Nonthermoelastic and Thermoelastic Martensitic Transformation Behavior Characterized by Acoustic Emission in An Fe-Pt Al
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The austenite grain size was measured as 150itm. Specimens for AE measurement and electrical resistivity of 0.5 X 1.5 X 20 mm 3 in size were cut form the heat-treated alloy. The specimen was attached directly to an AE transducer using a silicone vacuum grease. The AE transducer used was an M53 (Fuji Ceramics Co.), and its resonant frequency is approximately 300kHz. The specimen was cooled slowly at a cooling rate of approximately 2K/min. AE signals detected were amplified by a pre-amplifier and analyzed by an NF 9502 AE measurement system. The gain of the pre-amplifier was 40dB, and the threshold level was 50dB with reference to 0.1 [LV at the input of the preamplifier (or 31.6 RiV at the input of the preamplifier). Transformation behavior was also monitored in specimens of 3mm square by 0.5mm thickness by video-recorded observations of surface relief contrast in a cold-stage optical microscope. 0.8 ,RESULTS 0.762 0:.6 04 h 20.5 . 0.4 a 0.3 4 E 02volume -0 > 0.1 .22hrs, 0 ,I I 160 180 200 220 240 260 280 300 Temperature (K)
AND DISCUSSION Transformation Behavior Observed by Optical Microscope Figure 1 shows the vu-iume fraction of martensite as a function of temperature in the specimens ordered for 4, 22 and 108hrs. The fraction was measured on the printed images of each specimen at various temperatures during cooling. In the specimens ordered for 4 and volume fraction increases very abruptly with decreasing temperature and one nucleation event triggers other nucleation events by the mechanism of autocatalytic nucleation.but it Fig. 1 volume fraction of martensite formed increases more gradually in the specimens ordered for 108 hrs. during cooling as a function of temperature. Acoustic Emission Characteristics During Cooling Figures 2(a) shows the relation between total AE event counts, event rate and temperature during cooling in 4hrs ordered specimen. The Ms temperature measured by electrical resistivity change is also shown on the abscissa. In the 4 hr ordered specimen, no event is observed above 282K and AE events begin to be observed sporadically with decreasing temperature. AE event counts start to increase just below 278K and increase abruptly during further cooling. The Ms temperature determined by electrical resistivity change is 281K, and the generation of AE event counts corresponds to the martensitic transformation in this specimen. The test was stopped when the number of events exceeds approximately 6,000 events which is the limitation of the measurement system used in this study. Figures 2(b) and (c) show amplitude distribution of AE events observed in the temperature range between 282 and 278K, and that between 278 and 273K, respectively. AE event amplitude values are shown with reference to 0.1 [tV at the output of the sensor used. Amplitude values are distributed from 50 to 85 dB above 278K, while amplitude values below 278K are distributed from 50 to 106 dB. These results indicate that higher amplitude events are generated below 278K. Since the amplitude of AE event is related to the size of the
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