Rapid Solidification Of Deeply Undercooled Co-B Alloy Melts

Deeply undercooling alloy melts to far from the liquidus temperature by eliminating heterogeneous nucleation sites inside it is frequently used to study non-equilibrium solidification behavior,or even to prepare quasi-crystals,amorphous alloys and other metastable materials.Among all undercooling methods,the glass fluxing method is the most widely used in laboratory because of its simple operation,broad applicability and excellent purifying effect.In this dissertation,a glass purifier composed of 70.8%B2O3+29.2%Na2B4O7(wt.%)was prepared typically for the Co-B alloys,with which the undercooling could be dramatically increased compared with using pure B2O3 glass.The Co79.3B20.7 alloy with the nominal composition of metastable Co23B6 phase and the peritectic Co75B25 alloy with the nominal composition of stable Co3B phase were undercooled to different degrees.Their solidification structures were identified using X-Ray diffraction and the solidification paths were determined by analyzing the microstructures and cooling curves of the samples.For the Co79.3B20.7 alloy,there exist two critical undercoolings ?Tc1?60 K and ?Tc2?160 K.The alloy undercooled by ?T solidifies into primary Co3B and subsequently ?-Co/Co3B eutectic phases if ?T<?Tci,completely metastable Co23B6 phase if ?Tc1<?T<?Tc2,and merely ?-Co/Co3B eutectic phase if ?T>?Tc2.For the Co75B25 alloy,there is nothing other than ?-Co and Co2B phases to form during solidification,indicating that not only the peritectic reaction of L(liquid)and Co2B into Co3B but also the formation of Co3B as primary phase at large undercooling are inhibited.The peritectic reaction does not occur even when the melt solidifies at a very small undercooling and a cooling rate decreased to 5 K/min that is far smaller than the cooling rate of around 900 K/min in the normal solidification experiment.