The influence of interrupted cooling on the massive transformation in Ti46Al8Nb

Huang, Aijun; Hu, D.; Wu, Xinhua; Loretto, M. H.

doi:10.1016/j.intermet.2007.02.002

Cited by 27 publications

(22 citation statements)

References 12 publications

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“…c m phase transformation, where c m is the massive c phase, and subsequent ageing in the (a+c)/a phase field can provide a desirable refined (convoluted) lamellar structure, which is preferable from the viewpoint of the room temperature ductility and is expected to yield well balanced mechanical properties. [8][9][10][11][12][13] The main problem in developing the massive transformation as a useful processing route for c-TiAl alloys relates to the fact that high cooling rates and a very narrow cooling rate window are commonly required for the massive transformation. High cooling rates can lead to crack formation and is moreover hard to achieve in thick sections.…”

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confidence: 99%

Grain Refinement in Cast Ti‐46Al‐8Nb AND Ti‐46Al‐8Ta Alloys via Massive Transformation

Imayev¹,

Khismatullin

Oleneva

et al. 2008

Adv Eng Mater

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TiAl alloys are of increasing technical importance for high temperature applications in automotive and aerospace industries due to their low density combined with attractive high temperature properties. However, cast TiAl-based alloys usually suffer from poor room temperature ductility and a large scatter in other mechanical properties that impedes their wide industrial application. It is associated not only with intrinsic brittleness caused by directed type of interatomic bonding in the c-TiAl phase but also with a coarse columnar structure and a sharp casting texture which often evolves in castings during freezing and cooling. To overcome these deficiencies hot working (canned extrusion or forging) is usually applied in order to breakdown the ingot structure and to reach refined microstructure. However, this way is very laborious taking into account very high extrusion/forging temperatures. [1,2] Two approaches are now considered in the literature to refine the microstructures in cast TiAl alloys without involving any hot working: it is through solidification (during freezing and cooling) and through various heat treatments. The first approach can include: i) adding strong b-stabilizers (such as Re) and boron, [3,4] ii) adding boron in a level of about 1 at%, [4,5] iii) the use of b-solidifying alloys doped with b-stabilizing elements (such as Nb, Mo) and small boron additions. [6] The second approach is often associated with the use of the "massive transformation technique", which includes quenching from the single a phase field, followed by ageing in the temperature range of the (a+c)/a phase field. The most attractive advantages of this treatment are its simplicity and excluding boron as a grain refining agent necessary in the case of the first approach. The latter should be beneficial for ductility taking into account that coarse borides reduce the tensile ductility in cast TiAl alloys. [7] Quenching can lead to the massive a 7 ! c m phase transformation, where c m is the massive c phase, and subsequent ageing in the (a+c)/a phase field can provide a desirable refined (convoluted) lamellar structure, which is preferable from the viewpoint of the room temperature ductility and is expected to yield well balanced mechanical properties. [8][9][10][11][12][13] The main problem in developing the massive transformation as a useful processing route for c-TiAl alloys relates to the fact that high cooling rates and a very narrow cooling rate window are commonly required for the massive transformation. High cooling rates can lead to crack formation and is moreover hard to achieve in thick sections. Another problem is to know the temperature range at which c m transforms into a convoluted structure avoiding the re-transformation to a grains because this can lead to coarse lamellar c+a 2 structure during final cooling. Therefore, to utilize the massive transformation as a method of microstructural refinement in c-TiAl alloys special alloying additions such as niobium or tantalum are requested: they reduce the diffusivit...

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confidence: 99%

Grain Refinement in Cast Ti‐46Al‐8Nb AND Ti‐46Al‐8Ta Alloys via Massive Transformation

Imayev¹,

Khismatullin

Oleneva

et al. 2008

Adv Eng Mater

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“…Structure and properties of Ti 55 4,8,12. The γ phase is identified in all alloys subjected to X-ray diffraction (XRD) and the α 2 phase in most of them ( Table 2).…”

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confidence: 99%

Structure and properties of titanium-aluminum alloys doped with niobium and tantalum

Bondar

Witusiewicz

Hecht

et al. 2011

Powder Metall Met Ceram

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melted in a laboratory arc furnace from pure components (~99.9 wt.%) are studied by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), differential thermal analysis (DTA), and compression testing. The alloys are found to consist mainly of a superfine lamellar structure of decomposition β → α → α + γ → α 2 + γ and contain tiny islands of the γ phase and also the β phase at 12 at.% Nb or Ta. The CALPHAD approach is used to obtain a thermodynamic description of the Al−Nb−Ta−Ti system, which adequately reproduces the experimental data. The compression tests performed at room temperature show that the ternary alloys with 45 at.% Al are characterized by higher strength but lower plasticity than the alloys with 47 at.% Al. For the Ti 46 Nb 8 Al 46 and Ti 46 Ta 8 Al 46 alloys, high strength in the temperature range 20−800°C and a drop in plasticity to 2−4% at 200−600°C are revealed. Among the quaternary alloys examined, the best combination of strength and plasticity (σ 02 = 1043 MPa, σ ult = 1612 MPa, ε pl = 12.4%) is exhibited by the Ti 47 Nb 4 Ta 4 Al 45 alloy.

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“…[8] Owing to alloying by tantalum the range of cooling rates, over which massive gamma is formed, was significantly shifted towards lower cooling rates; therefore, air quenching instead of usually applied oil quenching was found to be enough to reach the massive g phase in small sample bars. [3][4][5][6][7][8] This finding seems to open up possibilities for manufacturing applications in respect of parts with thin sections. However, to promote this technique towards commercial application a better understanding of the near convoluted microstructure formation during ageing and its dependence on the ageing conditions is necessary.…”

mentioning

confidence: 75%

“…[1,2] One of the effective methods for breaking down coarse grained structures in g-TiAl alloys, at least in small-sized ingot bars, is the ''massive transformation technique,'' which includes quenching from the single a phase field followed by ageing in the temperature range of the (a þ g) phase field. [3][4][5][6][7][8] Quenching leads to the massive a ) g m phase transformation, where g m is the massive g phase, and subsequent ageing in the (a þ g) phase field can provide a desirable refined convoluted/lamellar structure, which is preferable from the viewpoint of the room temperature ductility and is expected to yield well-balanced mechanical properties. [3][4][5][6][7][8] As has been recently demonstrated, the ''massive transformation technique'' was effective in the alloys alloyed by elements with reduced diffusivity, particularly in the Ti-46Al-8Ta alloy.…”

mentioning

confidence: 99%

“…[3][4][5][6][7][8] Quenching leads to the massive a ) g m phase transformation, where g m is the massive g phase, and subsequent ageing in the (a þ g) phase field can provide a desirable refined convoluted/lamellar structure, which is preferable from the viewpoint of the room temperature ductility and is expected to yield well-balanced mechanical properties. [3][4][5][6][7][8] As has been recently demonstrated, the ''massive transformation technique'' was effective in the alloys alloyed by elements with reduced diffusivity, particularly in the Ti-46Al-8Ta alloy. [8] Owing to alloying by tantalum the range of cooling rates, over which massive gamma is formed, was significantly shifted towards lower cooling rates; therefore, air quenching instead of usually applied oil quenching was found to be enough to reach the massive g phase in small sample bars.…”

mentioning

confidence: 99%

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Formation and Stability of Near Convoluted Structure Obtained in the Ti–46Al–8Ta Alloy Via Air Quenching and Ageing

et al. 2010

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The critical issue of ingot-metallurgy g-TiAl-based alloys is low ductility and a large scatter in other mechanical properties which impedes their wide industrial application. These deficiencies are associated not only with intrinsic brittleness of the constituting g-TiAl and a 2 -Ti 3 Al phases caused by directed type of interatomic bonding but also with a coarse columnar structure, a high level of dendritic segregation and a sharp casting texture which often evolve in castings during freezing and cooling. To overcome these deficiencies hot working (canned extrusion or forging) is usually applied in order to breakdown the ingot structure and to reach refined microstructure. However, this way does not exclude texture and chemical inhomogeneities even if the microstructure is effectively refined. Another issue is a high cost of the hot working procedure. [1,2] One of the effective methods for breaking down coarse grained structures in g-TiAl alloys, at least in small-sized ingot bars, is the ''massive transformation technique,'' which includes quenching from the single a phase field followed by ageing in the temperature range of the (a þ g) phase field. [3][4][5][6][7][8] Quenching leads to the massive a ) g m phase transformation, where g m is the massive g phase, and subsequent ageing in the (a þ g) phase field can provide a desirable refined convoluted/lamellar structure, which is preferable from the viewpoint of the room temperature ductility and is expected to yield well-balanced mechanical properties. [3][4][5][6][7][8] As has been recently demonstrated, the ''massive transformation technique'' was effective in the alloys alloyed by elements with reduced diffusivity, particularly in the Ti-46Al-8Ta alloy. [8] Owing to alloying by tantalum the range of cooling rates, over which massive gamma is formed, was significantly shifted towards lower cooling rates; therefore, air quenching instead of usually applied oil quenching was found to be enough to reach the massive g phase in small sample bars. [3][4][5][6][7][8] This finding seems to open up possibilities for manufacturing applications in respect of parts with thin sections. However, to promote this technique towards commercial application a better understanding of the near convoluted microstructure formation during ageing and its dependence on the ageing conditions is necessary. Another critical issue revealed in our previous work was the stability of the near convoluted microstructure obtained by high-temperature ageing. [8] The present paper considers the influence of the ageing conditions on the formation of the convoluted/lamellar microstructure in the Ti-46Al-8Ta alloy. This work continues COMMUNICATION [*] Dr. V. Imayev, R. Gaisin Microstructure evolution after air quenching and variable ageing treatment has been investigated in small-sized ingot bars of the Ti-46Al-8Ta alloy. It was found that air quenching and subsequent ageing at 1200-1260 8C led to the formation of a homogeneous refined near convoluted microstructure. To increase the stability of t...

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The influence of interrupted cooling on the massive transformation in Ti46Al8Nb

Cited by 27 publications

References 12 publications

Grain Refinement in Cast Ti‐46Al‐8Nb AND Ti‐46Al‐8Ta Alloys via Massive Transformation

Grain Refinement in Cast Ti‐46Al‐8Nb AND Ti‐46Al‐8Ta Alloys via Massive Transformation

Structure and properties of titanium-aluminum alloys doped with niobium and tantalum

Formation and Stability of Near Convoluted Structure Obtained in the Ti–46Al–8Ta Alloy Via Air Quenching and Ageing

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