| Summary: | 博士 === 國立成功大學 === 材料科學(工程)學系 === 86 === TiAl-based intermetallics have a high potential to become an
alternative material to superalloys for the applications of
aerospace engine and structural parts due to their low density,
excellent high temperature strength, and high temperature
oxidation resistance. Their working temperature can be up to 850
~ 900 *C, howe?"v/ their ductility at room temperature is very
poor, increasing dramatically only above 700 *C. Therefore, it
is rather difficult to shape intermetallics by the conventional
hot working process due to their poor workability. A powder
metallurgy (PM) process is considered as an effective approach
to shape TiAl-based intermetallics in spite of their poor
workability. Also, the mechanical properties of PM parts are
considered superior to those of castings.The PM process of TiAl-
based intermetallics can be divided into two categories
depending on the starting materials, which are the elemental
powder metallurgy (EPM) and the alloy powder metallurgy (APM).
Because elemental powders are cheaper and easier to be obtained
than those of alloy powders, it is then desirable to study the
EPM process in producing TiAl-based intermetallics parts.
However, the EPM process cannot be conducted by pressureless
sintering due to the Kirkendall pores formed during the
interdiffusion between Ti and Al making the sintered billets
loose. Usually, the EPM process is conducted by hot isostatic
pressing (HIP). In this study, the EPM process is employed
through the solid state hot pressing process and the constrained
sintering process to prepare the TiAl-based intermetallics. In
the solid state hot pressing process, the effects of the process
variables such as pressure, time and temperature on the
densities of the billets as well as the phase transformation
from elemental powders to TiAl-based intermetallics are
investigated. In the constrained sintering process, the effects
of the constrained die on the densification of the billets and
the phase transformations during the procedure are also
investigated. Afterward, the hot pressed billets are forged with
the conventional forging process as well as the isothermal
forging process to evaluate the two forging methods and the
relationship between the forging temperature and the deformation
resistance.The results show that the billets sintered in the
pressureless sintering process usually swell with surface cracks
and are porous due to the Kirkendall effect, and then are not
acceptable in the practical application. However, dense billets
can be prepared with the two-stage solid state hot pressing
process. Fully dense billets with the partially lamellar
structure can be obtained after sintering under the condition of
630 *C/45 MPa/5 hr. + 630 *C/17 hr. + 1250 *C/45 MPa/2 hr.. The
duplex structure is formed after 4 hours of further heat-
treatment at 1250 *C. As to the results of the constrained
sintering process, the billets of 92% relative density with the
duplex structure can be obtained by pressureless sintering
through two-step sintering procedure which includes a
constrained sintering at 645 *C for 15 hr., and pressureless
sintering at 1250 *C for 6 hr.. The results obtained in this
study show that the EPM process can be conducted through hot
pressing and pressureless sintering instead of HIP in preparing
the TiAl-based intermetallics. The isothermal forging process is
preferred than the conventional forging process in the
deformation of the TiAl-based intermetallics. The temperature of
1200 *C is suitable for the isothermal forging of TiAl-based
intermetallics because the strain hardening dose not occur when
forged at this temperature. When the isothermal forging is
conducted at 1200 *C, the billets of 98 % relative density are
fully densified to about 100% under the 85% reduction in height.
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