C.R. Crowe

Twinning in TiAl

Abstract The results of this investigation are in agreement with the findings of Richards and Cahn which show that the formation of twin-related TiAl regions, TiAl T , and the Ti 3 Al phase, rather than TiAl deformation twins, will be favored energetically in a two-phase titanium aluminide alloy. Due to the formation of Ti 3 Al at the TiAl T interface, all possible {111} TiAl T , as opposed to only {111}[112] deformation twins, can be formed within TiAl lamellae without the destruction of the symmetry of L1 0 structure.

The formation of Ti3Al within TiAl during the deformation of XD™ titanium aluminide

Abstract A mechanism involving the formation of TiAl SF can explain how Ti 3 Al layers and TiAl T can be formed within TiAl during the deformation of TiAl. This mechanism is similar to the previously suggested for the formation of TiAl M /TiAl T laths which initiate at Ti 3 Al SF during the ordering transformation of Ti 3 Al (3) and can be applied to the formation of TiAl M /TiAl T during the deformation processing of Ti 3 Al.

XD™ Titanium Aluminide Composites

The advantages of reinforcing metals with ceramic particles to produce metal matrix composites are well known. The behavior of discontinuously reinforced intermetallic compounds, however, has not been extensively studied. Martin Marietta Laboratories has produced a new generation of discontinuously reinforced titanium aluminide composites using a proprietary casting process known as XD™ technology. These new materials possess enhanced properties at room and elevated temperatures and may be cast, extruded, or forged. The effects of matrix composition, reinforcing phase, and thermal mechanical processing on properties have been studied using optical and various electron microscopy and mechanical and physical property measurement techniques to characterize the alloys. To date, most work has been done on a two-phased lamellar Ti-45 a/o Al alloy reinforced with TiB2 ceramic having an equiaxed morphology. Data on temperature dependence of the dynamic Youngs modulus, coefficient of thermal expansion, deformation and fracture behavior, and microstructure are presented.

Microstructural characteristics of two-phase titanium aluminides

The microstructural characteristics of two-phase α2 + γ1 conventional and XD-processed, titanium aluminides were investigated by transmission electron microscopy. The observed microstructures show that the two-phase material composition promotes the formation of alternating lamellae of α2 and γ corresponding to twin-related γ laths. The microstructures of XD materials suggest that the influence of excess boron dissolved from TiB2 particles was to reduce the α2−γ interfacial mismatch. The microstructural results demonstrate that deformation which occurs via the formation of α2 layers and twin-related γ laths, rather than via large populations of mobile dislocations, plays the primary role in the deformation behavior of the two-phase titanium aluminides and show that twin-related γ lath formation was very important to the capability of two-phase materials for continuous deformation.

The effects of boron in TiAl/Ti3Al

Abstract The TiAl/Ti 3 Al interfacial misfit dislocations structures were investigated by TEM in Ti-45Al alloy and Ti-45Al/TiB 2 composite. For TiAl with c/a = 1.02, only a single set of misfit dislocation arrays are crystallographically possible; these were observed in Ti-45Al alloy. However, the observation of three sets of misfit dislocation arrays in the Ti-45Al/TiB 2 composite suggests that the occupation of octahedral sites in the TiAl structure by excess boron was responsible for a decrease in the c/a ratio leading to an increased fcc character of the TiAl at the TiAl/Ti 3 Al interface.

Strain-Induced Recrystallization of Tial

Transmission electron microscope investigation of extruded and rolled XD™ titanium aluminide sheet has shown the development of recrystallized TiAl regions adjacent to grains which exhibit submicron scale TiAl twins. Detailed diffraction analysis confirmed that the TiAl twins were separated by a layer of Ti 3 Al. This is consistent with previous observations of the microstructure of extruded XD™ titanium aluminide. In the extruded material, however, the scale of the twins was larger, but the extent of the recrystallized region was smaller. The evidence suggests that the recrystallization of the TiAl regions may result from the impingement of twins from adjacent grains.

Transformation of Miller indices in twinned structures

Abstract The parent–twin transformation equations derived by Mugge and subsequent workers have been reinvestigated in view of the observation that these equations produce similar, but not numerically identical, transformations. The current study suggests that, from the point of view of the classical orientation relations, the assumptions made in several derivations are applicable only to special transformation situations.