On the creep deformation of a cast near gamma TiAl alloy Ti48Al1 Nb

Abstract

Abstract--The steady-state creep deformation behavior of a cast two phase gamma TiA1 alloy having the composition Ti-48AI 1Nb (at.°) has been studied. Tension creep tests using the stress increment technique (a~ < cr 2 < or3) were conducted over the temperature range of 704-850°C at constant initial applied stress level of 103.4-241.3 MPa. The activation energy for creep over the temperature and stress regime of this study varied from 317.5 kJ/mol (137.8 MPa) up to 341.0kJ/mol (206.8 MPa) with an average value of 326.4 k J/tool. This is well within the range of values previously measured for gamma TiA1 alloys where creep controlled by volume diffusion has been suggested as rate controlling. The stress exponents measured were 5.0 at 704°C, 4.9 at 750°C, 4.7 at 800°C and 4.46 at 850°C. Using the activation energy of 326.4 kJ/mol, the temperature compensated steady-state creep rate was plotted against log stress with all temperatures collapsing onto a single line having a slope equal to 4.95. Using conventional creep analysis, this value of the stress exponent can be taken as suggestive of dislocation climb controlled power law creep as the operative deformation mechanism within the stress and temperature regime of the present study. The boundary separating the lamellar grains in two phase gamma TiA1 alloys having the duplex microstructure may be a very important aspect of this microstructure with respect to creep deformation resistance. The interlocking 7/~ 2 laths making up these boundaries are expected to be very resistant to grain boundary sliding which may contribute to creep deformation in the dislocation creep regime. Finally, some previous observations along with a comparison of the creep behavior of the Ti 48Al-INb alloy to that of a Ti-50.3AI binary have been discussed in terms of the pre-exponential constant A in the power law creep equation. TiA1 alloys having similar stress and temperature dependencies but differing steady-state strain rates over comparable stress-temperature regimes may be rationalized on the basis of differing power law creep constants which may reflect differences in stacking fault energies. R 6 s u m~) n &udie la d6formation par fluage en r6gime permanent d'un alliage TiA1 gamma biphas6 coul6 ayant la composition Ti-48AI 1Nb (en % atomique). Des essais de fluage, par la technique de l'incr6ment de contrainte (a t < o 2 < ~3), sont effectu6s sur une gamme de temp6ratures allant de 704 850°C pour des niveaux initiaux constants de la contrainte appliqu6e allant de 103,4 MPa 241,3 MPa. L'6nergie d'activation du fluage dans la gamme de contraintes et de temp6ratures de cette &ude varie entre 317,5 kJ/mol. (137,8 MPa) et 341,0 kJ/mol (206,8 MPa) avec une valeur moyenne de 326,4 kJ/mol. Ces r6sultats sont bien dans le domaine des valeurs d@i mesur6es pour les alliages TiAI gamma o0 l'on supposait que la vitesse de fluage 6tait contr616e par la diffusion en volume. Les exposants de contrainte mesur6s sont de 5,0 fi 704°C, 4,9 750°C, 4,7 ~i 800°C et 4,46/l 850C. En utilisant une energie d'...

Quantitative analysis of microstructures produced by creep of Ti–48Al–2Cr–2Nb–1B: Thermal and athermal mechanisms

“…[3][4][5][6][7][8] The increase in creep resistance of fully lamellar microstructures is generally attributed to the a 2 laths and g͞g interfaces acting as barriers to slip. 4,9 On the other hand, it has been suggested that the decreased creep resistance of the duplex microstructure compared to that of the equiaxed g microstructure is a result of the increased glide mobility of 1͞2 ͗110͘ dislocations within the g matrix 10 of the former. Although it is generally accepted that the alloys exhibit power law creep behavior, the values of the stress exponent, n, appear to increase with increasing applied stress.…”

Microstructural development and creep deformation in equiaxed γ, γ+α 2, and γ+α 2+B2 titanium aluminides

“…This may result in mixtures of regions with an equiaxed ␥ structure and others having varying degrees of lamellar grain formation. Final annealing of equiaxed structures in the ␣ ϩ ␥ phase field can lead to the formation of ␣ 2 plates in ␥ grains rather than lamellar grains on cooling similar to that shown by Wang et al [24] Microstructures containing these plate morphologies have been shown by several authors; [19,24,25] however, the resulting creep response of such microstructures has not been reported in detail.…”

“…The presence of multiple high-temperature deformation and recovery modes, including dislocation glide and climb, [1][2][3][4] deformation twinning, [3,5,6] dynamic recrystallization, [3,[7][8][9] and possible grain boundary sliding, [8,10] results in a deformation response that is highly dependent upon applied stress and temperature. This is apparent in single-phase binary ␥ TiAl alloys, as shown by Hamada et al [11] Other factors affecting creep rate, including aluminum content, grain size, interstitial and substitutional alloying additions, lamellar fraction, and lamellar spacing, have also been studied, [1,2,[12][13][14][15][16][17][18][19][20][21][22] but the interdependence of many of these factors makes understanding creep deformation in ␥-based alloys difficult.…”

Thermal-cycling creep of γ-TiAl-based alloys

“…2), the results of which are shown in Table 1. The calculated values are presented as a range of strain rates, corresponding to values of Q between 250 and 450 kJ/mol, which are reported for creep of various two-phase TiAl alloys [6,22,25,26]. The calculations are only weakly sensitive to these reasonable variations in Q.…”