Tension creep of wrought single phase γ TiAl

The metallurgical factors governing the solid-state diffusion bonding of TiAl alloys have been characterized using scanning electron microscopy together with energy-dispersive X-ray (EDX) spectroscopy and electron backscattered diffraction (EBSD) analysis. The investigations were performed on TiAl alloys with various compositions and microstructures, which had been thoroughly mechanically characterized. The process zone of the bonds typically consists of a fine-grained layer of a 2 (Ti 3 Al) phase at the former contact plane, followed by relatively large, defect-free c(TiAl) grains and a region of deformed parent material. The evolution of the process zone involves phase transformation and recrystallization processes, which are triggered by asperity deformation at the contact plane and the unavoidable contamination of the diffusion couple with oxygen and nitrogen. The structural details depend on the alloy composition and the bonding conditions. In the final section of the article, technical aspects, including the tensile strength of diffusion bonds, will be discussed.

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Diffusion Bonding of γ(TiAl) Alloys: Influence of Composition, Microstructure, and Mechanical Properties

Herrmann

Appel

2009

“…[2] A substantial amount of research effort has been directed toward the understanding of microstructure/property relationships and the micromechanisms of creep. [3][4][5][6][7][8][9][10][11] While differences of interpretation are common, there has been general agreement about the major features. As with many other materials, the creep characteristics of TiAl alloys involve primary, secondary (or steadystate), and ternary regions, the extent of which depend on stress and temperature.…”

Section: Introductionmentioning

confidence: 99%

Creep behavior of TiAl alloys with enhanced high-temperature capability

Appel

Paul

Oehring

et al. 2003

For high-temperature applications, creep strength is of major concern, in addition to oxidation and corrosion resistance, and determines the application range of titanium aluminide alloys in competition with other structural materials. Thus, this work was aimed at identifying mechanisms of creep deformation and microstructural degradation and at developing alloying concepts with respect to an enhanced high-temperature capability. The analysis shows that dislocation climb controls deformation in the range of the intended operation temperatures. Further, complex processes of phase transformations, recrystallization, and microstructural coarsening were observed, which contribute to microstructural degradation and limit component life in long-term service. By alloying with high contents of Nb, both room-and high-temperature strength properties can be improved as Nb increases the activation energy of diffusion and increases the propensity for twinning at ambient temperature. For alloys with enhanced hightemperature capability, microalloying with carbon is also of particular use, because carbide precipitates effectively hinder dislocation motion and are thought to increase microstructural stability.

“…Tensile deformation and fracture of near gamma titanium aluminide alloys at elevated temperatures have been investigated extensively. [1,2,[4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] These investigations can be broadly classified into two groups: (a) studies devoted to the evaluation of tensile properties of different alloys containing a variety of microstructures under potential service conditions, i.e., at temperatures below 1000 ЊC and at a fixed strain rate of ϳ10 Ϫ4 s Ϫ1 ; [4][5][6][7][8][9][10][11] and (b) studies dealing with superplasticity and related phenomena in wrought alloys containing fine, equiaxed aggregates of ␥, ␣ 2 , and ␤ 2 phases. [12][13][14][15][16][17][18] Accordingly, the latter group of studies includes tension tests conducted at temperatures ranging from 900 ЊC to 1250 ЊC and strain rates varying from 10 Ϫ5 to 10 Ϫ2 s Ϫ1 .…”

Section: Introductionmentioning

confidence: 99%

Plastic-flow and microstructure evolution during hot deformation of a gamma titanium aluminide alloy

Seetharaman,

Semiatin

1997

The hot workability of a near gamma titanium aluminide alloy, Ti-49.5Al-2.5Nb-1.1Mn, was assessed in both the cast and the wrought conditions through a series of tension tests conducted over a wide range of strain rates (10 Ϫ4 to 10 0 s Ϫ1 ) and temperatures (850 ЊC to 1377 ЊC). Tensile flow curves for both materials exhibited sharp peaks at low strain levels followed by pronounced necking and flow localization at high strain levels. A phenomenological analysis of the strain rate and temperature dependence of the peak stress data yielded an average value of the strain rate sensitivity equal to 0.21 and an apparent activation energy of ϳ411 kJ/mol. At low strain rates, the tensile ductility displayed a maximum at ϳ1050 ЊC to 1150 ЊC, whereas at high strain rates, a sharp transition from a brittle behavior at low temperatures to a ductile behavior at high temperatures was noticed. Dynamic recrystallization of the gamma phase was the major softening mechanism controlling the growth and coalescence of cavities and wedge cracks in specimens deformed at strain rates of 10 Ϫ4 to 10 Ϫ2 s Ϫ1 and temperatures varying from 950 ЊC to 1250 ЊC. The dynamically recrystallized grain size followed a power-law relationship with the Zener-Hollomon parameter. Deformation at temperatures higher than 1270 ЊC led to the formation of randomly oriented alpha laths within the gamma grains at low strain levels followed by their reorientation and evolution into fibrous structures containing ␥ ϩ ␣ phases, resulting in excellent ductility even at high strain rates.