| Summary: | 碩士 === 國立成功大學 === 機械工程學研究所 === 85 ===
The purpose of this study is to investigate the plastic deformation behaviour of Tungsten-Nickel-Iron (W 92.5 wt%＋Ni 5.25 wt%＋Fe 2.25 wt%) composite subjected to high temperature and high strain rate loading conditions. The mechanical testing is performed under strain rate ranging from 800 s-1 to 4000 s-1 andconstant temperatures in range of 25℃ to 1100℃ by means of a split-Hopkinson bar which is developed based on one dimensional elastic wave propagation theory. The tested temperatures are obtained by enclosing the specimen in a clam shell radiant-heating furnace. The O.M. and S.E.M. techniques are also used to analyze the fracture and microstructure characteristics of the deformed specimens. With the macroscopic and microscopic results, a constitutive equation is used to describe the plastic deformation behaviour of the material.
The experimental results indicate that the temperature and strain rate are influences on the mechanical properties of material. Flow stress increases with strain rate, but decreases with temperature. At the highest strain rate of 4000 s-1, flow stress increases firstly up to strain of 20%, then decreases rapidly with the augmentation of strain. The strain rate andtemperature sensitivities and work hardening coefficient change with the variety of strain, strain rate and temperature. From the microscopic analyzing, it is found that the microcrack densities and grain deformation parameter increase with the augmentation of strain rate and temperature. The material fracture includes four kinds of model: tungsten cleavage, matrix failure, tungsten-matrix separation and tungsten-tungsten grain boundary failure. Finally, by comparing the results of mechanical testing with those of Zerilli-Armstrong equation for BCC structure, the agreement between the experimental and simulated data was excellent.
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