Steady-state creep deformation of investment cast near-gamma titanium aluminide

Abstract

IntroductionThe ongoing search for increased aircraft engine performance has prompted the materials community to investigate intermetallic compounds as potential replacement materials for nickel-and cobalt-based superalloys. Of particular interest over the past decade has been near-gamma titanium aluminides due to their low density, high modulus and strength retention at elevated temperatures, and good environmental resistance (1,2).While many investigators have studied the microstructure/property relationships of gamma alloys, only a limited amount of work has been performed on their creep behavior (2-4). Since gamma titanium aluminides have been targeted for use at temperatures approaching 875°C, it is clearly important to understand their elevated temperature deformation characteristics.The vast majority of creep studies performed to date have concentrated on alloy systems prepared by wrought or powder metallurgy techniques (5-11).However, recent developments have identified investment casting as a viable method for producing complex, near-net shape gamma titanium aluminide components (12-15). As such, the need to examine the creep behavior of these alloys in the cast product form has prompted the current investigation. The stress dependency and activation energy for steady-state creep deformation were determined for cast, near-gamma alloy Ti-48AI-2Nb-2Cr (at.%). Post-test microscopy also was performed to characterize the microstructural evolution in these materials after prolonged high temperature exposure. The results are discussed with respect to potential creep deformation mechanisms operating within gamma titanium aluminides. Exoerimental ProcedureThe material for this study was produced by Vacuum Arc Remelting (VAR) and casting into 1.6 cm diameter x 15.2 cm test bars. Following casting, the material was hot isostatically pressed at 1260°C/172 MPa for 4 hours. The measured chemistry of the castings is given in Table I IntroductionThe ongoing search for increased aircraft engine performance has prompted the materials community to investigate intermetallic compounds as potential replacement materials for nicke1-and cobalt-based superalloys. Of particular interest over the past decade has been near-gamma titanium aluminides due to their low density, high modulus and strength retention at elevated temperatures, and good environmental resistance (l,2). While many investigators have studied the microstructure/property relationships of gamma alloys, only a limited amount of work has been performed on their creep behavior (2-4). Since gamma titanium aluminides have been targeted for use at temperatures approaching 875°C, it is clearly important to understand their elevated temperature deformation characteristics.The vast majority of creep studies performed to date have concentrated on alloy systems prepared by wrought or powder metallurgy techniques (5-1 I). However, recent developments have identified investment casting as a viable method for producing complex, near-net shape gamma titanium aluminide componen...

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

“…Figure 18 shows the plot ln ᠨ e versus ln s from which the stress exponent was obtained as the slope of the line. The values obtained, n 8, is typical of this type of alloy 11,14 and indicates that the secondary stage of creep cannot be described by simple recovery processes. 22 When such high stress exponents are obtained, it is possible to rationalize the creep behavior assuming the existence of an athermal threshold stress, s 0 , and express the strain rate as: ᠨ e A (s 2 s 0 ) n 0 exp͑2Q͞RT ͒.…”

“…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. 9,11 This has been interpreted as being due to a transition from diffusional creep at low stresses to dislocation glide creep at high stresses. 5,[12][13][14] Although mechanical twinning during creep deformation of TiAl alloys has been extensively reported, 5,[15][16][17] its contribution to the creep process is not easily interpreted since traditional creep theories do not include such contributions.…”

Effect of alloying and aging on morphological changes from lamellar to equiaxed microstructure of α 2+γ titanium aluminides

“…This is followed by the growth of secondary lamellae to form a discontinuous region on both sides of the grain boundary. The coarsening of lamellae in the alloy Ti-48Al-2Nb-2Cr after heat treatment at 1350 °C and after creep testing at 850 °C of heat-treated samples has been reported by Wheeler et al [24] Ramanujan [15] has studied the coarsening of primary lamellae in the alloy Ti-48Al-2Mn-2Nb annealing at 1200 °C, 1350°C, and 1420 °C for various annealing times followed by water quenching and has observed discontinuous and continuous coarsening. The annealing at 1200 °C shows dissolution of ␣/␣ 2 phases.…”

Microstructure stability during creep deformation of hard-oriented polysynthetically twinned crystal of TiAl alloy

“…[20,21] Microstructural changes have been observed during creep deformation such as dynamic recrystallization, [20][21][22][23] spherodization, [24,25,26] dissolution of ␣ 2 phase, [27][28][29][30][31] and coarsening of lamellar. [24,27,29,30] The dissolution of ␣ 2 lamellae results in disappearance of the ␣ 2 phase and coarsening of lamellar structure that can degrade the creep properties by increasing both the primary and secondary creep strain rates.…”