Creep Characteristics in Ti-50 mol%Al Single Phase Polycrystals

Takahashi, T.; Nagai, Hashiru; Oikawa, Hiroshi

doi:10.2320/matertrans1989.30.1044

Cited by 19 publications

(6 citation statements)

References 9 publications

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“…[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.…”

Section: Introductionmentioning

confidence: 99%

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

Morris

Leboeuf

1998

J. Mater. Res.

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A g-based TiAl alloy with equiaxed microstructure and fine grain size has been studied to analyze the deformation mechanisms responsible for the creep behavior. The microstructures produced by creep and high temperature deformation have been examined by TEM to obtain information about the different aspects characterizing the primary and secondary stages of creep. Mechanical twinning has been confirmed to occur in a fraction of the grains that never exceeds 50% while 1͞2 ͗110͘ dislocations are active within all the g grains. The twins are only responsible for a small amount of strain, but they lead to a subdivision of the microstructure and determine (directly or indirectly) the hardening process observed during the primary stage of creep. We have proposed that during the secondary stage the creep rate is determined by the unblocking of pinned dislocations by processes such as a pipe diffusion or cross slip that allow thermally activated glide of 1͞2 ͗110͘ dislocations on (001) planes.

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Section: Introductionmentioning

confidence: 99%

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

Morris

Leboeuf

1998

J. Mater. Res.

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“…[43] The comparison of creep curves in Figure 6(b) seems to indicate that second-phase morphology and alloying additions may have some effect on the extent of this transient strain, although the mecha- Fig. 1(b)); (c) alloy 2 100-m equiaxed ␥ (Fig.…”

Section: B Development Of Structures With Varying ␣ 2 Morphologymentioning

confidence: 95%

“…The presence of a minimum in the secondary creep rate followed by a constantly increasing rate reported by several authors in equiaxed ␥ [2,43] and duplex microstructures [2,3,44] has been attributed to microstructural instabilities such as dynamic recrystallization and cavity formation at grain boundaries. [3] In this study, no recrystallization occurred and cavity formation was not apparent until high creep strains were accumulated.…”

Section: B Development Of Structures With Varying ␣ 2 Morphologymentioning

confidence: 99%

Microstructural development and creep deformation in equiaxed γ,γ + α2 and γ + α2 + B2 titanium aluminides

Ott

Pollock

1998

Metall Mater Trans A

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The development of microstructure and its influence on creep properties have been studied for structures including equiaxed ␥, duplex, and other structures of varying ␣ 2 morphology in two Ti-48Al-2Cr-2Nb alloys. Heat treatments at 1125 ЊC have been utilized to produce equiaxed ␥ microstructures in alloys with or without Mo additions. The ␥ → ␣ transformation produces ␣ 2 plates with several orientation variants within ␥ grains during subsequent annealing of the equiaxed ␥ microstructures below the ␣ transus. Formation of this ␣ 2 morphology results from rapid up-quenching (UQ), and this structure persists through annealing, cooling, and creep testing. Differences in minimum creep rates for several microstructures containing varying amounts of multi-or single variant ␥/␣ 2 grains are shown to be minimal. The presence of Mo has also resulted in improved creep resistance in equiaxed ␥ and ␥ ϩ ␣ 2 ϩ B2 structures, as compared to similar microstructures in the Ti-48Al-2Cr-2Nb alloy. Deformation during creep at 760 ЊC at stresses between 200 and 400 MPa occurs by a combination of twinning and dislocation glide without recrystallization, resulting in power-law stress exponents in the range of 6 to 9. Only minimal strain path dependence of the minimum creep rate is detected in a comparison of creep rates in stress jump, stress drop, and single stress tests.

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“…[1,2] However, the current state of knowledge regarding the creep behavior of these alloys has recently been summarized as ''limited, lacking in the detailed understanding of the effects of specific microstructural variables.'' Published values of Q creep range from 192 to 700 kJ/mol, [5][6][7][8][9][10][11] and n has generally been measured to be much closer to 8 than 4.5. Attempts to resolve the measured values of the activation energy (Q creep ) and stress exponent (n) for creep with the activation energy for diffusion (Q diff ) and a pure metal exponent of ϳ4.5 have been largely unsuccessful.…”

Section: Introductionmentioning

confidence: 99%

“…[3,4] The comment is motivated, in part, by the fact that the majority of studies addressing the creep behavior of ␥-TiAl have been based on the phenomenological description of pure metal creep. [5][6][7]9,10,[12][13][14] Despite the fact that creep curves of TiAl have a general shape that is similar to pure metals, the limited nature of secondary creep and increased importance of tertiary or ''inverse'' creep suggest that the traditional method for describing pure metal creep is not valid for TiAl. Published values of Q creep range from 192 to 700 kJ/mol, [5][6][7][8][9][10][11] and n has generally been measured to be much closer to 8 than 4.5.…”

Section: Introductionmentioning

confidence: 99%

Microstructural evolution during creep of single-phase gamma TiAl

Hemker

1998

Metall Mater Trans A

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Mechanical experiments and transmission electron microscope (TEM) observations indicate that single-phase ␥-TiAl does exhibit primary, secondary, and inverse creep, but not steady-state creep. Constant stress creep tests of ␥-Ti-51Al-2Mn conducted at 550 ЊC, 597 ЊC, and 703 ЊC have been interrupted at different stages in the creep process. The TEM observations of these specimens were used to document the microstructural evolution that occurs during creep. Superdislocation motion was activated and subsequently exhausted during primary creep. Ordinary dislocations were observed to be pinned during primary creep, but with time, these dislocations began to bow past their pinning points. The extended region of inverse creep has been related to the bowing and multiplication of these ordinary dislocations. Quantitative measurements of dislocation density were performed, and while the density of superdislocations remained constant, the density of ordinary dislocations increased by an order of magnitude during the life of a creep test. The acceleration in the creep rate has been related to this increase in the density of ordinary dislocations, but the change in dislocation density was not high enough to account for the increase in the creep rate. This suggests that both the mobility and density of ordinary dislocations increase as creep progresses.

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Creep Characteristics in Ti-50 mol%Al Single Phase Polycrystals

Cited by 19 publications

References 9 publications

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

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

Microstructural development and creep deformation in equiaxed γ,γ + α2 and γ + α2 + B2 titanium aluminides

Microstructural evolution during creep of single-phase gamma TiAl

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