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

Abstract

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...

Microstructure Evolution of a Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y Alloy during Massive Transformation and Subsequent Annealing

“…Figure 3a shows the microstructure after oil-quenching from 1340 • C. This morphology is similar to that observed in the water-quenched sample shown in Figure 2a. The cooling rate of oil-quenching should be lower than water-quenching; thus, more massive structures were expected in this sample [11]. This result might be related to the fact that boron has an effect on the kinetics of massive transformation [18].…”

“…Several studies of the massive transformation in high Nb-TiAl alloys have been conducted, even in alloys with higher amount of Nb contents [17]. Saage et al [8] noted that the cooling rates required for initiating massive transformation apparently depend on the compositions, and the element Ta can increase the tendency of massive γ formation because of its lower diffusion rate than Nb, which is also supported by the results from Imayev et al [11]. However, the addition of B suppresses the formation of the massive γ phase and increases the tendency of lamellar structure formation by the refinement of the high-temperature α phase, whereas the cooling rated needed for the onset of massive γ formation is hardly changed [18].…”

“…For TiAl-based alloys, massive transformation, which is initiated by cooling from high temperature α phase in a controlled cooling rate to create massive γ phase, followed by subsequent annealing at α + γ two phase region is proven to be an effective method to refine the microstructures [5][6][7][8][9][10][11][12]. However, it is commonly agreed that a heat treatment window exists for each alloy composition; only in this window can the fully massive transformed microstructure be obtained that is suitable for the following annealing treatments, although the massive transformation is thought to be displacive only with atom rearrangement across interfaces [7,8,11,13]. For high Nb-TiAl alloys, although the microstructure is relatively finer than some other TiAl alloys during the common preparation process, further grain refinement is still necessary, since balanced mechanical properties usually exist in fine-grained fully lamellar microstructures [14][15][16].…”

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

“…[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.…”

“…[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.…”