| Summary: | 博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 85 === At the beginning of this work, the relations between
microstructural changes occurring during superplastic
deformation and superplastic deformation behaviors of an
annealed Ti-6Al-6V-2Sn rolled plate with 2mm in thickness were
investigated. Although the initial microstructure of test
material is characterized by mixed long, narrow and equiaxed
alpha grains, modification of grain shape was happen to occur
during high temperature annealing or during superplastic
deformation. Under the effect of thermal and/or mechanical
strain energy, long and narrow alpha grains were activated to
undergo recovery and to separate into several equiaxed grains by
the precipitations of beta phase through alpha subgrain
boundaries. It resulted in the achievement of high superplastic
deformation capability of test material. Increasing the
concentration of hydrogen in titanium alloy by hydrogenation is
able to effectively increase the volume fraction of beta phase
under fixed temperature condition, but is not necessary to
increase the superplasticity of the alloy at the same time.The
hydrogenation behavior of Ti-6Al-6V-2Sn alloy with starting
microstructures of coarse equiaxed alpha grains and of coarse
Widmanstatten alpha grains were evaluated. The effects of
hydrogenation on alpha grain transformation can be concluded
into two different ways : (1) In the initial peroid of
hydrogenation, hydrides precipitated from the alpha grain
boundaries and then from the interior of alpha grains. The
hydrides were kept going on transformation into beta phase
during hydrogenation. That is, the increasing of hydrogen
content of alloys initiated the transformation of α(H)＋γ(H)→
β(H) to occur. (2) After the volume fractions of alpha and beta
phases achieved equilibrium with the hydrogenating atmosphere,
the transformation of α(H)＋γ(H) into β(H) no more occurred.
In the following peroid of hydrogenating, the alpha phase was
oversaturated with hydrogen, and it resulted in hydride
precipitations remained stable within the alpha grain. In
additions, hydrogenating specimens at different hydrogen supply
rate resulted in different microstructures obtained. Original
alpha grains were refined as small alpha particles in hydrogen-
stabilizing beta phase matrix for specimens being hydrogenated
at high hydrogen supply rate. Indistinguishable grain structure
was observed for specimens being hydrogenated at low hydrogen
supply rate.Dehydrogenation caused the escape of hydrogen from
the hydrogenated alloy and then the dissolution of hydrides. It
resulted in highly defective microstructure with high density of
dislocations and stacking faults. Dehydrogenation also leads to
the formation of new recrystallized alpha grains and the
precipitation of fine beta phase from the locations of retained
hydride plates. Both are the mechanisms that contribute to the
refinement of alpha grains.
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