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Abstract
In the last decade, new procedures of thermochemical processing
( TCP )have been devised, by which use of hydrogen as a
temporary alloying element inTi alloys could refine the
microstructure and improve the properties of the alloys
effectively. However, the usefulness of TCP for Ti-6Al-6V-2Sn
has not been discussed yet. The material of this work is
commercial Ti-6Al-6V-2Sn sheets.In the present study, the
influence of two step TCP ( hydrogenation →dehydrogenation ) on
microstructure and superplasticity of Ti-6Al-6V-2Sn is
investigated. The results show that the α phase has been
refined during hydrogenation. The α phase has been refined by
the βH phase at high temperature hydrogenation, and refined by
the precipitation of hydrides at low temperature hydrogenation.
After high temperature hydrogenation ( above 750℃) and air
cooling to room temperature, the βH phase has been changed to
martensite structure, whereas the precipitation of hydrides were
not produced in the βH phase at all of the hydrogenation
temperature. After hydrogenation above βH transus
temperatureand then dehydrogenation, the βH phase may produce a
refinement structure ofmartensite, but it may not produce the
same refinement structure for the βH phase at hydrogenation
below βH transus temperature and sequent dehydrogenation. In
this study, the optimal TCP treatment is that after
hydrogenation at 750℃ for 1 hour and air cooling to room
temperature, dehydrogenation at 725℃ for 12 hours in vacuum
and air cooling to room temperature. The superplastic forming
height of the optimal TCP treatment specimen at 900℃ is 5mm
higher than that of the as-recieved materials at same
temperature, and the optimal TCP treatment give an excellent
superplastic elongation as large as 1140% at 800℃, more than
1040% of the as-recieved materials.
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