Structure,tensile deformation,and fracture of a Ti3Al-Nb alloy

The development of Ti₃Al-Nb alloys is an excellent example of the recent resurgence of interest in the use of intermetallics for high-temperature applications. We examine, in this contribution, the structure of a typical alloy Ti-24A1-11Nb and show it to consist primarily of the ordered α₂ phase (based on Ti₃Al, DO₁₉) and β_o, (based on Ti₂NbAl, B2) phases, with small amounts of a third phase, which is distorted slightly to an orthorhombic symmetry from the D0₁₉ (hexagonal) structure. Tensile properties have been examined on samples heat-treated to vary the size, shape, and volume fraction of α₂ phase and the deformation and fracture behavior of the ordered, two-phase mixture established. The tensile ductility is seen to maximize at intermediate volume fractions of the α₂ and β_o phases (∼30 pct) at values of 6 to 10 pct elongation to fracture, depending on the grain size of the β_o phase. A rationale incorporating the failure modes of the two phases—cleavage of α₂ and slipband decohesion of β_o—has been evolved to explain the trends in ductility with heat treatment.     

Martensitic phase transformations in IMI 550 (Ti-4Al-4Mo-2Sn-0.5 Si)

The influence of solution-treatment temperature on the martensitic phase transformations observed in IMI 550 (Ti-4Al-4Mo-2Sn-0.5Si) has been investigated. When solution treatment is conducted at temperatures above 1233 K, a hexagonal martensite (α′) is formed on rapid cooling. However solution treatment at temperatures between 1233 and 1123 K results in the formation of an orthorhombic martensite (α″) on rapid cooling. Finally, below 1123 K, the β phase is stable—no martensitic transformation occurs on rapid cooling. This transition from α′ → α _primary + (α′ + β _retained) → α _primary + (α″ + β _retained) → α _primary + β _metastable + ω, with decreasing solution-treatment temperature, is shown to be a result of alloy partitioning during solution treatment. Crystallographic analysis indicates that the transition in the martensite crystal structure with decreasing solution-treatment temperature is related to chemical short-range ordering (CSRO) in the high-temperature β phase.     

Microstructure and phase relations for Ti-Mo-Al alloys

The influence of aluminum additions to a Ti-7 at. pet Mo alloy on the phase equilibria was investigated. The microstructures of the alloys, Ti-7 pct Mo-7 pct Al and Ti-7 pct Mo-16 pct Al, were determined by light and electron microscopy. It was found that with increasing aluminum concentration the formation of the metastable w phase was suppressed. In the Ti-7 pct Mo-16 pct Al alloy the β phase decomposed upon quenching by precipitating coherent, ordered particles having a B2 type of crystal structure (β₂). At low temperatures the equilibrium phases for this alloy were β + α+ β ₂, whereas at high temperature (850° to 950°C) the Ti₃Al phase was in two-phase equilibrium with the β phase. The four-phase equilibrium which exists at a temperature of about 550°C involves the reaction β + Ti₃Al ⇌ α + β₂. G. LUETJERING, formerly Staff Member Materials Research Center, Allied Chemical Corp., Morristown, N. J.,     

Metastable phase equilibria in the lead-tin alloy system: Part II. Thermodynamic modeling

The lattice stability of Sn is reassessed in view of additional pressure-temperature data since the earlier assessment by Kaufman. The metastable phase boundary of the α-Pb phase reported in Part I in connection with available thermochemical data is used to define more accurately the model paraaeters for the stable L, α, β, and metastable α₁-phases. Using these models, the stable phase diagram of Pb−Sn, the metastable extension of the stable phase diagram, and several possible metastable phase diagrams are calculated. Thermodynamic and kinetic criteria are used to discuss the formation of various phases during rapid solidification processes.     

Transformations in a Ti-24Al- 15Nb alloy: Part II. a composition invariant βo → O transformation

This article explores the nature of a composition invariant phase transformation fromβ _o(B2, Ti₂AlNb) to an orthorhombic O phase (Cmcm) on aging aβ quenched Ti-24Al-15Nb alloy. The transformation product is shown to contain a variety of defect structures whose origins are analyzed and explained. The orthorhombic phase is shown in exist in two forms: (1) in which the Nb atoms occupy a specific sublattice and (2) in which the Nb and Ti atoms randomly occupy the same sublattice. The subsequent decomposition of the metastable O product into the equilibrium phases is also described.     

Phase transformations and modulated microstructures in Ti-Al-Nb alloys

The decomposition of alloys roughly based on the composition Ti₃Al with ternary additions of Nb has been studied through the use of transmission electron microscopy and selected area electron diffraction. It has been shown that a wide variety of transformation behavior in the metastable high temperature β phase can be produced through varying composition, cooling rates, and heat treatment. Transformations observed during quenching from the β phase field include the formation of hcp martensite in alloys with low Nb contents, the occurrence of B2(CsCl)-type ordering over a wide composition range in alloys richer in Nb, and the formation of an “ω-type≓ phase in the parent matrix subsequent to B2 ordering. In many of the as-quenched alloys a “tweed-like≓ or modulated micro-structure is observed with accompanying elaborate networks of rel rods and local diffuse maxima in electron diffraction patterns. During aging the decomposition proceeds through a complex sequence of reactions the nature of which will be discussed below. Many of the anomalies observed in these alloys are quite similar to behavior observed during decomposition of other ordered β(B2, DO₃)-type phases in a wide variety of other alloy systems. This paper is based on a presentation made in the symposium “Pre-transformation Behavior Related to Displacive Transformations in Alloys≓ presented at the 1986 annual AIME meeting in New Orleans, March 2–6, 1986, under the auspices of the ASM-MSD Structures Committee.     

Microstructural characterisation of near-α titanium alloy Ti-6Al-4Sn-4Zr-0.70Nb-0.50Mo-0.40Si

Microstructural stability in the near-α titanium alloy (alloy 834) containing Ti-6Al-4Sn-4Zr-0.70Nb-0.50Mo-0.40Si (in weight percent), in the β and(α + β) solution-treated and quenched conditions, has been investigated. The β transus for this alloy is approximately 1333 K. Solution treatment in the β phase field at 1353 K followed by quenching in water at room temperature resulted in the formation of α′ martensite platelets with high dislocation density and stacking faults. Thin films of β are found to be sandwiched between interface phases, which, in turn, are sandwiched at the interplatelet boundaries of lath martensite. The interface phase is a subject of much controversy in the literature. Solution treatment at 1303 K in the(α + β) phase field followed by quenching in water at room temperature resulted in the near-equiaxed primary α and transformed β. Both the β and(α + β) solution-treated specimens were aged in the temperature range of 873 to 973 K. While aging the —treated specimen at 973 K,(α + β)-treated specimen, even at a lower temperature of 873 K for 24 hours, caused precipitation of suicides predominantly at the interplatelet boundaries of martensite laths. Electron diffraction analysis confirmed them to be hexagonal suicide S₂ witha = 0.70₂ nm andc = 0.36₈ nm. The above difference in the precipitation could be attributed to the partitioning of a higher amount of β- stabilizing elements as well as silicide-forming elements to the transformed β in the(α + β) solution-treated condition. However, ordering of theα′ phase was observed under all of the aging conditions studied. The ordered domains were due to the longer aging times, which cause local increases in the level of theα-stabilizing elements. Formerly Research Associate, Department of Metallurgical Engineering, Baranas Hindu University.     

Microstructural Evolution During Laser Resolidification of Fe-25 Atom Percent Ge Alloy

The microstructural evolution of concentrated alloys is relatively less understood both in terms of experiments as well as theory. Laser resolidification represents a powerful technique to study the solidification behavior under controlled growth conditions. This technique has been utilized in the current study to probe experimentally microstructural selection during rapid solidification of concentrated Fe-25 atom pct Ge alloy. Under the equilibrium solidification condition, the alloy undergoes a peritectic reaction between ordered α ₂ (B2) and its liquid, leading to the formation of ordered hexagonal intermetallic phase ε (DO19). In general, the as-cast microstructure consists of ε phase and ε–β eutectic and α ₂ that forms as a result of an incomplete peritectic reaction. With increasing laser scanning velocity, the solidification front undergoes a number of morphological transitions leading to the selection of the microstructure corresponding to metastable α ₂/β eutectic to α ₂ dendrite + α ₂/β eutectic to α ₂ dendrite. The transition velocities as obtained from the experiments are well characterized. The microstructural selection is discussed using competitive growth kinetics. This article is based on a presentation made in the symposium entitled “Solidification Modeling and Microstructure Formation: In Honor of Prof. John Hunt,” which occurred March 13-15, 2006, during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the TMS Materials Processing and Manufacturing Division, Solidification Committee.     

Characterization of microstructure in laser-surface-alloyed layers of aluminum on nickel

In order to obtain basic understanding of microstructure evolution in laser-surface-alloyed layers, aluminum was surface alloyed on a pure nickel substrate using a CO₂ laser. By varying the laser scanning speed, the composition of the surface layers can be systematically varied. The Ni content in the layer increases with increase in scanning speed. Detailed cross-sectional transmission electron microscopic study reveals complexities in solidification behavior with increased nickel content. It is shown that ordered B2 phase forms over a wide range of composition with subsequent precipitation of Ni₂Al, an ordered ω phase in the B2 matrix, during solid-state cooling. For nickel-rich alloys associated with higher laser scan speed, the fcc γ phase is invariably the first phase to grow from the liquid with solute trapping. The phase reorders in the solid state to yield γ′ Ni₃Al. The phase competes with β AlNi, which forms massively from the liquid. The β AlNi transforms martensitically to a 3R structure during cooling in solid state. The results can be rationalized in terms of a metastable phase diagram proposed earlier. However, the results are at variance with earlier studies of laser processing of nickel-rich alloys.     

On the equilibrium silicide in beta Ti-V alloys containing Si

The addition of small amounts of silicon (usually less than R ~1 at. pct) to strengthen α(hcp)-β(bcc) and martensitic Ti alloys is well established. The equilibrium suicide formed in these alloys has been identified as hexagonal Ti₅Si₃ (Refs. 1, 2) (or (Ti,Zr)₅Si₃ phase in alloys containing Zr), although there is also a report of a tetragonal Ti₃Si phase. The use of Si to age harden β Ti alloys, specifically Ti-V-Si alloys, has also been reported. While the precipitation sequence in these alloys involves an identifiable hexagonal     

Microstructure and mechanical properties of Ti-40 Wt Pct Ta (Ti-15 At. Pct Ta)

Of the β-isomorphous Ti-X alloy systems, Ti-Ta is one of the least studied. In the current work, the microstructure and mechanical properties of Ti-40 wt pct Ta (Ti-15 at. pct Ta) are investigated. Annealing at 810 °C produces a two-phase microstructure consisting of Ti-richa idiomorphs in a continuous Ta-rich β matrix; this suggests the β-transus temperature is higher than indicated by the most recently published phase diagram. Water quenching from 810 °C causes the β phase to partially transform to orthorhombic martensite (α), while furnace cooling yields secondarya The primary α formed isothermally remains unchanged in both cases. Subsequent aging causes transformation of the martensite to type 1a plus residual β, with a corresponding increase in strength and decrease in ductility. The maximum ductility (20 pct elongation) occurs in the water-quenched condition in which metastable β is retained. Analysis of the true stresstrue strain data suggests that transformation-induced plasticity may contribute to the enhanced ductility of the water-quenched material.     

Effect of microstructure variations on the formation of deformation-induced martensite and associated tensile properties in a β metastable Ti alloy

This article focuses on the effect of the microstructure on the activity of different deformation mechanisms and the resulting mechanical behavior of a metastable β Ti alloy (β-Cez). Various types of microstructures were produced, with given volume fractions of β phase (100 or 90 pct). These microstructures differed in the size of their β grains as well as in the distribution, shape, and size of the primary α particles. A statistical approach was also developed to characterize small variations in chemistry of the β phase between the various microstructures. It is shown that, even for similar volume fractions of β phase, changes in the microstructure strongly affect the mechanical response of the alloy. The mechanical response is controlled by the interplay between the two deformation modes operating in this alloy: formation of α″ deformation-induced martensite and activation of slip. The easier formation of stress-induced martensite leads to lower apparent yield stresses and a better work-hardening response. On the contrary, very limited work hardening is obtained when slip is activated solely. The differences in the ability of the martensitic transformation to occur can be understood by considering the effect on M _s and T _o of both the chemistry of the β phase and of constraining effects due to grain sizes and dislocations.     

Microstructural stability on aging of an α + β titanium alloy: ti- 6ai- 1.6zr-3.3mo- 0.30si

The development of the microstructure on aging of an (α + β) type titanium alloy containing 6A1-1.6Zr-3.3Mo-0.3Si (VT9) (in weight percent) has been studied. The β-transus temperature of this alloy is approximately 1243 K. Solution treatment in the β-phase field of the alloy followed by quenching in water at room temperature resulted in the formation of a single-phase martensite struc-ture. The martensitic structure was confirmed to be orthorhombic (α″) using X-ray diffraction. The water-quenched (WQ) specimens were subjected to aging treatments at temperatures of 823, 873, and 973 K for various lengths of time. Aging at 823 K for times between 24 and 100 hours did not bring about any noticeable change in the microstructure. Aging at 823 K for 200 and 300 hours resulted in the heterogeneous precipitation ofs ₂ silicide particles and thin films of β sandwiched between the interplatelet boundaries of martensite. Electron diffraction analysis confirms that the crystal structure of silicide particles is hexagonal with lattice parameters α= 0.70(1) nm andc = 0.36(8) nm. Aging at 873 K for 12 and 24 hours resulted only in the precipitation ofs ₂ silicide particles, while aging at the same temperatures for longer times (48, 100, and 200 hours) and also at 973 K for 6 to 100 hours resulted in the precipitation of silicides and also thin films of β and acicular martensite. The relative sizes of silicide precipitates and width of thin films of β phase increase with increasing aging time. The sites for silicide precipitation are mainly at α′-α′ boundaries, α-β interfaces, and sometimes within regions of transformed β. The kinetics ofs ₂ silicide precipi-tation in this alloy is faster than in commercial near-α titanium alloys. This is attributed to the presence of Mo, a strong β stabilizer. Formerly Reader, Department of Metallurgical Engineering, Centre of Advanced Study, Institute of Technology, Banaras Hindu University, Varanasi-221 005, India     

Nonequilibrium behavior in the Al-Ge alloy system: Insights into the metastable phase diagram

The competitive formation of metastable and stable phases during nonequilibrium processing of Al-Ge alloys and the corresponding metastable phase equilibria have been investigated. For germanium concentrations in the range 30 to 50 at. pct, it is shown that the four metastable phases can be ranked in order of decreasing stability as follows: monoclinic (P2₁/c), rhombohedral (R-C), orthorhombic (Pbca), and hexagonal (P6/mmm). Their formation depends not only on the transformation temperature(e.g., the liquid undercooling), but also on the presence of appropriate heterogeneous nucleation sites. For example, the orthorhombic phase has only been observed in amorphous films after rapid annealing/crystallization treatments. It is also shown that all of these phases form metastable equilibria with α-aluminum only,i.e., no metastable phase equilibria appear to exist between any metastable phase and β-germanium or between any two metastable phases. Consequently, it is not possible to draw a single metastable phase diagram that incorporates all of these phases with phase boundaries that represent their metastable equilibria; rather, separate diagrams should be drawn for each metastable phase. It is noted that these diagrams should extend only to the metastable phase field rather than all the way to pure germanium: for compositions richer in germanium, the results indicate that the metastable phase forms and then remelts upon the formation of germanium or a more stable, germanium-enriched metastable phase. Furthermore, it is proposed that this behavior is rather general in nature. Finally, it is concluded that the production of metastable phases in bulk form, in systems such as this where so many reactions occur simultaneously and competitively, might be impossible using solidification processing approaches. Formerly with the Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195     

Synthesis of nanodispersed phases during rapid solidification and crystallization of glasses in Ti75Ni25 alloys

The formation of the metallic glass and crystalline phases and related microstructures and the decomposition behavior of rapidly solidified Ti₇₅Ni₂₅ alloys obtained under different processing conditions have been investigated in detail. The competition between glass transition and nucleation of β-Ti during rapid solidification leads to the possibility of synthesizing the nanocomposites of β-Ti and glass. Additionally, it is shown that the presence of a small amount of Si also promotes simultaneous nucleation of fine Ti₂Ni intermetallic compound. Thermodynamic calculation of the metastable phase diagram indicates the presence of a metastable eutectic reaction between α-Ti and Ti₂Ni. Evidence of this reaction at lower cooling rates has been presented. On heating, the glass decomposes through this reaction. Finally, on the basis of understanding of the microstructural evolution during decomposition, a new approach has been adopted to synthesize a nanodispersed composite of α-Ti in the crystalline Ti₂Ni matrix with a narrow size distribution by controlling the devitrification heat treatment of the metallic glass.     

Comparison of orthorhombic and alpha-two titanium aluminides as matrices for continuous SiC-reinforced composites

The attributes of an orthorhombic Ti aluminide alloy, Ti-21Al-22Nb (at. pct), and an alpha-two Ti aluminide alloy, Ti-24Al-11Nb (at. pct), for use as a matrix with continuous SiC (SCS-6) fiber reinforcement have been compared. Foil-fiber-foil processing was used to produce both unreinforced (“neat”) and unidirectional “SCS-6” reinforced panels. Microstructure of the Ti-24A1-11Nb matrix consisted of ordered Ti₃Al (α ₂) + disordered beta(β), while the Ti-21 Al-22Nb matrix contained three phases: α₂, ordered beta (β ₀), and ordered orthorhombic(O). Fiber/ matrix interface reaction zone growth kinetics at 982 °C were examined for each composite system. Although both systems exhibited similar interface reaction products(i.e., mixed Ti carbides, silicides, and Ti-Al carbides), growth kinetics in theα ₂ +β matrix composite were much more rapid than in theO +β ₀ +α ₂ matrix composite. Additionally, interfacial reaction in theα ₂ +β} composite resulted in a relatively large brittle matrix zone, depleted of beta phase, which was not present in theO +β ₀+α ₂ matrix composite. Mechanical property measurements included room and elevated temperature tensile, thermal stability, thermal fatigue, thermo-mechanical fatigue (TMF), and creep. The three-phase orthorhombic-based alloy outperformed the α₂+β alloy in all of these mechanical behavioral areas, on both an absolute and a specific(i.e., density corrected) basis.     

Martensitic phase transformations in IMI 550 (Ti-4Al-4Mo-2Sn-0.5 Si)

The influence of solution-treatment temperature on the martensitic phase transformations observed in IMI 550 (Ti−4Al−4Mo−2Sn−0.5Si) has been investigated. When solution treatment is conducted at temperatures above 1233 K, a hexagonal martensite α′ is formed on rapid cooling. However solution treatment at temperatures between 1233 and 1123 K results in the formation of an orthorhombic martensite (α″) on rapid cooling. Finally, below 1123 K, the β phase is stable—no martensitic transformation occurs on rapid cooling. this transition from with decreasing solution-treatment temperature, is shown to be a result of alloy partitioning during solution treatment. Crystallographic analysis indicates that the transition in the martensite crystal structure with decreasing solution-treatment temperature is related to chemical short-range ordering (CSRO) in the high-temperature β phase.