(0.19TL), gFe dendrites display a growth velocity of only 100 mm/s. The reduction of dendritic growth velocity is caused mainly by two factors: (1) the solute diffusion speed is slowed down if bulk undercooling is continually increased, and (2) the tiny Cu-rich globules formed by phase separation may hinder the progression of the solid/liquid interface. However, it should be noted that the absolute values of dendritic growth velocity are one order of magnitude larger than those at the moderate undercooling range, although the latter case shows an increasing tendency of dendritic growth velocity. Credit: ©Science China Press
The Stokes motion driven by the density difference between the Fe-rich and Cu-rich liquid phases plays the dominant role in the progression of macroscopic segregation. As the thermodynamic barrier against the nucleation of a secondary liquid phase is usually much smaller than in the case of solid phase nucleation, a great number of Fe-rich and Cu-rich globules can be formed in the liquid phase. Once the size of an individual globule becomes sufficiently large, its Stokes motion prevails over all other kinds of movement. The calculated results show that the Stokes motion velocity increases rapidly along with an increase in globule size. A small Fe-rich globule with a 10mm radius displays a moving velocity of only 8.1mm/s in the alloy sample undercooled by 305K, while a large globule with a 100mm radius achieves a floating velocity of 805.5mm/s. Considering that the phase separation time Dtps of this alloy sample attains 11.8s, the large globules can float for a distance of 9.5mm. Such a moving distance covers the entire alloy sample, confirming that the Stokes motion is indeed responsible for the progression of macroscopic segregation.The calculations also reveal that the Stokes motion velocity increases with bulk undercooling. The main reason for this lies in the fact that a greater bulk undercooling facilitates the achievement of macrosegregation and consequently leads to a larger density difference. It is evident that the subsequent solidification process of a macroscopically segregated Fe47.5Cu47.5Sn5 alloy should involve two successive stages: (1) the Fe-rich zone solidifies because of its higher liquidus temperature, and (2) the Cu-rich zone crystallizes at some lower temperatures T
Whether or not metastable phase separation occurs, the rapid dendritic growth of the primary gFe phase always induces a thermal recalescence effect. Since gFe dendrites spread quickly throughout he alloy sample in the course of recalescence, the alloy's growth velocity may be roughly measured as the ratio of the sample size divided by its recalescence time. As bulk undercooling increases from 112 to 236K, the actually measured dendritic growth velocity rises from 7.8 to 68.3mm/s. These results indicate that the primary dendritic growth of the gFe phase in a ternary Fe47.5Cu47.5Sn5 alloy is quite sluggish when compared to the dendtritic growth displayed by a similar Co-Cu-Ni alloy. One reason for this finding is that this Fe47.5Cu47.5Sn5 alloy has a broad solidification temperature range and hence its primary dendritic growth remains consistently in the regime of solute diffusion-controlled growth. On the other hand, the introdution of an Sn solute further suppresses the advancement of a solid/liquid interface.
The occurrence of macrosegregation imposes a drastic suppression effect on primary dendritic growth. The maximum dendritic growth velocity of the gFe phase attains 400 mm/s at the bulk undercooling of 246K. In contrast, under the maximum bulk undercooling of 329KPeritectic solidification requires a mutual reaction between the primary solid phase and the remnant liquid phase to form a new solid phase when the undercooling is below 196K. This is a typical solute diffusion-limited process, and it is rare for such a process to be completed. Two successive peritectic reactions are required for the successful completion of this process. First, primary gFe phase nucleates within the undercooled alloy must melt, grow dendritically, and react with the surrounding liquid phase to form the peritectic (Cu) phase, that is gFe+L®(Cu). This peritectic transformation is only partially fulfilled, resulting in the remains of many gFe dendrites that subsequently transform into the aFe phase. As a morphological feature of the peritectic product phase, a (Cu) solid solution always appears as haloes enveloping the dendritic fragments of the aFe phase. It is interesting to note that the (Cu) phase haloes often display two layers: an inner layer directly adjacent to the aFe phase that is designated as (Cu)i, and an outer layer marked as (Cu)o that surrounds the previous central part. This structural distinction results from the microsegregation of solutes within the peritectic (Cu) phase.
W.L. Wang | EurekAlert!
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