Growth temperatures and the limits of coupled growth in unidirectional solidification of Fe-C eutectic alloys

Abstract

Growth temperature has been measured as a function of applied growth velocityV for grey (austenite-flake graphite) eutectic, austenite dendrites and white (austenite-cementite) eutectic in Fe-4.28 wt pct C at a temperature gradientG of ∼7 K/mm. Grey eutectic (0.4 to 65 µm/s) required an undercoolingΔTfor growth equal toK1V1/2 withK1 as 3.4 ± 0.1 Ks1/2/µm1/2, giving values ofΔT nearly an order of magnitude larger than predicted for growth at the extremum, as shown previously by Toloui and Hellawell for the related Al-Si eutectic. Austenite dendrites growing together with the grey eutectic atV between 11 and 65 µ m/s exhibited a range of tip temperatures giving rise to average undercoolings of magnitude [GD/V +K2Vn] withD as diffusion coefficient of carbon in the melt andK2 as 0.73 Ksn µm−n withn as 0.46 predicted by Ivantsov’s theory for growth of an austenite needle. White eutectic displaced both grey eutectic and austenite dendrites atV of 100 µm/s. Except for the persistence of grey eutectic rather than white eutectic in the presence of austenite dendrites (11 <V < 65 µm/s), these observed growth transitions are consistent with the derived relationships governing growth temperatures on the basis of competitive growth,i.e. that the growth form with the highest growth temperature at a particularV should prevail. The results are then applied to derive the limiting conditions for growth of grey and white eutectics as a function of composition in Fe-C (i.e. their coupled zones). The significance of observed dependences onV of volume fraction and spacing of austenite dendrites is discussed.