Microstructural effects of martensitic transformation cycling of a Cu-Zn-Al alloy: Vestigial structures in the parent ph
- PDF / 3,026,383 Bytes
- 9 Pages / 594 x 774 pts Page_size
- 97 Downloads / 169 Views
I.
INTRODUCTION
RECENTLY, several papers have reported on the effects of cyclic transformation in martensitic alloys. 1-9 Among the various effects reported have been kinetic changes, such as shifts in M , and substructural changes, such as the appearance of dislocation 'debris'. The present work attempts to correlate and explain these various effects by means of a combined kinetic and microstructural study of transformation cycling in a Cu-Zn-A1 alloy. II.
EXPERIMENTAL
The alloy material used in this work was provided by Delta Metals Research, Ltd., Ipswich, Suffolk, England, with a nominal composition 66.2 at. pet Cu-24.8 at. pet Zn-9.0 at. pet AI and a measured Ms temperature of 268 K. This material, originally in the form of 1 cm diameter hotworked bar, was machined into rods 3 mm in diameter which were sealed in evacuated quartz tubes, solution treated for 15 minutes at 1173 K, and quenched into ice water. The rods were then wafered into 0.25 mm thick discs using a lowspeed diamond saw. Transformation cycling was carried out, prior to electropolishing to thin foils, in a Perkin-Elmer Model DSC-2 differential scanning calorimeter, at a scan rate of 10 K per minute between 230 K and 300 K, which spans the Mf to Pf range for this alloy. Three-to-five discs were cycled as a group to enhance the DSC sensitivity and to ensure that at least one good thin foil could be obtained when the discs were subsequently electropolished. Several groups of samples were cycled more than one hundred times. Thin foils were then prepared with a Struers TENUPOL unit, using a 3 pet perchloric acid-methanol solution at a temperature of 240 K and a voltage of 70 to 75 VDC. The thin foils were examined in a JEM 100-CX II transmission electron microscope. JEFF PERKINS, Professor of Materials Science, Department of Mechanical Engineering, and PAUL BOBOWIEC, Graduate Student, are with Naval Postgraduate School, Monterey, CA 93943. Manuscript submitted April 5, 1984.
METALLURGICALTRANSACTIONS A
III. RESULTS AND DISCUSSION: TRANSFORMATION KINETICS The sample groups were thermally cycled more than 100 times while continuously monitoring the transformation in the DSC. Based on experience in earlier work, 3 the temperatures corresponding to the maxima of the DSC peaks were taken as sufficient to assess any cyclic effects. These temperatures are designated as MMAX, corresponding to the temperature of the maximum rate of heat evolution during the exothermic parent-to-martensite (cooling) transformation, and PMAX, corresponding to the temperature of the maximum rate of heat absorption during the endothermic martensite-to-parent (heating) transformation. The temperature MMAX is roughly halfway between M, and Mr, while PMAXis roughly halfway between Ps and Pc.3 Shifts of the transformations with cycling are reflected in Figures 1 and 2 by plotting the difference between MMAXon the initial cycle with MMAX on subsequent cycles, and similarly for PMAX. The initial cycle MMAXwas 256.5 K for Figure 1, and the initial cycle PMAXwas 273.4 for Figure 2.
Data Loading...
