 3–MoSi 2 eutectic alloy, hypoeutectic alloy and hypereutectic alloy were prepared by arc-melting pure raw materials of Mo and Si. Annealing at 1200 °C for different times was conducted on all of the alloys to investigate the effect of annealing on the microstructure and mechanical property including Vickers hardness and indentation toughness at room temperature. Lamellar structure consisted of Mo 5Si 3 (D8m) phase and MoSi 2 (Cllb) phase was observed in all the alloys. For Mo 5Si 3–MoSi 2 eutectic alloy, a typical eutectic microstructure with primary Mo 5Si 3 phase and Mo 5Si 3/MoSi 2 lamellar structure was observed, and the primary Mo 5Si 3 phase was eliminated when annealing at 1200 °C for over 24 h. For Mo 5Si 3–MoSi 2 hypoeutectic alloy, the lamellar structure was found only after annealing. A full lamellar structure throughout the alloy developed well with fine spacing on the order of 100 nm after annealing at 1200 °C for 48 h. For Mo 5Si 3–MoSi 2 hypereutectic alloy, the lamellar structure was found both before and after annealing. Primary MoSi 2 was observed in the alloy even after annealing at 1200 °C for 96 h, although the volume fraction of lamellar structure was increased with prolonging annealing time. The effect of the formation, development and destruction of lamellar structure on Vickers hardness and indentation toughness of alloys was investigated. The Vickers hardness and indentation toughness of Mo 5Si 3–MoSi 2 hypoeutectic alloy show a same regularity, that is, they increase with prolonging annealing time and development of lamellar structure. The highest Vickers hardness and indentation toughness are about 1470 HVN and 4.8 MPa m 1/2, respectively. The Vickers hardness and indentation of Mo 5Si 3–MoSi 2 eutectic alloy and Mo 5Si 3–MoSi 2 hypereutectic alloy are all roughly constant with prolonging annealing time, that is, about 1400 and 1270 HVN for Vickers hardness and 4 and 3.2 MPa m 1/2 for indentation toughness.

<B> In-Situ</B> Formation of Nitride Particle Reinforced Titanium Aluminide by Reactive Plasma Arc Melting Process

A reactive plasma arc melting process using a mixture of Ar and N2 as the carrier gas for titanium and aluminum powders has been applied to synthesize nitride particle reinforced Ti–34 mass%Al intermetallic matrix composites (IMCs). This technique successfully allowed to produce in-situ nitride particle dispersed IMCs and the volume fractions of nitride increased from 6 vol% to 54 vol% with increasing mixing ratios of N2 gas from 10 vol% to 100 vol%. In the IMCs both rod-like Ti2AlN and coarse two-phase nitride particles consisting of the core of TiN and the outer shell of Ti2AlN were formed in the matrix of a full lamellar structure or a lamellar containing small amount of equiaxed γ phase structure. The IMCs had significantly fine grains of which size was about 1/4 that of the unreinforced Ti–34 mass%Al. The Rockwell hardness of the IMCs increased abruptly from 36.5 to 48.4 HRC with increasing volume fraction of nitride. Unlike the hardness, the tensile strength of the IMCs had a maximum value of 507 MPa, which was approximately 170 MPa higher than that of the unreinforced Ti–34 mass%Al, at 13 vol% nitride, beyond which the strength decreased. The strengthening is derived from complex reinforcing effects of both direct strengthening effects of nitride particles due to the interaction of dislocations with the particles and grain refinement. The degradation in tensile strength at higher volume fractions of nitride is considered to be attributed to higher population of clustered coarse two-phase nitride particles, which can act as crack initiation and propagation sites. As for the strength at elevated temperatures, the IMC with 13 vol% nitride had higher tensile strengths than the unreinforced Ti–34 mass%Al by 100 MPa at 1173 K and 61 MPa at 1273 K.

Effect of minor Sc on high temperature mechanical properties of Ti–Al based alloys

Abstract High temperature compression properties and microstructure of two Ti–Al based alloys with and without Sc were studied The results show that minor addition may remarkably improve the high temperature yield strength of Ti–Al and Ti–48Al based alloy which have either duplex structure or full lamellar structure, the yield strength increment is 120 and 125 MPa, respectively. Minor Sc exists as Ti 3 (Al,Sc) in plate precipitates which distribute mainly along with α 2 /γ or γ/γ T boundaries, partly within γ phases. At high temperature, deformation of γ phases is easier than that in the α 2 phase, the deformation mode is dislocation slipping. The obstacle to interlamellar sliding and dislocation slipping caused by Ti 3 (Al, Sc) precipitates should be responsible for yield strength increment of Ti–Al alloys with Sc.