Search

Positron Annihilation Techniques have been proved to be a characteristic tool in the research of lattice defects in metals and alloys. Especially, vacancy type point defects and dislocations can be detected. During recent years, researchers have completed Doppler broadening and positron lifetime measurements to study the phase transformations and behavior of vacancies in CuZnAl, CuAlNi, CuAlMn and NiTi shape memory alloys. Also, research has demonstrated that positrons are sensitive to the evolution of defects, during aging and thermal cycling. From common knowledge, the authors know that copper base shape memory alloys contain a high concentration of excess vacancies once the alloys are quenched from the high temperature single beta phase. Also, the excess vacancies play a very important role in the complicated aging effects of both the parent and martensite phases. The present work intends, by means of Doppler broadening positron annihilation measurements, to investigate the behavior of quenched-in vacancies and the evolution of defects during martensitic transformation cycling.

A dynamic simulation model of magnetostrictive actuators and transducers has been used to study the performance and thermal balance of an ultrasonic magnetostrictive power transducer. Electrically resistive, eddy current, mechanically resistive and hysteresis losses have been deduced when the transducer works against a purely mechanically resistive load of r a moderate driving current of 10 A and a frequency of 21 kHz. The eddy current losses can be reduced significantly by laminating the active material. However the hysteresis losses are the main source for heating the transducers. The power losses obtained from the dynamic simulations have been used as thermal sources in electro-thermal finite element calculations. The calculations show that free air convection is not enough to cool the actuator. Water cooling of the actuator with a flow of 6.8 l/min will decrease the active materials temperature to around 80 degrees C. This has been obtained by estimating the heat transfer and use the heat flow as sinks in the finite element calculations. The design of a magnetostrictive ultrasonic transducer must therefore comprise an optimal working point regarding magnetic biasing and mechanical pre-stress to minimize the hysteresis.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The dynamic behavior of Terfenol-D actuators with laminated magnetostrictive materials for ultrasonic applications has been modeled using a new technique developed on the Simulink platform. The nonlinear simulation model is based on the finite difference method and lumped-circuit approaches with modified constitutive equations, which can simultaneously handle the non-linearities, eddy currents, hysteresis effects, and the electrical, magnetic and mechanical system interactions. Because the power losses are so considerable at ultrasonic frequencies, a thermal lumped circuit approach of the electro-thermal performance has been incorporated in the model and the thermal balance of the actuators with possible cooling system thus can be taken into account in the whole system design. The nonlinear properties of Terfenol-D, obtained from static material characterizations, were used as numerical input to the simulation models. The model integration and some application examples including the numerical estimation of the effects of frequency, number of lamination, magnetostrictive rod dimensions, and eddy currents were presented. The preliminary result indicate that the new design and simulation package is very straightforward and the dynamic behavior of the laminated magnetostrictive actuators at high frequencies can be predicted and estimated fairly well from the simulation model.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.