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1.
The strong dependence of the superplastic behavior of metals and alloys on grain size has been demonstrated, and it is now
well known that a fine grain size is normally a requirement for superplasticity. However, the microstructure of certain alloy
systems such as Ti-6A1-4V cannot always be adequately characterized by a single parameter such as grain size. In two-phase
α β alloys such as Ti-6A1-4V, other microstructural parameters such as volume fractions of the two phases, grain aspect ratio,
grain size distribution and crystallographic texture may also influence superplasticity. For example, if “grain switching”
is an important deformation mechanism in superplastic flow as suggested by Ashby and Verall, then factors such as grain aspect
ratio and range of grain sizes would be expected to have an effect on superplastic behavior. In this study, these microstructural
features were determined for several different heats of Ti-6Al-4V, and the corresponding superplastic properties were evaluated
in terms of their fully characterized microstructure. The flow stress as a function of strain rate, strain rate sensitivity
exponent (m) as a function of strain rate and total elongation on properties were found to be strongly influenced by microstructural
parameters such as grain aspect ratios, grain size and grain size distribution. 相似文献
2.
The effect of alloying additions on the superplastic properties of Ti-6 pct Al-4 pct V 总被引:1,自引:0,他引:1
The superplastic deformation properties of Ti-6 pct Al-4 pct V and modified alloys containing 1/4 pct, 1/2 pct, 1 pct, and
2 pct of either cobalt or nickel have been investigated in the temperature range 950 to 750 °C. The results show that both
cobalt and nickel modified alloys have reduced flow stresses, in comparison with Ti-6 pct Al-4 pct V, the reductions being
particularly marked at the lower temperatures and lower strain rates. The results are shown to be consistent with an isostress
model for the deformation of (α + β) two-phase alloys in which the varying β volume fractions and differing diffusivities
of titanium, cobalt, or nickel in the β phase are taken into account. 相似文献
3.
Y. G. Ko C. S. Lee D. H. Shin S. L. Semiatin 《Metallurgical and Materials Transactions A》2006,37(2):381-391
The low-temperature superplasticity of ultra-fine-grained (UFG) Ti-6Al-4V was established as a function of temperature and
strain rate. The equiaxed-alpha grain size of the starting material was reduced from 11 to 0.3 μm (without a change in volume
fraction) by imposing an effective strain of ∼4 via isothermal, equal-channel angular pressing (ECAP) at 873 K. The ultrafine microstructure so produced was relatively stable
during annealing at temperatures up to 873 K. Uniaxial tension and load-relaxation tests were conducted for both the starting
(coarse-grained (CG)) and UFG materials at temperatures of 873 to 973 K and strain rates of 5 × 10−5 to 10−2 s−1. The tension tests revealed that the UFG structure exhibited considerably higher elongations compared to those of the CG
specimens at the same temperature and strain rate. A total elongation of 474 pct was obtained for the UFG alloy at 973 K and
10−4 s−1. This fact strongly indicated that low-temperature superplasticity could be achieved using an UFG structure through an enhancement
of grain-boundary sliding in addition to strain hardening. The deformation mechanisms underlying the low-temperature superplasticity
of UFG Ti-6Al-4V were also elucidated by the load-relaxation tests and accompanying interpretation based on inelastic deformation
theory. 相似文献
4.
The addition of vanadium to polycrystalline, single-phase γ-TiAl significantly enhances stiffness from room temperature (RT)
to 1173 K. The maximum yield stress in Ti-55A1-10V alloy is centered near 1073 K, in contrast with those of single-crystal
Ti-54A1 at 873 and 1073 K, depending upon deformation orientation. The effect of strain rate on yield stress is present over
the entire temperature range, but it is more prominent above 873 K, while the effect becomes significantly small between 673
and 873 K, similar to that observed in Ll2 type Ni3Al. The microstructure of deformed Ti-55A1-10V and Ti-49.5A1-10V alloys evolved by slip, twinning, and formation of stacking
faults. Twinning is a major deformation mode in Ti-49.5-10V alloy and a minor mode in Ti-55A1-10V alloy. It appears that the
amount of twins for a given deformation decreases with increasing Al content. The twin structures in both alloys were found
to be true twins of {111}〈112]. No pseudotwinning was observed in the alloys deformed in the temperature range of RT to 1273
K. The stacking faults found in the deformed alloys had an extrinsic character. No intrinsic type of stacking faults were
found in these alloys. In addition to the cross slip of superdislocations, the anomalous hardening up to 1073 K appears attributable
to an ordinary superdislocation pinning structure, dislocation loops, and nonglissile “square shaped dislocations,” which
are products of super-superdislocation reaction. These nonglissile or pinning structures are unwinded in the deformed Ti-55A1-10V
alloy at 1273 K, consistent with the softening behavior at this temperature. 相似文献
5.
R. K. Rayudu S. S. Bhattacharya V. P. Deepkumar 《Transactions of the Indian Institute of Metals》2011,64(1-2):21-25
Superplasticity is the ability of a polycrystalline material to exhibit, in a relatively isotropic manner, large elongations when deformed in tension. This property is exploited during superplastic forming in the fabrication of complex shaped components which are otherwise technically difficult or economically costly to form by conventional methods. The ability of some titanium alloys to undergo superplastic deformation coupled with their diffusion bonding capability (SPF/DB) provides excellent opportunities to fabricate intricate parts in a single operation resulting in significant cost and weight savings, particularly in the manufacture of aerospace structures. In the present work, experimental studies to characterize the superplastic behaviour of an as-received titanium Ti-6Al-4V alloy sheet commonly used in aerospace structural applications are reported. Tensile test coupons prepared from the alloy sheet were subjected to high temperature tensile tests in the temperature range of 1123 K (850°C) to 1223 K (950°C) and strain rate range of 10?4 s?1 to 10?2 s?1 in order to characterize the superplastic deformation behaviour. Suitable dies, for superplastic forming of 80 mm × 80 mm square components to depths of 43 and 50 mm, were designed and fabricated. Components were superplastically formed at a temperature of 1200 K (927°C) and 0.7 MPa constant argon pressure. The components were characterized for their thickness distribution, mechanical and metallurgical properties and the results are presented. 相似文献
6.
It was the purpose of this study to identify what metallurgical processes could be applied to commercial structural titanium alloys to increase the hot salt stress corrosion (HSSC) threshold stress and therefore increase their range of application. Toward this purpose Ti-6A1 and Ti-6A1-4V were evaluated as a function of microstructural variables. Specifically, it was shown that both increasing amounts of cold work and increasing grain size decrease HSSC resistance of Ti-6A1. Also, for Ti-6A1-4V it was shown that preferred orientation can have a profound effect on the HSSC resistance. Crack initiation time, crack growth rate, and stress rupture life were evaluated in Ti-6A1-4V as a function of applied stress at 727 K. These results indicate that HSSC cracking can be described by a critical resolved shear stress criterion, and that increased high temperature creep resistance and decreased room temperature notch rupture strength combine to increase HSSC susceptibility and embrittlement. 相似文献
7.
It was the purpose of this study to identify what metallurgical processes could be applied to commercial structural titanium
alloys to increase the hot salt stress corrosion (HSSC) threshold stress and therefore increase their range of application.
Toward this purpose Ti-6A1 and Ti-6A1-4V were evaluated as a function of microstructural variables. Specifically, it was shown
that both increasing amounts of cold work and increasing grain size decrease HSSC resistance of Ti-6A1. Also, for Ti-6A1-4V
it was shown that preferred orientation can have a profound effect on the HSSC resistance. Crack initiation time, crack growth
rate, and stress rupture life were evaluated in Ti-6A1-4V as a function of applied stress at 727 K. These results indicate
that HSSC cracking can be described by a critical resolved shear stress criterion, and that increased high temperature creep
resistance and decreased room temperature notch rupture strength combine to increase HSSC susceptibility and embrittlement. 相似文献
8.
G. M. Ludtka R. E. Oakes Gail Mackiewicz-Ludtka R. L. Bridges J. L. Griffith 《Metallurgical and Materials Transactions A》1993,24(2):379-388
A uranium-6 niobium alloy was shown to exhibit superplasticity. A thermomechanical processing (TMP) sequence was used to develop
the ultrafine grain size essential for superplastic behavior. Strain-rate sensitivity, maximum elongation, and flow curve
data indicated that this alloy is superplastic above the monotectoid temperature (647 °C) in the equilibrium γ1, single-phase, temperature regime. The existence of superplasticity in the single-phase temperature regime was explained
by the presence of metastable γ2 at these higher temperatures. Sluggish niobium diffusion and the resultant slow dissolution kinetics were shown to be responsible
for this anomalous “single-phase” superplastic behavior. An engineering elongation of 658 pct was obtained at 685 °C for a
constant true strain rate of 2.5 × 10-4 s-1 which required an initial flow stress of only 2.8 MPa. A grain growth kinetic study, along with flow curve information, has
also shown that superplastic forming (SPF) must be completed within 2 hours at 670 °C to obtain maximum ductility with the
lowest forming pressure. 相似文献
9.
Jeoung Han Kim S. L. Semiatin You Hwan Lee Chong Soo Lee 《Metallurgical and Materials Transactions A》2011,42(7):1805-1814
The flow behavior of the α and β phases in Ti-6Al-4V was interpreted in the context of a self-consistent modeling formalism. For this purpose, high-temperature
compression tests were conducted at various temperatures for a single-phase α alloy (Ti-7Al-1.5V), a variety of near-β alloys, and the two-phase alloy Ti-6Al-4V, each with an equiaxed microstructure. The flow behavior of the α phase in Ti-6Al-4V was deduced from the experimental results of the single-phase α alloy. The flow behavior of the β phase, which was predicted by using the self-consistent approach and the measured flow behaviors of Ti-6Al-4V and Ti-7Al-1.5V,
showed good agreement with direct measurements of the various near-β alloys. From these results, it was shown that the strength of the α phase is approximately three times higher than that of the β phase at temperatures between 1088 K and 1223 K (815 °C and 950 °C). It was also concluded that the relative strain rates
in the two phases varies significantly with temperature. The usefulness of the approach was confirmed by comparing the predicted
and measured flow stresses for other Ti-6Al-4V and near-α alloys. 相似文献
10.
Megan Frary Christopher Schuh David C. Dunand 《Metallurgical and Materials Transactions A》2002,33(6):1669-1680
By thermally cycling through their transformation temperature range, coarse-grained polymorphic materials can be deformed
superplastically, owing to the emergence of transformation mismatch plasticity (or transformation superplasticity) as a deformation
mechanism. This mechanism is presently investigated under biaxial stress conditions during thermal cycling of unalloyed titanium,
Ti-6Al-4V, and their composites (Ti/10 vol. pct TiC
p
, Ti-6Al-4V/10 vol. pct TiC
p
, and Ti-6Al-4V/5 vol. pct TiB
w
). During gas-pressure dome bulging experiments, the dome height was measured as a function of forming time. Adapting existing
models of biaxial doming to the case of transformation superplasticity where the strain-rate sensitivity is unity, we verify
the operation of this deformation mechanism in all experimental materials and compare the biaxial results directly to new
uniaxial thermal cycling results on the same materials. Finally, existing thickness distribution models are compared with
experimentally measured profiles. 相似文献
11.
Transformation Superplasticity of Cast Titanium and Ti-6Al-4V 总被引:1,自引:0,他引:1
Qizhen Li Edward Y. Chen Douglas R. Bice David C. Dunand 《Metallurgical and Materials Transactions A》2007,38(1):44-53
Samples of unalloyed titanium and Ti-6Al-4V with a cast, coarse-grain structure were subjected to simultaneous mechanical
loading and thermal cycling about their transformation range to assess their capability for transformation superplasticity.
Under uniaxial tensile loading, high elongations to failure (511 pct for titanium, and 265 pct for Ti-6Al-4V) and an average
strain-rate sensitivity exponent of unity are observed. Samples previously deformed superplastically to a strain of 100 pct
show no significant degradation in room-temperature mechanical properties as compared to the undeformed state. Biaxial dome
bulging tests confirm that transformation superplasticity is activated under thermal cycling and faster than creep deformation.
The cast, coarse-grained titanium and Ti-6Al-4V have similar transformation-superplasticity characteristics as wrought or
powder-metallurgy materials with finer grains. This may enable superplastic forming of titanium objects directly after the
casting step, thus bypassing the complicated and costly thermomechanical processing steps needed to achieve fine-grain superplasticity.
相似文献
David C. Dunand (Professor)Email: |
12.
Yong Namkwon Hyang Jin Koh Sunghak Lee Nack J. Kim Young Won Chang 《Metallurgical and Materials Transactions A》2001,32(7):1649-1658
This study is concerned with the effects of microstructural modification on superplastic deformation characteristics of a
rapidly solidified (RS) Al-3Li-1Cu-0.5Mg-0.5Zr (wt pct) alloy. This Al-Li alloy has a very fine grain structure desirable
for improved superplasticity. The results of superplastic deformation indicated that the alloy exhibited a high superplastic
ductility, e.g., elongation of approximately 800 pct, when deformed at temperatures above 500 °C and at the strain rates of 10−2/s to 10−1/s. Such a high strain rate is quite advantageous for the practical superplastic forming application of the alloy. Stress-strain
rate curves were obtained by performing a series of load relaxation tests in the temperature range from 460 °C to 520 °C in
order to examine the superplastic deformation behavior and to establish its mechanisms. The stress-strain rate curves could
be separated into two parts according to their respective physical mechanisms, i.e., grain matrix deformation and grain boundary sliding, as was proposed in a new superplasticity theory based on internal deformation
variables. The microstructural evolution during superplastic deformation was also analyzed by using transmission electron
microscopy. During superplastic deformation, grains were kept fine and changed into equiaxed ones due to the presence of fine
secondary phase particles and the continuous recrystallization due to the development of subgrains. Consequently, the rapidly
solidified (RS) alloy showed much improved superplasticity compared to the conventional ingot cast 8090 alloy. 相似文献
13.
Implantation of HA into Superplastic Ti-6Al-4V: Kinetics and Mechanical Behaviors of Implanted Layer
Sanaz Yazdan Parast Iswadi Jauhari Mohsen Asle Zaeem 《Metallurgical and Materials Transactions A》2011,42(1):219-226
An implanted layer is produced by implantation of hydroxyapatite (HA) into superplastic Ti-6Al-4V. X-ray diffraction (XRD)
analysis indicates that the surface of the implanted layer is composed of HA and Ti-6Al-4V, and line-scanning analysis confirms
a mutual elemental diffusion of HA and Ti-6Al-4V. According to the scanning electron microscope (SEM) images, by increasing
the implantation temperature, the thickness of the implanted layer increases. The bonding strength between implanted layer
and titanium substrate is examined by conducting a friction wear test. Higher surface removal of an implanted layer is observed
when as-received Ti-6A1-4V was used in the implantation process, which is an indication of higher bonding strength between
implanted layer and superplastic Ti-6A1-4V. The effect of implanted layer thickness on the wear resistance is also investigated.
The reduction in thickness of the implanted layer is more evident in thicker implanted layers. The results suggest that the
adhesion between the implanted layer and titanium substrate is stronger than the cohesion within the implanted layer. 相似文献
14.
Forming of automotive leightweight parts using aluminium offers numerous advantages. Compared to other wrought aluminium alloys, in particular AlMg‐alloys generally show a good formability which is favourable for the production of complex parts. However, forming of Mg‐containing alloys at room temperature leads to yielding patterns preventing their implementation for class‐A‐surface applications. Furthermore, the formability of steel still exceeds that of AlMg‐alloys at room temperature. Thus, in the present study, sheet metal forming is applied at a temperature range that is typical for warm forming. It is supposed to profit from the advantages of warm forming like high achievable strains and improved surface quality of the formed part, while not having the disadvantages of long production times and high energy consumption, which is correlated with superplastic forming. Applying fluid‐based sheet metal forming in this paper, nitrogen is used as fluid working medium to satisfy the demand on high temperature resistance. Concerning the blank material used, formability of Mg‐containing aluminium alloys shows strong strain rate sensitivity at elevated temperatures. To figure out the optimal strain rates for this particular process, a control system for forming processes is developed within the scope of this paper. Additionally, FE‐simulations are carried out and adapted to the experiment, based on the generated process data. FE‐investigations include forming of domes (bulging) as well as shape‐defined forming, having the objective to increase formability in critical form elements by applying optimal strain rates. Here, a closed‐loop process control for gas‐pressure forming at elevated temperatures is to be developed in the next stages of the project. 相似文献
15.
J. R. Kennedy P. N. Adler H. Margolin 《Metallurgical and Materials Transactions A》1993,24(1):2763-2771
The effect of alloy composition on hydrogen activity was measured for seven titanium alloys as a means to determine the tendency
for hydrogen migration within dissimilar metal welds. The alloys were: Ti-CP, Ti-3A1-2.5V, Ti-3Al-2.5V-3Zr, Ti-3Al-2Nb-lTa,
Ti-6A1, Ti-6A1-4V, and Ti-6Al-2Nb-lTa-0.8Mo. Hydrogen pressure—hydrogen concentration relationships were determined for temperatures
from 600 °C to 800 °C and hydrogen concentrations up to approximately 3.5 at. pct (750 wppm). Fusion welds were made between
Ti-CP and Ti-CP and between Ti-CP and Ti-6A1-4V to observe directly the hydrogen redistribution in similar and dissimilar
metal couples. Hydrogen activity was found to be significantly affected by alloying elements, particularly Al in solid solution.
At a constant Al content and temperature, an increase in the volume fraction of β reduced the activity of hydrogen in α-β
alloys. Activity was also found to be strongly affected by temperature. The effect of temperature differences on hydrogen
activity was much greater than the effects resulting from alloy composition differences at a given temperature. Thus, hydrogen
redistribution should be expected within similar metal couples subjected to extreme temperature gradients, such as those peculiar
to fusion welding. Significant hydrogen redistribution in dissimilar alloy weldments also can be expected for many of the
compositions in this study. Hydride formation stemming from these driving forces was observed in the dissimilar couple fusion
welds. In addition, a basis for estimating hydrogen migration in titanium welds, based on hydrogen activity data, is described. 相似文献
16.
J. R. Kennedy P. N. Adler H. Margolin 《Metallurgical and Materials Transactions A》1993,24(12):2763-2771
The effect of alloy composition on hydrogen activity was measured for seven titanium alloys as a means to determine the tendency
for hydrogen migration within dissimilar metal welds. The alloys were: Ti-CP, Ti-3A1-2.5V, Ti-3Al-2.5V-3Zr, Ti-3Al-2Nb-lTa,
Ti-6A1, Ti-6A1-4V, and Ti-6Al-2Nb-lTa-0.8Mo. Hydrogen pressure-hydrogen concentration relationships were determined for temperatures
from 600 ‡C to 800 ‡C and hydrogen concentrations up to approximately 3.5 at. pct (750 wppm). Fusion welds were made between
Ti-CP and Ti-CP and between Ti-CP and Ti-6A1-4V to observe directly the hydrogen redistribution in similar and dissimilar
metal couples. Hydrogen activity was found to be significantly affected by alloying elements, particularly Al in solid solution.
At a constant Al content and temperature, an increase in the volume fraction ofΒ reduced the activity of hydrogen in α-@#@ Β alloys. Activity was also found to be strongly affected by temperature. The effect of temperature differences on hydrogen
activity was much greater than the effects resulting from alloy composition differences at a given temperature. Thus, hydrogen
redistribution should be expected within similar metal couples subjected to extreme temperature gradients, such as those peculiar
to fusion welding. Significant hydrogen redistribution in dissimilar alloy weldments also can be expected for many of the
compositions in this study. Hydride formation stemming from these driving forces was observed in the dissimilar couple fusion
welds. In addition, a basis for estimating hydrogen migration in titanium welds, based on hydrogen activity data, is described. 相似文献
17.
Structural superplasticity in two phase alloys of the copper-nickel-zinc system (nominal composition in wt. pct Cu-15Ni-38
Zn-0.2 Mn) occurs over a wide range of strain rates in the temperature range 850 to 1050∮F (454 to 565°). The upper temperature
limit for super-plastic behavior in this system is determined by the reversion of the fine-grained two-phase structure to
a single phase structure in which extensive grain growth is possible. Residual room temperature tensile properties and microstructure
of the microduplex alloy after superplastic straining have been studied as a function of test temperature and total super-plastic
strain. At test temperatures sufficiently removed from the phase transformation temperature, the high tensile properties and
fine microstructure of the starting material are essentially retained after superplastic strains approaching 200 pct. In the
immediate vicinity of the phase transformation temperature, rapid degradation of the microduplex structure occurs during superplastic
deformation with a consequent severe degradation of the residual room temperature tensile properties.
Formerly with The International Nickel Company, is now with Gulf Energy and Environmental Systems, Materials Science Department,
P. O. Box 608, San Diego, Calif. 92112. 相似文献
18.
19.
The effect of deformation-induced transformation on the fracture toughness of commercial titanium alloys 总被引:2,自引:0,他引:2
M. Niinomi T. Kobayashi I. Inagaki A. W. Thompson 《Metallurgical and Materials Transactions A》1990,21(6):1733-1744
The effect of deformation-induced transformation of metastableβ phase on the ductility and toughness of four commercial titanium alloys was investigated. Tensile tests, Charpy impact tests,
and both static and dynamic fracture toughness tests were carried out at temperatures between 77 and 473 K on four titanium
alloys containing metastableβ phase. Deformation-inducedα″ (orthorhombic martensite) was observed in an (α + β)-type Ti-6Al-2Sn-4Zr-6Mo alloy. The dynamic fracture toughness of this
alloy increased considerably at 223 K compared to those at other temperatures. In another (α + β)-type Ti-6A1-4V alloy, the
static fracture toughness at 123 K and the dynamic fracture toughness at 223 K were increased considerably by the presence
of deformation-induced martensite compared to those at other temperatures. The strength increased as the temperature decreased
in this alloy. An abnormal elongation of aβ-type alloy, Ti-15V-3Al-3Sn-3Cr, at 123 K was attributed to the mechanical twinning of theβ phase. However, the effect of deformation-induced transformation on the fracture toughness of Ti-3Al-8V-6Cr-4Mo-4Zr alloy
was not observed.
Formerly Visiting Associate Professor, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University,
Pittsburgh, PA.
Formerly with the Department of Production Systems Engineering, Toyohashi University of Technology. 相似文献
20.
I. A. Akmoulin M. Niinomi T. Kobayashi 《Metallurgical and Materials Transactions A》1994,25(8):1655-1666
The effect of stability of the body-centered cubic (bcc) β phase on the dynamic fracture behavior of Ti-6Al-4V alloy at room
temperature and 77 K has been studied. The presence of a highly unstable β phase in the quenched alloy leads to a decrease
in both the dynamic fracture toughness and the crack propagation energy, and this decrease bccomes more pronounced when test
temperature is reduced to 77 K. Somewhat improved fracture characteristics were obtained by applying anneal procedure to receive
a fully stable β phase. The highest fracture toughness as well as the greatest crack propagation resistance were observed
in the air-cooled grade, where the lattice parameter of the bcc phase was intermediate between those pertaining to quenched
and annealed Ti-6Al-4V alloys. The effect is attributed to the vanadium content in the β phase, which is sufficiently high
to suppress deformation-induced transformation. On the other hand, the V content should be low enough to retard ductile-brittle
transition, typical for the bcc metals at cryogenic temperatures. As a result, marked toughening can be achieved, so that
the lowest application temperature of high-strength titanium alloys containing the bcc phase can be decreased significantly.
Formerly Assistant Professor, Department of Production Systems Engineering, Toyohashi University of Technology 相似文献