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Preferred growth orientation and microsegregation behaviors of eutectic in a nickel-based single-crystal superalloy

View Article: PubMed Central - PubMed

ABSTRACT

A nickel-based single-crystal superalloy was employed to investigate the preferred growth orientation behavior of the (γ + γ′) eutectic and the effect of these orientations on the segregation behavior. A novel solidification model for the eutectic island was proposed. At the beginning of the eutectic island’s crystallization, the core directly formed from the liquid by the eutectic reaction, and then preferably grew along [100] direction. The crystallization of the eutectic along [110] always lagged behind that in [100] direction. The eutectic growth in [100] direction terminated on impinging the edge of the dendrites or another eutectic island. The end of the eutectic island’s solidification terminates due to the encroachment of the eutectic liquid/solid interface at the dendrites or another eutectic island in [110] direction. The distribution of the alloying elements depended on the crystalline axis. The degree of the alloying elements’ segregation was lower along [100] than [110] direction with increasing distance from the eutectic island’s center.

No MeSH data available.


Solidification model of the (γ + γ′) eutectic island.
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Figure 2: Solidification model of the (γ + γ′) eutectic island.

Mentions: Based on the examination and the previous studies [6, 14], a novel formation model of the eutectic island can be depicted in figure 2. On commencing, the (γ + γ′) eutectic core forms directly from the remaining liquid by eutectic reaction. At this stage, the anisotropy has no effect on the formation of the core. As this reaction proceeds, the eutectic prefers to grow in [100] direction, and the advancing velocity of the solidification front is the higher than [110] direction. When the liquid/solid interface of the eutectic in [100] direction impinges on the peripheral dendrites or another eutectic island, and there is no more free space for further growth, the eutectic reaction will cease in this direction, and the eutectic growth continues in [110] direction until the end of solidification. That is, the eutectic growth in [110] direction always lags behind that in [100] direction. Previous studies [15, 16] suggest that a long-range solute diffusion boundary layer established ahead of the eutectic solid/liquid interface may destabilize the morphology of the eutectic interface as a whole. During the solidification of the (γ + γ′) eutectic, owing to the crystallization of γ dendrite around the eutectic, the positive segregation elements such as Al, Ta, Ti and Hf are rejected in the interdendritic remaining liquid, and some of them can enrich ahead of the eutectic interface by a long-distance diffusion. Consequently, a compositional gradient of these elements perpendicular to the interface is built as shown in figure 3. In the present investigation, the longitudinal compositional gradient causes a concave eutectic liquid/solid interface (isotherm) to the remaining liquid. In addition to this, due to the formation of the (γ + γ′) core, the latent heat is gradually released, which causes a reduction in the nucleating undercooling and retards the eutectic reaction. The γ′ and γ lamellas have sufficient time to thicken in the final solidification, which will gradually widen the eutectic liquid/solid interface. Since the growth directions of the eutectic lamellas are usually vertical to the isotherm, the eutectic lamella emanates with increasing distance from its center.


Preferred growth orientation and microsegregation behaviors of eutectic in a nickel-based single-crystal superalloy
Solidification model of the (γ + γ′) eutectic island.
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC5036469&req=5

Figure 2: Solidification model of the (γ + γ′) eutectic island.
Mentions: Based on the examination and the previous studies [6, 14], a novel formation model of the eutectic island can be depicted in figure 2. On commencing, the (γ + γ′) eutectic core forms directly from the remaining liquid by eutectic reaction. At this stage, the anisotropy has no effect on the formation of the core. As this reaction proceeds, the eutectic prefers to grow in [100] direction, and the advancing velocity of the solidification front is the higher than [110] direction. When the liquid/solid interface of the eutectic in [100] direction impinges on the peripheral dendrites or another eutectic island, and there is no more free space for further growth, the eutectic reaction will cease in this direction, and the eutectic growth continues in [110] direction until the end of solidification. That is, the eutectic growth in [110] direction always lags behind that in [100] direction. Previous studies [15, 16] suggest that a long-range solute diffusion boundary layer established ahead of the eutectic solid/liquid interface may destabilize the morphology of the eutectic interface as a whole. During the solidification of the (γ + γ′) eutectic, owing to the crystallization of γ dendrite around the eutectic, the positive segregation elements such as Al, Ta, Ti and Hf are rejected in the interdendritic remaining liquid, and some of them can enrich ahead of the eutectic interface by a long-distance diffusion. Consequently, a compositional gradient of these elements perpendicular to the interface is built as shown in figure 3. In the present investigation, the longitudinal compositional gradient causes a concave eutectic liquid/solid interface (isotherm) to the remaining liquid. In addition to this, due to the formation of the (γ + γ′) core, the latent heat is gradually released, which causes a reduction in the nucleating undercooling and retards the eutectic reaction. The γ′ and γ lamellas have sufficient time to thicken in the final solidification, which will gradually widen the eutectic liquid/solid interface. Since the growth directions of the eutectic lamellas are usually vertical to the isotherm, the eutectic lamella emanates with increasing distance from its center.

View Article: PubMed Central - PubMed

ABSTRACT

A nickel-based single-crystal superalloy was employed to investigate the preferred growth orientation behavior of the (γ + γ′) eutectic and the effect of these orientations on the segregation behavior. A novel solidification model for the eutectic island was proposed. At the beginning of the eutectic island’s crystallization, the core directly formed from the liquid by the eutectic reaction, and then preferably grew along [100] direction. The crystallization of the eutectic along [110] always lagged behind that in [100] direction. The eutectic growth in [100] direction terminated on impinging the edge of the dendrites or another eutectic island. The end of the eutectic island’s solidification terminates due to the encroachment of the eutectic liquid/solid interface at the dendrites or another eutectic island in [110] direction. The distribution of the alloying elements depended on the crystalline axis. The degree of the alloying elements’ segregation was lower along [100] than [110] direction with increasing distance from the eutectic island’s center.

No MeSH data available.