Interplay between oxidation and wear behavior of the Ti-48Al-2Cr-2Nb-1B alloy

This study deals with some aspects of the dry sliding behavior of a gamma TiAl-based alloy with the following composition: Ti-48Al-2Cr-2Nb-1B (at. pct). The tribological system consisted of an AISI M2 steel disk sliding against TiAl alloy sheets. Different alloy microstructures—duplex, equiaxed, and lamellar—were considered, in order to check for their possible effect on the wear behavior. At low loads, a mainly abrasive mechanism was inferred from the phases present in the debris collected at the end of each test. An additional contribution from oxidational phenomena was found at higher loads. In such conditions, the equiaxed samples displayed a slightly better wear resistance. A remarkable improvement in wear resistance was observed when the alloy specimens were tested without a preliminary removal of the surface layer which resulted from the heat treatments carried out to obtain different microstructures. Again, some differences were found in the wear behavior of alloys with different microstructures. In this case, though, the difference can rather be attributed to the different morphology of the surface layers. Better performances were observed in those samples, both duplex and equiaxed, in which a good mechanical matching between the alloy and the outer scale was accomplished, thanks to the presence of a hardened interdiffusion zone. An aspect which should be considered further is the tribological coupling of the as-treated alloys. Indeed, the elevated hardness and surface roughness of the coating yields a significant wear of the sliding counterface.     

Autogenous gas tungsten arc weldability of cast alloy Ti-48Al-2Cr-2Nb (atomic percent) versus extruded alloy Ti-46Al-2Cr-2Nb-0.9Mo (atomic percent)

This study examines procedures for consistently producing sound (crack and void free) welds using the autogenous (without filler metal) gas tungsten arc (GTA) welding process. Cast alloy Ti-48Al-2Cr-2Nb (at. pct) and extruded alloy Ti-46Al-2Cr-2Nb-0.9Mo (at. pct) have been examined to determine if sound welds can be produced using autogenous GTA welding without any preheat. Experimentation consisted of GTA spot welding samples of gamma titanium aluminide at weld current levels of 45, 55, 65, and 75 A for a duration of 3 seconds. For the cast alloy, current levels of 45, 55, and 65 A for 3 seconds produced similar fusion zone microstructures, which consisted of a dendritic solidification structure. The fusion zone microstructure of the 75 A for 3 seconds current level differed significantly from the lower current levels. It also consisted of a dendritic solidification structure; however, the morphology was quite different. For the extruded alloy, current levels of 45 and 55 A for 3 seconds produced fusion zone microstructures similar to the lower current level samples of the cast γ-TiAl, which consisted of a dendritic solidification structure. The fusion zone microstructures of the 65 and 75 A samples were similar to each other, but they had a dendritic solidification structure of a different morphology than that of the 45 and 55 A samples. For both alloys at all current levels, microhardness profiles showed an increase in hardness from the base metal to the fusion zone. There were no significant differences in the average fusion zone hardness as a function of increasing current level. However, nanoindentation testing did show that certain phases and microconstituents in the fusion zone did have significant variations in hardness in relation to the enrichment and depletion of chromium. This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations Committees.     

Grain-size control in Ti-48Al-2Cr-2Nb with yttrium additions

A gas-atomized (GA) prealloyed powder of the Ti-48Al-2Cr-2Nb intermetallic and 1.6 wt pct Y were mechanically alloyed (MA) and hot isostatically pressed (hipped) to produce a fully dense nanocrystalline material. Mechanical alloying of the as-blended powder for 16 hours resulted in the formation of a disordered fcc phase. Hipping of the alloy powder produced a single-phase nanocrystalline TiAl intermetallic, containing a distribution of 20 to 35-nm-sized Al₂Y₄O₉ particles. The formation of oxide particles occurred by the chemical combination of Al and Y with oxygen, which entered as a contaminant during milling. Oxide particles increased the hardness of the intermetallic compound and minimized grain growth even at 0.8 T _m , where T _m is the melting point of the compound.     

Study of microstructure and solute partitioning in a cast Ti-48Al-2Cr-2Nb alloy by quenching during directional solidification technique

One major hindrance to effective implementation of cast gamma TiAl-based intermetallic alloys in aircraft engines lies in the variability of their mechanical properties resulting from chemical and microstructural heterogeneities. In the present work, the buildup of microsegregation in a cast Ti-48Al-2Cr-2Nb alloy is investigated through experiments of quenching during directional solidification (QDS). The solidification process, as well as the partitioning of alloying elements, between the solid and liquid phases, is investigated. Considering experimental conditions, the α-hcp phase is found to be the primary solidifying phase. A low dendrite tip temperature of 1475 °C was estimated from thermal recordings. These observations could be explained considering the value of the thermal gradient (around 4 °C/mm). Quantitative values of partition coefficients are proposed for Al, Cr, and Nb. In addition to Al, Cr is found to segregate in interdendritic regions, whereas Nb tends to be retained in the Ti-rich inner dendrites. Considering experimental cumulative solute distributions, the buildup of microsegregation can be satisfactorily represented on the basis of Gulliver-Scheil assumptions. Due to high-temperature quenching, the QDS experiments are also found to be appropriate to the study of high-temperature phase transformations and microstructural development of TiAl-based alloys. The results of QDS experiments are discussed with regard to the range of microstructural and chemical heterogeneities determined within Ti-48Al-2Cr-2Nb investment castings. Finally, regarding solid-state phase transformations subsequent to solidification, the study attempts to explain the formation of B2 phase particles stabilized by the ternary additions. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 17–21, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

High temperature deformation behavior and processing map of hot isostatically pressed Ti-47.5Al-2Cr-2Nb-0.2W-0.2B alloy using gas atomization powders

The hot compressive deformation behavior of hot isostatically pressed Ti-47.5Al-2Cr-2Nb-0.2W-0.2B al-loy using gas atomization powders was systematically investigated and the processing map was obtained in the temperature range of 1323-1473 K and strain rate range of 0.001-0.5 s-1 .The calculated activa-tion energy in the above variational ranges of temperature and strain rate possesses a low activation energy value of approximately 365.6 kJ/mol based on the constitutive relationship models developed with the Ar-rhenius-type constitutive model respectively considering the strain rate and deformation temperature.The hot working flow behavior during the deformation process was analyzed combined with the microstructural evolution.Meanwhile, the processing maps during the deformation process were established based on the dynamic material model and Prasad instability criterion under different deformation conditions.Finally, the optimal hot processing window of this alloy corresponding to the wide temperature range of 1353-1453 K and the low strain rate of 0.001-0.1 s-1 was obtained.     

An investigation of the fatigue and fracture behavior of a Nb-12Al-44Ti-1.5Mo intermetallic alloy

This article presents the results of a study of the fatigue and fracture behavior of a damage-tolerant Nb-12Al-44Ti-1.5Mo alloy. This partially ordered B2 + orthorhombic intermetallic alloy is shown to have attractive combinations of room-temperature ductility (11 to 14 pct), fracture toughness (60 to 92 MPa√m), and comparable fatigue crack growth resistance to IN718, Ti-6Al-4V, and pure Nb at room temperature. The studies show that tensile deformation in the Nb-12Al-44Ti-1.5Mo alloy involves localized plastic deformation (microplasticity via slip-band formation) which initiates at stress levels that are significantly below the uniaxial yield stress (∼9.6 pct of the 0.2 pct offset yield strength (YS)). The onset of bulk yielding is shown to correspond to the spread of microplasticity completely across the gage sections of the tensile specimen. Fatigue crack initiation is also postulated to occur by the accumulation of microplasticity (coarsening of slip bands). Subsequent fatigue crack growth then occurs by the “unzipping” of cracks along slip bands that form ahead of the dominant crack tip. The proposed mechanism of fatigue crack growth is analogous to the unzipping crack growth mechanism that was suggested originally by Neumann for crack growth in single-crystal copper. Slower near-threshold fatigue crack growth rates at 750 °C are attributed to the shielding effects of oxide-induced crack closure. The fatigue and fracture behavior are also compared to those of pure Nb and emerging high-temperature niobium-based intermetallics.     

Thermal-Mechanical fatigue of Ti-48Al-2V alloy and its composite

Thermal-mechanical fatigue (TMF) and isothermal fatigue (IF) of a Ti-48Al-2V alloy and its composite, reinforced with TiB₂ particles, were studied. In-phase TMF testing was conducted under the condition of a minimum temperatureT _min = 100 °C and a maximum temperatureT _max, which ranged from 750 °C to 1400 °C. The applied cyclic stress ranges were 2.8 to 28 MPa and 4.2 to 42 MPa. The IF tests were carried out at aT _max. The TMF and IF lives are longer for lowerT _max and for smaller stress ranges in both the matrix alloy and its composite. The IF life at a givenT _max is shorter than the TMF life in the matrix alloy at all temperatures employed and in the composite only at higher temperatures. At lower temperatures, the TMF and IF lives are essentially the same in the composite. The resistance to TMF is similar in the matrix alloy and the composite, but the IF resistance is greater in the composite than in the matrix alloy. The proposed TMF mechanism is nucleation and growth of voids on interlamellar plate, twin, and grain boundaries; their interlamellar, translamellar, and intergranular linkage; intergranular separation; and disintegration of lamellar structure.     

Microstructure and creep behavior of an orthorhombic Ti-25Al-17Nb-1Mo alloy

Microstructural evolution during three heat-treatment schedules and the terminal microstructures in an orthorhombic alloy of Ti-25Al-17Nb-1Mo were observed and analyzed with optical microscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The creep behavior of the alloy with three different microstructures (a coarse-lath, fine-lath, and fine equiaxed microstructure) was studied over a temperature range of 600 °C to 750 °C and over a stress range of 150 to 400 MPa in air. The steady-state creep rates, apparent stress exponents, and apparent creep activation energies of the various samples have been determined. The results show that creep behaviors in the alloy are strongly influenced by microstructure. The effect on creep by some of the microstructural features, such as the multivariants within the coarse laths and the interfaces of the laths and the equiaxed grains, is also discussed.     

Mechanisms of ambient temperature fatigue crack growth in Ti-46.5Al-3Nb-2Cr-0.2W

Fatigue crack growth studies have been conducted on a two-phase alloy with a nominal composition of Ti-46.5Al-3Nb-2Cr-0.2W (at. pct), heat treated to produce duplex and lamellar microstructures. Fatigue crack growth tests were conducted at 23 °C using computer-controlled servohydraulic loading at a cyclic frequency of 20 Hz. Several test methods were used to obtain fatigue crack growth rate data, including decreasing-load-range-threshold, constant-load-range, and constant-K _max increasing-load-ratio crack growth control. The lamellar microstructure showed substantial improvement in crack growth resistance and an increase in the threshold stress intensity factor range, ΔK _th , when compared with the behavior of the duplex microstructure. The stress ratio had a significant influence on crack growth behavior in both microstructures, which appeared to be a result of roughness-induced crack closure mechanisms. Fractographic characterization of fatigue crack propagation modes indicated a highly tortuous crack path in the fully lamellar microstructure, compared to the duplex microstructure. In addition, limited shear ligament bridging and secondary cracking parallel to the lamellar interfaces were observed in the fully lamellar microstructure during fatigue crack propagation. These observations were incorporated into a model that analyzes the contribution of intrinsic vs extrinsic mechanisms, such as shear ligament bridging and roughness-induced crack closure, to the increased fatigue crack growth resistance observed for the fully lamellar microstructure.     

Development of Non-reactive Mold for Investment Casting of Ti-48Al-2Cr-2Nb Alloys

Metallurgical and Materials Transactions B - The interfacial reactions between liquid Ti-48Al-2Cr-2Nb and various shell mold materials were investigated, and a cost-effective shell mold, containing...     

The effect of processing on the hot workability of Ti-48Al-2Nb-2Cr alloys

The hot compression behavior and microstructure evolution of ingot metallurgy (I/M) and powder metallurgy (P/M) processed samples of the near-γ Ti-aluminide alloy, Ti-48Al-2Nb-2Cr (at. pct), were determined. Three I/M conditions and two P/M conditions were examined in this study. Hot compression tests were performed in the temperature range of 1100 °C to 1300 °C at strain rates ranging from 1.67×10⁻¹/s to 1.67×10⁻⁴/s. The P/M materials consolidated by either hot isostatic pressing (“hipping”) or extrusion exhibited the best hot workability in most cases. The P/M materials possessed finer, more homogeneous microstructures than the I/M materials. It was also noted that improved workability was observed in materials with equiaxed microstructures without any lamellar constituents.     

High-temperature low-cycle fatigue of a gamma titanium aluminide alloy Ti-46Al-2Nb-2Cr

The low-cycle fatigue (LCF) behavior of a gamma titanium aluminide alloy Ti-46Al-2Nb-2Cr in fully lamellar (FL) and nearly lamellar (NL) microstructural conditions is studied at 650 °C and 800 °C, with and without hold times. At 650 °C and 800 °C, the alloy in either condition exhibits cyclic stability at all strain levels studied, excepting the NL structure which shows slight cyclic hardening at higher strain levels at 650 °C. Fracture in the FL condition occurs by a mixed mode comprising delamination, translamellar fracture, and stepwise fracture. On the other hand, fracture occurs mostly by translamellar mode in the NL condition. At both test temperatures, the alloy in the FL condition obeys the well-known Manson-Coffin behavior. The fatigue resistance of the alloy at 650 °C in the FL condition is very much comparable to, while in the NL condition it is superior to, that of Ti-24Al-llNb alloy. At 650 °C, a 100-second peak tensile strain hold doubles the fatigue life of the alloy in the FL condition, while a 100-second hold at compressive peak strain or at both tensile and compressive peak strain degrades fatigue life. The observed hold time effects can primarily be attributed to mean stress. Irrespective of the nature of the test, the hysteretic energy (total as well as tensile) per cycle remains nearly constant during the majority of its life. The total and tensile hysteretic energy to fracture, at both test temperatures, increase with cycles to failure, and the variation follows a power-law relationship. Formerly NRC Senior Resident Associate, Wright Laboratory.     

Changes in microstructure during primary creep of a Ti-47Al-2Nb-1Mn-0.5W-0.5Mo-0.2Si alloy

Cast gamma titanium aluminides are gaining acceptance as potential replacements for superalloy and steel components in many applications. One particular alloy with W, Mo, and Si additions has shown exceptional primary creep resistance. Quantitative microscopic comparisons were made between microstructures in undeformed and deformed regions in creep specimens deformed to strains between 0.1 and 1.5 pct strain, using optical microscope, scanning electron microscope (SEM), and transmission electron microscope (TEM) techniques. As-hot isostatically pressed (“hipped”) and heat-treated (1010 °C for 50 hours) conditions were compared. The as-hipped specimen had a higher lamellar volume fraction, and it crept more than 100 times faster. The lamellar spacing in the lamellar grains systematically decreased by 15 to 35 pct, with increasing stress, during the first 0.1 to 2 pct strain. Precipitates containing W, Mo, and/or Si were observed in the deformed gage and undeformed grip sections of the heat-treated specimens. Precipitation is nucleated by heat treatment, but, during creep deformation, a more homogeneous and faster growth process occurs in the gage section than in the aged but undeformed grip section. The gage section had a 35 pct higher precipitate volume fraction, but their average size was smaller. A lower volume fraction of lamellar grains and the presence of precipitates account for the excellent creep resistance in the heat-treated alloy. This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations Committees.     

Mathematical modeling of aluminum evaporation during electron-beam cold-hearth melting of Ti-6Al-4V ingots

A mathematical model was developed to describe the kinetics of aluminum evaporation during the electron-beam cold-hearth melting of titanium alloys. The analysis treated the diffusion of aluminum to the surface of the melt, chemical reaction between the melt and vapor phases at the surface, and the transport of aluminum into the vacuum chamber. The kinetics equation was combined with appropriate mass and energy balance equations to determine the effect of melting rate, the electron-beam power input to the melt pool in the hearth and the mold, and charge chemistry on the composition of the cast ingot. The model was validated by comparison to measurements for Ti-6Al-4V processed under a wide range of conditions.     

Phase transformation and microstructural evolution in Ti-44Al-4Nb-4Zr alloy during heat treatment

Phase transformation and microstructural evolution have been studied in Ti-44Al-4Nb-4Zr-0.2Si-0.1B alloys that were cooled from theα +β phase region with various cooling rates. It has been shown that the cooling rates have different influence on the morphology of the transformation products for the three phase transformations studied,α →α ₂, B2 →ω, andα →γ. Under slow cooling, all three transformations can be fulfilled. Under rapid cooling, B2 →ω is partially detained and a diffuseω phase forms as metastable phase, butα →γ is almost completely suppressed, which supports that theγ lamellae formation is diffusion controlled.     

Flow Behavior and Hot Workability of Nb-15Si-22Ti-5Cr-3Al-2.5Hf Alloy

Constitutive models for flow behaviors of an arc-melted Nb-15Si-22Ti-5Cr-3Al-2.5Hf alloy at temperatures of 1350 °C to 1500 °C and strain rates of 0.001 to 0.1 s⁻¹ have been successfully established during work hardening and dynamic softening stages, respectively, and relatively small average absolute relative errors of the predicted flow stresses are reached (7.7 pct for the work hardening stage and 5.7 pct for the dynamic softening stage). The hot processing map has also been established successfully for this Nb-Si-based ultrahigh temperature alloy. The favorable conditions for hot working of this alloy have been determined as 1350 °C to 1380 °C/0.001 to 0.003 s⁻¹ and 1380 °C to 1440 °C/0.001 to 0.01 s⁻¹. The deformed microstructures under different conditions have been explored and the mechanisms for flow instability of this alloy have been revealed. Flow instability at relatively low temperatures and high strain rates (1350 °C and 1410 °C, 0.1 s⁻¹) is mainly derived from the cracking of Nb₅Si₃ and the debonding of Nbss/Nb₅Si₃ interfaces, while flow instability at higher temperatures (1500 °C) should primarily result from the development of cracks and holes within the Nbss phase because of excessive strain accumulation.

     

Creep behavior of Ti-25Al-10Nb-3V-1Mo

A study has been made of the role of microstracture in room-temperature tensile properties as well as elevated-temperature creep behavior of an advanced Ti₃Al-base alloy, Ti-25Al-10Nb-3V-lMo (atomic percent). Creep studies have been performed on this alloy as a function of stress and temperature between 650 °C and 870 °C, since the use of conventional titanium alloys has generally been restricted to temperatures below 600 °C. A pronounced influence of microstructure on creep resistance was found. Generally, the β solution-treated colony-type (slow-cooled or SC) microstructure showed superior creep resistance. This improved creep resistance in β/SC is accompanied by lower room-temperature tensile strength and ductility. Study of the stress dependence of steady-state creep rate indicates that increasing temperature caused a gradual decrease in the stress exponentn and a transition in creep mechanism at 870 °C, depending on applied stress level. Transmission electron microscopy observations of deformed dislocation structures developed during steady-state creep and room-temperature tensile tests, as well as the corresponding fracture modes, were used to interpret properties as a function of temperature. Finally, creep behavior of the present Ti₃Al alloy was found to be superior to that of conventional near-α titanium alloys. WONSUK CHO, formerly with Carnegie Mellon University, is Senior Research Staff Member, Kia Technical Center, Yeoeuido, P.O. Box 560, Seoul, Korea. JAMES WILLIAMS, formerly Dean of Engineering, Carnegie Mellon University.     

Ti-44Al-5V-1Cr-0.3Ni-0.1Hf-0.15Gd合金的微观组织和拉伸性能

针对变形量达86%的锻造Ti-44Al-5V-1Cr-0.3Ni-0.1Hf-0.15Gd(原子分数,%)合金,对其进行热处理获得近层片组织,研究变形合金及其近层片组织的微观组织特征,并进行了室温、700℃和800℃拉伸实验.组织观察发现,近层片组织由层片团、分布于层片团界的β相以及弥散分布于基体的椭圆状Gd析出物组成.层片团的平均尺寸为40μm,层片组织、β相和Gd析出物的体积分数分别为93.73%、5.25%和1.02%.拉伸结果显示,室温下合金试样的平均抗拉强度为865MPa,平均延伸率可达4.17%,700℃时其平均抗拉强度和延伸率分别为643MPa和22%.Ti-44Al-5V-1Cr-0.3Ni-0.1Hf-0.15Gd合金不仅具有与高β相TiAl合金相当的塑性变形能力,且室温塑性也得到显著提高,这主要归因于β相体积分数的下降和Gd化合物对微观组织室温塑性的贡献.     

The effect of ballistic impacts on the high-cycle fatigue properties of Ti-48Al-2Nb-2Cr (atomic percent)

The ability of γ-TiAl to withstand potential foreign object damage (FOD) and/or domestic object damage (DOD) is a technical risk to the implementation of γ-TiAl in low-pressure turbine (LPT) blade applications. The overall purpose of the present study was to determine the influence of ballistic impact damage on the high-cycle fatigue strength of γ-TiAl-simulated LPT blades. Impact and specimen variables included ballistic impact energy, projectile hardness, impact temperature, impact location, and leading-edge thickness. The level of damage induced by the ballistic impacting was studied and quantified on both the impact (front) and backside of the specimens. Multiple linear regression was used to model the cracking and fatigue response as a function of the impact variables. Of the impact variables studied, impact energy, had the largest influence on the response of γ-TiAl to ballistic impacting. Backside crack length (BSCL) was the best predictor of remnant fatigue strength for low-energy impacts (<0.74 J), whereas Hertzian crack length (HCL) (impact side damage) was the best predictor for higher-energy impacts. The impacted γ-TiAl samples displayed a classical mean stress dependence on the fatigue strength. For the fatigue design stresses of a sixth-stage LPT blade in a GE90 engine, a Ti-48Al-2Nb-2Cr LPT blade would survive an impact of normal service conditions.