Hysteresis of Long-Range Ordering in CuAu
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ABSTRACT The effect of heating and cooling on the long-range order transformation in stoichiornetric CuAu is investigated by several complementary measuring methods. Measurements of heat flow, resistometry and acoustic emission are done dynamically by linear heating/cooling. It is shown that measuring dynamically yields the expected effect of undercooling, which decreases with decreasing cooling rate. The dependence of undercooling on cooling rate is compared with the concept of continuous cooling for glass forming. A small influence of heating rate on disordering temperature is reported (retro-effect). INTRODUCTION The effect of undercooling a phase into a range of instability is a well known phenomenon of first order phase transitions. Since this effect is related to the process of generating supercritical nuclei of the new phase it is dependent on the specific nucleation process and on the cooling rate, and should vanish for cooling rates equal to zero. Although there are a lot of theoretical and experimental studies on the nucleation of solid phase from undercooled melts, there are only a few investigations of the similar effect of nucleation of ordered domains from disordered solid phase. Because of experimental difficulties studies of undercooling effects for continuous cooling as a function of cooling rate are rare and there is limited experimental evidence that the effect of undercooling vanishes for very small heating rates. It is the aim of the present paper to show the influence of cooling rate on the value of undercooling observed for (nearly) stoichiometric CuAu. Several measuring methods, including differential scanning calorimetry, electrical resistivity and acoustic emission (AE), have been used. These methods are complementary with respect to their sensitivity for changes in order/disorder. The results reported here in a very condensed form will be published in full length elsewhere. EXPERIMENTAL Sample material (chemical analysis: 49.4±0.5at%Cu, 50.6+0.6Au) was supplied by DEGUSSA via the Institut fur Physikalische Chemie, University of Munich, Germany. An additional sample for acoustic emission (chemical analysis: 49.2±0.2 at% Cu, 50.8+0.2 at% Au) was made in the Institut of Metal Physics, Charles University, Praha. Samples were rolled at room temperature to about 0.3mm thickness with intermediate and final annealings at 873K in a purified Argon atmosphere. Samples of 3mm diameter were mechanically punched-out for calorimetry and AE measurements. Meander shaped resistivity samples were spark eroded. Calorimetric measurements were done in a Perkin Elmer DSC7 apparatus. After inserting sample and reference sample (99.99% pure Au) the measuring chamber was first evacuated and then filled with purified 99.999% Argon gas to prevent sample corrosion. In-situ resistometry was done under Argon atmosphere in an apparatus described in [ I]. 581 Mat. Res. Soc. Symp. Proc. Vol. 398 01996 Materials Research Society
The AE measurements during thermal cycling were realised in a furnace under Ar atmosphere. To
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