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DSC analySIS of orDer-DISorDer tranSItIon In ni3al baSeD alloyS from ni-al-Cr SyStemAnAlizA DSC przemiAn typu porząDek-nieporząDek w StopACh nA oSnowie ni3al z ukłADu ni-al-CrNi-Al-Cr system is significant for industrial alloys based on intermetallic phase Ni3Al, that crystallizes in crystal lattice L12. It is indicated as γ′, contrary to disordered phase γ which occurs with addition of chromium. DSC analysis was performed on alloys of chemical composition from Ni75Al25÷Ni75Cr25 range of Ni-rich part of Ni-Al-Cr system. In addition, few measurements were conducted using DTA method. Based on curves registered, it was possible to identify characteristic temperature during heating and cooling of the sample. It corresponds to existence of phase boundary γ′+γ / γ in examined alloys. Results of thermal analysis were compared with results obtained with other method performed by authors which is solution calorimetric method.Keywords: Ni-Al-Cr system, intermetallic phase Ni3Al , order-disorder, DSC analysisUkład Ni-Al-Cr jest ważnym układem dla przemysłowych stopów na osnowie fazy międzymetalicznej Ni3Al, która krystalizuje w uporządkowanej sieci L12. Oznacza się ją jako γ′ w odróżnieniu od fazy nieuporządkowanej γ, która pojawia się wraz z dodatkiem stopowym chromu. Wykonano serię stopów, których skład chemiczny leży na linii Ni75Al25÷Ni75Cr25 w bogatej w nikiel części układu Ni-Al-Cr i poddano je analizie DSC. Dodatkowo wykonano kilka pomiarów metodą DTA. Na podstawie zarejestrowanych krzywych udało się zidentyfikować charakterystyczne temperatury podczas nagrzewania i studzenia próbki. Odpowiadają one miejscom występowania granicy międzyfazowej γ′+γ / γ w stopach. Wyniki analiz termicznych zestawiono z wynikami innej metody zastosowanej przez autorów tj. kalorymetrycznej metody typu rozpuszczania.

The paper presents the results of DSC and microstructural investigations of Elektron 21 magnesium alloy in as cast condition and after solution hardening. Elektron 21 is a magnesium based casting alloy containing neodymium and gadolinium for used to at 200°C in aerospace application. The solution heat treatment was performed at 520°C/8h/water. Ageing treatment was performed at different temperatures 200, 250, 300 and 350°C, then quenched in air. The microstructure of Elektron 21 in as cast condition consists of primary solid solution α -Mg grains with eutectic α-Mg + Mg3(Nd,Gd) phase and regular precipitates of MgGd3 phase. After DSC investigations three exothermal signals has been observed. First exothermal signal at ~170÷245°C assigned to an undifferentiated formation of the metastable phases β” and β’ and the second one at ~280°C corresponded to the formation of a stable β (Mg3Nd) phase. The last signal at ~300°C was connected to the formation of Mg41Nd5 phase. Regular precipitates of MgGd3 phase have been also observed. TEM investigation confirmed that the Elektron 21 alloy precipitate from the solid solution according to the sequence of the following phases: α–Mgβ”β’β(Mg3Nd)Mg41Nd5

In the present study, the isothermal early oxidation behaviour of the WE54 and Elektron 21 alloys were studied at a temperature of 773 K in pure O2 up to 150 min. The results showed that the oxidation kinetics depending on the chemical composition and microstructure of the investigated alloys. The oxidation kinetics of these alloys in as-cast and T6 conditions obtained a parabolic law, while in supersaturated state these alloys exhibited a linear kinetics. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses indicated that an oxide film, composed of MgO and (Y,Dy)2O3 in WE54 alloy and (Nd,Gd)2O3 in Elektron 21 alloy, had been formed.

The specific heat capacity was calculated by DSC and the kinetics of oxidation was determined for alloys from the NiAl, FeAl and TiAl phase equilibrium system. The Ti48Al alloy contained an addition of 2 at.% of chromium and niobium. The highest value of heat capacity had the Ti-Al and Fe-Al alloys. At temperatures above 673K, the Ti48Al alloy showed an anomaly, that is, a lower value of the heat capacity. The change in heat capacity was related with an exothermic reaction. The oxidation of alloys was running according to either parabolic or cubic law. Alloys characterised by high thermal capacity showed a higher gain in weight. The slowest to oxidise was the Ni3Al intermetallic phase.