Microstructure,creep, and tensile deformation in Ti-6Al-2Nb-1Ta-0.8Mo

The effects of microstructure, temperature, and stress level on the creep response of Ti-6211 have been investigated. A variety of microstructures simulating the heat affected zone of a weld (HAZ), as well as the as-received structure, were tested in a temperature range of 298 K to 873 K. At stress levels below the tensile yield strength, creep curves saturate in the ambient temperature regime. The colony type Widmanstätten alpha + beta as-received structure exhibited the highest creep strains at ambient temperatures. Long slip lengths associated with the large colony size, and sliding along various interfaces account for the relatively high creep strains to saturation. Planar arrays of straight dislocations operating on a single slip system were observed for samples crept at 298 K while thermally activated cross slip was observed for samples crept at 453 K. Beta-annealed martensitic micro-structures displayed enhanced creep resistance, out-performing other recrystallized HAZ structures. Above 778 K the activation energy of creep is close to that for self-diffusion in titanium, suggesting that diffusion-controlled dislocation mechanisms are the rate-controlling processes at elevated temperatures. Creep rupture at elevated temperatures occurred by microvoid nucleation and growth. Fracture occurred along colony boundaries in the as-received structure but appear to be intergranular with the crack propagating along G. B. alpha/matrix interfaces in the equiaxed microstructures. Sliding along alpha/beta interfaces, colony boundaries, prior beta grain boundaries, and slip traces contributed to the creep strain and rupture process. Cyclic creep with a loading-unloading sequence was also performed at room temperature and cyclic creep acceleration was observed.     

The effect of microstructure on the deformation modes and mechanical properties of Ti-6Al-2Nb-1Ta-0.8Mo: Part II. Equiaxed structures

The Ti-6Al-2Nb-lTa-0.8Mo alloy was processed to develop both near-basal and transverse textures. Samples were annealed at different temperatures to vary the equiaxed alpha grain size and the thick-ness of the grain boundary beta, and subsequently quenched in order to transform the beta phase to either martensite, tempered martensite, or Widmanstätten alpha + beta. The effect of microstructure and texture on tensile properties and on fracture toughness was investigated. In addition, yield locus diagrams were constructed in order to study the texture strengthening effect. The yield strength was found to be strongly dependent on the thickness and Burgers relationship of the transformed beta phase surrounding the alpha grains. A texture hardening effect as large as 60 pct was found for the basal-texture material but only 15 pct for the transverse texture material. These variations are asso-ciated with differences in deformation behavior.     

The effect of microstructure on the deformation modes and mechanical properties of Ti-6Al-2Nb-1Ta-0.8Mo: Part I. Widmanstätten structures

An alpha + beta Ti-6Al-2Nb-lTa-0.8Mo alloy with an initial Widmanstätten structure was thermally treated to produce a wide range of microstructures. The effects of individual microstructural parameters on deformation behavior and mechanical properties were investigated. The results show that the Widmanstätten colony boundaries are major barriers to slip. However, the slip distance can be decreased to a distance equal to the thickness of acicular alpha by transforming the beta phase in the Widmanstätten structure to martensite by quenching from 950°C. The decrease in slip distance is accompanied by a 25 pct increase in yield strength with no loss in ductility. A large decrease in ductility occurs after excursions above the beta-transus. The development of both equiaxed beta grains during heating in the beta phase field and continuous grain boundary alpha during cooling in the alpha + beta phase field leads to strain localization along prior beta grain boundaries.     

Elevated temperature flow strength,creep resistance and diffusion welding characteristics of Ti-6Al-2Nb-1Ta-0.8Mo

A study of the flow strength, creep resistance and diffusion welding characteristics of the titanium alloy Ti-6Al-2Nb-1Ta-0.8Mo has been conducted. Two mill-processed forms of this alloy were examined. The forged material had been essentially processed above the beta transus (∼1275 K) while the rolled form had been subjected to work below the beta transus. Between 1150 and 1250 K, the forged material was stronger and more creep resistant than the rolled alloy. Both forms exhibit superplastic characteristics in this temperature range. Strain measurements during diffusion welding experiments at 1200 K reveal that weld interfaces have no measurable effect on the overall creep deformation. Significant deformation appears to be necessary to produce a quality diffusion weld between superplastic materials. A “soft” interlayer inserted between faying surfaces would seemingly allow manufacture of quality diffusion welds with little overall deformation.     

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.     

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.     

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.     

The influence of microstructure and strain rate on the compressive deformation behavior of Ti-6Al-4V

This article reports on a study of deformation of Ti-6Al-4V in compression. In particular, two different microstructures, the equiaxed microstructure and the Widmanstätten microstructure, were generated from the same parent material and their properties were measured. The results show that at small strains, the mechanical response of samples with these microstructures is similar. The yield strength and the flow stress at a 0.05 true strain have similar values; these increase with increasing strain rate over the range of 0.1 to 1000 s^?1. However, samples with the Widmanstätten microstructure failed at a smaller strain than their counterparts with the equiaxed microstructure, and this difference increased with increasing strain rate. Examination of cross sections of samples deformed to different levels of strain showed that the deformation was inhomogeneous. As the sample barreled, the deformation built up on the surfaces of two cones of material whose apices met in the center of the sample. Cracks formed in the corners of the samples and propagated in toward the center. In samples with the equiaxed microstructure, short cracks and voids formed, but they were usually blunted at the grain boundaries. Long cracks were only observed immediately before failure. In samples with the Widmanstätten microstructure, cracks could grow within the laths more easily, and, as a result, longer cracks formed at lower strains. We propose that this difference leads to the differences in the failure strains for these two microstructures. Finally, examination of data in the literature, along with our own results, indicates that the interstitial content plays an important role in determining the yield stress of the material.     

Effects of rolling deformation on microstructure and hardness of Ti-45Al-9Nb-0.3Y alloy

The microstructure evolution of as-rolled Ti-45Al-9Nb-0.3Y alloy as well as the nanohardness of β/B2 matrix was investigated by means of scanning electron microscopy(SEM) in backscattered electron mode(BSE) mode, transmission electron microscopy(TEM) and nanoindentation. This high Nb containing Ti Al based alloy was rolled with 50%, 60%, 65% reduction, respectively. Omega phase precipitated in B2 phase with an orientation relationship of {110}_β//{11 2 0}_ω and 11 1_β//0001_ω. Moreover, with the increase of deformation reduction, rod-like structure which was formed in γ grain transformed from(α_2+γ) lamellae structure into α_2 phase only. Additionally, nanoinentation experiment revealed that the precipitation hardening of ω phase increased the hardness of β/B2 phase.     

轧制变形量对Ti-45Al-7Nb-0.3W合金组织与性能的影响

利用X衍射分析(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、室温拉伸试验等手段,研究粉末冶金Ti-45Al-7Nb-0.3W(原子分数,%)合金包套轧制过程中的显微组织和力学性能的变化规律。结果表明:热等静压法态的Ti-45Al-7Nb-0.3W合金组织为近γ组织,主要由块状的γ相组成,同时包括少量的α2相及极少量的B2相。轧制后Ti Al合金板材为双态组织,B2相消失。随轧制变形量增加,合金板材强度增加,变形量为40%时,板材抗拉强度最大,达到955 MPa。继续增加变形量合金板材的力学性能有所降低。当变形量较小时,合金的塑性变形主要通过位错滑移和攀移来实现。随变形量增加,孪生和动态再结晶机制发挥作用。     

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.     

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.     

The effect of cooling conditions on the microstructure of rapidly solidified Ti-6Al-4V

The effect of cooling conditions, giving estimated cooling rates in the range 10⁴ °C per second to 10⁷ °C per second, on the microstructure of Ti-6Al-4V has been evaluated. The microstructures of as-solidified particulates were martensitic, with the martensite lath length decreasing with beta grain size,L, which in turn decreased with increasing cooling rate. For material alpha + beta heat-treated or vacuum hot pressed, the alpha morphology was dependent on the prior cooling rate. For materials cooled at <5 × 10⁵ °C per second martensite transformed to lenticular alpha, while material cooled at >5 × 10⁵ °C per second developed an equiaxed alpha morphology. This change in morphology was explained in terms of high dislocation density or grain size refinement, both of which result from the high cooling rate. When the beta grain size (L) was plottedvs section thickness (z), and estimated cooling rate (T), power law relationships analogous to those reported for secondary dendrite arm spacing were found:L = 1.3 ± 0.4z^089±006 (thin, chill-substrate quenched),L = 0.17 ± 0.05z^0.86±0.01(thick, convection-cooled material), andL = 3.1 × 10⁶ T^{−0.93±0.12} (all material), whereL and z are in μm andT is in K/s. The last relationship is in agreement with the 0.9 exponent predicted using a model developed for the effect of grain size on cooling rate assuming classical homogeneous nucleation and isotropic linear growth during solidification. The first two relationships were rationalized by assuming that the two materials cooled under near-Newtonian conditions.     

Elevated temperature tensile behavior of Ti-25AI-10Nb-3V-1Mo

The effects of microstructure and temperature on tensile and fracture behavior were explored for the titanium aluminide alloy Ti-25Al-10Nb-3V-lMo (atomic percent). Three microstructures were selected for study in an attempt to determine the role of the individual microstructural constituents in this α₂ + B2 alloy. Tensile testing of both round and flat specimens in vacuum indicated a change in deformation behavior from 25 °C to 450 °C. Observations suggested that this change in deformation behavior occurred within the α₂ phase. Failure initiation at 450 °C and above was by a ductile process and was associated with the B2 phase. Above 600 °C and at high strains, plastic deformation occurred predominantly in the B2 phase. Strain localization was observed above 600 °C and found to be due to the lower work-hardening rate of the B2 phase. Strain localization at slip band intersections with prior β grain boundaries resulted in rapid strain accumulation in the B2 phase. Alignment of secondary α₂ laths with the tensile axis at high deformation levels appeared to inhibit shear band localization between voids due to a lack of participation of the α₂ phase in deformation. Formerly Materials Scientist, Materials Directorate, Wright Laboratory, Wright-Patterson AFB, Dayton, OH 45433, is Program Manager, Air Force Office of Scientific Research, Boiling AFB, Washington, DC 20332. Formerly Professor, Carnegie Mellon University, Department of Materials Science and Engineering, Pittsburgh, PA 15213, is Scientist, Lawrence Berkeley Laboratory, Berkeley, CA 94720.     

Effect of strain reversal on the dynamic spheroidization of Ti-6Al-4V during hot deformation

The effect of strain-path reversal on the kinetics of dynamic spheroidization during subtransus hot working was determined for Ti-6Al-4V with a colony α structure. Isothermal torsion tests were conducted at a temperature of 815 °C and a strain rate of 0.001 s⁻¹; strain-path reversals were achieved by applying forward and reverse torsion sequentially. The kinetics of spheroidization were measured as a function of the local (macroscopic) strain for monotonic-deformation, reversed-torsion, and double-reversed-torsion tests. Strain-path reversal led to a reduction in the spheroidization kinetics compared with monotonic deformation for a given total strain. The slower rate of dynamic spheroidization associated with strain-path reversals was ascribed to a reduced rate of sub-boundary formation/lower sub-boundary energies, which drive the boundary splitting process, and less sharp α/β interface curvatures, which control the coarsening process that also contributes to spheroidization.     

Hot ductility of DSS and its microstructure observation during hot deformation

selecting several typical DSS 00Cr22Ni5Mo3N,00Cr21Ni2Mn5N and 00Cr25Ni7Mo4N as research materials,hot ductility characteristic of DSS was studied and microstructure evolution during hot compression was observed.The results show that the optimum hot ductility temperature range of DSS is 1 050~1 200℃.00Cr25Ni7Mc4N exhibits the worst hot ductility and 00Cr21Ni2Mn5N has similar hot ductility to 00Cr22Ni5Mo3N.During hot compression,austenite of DSS mainly occurs dynamic recovery,the ferrite of 00Cr22Ni5Mo3N,00Cr21Ni2Mn5N can perform dynamic recovery and recrystallization,but only dynamic recovery can be observed in the ferrite of 00Cr25Ni7Mo4N.