Effects of cooling rate and precipitation kinetics in shape memory CuZnAl alloys

IV SUMMARY Master Thesis EFFECTS OF COOLING RATE AND PRECIPITATION KINETICS IN SHAPE MEMORY CuZnAI ALLOYS Murat ESKİL Fırat University Graduate School of Natural and Applied Sciences Department of Physics 2000, Page:XIV+97 Copper-based shape memory alloys are very sensitive to the thermal effects before and after the transformation, and this effects may cause important changes on crystallographic, morphologic and mechanical properties or the other transformation parameters of the alloys. In this study, the changes which occured on some transformation parameters of two shape memory CuZnAI alloys due to the effects of various thermal procedures have been investigated by using X-ray diffraction, differantial scanning calorimetry, scanning electron microscope, optical metallography and hardness measurements. At the specimens cooled from high temperature ft phase region, the L2ı- »M18R martensitic transformation was observed. The lattice parameters of M18R monoclinic martensitic structure and the changes occured by cooling rate on lattice parameters were defined. In addition, spacing differences (Ad) between some selected pairs of diffraction planes with [hikjl) ve (/z^/) Miller indices satisfying the relation (h}2-h2 ) / 3 = (£/-£/) I n («=1 for 9R and n=A for 1 8R structure) were examined due to the cooling conditions. From the differential scanning calorimetry measurements, for the first thermal cycles, the austenitic transformation temperatures were found to be very high rather than the cycles which were after the first cycle. This result was interpreted that martensite structure has very strong stabilization before the first cycle. From the various cooling or heating rate measurements, it was observed that by the increase of the heating rate, the reverse transformation temperatures were increased, but by the increase of the cooling rate, the forward transformation temperatures were seen to be decreased. At the microscopic observations performed by two different methods named scanning electron microscope and optical microscope, the grain structures were observed; self accommodating martensite plate groups, plate martensites, V-type martensite structures, twin-like bands, precipitation phases and microcracs were found. It has been seen that usually these types of morphological characteristics have no influence by cooling rate from the (3 phase region. By the thermal cycling effects, the increases observed in the hardness of the specimens were thought to be the result of the structural defects like dislocations which formed during the irreversible processes and commented on that this process is the reflection of the transition to a more stabilized structure. In addition, it is suggested that the reason why the hardness result of Alloy 2 is higher than Alloy 1 is that the Al content of the AHoy 2 is higher than Alloy 1. Key Words: CuZnAI alloys, shape memory effect, M18R martensitic structure, cooling rate effects, interplanes distance difference, transformation temperatures, martensite morphology, precipitation kinetics, Vickers micro hardness.