Transformation Behavior and its Effect on Damping Capacity in Fe-Mn Based Alloys

7 -+ E transformation behavior in a Fe-21Mn alloy with different grain size and a Fe-32Mn-6Si alloy with various degrees of wld rolling is investigated and correlated with damping capacity. Effect of microstructure on damping capacity is discussed on the assumption that the capacity is proportional to volume swept by 7 / e boundaries. 1.INTRODUCTION Fe-Mn based alloys have been well known for shape memory effect which is based on the formation of e martensite when the alloys are deformed [1,2]. Thermoelastic martensitic alloys which experience phase boundary movement by deformation have been found to generate damping capacity [3,4]. Recently, there have been some reports that Fe-Mn based alloys undergoing non-thermoelastic martensitic transformation exhibit damping capacity [5]. This work is aimed at correlating y -+ e transformation behavior with damping capacity in Fe-Mn based alloys. The transformation takes place during cooling or by deformation according to alloy composition,. Two Fe-Mn based alloys, Fe-21% (wt%) and Fe-32Mn-6Si (wt%) which undergo the transformation during cooling and by deformation respectively, are used in the study. The variation in the transformation behavior is made by changing grain size for the Fe-21Mn alloy and degree of cold rolling for the Fe-32Mn-6Si alloy. 2.EXPERIMENTAL PROCEDURE The alloys were prepared by melting in a magnesia crucible in a vacuum induction furnace. The ingots were homogenized at 1000°C for 2hrs and hot-rolled at 900°C. After hot rolling, the Fe-21Mn alloy was subjected to 30% cold rolling, followed by heat treatment at various temperatures (700-1200°C) for lhr to vary grain size. The Fe-32Mn-6Si alloy was cold rolled with various thickness reduction (1, 3, 5, 10, 15%) at a rate of 0.1% a pass to minimize the microstructural inhomogeneity across the thickness. The free vibration method was applied to measure damping capacity with a strain gauge attached on the specimen [6]. Damping capacity was evaluated as the logarithmic decrement. Measurement of volume ffaction of e martensite was made by comparing the volume change during reverse transformation with the specific volume difference which was Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1995857 (3-386 JOURNAL DE PHYSIQUE IV determined by X-ray diffraction. Since E martensite in the Fe-21Mn alloy exhibits no anisotropy, its volume change is obtained as three times the length change due to the reverse transformation by 3. In the Fe-32Mn-6Si alloy, however, the sum of the length changes in the rolling (RD), transverse (TD) and normal (ND) directions was taken as volume change because of direction-dependent dilatometric behavior [7]. The specimens for TEM were prepared by eletropolishing in 10% HCl04/methanol solution at 223K. The microscope used is Phillips CM-30 with an accelerating voltage of 300kV. 3.RESULTS AND DISCUSSION Fig. 1 shows optical microstructure of the Fe-21Mn alloy heat treated at various temperatures. As the temperature increases, grain size and E martensite amount increases. No a ' martensite is observed in the alloy. Variation in grain size, from 3 . 5 ~ to IOllrm, with heat treatment temperature is shown in Fig. 2. Fig. 3 represents dilatometric behavior of the alloy subjected to heat treatment at various temperatures. As and Af temperatures as well as volume fiaction of E martensite formed during cooling from the heat treatment temperatures are determined from the heating curves. Ms temperatures are measured from the cooling curves. As, Af and Mi temperatures me, irrespective of grain size, measured to be 185, 200 and 133 "C, respectively. This result is inconsistent with the previous study on an Fe-1SMn alloy that Ms increases with the increase in grain size [8]. With the result of X-raydiffraction that 1.98% volume expansion accompanies e -7 transformation in the Fe-21Mn alloy, e martensite amount, calculated from Fig. 3, is shown in Fig. 4. Fig. 5 shows the effect of heat treatment temperature on damping behaviorof the Fe-21Mn alloy. Damping capacity of all the specimens increases with increasing strain amplitude. Some therrnoelastic martensitic alloys like Cu-based alloys [3,4] and Mn-Cu Fig. 1. Optical microstructure of the Fe-21Mn alloy heat treated at 700°C (a), 800°C (b), 900°C (c), 100O'C (d), 1100°C (e) and 1200°C (0 [9] are reported to exhibit the same behavior. But Fe-Mn alloys shows stronger strain amplitude dependence. As heat treatment temperature increases, damping capacity improves, reaching its maximum around 1000'C. Further increase in the temperature, however, aggravates damping capacity. Fig. 6 shows transmission electron microstructure of the alloys heat treated at 700, HEAT TREATMENT TEMP.(OC) Fig. 2. Variation in grain size with heat treatment temperature TEMPERATURE (OC) Fig. 3. Dilatometric behavior of the alloy subjected to heat treatment at various temperatures 700 800 90