Peritectic reaction and solidification in iron-nickel alloys
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31/7/03
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Peritectic Reaction and Solidification in Iron-Nickel Alloys N.J. McDONALD and S. SRIDHAR A high-temperature confocal scanning laser microscope (CSLM) has been used to study austenite () precipitation and growth on -ferrite during peritectic reaction and solidification in the Fe-Ni system. The two stages of the peritectic transition involving the liquid phase—the reaction, which sees austenite grow along the ferrite/liquid boundary, and the direct solidification of austenite—were observed. It was found that, for both the hypoperitectic (4.2 pct nickel) and hyperperitectic (4.7 pct nickel) alloys, the reaction rate increases with increased undercooling. It was also observed that the reaction rates were faster in the hypoperitectic alloy. However, for both grades, the solidification rate was found to be a function of local-temperature gradients rather than undercooling. Comparisons to available models are made, and a description of possible rate-controlling phenomena is given.
I. INTRODUCTION
THE peritectic reaction, L , is extremely important for a number of material systems. These include copper and aluminum alloys as well as magnetic and electronic materials.[1] It is perhaps most important to understand for iron-based alloys solidified in nonequilibrium processes. In processes such as ingot casting, it is assumed that steel compositions that fall within the peritectic range will eventually pass through the austenite phase and negate the delta-ferrite phase that formed earlier.[2] However, in processes such as continuous casting, the volume change associated with the peritectic transition causes the shell to pull away from the mold, leading to a decrease in heat flux, which, in turn, leads to thin spots. These thin areas on the surface of the casting then have a higher chance of cracking or causing a breakout.[3] Currently, this problem is prevented by reducing the rate at which the casting is pulled from the mold, to ensure adequate wall thickness.[4] A magnification of the peritectic region of a binary-phase diagram is shown in Figure 1(a). The peritectic composition is Cp at the temperature Tp and is the composition at which all ferrite () will convert to austenite (). The peritectic region is described as any alloy with a composition that falls between C and CL, as shown in Figure 1(a). Hypoperitectic alloys are those between C and Cp, while hyperperitectic alloys have a composition range of Cp to CL.[2] It should be noted that the equilibrium partition coefficient (keq CS /CL) for the iron-nickel system—shown in Figure 1(b)—is less than unity, so the product is not an intermetallic species.[1] The peritectic transition is here defined as all stages between the initial formation of austenite and the final solidification. Kerr et al.[5] have split this transition into three distinct regimes: reaction, transformation, and solidification. The peritectic reaction requires that all three phases be in contact with one another while the grows, while the peritectic trans
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