C.R. Feng

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.

Antiphase domains and {011} twins in TiAl

Abstract Titanium aluminides with a TiAl/Ti3Al two-phase microstructure exhibit mechanical properties which are superior to those for either single-phase TiAl or Ti3Al. The TiAl lamella in the two-phase microstructures show differently oriented TiAl regions which are frequently observed to coexist within the same TiAl lamella. The atomic arrangements of these regions are similar to antiphase domains/{011} twins of TiAl. However, the mechanism by which these regions are formed cannot be derived from the TiAl phase alone. This study suggests that, for two-phase titanium aluminides, Ti3Al has a strong influence on the development of the TiAl microstructure.

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.

Grain Size Effect on the Creep Behavior of Monolithic MoSi 2

The grain size effect on the creep behavior of hot-pressed monolithic molybdenum disilicide was investigated at 1,200 C in a 19--255 MPa stress range. The creep-stress exponent, n, increased from 1 at low stresses to 4 at high stresses. The grain size exponent, p, varied from 0, to 3.5 and to 8 depending on the grain size, the creep-stress exponent, and creep history.

Microstructure of extruded XDTM Nb-26Ti-48Al+ (Nb, Ti) B

Titanium aluminides, {gamma}-TiAl and {alpha}{sub 2}-Ti{sub 3}Al, are of considerable interest for use in selected applications as the result of their excellent mechanical properties at elevated temperature. With a refractory metal addition, such as either Nb or Ta, these new alloys, based on a Nb-25 at.% Ti-50at%Al composition, exhibit superior oxidation resistance at elevated temperatures when compared to {gamma}-Ti-Al and {alpha}{sub 2}-Ti{sub 3}Al + {gamma}-TiAl alloys. The improvement of mechanical properties by reinforcing metals with ceramic particles to produce metal matrix composites has been demonstrated repeatedly. An exothermic process termed XD technology was recently developed where ceramic particles/whiskers, such as borides, carbides, nitrides and silicides, may be formed in-situ in molten metals. It was suggested that this process may be used to produce materials with designed microstructures and improved mechanical properties. The purpose of this paper is to report the results of a transmission electron microscopy study of the microstructures of extruded Nb-Ti-Al alloys with and without a (Nb,Ti)B whisker addition.

On ripple-load, stress-corrosion, and sustained-load cracking behavior in a high strength beta titanium alloy

Both overaged and peakaged TIMETAL 21S beta titanium alloys exhibit significant ripple-load cracking susceptibility in salt water and in ambient air environments. At R = 0.90, the ripple-load cracking thresholds of the overaged alloy are 67% and 72% lower than the stress-corrosion cracking and sustained-load cracking thresholds. For the peakaged alloy, the reductions are 55% and 61%. The stress-corrosion cracking threshold in salt water and the sustained-load cracking threshold in air of peakaged TIMETAL 21S are significantly lower while the ripple-load cracking threshold is slightly lower than those of the overaged alloy. The stress-corrosion cracking, sustained-load cracking, and ripple-load cracking resistance of peakaged TIMETAL 21S are significantly inferior to those of both beta-annealed Ti-6Al-4V and Ti-15V-3Cr-3Al-3Sn. The ripple-load cracking resistance of overaged TIMETAL 21S, though better than Ti-15V-3Cr-3Al-3Sn, is still inferior than that of beta-annealed Ti-6Al-4V.

Twins and lamellar structures in the Ti-Al system with managanese addition

The purpose of this study was to determine the effects of Mn on twin formation and lamellar structure formation in an off stoichiometric TiAl alloy

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.

Creep Deformation of Titanium Aluminides with TiB 2

Creep deformation of as-forged and heat-treated titanium aluminides, Ti-48Al with 5 and 10% volume fraction of TiB 2 , has been studied under tension in the temperature range of 649°-871°C to determine the rate-controlling mechanism. Microstructure of these alloys correspond to a mixture of equiaxed grains of γ and lamellar regions containing eutectoid γ+α 2 . Data indicate that with increase in load, stress exponent increases from one to seven. The activation energy for creep varies from 340kJ/mol to 455kJ/mol in the temperature range investigated. Creep data of these alloys were analyzed using several theoretical models.