共查询到20条相似文献,搜索用时 15 毫秒
1.
R. M. Imayev V. M. Imayev M. Oehring F. Appel 《Metallurgical and Materials Transactions A》2005,36(13):859-867
The hot-working behavior of γ(TiAl)-based alloys was investigated in order to understand fundamental aspects of the evolution
of the microstructure and to establish guidelines for advanced alloy design and processing. The investigations involved a
wide range of Al compositions and are based on metallographic investigations of the deformed samples. Particular emphasis
was placed on the effects of phase composition and casting texture. It was found that the behavior of dynamic recrystallization
was significantly influenced by the Al content of the alloys. Under the same deformation conditions. dynamic recrystallization
was fastest for alloys with nearly stoichiometric composition, whereas the recrystallization kinetics decreased for lower
or higher Al contents. This result can be attributed to the effect of the Al concentration on the micromechanisms of deformation
and diffusion as well as on the initial cast microstructure, which changed from fully lamellar to equiaxed near-γ microstructures
by raising the Al content from 45 to 50 at. pct. Further, it was observed that the casting texture, i.e., the orientation of lamellae with respect to the deformation axis, significantly influenced the recrystallization behavior.
In this respect, the development of shear bands due to kinking and bending of lamellae is concluded to play an important role
in the recrystallization behavior and seems in general, to be a particular feature of the microstructural evolution of lamellar
alloys on hot working.
R.M. IMAYEV and V.M. IMAYEV, Senior Scientists, formerly with the GKSS Research Centre, Institute for Materials Research 相似文献
2.
H. -G. Brokmeier M. Oehring U. Lorenz F. Appel H. Clemens 《Metallurgical and Materials Transactions A》2004,35(11):3563-3579
The evolution of preferred orientations during processing appears to be of significant importance for the use of γ-titanium aluminide alloys, since the desired lamellar microstructures exhibit a strong anisotropy of mechanical properties.
In this work, texture development has been investigated after hot extrusion and sheet rolling, which are considered to be
technologically relevant wrought processes. As texture evolution certainly is dependent on several factors, involving deformation
properties, recrystallization kinetics, and, particularly, the phase constitution at hot-working temperature, different processing
conditions and alloy compositions were investigated. By comparing the results, it is indicated that the determined textures
can be understood by the deformation modes of the dominating phase at hot-working temperature and the subsequent phase transformations.
However, the current understanding of texture evolution is far from being complete, as no model can be presented which quantitatively
accounts for the contribution of the different processes mentioned. 相似文献
3.
4.
Hot deformation studies using torsion testing were conducted on high purity Al and Al-4 at. pct Mg alloy systems in the strain
rate range of 0.1 to 1.0 s−1 and temperatures up to 810 K (1000‡F). At all test temperatures, the flow stress of the Al-Mg alloy was higher than that
of pure AL The strengthening in hot working (above 522 K (480°F)) is suggested to be due to a higher equilibrium subgrain
forest dislocation density. Special quenching procedures were required to show this correlation. Conventional quenching fails
to show this because structural details are lost when quenching from high temperatures.
Formerly with Olin Metals Research Laboratories. 相似文献
5.
Jun-Hyun Han Kyu Hwana Oh Jae-Chul Lee 《Metallurgical and Materials Transactions A》2003,34(8):1675-1681
The effect of the initial textures prior to dissimilar channel angular pressing (DCAP) on the texture evolution of the 1050
Al alloy sheets, processed by the continuous confined strip shearing (C2S2) process, were studied. The four different specimens,
i.e., cold rolled, heat treated, warm rolled, and as-cast 1050 Al alloy sheets, having various initial textures were prepared
using different thermomechanical routes. Although the major texture types were significantly affected by the initial textures
prior to DCAP, DCAP always promoted both the 〈111〉//normal direction (ND) textures and the {001}〈110〉 rotated cube texture
regardless of the initial texture status. Effects of the texture evolutions due to equal channel angular pressing (ECAP) on
deep drawbility and planar anisotropy were analyzed based on the -r value and the Δr value determined from the measured pole figures. A feasibility for producing the 1050 Al alloy sheets having high deep drawbility
and low planar anisotropy was demonstrated. 相似文献
6.
T. H. Courtney 《Metallurgical and Materials Transactions A》1977,8(5):685-689
During liquid phase sintering, microstructural coarsening takes place. One mechanism by which this occurs is Ostwald ripening.
Alternatively, particle coalescence also leads to a concomitant reduction in the solid particle surface area per unit volume.
In isolated structures in which particle-particle contacts are made, the rate of coarsening by coalescence is limited by the
time between particle contacts, for this is long compared to the time to fuse two particles together. In skeletal structures
the “coalescence time” limits coarsening by coalescence since this is long in comparison to the time between contacts. Expressions
for the rate of particle coarsening are developed for the different mechanisms and different particle morphologies. The results
of these calculations are combined with the microstructure maps developed in Part I of this paper to refine these maps so
that they predict both the morphology developed and the dominant mechanism of coarsening in liquid phase sintered systems. 相似文献
7.
《钢铁冶炼》2013,40(3):230-236
AbstractThe hot direct rolling (HDR) of thin slabs introduces some new microstructural phenomena with respect to conventional hot rolling of steels. This paper aims to investigate the microstructural changes of as cast austenite under these conditions. Current laboratory techniques for HDR simulation require a freshly cast slab for every experiment and a perfect link between casting and hot deformation. The present work adopted a new approach; the C–Mn steel is substituted by austenitic Fe–30Ni alloy, Conventional reheating before rolling replaces the direct link. The experimental ingot casting of Fe–30Ni alloy resulted in a solidification structure in good agreement with that of thin slabs of C–Mn steels. From metallographic observations, a mixed softening process and a strong grain refinement and homogenisation characterise the microstructural changes during HDR simulation. The microstructural behaviour and the grain refinement measured for the Fe–30Ni alloy is closely comparable with that predicted for C–Mn steels for the same conditions. The steel substitution appears to constitute a suitable and advantageous experimental approach for HDR simulation. 相似文献
8.
Plastic-flow and microstructure evolution during hot deformation of a gamma titanium aluminide alloy 总被引:3,自引:0,他引:3
The hot workability of a near gamma titanium aluminide alloy, Ti-49.5Al-2.5Nb-1.1Mn, was assessed in both the cast and the
wrought conditions through a series of tension tests conducted over a wide range of strain rates (10−4 to 100 s−1) and temperatures (850 °C to 1377 °C). Tensile flow curves for both materials exhibited sharp peaks at low strain levels
followed by pronounced necking and flow localization at high strain levels. A phenomenological analysis of the strain rate
and temperature dependence of the peak stress data yielded an average value of the strain rate sensitivity equal to 0.21 and
an apparent activation energy of ∼411 kJ/mol. At low strain rates, the tensile ductility displayed a maximum at ∼ 1050 °C
to 1150 °C, whereas at high strain rates, a sharp transition from a brittle behavior at low temperatures to a ductile behavior
at high temperatures was noticed. Dynamic recrystallization of the gamma phase was the major softening mechanism controlling
the growth and coalescence of cavities and wedge cracks in specimens deformed at strain rates of 10−4 to 10−2 s−1 and temperatures varying from 950 °C to 1250 °C. The dynamically recrystallized grain size followed a power-law relationship
with the Zener-Hollomon parameter. Deformation at temperatures higher than 1270 °C led to the formation of randomly oriented
alpha laths within the gamma grains at low strain levels followed by their reorientation and evolution into fibrous structures
containing γ + α phases, resulting in excellent ductility even at high strain rates. 相似文献
9.
Baolute Ren Ph.D. Candidate James G. Morris 《Metallurgical and Materials Transactions A》1995,26(1):31-40
The evolution of microstructure and texture of commercial purity Al during hot and cold rolling has been studied. The results
show that the dynamic restoration mechanism for Al rolled to a total equivalent strain of 2.66 at a mean equivalent strain
rate of 14.4 s-1 at 510 °C is essentially dynamic recrystallization (DRX), whereas for those materials deformed to lower strains at lower
strain rates at this temperature, the restoration mechanism is mainly dynamic recovery (DRV). This is confirmed by examining
the microstructures, textures, and substructures of the material developed during hot rolling as well as by comparing the
results with those developed during cold rolling and annealing. The texture analysis using orientation distribution functions
(ODFs) indicates that the dynamically recrystallized material has a random orientation distribution, whereas dynamically recovered
materials have a developed deformation texture with a predominantD component and a Cu component. The substructure observation by transmission electron microscopy (TEM) indicates that the subgrains
in the dynamically recrystallized material are completely dynamically recovered, which is very similar to those subgrains
in cold-rolled material. However, the annealed material shows a completely different substructure. By studying all of these
structural features, which are closely associated with the dynamic restoration mechanism, it is confirmed that Al undergoes
DRX in the present work, which is different from either DRV or static recrystallization (SRX). 相似文献
10.
M. A. Wells I. V. Samarasekera J. K. Brimacombe E. B. Hawbolt D. J. Lloyd 《Metallurgical and Materials Transactions B》1998,29(3):611-620
A comprehensive mathematical model of the hot tandem rolling process for aluminum alloys has been developed. Reflecting the
complex thermomechanical and microstructural changes effected in the alloys during rolling, the model incorporated heat flow,
plastic deformation, kinetics of static recrystallization, final recrystallized grain size, and texture evolution. The results
of this microstructural engineering study, combining computer modeling, laboratory tests, and industrial measurements, are
presented in three parts. In this Part I, laboratory measurements of static recrystallization kinetics and final recrystallized
grain size are described for AA5182 and AA5052 aluminum alloys and expressed quantitatively by semiempirical equations. In
Part II, laboratory measurements of the texture evolution during static recrystallization are described for each of the alloys
and expressed mathematically using a modified form of the Avrami equation. Finally, Part III of this article describes the
development of an overall mathematical model for an industrial aluminum hot tandem rolling process which incorporates the
microstructure and texture equations developed and the model validation using industrial data. The laboratory measurements
for the microstructural evolution were carried out using industrially rolled material and a state-of-the-art plane strain
compression tester at Alcan International. Each sample was given a single deformation and heat treated in a salt bath at 400
°C for various lengths of time to effect different levels of recrystallization in the samples. The range of hot-working conditions
used for the laboratory study was chosen to represent conditions typically seen in industrial aluminum hot tandem rolling
processes, i.e., deformation temperatures of 350 °C to 500 °C, strain rates of 0.5 to 100 seconds and total strains of 0.5 to 2.0. The semiempirical
equations developed indicated that both the recrystallization kinetics and the final recrystallized grain size were dependent
on the deformation history of the material i.e., total strain and Zener-Hollomon parameter (Z), where
and time at the recrystallization temperature. 相似文献
11.
The kinetics of dynamic spheroidization of the lamellar microstructure and the associated flow-softening behavior during isothermal,
constant-strain-rate deformation of a gamma titanium aluminide alloy were investigated, with special emphasis on the role
of the prior-alpha grain/colony size. For this purpose, fully lamellar microstructures with prior-alpha grain sizes between
80 and 900 μm were developed in a Ti-45.5Al-2Nb-2Cr alloy using a special forging and heat-treatment schedule. Isothermal hot compression
tests were conducted at 1093 °C and strain rates of 0.001, 0.1, and 1.0 s−1 on specimens with different grain sizes. The flow curves from these tests showed a very strong dependence of peak flow stress
and flow-softening rate on grain size; both parameters increased with alpha grain/colony size. Microstructures of the upset
test specimens revealed the presence of fine, equiaxed grains of γ + α
2 + β phases resulting from the dynamic spheroidization process that initiated at and proceeded inward from the prior-alpha grain/colony
boundaries. The grain interiors displayed evidence of microkinking of the lamellae. The frequency and severity of kinking
increased with strain, but were also strongly dependent on the local orientation of lamellae with respect to the compression
axis. The kinetics of dynamic spheroidization were found to increase as the strain rate decreased for a given alpha grain
size and to decrease with increasing alpha grain size at a given strain rate. The breakdown of the lamellar structure during
hot deformation occurred through a combination of events, including shear localization along grain/colony boundaries, microbuckling
of the lamellae, and the formation of equiaxed particles of γ + β
2 + α
2 on grain/colony boundaries and in zones of localized high deformation within the microbuckled regions. 相似文献
12.
Ch. A. Gandin M. Rappaz D. West B. L. Adams 《Metallurgical and Materials Transactions A》1995,26(6):1543-1551
The grain selection that operates in the columnar zone of a directionally solidified (DS) INCONEL X750 superalloy has been
investigated using standard metallography and an automatic indexing technique of electron backscattered diffraction patterns
(EBSPs). From the crystallographic orientations measured at 90,000 points in a longitudinal section, the grain structure was
reconstructed. The grain density as measured by the inverse of the mean linear intercept was found to be a decreasing function
of the distance from the chill. The evolution of the 〈100〉 pole figures along the columnar zone of the casting and the distribution
of the angle θ characterizing the 〈100〉 direction of the grains that is closest to the temperature gradient were then deduced
from the EBSPs measurement. It was found that, near the surface of the chill, the θ distribution was close to the theoretical
curve calculated for randomly oriented grains. As the distance from the chill increased, the measured θ distribution became
narrower and was displaced toward smaller θ values. At 2 mm from the chill, the most probable orientation of the grains was
found to be about 0.21 rad (12 deg). The information obtained with the EBSPs was then compared with the results of a three-dimensional
stochastic model (3D SM) describing the formation of grain structure during solidification. This model accounts for the random
location and orientation of the nuclei, for the growth kinetics and preferential 〈100〉 growth directions of the dendrites.
Although this model assumes a uniform temperature within the specimen, the simulation results were found to be in good agreement
with the EBSPs measurement. 相似文献
13.
采用DSC(differential scanning calorimetry) 、金相、扫描电镜及其能谱分析等方法,研究了7050超高强铝合金φ200 mm半连续圆铸锭中非平衡凝固共晶体在DSC实验过程的变化.结果表明:7050超高强铝合金φ200 mm半连续铸锭中仅有一种非平衡凝固的低熔点共晶体,其开始熔化温度为477 ℃;当DSC分析的加热速度低于20 ℃/min时,在其加热过程,半连续铸锭中的非平衡凝固共晶体发生向具有更高熔化温度的Al2CuMg(S相)的转变,该合金中平衡S相的熔化温度为490 ℃;采用DSC方法研究非平衡凝固产物时,应考虑其在加热过程发生的转变. 相似文献
14.
《Acta Metallurgica Materialia》1992,40(6):1121-1134
A two-phase gamma titanium aluminide alloy, Ti-47Al-1Cr-1V-2.5Nb (in at.%), was studied under forged and various subsequent heat treatment conditions, to investigate the microstructural evolution and the effect of microstructure on room temperature (RT) tensile properties and fracture toughness behavior. Four classes of microstructure and three types of lamellar formation were identified, and their formation mechanisms were analyzed using various analytical techniques including metallography, electron optics, differential thermal analysis (DTA), and crystallography. It was found that both tensile and toughness behavior were profoundly affected by the microstructural variations. 相似文献
15.
Y. F. Gu Y. Ro T. Kobayashi H. Harada 《Metallurgical and Materials Transactions A》2005,36(3):577-582
In an effort to enhance ductility and strength of Cr-base alloys, a series of Cr-Ru alloys with Ru contents ranging from 3
to 30 at. pct were made to study their microstructure evolution and mechanical properties. The microstructure of the alloys
with 6 to 20 at. pct Ru showed signs of a eutectic structure. However, no corresponding eutectic reaction is indicated in
the published Cr-Ru phase diagram. The yield strength of the Cr-Ru alloys increased with increasing Ru content at both room
temperature and 1200 °C. The tensile ductility of Cr-3 at. pct Ru is about 1.5 pct at room temperature, while the alloys containing
6 at. pct or more Ru showed zero tensile elongation. The deformation mechanisms of the Cr-Ru alloys are discussed in terms
of the microstructure and fracture behavior.
This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place
March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects
Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory
Metals Committee. 相似文献
16.
Jinkeun Oh W. C. Lee Sung Gyu Pyo Woojin Park Sunghak Lee Nack J. Kim 《Metallurgical and Materials Transactions A》2002,33(12):3649-3659
This study is concerned with the microstructural analysis of multilayered or bulk Ti aluminide sheets fabricated by the self-propagating
high-temperature synthesis (SHS) reaction using hot rolling and heat treatment. Multilayered Ti/Al sheets were prepared by
stacking thin Ti and Al sheets alternately, and a good Ti/Al interfacial bonding was achieved after rolling at 500 °C. When
these sheets were held at 1000 °C, spheroidal TiAl3 phases were formed by the SHS reaction at Ti/Al interfaces and inside Al layers. Microstructural analysis on the hot-rolled,
multilayered Ti/TiAl3 sheets revealed that intermetallic phases such as TiAl2, TiAl, and Ti3Al were formed at Ti/TiAl3 interfaces due to interaction between Ti and TiAl3 and that pores formed in the TiAl3 layer were significantly reduced during hot rolling. When multilayered Ti/Ti aluminide sheets were heat treated at 1000 °C,
Ti3Al, TiAl, and TiAl2 were grown as Ti and TiAl3 were consumed. As the heat treatment proceeded, TiAl grew further, eventually leading to the fabrication of multilayered
sheets composed of Ti3Al and TiAl. Bulk Ti aluminide sheets, having a lamellar structure of Ti3Al and TiAl, instead of multilayered sheets, were also fabricated successfully by heat treatment at 1400 °C. This fabrication
method of the bulk sheets had several advantages over the method by hot forging or rolling of conventional cast Ti aluminides.
From these findings, an idea to fabricate multilayered or bulk Ti aluminide sheets by hot rolling and heat treatment is suggested
as an economical and continuous fabrication method, and the formation and growth mechanisms of interfacial phases are elucidated
in this study. 相似文献
17.
Suyitno V. I. Savran L. Katgerman D. G. Eskin 《Metallurgical and Materials Transactions A》2004,35(11):3551-3561
Effects of casting speed and alloy composition on structure formation and hot tearing during direct-chill (DC) casting of
200-mm round billets from binary Al-Cu alloys are studied. It is experimentally shown that the grain structure, including
the occurrence of coarse grains in the central part of the billet, is strongly affected by the casting speed and alloy composition,
while the dendritic arm spacing is mostly dependent on the casting speed. The hot cracking pattern reveals the maximum hot-tearing
susceptibility in the range of low-copper alloys (1 to 1.5 pct) and at high casting speeds (180 to 200 mm/min). The clear
correlation between the amount of nonequilibrium eutectics (representing the reserve of liquid phase in the last stage of
solidification) and hot tearing is demonstrated. A casting speed-copper concentration-hot-tearing susceptibility chart is
constructed experimentally for real-scale DC casting. Computed dimensions of the solidification region in the billet are used
to explain the experimentally observed structure patterns and hot cracking. Thermomechanical finite-element simulation of
the solidifying billet was used as a tool for testing the applicability to DC casting of several hot-tearing criteria based
on different principles. The results are compared to the experimentally observed hot tearing. It is noted that hot-tearing
criteria that account for the dynamics of the process, e.g., strain rate, actual stress-strain situation, feeding rate, and melt flow, can be successfully used for the qualitative prediction
of hot tearing. 相似文献
18.
T. H. Courtney 《Metallurgical and Materials Transactions A》1977,8(5):679-684
During liquid phase sintering, numerous solid-solid particle contacts can be generated by particle motion within the fluid.
It is shown that, somewhat surprisingly, Brownian motion can produce such contacts. If such contacts are accompanied by particle
adherence, the particles can then subsequently fuse into one (i.e., coalesce) by the liquid state analog of the evaporation-condensation mechanism of sintering. An isolated microstructure
will develop if the time for particle coalescence is much less than the time between contacts. A highly skeletal arrangement
of particles will form under the converse condition. Using these principles, a “microstructure map” is calculated in which
the expected morphology of microstructure (i.e., skeletal or isolated) is related to the solid particle volume fraction, the kinetic and thermodynamic parameters affecting
particle coalescence, and the frequency of particle contacts by Brownian motion. Some discussion of the thermodynamic and
morphological factors affecting the probability of particle adherence after contact is presented. 相似文献
19.
C. Devadas I. V. Samarasekera E. B. Hawbolt 《Metallurgical and Materials Transactions A》1991,22(2):335-349
A mathematical model has been developed to compute the changes in the austenite grain size during rolling in a hot-strip mill.
The heat-transfer model described in the first of this series of papers has been employed to calculate the temperature distribution
through the thickness which serves as a basis for the microstructure model. Single-and double-hit compression tests have been
conducted at temperatures of 900 °C, 850°C, 950 °C, and 875 °C on 0.34 and 0.05 pct carbon steels to determine the degree
of recrystallization by metallographic evaluation of quenched samples and by measuring the magnitude of fractional softening.
The Institut de Recherches de la Sidérurgie Francaise, (IRSID) Saint Germain-en-Laye, France equation has been found to yield
the best characterization of the observed recrystallization kinetics. The equations representing static recrystallization
kinetics, recrystallized grain size, and grain growth kinetics have been incorporated in the model. The principle of additivity
has been invoked to permit application of the isothermal recrystallization data to the nonisothermal cooling conditions. The
model has been validated by comparing predicted austenite grain sizes with measurements made on samples quenched after one
to four passes of rolling on the CANMET pilot mill. The austenite grain size evolution during rolling of a 0.34 pct carbon
steel on Stelco’s Lake Erie Works (LEW) hot-strip mill has been computed with the aid of the model. The grain size decreased
from an initial value of 180μm to 35μm in the first pass due to the high reduction of 46 pct. The changes in austenite grain size in subsequent passes were found
to be small in comparison because of the lower per pass reductions. It has been shown that the equation employed to represent
grain growth kinetics in the interstand region has a significant influence on the computed final grain size. Altering the
rolling schedule had a negligible influence on the final grain size for a given finished gage. A 200°C increase in entry temperature
to the mill resulted in a 20μm increase in final grain size, which is significant. This can be attributed to increased grain growth at the higher temperature.
Formerly Graduate Student, The Centre for Metallurgical Process Engineering, The University of British Columbia Metallurgical
transactions a 相似文献
20.
Flow softening and microstructure evolution during hot working of wrought near-gamma titanium aluminides 总被引:2,自引:0,他引:2
S. L. Semiatin N. Frey S. M. El-Soudani J. D. Bryant 《Metallurgical and Materials Transactions A》1992,23(6):1719-1735
The hot-working behavior of two wrought ingot-metallurgy near-gamma titanium aluminides was established using the isothermal,
hot compression test. Experiments were conducted in both the two-phase (alpha+gamma) and single-phase (alpha) regimes at strain
rates typical of conventional metalworking operations (0.1 to 10 s−1). As for conventional titanium alloys, the flow stress showed a strong dependence on temperature and strain rate. In addition,
the stress-strain curves revealed substantial levels of flow softening. Under subtransus conditions, the majority of this
softening was ascribed to deformation heating effects and, secondarily, to microstructural effects. By contrast, microstructural
changes, associated with the dissolution of remnant gamma grains and redistribution of solute, appeared to lead to the majority
of the softening observed in the nominally single-phase alpha regime. Metallography on as-received (isothermally forged),
heat-treated, isothermally upset, and upset and heat-treated samples revealed the persistence of segregation which appears
to be a signature of the solidification process characteristic of near-gamma titanium aluminides.
S.L. S-upemiatin, formerly Senior Research Scientist, Metals and Ceramics Department, Battelle Memorial Institute, is Senior Scientist, Metals
and Ceramics Division, Materials Directorate, Wright Laboratory, WL/MLLN, Wright-Patterson Air Force Base, OH 45433-6533. 相似文献