Fundamentals of Gamma Titanium Aluminides

The emergence of a new class of aerospace alloys based on the ordered, face-centered tetragonal, or gamma, phase of the titanium-aluminum system has brought the promise of exceptional improvements in the performance of jet engines. Intended for service at high temperatures approaching or equaling those for which super alloys have typically been used, these so-called gamma titanium aluminide alloys pose significant challenges during processing to useful shapes.

Among the most serious obstacles to overcome have been those associated with the wrought processing of ingot metallurgy product forms. Such difficulties are not unexpected in view of the restricted temperature range for deformation processing and the limited hot work- ability which these alloys exhibit. Nevertheless, a fair amount of success has been achieved in both primary processing of these materials via extrusion or forging as well as during secondary processing such as by sheet rolling, closed-die forging, or super plastic sheet forming.

Microstructure evolution in a wrought near-gamma titanium alloy, Ti-45AI-2Cr-2Nb, was investigated by a series of heat treatments comprised of initial heating high in the alpha-plus-gamma phase field followed by short-time heating in the single-phase alpha field.

The initial heating step led to a dispersion of gamma particles which pinned the alpha grain boundaries. The kinetics of the gamma grain dissolution during subsequent heating in the single-phase field was interpreted in terms of models for both interface reaction-controlled and diffusion-controlled processes. The model for diffusion-controlled dissolution yielded predictions comparable to the observed times, whereas the model for interface reaction-controlled behavior predicted dissolution kinetics over an order of magnitude slower than observed. The growth of the alpha grains, both before and after the dissolution of the gamma phase, was also modeled.

Section size limitations to the ability to use supertransus heating to obtain uniform and moderately fine alpha grain sizes were examined using the transformation models and a simple heat transfer analysis approach. The results were validated through the heat treatment of subscale and full-scale forgings.

Much of the recent research and development on the thermo mechanical processing of the gamma titanium aluminide alloys has dealt with materials containing a small amount of the alpha-two (or beta) phase in addition to the gamma phase. The incorporation of the second phase in these near-gamma alloys, coupled with working in the two- phase (alpha plus gamma) field, has led to substantial improvements in hot workability and the ability to obtain very fine microstructures of equiaxed gamma analogous to the equiaxed alpha structures found in many conventional titanium alloy mill products.

The equiaxed gamma materials provide good ductility and strength, but inferior fracture toughness and creep resistance. For this reason, considerable effort is now being expended to develop approaches for obtaining alternate microstructures with a better balance of properties.

Near-gamma alloys containing a fully lamellar microstructure with a moderately small (≈50 to 200 µm) alpha grain size have been found to provide the desired better property mix. Several processing techniques have been developed to obtain such microstructures. These methods include extrusion at temperatures within a narrow window around the alpha transus (temperature at which alpha + gamma → alpha), or the supertransus heat treatment of alloys containing grain growth-inhibiting elements, such as boron, in solid solution or in the form of precipitates.

The general problem of using the supertransus heat treatment of near-gamma titanium aluminide alloys is to obtain fully lamellar microstructures. In particular, the control of the alpha grain size in alloys not containing grain growth-inhibiting elements was the focus of metallurgist.

The specific objective of metallurgist was to develop models of gamma phase dissolution and alpha grain growth during supertransus heating. To this end, samples of a typical near-gamma titanium aluminide were supertransus heat treated using various temperatures and times to establish particle dissolution and grain growth behaviors. The data were interpreted in terms of a suite of phase transformation models as well as a simple heat transfer analysis utilized to quantify temperature-transient effects.

Based on the superior tensile strength and ductility of the fine-grained duplex microstructure and superior toughness and creep resistance of the coarse-grained lamellar microstructure, a hybrid fine-grained lamellar microstructure would be expected to produce a material with balanced properties. Three basic methods have been reported to develop the fine-grained lamellar microstructure, sometimes also referred to as refined fully lamellar (RFL) microstructures.

The first method was to heat treat the sample for very short times (i. e., on the order of a few minutes) above the a transus and use rapid cooling to form the lamellar microstructure. While at the heat treatment temperature in the a-Ti single-phase field, though, the heat treatment time must be limited to prevent excessive grain growth. However, accurate reproduction of control heating rate, cooling rate, hold times, and temperatures in samples with varying thicknesses has been difficult.

The second method used an alloying addition to form a second phase to restrict grain growth at temperatures above the α transus. The second phase can be discrete grains (e.g., small additions of W to form stable β grains) or fine particles or dispersoids (e.g., additions of B to form TiB₂.

The third method of producing RFL microstructures involves combining heat treatment and thermo mechanical processing. This method, referred to as supertransus processing (STP), uses hot working of the material above the a transus to stimulate recrystallization of a fine-grained material and then cooling the sample before grain growth occurs.

The development of near-γ Ti-aluminide alloys with useful engineering properties has been possible due to a better understanding of the effects of alloying additions, thermo mechanical processing, and heat treatment on the microstructure and properties of TiAl-based alloys. The ability to produce hypostoichiometrlc (e.g., Ti-48Al at. pct) alloys with duplex microstructures resulted in significant improvements in tensile strength and ductility.

However, duplex microstructures exhibit relatively low fracture toughness and creep resistance. Duplex microstructures typically contain fine equiaxed γ- TiAl grains with low volume fractions (5 to 20 pct) of relatively fine lamellar γ-TiAl/α₂-Ti₃Al grains and/or very fine equiaxed α₂-Ti₃Al grains and are generated by heat treatment in the α + γ phase field, typically at temperatures ranging from 1275°C to 1325°C. If the alloy contains large amounts of β stabilizers (e.g., Cr, W, etc.), then limited amounts of fine equiaxed grains of β-Ti₂Alx, where x = Cr, W, etc., are observed.

Heat treatment above the α transus results in a fully lamellar microstructure comprised entirely of lamellar γ-TiAl/α₂-Ti₃Al grains. Typically, fully lamellar microstructures have very coarse grain size, due to the rapid grain growth in the α- Ti single-phase field, and exhibit poor tensile strength and poor ductility. However, the lamellar microstructure produces very high fracture toughness and creep resistance. In addition, many alloys exhibit lamellar microstructures with serrated grain boundaries, which further improve creep resistance.

The following conclusions were drawn from various testing:

1. Subtransus heat treatment just below the alpha transus gives rise to a microstructure of equiaxed alpha grains pinned by remnant gamma particles. The limiting size of the alpha grains can be predicted by the model of Hazzledine and Oldershaw.
2. Microstructure development during a supertransus heat treatment immediately following the subtransus heat- treatment results in the dissolution of remnant gamma particles, after which rapid growth of single-phase alpha grains occurs. Prior to complete dissolution of the gamma particles, relatively little alpha grain growth occurs, as predicted by the application of the results of the Hazzledine and Oldershaw model in conjunction with an assumption of uniform dissolution of all remnant gamma particles.
3. The observed kinetics of gamma particle dissolution during supertransus heat treatment is better predicted by a diffusion analysis; an interface reaction model greatly overestimates the required dissolution times.
4. The short-time kinetics of alpha grain growth follow the same trend as the longer time grain growth kinetics measured previously.
5. Within the limits determined by temperature uniformity and heating rate, full scale forgings can be supertransus heat treated to produce microstructures with small-to- moderate alpha grain sizes.