Ordering kinetics of Fe 3 Pt austenites and reversed austenites
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treating temperatures below T c. The T c for 22 and 24 at. pct Pt alloys are 765 ~ and 835 ~ respectively. 6 At the end of heat treatment the foils were water quenched by breaking the capsules. The M s temperature was then measured by a D u P o n t differential scanning calorimeter (DSC). Since the austenite to martensite transformation is an exothermic reaction the M, temperature is the start of the exothermic peak. Of course, in this heat treatment an M, temperature above room temperature cannot be measured. For heat treating of reversed austenite the foils were first water quenched from 1300 ~ then cooled to liquid nitrogen temperature to convert all the austenite to martensite. They were then heat treated in two different ways. For short heat treating time where the M s temperature is above room temperature the DSC were used to heat treat the foils and subsequently to measure the M s temperature. The DSC was operated in an argon atmosphere. A high heating and cooling rate was used to minimize annealing or ordering during the course of heating or cooling. In the temperature range between 300 ~ and 600 ~ the heating rate and cooling rate were measured to be 115 ~ and 300 ~ respectively. A liquid nitrogen cooling jacket was used to cool the DSC. For longer heat treating times where the M, temperature is below room temperature the foils sealed in the capsules were heat treated in the regular furnace and then water quenched by breaking the capsules. Again, the M s temperature was measured by the DSC.
RESULTS AND DISCUSSION Figure 1 shows the resistance-transformation curves of 22 and 24 at. pct Pt samples quenched from 1300 ~ into a silicone oil bath. As indicated, the M s temperatures of virgin austenite for the two alloys are 91 ~ and 43 ~ respectively. After heating the martensite samples in the DSC above the A : temperature to 600 ~ and then immediately cooling, the M s temperature of reversed austenite is 124 ~ and 89 ~ respectively, Fig. 2. The increase of M s temperature in this second cycle is due to the
ISSN 0360-2133/80/0611-0905500.75/0 METALLURGICAL TRANSACTIONS A 9 1980 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME
VOLUME 11A, JUNE 1980--905
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.... .d . ,i,,,,,l , ,,,,,,, 10 100 1000 TIME,HOUR F i g . 4 - - T h e variation of M s temperature with heat treating time at 575 ~ for 22 and 24 at. pct Pt austenites and reversed austenites. -2oc,
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80 100 120 140 160 180 200 TEMPERATURE~ Fig. 2 - - T h e exothermic peaks of reversed austenite to martensite transformation. The reversed austenites are obtained by heating the martensites t
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