Undercooling Experiments in a High Temperature Differential Scanning Calorimeter
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In the present paper we report some enthalpy data of undercooled pure elements and alloys obtained by DSC. The behaviour of the heat capacity of undercooled liquid Ni as a function of temperature is discussed comparing the experimental enthalpies with literature data and models.
81 Mat. Res. Soc. Symp. Proc. Vol. 398 0 1 99 6 Materials Research Society
Undercooling experiments on some Ni-B alloys will be presented together with the description of a portion of a metastable Ni-B phase diagram. EXPERIMENTAL Pure elements have been used as-received and various alloys have been prepared by arc melting. The composition of relevant alloys has been determined by plasma atomic emission. A high temperature differential scanning calorimeter (Setaram HTDSC) has been employed for measurements in the temperature range 600-1870 K. It consists of twin alumina cells surrounded by Pt-13% Rh thermopiles which are inserted in a graphite furnace. The samples (100-300 mg) have been introduced in the crucible together with allumina powder in order to avoid sticking to the crucible walls. After repeated evacuation and filling with He, a constant flow of pure He has been sent in the calorimetric cell in order to guarantee a protected atmosphere during the experiments. The heating and cooling rates were between 1 and 10 K min' and a temperature of about 100 K above the melting point was reached before each undercooling experiments. The calibration of the DSC cell has been performed by melting several pure elements (Zn, Al, Ag, Au, Ni), to cover the whole temperature range explored during the experiments. The reproducibility for the values of the enthalpies used for calibration was within 1.5%. After a correction for the effect of heating rate, a polynomial of fourth degree was fitted to the point and a calibration curve has been determined. The microstructure of the samples before and after the experiments has been checked by SEM (Leika Stereoscan 420) and optical microscopy. Phase structure analysis has been performed by X-ray diffraction, using Co Kct incident radiation. RESULTS AND DISCUSSION Metals No undercooling has been obtained in the present conditions for Ag, Au, Al and Zn whereas Ni, Cu, Fe, Pd and Co were substantially undercooled [7]. Thermograms of melting and solidification of pure Ni are reported in fig. 1. Melting is located at 1728 K and exothermic signals due to solidification occur on undercooling. The degree of undercooling depends strongly on the occurrence of heterogeneous nucleation events in I each experiment, so there is no 1 correlation between AT and a= 5 K minC b b=1OKmin-1 cooling rate or sample mass. A typical AT value of 180 K has Kmin-' 2 c= a Cd• been obtained for liquid Ni. It is "significantly lower than the value 4estimated for homogeneous Snucleation limit [1] and I experimentally obtained on small I---i particles [8] or on levitated bulk samples [9]. On the other hand, it is comparable to the results of 1500 1600 1700 1800 experiments on calorimetry of levitated droplets [4]. The Temperature / K het
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