nice的中文意思Mechanical Properties and Microstructures of Al-Fe Alloys Procesd by High-Pressure Torsion
JORGE M.CUBERO-SESIN and ZENJI HORITA
Al-Fe alloys in the form of thin disks10mm in diameter,with Fe nominal weight fractions of
0.5,1,2,and5pct,were extracted from bulk-extruded rods and procesd by high-pressure
torsion(HPT).A group of the bulk samples was procesd in an as-received state,whereas
another group was annealed at773K(500°C)for a period of1hour,prior to the application of
HPT.An additional t of samples was prepared by mixing high-purity powders with similar Fe郑州英语翻译
contents and consolidated directly in the HPT facility.The samples were procesd up to
10revolutions.Vickers microhardness,tensile strength,and elongation to failure were evaluated
for all cas along with obrvations of the Al matrix by transmission electron microscopy
(TEM).Basic characterization of the microstructures was carried out by X-ray diffraction
(XRD)and optical microscopy(OM).Significant strengthening with ductility retained was
achieved in the bulk samples as a conquence of grain refinement and dispersion of interme-cleanup
tallic phas.Powder samples had a more gradual increa in strength but incread ductility as
a result of the impod strain.
DOI:10.1007/s11661-012-1341-z
ÓThe Minerals,Metals&Materials Society and ASM International2012
I.INTRODUCTION
A LUMINUM(Al)alloys have gained widespread u in modern engineering applications due to veral advantages,such as weight reduction and good ductility, but especially due to the increa in strength when combined with other elements as Mg,Si,Cu,and Zn. This has been possible by the u of traditional methods such as work(deformation)hardening,solid-solution hardening,grain refinement,andfine dispersion of precipitate particles.[1,2]However,for the ca of iron (Fe),its application to commercial Al alloys is limited. Often considered one of the most common impurities d
mercaue to its general abundance,Fe appears as a leftover during production,casting,and other processing tech-niques.[3]Its common u in devices and components increas the contamination of Al with Fe during recycling.[3,4]According to the equilibrium condition, the solubility of Fe in Al is very low(<0.052wt pct),[3,5,6] which leads to the formation of hard,brittle intermetal-lics and a subquent reduction in formability.Besides the early nucleation of intermetallic cond phas,there is a eutectic reaction between Al(solid solution)and the Al3Fe compound.It is said that the eutectic composition ranges from1.7to2.2wt pct Fe.[1–3]The solidification of the Al rich Al-Fe system under veral cooling rates and its deviation from equilibrium was studied using meth-ods such as rapid quenching(RQ),[7–9]mechanical alloying,[7,8,10–12]and vere plastic deformation (SPD).[7–9,13–15]The application of such out-of-equilib-rium methods documented the attainment of a supersat-urated solid solution as well as the formation of veral stable and meta-stable phas,and it was shown that Fe in solid solution and/orfinely disperd particles may provide a potential for higher strength.
High-pressure torsion(HPT)is now well known as a typical processing procedure of SPD,where a sample in a form of a disk or a ring is placed between two anvils and shear strain is introduced in the sample by rotating the anvils with respect to each other under a high compressive load.[16]It was sh
own that significant grain refinement occurs through the application of HPT,and considerable increa in strength is attained.[17]HPT is also capable of consolidating powder mixtures consist-ing of different elements and/or metal-ceramic compos-ites so that in situ production of bulk metallic alloys and/or composites is feasible at lower temperatures through solid-state reactions,often without requiring sintering process.[16–27]
It was reported[14]that an Al-11wt pct Fe alloy incread its strength by processing with HPT,and the subquent aging led to a further increa in strength.It was suggested that not only the grain refinement but also dissolution of Fe occurred.Another study[7] employed a combination of RQ and HPT,and it obtained significant strengthening in an Al-8wt pct Fe alloy with incread solid solubility.Subquent work by the same group[8,9]reported additional results by applying RQ and HPT to Al-Fe alloys with different Fe concentrations(2,8,and10wt pct)to show an increa in microhardness with aging.Another study employing equal-channel angular pressing(ECAP)to process a cast Al-5wt pct Fe alloy[15]reported grain refinement of the Al matrix and dispersion of cond-pha particles so
cosby
JORGE M.CUBERO-SESIN,Graduate Student,and ZENJI HORITA,Professor,are with the Department of Materials Science and Engineering,Faculty of Engineering,and also with International
Institute for Carbon-Neutral Energy Rearch(WPI-I2CNER),Kyushu University,Fukuoka,819-0395Japan.Contact e-mail:cubero@zaiko6. zaiko.kyushu-u.ac.jp
Manuscript submitted January3,2012.
Article published online August23,2012
that the process led to the improvement of not only the microhardness but also the tensile strength and ductility.The ECAP study by Stolyarov et al.[15]indicates the importance of back pressure in ECAP processing so that fragmentation of the particles proceeds to give ri to an additional increa in the strength.Becau higher pressure can be applied by using HPT compared to ECAP,it is anticipated that finer fragmentation may be achieved.It is also anticipated that the initial state of the sample may affect the fragmentation of the particles.In this study,thus,HPT was applied to Al-Fe alloys with four different compositions having different microstruc-tural states after extrusion and annealing including mixtures of Al and Fe powders.The evolutions of microstructure and mechanical properties with straining by HPT are examined to obtain an optimum state of sample for HPT processing.
II.
EXPERIMENTAL MATERIALS
AND PROCEDURES
This study compared samples in the form of bulk and powder.Powder mixtures were first prepared from powders of high-purity Al (99.99pct)sieved through 75l m mesh and of Fe (>99.9pct)through 53l m mesh,and then they were manually agitated for 5minutes.Designated compositions of the powders were deter-mined by using an electronic scale with the following Fe contents:0.5pct,1pct,2pct,and 5pct.A sufficient amount of the powder mixtures was then placed between the upper and lower anvils in the HPT facility
for direct consolidation at room temperature.Each anvil has a shallow circular cavity with 10mm in diameter and 0.25mm in depth at the center of the anvil.Figure 1shows the appearance of a disk sample consolidated after HPT processing for 1revolution.Density mea-surements were carried out using an Archimedes tech-nique to validate the effectiveness of the consolidation process.The thickness of the samples after HPT is 0.7±0.1mm,given that some material flows out of the cavity during the HPT processing.
Bulk material was supplied by Kobe Steel,Ltd.(Tokyo,Japan)in the form of extruded rods measuring
20mm in diameter.The rods were obtained by extru-sion from cast ingots with 155mm in diameter and 200mm in length at an extrusion speed of 2mm/minutes at 723K (450°C).The ingots have four different nominal weight fractions (0.5pct,1pct,2pct,and 5pct Fe),and they were verified using inductively coupled plasma atomic emission spectroscopy,including the analysis of other impurities.Table I summarizes the chemical com-positions of the cast samples ud in this study.
The 0.5pct and 1pct samples are hypoeutectic,whereas the 2pct appears to be well within the eutectic composition.[3]The Al-5pct Fe material had a final composition of 3.72pct,which is below the nominal value.However,this sample is well in the hypereutectic range,and for practical purpos,it will be compared to its powder counterpart,taking into account the slight difference in the Fe content.
For HPT processing,the rods were first sliced into disks with 0.9mm thickness using a metal cutting wheel.Thereafter,a wire-cutting electrical discharge machine (EDM)was ud to extract the 10-mm diam-eter disks.A t of disk samples,containing each of the above mentioned compositions,was annealed at 773K (500°C)for 1hour,whereas a parate t was left as received,to evaluate the effect of the previous extrusion process.Annealing was conducted in air,and the samples were left inside the furnace to cool to room temperature.Both ts of the heat-treated and as-received disks were procesd by HPT.The physical appearance of the bulk samples after HPT is
very similar to the powder sample in Figure 1.
HPT was conducted on the powder and bulk samples at room temperature under an applied pressure of 6GPa with a rotation speed of 1rpm for different numbers of revolutions as N =1,5,and 10.Basic mechanical properties such as Vickers microhardness (H v ),tensile strength,and ductility were evaluated.Figure 2illus-trates the dimensions of specimens for evaluation of mechanical properties.HPT-procesd disks are polished and mechanically buffed to a mirror-like surface for hardness testing.Indentations were made from the
center
Fig.1—Appearance of powder sample after HPT processing for N =1revolution.No visible differences are obrved between bulk and powder samples after HPT.
Table I.
Chemical Composition of Al-Fe Bulk Samples After Casting (in Wt Pct)Material Fe Si Mn Mg Cr Zn Ti Ni Al-0.5pct Fe 0.50<0.0030.006<0.1720.0040.004<0.001<0.022Al-1pct Fe 0.98<0.0030.008<0.1780.0040.004<0.001<0.022Al-2pct Fe 1.99<0.0040.012<0.1890.0030.002<0.001<0.041Al-5pct Fe
3.72
<0.008
0.022
<0.220
0.004
0.002
<0.001
免费英语听力<0.041
健康管理师培训中心to the periphery of the disk along12radial directions:the first data point at0.1mm from the disk center and the rest every0.5mm.The12measurements at equal distances from the center were averaged.The hardness measurements were made using an Akashi MVK-E3 tester(Akashi Corporation,Kanagawa,Japan)applying a load of50g for duration of15conds.
Tensile specimens with dimensions of1.5mm long, 0.7mm wide,and0.6±0.1mm thick were extracted from the disks as shown in Figure2.They were pulled at room temperature with an initial strain rate of2.2910À3 condsÀ1using a Shimadzu AG-10kNE tester(Shima-dzu Corporation,Kyoto,Japan).It should be noted that the thickness of each tensile specimen was measured before tensile testing.Additionally,due to the miniature-size specimens,certain care should be taken,especially when comparing with results from other studies using specimens with different dimensions,as it was described by Zhao et al.[28]Thus,tensile tests from specimens of high-purity Al procesd by HPT,both from powder and bulk,were carried out for comparison.Fractographs of the failed surfaces were obrved with a Hitachi S-4300SE (Hitachi High Technologies,Tokyo,Japan)scanning electron microscope(SEM)operating at20kV.
For optical microscopy(OM),transmission electron microscopy(TEM),and X-ray diffraction(XRD)anal-ysis,disks with3mm in diameter were punched out from the edge of the HPT disks at3.5mm from the center,as shown in Figure2.X-ray profiles were obtained from the disks with a Rigaku X-ray diffrac-tometer(Rigaku Corporation,Tokyo,Japan)using the Cu-target K a radiation.The disks are later ground mechanically to0.15mm thickness and further thinned for TEM using a solution of20pct H2SO4and 80pct CH3OH in a twin-jet electropolishing apparatus. TEM was undertaken using a Hitachi H8100micro-scope operating at200kV.Selected-area electron dif-fraction(SAED)patterns were obtained from areas covering~6.3l m in diameter.For obrvation in OM, the samples were electropolished with the same solution as the TEM samples mentioned above and then electro-etched using a solution of5pct HF+95pct H2O.
III.RESULTS AND DISCUSSION
The results from the density measurements as a function of Fe content are shown in Figure3,for powder samples after HPT processing for N=1and N=10revolutions.Theoretical densities of the alloys were calculated using the Vegard’s law[29]where a linear relationship exists with the concentration as follows:
q AlÀFe¼q Fe xþq Al1Àx
ðÞ½1 where x is the volume fraction of Fe,and q Al-Fe,q Al,and q Fe are the densities of the alloy,pure Al,and pure Fe, respectively.The alloy density calculated using Eq.[1]is drawn as a solid line in Figure3.A comparison between the measured and calculated densities shows that all measured densities reasonably follow the Vegard’s law. The maximum deviation of the data points in Figure3 from Vegard’s law corresponds to a relative density of 99.34pct.This agreement suggests that consolidation of powder samples was sufficiently achieved by HPT processing,already after N=1revolution.
Figure4shows the measured values of Vickers microhardness(H v)plotted against the distance from the disk center.Each point reprents the average of the measured values at equal spacing in the disk,while the error bars indicate the standard deviation between the values.Thus,the graphs plotted against the distance from the disk center provide a straightforward interpre-tation of the variation of hardness along the radial direction.Figures4(a)through(d)correspond to plots of the extruded(EXT+HPT)bulk samples,the annealed(EXT+ANN+HPT)bulk samples,and mixed powder(HPT)samples for each Fe content. The initial levels of hardness in the as-extruded and annealed conditions are plotted in eachfigure accord-ingly.Hardness increas in general with increasing distance from the disk center and with increasing content of Fe.Such increas in hardne
ss are most prominent in the as-extruded samples,although the trends are similar between the two types of the bulk samples.For the powder samples,the hardness levels as well as the effect of Fe addition appears to be low when compared with the two types of the bulk samples.The results suggest that there is a difference in the state of
Fe
in which the prence of Fe as an elemental state is less effective for hardening within the strain impod by HPT.
The difference in the trends is more clearly demon-strated when the hardness is plotted against the equiv-alent strain in Figures5(a)through(c)for the corresponding samples.Here,the equivalent strain e is given by the following functional form with the distance from the disk center r,the number of revolutions N,and the sample thickness t[16]:
e¼2p rN
ffiffiffi
3
stsp
t
½2
High-purity Al(99.99pct)samples in bulk and powder forms procesd under the same conditions are also plotted for comparison purpos.For the samples of pure Al and of lower contents of Fe,the hardness saturates to constant levels with straining.Prence of the constant hardness level was reported in many pure metals and alloys[30–33]when they were subjected to HPT processing. In the ca of Al,it was considered that the appearance of the constant hardness is due to a balance between hardening by the generation of dislocations and softening by the recovery of dislocations.With the increasing addition of Fe,however,the constant level no longer appears as in Figure5(a)and(b),but the hardness increas continuously with strain.This increa is more inten f
or higher contents of Fe and is more prominent in the bulk samples.It is suggested that the microstructure is still changing even after N=10revolutions probably becau intermetallic phas are fragmented into small pieces of particles as described below.
There is also an important aspect to be highlighted, which is that the hardness levels for the bulk2pct and 5pct Fe alloys are similar.Two reasons are considered for this similarity.First,the actual Fe content between the two samples,as stated in Table I,differs just by 1.73pct.Second,in the solidification of the hypereutec-tic Fe alloy,the excess Fe is originally prent as a coar intermetallic pha,where as in the clo-to-eutectic2pct Fe alloy,most of Fe exists in afiner intermetallic pha,forming a lamellar structure.After HPT processing,thefiner intermetallic pha is easier to disper in the Al matrix in contrast to the hypereutectic ca.It ems that the coar intermetallic pha is not
sufficiently refined,by the extent of strain imparted by HPT in this study,to a size for the enhancement of the overall hardness level.
The effect of finer Fe-containing phas on the hardness increa can be supported from the data for powder samples in Figures 4(c)and 5(c),since no intermetallic pha exists,at least at the initial state.The powder samples show an initial sharp increa in hardness with impod strain.However,the plateau-like trend obrved after N =10revolutions ems to remble the saturation phenomena stated before for the bulk samples with the lower Fe contents less than 2pct.The overall lower hardness levels in the powder samples when compared to the bulk indicate th
at a large fraction of the Fe remains in the form of elemental particles from the initial mixtures of powders.英文动画片大全
Table II summarizes the mechanical properties of the bulk EXT+HPT,bulk EXT+ANN+HPT,and pow-der HPT samples procesd in this study for N =1and N =10revolutions.The maximum hardness values obtained from each of the curves in Figures 4and 5are prented in the first column for each type of samples.
TEM obrvations may be helpful for understanding the difference in the strengthening behavior between the EXT samples and the EXT+ANN samples shown in Figures 5(a)and (b).This difference
is prominent on the samples containing 5pct Fe,and thus,TEM micro-graphs are shown in Figures 6(a)and (b)from the two samples with 5pct Fe before HPT.It is apparent that the EXT sample consists of a finer structure than the EXT+ANN sample:finer dispersion of smaller inter-metallic particles in the fine grain matrix in the former samples when compared with the latter samples.It is found that the finer microstructures,in terms of particle size and dispersion including grain size,evolve more efficiently during the application of HPT.
Figure 7shows TEM micrographs from four repre-ntative samples after HPT.Figure 7(a)shows a bright-field image as well as a dark-field image of the Al-0.5pct Fe bulk sample without prior annealin
g,procesd for N =1revolution.In this condition,the average grain size was d %900nm,which corresponds to a hardness level of 41HV.Few dislocations are obrved within grains and the grain boundaries are well defined.However,the microstructure is not homogeneous:Smaller grains with the size of ~300nm are obrved in the upper left corner of the dark-field image,which was obtained from the diffracted beam indicated by the arrow in the innermost Al fundamental ring in the SAED pattern.Figure 7(b)shows a micrograph from a sample with the Fe content of 5pct after processing for N =10revolutions.A finer microstructure is appreci-ated,both from the abundance of diffracted beams with the form of rings in the SAED pattern and from the dark-field image.The average grain size is of d %240nm yielding the highest hardness level,134HV,in all measured samples.The formation of ill-shaped boundaries,which are characteristic of the samples procesd by vere plastic deformation,is also
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V i c k e r s M i c r o h a r d n e s s (H V )
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Fig.5—Plots of Vickers microhardness against equivalent strain for samples of (a )bulk EXT+HPT,(b )bulk EXT+ANN+HPT,and (c )powder HPT samples.
