Lamellar Colonies Research Articles

In-Situ Regulation to Tailor Additive Manufacturing of TiAl Alloy with Homogeneous Fine Fully Lamellar Colony

In-Situ Regulation to Tailor Additive Manufacturing of TiAl Alloy with Homogeneous Fine Fully Lamellar Colony

Quantitative analysis on microstructure characteristic of pre-strained β-solidified TiAl alloy during post-heat treatment

The β/βo phase in β-solidified TiAl alloys plays a contradictory role in improving processability at high temperatures but deteriorating performance at service temperature due to its ordering transformation. After exerting its positive role during thermomechanical processing, it must be eliminated or reduced to a minimum through post-heat treatment. In this study, the microstructural evolution of the pre-strained Ti-43.24Al-8.42Nb-0.20W-0.21B-0.24Y alloy on post-heat treatment is investigated, and a quantitative relationship between βo phase content and the pre-strain is established. Due to the difference in driving force of phase transformation, with decreasing pre-strain the microstructure displays varied characteristics from a mixed structure comprising (α2+γ) lamellar colonies, γ blocks, and βo phase, to a nearly-lamellar structure after post-heat treatment at 1270 °C/4h/FC. Only if the pre-strain is less than 0.78, a refined nearly-lamellar structure can be achieved. This work provides important theoretical guidance for practical forging processing of β-solidified TiAl alloys.

High-temperature tensile rupture property of (Nb,W) co-alloying TiAl-based alloys

In order to explore whether (Nb,W) co-alloying TiAl-based alloys with relatively higher W addition have better high-temperature tensile rupture property, Ti-44Al-4Nb-1 W-0.1B alloy is designed and prepared. Ti-44Al-8Nb-0.1B alloy and Ti-44Al-7.2Nb-0.2 W-0.1B alloy are also prepared for comparative study. The rupture property testing is carried out at 800 °C and different tensile stresses. The property data, macro/microstructure evolution, fracture surface, W content, crack failure behaviors are studied. The results show that the (Nb,W) co-alloying alloys have better rupture property than the pure Nb alloying alloy. For the (Nb,W) co-alloying alloys, the higher W contained Ti-44Al-4Nb-1 W-0.1B alloy has the better property under lower tensile stress, and the lower W contained Ti-44Al-7.2Nb-0.2 W-0.1B alloy has the better property under higher tensile stress. The relationships of rupture life t and stress σ for the three alloys are given. All the three alloys have a coupling fracture mode of ductile fracture and brittle fracture. The ductile fracture exhibits the typical dimple characteristics. The brittle fracture exhibits the typical trans-granular cleavage, river-like pattern and trans-lamella fracture characteristics. The higher the stress, the more brittle fracture characteristics there are. After rupture property testing, the (α2 + γ) lamella colony sizes of the three alloys all decrease, indicating that DRX and grain boundary slip occur not only along (α2 + γ) lamella colony boundary, but also inside it. The colony boundary regions have the stress concentration, where the B2 phase produces the better buffering and coordination, and as well as the dislocation tangles, DRX and grain boundary slip can be found by EBSD and TEM. EPMA results show that the more W added in the alloy, the more W content is in the (α2 + γ) lamella matrix, which is beneficial for the rupture property. However, more W addition will also lead to the formation of more B2 phase in the initial as-cast microstructure. So that, under higher tensile stress, when the stress intensity factor K is higher, the crack failure is more likely to occur.

Metastable phase transformations and mechanical properties of an as-extruded TiAl-based alloy during two-step heat treatment

Metastable phase transformations of TiAl-based alloys provide a pathway for grain refinement and performance improvement. In this paper, the microstructure evolution of an as-extruded Ti-47Al-2Cr-0.2Mo alloy during quenching and tempering was investigated by scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM), and the mechanical properties of the alloy were further evaluated by tensile tests. The results indicated that the analogous Σ3 coincidence site lattice (CSL) and γ/γ true twin interface appeared in both the Widmanstätten structure (γw) and feathery structure (γf), implying that sympathetic nucleation mechanism was responsible for the nucleation of the metastable phases. A 6 H intermediate phase with the long period stacking ordered structure (LPSO) was discovered after short-term water quenching, which provided an evidence supplement of the sympathetic nucleation mechanism. The transformation path of the metastable γw phase was proposed to follow the sequence of α/α2→6 H→γ. Apparent refinements of both grains and α2/γ lamellar colonies ((α2+γ)L) were exhibited after metastable phase transformation. The ultimate tensile strengths of the heat-treated alloy at whether room temperature (RT) or 800 °C were obviously improved, and the elongation at RT also kept at a considerable level.

Induction mechanisms of high-density nano twins during solidification process: Reducing stacking fault energy of γ phase by Re and forming highly mismatched B2(Re)/α2 interface

It is extremely difficult to introduce high-density nano twins during the solidification process of TiAl alloy. In this study, high-density nanotwins are inducted in the as-cast Ti48Al2Cr alloyed by adding Re element. Phase transformation, morphology characteristics of nano twins, compressive and tensile properties, and the related mechanisms have been studied. Results show that B2 phase enriched with Re tends to precipitate along the α2/γ interface within lamellar colony. The stacking fault energy (SFE) of γ phase decreases from 43 mJ/m2 to 16 mJ/m2 as Re content increases from 0 at.% to 0.6 at.%, decreasing the critical shear stress for twin formation. Compared to the mismatch value of α2/γ interface (0.004), which of B2/α2 and B2/γ interfaces increase to 0.247 and 0.149, respectively. Driven by high interfacial stress, high-density dislocations are generated at the B2/α2 interface, providing the dislocation slip channel for the formation of stacking faults (SFs) and nanotwins at the B2/γ interface. Therefore, the mechanism of inducting high-density nanotwins is to reduce the stacking fault energy of γ phase by Re and form highly mismatched B2/α2 interface. Compressive strength and the strain increase from 1723 MPa to 2398 MPa and 29 % to 39 % as Re content increases from 0 at.% to 0.6 at.%, respectively. Tensile strength increases from 356 MPa to 452 MPa without sacrificing plasticity. The improvement in strength and plasticity are attributed to the nano-twinning strengthening and interfacial thermal mismatch strengthening. Forming nanotwins during solidification process serve as the nucleation sites for newly formed twins during deformation process, increasing the deformation tolerance of TiAl alloy.

Thermal stability and coarsening of eutectic and near-eutectic Ni–Ce alloys

Ni–Ce alloys are currently being studied as a castable alternative to conventional Ni-based superalloys. To understand the evolution of microstructure and mechanical behavior at elevated temperatures, this study investigates the mechanism of coarsening behavior in eutectic and near eutectic Ni–Ce at 900 °C. The as-cast eutectic Ni–Ce consists of a combination of fine lamellar and rod eutectic colonies with coarser boundary regions. Upon annealing, continuous coarsening, via process of globularization, initiates at colony boundaries or primary dendrites where faults (termination and branches) in the lamellar structure are plentiful. Coarsening then proceeds by a combination of fault migration mechanism and lamellar boundary splitting, growing toward the center of the eutectic colonies with increasing annealing time. Coarsening near the center of a colony is extremely slow, indicating the eutectic microstructure itself is very stable. The near-eutectic alloys containing primary dendrites coarsen and stabilize faster compared to the fully eutectic alloy. Moreover, an anomaly is observed in the hardness of the fully eutectic alloy after long exposure at elevated temperature compared to the near-eutectic alloys.

Lamellar orientation control of TiAl-based alloy by uniaxial compressive deformation at high-temperature in (α+β) two-phase region

In this study, the orientation of (α+γ) lamellar structure in the TNM alloy (Ti-43.7Al-4 Nb-1Mo-0.1B) is controlled by high-temperature uniaxial compression conducted in the (α+β) two-phase region. The control of the lamellar orientation is focused on a control of α phase orientation because of being decisive for lamellar γ orientation. All true stress-true strain curves indicated the work softening due to dynamic recrystallization. The stress-strain curve behaviors were consistent with microstructure observation. The peak stress increased with a decrease in lamellar colony size. After deformation in the (α + β) two-phase region at 1543 K and 1573 K, where the content of α phase was dominant, a fiber texture with the (0001)α2 plane tilted 35° away from the compression plane was formed by the deformation mechanism of α phase. On the other hand, at 1623 K where the content of β phase was dominant, a double (0001)α2 fiber texture tilted at 35°- 45° and 80°- 90° away from the compression plane was observed. The double (0001)α2 fiber texture is considered to be caused by β→α phase transformation during cooling according to Burgers orientation relationship. Single-step uniaxial compressions conducted at the (α + β) two-phase region at different temperature (1543 K−1623 K) led to an inclination of (0001)α2to compression plane and subsequently lamellar interfaces of one colony unparallel to those of other colonies.

Interaction between dynamic recrystallization and phase transformation of Ti-43Al-4Nb-1Mo-0.2B alloy during hot deformation

The β solidified γ-TiAl alloy holds important application value in the aerospace industry, while its complex phase compositions and geometric structures pose challenges to its microstructure control during the thermal-mechanical process. The microstructure evolution of Ti-43Al-4Nb-1Mo-0.2B alloy at 1200 °C/0.01 s−1 was investigated to clarify the coupling role of dynamic recrystallization (DRX) and phase transformation. The results revealed that the rate of DRX in α2+γ lamellar colonies was comparatively slower than that in βo+γ mixed structure, instead being accompanied by intense lamellar kinking and rotation. The initiation and development rates of DRX in α2, βo, and γ phases decreased sequentially. The asynchronous DRX of the various geometric structures and phase compositions resulted in the uneven deformed microstructure, and the dynamic softening induced by lamellar kinking and rotation was replaced by strengthened DRX as strain increased. Additionally, the blocky α2 phase and the terminals of α2 lamellae were the preferential DRX sites owing to the abundant activated slip systems. The α2→βo transformation within lamellar colonies facilitated DRX and fragment of α2 lamellae, while the α2→γ transformation promoted the decomposition of α2 lamellae and DRX of γ lamellae. Moreover, the varied βo+γ mixed structures underwent complicated evolution: (1) The γ→βo transformation occurred at boundaries of lamellar colonies, followed by simultaneous DRX of γ lamellar terminals and neighboring βo phase; (2) DRX occurred earlier within the band-like βo phase, with the delayed DRX in enclosed γ phase; (3) DRX within the βo synapses and neighboring γ phase was accelerated owing to generation of elastic stress field; (4) Dispersed βo particles triggered particle stimulated nucleation (PSN) of γ phase. Eventually, atomic diffusion along crystal defects in βo and γ phases caused fracture of band-like βo phase and formation of massive βo particles, impeding grain boundary migration and hindering DRXed grain growth of γ phase.

Improved hot workability of TiAl composite with core-shell structure via in-situ synthesized multi-phases ceramic particles

TiAl composite with core-shell structure was in-situ synthesized incorporating BN nanosheets and graphene oxide into TiAl alloy using SPS, and the hot deformation behaviors were subsequently investigated on a Gleeble-1500D thermal simulation machine. The microstructural evolution and the influence of ceramic particles on hot deformation were revealed using OM, SEM, TEM, and EBSD. The results demonstrated a refined microstructure of TiAl composite induced by ceramic particles providing extra nucleation sites for DRX, contributing to lower flow stress, with peak stresses of 387 and 360 MPa for TiAl alloy and TiAl composite deformed at 1200 °C/1s−1, respectively. Moreover, dislocation slip was inhibited by ceramic particles leading to dislocation pile-up, facilitating the nucleation and growth of DRX at lamellar colony boundaries and α2/γ interfaces. Notably, the “shell” composed of TiB2 and Ti2AlN nanoparticles coordinated the deformation of lamellar colonies with different orientations, improving the hot workability of TiAl composite.

Hierarchically heterogeneous microstructure and mechanical properties in laser-directed energy deposition of γ-TiAl alloy through intrinsic cyclic heat treatment

The complex thermal effects of laser direct energy deposition (L-DED) provide significant advantages for production of high-performance γ-TiAl alloy, whereas challenges still exist in microstructure tailoring and mechanical properties. This work focuses on the influence of intrinsic cyclic heat treatment (ICHT) on the evolution of microstructures and mechanical properties in different building height regions of an L-DED TiAl alloy thin-wall sample. The results indicate that ICHT induces discontinuous coarsening (DC) effects by consuming the lamellar interface energy of the initial lamellae, resulting in grain refinement. With the increase in building height, the thermal accumulation leads to an increase in the initial lamellar spacing, which weakens the DC effect. At the top, insufficient thermal cycles prevent fine grain growth and erode coarse grains, hindering overall grain refinement. Ultimately, the hierarchically heterogeneous microstructure forms in the L-DED TiAl alloy, including refinement of hierarchical lamellar colony structure decorated with fine (α2+γ) lamellar structure and (α2+6H-LPS) lamellar structure (6H-LPS refers to 6H long-period structure phase). Specifically, the bottom region exhibits the best combination of ultimate tensile strength (706 ± 33 MPa) and strain (0.68 ± 0.05 %). The high tensile strength can be attributed to the hindrance of dislocation movement by the fine lamellae and the 6H-LPS phase ,caused by the high cooling rate of L-DED, while the elongation is attributed to the reduction in crack propagation energy due to the presence of fine grains.

Effect of trace boron addition on the microstructural evolution and mechanical properties in Ti45Al8Nb2Cr-B alloys

High Nb-containing TiAl alloys have been deemed ideal candidates for demanding structural applications under high temperatures due to their appealing mechanical qualities and low density. Nonetheless, its inherent brittleness at room temperature remains a significant impediment to its processability. Herein, different concentrations of boron were added into the Ti45Al8Nb2Cr (Ti4582) alloy by vacuum arc melting to tailor the microstructure and corresponding mechanical properties. The coarse lamellar colonies were significantly refined via boron addition, while excessive boron addition did harm to the mechanical properties. The different morphologies of the borides (Bf or B27) profoundly influenced the microstructure and mechanical properties of as-prepared alloys. Further elemental distribution, phase constitution, lamellar colony size, and mechanical properties were systematically investigated. Noteworthily, trace boron additions effectively improved the mechanical properties of Ti4582-xB alloys with an optimal adjustment in Ti4582–0.2B, showing 673 MPa for room temperature ultimate tensile strength (UTS) with elongation of 1.3 % and 1623 MPa for ultimate compressive strength (UCS) with compressive strain of 32.5 %. The improved mechanical properties of Ti4582-xB were mainly attributed to grain refining and second phase strengthening. Moreover, the hindrance and accommodation for dislocations by deformed twins and reinforcements effectively enhanced the mechanical properties.

Multiple ceramic particles help to improve oxidation resistance of TiAl alloy

A novel microstructural regulation strategy was utilized to improve the oxidation resistance of TiAl alloy by the controllable multiple ceramic particles. TiAl composites were prepared using spark plasma sintering (SPS) by introducing SiC and graphene oxide (GO). TiAl composites showed nearly full lamellar microstructure, and the Ti5Si3 particles in-situ precipitated at the lamellar colony boundaries and Ti2AlC in-situ precipitated at the α2/γ interfaces. Grain refinement of TiAl composite was obvious, displaying a decrease of lamellar colony size from 259 to 81 μm after addition 0.3 wt% SiC and 0.5 wt% GO. The isothermal oxidation results showed that the mass gain of TiAl composite was decreased by 56% compared to TiAl alloy after oxidation at 950 ℃ for 100 h. This was mainly attributed to the in-situ precipitated Ti2AlC and Ti5Si3 ceramic particles inhabiting the interior diffusion of O atoms and the outward diffusion of Ti and Al atoms, then the activity of O and Ti was decreased. Furthermore, the Ti2AlC and Ti5Si3 ceramic particles contributed to release the internal stress between the oxide layer and matrix, strengthening the bondability between the oxide layer and matrix.

The Effect of Heat Treatment on the Microstructure and Mechanical Properties of Powder Metallurgy Ti-48Al Alloy

Heat treatment is the critical step in achieving a refined microstructure and enhanced mechanical properties of TiAl-based alloys. This study investigated the influence of heat treatment temperature, cooling method, and heat treatment time on the microstructure and mechanical properties of an extruded powder metallurgy Ti-48Al alloy, and achieved the control of fully lamellar fine microstructures and the enhancement of performance through a simple heat treatment, rather than the traditional approach of homogenization followed by heat treatment. The results indicate that the heat treatment temperature determines the type of microstructure, while the cooling rate dictates the lamellar width. As the heat treatment temperature was increased from the two-phase region to the α single-phase region, the microstructure transitioned from duplex to near lamellar, and the alloy strength initially increased and then decreased, influenced by both the lamellar colony ratio and grain size. A rapid cooling rate (water quenching) induces a non-diffusive massive phase transformation, whereas a slow cooling rate (air cooling) gradually forms α2/γ lamellar colonies. Therefore, a suitable heat treatment regime for the powder metallurgy Ti-48Al alloy was determined to be 1340 °C/5 min/air cooling. The microstructure of the alloy was near lamellar, consisting of lamellar colonies approximately 50 μm and a small number of γ equiaxed grains of about 10 μm. Subsequently, the alloy exhibited a room temperature tensile strength of 784 MPa and a yield strength of 763 MPa, representing improvements of 17.0% and 38.7% over the extruded alloy, respectively. This research provides a reference for establishing a heat treatment process for powder metallurgy TiAl alloys.

Effect of forging routes on the microstructure and mechanical properties of newly-developed Ti-44Al-5.5Nb-0.5W-0.5Cr-0.3Si-0.1C alloys

In this study, the effects of forging routes on the microstructure and tensile properties of Ti-44Al-5.5 Nb-0.5 W-0.5Cr-0.3Si-0.1 C (at%) alloys was comparatively investigated. Results showed that the bare ingot forged at 1400 ℃ (BH) exhibited refined lamellar colonies with narrow lamellar spacing. In contrast, dynamic recrystallized γ grains and lamellar structure with thick lamellar spacing formed in the canned sample deformed at 1320 ℃ (CA). In addition, the heat-treated BH-HT sample showed fully recrystallized texture, while the lamellar structure in the CA-HT presented a preferred orientation, i.e., lamellar interfaces were parallel to rolling direction (RD). The mechanical properties of the CA-HT sample test results showed superior room-temperature ductility of 1.68 % and tensile strength of 515 MPa at 950 ℃ due to the lamellar orientation alignment to tensile direction. According to the temperature drop calculation results, both BH and CA forging routes exhibited rapid cooling rates. The initial microstructure at the pre-heating temperature was maintained until a hot deformation sequence was initiated. Therefore, the BH route could be performed in the α+β region, while the CA route could be carried out in the α-forging condition with the 1101̅12 γ texture.

Origin of surface oxidation induced the nucleation and propagation of microcracks in TNM alloy

This study investigated the impact of surface degradation caused by oxygen (O) and nitrogen (N) elements in the environmental atmosphere on the service failure of nearly lamellar TNM alloy. The results show that oxidation-induced phase transformations at the surface and subsurface regions strongly affect the alloy's tensile properties at high temperatures. The precipitation of Al2O3, which disrupts the integrity of the Ti2AlN layer, has a weaker deformation resistance capacity compared to the Z-phase. Under stress conditions, numerous dislocations concentrate at the Al2O3 phase, causing Al2O3 particles to easily detach and initiating crack nucleation at the Z-phase/Al2O3 interface. This is the origin of oxidation failure. Cracks nucleate at the Al2O3 detachment region, propagate, and connect with surface microcracks induced by the rough and loose outer oxide layer structure. This leads to the rapid propagation of cracks into the subsurface. The consumption of β-stabilizing elements at the surface, along with the internal diffusion of O and N elements into the subsurface region, results in the formation of the hard brittle Z-phase at the subsurface lamellar structure and a transition from β0 to α2 at the lamellar colony boundaries. The precipitation of α2 and Z-phase also accelerates crack propagation. In summary, the failure mode shifts from toughness fracture to brittle fracture.

Microstructure and mechanical properties of high relative density Ti-48Al-1Fe alloy using irregular blend powder

Conventional pressing and sintering methods provide difficulties in achieving high density for PM TiAl alloys. The Ti-48Al-1Fe alloy was prepared via vacuum pressure-less sintering, and the microstructure evolution and mechanical properties during the sintering densification process of the mixed powder system were investigated. The results shows that the density of the mixed powder reached 98.3 ± 0.14% after pressure-less sintering for 3 hours under high vacuum conditions at 1300°C. The microstructure of the as-sintered alloy is a fine duplex structure composed of α2/γ lamellar colonies and the composite structure. The ultimate tensile strength (UTS) of the as-sintered alloy at room temperature is 315 MPa. The density of the as-sintered alloy reach more than 99.9% after non-capsule hot isostatic pressing (HIP). The as-HIPed alloy exhibits an UTS of 332 MPa at room temperature. At high temperatures, it maintains high strength, with an UTS of 408 MPa and yield strength of 386 MPa at 900°C, which is comparable to that of forged TiAl alloy. Furthermore, the elongation of the sample at 900°C is 20.0%. This process offers a novel method for sintering PM TiAl with high density and mechanical properties at a low cost and near-net shape.

Modification of microstructure and mechanical properties for twin-wire directed energy deposition-arc fabricated Ti–48Al–2Cr–2Nb alloys via current regulation

Considering the advantages of low density and excellent high-temperature performance, titanium aluminide is identified as an ideal aerospace structural material. In recent years, twin-wire directed energy deposition-arc (TW-DED-arc), which possesses the characteristics of low cost but high deposition efficiency, has continuously attracted worldwide attention. However, obtaining equiaxed grains with small Al variation for titanium aluminide using TW-DED-arc method is still a key concern. In the present research, Ti–48Al–2Cr–2Nb alloy with equiaxed lamellar colonies and less than 1 at.% of Al content was successfully fabricated by regulating deposition current of TW-DED-arc. Also, the effect of deposition current on microstructure and mechanical properties was systematically investigated. The experimental results that focus on top and middle regions of as-fabricated TiAl-4822 alloys, indicate that increasing current favors in generation of equiaxed grains and γ lamellae, which leads to columnar to equiaxed transition and higher γ phase content, respectively. Also, elemental homogeneity can be significantly homogenized. As deposition current increases, tensile strength increases while degree of tensile anisotropy decreases significantly, benefiting from the equiaxed grains with satisfying size. Importantly, microstructure evolution, tensile anisotropy and fracture characteristics of TW-DED-arc fabricated TiAl-4822 alloys are discussed in detail. Generally, these findings provide an effective process method to optimize microstructure and mechanical properties of TW-DED-arc fabricated TiAl-4822 alloys.

Refinement and enhancement of high-Nb TiAl alloy via in-situ precipitation of Ti2AlC and TiB2 nanoparticles

An in-situ dual morphology of Ti2AlC/TiB2 nanoparticles was refined and enhanced to fabricate Ti-47.7Al-7.1Nb-2.3V-1.1Cr composites via induction hot-pressing sintering with nano-B4C addition. The generation process of nanoparticles, lamellar refinement mechanism, and effect of nanoparticles on the microstructure evolution/mechanical properties were meticulously analysed. With 0.2 at% nano-B4C addition, Ti2AlC/TiB2 nanoparticles were uniformly distributed in the matrix following sintering. In addition, upon sintering at 1425 °C for 10 min under a pressure of 50 MPa, the lamellar colony size decreased from 280 ± 145 μm to 55 ± 16 μm, and the ultimate tensile strength (UTS) increased by 116 MPa, 142 MPa, and 176 MPa, when tested at room temperature, 800 °C, and 900 °C, respectively. The enhanced mechanical properties were attributed to the synergistic effects of grain refinement strengthening, solid solution strengthening, the Orowan mechanism, and twinning strengthening.

In-situ synthesized multi-component particle reinforced TiAl composite with excellent mechanical properties

In order to overcome the inherent trade-off between strength and ductility of TiAl alloys, the BN nanosheets were added in Ti-48Al-2Cr-2Nb alloy. Ti-48Al-2Cr-2Nb-xBN (x = 0, 0.1, 0.5, 1, 2 wt%) alloys were prepared by vacuum arc melting (VAR), and the microstructures displayed a fully lamellar structure accompanied by simultaneous precipitation of Ti2AlN and TiB2 phases. The reaction of BN nanosheets with the Ti and Al elements resulted in formation of Ti2AlN and TiB2 phases. The lamellar colony size of TiAl composites was refined from 276.7 to 29.4 µm with BN content increasing from 0 to 2 wt%. While the lamellar spacing displayed an initial decrease followed by an increase, which was attributed to the synergistic effect of Ti2AlN and TiB2. Room temperature compression results showed that the compressive strength surged from 1653 to 2272 MPa as BN content rose from 0 to 0.5 wt%, with the highest compression strain of 34.7 % at 0.5 wt% BN. The high temperature compression results showed that the compressive strength increased from 887 to 1399 MPa as the BN content increasing from 0 to 1 wt%. Crack extension took the form of both trans-granular and inter-granular modes. The strengthening effect of TiAl composites was attributed to grain refinement strengthening and secondary phase strengthening via Ti2AlN and TiB2.

Quasi-in-situ investigation on complete lamellar fragmentation of β-solidified TiAl alloy during uniaxial isothermal compression

The coarse as-cast lamellar microstructure in TiAl alloys is difficult to be broken completely by thermomechanical processing. Some remnant lamellar colonies in the deformed microstructure seriously affect the microstructural homogeneity and deteriorate the properties. In this study, it is found that by isothermal compression at 1230 °C and 1250 °C, the lamellar colonies of Ti-43.5Al-4Nb-1Mo-0.1B (TNM) alloys can be completely broken. This is attributed to the weakened anisotropic deformation behavior of the lamellar colonies due to the isothermal holding treatment before deformation. The deformation behavior at 1230 °C was investigated by quasi-in-situ experiments. It is observed that the regions near lamellar colony boundaries first undergo dynamic recrystallization at small strain, while the lamellar colonies gradually break down with increasing strain. The adequate fragmentation of lamellar colonies mainly depends on the recrystallization of α lamellae (αL). The isothermal holding at 1230 °C leads to an increase in the content and thickness of αL, which allows it to assume more deformation and promotes its recrystallization by reaching critical strain. The interrupted γ lamellae (γL) formed by decomposition during isothermal holding facilitates the occurrence of α recrystallization within the lamellar colonies by hindering dislocation movement. In addition, recrystallized γ grains (γR) are gradually dissolved by the formation of α precipitates inside them through the γ → α phase transformation and the subsequent consumption of α precipitates by the recrystallized α grains.