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.     

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...     

Effects of interstitial oxygen on microstructure and mechanical properties of Ti-48Al-2Cr-2Nb with fully lamellar and duplex microstructures

The effect of added oxygen in the range of 1000 to 4000 wt ppm on the microstructures of a Ti-48Al-2Cr-2Nb alloy has been investigated and compared to the microstructures for a high-purity alloy. For specimens cooled from theα phase, interstitial oxygen stabilizes the lamellar microstructure with respect toγ grains, with higher than equilibrium values for theα ₂ volume fraction. For specimens cooled from theα +γ phase field, the lamellar microstructure still tends to be favored by oxygen, but it is found that the phase volume fractions are not significantly different from equilibrium values. This suggests that interstitial O essentially reduces the kinetics of theα toα +γ transformation. Thus, interstitial oxygen will have a strong effect on microstructures obtained by continuous cooling fromα, but significantly less on those, such as the duplex microstructure, obtained by long treatment in theα +γ phase field. In general, increased O content is well correlated with reduced ductility. Finally, the role of interstitial oxygen on this phase transformation is discussed.     

Elimination of inclusions and evaporation control during melting of Ti-48Al-2Cr-2Nb-1B intermetallic alloy

Titanium aluminide intermetallic compounds have an excellent capability for use in engineering structures at high temperatures. In the present work the formation of Nb rich inclusions in microstructure and evaporation of Al during melting of γ-TiAl based alloy (Ti-48Al-2Cr-2Nb-1B (at %)) was studied. The results show that the inclusions cannot be removed even with a four-stage melting process, when elemental Nb is used as raw material. However, by replacing Nb with NbAl₃ and using a three-stage melting process, the inclusions were removed from microstructure and also evaporation of Al was reduced remarkably. Otherwise, with removing elemental Al from raw material by using TiAl compound, evaporation of Al will be very low. Increasing vessel pressure from 400 to 600 mbar will not influence evaporation of Al. The article is published in the original.     

The mechanism of formation of a fine duplex microstructure in Ti-48Al-2Mn-2Nb alloys

The mechanism of formation of the fine duplex microstructure resulting from the α → γ transformation in water-quenched Ti-48Al-2Mn-2Nb alloys was studied using transmission and analytical electron microscopy. As-cast Ti-48Al-2Mn-2Nb alloys were heat treated in the α phase field and water quenched to room temperature. The resulting microstructure (referred to as a fine duplex microstructure) consisted of equiaxed grains and abutting lath colonies. Both the colonies and the grains were composed of the γ phase, twinned γ laths, and α₂ laths. It was found that the transformation from α to γ in the fine duplex microstructure took place through long range diffusional processes, and compctitive growth between the equiaxed and lath morphology occurred. Nucleation of they phase from the α matrix can occur through nucleation on stacking faults, followed by growth through the sympathetic nucleation and growth of new γ laths on a substrate lath. The observed misorientations and the interfacial structures between the laths were found to be consistent with such a mechanism. Compctition between such nucleation and growth mechanisms for the equiaxed and lath morphologies of γ leads to the formation of lath colonies (of γ and α₂) interspersed with equiaxed grains in these alloys. Formerly Visiting Scientist, Metals and Ceramics Division, Oak Ridge National Laboratory This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.     

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.     

Deformation structure in a Ti-24Al-11Nb alloy

A Ti-24Al-11Nb alloy has been heat-treated so as to obtain a microstructure of coarse α₂ particles (D0₁₉ structure based on Ti₃Al) in a matrix of the ordered β_o phase (B2 structure based on Ti₂AlNb). Dislocation structures generated by tensile strains of ∼2 pct at room temperature have been analyzed by transmission electron microscopy The β_o phase is shown to deform inhomogeneously on {110}, {112}, and {123} planes by α/〈211〉 slip. The slipband structure is complex, consisting of segments of heavily pinned edge dislocations with periodic cross slip of screw components on to secondary slip planes. Incompatibility stresses at α₂/β_o interfaces can generate fine α[100] slip as well. The α₂ phase deforms independently by α dislocation slip. Slipbands in the β_o phase can shear the α₂ phase by activatingc +a/2 slip on and slip planes, as well asa slip on higher order pyramidal planes, where the parallelism of the specific slip system is permitted by the Burgers relationship between the two phases.     

外加应力对Ti-25Al-10Nb合金显微组织几何特征的影响

利用相场法显微组织模拟研究了外加应力对Ti-25Al-1ONb(摩尔分数/%)合金中a2→O相相变所产生显微组织几何特征的作用.结果表明,当外加应力达到390MPa时,就开始对O相析出形貌具有显著作用.外加应力改变O相析出形貌是因为:(1)外加应力使O相析出的总量增加,(2)外加应力诱发取向有利的O相变体析出并抑制其它O相变体的析出.外加应力大小对O相析出平均尺寸的影响可达106%,并且O相析出的尺寸标准差随外加应力增加而显著增大.     

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.     

Electron microscopy of the primary microstructure of rapidly solidified Ti-46Al-8Nb alloy

The contradictory data on the formation of the microstructure of a refractory intermetallic Ti-46 at % Al-8 at % Nb alloy and on the phase transformations proceeding within in the Ti-Al-Nb phase diagram are analyzed and improved experimentally. To determine the primary solidified phase, a set of experiments is performed on melting of the alloy, which is synthesized in a high-purity argon atmosphere using crucibles made of an oxygen-free ceramics (99.99% AlN), and subsequent rapid volumetric isothermal solidification. Cooling from 1943 K at rates of 5, 10, and 20 K/s and subsequent quenching from 1763 K are used. Polished sections of ingots are studied by scanning electron microscopy using backscattered electron mode. Scanning electron microscopy micrographs demonstrate contrast regions of nonuniform niobium segregation, which are fixed by quenching and decorate the primary polycrystalline microstructure formed in the temperature range from 1843 (liquidus) to 1773 K (solidus). The primary crystalline phase is shown to be represented by β(Ti) dendrites, which have clearly pronounced fourfold symmetry that form during the development of secondary arms.     

A simple model of deformation of Ti48Al-2Cr-2Nb at high temperatures

The point of maximum deformation resistance (minimum creep rate) and the subsequent softening (tertiary creep) can be modelled on the basis of the tranformation of the fully lamellar structure into the globular structure by dynamic recrystallization. The globular material contains a steady-state dislocation structure with subgrains of size w = 8bG/σ in fine grains of size d ≈ 2 w. As the volume fraction with globular structure increases with strain at the expense of the lamellar structure, the deformation resistance approaches the steady state resistance of the globular structure which is of the order of magnitude predicted by the natural creep law for a solid solution.     

Improvement in mechanical properties of dental cast Ti-6Al-7Nb by thermochemical processing

Hydrogenation and dehydrogenation, that is, thermochemical processing (THP) and its variation with a post-heat treatment (THPH), are investigated in order to improve the balance of strength, elongation, and fatigue strength of cast Ti-6Al-7Nb and Ti-6Al-4V for dental applications. Microstructures of both cast alloys change from coarse Widmanst?tten α structure to super fine α structure with an average diameter of 3 μm by conducting THP or THPH. Tensile strength and fatigue limit of cast Ti-6Al-7Nb and Ti-6Al-4V increase by around 10 and 40 pct, respectively, as compared with those of both as-cast alloys. The balance of strength and ductility of cast Ti-6Al-7Nb is improved by conducting THPH as compared with the case where THP is conducted. This improvement is due to the plastic deformability of unstable β phase because the lattice constant of β phase in each alloy conducted with THPH is much greater than that of each as-cast alloy.     

注射成形Ti-6Al-4V合金的显微组织和力学性能

采用金属注射成形方法制备Ti-6Al-4V合金坯体,然后利用溶剂脱脂和热脱脂工艺脱除坯中粘结剂,研究了合金在真空烧结和热等静压烧结条件下的显微组织和力学性能.结果表明:真空烧结Ti-6Al-4V合金具有典型的魏氏体组织,其初始的β晶粒粗大,β晶粒内为次生片状α和薄β相片,空隙较多,合金的强度和塑性较低;合金经热等静压处理后,组织明显细化且均匀,空隙很少或几乎没有,从而强度和塑性都有所提高.     

VIGA-CC法制备球形Ti-35.8Al-18.4Nb合金粉末及其性能研究

以真空自耗电弧熔炼的Ti-35.8Al-18.4Nb（质量分数）合金铸锭为原料,采用水冷铜坩埚真空感应熔炼气雾化制粉技术（water-cooled copper crucible vacuum induction melting-gas atomizing,VIGA-CC）制备球形Ti-35.8Al-18.4Nb合金粉末,利用振动筛分法、扫描电子显微镜（scanning electron microscope,SEM）观察、X射线衍射（X-ray diffraction,XRD）分析等手段对所制备的粉末进行性能表征。结果表明,VIGA-CC技术制备的粉末粒度分布较宽,主要分布在45~150 μm之间,呈正态分布,其中粒径不高于45 μm粉末收得率为15.8%,粒径不低于150 μm粉末收得率为12%;粉末流动性为27.2[s·(50 g)-1],粉末中氧质量分数的增量小于0.01×10-6,粉末整体氧质量分数小于0.06×10-6;TiAlNb合金粉末主要以γ（TiAl）相和α₂（Ti₃Al）相为主,随着粉末粒径的减小,冷却速率逐渐提高,γ（TiAl）相逐渐减少,α₂（Ti₃Al）相逐渐增加;大颗粒粉末表面为枝状冷凝组织,小颗粒粉末为光滑表面。     

High-temperature low-cycle fatigue and lifetime prediction of Ti-24Al-11Nb alloy

The influence of hold time on low-cycle fatigue (LCF) of Ti-24Al-11Nb was studied at 650 °C. At 0.167 Hz, the alloy exhibits cyclic hardening at all strain levels studied and obeys the well-known Manson-Coffin behavior. A 100-second hold at peak tensile or compressive strain at ±0.6 pct strain has no observable effect on cycles to failure. For hold times at ±0.5 pct strain, however, the fatigue lives are nearly halved and specimens show secondary cracking normal to the stress axis. The increase in inelastic strain as a result of hold time appears to be primarily responsible for the observed loss in fatigue life. A linear life fraction model, which considers both fatigue and creep damage, is found to provide good correlation of measured lives with predictions. For the range of test conditions employed, the total and the tensile hysteretic energy per unit volume, absorbed until fracture, remain nearly constant. The tensile hysteretic energy appears to be a more useful measure of fatigue damage for life prediction. On leave from Defence Metallurgical Research Laboratory, Hyderabad, India 500-258     

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.