Calorimetric Study of the y-loop Region of the Iron-Chromium System BY A. S. NORMANTON* Dept. of Metallurgy University of SheffieId Received 16th January 1974 A high-temperature adiabatic calorimetric study of the y-loop region of the iron-chromium system is briefly described. Heats of the a +-y and y -+high-temperature-a transformations are given for appropriate mean transformation temperatures. As part of a programme of research on the thermodynamic properties of iron- base alloys heat capacities and heats of transformation have been measured for iron- chromium alloys containing up to 16 at. %Cr. There are only limited similar calorimetric data for this region of the Fe-Cr system 1* and until recently 3* the phase boundaries of the closed y-loop were not known with any certainty.The results of some recent measurements are brieff y described. EXPERIMENTAL The measurements were made in a spherical adiabatic calorimeter capable of operating in the range 600-1700K. The calorimeter and its operation together with data obtained for high-purity iron have been In the current study alloys were prepared by vacuum-melting appropriate quantities of 99.9 % iron and chromium. All alloys con- tained < 0.004wt %C and < 120p.p.m. O2and < 10p.p.m. N2. Specimens weighing approx. 60 g were machined from a hot rolled bar and were subsequently homogenized. Heat-capacity measurements were made discontinuously with the heat input to the specimen of NN 0.35 J s-I for any one measurement of lo3s duration and heating rates were > 10-14 K h-l.Heats of transformation for a -+y and y -+high-temperature-a were measured using heating rates of M 7-15 K h-l. The heat capacities were easily calcu- lated from the temperature rise in the specimens produced by a measured power input together with a knowledge of the specimen weight and " thermal leak " constant of the calorimeter. The heats of transformation were calculated from the total power input during the time of the transformation but because the transformation occurs over the tempera- ture range of the (a+y) two-phase field this total power includes a term to account for the change in temperature and heat capacity as well as the actual transformation heat and this must be taken into consideration. RESULTS A typical set of heat-capacity data are shown in fig.1 for an Fe-3.01 at. %Cr alloy. These show the increase in heat capacity as the Curie temperature is ap- proached and the decrease afterwards as well as the discontinuities at the phase transformations. A typical heating curve for an a -+y transformation is shown in fig. 2 the start and finish of the transformation being respectively taken as the points of departure and * now at British Steel Corporation Advanced Process Laboratory Hoyle Street Sheffield S3 7EY. 52 A. S. NORMANTON 53 I . I I I I 1 1 ] 1 I ; ; ! I ! 1 A ' - 7O- r(I CI2 & 53-- b 0" - 3ot,1 ; 1 I d--z -_ -%-I t 1 sI I 18 ::'!-I ! i --1 - _--I1 f if& ! ; time/min FIG.2.-Typical heating plot of temperature (T/K) against time/& for the a +y transformation of an Fe-7.83 at.%Cr alloy. The dots (0)indicate experimental points some of which are omitted for clarity. return to linearity on this temperature-time graph. The heats of transformation calculated at appropriate mean transformation temperatures are shown in fig. 3. The phase boundaries of the y-loop as determined from the calorimetric measure- ments agree well with the recent measurements of Kirchner et aL3 and are shown in fig.4 CALORIMETRIC STUDY OF Fe-Cr -I I 0 I I ... at. %chromium FIG. 3.-Plot of heat of transformation (kJmol-') against at. %Cr at the mean transformation temperature -x - cc -+y transformation ; - - o -,y-high-temperature-a transformations.The symbols x and 0 on the abscissa represent the values of the o! -+ y and y -+6 transformation for pure Fe. I700 1500-1300. I too. +I I I t I f i; 2 4 6 0 10 12 14 at. %chromium FIG.4.-PIot of temperature (T/K) against at. %Cr showing the phase boundaries of the y-loop according to Kir~hner,~ - and those determined in the current study a. A. S. NORMANTON DISCUSSION The thermodynamic data obtained from a calorimetric study of alloys in the y-loop region of the Fe-Cr system have been briefly described. The heat capacity data are currently being analyzed to yield information on the magnetic transformations. The heats of the a-y transformations show little variation with composition until a marked decrease begins at x 7 at.%Cr. Any comparison of these values with that for pure iron is not easy because of the difference in transformation temperatures and the difference in the degree of magnetic order remaining in the alloys at the start of the transformation. The heats of the y -+ high-temperature-a transformation appear to decrease initially on adding Cr as compared to the heat of the y -+6 transformation in pure iron. This is followed by a rapid rise and then a decrease for Cr contents > 8 at. %. A viable procedure for the correction of the a -+ y heats is currently under consideration. Also it is hoped to determine y + high-temperature-a heats of transformation for a 1 at. %Cr alloy to verify the initial fall in the heats as in fig.3. Other iron binary systems exhibiting closed y-loops have also been studied and it is hoped that data for Fe-V and Fe-Si systems will help in the interpretation of the current results. It is intended to examine the results in terms of thermodynamic models of solid solutions including the applicability of the " two-y-state " approach. The author thanks the Science Research Council for financial support and Professor B. B. Argent for his help and encouragement. W. B. Kendall and R. Hultgren Trans. Amer. Suc. Metals 1961 53 207-14. K. Schroder Phys. Rev. 1962,125,1209. G. Kirchner T. Nishizawa and B. Uhrenius Met. Trans. 1973 4 167. P. Poyot G. Guiraldenq and J. Hochmann Mem. Sci. Rev. Met. 1972 69 no. 77 p. 775. F. R. Sale J. Phys. E 1970 3 646. G. R. Smith and F. R. Sale J. Phys. E 1971,4,366. F. R. Sale and A. S. Normanton Metallurgical Chemistry Symp. Ed. 0. Kubaschewski N.P.L.,1971 (London H.M.S.O. 1972) p. 19.