Abstract: The mechanisms of creep and superplasticity occurring in conventional large-grained
materials are now understood reasonably well. However, very recent advances in the processing of
theoretically-dense metals with submicrometer grain sizes have provided the opportunity to extend
the understanding of flow behavior to include materials where the grains are exceptionally small.
Using processing through the application of severe plastic deformation, as in procedures such as
equal-channel angular pressing, it is now feasible to fabricate relatively large samples having
ultrafine grain sizes in the submicrometer or nanometer range. This paper examines these recent
advances and gives examples of the flow behavior in ultrafine-grained aluminum alloys.
Abstract: A TiB particulate-reinforced Ti-22Al-27Nb (mol%) alloy, based on the orthorhombic
intermetallic phase, was prepared using gas atomization powder metallurgy method. In the
as-atomized condition, extremely fine TiB particulates of less than 1-μm diameter and 5-μm length
were dispersed in the matrix. After annealing heat treatment (heat treated at 1423 K with subsequent
furnace cooling), this composite exhibited a lamellar matrix microstructure and showed better creep
properties than a composite produced using conventional ingot metallurgy method, with coarse TiB
particulates of 5-μm diameter and 40-μm length. Coarsening of the matrix microstructure and
growth of TiB particulates occurred after annealing heat treatment at higher temperature (ca. 1473
K). Creep-resistance improvement was also observed, which seemed to be mainly attribute to the
effect of the matrix microstructure. From measurements of stress components and activation energy,
all composites showed an identical creep mechanism: dislocation-controlled creep.
Abstract: It is known that Ti-6Al-4V alloy is one of the excellent candidates for aerospace
structure due to their high specific strength.However, its higher cost and low formability relative to
other materials tend to limit the wide usage of the material.The purpose of this study is to characterize
the superplasticity of this alloy so to obtain materials and process parameters for superplastic forming
and diffusion bonding for industrial application. High temperature tensile tests was carried out at the
strain rate range of 10-4 to 10-2 s-1 and temperature range of 1123°C to 1223°C. According to the results of
the experiment, the optimum diffusion bonding condition was obtained at 1148°C, applying pressure
of 4MPa for 1 hour in argon gas environment, which condition is more practical than expensive
vacuum condition. It is shown that at the optimum condition for diffusion bonding with parent metal,
the oxide film becomes unstable and the oxygen is diffused into the bulk. At this condition, the
mechanical and microstructural integrity at the bonding interface was observed in a sandwich
structure and a heavy block of titanium part from massive diffusion bonding process.
Abstract: Recent researches have shown the premature breakdown of creep rupture strength in
long term creep region of advanced high Cr ferritic steels. As safe operation of power plants
becomes a serious problem we should be able to detect and predict the breakdown transition of
creep rupture strength. Some methods for detecting the breakdown transition have been presented
till now like the measurement of reduction of area after creep rupture and particle size of laves
phase. However it will be more economic if we make use of non-destructive tests, for example,
hardness testing. In this paper 3 types of ferritic steels with different Cr concentration have been
studied. The results suggest that the hardness of aged structures is constant independently of
exposure time in short term region, whereas the hardness breaks down in long term region. The
boundary of breakdown in hardness coincides with that of breakdown in creep rupture strength.
Abstract: Influences of Ca addition on microstructures and mechanical properties at room and elevated
temperatures have been investigated for Mg-1.5%Nd-1.0%RE-0.5%Zn-(0~1.0)%Ca casting alloys, on
basis of experimental results from X-ray diffractometry (XRD), scanning electron microscopy (SEM)
with energy dispersive X-ray spectroscopy (EDS), tensile and creep tests. Microstructures of the alloys
are characterized by dendritic α-(Mg) grains surrounded by Mg12Nd-Zn-(Ca) eutectic network phase.
The average size of α grains decreases gradually with an increase in Ca content. At room temperature,
yield strength (YS) is enhanced with increasing Ca content with a decrease in ultimate tensile strength
(UTS) and elongation to fracture, whereas the Ca addition leads to greater YS and UTS at 175oC. The
tensile creep strain and secondary creep rate, measured at 150 and 200oC under 100MPa for 100hrs,
become lower with the increase in Ca content. The obtained tensile properties at elevated temperature
demonstrate that the addition of Ca plays a role in improving high temperature mechanical properties
including creep resistance for the Mg-Nd-RE-Zn-(Ca) alloys. In view of microstructural evolution, this
would be attributed to the refined primary α grains and higher thermal stability of the Mg12Nd-Zn-Ca
eutectic strengthening phase.
Abstract: The development of new creep resistant magnesium alloys has become a major issue in
recent years. The alloys investigated in the present work are based on the binary system Mg-Sn. Sn
as major alloying element was chosen due to its high solid solubility over a wide temperature range
and due to the possible formation of Mg2Sn intermetallic precipitates with a high melting
temperature of about 770°C. These characteristics suggest that a fairly large volume fraction of
thermally stable Mg2Sn particles can be formed during solidification. This makes it possible that the
Mg-Sn alloys can be developed as creep resistant magnesium alloys. In fact, previous investigations
indicate that the Mg-Sn alloys have a comparable or even better creep property than AE42 alloy.
The present work investigates the microstructure of Mg-Sn alloys with and without creep
deformation using SEM and TEM technique. The effects of microstructural inhomogeneity on the
creep response are presented. Based on the microstructural analysis, the mechanism responsible for
improving the creep resistance will be discussed. It is shown that the grain boundary sliding is a
dominant creep mechanism for the Mg-Sn binary alloy.
Abstract: For steady-state deformation caused by grain-boundary diffusion and viscous
grain-boundary sliding, the creep rate of regular polyhedral grains is analyzed by the
energy-balance method. For the microstructure, the grain-grain interaction increases the degree of
symmetry of diffusional field, resulting in a decrease of the effective diffusion distance. Meanwhile,
the viscous grain-boundary sliding is found to decrease the creep rate. The present analysis reveals
that the grain-size exponent is dependent on the grain size and the grain-boundary viscosity: the
exponent becomes unity for small grain sizes and/or high viscosity, while it is three for large grain
sizes and/or low viscosity.
Abstract: The effect of boron on microstructure evolution and creep deformation behavior has been
investigated for a tempered martensitic 9Cr-3W-3Co-0.2V-0.05Nb steel at 650oC. Creep tests were
carried out at 650oC for up to about 6 x 104 h. The addition of boron retards the onset of acceleration
creep at low stress and long time conditions, which results in lower minimum creep rate and longer
time to rupture. The addition of boron also retards the Ostwald ripening of M23C6 carbides near prior
austenite grain boundaries (PAGBs) during creep. The retardation of the onset of acceleration creep
results from the retardation of the recovery of martensitic microstructure near PAGBs by pinning
effects due to fine M23C6 carbides. The main effect due to boron is considered to occupy vacancies
near growing M23C6 carbides, which makes it difficult to accommodate local volume change around
the growing carbides. This reduces the rate of Ostwald ripening of M23C6 carbides.
Abstract: The effect of nanocrystalization on superplastic flow was examined in ZrO2-30vol%spinel
composite. The nanocrystalization can increase the strain rate by one order of magnitude or lower the
deforming temperature by about 100 K. Irrespective of the lowered flow stress, however, the tensile
elongation to failure of nanocrystalline composite is lower than that of submicrom-grain composite.
The limited tensile elongation in nanocrystalline composite can be ascribed mainly to accelerating
cavity damage accumulation.
Abstract: A unified interpretation of super plastic flow (SPF) and cosmic micromechanics in
spatially extended single and polycrystalline systems (SESPS) allows determined that the nature of
the hyperbolic granular flow in SESPS is assisted by the movement of dislocations as the pattern of
the inner dimension flow. Consequently in this work a mathematical model related with relativistic
cosmology and quantum mechanics is used in order to obtain the activation energy for super plastic
flow in SESPS. This correspondence law between SPF and cosmic micromechanics is important in
the light of recent cosmological theories of the existence of dark matter and dark energy in the
cosmic structure, because in this new interpretation of the universe the planets, stars, galaxies,
clusters of galaxies, etc., are considered as precipitates on dislocations in the cosmic structure,
which is formed in a nature way by the dark matter and dark energy, in a similar form of
precipitates on dislocations in a SESPS of metals. Physically in this context the expansion process
of the universe is highly dependent upon the volume fraction, size and distribution of precipitates on
dislocations in the cosmic structure. Therefore, in this work the main results obtained in cosmic
micromechanics and cosmic macromechanics are related with the Max Planck’s scale (MPE) and
Edwin Hubble’s scale (EHS) respectively.