| Summary: | 碩士 === 大同大學 === 材料工程學系(所) === 92 === This research adopted two kinds of starting powders, one is pure Ti, Al, Si powders and the other is Al3Ti, TiH2, Si powders. Mechanical alloying was used to mill the powders, and the milled powders were vacuum annealed or subjected to vacuum hot-pressing to synthesize Ti5Si3/TiAl and Ti5Si3/Ti3Al composite powders and bulk materials. It is intended to study the influence of various starting powders on the structure evolution of Ti5Si3/TiAl and Ti5Si3/Ti3Al composites by using XRD, DTA thermal analysis and TEM analysis. The microstructures of the bulk alloys were also observed by using SEM and optical microscope, and determined by using EDS composition analysis.
The results show that the formation of Ti5Si3/TiAl composite is achieved by milling the elemental Al, Ti and Si for 3 hours and annealing the powder mixture at 750℃ for 1 hour. As for the Ti5Si3/Ti3Al composite from elemental powders, the composite powders can be obtained by milling the powder mixtures for 3 hours followed by annealing at 600℃ for 1 hr.
For the formation of Ti5Si3/TiAl from compound TiH2, Al3Ti and Si powders, the results show that with continued milling up to 20 hours, amorphization of the powders begins to appear. After milling for 30 hr, the amorphous phase Ti1-XAlXHY was formed. The Ti5Si3/TiAl composites were synthesized from the amorphous powders during the subsequent anneal treatment through a mechanism similar to the HDDR process. For the Ti5Si3/TiAl composite, the product evolution upon heating was first the formation of Ti solid solution followed by desorption of hydrogen from the solid solution. The second step is the formation of Ti5Si3 and the last step is the crystallization of Ti3Al phase.
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