R. Mehrabian

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.

Ductile reinforcement toughening of γ-TiAl: Effects of debonding and ductility

Abstract The effects of reinforcement debonding and work hardening on ductile reinforcement toughening of γ-TiAl have been examined. Debonding has been varied by either the development of a brittle reaction product layer or by depositing a thin oxide coating between the reinforement and matrix. The role of work hardening has been explored by comparing a Nb reinforcement that exhibits high work hardening with a solution hardened TiNb alloy that exhibits negligible work hardening. It is demonstrated that a high work rupture is encouraged by extensive debonding when the reinforcement exhibits high work hardening. Conversely, debonding is not beneficial when the reinforcement exhibits low intrinsic ductility due to an absence of work hardening.

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.

A test procedure for characterizing the toughening of brittle intermetallics by ductile reinforcements

Abstract A cylindrical test specimen for evaluating the toughening of brittle intermetallics by ductile reinforcements has been evaluated. A processing procedure capable of producing specimens has been devised, using HIPing, and a method for the tensile testing of the specimen has been established. Results are presented for γ-TiAl reinforced with Nb and a Ti-33 at.% Nb alloy. The toughening imparted by these materials is interpreted in terms of their strength, ductility and interface reactions, as well as loss of constraint mediated by the extent of debonding along the reaction product layers. Finally, the results are compared with those previously obtained on actual composites.

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 PROCESSING AND MECHANICAL BEHAVIOR OF AN ALUMINUM MATRIX COMPOSITE REINFORCED WITH SHORT FIBERS

Abstract An aluminum alloy composite reinforced with chopped Al 2 O 3 fibers randomly oriented in a plane has been produced by a modified squeeze casting process and its mechanical behavior compared with the corresponding behavior of the matrix. The specific characteristics of the compositing process have been examined vis-a-vis the characteristics of liquid metal infiltration into the capillaries of the fiber bundle. The flow stresses in the as-cast composite have been correlated with predictions based on continuum plasticity. Fracture of the fibers has also been characterized and interpreted using a weakest link statistical model, in conjunction with trends in the stress in the fibers. Changes in the composite flow stress caused by such fractures have been addressed. Finally, residual stress effects have been analyzed.

Characterization of Al2O3ZrO2 powders produced by electrohydrodynamic atomization

Abstract The roles of supercooling and alloy composition in the microstructure evolution of rapidly solidified powders have been systematically examined across the Al2O3ZrO2 binary system. Powders ranging in diameter from 10 nm to over 100 μm were produced by electrohydrodynamic atomization of Al2O3ZrO2 rods prepared by colloidal techniques. At the largest supercoolings (powders less than 1 μm), single-phase homogeneous structures, both amorphous and crystalline, may be observed across the entire phase diagram. With increasing particle size, segregation gives rise to duplex microstructures which consist of metastable alumina and monoclinic or tetragonal zirconia, or of tetragonal zirconia and glass. Coarse (greater than 10 μm) powders exhibit different morphologies in the primary phase and the eutectic constituent, with varying supercoolings and cooling rates. The trends in phase selection and segregation are examined with the aid of available information on Tg and schematic T0 curves for the different phases.

A thermogravimetric study of the oxidative growth of Al 2 O 3 /Al alloy composites

The oxidation of liquid Al-Mg-Si alloys at 900-1400 °C was studied by thermogravimetric analysis (TGA). The development of a semi-protective surface layer of MgO/MgAl 2 O 4 allows the continuous formation of an Al 2 O 3 -matrix composite containing an interpenetrating network of metal microchannels at 1000-1350 °C. An initial incubation period precedes bulk oxidation, wherein Al 2 O 3 grows from a near-surface alloy layer by reaction of oxygen supplied by the dissolution of the surface oxides and Al supplied from a bulk alloy reservoir through the microchannel network. The typical oxidation rate during bulk growth displays an initial acceleration followed by a parabolic deceleration in a regime apparently limited by Al transport to the near-surface layer. Both regimes may be influenced by the Si content in this layer, which rises due to preferential Al and Mg oxidation. The growth rates increase with temperature to a maximum at ~1300 °C, with a nominal activation energy of 270 kJ/mole for an Al-2.85 wt. % Mg-5.4 wt. % Si alloy in O 2 at furnace temperatures of 1000-1300 °C. An oscillatory rate regime observed at 1000-1075 °C resulted in a banded structure of varying Al 2 O 3 -to-metal volume fraction.

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.

Abrasive Wear of Aluminum-Matrix Composites

Abrasive wear resistance of aluminum matrix composites containing Al2O3 and SiC was investigated using a dry sand/rubber wheel abrasion tester. Composites containing Al2O3 were found to be superior to those containing SiC. This behavior was attributed to the formation of a brittle bond at the interface between aluminum matrix and SiC. Wear resistance of a composite containing large 142 μm Al2O3 was beter than that of composites containing smaller Al2O3 particles, and was comparable to AISI 1345 steel heat treated to a hardness of 57 HRC.