| Summary: | 博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 86 === Abstract
At the beginning of this work, the influence of hydrogenation on the
microstructural evolution process of the Ti-6Al-4V alloy at elevated
temperatures was investigated. The results indicate that two different
procedures involving hydrogen can be used to modify microstructures. [1]
Sluggish changes in the hydrogenated microstructure and gradual
modification in the alpha/beta volume ratio occurred at a low hydrogenation
temperature (600℃). However, [2] at elevated hydrogenation temperatures
(700, 800℃), an abrupt change in the microstructure, that is, from an alpha
matrix plus a small amount of beta phases to a beta matrix plus
a small amount of
alpha phases, and a new alpha/beta volume ratio are achieved
throughout a fast re-
equilibrium process. Furthermore, models are constructed to interpret the
microstructural evolution process at various hydrogenation temperatures; a
hydride''s transformation is also illustrated through a decelerated
microstructural evolution process at 600℃.
Secondly, thermochemical processing (TCP) with hydrogen acting as a
temporary alloying element to refine the microstructure of a Ti-6Al-4V alloy
was evaluated in order to examine the microstructural variations. It included
a hydrogenation treatment, an elevated temperature beta solution treatment, a
moderate temperature eutectoid decomposition, and finally a vacuum
dehydrogenation treatment. After the hydrogenation treatment, an
atmosphere containing hydrogen was introduced to stop the hydrogen from
escaping from the hydrogenated specimens in the subsequent solution
treatment and eutectoid decomposition treatment, which is different from
conventional thermochemical processing. The results have shown that gama
titanium hydride precipitated in the alpha phase in the hydrogenated specimen.
Also, during the eutectoid decomposition treatment, a mixture consisting of
alpha + gama hydride nucleated from the beta matrix while being
continuously cooled
to the decomposition temperature. The mixture continued to nucleate in the
same way during the subsequent isothermal aging treatment. Finally, during
the dehydrogenation treatment, the gama hydrides gradually lost their hydrogen
content, resulting in the recrystallization of fine alpha/beta
phases. In addition, the
microstructure resulting from each step of TCP was carefully examined, and
particular attention was paid to the control of eutectoid decomposition
conditions to shorten the time required to achieve a fully transformed
microstructure. In addition, the optimal conditions pertaining to eutectoid
decomposition treatment are proposed in this study.
Finally, in order to evaluate the hydrogen effect on the mechanical and
superplastic property of hydrogenated Ti-6Al-4V alloy, tensile tests were
conducted at room temperature and at elevated temperatures, separately.
The results indicate that hydrogen embrittlement occurred at room
temperature, and resulted in the decreasing of strength and ductility. For
high temperature superplastic tensile test, the results indicate that two
opposite ways involving hydrogen can be used to affect the superplastic
property. [1] Hydrogen strengthened the beta phases and resulted in the
increasing of flow stress during superplastic forming. [2] Hydrogen
stabilized the beta phases and lowered the beta transus temperature, and thus,
resulting in the decreasing of flow stress. Therefore, according to the
adopted forming temperature, an appropriate controlling of the hydrogen
concentration in the matrix is necessary.
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