Discontinuous Coarsening Research Articles

Investigation of Precipitation Behavior of a Novel Ni-Fe-Based Superalloy during High-Temperature Aging Treatment.

The precipitation behavior of a novel Ni-Fe-based superalloy developed for advanced ultra-supercritical (A-USC) coal-fired power plant applications during high-temperature aging treatment was investigated. The results showed that the major precipitates in the novel alloy were randomly distributed MC carbides, M23C6 carbides at grain boundaries, and the γ'-Ni3 (Al, Ti) phase in grain interiors after aging. MC remained relatively stable during both short-term and long-term aging. M23C6 quickly precipitated and exhibited a discrete distribution at grain boundaries during short-term aging, and partly developed into continuous films during long-term aging. After uniform precipitation, the shape of γ' remained spherical, and the size kept increasing with aging time according to the Lifshitz-Slyozov-Wagner (LSW) model. The hardness of the novel alloy was mainly associated with the precipitation behavior of γ'; as γ' gradually precipitated, the hardness steadily increased; after complete precipitation, as the size of γ' increased, the hardness first increased and then decreased, reaching the peak hardness when the average radius of γ' achieved the critical size. In addition, the novel alloy exhibited abnormal coarsening behavior at grain boundaries during both short-term and long-term aging. The coarsened grain boundaries were actually precipitate-free zones (PFZs) and the coarsened and elongated rod-like particles inside were identified as γ' precipitates. The mechanism of strain-induced grain boundary migration and the discontinuous coarsening reaction is proposed for the formation of PFZs. Furthermore, PFZs were considered to be potential crack sources during the creep rupture test, leading to earlier failure of the material.

Discontinuous coarsening behavior and kinetics of L12 precipitate in high-entropy alloys

Coarsening is a metallurgical process that is related to solute transmission and system energy decrease. However, for the coarsening behavior of the L12 precipitate in high-entropy alloys, the influence of moving boundary is still an open question. Here, we found the moving boundary accelerates the discontinuous coarsening of L12 phase. The multiple defects-accelerated diffusion and continual particle-capturing ability of the moving boundary provide a high-efficiency solute transport for the fast-coarsening kinetics of L12 phase. In addition, a boundary-encountered assisted precipitate formation mechanism is also validated in the recrystallized area.

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.

Discontinuous coarsening leads to unchanged tensile properties in high-entropy alloys with different recrystallization volume fractions

Heterogeneous microstructure alloys provide a possibility for the combination of strength and ductility. As a typical heterogeneous microstructure, partially recrystallized microstructure is attractive because of the convenient processing route and great potential for industrial applications. However, the mechanical properties of this microstructure vary dramatically with different morphology factors, especially the recrystallization fraction, which restricts the processing window and the property consistency. In this study, we found that the yield strength (∼1.3 GPa) and ductility (∼20 %) of the partially recrystallized Ni2CoCrFeTi0.24Al0.2 do not change when the recrystallization fraction increases from 21 % to 72 %. This novel phenomenon is attributed to an additional hetero-deformation induced (HDI) strain hardening effect produced by heterogeneous precipitates. With the increased recrystallization fraction, this additional HDI stress keeps the ductility unchanged by offsetting the decreased HDI stress arising from the hetero-deformation between recrystallized (RX) and non-recrystallized (NRX) areas. The unchanged yield strength comes from the increased strengthening effects of the lamellar precipitates and grain boundaries. We also confirmed that the discontinuous coarsening contributes to the formation of heterogeneous precipitates. These findings would open a new pathway for enhancing the consistency and processability of hetero-structured alloys and thus promote their broader industrial applications.

Effect of compositional minor adjustment on localized lamellar structure coarsening of TiAl alloys with different solidification modes

Possible lamellar coarsening in the as-cast microstructure of TiAl alloys significantly affects the subsequent processing of ingot metallurgy, and a systematic study of it is necessary. In this paper, two TiAl alloys with different solidification modes (TNM-45Al alloy and TNM-0.3Si-0.7 C alloy) were obtained by compositional minor adjustment basing on TNM alloy. The effect of different adjustment routes on the localized lamellar structure coarseing behaviour and the resulting differences in the uniformity of the nanohardness distribution were investigated by comparing with TNM alloy. It is found that increasing the Al content of the TNM alloy while keeping the solidification mode as β-solidification unchanged leads to severe localised lamellar coarsening in the S-segregation zone of TNM-45Al alloy, including continuous coarsening (CC) and discontinuous coarsening (DC). In contrast, the solidification mode of the TNM-0.3Si-0.7 C alloy with the addition of the strong α stabilizer elements Si and C is transformed into hypoperitectic solidification with severe peritectic segregation, but the lamellar structure exhibits surprising full-domain stability. The results show that the thermodynamic tendency caused by high Al content and the absence of net-like β-segregation leads to severe localised lamellar coarsening in TNM-45Al alloy. CC can be accomplished through the migration of interface defects. DC is formed by the nucleation and expansion of γ-phase Shockley incomplete dislocations, and the interfaces consist of the α2 /γ interface and the first discovered γ /γ twin interface. The lamellar coarsening of TNM-0.3Si-0.7 C alloy can be effectively suppressed by the solid solution of Si/C atoms and the near net-like β-segregation in the primary β-phase region. As a result, the distribution uniformity of the nanohardness of the lamellar structure in TNM-0.3Si-0.7 C alloy is excellent, with the variance decreasing from ±0.24 of TNM alloy to ±0.20, while it increases to ±0.37 for the TNM-45Al alloy.

Interplay between high-temperature creep deformation behavior and nanoscale precipitate evolution in a newly designed Fe–Ni-based superalloy

In an endeavor to achieve the critical goals of carbon neutrality, an explorative study was conducted to scrutinize the precipitate evolution and high-temperature creep behavior of an innovative and cost-effective Fe–Ni-based superalloy that was meticulously engineered for the next-generation ultra-supercritical (USC) power plants. The creep experiments conducted at temperatures and stresses of 650 °C/280 MPa (17,179 h), 675 °C/250 MPa (10,271 h), and 700 °C/250 MPa (2378 h), coupled with proficient data fitting were employed to accurately assess the long-term service performance of the superalloy under real-world operational environments. Though it can withstand theoretically an intense stress of 130 MPa at 700 °C for approximately 30 years as calculated by the Larson-Miller equation, its performance was not without a degree of perceived vulnerability. This was primarily traced to the conspicuous coarsening behavior of the precipitate-free zones (PFZs)/discontinuous coarsening zones (DCZs) formed in the grain boundaries (GBs) on a longer time scale. It had a significant diminishing impact on the overall strength of the GBs, resulting in a recorded creep life that was considerably shorter than initially projected. Moreover, a synergistic mechanism between PFZs/DCZs with TiC carbides was advanced to interpret the intergranular fracturing dynamic, which was postulated due to the evidence gathered from electron backscatter diffraction (EBSD), highlighting that the banded distribution of TiC carbides circumambient to GBs engendered considerable stress aggregation. Conversely, the plentiful supply of coincidence site lattice (CSL) boundaries disrupted the continuous network structure of random high-angle grain boundaries (HAGBs), thereby creating an effective barricade against the proliferation of cracks. These empirical findings paved the way for the proposition of an enhanced blueprint and the industrial utilization of such.

Effects of Thermal Exposure on the Microstructure and Mechanical Properties of a Ti-48Al-3Nb-1.5Ta Alloy via Powder Hot Isostatic Pressing.

Research on how thermal exposure affects the microstructure and mechanical properties of the Ti-48Al-3Nb-1.5Ta (at. %) alloy, which is prepared via powder hot isostatic pressing (P-HIP), is essential since this low-density alloy shows promise for use in high-temperature applications, particularly for aero-engines, which require long-term stable service. In this study, a P-HIP Ti-48Al-3Nb-1.5Ta (at. %) alloy was exposed to high temperatures for long durations. The phase, microstructure and mechanical properties of the P-HIP Ti-48Al-3Nb-1.5Ta alloy after thermal exposure under different conditions were analyzed using XRD, SEM, EBSD, EPMA, TEM, nanomechanical testing and tensile testing. The surface scale is composed of oxides and nitrides, primarily Al2O3, TiO2, and TiN, among which Al2O3 is preferentially generated and then covered by rapidly growing TiO2 as the thermal exposure duration increases. The nitrides appear later than the oxides and exist between the oxides and the substrate. With increasing exposure temperature and duration, the surface scale becomes more continuous, TiO2 particles grow larger, and the oxide layer thickens or even falls off. The addition of Ta and Nb can improve the oxidation resistance because Ta5+ and Nb5+ replace Ti4+ in the rutile lattice and weaken O diffusion. Compared with the P-HIP Ti-48Al-3Nb-1.5Ta alloy, after thermal exposure, the grain size does not increase significantly, and the γ phase increases slightly (by less than 3%) with the decomposition of the α2 phase. With increasing thermal exposure duration, the γ phase exhibits discontinuous coarsening (DC). Compared with the P-HIP Ti-48Al-3Nb-1.5Ta alloy, the hardness increases by about 2 GPa, the tensile strength increases by more than 50 MPa, and the fracture strain decreases by about 0.1% after thermal exposure. When the depth extends from the edge of the thermally exposed specimens, the hardness decreases overall.

Effect of variation in Zr content on microstructure and high-temperature tensile properties of a γ-TiAl alloy

A series of Ti-46Al-xZr (x = 0, 1, 2, 3, 4, 5 at.%) alloys were prepared to systematically investigate the effect of variation in Zr content on the microstructure and high-temperature tensile properties. The results show that the addition of Zr intensifies the solidification segregation and promotes the formation of blocky γ phase in TiAl alloys. With the increase in Zr content from 1 to 5 at.%, the volume fraction of the blocky γ phase increases from 0.2 to 12.8 %, and the solidification microstructure is transformed from a fully lamellar to a nearly lamellar microstructure composed of the blocky γ phase and lamellar colonies. Because Zr stabilises the γ phase, continuous and discontinuous coarsening occur simultaneously; the former is observed inside the colony, whereas the latter occurs between two adjacent lamellar colonies. The high-temperature tensile results show that the ultimate tensile strength at 800 °C of the Ti-46Al-4Zr alloy (692 MPa) is significantly greater than that of the Ti-46Al alloy (351 MPa), which is attributed to solid solution strengthening and the coordinated deformation of blocky γ phase.

Development and performance of INCONEL® alloy 740H® seam-welded piping

INCONEL® alloy 740H® is an age-hardenable nickel-based alloy approved for pressure vessels and piping within ASME Section I, VIII, and B31.1. Currently, the code applies a weld strength reduction factor (WSRF) of 0.7 to the allowable stresses for longitudinal seam welds in the time dependent creep regime. In this work, a full-scale seam weld was successfully produced using typical industrial practices. The component was solution heat-treated and aged after fabrication to improve the performance of the weldment. Tensile, bend, impact, and low-cycle fatigue tests showed the component met all the specification requirements and elevated temperature properties were within base metal expectations with failures predominately in the weld metal. Long-term creep tests, including large samples more representative of the entire weldment, were fabricated and tested to times in excess of 10,000 h at multiple temperatures. Analysis of the creep data supports a WSRF of 0.9 for the solution annealed + aged weldments in contrast to the WSRF of 0.7 currently applied to welded + aged weldments. Metallurgical analysis shows that the solution annealing causes recrystallization of the weld metal and reduces the chemical compositional and microstructural gradients in the weldments resulting in acceptable short-term performance. In long-term creep, evidence for coarsened zones in the weldments due to discontinuous coarsening reactions were identified as the mechanism leading to accelerated creep damage formation in the weld metal and sample failure.

Comparison of discontinuous precipitation behavior of Cu-15Ni-8Sn alloy with and without Co addition

The microstructure evolution, phase interface relationships, and corresponding hardness changes of discontinuous precipitation in Cu-15Ni-8Sn alloy during the aging process were investigated. The samples were isothermally aged for 15 h at 500–700 °C to yield discontinuous precipitates with varying lamellar spacings. The results showed that discontinuous precipitation with a lamellar structure is composed of the α-Cu phase and γ-DO3 phase, which has an incoherence interface and satisfies the K-S orientation relationship. With the increase of aging temperature, the lamellar spacing of the discontinuous precipitation increases, the length of lamellar shortens, and the lamellar nature weakens. Simultaneously, the discontinuous coarsening reaction is accelerated. The addition of the trace Co element can inhibit the formation of discontinuous precipitation and discontinuous coarsening phase. Co enrichment was found in the γ-DO3 phase, which can alter the lamellar spacing and then affect the hardness of the Cu-15Ni-0.5Co alloy. In addition, the hardness of discontinuous precipitation in the Cu-15Ni-8Sn- (0.5Co) alloy is negatively correlated with the lamellar spacing and can follow the Hall-Petch type relation.

Correlation Between Local Chemical Composition and Formation of Different Types of Ordered Phases in the Polycrystalline Nickel‐Base Superalloy A718Plus

Allvac 718Plus is a derivative of alloy 718 for applications at higher temperatures in rotating and static components of gas turbines. According to literature, various phases as γ, γ′, η, δ, and γ″ can form with so far contradicting results. Herein, the precipitation behavior in different alloy conditions is investigated using transmission electron microscopy and atom probe tomography. The aim is to clarify how local variations in chemical composition alter the sequence of precipitation and type of precipitates that form. Material conditions with a minor volume fraction of η/δ phase and a high fraction of primary γ′ precipitates form γ″ phase due to excess of available Nb. Spherical γ′ precipitates with disc‐shaped γ″ precipitates at γ′ interfaces are observed. During discontinuous coarsening of η/δ plates mainly Nb is consumed which leads to an enrichment of γ′ forming elements and hence the heterogeneous nucleation of γ′ at the η/γ interface. The coexistence of thin η plates embedded in δ phase in grain boundary pinning precipitates could additionally be confirmed. These results show under which conditions certain primary phases form and how the resulting consumption and redistribution of phase promoting elements trigger the formation of other secondary phases.

Metastable phase and microstructural degradation of a TiAl alloy produced via selective electron beam melting

Selective electron beam melting (SEBM) is an attractive rapid prototyping technology to fabricate TiAl alloys with a complex structure. A near fully dense Ti–47Al–2Cr–2Nb alloy was fabricated by SEBM by using a low-energy density of 20.04 J/mm3. The phase transformation, microstructural degradation and microhardness of the SEBM-produced TiAl alloy were investigated. The results showed that the microstructure was inhomogeneous along the building direction. Multiple phase transformations were observed during the in-situ thermal cycle. Disordered α phase was observed in the last solidifying tracks in the top surface due to the rapid cooling rate. A metastable Ti2Al phase was observed due to the relatively high residual stress that was caused by a quick cooling rate when fabrication occurred under a low energy density of 20.04 J/mm3. The decomposition of α2 lamellae, continuous coarsening and discontinuous coarsening of lamellar structure were the three main routes for microstructural degradation for SEBM-produced TiAl alloys. The microhardness exhibited an increasing tendency from a height of 18 mm to the top surface due to the increasing content of the lamellar colony. Finally, the degradation mechanism of the lamellar colony was discussed in detail.

Effect of multi-stage heat treatment on mechanical properties and microstructure transformation of Ti–48Al–2Cr–2Nb alloy

Refining the grain size of γ -TiAl alloys to improve their strength and ductility is of academic interest. Thus, we report a multi-stage heat treatment method consisting of solution treatment, cyclic heat treatment, annealing, short heat treatment, and aging to refine the microstructure of the Ti–48Al–2Cr–2Nb alloy. The solution-treated microstructure was refined from 1100 to 191 μm by cyclic heat treatment, promoting feathery γ packets. Fine duplex grains below 23 μm were obtained through discontinuous coarsening, which was accelerated in the cyclic-heat-treated alloys owing to the high driving force resulting from the presence of feathery γ packets and fine interlamellar spacing. Through the short heat treatment and aging at single α and α 2 + γ fields, a tailored duplex structure with a grain size of 24 μm and interlamellar spacing of 42 nm was achieved. Through microstructure refinement, tensile strength and elongation were improved to 697 MPa and 2.1%, respectively, compared to those of the conventional forged specimen (622 MPa and 1.3%, respectively). We believe that our study provides a simple pathway to refine the grain size and interlamellar spacing of γ -TiAl alloys.

Preparation of large-size Al2O3/GdAlO3/ZrO2 ternary eutectic ceramic rod by laser directed energy deposition and its microstructure homogenization mechanism

Laser 3D printing based on melt growth has great potential in rapid preparation of Al2O3-based eutectic ceramics. In this work, large-scale Al2O3/GdAlO3/ZrO2 ternary eutectic ceramic rod with diameter of 4–5 mm and height higher than 250 mm was additively manufactured by laser directed energy deposition. Especially, heat treatment was applied to eliminate the microstructure heterogeneity in the as-deposited eutectic ceramic, and the microstructure homogenization mechanism was studied in depth. The results indicate that colonies and banded structures completely disappear after the heat treatment, producing a homogeneous network eutectic structure. The microstructure homogenization is revealed to experience three stages of discontinuous coarsening, continuous coarsening and microstructure coalescence. Additionally, it is found that the eutectic spacing linearly increases with the heat treatment time, meaning that the coarsening behavior of the laser 3D-printed Al2O3/GdAlO3/ZrO2 eutectic ceramic satisfies well with the Graham-Kraft model.

Effects of Ru content on phase transformation and compression property of cast TiAl alloys

Ruthenium (Ru) is a promising element to heighten the comprehensive mechanical properties of TiAl alloys. In the present study, the phase transformation during the cooling of Ti-47Al-2Cr-2Nb-xRu (x=0, 0.1, 0.5, 1.0, at.%) alloys was investigated. The results show that Ru tends to segregate in the interdendritic region during solidification, and therefore, refines the as-cast microstructure of the alloys. As a β-stabilizer, Ru does not induce a novel solidification reaction but maintains the peritectic reaction of L+β→α. An excessive Ru content would cause the formation of (γ+τ1+B2) and (α+γ+τ1+B2) phase regions below 1,250 °C. The precipitation of τ1-containing mixture is attributed to the Ru-segregation, which inhibits the reaction of α→γ and facilitates the formation of B2 phase. The discontinuous coarsening of γ phase and block τ1 phase formed alternately in a manner of analogous eutectoid decomposition within B2 phase. In addition, the effect of Ru content on compression property of the alloys was studied. The yield strength increases up to 427 MPa at 800 °C with the addition of 1.0at.% Ru, which is mainly due to the solution strengthening effect of Ru.

Influence of heat treatment on microstructure and nanohardness of TiAl alloy solidified under high pressure

The Ti-48at.%Al alloy solidified under different pressures was prepared using a tungsten-carbide six-anvil apparatus and then heat treated at different temperatures. The effect of heat treatment on microstructure and mechanical properties of high pressure solidified Ti-48Al alloy was investigated. The results showed that the phase constitution of the Ti-48Al alloy solidified under high pressure did not change after heat treatment. After 1,100 °C/12 h AC (air cooling), the value of c/a showed a slight increase. The Widmanstatten structure was formed in the samples solidified under 2 GPa and 4 GPa. More defects were found in the high-pressure samples. The volume fraction of α2 particles increased with an increase in pressure. After 1,150 °C/4 h AC, the discontinuous coarsening phenomenon could only be found in the 4 GPa solidified samples. The formation of α2 phase can lead to the increase of nanohardness. Compared with the samples before heat treatment, the nanohardness increased about 1.0–1.2 GPa.

Continuous-Cooling-Transformation (CCT) Behaviors and Fine-Grained Nearly Lamellar (FGNL) Microstructure Formation in a Cast Ti-48Al-4Nb-2Cr Alloy

The CCT behaviors of a high-Al mid-Nb-containing cast TiAl alloy were experimentally studied in the current work. Samples with the nominal composition of Ti-48.2Al-4Nb-2Cr (at.pct) were preheated to 1420 °C and then cooled at different cooling rates. With preheating time of 8 minutes, the original microstructure was replaced by coarsening α grains. Interrupted tests demonstrated the significant decrease of microsegregation which was also predicted by theoretical calculations. The water-quenched microstructures during continuous cooling were characterized. With the increase of cooling rate from 5 to 700 °C/min, the lamellar spacing was refined from 1.77 to 0.43 μm and the microhardness was heightened from 299 to 426 HV, showing the solute strengthening effect of Nb addition compared with 4722 and Ti-48Al alloys. The CCT diagram of the current alloy was established to guide the microstructure control. Slow cooling contributed to the discontinuous coarsening of γ lamellae, suggesting a nearly lamellar (NL) microstructure. The addition of 4 at. pct Nb increased the temperatures of phase transformations, moved the CCT curves to higher temperature, and decreased the critical cooling rate for the formation of Widmanstatten and feathery structures. Finally, a fine-grained NL (FGNL) microstructure with a potential high performance was obtained.

Evolution of Metastable α2 Phase in a Quenched High‐Nb‐Containing TiAl Alloy at 800 °C

Fast cooling from high temperatures is a common way in heat treatments and hot processing, which could introduce excess metastable α2 phase in the microstructure. Herein, the evolution of the metastable α2 phase in a water‐quenched Ti–45Al–8.5Nb–(W, B, Y) alloy from different single‐phase regions is experimentally studied. After water quenching from the β single‐phase region, Widmanstätten α2, as well as residual βo, γ grains form from β phase and thin γ laths in α2 phase are observed. However, the water‐quenched microstructure from the α single‐phase region consists of equiaxed α2 grains, residual βo, and massive γ phase. When annealing the quenched samples at 800 °C, similar microstructural evolutions are found in both samples quenched from different temperatures. 1) α2 phase transform into α2/γ nanolamellar structure immediately. 2) Recrystallization and discontinuous coarsening occur at the colony boundaries. 3) The nanometer‐scale γ/γT lamellae transformed from α2 grains are also unstable, which merge into thicker γ lamellae, and the interfaces of coarsened γ lamellae become curvy and blurry.

Phase reversion kinetics of thermally decomposed (α + γ′) phases to γ-phase in U – 10 wt% Mo alloy

Thermally induced phase reversion kinetics of decomposed α + γ′ phases to γ phase was investigated for the U – 10 wt% Mo (U–10Mo) hypoeutectoid alloy using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The as-cast U–10Mo alloy was homogenized to the γ phase at 1073 K for 96 h, and decomposed at sub-eutectoid temperature of 773 K as a function of time from 240 to 1200 h. The decomposition of γ phase initiated via two mechanisms: (1) discontinuous precipitation (DP) or cellular precipitation along the grain boundaries and inclusions; and (2) continuous precipitation (CP) within the γ grains as γ′ needles. After 720 h at 773 K, the DP colonies completely consumed the continuous γ matrix, after which discontinuous coarsening (DC) was observed. The completely-decomposed U–10Mo alloy samples were then annealed at 843, 853, and 863 K as a function of time to document the kinetics of α+γ’ → γ phase reversion. The reference intensity ratio analysis was employed to XRD patterns to quantify the phase amount after each heat treatment. A combinatorial kinetic model, consisting of the classical JMAK nucleation and growth, and spherical particle dissolution, was employed to describe the α+γ’ → γ phase reversion as a function of time and temperature. This phenomenological model provided a good approximation for the phase reversion within the temperature and time ranges examined in this study. Furthermore, a diffusion enhancement approach was employed for a set of typical irradiation parameters to estimate the irradiation effect on the α+γ’ → γ phase reversion kinetics.

The influence of niobium, tantalum and zirconium on the microstructure and creep strength of fully lamellar γ/α2 titanium aluminides

Titanium aluminides are excellent materials for light-weight applications at high temperatures. To enhance their creep strength, various alloying elements are added. In this work the effect of 5 at% Niobium (Nb), Tantalum (Ta) and Zirconium (Zr) on the creep properties of ternary Ti-44Al-5X (X = Nb, Ta, Zr) with a fully lamellar microstructure, is systematically studied. These alloying elements have a high solubility in titanium aluminides, and show a different α2/γ partitioning behavior as well as a different influence on the lattice parameters of both phases. The average interface distance between the α2- and γ-phase is adjusted to about 300 nm by the cooling rate to evaluate the influence of the three alloying elements. The creep tests reveal that all three alloying elements increase the creep resistance of titanium aluminides, particularly Ta and Zr. Ta seems to extend the pronounced primary creep regime, whereas Zr seems to reduce it. The alloying elements decrease also the lamellar spacing compared to the binary alloy. Microstructural investigations after the creep tests show a beneficial effect of the alloying elements on the microstructural stability during creep exposure, since they retard the discontinuous coarsening rate.