C. McCullough

Phase equilibria and solidification in Ti-Al alloys

High temperature phase equilibria and microstructure evolution during solidification were investigated for Ti-Al alloys in the range 40–55 at.%Al. In situ high temperature X-ray diffraction was used to study the phases present at elevated temperatures. It was found that a hexagonal close-packed α-phase exists close to the melting point for alloys containing 46–50 at.%Al, while leaner alloys (<44%Al) are in a cubic β-phase field at similar temperatures. Examination of dendritic morphologies in arc-button shrinkage cavities revealed the crystallography of the primary solidification phase. These observations were coupled to TEM analysis of the final microstructures to deduce phase sequencing during solidification and solid-state transformations. For alloys in the range 40–49 at. %Al, β was found to be the primary phase in local equilibrium with the liquid, and from 49 to 55 at.%Al, the primary phase was a. α. γ-segregate was first observed at 46 at.%Al and its fraction increased with Al content. Both β and α dendrites transformed in the solid-state to a lath structure consisting of layers of α2 and γ. The γ-segregate did not transform further. A revised phase diagram is proposed, for the composition range studied, incorporating two peritectics L + β → α, and L + α → γ, together with a high temperature α-phase field.

Microstructure evolution in tial alloys with b additions: Conventional solidification

Solidification microstructures of arc-melted, near-equiatomic TiAl alloys containing boron additions are analyzed and compared with those of binary Ti-Al and Ti-B alloys processed in a similar fashion. With the exception of the boride phase, the matrix of the ternary alloy consists of the same α2 (DO19) and γ (Ll0) intermetallic phases found in the binary Ti-50 at. pct Al alloy. On the other hand, the boride phase, which is TiB (B27) in the binary Ti-B alloys, changes to TiB2 (C32) with the addition of Al. The solidification path of the ternary alloys starts with the formation of primary α (A3) for an alloy lean in boron (∼1 at. pct) and with primary TiB2 for a higher boron concentration (∼5 at. pct). In both cases, the system follows the liquidus surface down to a monovariant line, where both α and TiB2 are solidified concurrently. In the final stage, the α phase gives way to γ, presumably by a peritectic-type reaction similar to the one in the binary Ti-Al system. Upon cooling, the α dendrites order to α2 and later decompose to a lath structure consisting of alternating layers of γ and α2.

Evolution of boride morphologies in TiAl-B alloys

The solidification of γ-TiAl alloys with relatively low (<2 at. pct) additions of boron is discussed. Binary Ti-Al alloys containing 49 to 52 at. pct Al form primary α-(Ti) dendrites from the melt, which are subsequently surrounded by γ segregate as the system goes through the peritectic reactionL + α →γ. Alloys between 45 and 49 at. pct Al go through a double peritectic cascade, forming primary β-(Ti) surrounded by α-(Ti) and eventually by γ in the interdendritic spaces. Boron additions to these binary alloys do not change the basic solidifi-cation sequence of the matrix but introduce the refractory compound TiB2 in a variety of mor-phologies. The boride develops as highly convoluted flakes in the leaner alloys, but needles, plates, and equiaxed particles gradually appear as the B content increases above ∼1 at. pct. Increasing the solidification rate initially promotes the formation of flakes over plates/needles and ultimately gives way to very fine equiaxed TiB2 particles in the interdendritic spaces of the metallic matrix. Furthermore, the primary phase selection in the 49 to 52 at. pct Al range changes from α-(Ti) to β-(Ti) at supercoolings of the order of 200 K. The different boride morphologies are fully characterized, and their evolution is rationalized in terms of differences in their nucleation and growth behavior and their relationship to the solidification of the inter-metallic matrix.

The high temperature α field in the titanium-aluminum phase diagram

Il est etabli de facon definitive que la phase en equilibre avec un alliage liquide Ti-50% Al est la solution solide hexagonale compacte basee sur la structure du Tiα, plutot que sur une forme cubique centree suggeree par le diagramme de phases. Revision de ce dernier pour y inclure un champ α haute temperature et une seconde reaction peritectique L+α→γ

Solidification microstructure of supercooled Ti-Al alloys containing intermetallic phases

Abstract Titanium-aluminum alloys (45, 50 and 55 at.% Al) containing the intermetallic phases α2-Ti3Al and γ-TiAl were melted, supercooled and solidified in an electromagnetic levitation device. The thermal history was recorded with the aid of a fast-response two-color pyrometer and the supercooling achieved was determined from the thermal excursion during recalescence. Maximum supercoolings were 262, 286 and 348 K for the 45, 50 and 55 at.% Al alloys, respectively. Microstructural analysis reveals that the 45 at.% Al alloy selects the primary β-(Ti) phase at all supercoolings, while the 50 at.% Al alloy changes from primary α-(Ti) to primary β-(Ti) at supercoolings of the order of 85 K. The relative amount of second phase segregate (γ) decreases with increasing supercooling in the 50 at.% Al alloy, and is absent in Ti-45 at.% Al. The 55 at.% Al alloy microstructure consists of primary a dendrites in a matrix of γ segregate, but in this case the amount of γ increases with increasing supercooling. The primary β α dendrites normally transform upon cooling to an α2 + γ lath microconstituent, although the transformation is largely suppressed by increasing the cooling rate and/or decreasing the aluminum content.

The evolution of metastable Bƒ borides in a TiAlB alloy

Additions of boron to titanium aluminides in the α2 to γ composition range leads to the formation of a variety of boride particles. Information available on the ternary phase diagram indicates that the equilibrium borides are TiB(B27), Ti3B4(D7b) and TiB2(C32). Conventional and high resolution transmission microscopy of the borides produced in a dilute Ti-40.97Al-0.97B alloy revealed the presence of numerous monoboride particles with the Bƒ(CrB-type) structure. This metastable form of TiB evolves during solidification, nucleating on preexisting particles of the same compound with the normal B27 structure. In addition, the Bƒ particles contain faults equivalent to nanoscale intergrowths of the D7b and C32 structures. The evolution of the boride phases in this system was analyzed in terms of crystallographic relationships elicited from structural models, all of which are based on th same trigonal building block of 6 metal atoms around each B atom. A Mie potential calculation was used to assess the relative stability of the monoboride forms. It is proposed that the Bƒ structure arises in response to a change in the melt chemistry which would produce Ti3B4 or TiB2 under equilibrium conditions. The preferential formation of Bƒ is ascribed to its closer crystallographic relationship with the B27 structure, which facilitates its nucleation on existing TiB crystals.

Microstructural analysis of rapidly solidified TiAlX powders

Abstract Microstructural evolution in Ti-48at.%Al powders with additions of tantalum and carbon is analyzed with the aid of levitation melting-solidification experiments on Ti-48at.%Al and Ti-45at.%Al alloys with and without additions of carbon, where the supercooling can be quantified and related to the observed microstructures. Supercoolings up to 258 K were achieved in the binary alloys; in all cases the primary solidification phase was β, and the relative amount of γ segregate decresed with increasing ΔT. Analysis of powders prepared by the plasma-rotating-electrode process (PREP) and XSR of nominally identical compositions revealed major differences in the primary phase selection. The PREP powders showed evidence of β formation at all particle sizes, while the XSR powders were overwhelmingly α in the coarser powder sizes, with the proportion of primary β increasing with decreasing particle size. This effect was ascribed primarily to the carbon contamination introduced in the XSR process. Carbon appears to shift the phase equilibria so that α is the stable primary phase in the ternary alloys. There is evidence of Ti 2 AlC formation if the carbon content is larger than about 1 at.% (0.3 wt.%), but the carbide appears to have a marginal role in the phase selection process. Even at the higher carbon contents, the phase selection reverts to primary β at high supercoolings, presumably owing to a kinetic preference for the latter phase. The decomposition of the α 2 phase—resulting from solid state transformations of the primary dendrites—into the α 2 + γ lath microconstituent, appears to be suppressed by tantalum additions and the increased cooling rate associated with reduction in powder particle size. It was also observed that martensite forms in segregate-free powders, presumably because the particles solidify completely as single-phase β. When martensite is formed, the room temperature microstructure is disordered α, rather than the α 2 or α 2 + γ commonly observed in the dendritic powders.

Peritectic solidification of Ti—Al—Ta alloys in the region of γ-TiAl

An investigation was made into the solidification behavior and phase equilibria of titanium alloys with 45–70 at.% Al and 0–27 at.% Ta, in which γ-TiAl is the predominant phase. A partial liquids surface was determined from studies on arc buttons. The principal features are peritectic cascades of the type L+β→α, and L+α→γ, emerging from the Ti—Al binary into the ternary, and a line of twofold saturation between γ and η. For alloys solidifying with β, α or γ as the primary phase, segregation led to significant enrichment of tantalum and depletion of aluminum in the dendrites. This gave rise to precipitation of σ, even for bulk alloy compositions within the equilibrium single-phase γ field. Transformations involving solute partitioning were frequently suppressed at the rather modest cooling rates characteristic of arc melting, presumably because of the sluggish diffusivity of tantalum in the various phases. The ensuing supersaturation results in further decomposition of the as-cast structure in the early stages of heat treatment, producing non-equilibrium precipitates which must be redissolved in order to achieve subsequent homogenization of the alloy. Typically, primary β dendrites decompose into, α, γ and σ, depending on the composition, whilst α may transform to either α2+γ or γ+σ. The γ phase is stable at low aluminum contents, but precipitates η in the richer aluminum alloys. Long-term homogenization treatments at 1373 K produced microstructures in close agreement with currently available information on the ternary phase diagram.

In-situ-grown reinforcements for titanium aluminides

Abstract Refractory compounds grown from the melt offer significant potential for high temperature reinforcement of titanium aluminides. In principle, the strengthening arises from the constrained plastic flow of the matrix around the refractory second phases, which must be thermodynamically stable, have high aspect ratios and be of a scale larger than the dislocation cell size. The reinforcing phase morphology and size scale may be tailored by suitable control of the alloy chemistry and solidification parameters. Additions of boron to γ-TiAl produce primary TiB 2 as equiazed particles but, if tantalum or niobium is also added, the morphology changes to elongated rods as the crystal structure of the primary phase changes to that of TiB. The latter, however, contains substantial amounts of niobium or tantalum substituting for titanium in the metal sublattice. The effects of alloy composition on the microstructural development of these materials is discussed. Preliminary creep data indicate that the composite alloys are substantially stronger than the binary γ matrix, but the strengthening contribution of the rod reinforcements is rather modest. The results are discussed in terms of recent developments in micro-mechanics models.

Solidification paths of TiTaAl alloys

Abstract Microstructure evolution during solidification and high temperature phase equilibria were investigated for TiTaAl alloys in the vicinity of the 50 at.%Al isoconcentrate. Examination of dendrite morphologies and segregation profiles were used to deduce the phase sequencing during solidification and the boundaries of the relevant liquidi surfaces. In situ high-temperature X-ray diffraction and isothermal annealing experiments were conducted to determine the phases present at elevated temperatures and coupled with extensive characterization by transmission electron microscopy to elucidate the solid state transformations during cooling of the solidification microstructure. For approximately equiatomic TiAl, the primary phase selected from the liquid was α for the leaner Ta concentrations ( 10 %). With increasing Al and Ta the α liquidus penetrates deeply into the ternary. The interdendritic segregate was always γ. Dendrites of the β-forming alloys were heavily segregated leading to different microconstituents in the core and bulk dendrite regions during post-solidification cooling. In alloys with 10% Ta). With increasing Ta levels the lath is gradually replaced by polycrystalline γ which grows into the dendrite bulk, and the core decomposes into a lamellar (γ + σ) microstructure from the decomposition of σ. The γ segregate does not transform further.