The massive transformation in TiAl alloys: Mechanistic observations

Zhang, X.D.; Godfrey, S.P.; Weaver, M.L.; Strangwood, Martin; Threadgill, P. L.; Kaufman, Michael J.; Loretto, M. H.

doi:10.1016/1359-6454(95)00453-x

Cited by 62 publications

(45 citation statements)

References 8 publications

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“…The results atomic jumps across the IPBs. The abrupt changes in the of irrational ORs across growth IPBs are consistent with orientation of the IPBs and the occurrence of twin-related those of others [23,38,41,42] reported in Ti-Al alloys, as well as variants within a grain of ␥ M are also difficult to explain by those reported in other alloy systems. [7,8,39,40] These observaa ledge-type mechanism.…”

Section: A General Remarkssupporting

confidence: 71%

“…For the ␥ M grains formed at of massive ␥ M phase is accomplished not by the classical grain edges and corners, no low-index ORs were obtained ledge mechanism, but rather by short-range atomic jumps with any of the surrounding ␣ grains, and it was established more or less continuously across these structurally rough, that the IPBs were incoherent. Similarly, Zhang et al, [23] in incoherent IPBs. their study of an electron-beam welded and cooled Ti-48Al…”

Section: A General Remarksmentioning

confidence: 91%

“…pct Al alloys on rapid cooling is now well established. [17][18][19][20][21][22][23][24][25][26] (The central portion of the TiAl phase diagram [27] is shown in Figure 1). In a recent article, continuous cooling studies using a novel temperature and electrical resistivity measurement system were performed to study this transformation in a Ti-47.5Al (in at.…”

Section: Introductionmentioning

confidence: 99%

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Massive-parent interphase boundaries and their implications on the mechanisms of the α → γ M massive transformation in Ti-Al alloys

Wang

Veeraraghavan

Kumar

et al. 2002

Metall Mater Trans A

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The massive-parent interphase boundaries associated with the ␣ → ␥ M massive transformation in a Ti-46.5 at. pct Al alloy were studied. Special experiments were performed to arrest the transformation at an early stage. Orientation relationships (ORs) between the ␥ M and parent ␣ (retained as ␣ 2 ) phases were determined using electron backscattered diffraction (EBSD) in a scanning electron microscope and by electron diffraction, and the interphase boundaries were characterized by two-beam brightfield/weak-beam dark-field (WBDF) transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The results reveal that the ␥ m nucleates at grain boundaries generally with a low-index Burgers orientation relation and a coherent interface with one parent grain, but grows into the adjacent grain with a high-index/irrational orientation relation. The growth interfaces between the two phases are generally free of misfit dislocations or other defects and consist of curved parts as well as planar facets, whose macroscopic habit planes are of generally high-index/irrational orientation and deviate substantially from the close-packed planes. On an atomic scale, the growth interfaces are sometimes found to be faceted along {111} planes, as well as along other planes, with closely spaced steps, but are concluded to be incoherent with respect to the parent grain into which growth occurs. The implications of these results on the nucleation and growth mechanisms associated with the ␣ -to-␥ M massive transformation are discussed. In particular, the nature of the interphase boundaries and their relation to whether growth occurs by a ledgewise motion of the interfaces or by continuous growth are addressed.

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Section: A General Remarkssupporting

confidence: 71%

Section: A General Remarksmentioning

confidence: 91%

Section: Introductionmentioning

confidence: 99%

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Massive-parent interphase boundaries and their implications on the mechanisms of the α → γ M massive transformation in Ti-Al alloys

Wang

Veeraraghavan

Kumar

et al. 2002

Metall Mater Trans A

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“…Although bulk nucleation has been reported [9,19], it is generally accepted that nucleation of massive g occurs predominantly at grain boundaries. Gamma nuclei generally exhibit a Blackburn orientation relationship with one of the adjacent a grains while they grow at the expense of the other grain by the movement of highly mobile incoherent interfaces [20][21][22]. It has been suggested also that lamellae nucleated on grain boundaries serve as a precursor of massive g which would develop from the incoherent interface of the lamella in the adjacent grain where it has no orientation relationship [20].…”

Section: Introductionmentioning

confidence: 99%

“…Gamma nuclei generally exhibit a Blackburn orientation relationship with one of the adjacent a grains while they grow at the expense of the other grain by the movement of highly mobile incoherent interfaces [20][21][22]. It has been suggested also that lamellae nucleated on grain boundaries serve as a precursor of massive g which would develop from the incoherent interface of the lamella in the adjacent grain where it has no orientation relationship [20]. Massive grains nucleated on grain boundaries grow with a hemispherical morphology owing to the immobile coherent interface with one of the adjacent a grains [21,22].…”

Section: Introductionmentioning

confidence: 99%

A numerical model for the description of the lamellar and massive phase transformations in TiAl alloys

Rostamian

Jacot

2008

Intermetallics

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a b s t r a c tA phenomenological modelling approach has been developed to describe the massive transformation and the formation of lamellar microstructures during cooling in binary g titanium aluminides. The modelling approach is based on a combination of nucleation and growth laws which take into account the specific mechanisms of each phase transformation. Nucleation of massive and lamellar g is described with classical nucleation theory, accounting for the fact that nuclei are formed predominantly at a/ a grain boundaries. Growth of the massive g grains is based on theory for interface-controlled reactions. A modified Zener model is used to calculate the thickening rate of the g lamellar precipitates. The model incorporates the effect of particle impingement and coverage of the nucleation sites by the growing phase. The driving pressures of the phase transformations are obtained from Thermo-Calc based on the actual temperature and matrix composition. CCT diagrams and lamellar spacings calculated with the model are in good agreement with experimental data obtained from dedicated heat treatment experiments and from the literature. The model permitted investigating the influence of cooling rate, alloy chemistry and average a grain size upon the amount of massive g and the average thickness and spacing of the lamellae. In particular it indicates that the Al depletion of the a phase during lamellar precipitation seems to play an important role in the suppression of the massive transformation at moderate cooling rate and in the large lamellar spacings observed at low cooling rate.

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