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1.
Influence of pressing speed on microstructural development in equal-channel angular pressing 总被引:8,自引:0,他引:8
Patrick B. Berbon Minoru Furukawa Zenji Horita Minoru Nemoto Terence G. Langdon 《Metallurgical and Materials Transactions A》1999,30(8):1989-1997
The influence of pressing speed in equal-channel angular (ECA) pressing was investigated using samples of pure Al and an Al-1
pct Mg alloy and a range of pressing speeds from ∼10−2 to ∼10 mm s−1. The results show that the speed of pressing has no significant influence on the equilibrium grain size, at least over the
range used in these experiments. Thus, the equilibrium grain sizes were ∼1.2 μm for pure Al and ∼0.5 μm for the Al-1 pct Mg alloy for all pressing conditions. However, it is shown that the nature of the microstructure is dependent
on the pressing speed, because recovery occurs more easily at the slower speeds, so that the microstructure is then more equilibrated.
There is also indirect evidence for the advent of frictional effects when the cross-sectional dimensions of the samples are
at or below ∼5 mm. 相似文献
2.
An investigation of microstructure and grain-boundary evolution during ECA pressing of pure aluminum 总被引:2,自引:0,他引:2
S. D. Terhune D. L. Swisher K. Oh-Ishi Z. Horita T. G. Langdon T. R. McNelley 《Metallurgical and Materials Transactions A》2002,33(7):2173-2184
High-purity aluminum (99.99 pct) was processed by equal-channel angular pressing (ECAP) at room temperature through a die
with a 90 deg angle between the die channels. Samples were examined by transmission electron microscopy (TEM) and orientation
imaging microscopy (OIM) methods after one, four, and 12 passes through the die. Repetitively pressed samples were rotated
by 90 deg in the same sense between successive pressing operations (route BC). After one pressing, TEM showed a subgrain structure which was elongated in the shearing direction. Corresponding OIM data
illustrated an inhomogeneous microstructure in which bandlike features were also aligned with the shearing direction. The
lattice orientation varied from location to location in the material. The boundary disorientation distribution determined
from the OIM data exhibited a peak at 2 to 5 deg, in agreement with a predominance of subgrains in the microstructure. After
four pressings, the microstructure data obtained by TEM and OIM were mutually consistent. The disorientation data revealed
a decrease in the population of 2 to 5 deg boundaries accompanied by an overall upward shift in the distribution. Two orientations
were generally apparent in the texture, although specific orientations varied with location. Often, a 〈111〉 orientation tended
to align with the shear direction. Following 12 ECA passes, the grain size was reduced further to about 1.0 μm. The populations of high-angle boundaries (≥15 deg) increased in the disorientation distribution. A texture characteristic
of shear deformation of fcc metals became apparent, although the orientations and particular components varied with location.
Microstructural refinement during severe straining includes the development of large fractions of high-angle boundaries. 相似文献
3.
Patrick B. Berbon Terence G. Langdon Nikolai K. Tsenev Ruslan Z. Valiev Minoru Furukawa Zenji Horita Minoru Nemoto 《Metallurgical and Materials Transactions A》1998,29(9):2237-2243
Ultrafine grain sizes were introduced into samples of an Al-3 pct Mg solid solution alloy and a cast Al-Mg-Li-Zr alloy using
the process of equal-channel angular (ECA) pressing. The Al-3 pct Mg alloy exhibited a grain size of ∼0.23 μm after pressing
at room temperature to a strain of ∼4, but there was significant grain growth when the pressed material was heated to temperatures
above ∼450 K. The Al-Mg-Li-Zr alloy exhibited a grain size of ∼1.2 μm, and the microstructure was heterogeneous after pressing
to a strain of ∼4 at 673 K and homogeneous after pressing to a strain of ∼8 at 673 K with an additional strain of ∼4 at 473
K. The heterogeneous material exhibited superplastic-like flow, but the homogeneous material exhibited high-strain-rate superplasticity
with an elongation of >1000 pct at 623 K at a strain rate of 10−2 s−1. It is concluded that a homogeneous microstructure is required, and therefore a high pressing strain, in order to attain
high-strain-rate superplasticity (HSR SP) in ultrafine-grained materials.
This article is based on a presentation made in the symposium “Mechanical Behavior of Bulk Nanocrystalline Solids,” presented
at the 1997 Fall TMS Meeting and Materials Week, September 14–18, 1997, in Indianapolis, Indiana, under the auspices of the
Mechanical Metallurgy (SMD), Powder Materials (MDMD), and Chemistry and Physics of Materials (EMPMD/SMD) Committees. 相似文献
4.
Equal-channel angular pressing of commercial aluminum alloys: Grain refinement, thermal stability and tensile properties 总被引:15,自引:0,他引:15
Zenji Horita Takayoshi Fujinami Minoru Nemoto Terence G. Langdon 《Metallurgical and Materials Transactions A》2000,31(3):691-701
Using equal-channel angular (ECA) pressing at room temperature, the grain sizes of six different commercial aluminum-based
alloys (1100, 2024, 3004, 5083, 6061, and 7075) were reduced to within the submicrometer range. These grains were reasonably
stable up to annealing temperatures of ∼200 °C and the submicrometer grains were retained in the 2024 and 7075 alloys to annealing
temperatures of 300 °C. Tensile testing after ECA pressing through a single pass, equivalent to the introduction of a strain
of ∼1, showed there is a significant increase in the values of the 0.2 pct proof stress and the ultimate tensile stress (UTS)
for each alloy with a corresponding reduction in the elongations to failure. It is demonstrated that the magnitudes of these
stresses scale with the square root of the Mg content in each alloy. Similar values for the proof stresses and the UTS were
attained at the same equivalent strains in samples subjected to cold rolling, but the elongations to failure were higher after
ECA pressing to equivalent strains >1 because of the introduction of a very small grain size. Detailed results for the 1100
and 3004 alloys show good agreement with the standard Hall-Petch relationship. 相似文献
5.
Shogo Komura Zenji Horita Minoru Furukawa Minoru Nemoto Terence G. Langdon 《Metallurgical and Materials Transactions A》2001,32(13):707-716
An Al-3 pct Mg-0.2 pct Sc alloy was fabricated by casting and subjected to equal-channel angular pressing to reduce the grain
size to ∼0.2 μm. Very high tensile elongations were achieved in this alloy at temperatures over the range from 573 to 723
K, with elongations up to >2000 pct at temperatures of 673 and 723 K and strain rates at and above 10−2 s−1. By contrast, samples of the same alloy subjected to cold rolling (CR) yielded elongations to failure of <400 pct at 673
K. An analysis of the experimental data for the equal-channel angular (ECA)—pressed samples shows consistency with conventional
superplasticity including an activation energy for superplastic flow which is within the range anticipated for grain boundary
diffusion in pure Al and interdiffusion in Al−Mg solid solution alloys.
MINORU NEMOTO, formerly Professor, Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University. 相似文献
6.
Minoru Furukawa Patrick B. Berbon Terence G. Langdon Zenji Horita Minoru Nemoto Nikolai K. Tsenev Ruslan Z. Valiev 《Metallurgical and Materials Transactions A》1998,29(1):169-177
Experiments were conducted to determine the age-hardening characteristics and the mechanical properties of an Al-5.5 pct Mg-2.2
pct Li-0.12 pct Zr alloy processed by equal-channel angular (ECA) pressing to give a very fine grain size of ∼1.2 μm. The results show that peak aging occurs more rapidly when the grain size is very fine, and this effect is interpreted in
terms of the higher volume of precipitate-free zones in the fine-grained material. Mechanical testing demonstrates that the
ECA-pressed material exhibits high strength and good ductility at room temperature compared to conventional Al alloys containing
Li. Elongations of up to ∼550 pct may be achieved at an elevated temperature of 603 K in the ECA-pressed condition, thereby
confirming that, in this condition, the alloy may be a suitable candidate material for use in superplastic forming operations. 相似文献
7.
Shogo Komura Zenji Horita Minoru Furukawa Minoru Nemoto Terence G. Langdon 《Metallurgical and Materials Transactions A》2001,32(3):707-716
An Al-3 pct Mg-0.2 pct Sc alloy was fabricated by casting and subjected to equal-channel angular pressing to reduce the grain
size to ∼0.2 μm. Very high tensile elongations were achieved in this alloy at temperatures over the range from 573 to 723 K, with elongations
up to >2000 pct at temperatures of 673 and 723 K and strain rates at and above 10−2 s−1. By contrast, samples of the same alloy subjected to cold rolling (CR) yielded elongations to failure of <400 pct at 673
K. An analysis of the experimental data for the equal-channel angular (ECA)-pressed samples shows consistency with conventional
superplasticity including an activation energy for superplastic flow which is within the range anticipated for grain boundary
diffusion in pure Al and interdiffusion in Al-Mg solid solution alloys. 相似文献
8.
David Witkin Bing Q. Han Enrique J. Lavernia 《Metallurgical and Materials Transactions A》2006,37(1):185-194
Room-temperature mechanical properties of cryomilled Al-7.5 pct Mg and Al 5083 alloys are discussed in the context of a duplex
microstructure, which arises during processing. After consolidation via hot isostatic pressing (“hipping”), coarse-grained regions are formed in former interparticle void volumes, and these regions
become elongated during extrusion. Comparison of tensile and compression testing results on both “as-hipped” and extruded
materials shows that tension-compression asymmetry is the result of these coarse-grained regions and not necessarily a fundamental
property of ultrafine grained Al. The strength of the extruded materials is consistent with the Hall-Petch model of strengthening
by grain size refinement, but the hipped material deviates from this trend, with a lower strength despite finer average grain
size. This can also be attributed to the presence of coarse-grained regions, which substract from the strength in a predictable
manner and also enhance the ability of the cryomilled material to work harden. 相似文献
9.
We have performed Bauschinger experiments on a variety of fcc metals and alloys, after large amounts of prestrain, using torsion
and a short thin-walled tube geometry. The materials we studied were 99.99 pct Al, OFE copper, 70:30 brass, Al-1 pct Mg, Al-2
pct Mg, Al-0.17 pct Fe-0.07 pct Si, Al-0.8 pct Mn, and two Al-Cu alloys (Al-2.6 pct Cu and Al-4 pct Cu) given different heat
treatments. For the material systems other than the Al-Cu alloys, the stress reversal was after a prestrain in shear of ≈3.0.
Two stress reversals were performed on the Al-Cu alloys. The first was at γ = 0.3 and the second at γ = 1.2. Thus, for the
Al-Cu, the prestrain and the final increment of deformation were in the same direction. The Bauschinger yield stress in these
experiments was characterized by a very large offset shear strain of 0.05. This definition of reverse yield minimizes the
effects of heterogeneous deformation and long-range internal elastic stresses that arise mainly from second-phase particles.
We attributed the effects we observed to “isotropic hardening” associated with the dislocation substructures that developed
in the different materials. We found that the behavior of these materials could be divided into two categories: those which
deform by planar slip and those that form a “cell” structure and are characterized as having wavy slip. When the deformation
was wavy in nature, we attributed the observed Bauschinger effects to be a result of the untangling of the “cells” formed
during the prestrain. Different morphologies of cells had different behaviors when the stress was reversed. The behavior of
the planar slip alloys depended on whether or not the barriers to dislocation activity were rigid or shearable. The θ′ precipitates
in the Al-Cu alloys and the twin boundaries in the 70:30 brass constituted rigid barriers to dislocation motion, and a very
large Bauschinger effect was observed. The solid solution Al-Cu material and that containing Guinier-Preston (GP) zones and
θ″ had almost no Bauschinger effect when the yield stress in reverse deformation was considered. After yield, these materials
hardened very rapidly and the flow stress in the reverse direction exceeded that for the equivalent amount of monotonie deformation. 相似文献
10.
Kiyoshi Matsubara Yuichi Miyahara Zenji Horita Terence G. Langdon 《Metallurgical and Materials Transactions A》2004,35(6):1735-1744
Experiments were conducted to evaluate the utility of a new processing procedure developed for Mg-based alloys in which samples
are subjected to a two-step processing route of extrusion followed by equal-channel angular pressing (designated as EX-ECAP).
The experiments were conducted using a Mg-0.6 wt pct Zr alloy and, for comparison purposes, samples of pure Mg. It is shown
that the potential for successfully using ECAP increases in both materials when adopting the EX-ECAP procedure. For the Mg-Zr
alloy, the use of EX-ECAP produces a grain size of ∼1.4 μm when the pressing is undertaken at 573 K. By contrast, using EX-ECAP with pure Mg at 573 K produces a grain size of ∼26
μm. Tensile testing of the Mg-Zr alloy at 523 and 573 K after processing by EX-ECAP revealed the occurrence of significantly
enhanced ductilities with maximum elongations of ∼300 to 400 pct. 相似文献
11.
The effects of chromium and copper additions on precipitation in several Al?Zn?Mg alloys have been investigated. Results show that chromium additions heterogenize precipitation in aged Al?Zn?Mg alloys by creating special nucleation sites. Multirowed bands of incoherent precipitates appeared in the grain boundaries and subboundaries in an Al-5 pct Zn-2 pct Mg-0.1 pct Cr alloy. It is believed that fine nuclei associated with the existence of chromium-rich regions are formed during solidification and are retained after solution heat treatment. These nuclei would lead to the formation of incoherent precipitates during quenching and aging. Chromium is, therefore, considered to causehigh temperature nucleation. Copper additions to Al?Zn?Mg alloys accelerate precipitation at lower aging temperatures and increase the density of G. P. zones nucleated at relatively lower temperatures (20 to 90°C). In this way copper considerably strengthens Al?Zn?Mg alloys. Copper, in contrast to chromium, causes increased low-temperature nucleation of G. P. zones. 相似文献
12.
Øyvind Ryen Hans Ivar Laukli Bjørn Holmedal Erik Nes 《Metallurgical and Materials Transactions A》2006,37(6):2007-2013
The work-hardening behavior of a range of aluminum-magnesium alloys, from 0.5 to 4.55 wt pct Mg, is followed up to large strains
using compression testing and cold rolling. At large strains, stage IV, an unexpectedly low work-hardening rate of high-Mg
alloys is observed, and the work-hardening rate in stage IV is almost unaffected by the Mg content. A model for work hardening
is applied and discussed in relation to the experimental observations. Based on microstructural observations of the cold-rolled
materials, the low work-hardening rate of high-Mg alloys is ascribed to a different storage pattern of dislocations caused
by an increased amount of shear bands and a higher dislocation density inside subgrains. 相似文献
13.
Ehab Samuel John J. Jonas Fawzy H. Samuel 《Metallurgical and Materials Transactions A》2011,42(4):1028-1037
Conventionally, superplasticity requires the presence of a fine-grained microstructure to enable grain-boundary sliding to
take place during deformation. However, coarse-grained materials have also been shown to exhibit higher than normal amounts of ductility, provided they possess a high-enough strain
rate sensitivity. In this work, coarse-grained Al-3 pct Mg, Al-5 pct Mg, and AA 5056 alloys were tested for enhanced ductility.
The dependence of flow stress on temperature was found to display some unusual characteristics; these were interpreted as
resulting from the occurrence of dynamic strain aging (DSA). In these materials, a local peak in elongation coincided with
the presence of an unusual peak in rate sensitivity. This region of higher than normal rate sensitivity was coupled with the
usual region of negative rate sensitivity found in DSA-prone materials, such as the Al-Mg alloys. A maximum ductility of 170 pct
was recorded at 723 K (450 °C) and a strain rate of 5 × 10−2 seconds−1 was found in the vicinity of the rate sensitivity peak. This was found to increase to nearly 300 pct when the gage length
was shortened. These peaks in elongation occurred below the maximum test temperatures. 相似文献
14.
G. A. Henshall M. E. Kassner H. J. McQueen 《Metallurgical and Materials Transactions A》1992,23(3):881-889
An Al-5.8 at. pct Mg (5.2 wt pct Mg) alloy was deformed in torsion within the solute drag regime to various strains, up to
the failure strain of 10.8. Optical microscopy (OM) and transmission electron microscopy (TEM) were used to analyze the evolution
of the microstructure and to determine the dynamic restoration mechanism. Transmission electron microscopy revealed that subgrain
formation is sluggish but that subgrains eventually (ε ≈ 1) fill the grains. The “steady-state” subgrain size (λ ≈ 6 μm) and
misorientation angle (θ ≈ 1.6 deg) are reached by ε ≈ 2. These observations confirm that subgrains eventually form during
deformation in the solute drag regime, though they do not appear to significantly influence the strength. At low strains,
nearly all of the boundaries form by dislocation reaction and are low angle (θ < 10 deg). At a strain of 10.8, however, the
boundary misorientation histogram is bimodal, with nearly 25 pct of the boundaries having high angles due to their ancestry
in the original grain boundaries. This is consistent with OM observations of the elongation and thinning of the original grains
as they spiral around the torsion axis. No evidence was found fordiscontinuous dynamic recrystallization, a repeating process in which strain-free grains nucleate, grow, deform, and give rise to new nuclei.
It is concluded that dynamic recovery in the solute drag regime gives rise togeometric dynamic recrystallization in a manner very similar to that already established for pure aluminum, suggesting that geometric
dynamic recrystallization may occur generally in materials with a high stacking-fault energy (SFE) deformed to large strains. 相似文献
15.
M. Raghavan 《Metallurgical and Materials Transactions A》1980,11(6):993-999
In order to better understand the formation of Precipitate Free Zones (PFZ), microanalysis was conducted on heat treated Al-2.2
at. pct Zn-4.7 at. pct Mg and Cu-30 at. pct Ni-0.9 at. pct Nb alloys. In both the alloys, no appreciable solute depletion
at the grain boundaries was observed in the as-quenched condition. After aging, marked solute depletion was observed in the
PFZ of both the alloys. In the Al-Zn-Mg alloy, the PFZ were supersaturated with respect toη andT phases up to 4 h of aging at 473 K. In the Cu-Ni-Nb alloy, the PFZ were supersaturated only with respect to theβ phase but not the metastable γ″ phase. Based on the results, the factors affecting the formation and growth of PFZ are discussed. 相似文献
16.
S. Hariprasad S. M. L. Sastry K. L. Jerina R. J. Lederich 《Metallurgical and Materials Transactions A》1993,24(4):865-873
Dispersion-strengthened high-temperature Al-8.5 pct Fe-pct Si-pct V alloys were produced by atomized melt deposition (AMD)
process. The effects of process parameters on the evolution of microstructures were determined using optical metallography
and scanning and transmission electron microscopy. The extent of undercooling and the rate of droplet solidification were
correlated with process parameters, such as melt superheat, metal/gas flow rates, and melt stream diameter. The size distribution
and morphology of silicide dispersoids were used to estimate the degree of undercooling and the cooling rate as functions
of process parameters. The tensile properties at 25 °C to 425 °C and fracture toughness at 25 °C of these alloys produced
with wide variations in dispersoids size and grain size were determined. Under optimum conditions, the alloy has ultimate
tensile strength of 281 MPa and 9.5 pct ductility in the as-deposited condition. Upon hot-isostatic pressing and extrusion,
the ultimate tensile strength increased to 313 MPa and ductility increased to 18 pct. 相似文献
17.
Experiments were conducted on a plain low carbon steel with an initial grain size of ~ 30μm to investigate the changes of microstructure and mechanical properties by repetitive equal channel angular pressings. Under the pressing conditions of giving a strain of ~ 1 and rotating samples 180° between each pass, the yield strength significantly increases from 310 to 750 MPa after single pass, and it reaches 1050 MPa after 12 passes. The increment of yield strength gradually decreases as the number of passes increases. The examination of microstructure by transmission electron microscopy shows that ferrite consists of parallel bands of elongated subgrains having a width of 0.3 μm and a length of 2 μm after a single pass. The subgrains are further divided by boundaries with low angle misorientation on subsequent passages. Low angle boundaries turn to high angle boundaries without noticeable grain refinement as the number of pass increases. In addition, lamellar cementites in pearlite are broken up into fragments within a pearlite colony. Analyses of structural and mechanical changes in a plain low carbon steel by equal channel angular pressing indicate that the strength enhancement is mainly due to the grain refinement of ferrite. 相似文献
18.
Yuichi Miyahara Kiyoshi Matsubara Zenji Horita Terence G. Langdon 《Metallurgical and Materials Transactions A》2005,36(7):1705-1711
The extrusion/equal channel angular pressing (EX-ECAP) processing procedure, in which magnesium-based alloys are subjected
to extrusion followed by ECAP, was applied to a Mg-7.5 pct Al-0.2 pct Zr alloy prepared by casting. Microstructural inspection
showed the EX-ECAP process was effective in reducing the grain size from ∼21 μm after extrusion to an as-pressed grain size of ∼0.8 μm. It is shown through static annealing that these ultrafine grains are reasonably stable up to 473 K, but grain growth occurs
at higher temperatures. Tensile specimens were cut from the billets prepared by EX-ECAP and testing showed these specimens
exhibited superplasticity at relatively low temperatures with maximum elongations up to >700 pct. By processing through EX-ECAP
to a higher imposed strain and thereby increasing the area fraction of high-angle boundaries, it is demonstrated that there
is a potential for achieving high-strain-rate superplasticity.
This article is based on a presentation made at the Symposium entitled “Phase Transformations and Deformation in Magnesium
Alloys,” which occurred during the Spring TMS meeting, March 14–18, 2004, in Charlotte, NC, under the auspices of the ASM-MSCTS
Phase Transformations Committee. 相似文献
19.
Mechanical properties and microstructures of Al-Mg-Sc alloys 总被引:8,自引:0,他引:8
The mechanical properties of Al-(Mg)-0.5Sc alloys have been investigated. Room-temperature tensile and toughness properties
were found to reflect a superposition of the properties of Al-Mg and Al-0.5Sc alloys and are quite competitive with high-performance
Al alloys. A combination of substructure refinement by Mg and stabilization by Al3Sc precipitates produces exceptional superplasticity as exemplified by superplastic forming (SPF) elongations in excess of
1000 pct at a strain rate of 0.01 s-1. Overall, these alloys demonstrate an extremely attractive combination of strength, toughness, density, and SPF fabricability. 相似文献
20.
C. M. Dinnis J. A. Taylor A. K. Dahle 《Metallurgical and Materials Transactions A》2006,37(11):3283-3291
Sand-cast plates were used to determine the effect of iron and manganese concentrations on porosity levels in Al-9 pct Si-0.5
pct Mg alloys. Iron increased porosity levels. Manganese additions increased porosity levels in alloys with 0.1 pct Fe, but
reduced porosity in alloys with 0.6 and 1 pct Fe. Thermal analysis and quenching were undertaken to determine the effect of
iron and managanese on the solidification of the Al-Si eutectic. At high iron levels, the presence of large β-Al5FeSi was found to reduce the number of eutectic nucleation events and increase the eutectic grain size. The preferential formation
of α-Al15Mn3Si2 upon addition of manganese reversed these effects. It is proposed that this interaction is due to β-Al5FeSi and the Al-Si eutectic having common nuclei. Porosity levels are proposed to be controlled by the eutectic grain size
and the size of the iron-bearing intermetallic particles rather than the specific intermetallic phase that forms. 相似文献