High temperature mechanical and microstructural behavior of A356\15 vol% SiCp and A356 alloy

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Canadian Metallur`ical Quarterly\ Vol[ 26\ No[ 1\ pp[ 014Ð028\ 0887 Þ 0887 Canadian Institute of Mining and Metallurgy[ Published by Elsevier Science Ltd Printed in Great Britain[ All rights reserved 9997Ð3322:87 ,08[99¦9[99

Pergamon

PII ] S9997Ð3322"87#99996ÐX

HIGH TEMPERATURE MECHANICAL AND MICROSTRUCTURAL BEHAVIOR OF A245:04 VOL) SiCp AND A245 ALLOY H[ J[ MCQUEEN\$ M[ MYSHLYAEV\% E[ KONOPLEVA$ and P[ SAKARIS$ $ Mech[ Eng[\ Concordia University\ Montreal\ Canada H2G 0M7 % Baikov Institute of Metallurgy\ RAS 006 223 Moscow\ Russia "Received 02 November 0886# Abstract*A metal!matrix composite "MMC\ 04 vol) SiCp:A245 Al# and its matrix alloy were subjected to hot torsion over the range 299Ð439>C and 9[0Ð4[9 s−0[ Flow stresses of the A245 MMC were found to be much higher than A245 alloy at low temperatures but the di}erence was quite small at higher tempera! tures[ Flow stresses were found to depend on the strain rate through a sinh function and on temperature through an Arrhenius term with activation energies of 152 kJ:mol for the composite and 050 kJ:mol for the matrix ^ the increased value for the composite suggests that the SiC particles cause the matrix to undergo additional strain hardening[ The substructures in both materials increase in cell size and decrease in internal and wall density\ as temperature T rises and strain rate o¾ falls ^ the composite shows much greater and less uniform dislocation density to which the strengths of the two materials are related[ Dynamic recovery seems to be predominant in A245 ^ however\ dynamic recrystallization likely nucleates in the vicinity of silicon carbide particles in 04 vol) SiCp:A245 Al[ Ductility of the composite\ about 14) below that of the alloy\ rose by a factor of 3 between 399 and 499>C to become higher than many wrought alloy composites[ The low ductility of A245 was shown to result from linking up of the cracks nucleated at coarse Si particles\ whereas linkage of the decohesion voids at the SiC was associated with more plastic ~ow in the matrix which had much _ner Si particles than the bulk alloy[ Þ 0887 Canadian Institute of Mining and Metallurgy[ Published by Elsevier Science Ltd[ All rights reserved[ Resume*On a soumis le composite a matrice metallique "CMM\ 04 vol) SiCp:A245 Al# et l|alliage de sa matrice a une torsion a haute temperature entre 299 et 439>C et entre 9[0 et 4[9 s−0[ L|ecoulement plastique du CMM A245 etait beaucoup plus eleve que pour l|alliage A245\ a basse temperature\ mais la di}erence etait pluto¼t petite aux temperatures plus elevees[ L|ecoulement plastique dependait du taux de deformation par l|intermediaire d|une fonction sinh ainsi que de la temperature par l|intermediaire d|un terme d|Arrhen! ius\ avec des energies d|activation de 152 kJ:mol pour le composite et de 050 kJ:mol pour la matrice ^ la valeur plus elevee pour le composite suggere que les particules de SiC forcent la matrice a subir en plus un durcissement par ecrouissage[ La taille de cellule des sous!structures des deux materiaux a augmente et la densite interne et des parois a diminue a mesure que la temperature s|eleviat et que la vitesse de deformation\ se diminuait ^ le composite montre une densite de dislocation beaucoup plus elevee et beaucoup moins uniforme[ La resistance des deux materiaux est reliee a densite des dislocation[ La restauration dynamique semble predominer chez A245 ^ cependant\ des noyaux se forment probablement par recristallisation dynamique dans le voisinage des particules de carbure de silicium chez le 04 vol) SiCp:A245 Al[ La ductilite du composite\ environ 14) au!dessous de celle de l|alliage\ s|est elevee par un facteur de 3 entre 399 et 499>C\ surpassant ainsi celle de plusieurs composites des alliage corroyes[ La faible ductilite de A245 resultait de la liaison des criques nucleees aux grosses particules de Si\ alors que la liaison des lacunes de decohesion pres du SiC etait associee avec un ecoulement plus plastique dans la matrice\ qui avait des particules de Si beaucoup plus _nes qu|au coeur de l|alliage[ Þ 0887 Canadian Institute of Mining and Metallurgy[ Published by Elsevier Science Ltd[ All rights reserved[

INTRODUCTION Interest in particulate reinforced metal matrix composites stems from the high speci_c sti}ness and strength which can be pro! duced through combining a light alloy matrix such as aluminum and a reinforcement of high sti}ness and strength and low density such as silicon carbide ð0Ð3Ł[ As an alternative to powder metallurgy\ which requires hot pressure consolidation of mixed

 To whom all correspondence should be addressed[ 014

metal powders and ceramic particles\ particulate reinforced MMCs have been developed by incorporating the particles into the melt and casting in the conventional continuous manner ð0Ð 5Ł[ Although the strengths of particulate composites are usually inferior to composites with continuous _bers\ there are advan! tages ] they have isotropic properties\ can be fabricated by con! ventional metal working techniques such as extrusion\ forging and rolling and consequently are cheaper ð0Ð8Ł[ When strong particles are introduced into the matrix\ it is found that the elastic modulus\ strength and wear resistance increase linearly with the volume fraction ð0\ 4\ 6\ 09Ð02Ł[ Because the ductility

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H[ J[ MCQUEEN et al[ ] BEHAVIOR OF A245:04 VOL) SiCp AND A245 ALLOY

and fracture toughness generally decrease\ the maximum tol! erable volume fraction has been shown to lie between 04 and 29 vol) ð6Ł[ Deformation processing of MMCs is usually carried out at elevated temperatures where the ductility is high and the ~ow stress reduced\ although the ~ow stress remains signi_cantly higher than the matrix alloy ð4Ð6\ 8\ 09\ 03Ð10Ł[ The work hardening rate of the matrix is reduced primarily because of dynamic recovery "DR# which results in the formation of large subgrains with walls of low energy dislocation arrays ð4\ 5\ 06Ð27Ł[ However\ it is also possible that particle stimulated nucleation PSN of dynamic recrystallization "DRX# makes a substantial contribution ð4\ 5\ 00\ 06\ 15Ð25\ 28\ 39Ł[ Avoidance of matrix microcracks at particles and particle decohesion or cracking under reduced forming stresses is very important for sound products ð4Ð6\ 09\ 03Ð06\ 13\ 24\ 28Ł[ The particles in composites are su.ciently large that they do not interact with dislocations individually but\ being much harder than the matrix and not undergoing deformation themselves\ they force the matrix to undergo non!uniform shear thus causing additional strain hardening ð4\ 5\ 8Ł[ At temperatures near melt! ing\ the ~ow stress of the composite may fall below that of the matrix alloy and the strain rate sensitivity may increase[ These e}ects have been attributed to grain boundary sliding in _ne grained composites ð4Ð6\ 30Ð32Ł[ Powder based composites are usually hot worked following consolidation to improve the mechanical behavior by eliminating unmixed regions of reinforcement and matrix thus augmenting the reinforcement! matrix bond ð4\ 5\ 08Ł[ The distribution of the second phase in the matrix becomes more uniform as the amount of defor! mation is higher[ Through reduced stress\ elevated temperature improves matrix ~ow retarding decohesion at particles and rupture\ and reduces particle degradation ð0\ 2\ 09\ 06\ 13\ 22Ð 27Ł[ Such behavior in hot working indicates that it would be much more suitable than cold working for the fabrication of composites since the matrix has more ability to ~ow around the particles without damaging them ð4\ 8\ 03\ 06Ł[ Through torsion testing of A245:SiCp and A245\ the depen! dence of peak stress s and fracture strain of on temperature T and strain rate o¾ was found and expressed by a suitable consti! tutive equation ð12Ð15Ł[ Through microscopic examination\ the following features were de_ned ] distribution and size of Si and SiC particles\ cracking and decohesion at particles\ and the paths followed by both the main crack and auxiliary cracks[ The in~uences of the matrix constituents and the particles on the deformation mechanisms are explained\ as well as the relative ductilities of composite and bulk alloy[ Finally\ comparison is made of the hot workabilities of SiCp:A245 Al and SiCp:5950 Al composites\ and their matrix alloys[ A245 is a hypoeutectic AlÐ6[9) Si casting alloy that is streng! thened by about 5[4) Si particles\ which form as the eutectic solidi_es ^ there is some further strengthening from Mg solute and a little Mg1Si "about one third compared to 5950 "0[9 MgÐ 9[5 Si#\ which is also a common matrix alloy ð33Ł[ Its 19>C strength is only 59) that of heat treated 5950\ but the elon! gations are similar\ about 09) ^ at 269>C\ the tensile ductility is about 79)[ The A245 does not react with SiCp because of free Si providing a castable metal matrix composite[ However as an extrusion or forging\ it has potentially better creep resistance than a 5950 based MMC[

EXPERIMENTAL TECHNIQUES The SiCp:A245 composite and the bulk A245 matrix alloy "6[9 SiÐ9[24 MgÐ9[1 CuÐ9[1 FeÐ9[0 Mn# in the as!cast condition were provided by Dural Aluminum Composite Corp[ through Alcan Kingston Laboratories[ The 04 vol) SiC particles has an average size of 01 mm[ Testing was conducted on a computer! directed\ servo!controlled\ hydraulic torsion machine at three strain rates o¾ between 9[0 and 4[9 s−0 and four temperatures T between 299 and 439>C ð12\ 29\ 34Ł[ Torsional specimens with a gauge length L of 14[3 mm and diameter 1r of 5[21 mm were threaded into the stationary bar _xed to the torque cell at one end and inserted into a rectangular slot in the rotating grip at the other[ This arrangement allowed for easy alignment and heating up without accidentally straining the specimen[ The samples were twisted without interruption to fracture[ Heating was provided by a quadruple elliptical radiant furnace con! nected to a programmable controller which automatically varied power input to maintain test temperature[ Control by a thermocouple pressed against the gauge section at the _xed end permitted rapid temperature adjustment in relation to defor! mation heating[ The testing is controlled by a computer pro! gram and the data are acquired by the computer to keep them available for display "as torque G versus angle u#\ analysis and plotting ð12\ 29\ 34Ł[ Torsion is preferable to tension for composites because the low ratio of normal to shear stress reduces cracking[ In torsion\ constant o¾ can be applied over large strains o ^ however\ there is a linear gradient of o and o¾ from center to surface[ In conse! quence\ the following formulae are used to calculate a com! pensated von Mises equivalent stress s and equivalent surface strain o from the torque G and rotation u ð03\ 12\ 29\ 28\ 34Ł ] s  z2G"2¦m¦n?#:1pr2

"0#

o  ru:z2L

"1#

The strain rate sensitivity m is determined from the o¾ dependence of G "Fig[ 0#[ The strain hardening coe.cient n? is taken as zero at the peak of the curve\ which gives rise to a small error "½4)# at other points[ The specimens\ which were protected by argon\ were quen! ched in water within 09 s after straining ended[ To examine the microstructure and especially the cracking\ specimens were polished on transverse\ longitudinal "near axis# and tangential surfaces ð14Ð21Ł[ The tangential section shows elongation of the grains\ alignment of particles and cracks but not their depth[ On the longitudinal section\ the main crack edge can be inspected as well as branches and also additional cracks progressing from the surface[ The specimens were cut with a diamond saw and polished with diamond paste[ They were examined in the as! polished condition since the phases could be distinguished and enlargement of cracks and decohesions at particles were avoided[

EXPERIMENTAL MECHANICAL RESULTS Plots of torque versus strain rate "Fig[ 0# reveal similar behavior for reinforced and unreinforced materials as strain rate sensitivity varies from 9[996 and 9[924 at 299>C to 9[097

H[ J[ MCQUEEN et al[ ] BEHAVIOR OF A245:04 VOL) SiCp AND A245 ALLOY

016

In an almost inverse manner to s\ of increases with rising T\ most rapidly between 399 and 499>C "with a maximum of about 3[1 above 399>C for o¾  9[0 s−0# ð12\ 13Ł[ Unlike the composite\ the unreinforced alloy displayed maximum strain of 4[5 and 3[6 for 9[0 and 0[9 s−0 respectively at 439>C[ From 499Ð439>C\ of decreases for the composite but for A245 only at 4 s−0[ At 499 and 439>C\ of increases as o¾ decreases\ much more for the composite than for the bulk alloy "Fig[ 2"b##[ Below 399>C\ the ductility does not greatly vary with o¾ and is only slightly lower for the composite ^ being near 0\ it is much higher than in tension[ Here\ the composite ductility is about half that of the alloy alone\ except for 9[0 s−0 where it is only 09Ð19) less[ The constitutive analysis for the peak stress was carried out with the temperature and strain rate dependence of the ~ow stress ^ this can be expressed by the following equation which has a broader stress range than the creep power law ð4\ 5\ 8\ 12\ 29\ 26Ð28Ł ] A" sinh as#n  o¾ exp "QHW:RT#  Z

Fig[ 0[ Plot of log G versus log o¾ to determine the slope equaling the strain rate sensitivity m for A245:04 vol) SiCp and for A245 in order to calculate the stress[

and 9[012 at 499>C for SiCp:A245 and A245 alloy respectively[ Using eqns "0\ 1#\ stress!strain curves for the SiCp:A245 Al composite and the unreinforced A245 aluminum alloy were generated and are shown in Fig[ 1[ The ~ow stresses of the composite and the matrix alloy both decrease with increasing deformation temperature ð12Ł[ There is a progressive rise in ~ow stress when the strain rate is increased for all temperatures[ Similar behavior is exhibited for both composite and unre! inforced alloy such that up to 299>C\ they strain harden to failure[ A degree of ~ow softening to a steady state regime before fracture is observed at higher temperatures[ Although distinct peak stresses in the ~ow curves for both materials at 299>C may indicate dynamic recrystallization\ it is considered to be the result of deformation heating which is greater than at 399\ 499 or 439>C[ Alteration of phase distribution as a cause of this is unlikely here ^ however\ cracking and decohesion could be signi_cant in tests with low fracture strains of ð12\ 13Ł[ Vari! ation in crack initiation with vagaries of particle distribution especially at high o¾ and low T could account for the scatter in both op and sp[ However\ the A245 composite strength is approximately 14Ð49) higher than that of the matrix at 299>C\ but not as much at 399>C and even less at 499>C\ whereas the strength of 5950 composite is greater between 299 and 399>C ð12Ł[ In Fig[ 2"a#\ sp of alloy or of composite at 0 s−0 is seen to decline with rising T\ varying more slowly at high T ^ values for 9[0 and 3 s−0 vary in essentially the same way being lower and higher\ respectively[ The composite is much stronger than the alloy at low T but becomes only slightly stronger above 499>C[

"2#

where a"9[941 MPa−0# ð8\ 12\ 28Ł A\ n\ and QHW are material constants "R  7[20 J:mol K#[ In the plot of log "sinh as# versus log o¾ "Fig[ 3#\ lines of constant T are drawn parallel guided by the high T data because of the low T scatter especially for the MMC ^ the values of n being the reciprocals of the slopes\ about 2[1 for the composite and 1[8 for the alloy[ In the graph of log "sinh asp# against 0:T\ the slopes s of the parallel constant o¾ lines are used to calculate the activation energies "QHW  1[2 R n s# ] 152 kJ:mol for SiCp:A245 and 050 kJ:mol for A245[ With QHW determined\ it is possible to plot log "sinh as# against log Z in order to draw the data into single lines with slopes n and intercepts A "Fig[ 3#[ The present constants n and QHW have been corrected from previously reported values ð12\ 14\ 22\ 23Ł[ The Zener!Hollomon parameter\ Z\ combines the two control factors[ For A245\ QHW "¼050 kJ:mol# is relatively close to those of Al "½049 kJ:mol# and of 5950 ð12\ 28Ł\ since neither alloy contains high levels of solute[ For the composite\ QHW "¼152 kJ:mol# is 49) higher than the bulk alloy re~ecting the greater decrease in s from 199 to 439>C ^ it is similar to that of 5950 −04 vol) SiC ð12\ 22\ 23Ł[ The high apparent activation energy arises from the increase in dislocation density as T declines\ due to the di}erence in thermal expansion coe.cient "DCTE# ð01\ 02Ł ^ more dislocations have been generated at lower T and there is less opportunity for any recovery[ The ductility of rises as o¾ declines and as T rises\ indicating that cracking declines due to stress concentration as sp decreases ð12\ 13\ 24\ 28Ł[ With increasing T at 439>C\ the slight decrease for the composite and the bulk alloy can be ascribed to the approach to melting near 459>C[ Fracture initiates when defor! mation heating produces localized melting of segregates[ In remarkable contrast to 5950 composite and alloy\ the ductility of the A245 composite is almost equal to the alloy ð12\ 13Ł[ The A245 MMC is much more ductile than the 5950 MMC\ even though the A245 alloy is much less ductile than 5950[ The causes will be discussed after presentation of the microstructural observations[ EXPERIMENTAL MICROSTRUCTURAL RESULTS The microstructures of the unetched alloy are presented in Fig[ 4 and for the composite in Fig[ 5[ The most notable feature

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H[ J[ MCQUEEN et al[ ] BEHAVIOR OF A245:04 VOL) SiCp AND A245 ALLOY

Fig[ 1[ The torsional equivalent stress versus equivalent strain curves for "a# A245:04 vol)SiCp\ and "b# A245 exhibits declining sp and mounting of as T increases and o¾ decreases[ At high T\ a steady state regime develops with a slight decline related to deformation heating[

is the di}erence in the size and cellular distribution of the facetted Si particles ð13\ 14Ł[ In the alloy\ there are many large Si particles\ about 8Ð01 mm in the long direction "½1 mm thick# and small ones\ about 1Ð3 mm[ The particles outline proeutectic dendrite regions ^ there is some evidence of very _ne grains in the eutectic regions[ In the composite\ the SiC particles are about 09 mm when equiaxed\ but some are as long as 19 mm ^ however\ the Si particles are small\ 9[4Ð1 mm[ Evidently\ the solidi_cation rate for the composite was much faster than for the alloy or it has been modi_ed by strontium[ The grain size or dendrite spacing\ as inferred from the particle spacing\ is much _ner\ about 1Ð4 mm\ in comparison to 09Ð19 mm in the alloy[ After deformation with rising T and of\ the alloy shows increased alignment of the particles and limited evidence of particle break up ð13\ 24Ł[ Moreover\ there is some evidence of localized banding with di}erent quantities of particles[ In the composites\ there is much less evidence of particle alignment and no evidence of banding[ The particles do not seem to have changed markedly in aspect ratio\ although there may be some slight decrease in average size[ There are not many cracked particles\ if cracks are de_ned as parallel particle interfaces with little or no matrix between them[ There may be slightly more cracked particles "¼09)# at 299>C after a low of\ than at 439>C after high of[ The many instances of groups of particles touching to enclose triangular volumes of matrix\ indicate that they are agglomerates from the solidi_cation process[ Their redis! tribution is only a little improved after high of at 439>C\ 9[0 s−0[ In the alloy "Fig[ 6#\ the fracture surface was quite irregular exhibiting sharp angular features at 299>C and curved ones at

499>C "Fig[ 6"a\e##[ The main fracture surface lies in the trans! verse plane of maximum shear stress even though the normal stress is nominally zero ð13Ł[ Adjacent to the fracture\ there were additional short cracks or tears with similar features[ Near the fracture\ the Si particles gave the appearance of being alig! ned parallel to the main crack direction[ Examination at high magni_cations con_rmed what was evident even at low\ that the crack path went through particle!matrix interfaces ð13\ 24Ł[ Decohesion was observed at many particles\ commonly at larger ones ^ the frequency of cracked particles was less than 4) of decohesions[ At 299>C "Fig[ 6"a##\ numerous small cracks were observed at the surface ^ these were less frequent at higher T[ At 499 and 439>C\ some cracks seemed to lie along grain bound! aries "GB# with evidence of triple junctions[ In one transverse section cracks were observed in several planes parallel to the axis and with a common intersection "Fig[ 6"b## ^ while such planes are subject to high shear stresses "as in the transverse#\ the cracks could be casting defects[ At high T the surface was somewhat dimpled but not to such an extent as in the composite[ In the composite "Figs 7 and 8#\ the fracture path appears more irregular than in the alloy and gives more impression of being dimpled[ There is also less branching of the main crack[ While the _ne Si particles give rise to little _ssure initiation\ the main and auxiliary cracks frequently lie along the particle matrix interfaces\ with little evidence of cracked particles ð13Ł[ At high T\ there is evidence of crack initiation at grain bound! aries and triple junctions[ Short cracks were found at clusters of _ne Si particles\ but ductile tearing in dendrite regions was more signi_cant[ At both low and high temperatures\ the _s! sures were in contact with particles along less than a quarter of

H[ J[ MCQUEEN et al[ ] BEHAVIOR OF A245:04 VOL) SiCp AND A245 ALLOY

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Fig[ 2[ The dependencies for composite and alloy of both sp and of "a# on T and "b# on o¾ have many similarities[ "a# Despite considerable di}erences in sp at low T\ the low of are almost the same\ whereas equally reduced sp is associated with variously improved of at high T[ "b# The ductility decreases as o¾ rises for the A245 MMC\ but clearly only at 439>C for the A245 ^ the curves are higher as T increases except for a slight decline for the composite at 439>C[ For A245 alloy\ the peak and minimum at 0 s−0 for 399 and 499>C are probably random scatter ^ lines more consistent with the general trend have been estimated[

Fig[ 3[ The constitutive analysis shows that the sinh function relates s to log o¾ quite well at high T "a# and to 0999:T at low o¾ "b#[ Despite considerable scatter\ where s is high\ the relation to log Z "c# is satisfactory for both composite and matrix alloy[

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H[ J[ MCQUEEN et al[ ] BEHAVIOR OF A245:04 VOL) SiCp AND A245 ALLOY

Fig[ 4[ Microstructures "unetched# of A245 exhibit a bimodal dis! tribution of Si eutectic ] "a# 399>C\ 4 s−0 of ¼ 0[4\ some _ssures aligned with particles ^ and "b# 439>C\ 0 s−0\ of ¼ 3[5\ some debonding at particles[ "Magni_cation 499×#[

the length[ The dimples likely indicated where particles had been ^ they were possibly in the matching surface or had fallen out during polishing "Fig[ 7#[ However\ in a separate long _ssure found at 439>C "Fig[ 8# the crack runs mainly through the matrix with limited evidence of lost particles and is much smoother than at 299>C[ In addition to decohesions with exten! sions at perhaps one particle in a hundred\ clearly the larger ones\ short cracks were often seen in the regions where there were clusters of Si particles[ Because of the _ne scale of the Si particles\ one could not see obvious cases of decohesion or the association of an individual particle with the crack\ as could be seen in the alloy at the same or lower magni_cation[ The examination by transmission electron microscopy revealed the formation of a fairly uniform subgrain structure in the A245 alloy ^ this became more recovered as T rose and o¾ decreased\ although there were constraints from the particles "Fig[ 09#[ A TEM survey of A245 showed that subgrains had the following sizes for 9[0 and 4 s−0 respectively ] 299>C\ 9[60 and 9[68 mm ^ 399>C\ 1[23 and 0[35 mm ^ 499>C\ 4[38 and 2[04 mm ^ 439>C 5[90 and 5[00 mm ð14\ 15Ł[ At 299>C\ the cells are elongated and deformation bands are observed[ Along with smaller size at lower T and higher o¾\ the dislocation densities\ both in the subgrains and in their walls\ are higher and less

Fig[ 5[ Microstructures unetched of A245¦04 Vol) SiCp exhibit large SiCp particles and small Si eutectic particles ] "a# 399>C\ 4 s−0 of ¼ 0[2 ^ and "b# 499>C\ 9[0 s−0\ of ¼ 3[1[ The fraction of broken particles or debonding is very low and there is little change in agglomeration as of rises\ but Z and s decline[ "Magni_cation 499×#[

uniform than in Al and even in AlÐ9[54 Fe with 9[1 mm particles ð28Ł[ In addition to the large Si particles\ there were _ne spherical Mg1Si particles usually associated with dislocations ^ they are most noticeable at 399>C which enhances growth but limits solubility[ In A245¦04 vol) SiCp\ the dislocation dis! tributions are much more inhomogeneous "Figs 00 and 01# because of the large rigid particles and the high initial DCTE dislocation densities ð01\ 02Ł[ In some regions of the MMC\ the densities become so high that DRX grains appear to have for! med "Figs 01 and 02# ^ however\ these did not grow to com! pletely replace the original grains ð14Ð16Ł[

DISCUSSION A peak\ according to the theory of hot deformation\ followed by considerable work softening in the ~ow curve may be indica! tive of dynamic recrystallization ð35Ł[ By contrast\ for aluminum alloys that readily undergo dynamic recovery\ there is no peak in the sÐo curve except as a result of microstructural changes such as precipitate coarsening or of deformation heating ð28Ł[

H[ J[ MCQUEEN et al[ ] BEHAVIOR OF A245:04 VOL) SiCp AND A245 ALLOY

020

Fig[ 6[ Optical micrographs of unetched longitudinal sections of A245 showing the Si eutectic particles and the cracks "all torsioned at 0[9 s−0# ] "a# 299>C\ main fracture surface\ decohesion at large particles\ aligned _ne particles ^ "b# 399>C\ of  1[3\ transverse section showing longitudinal cracks possibly related to a casting defect ^ "c# 499>C\ of  2[4\ main and auxiliary fractures\ somewhat dimpled appearance ^ "d# 439>C\ of  3[5\ Si particles adjacent to fracture path or to decohesion and extended cracks possibly along GB[

Although peaks are seen in Fig[ 1 for the composite and matrix alloy\ it is unlikely that DRX is the predominant restoration mechanism but rather DRV ð36Ł[ For the alloy\ the gradual decline during steady state due to deformation heating is con! sistent with DRV ð28\ 36Ł[ The softening is unlikely due to coalescence of particles as in some precipitation alloys ð28Ł because of the stability of the eutectic particles\ for which there is evidence in this microstructural study[ The observations of unetched specimens for the cracking did not determine whether the grains were elongated or not[ The Al rich phase has dilute solute so is likely to undergo a high level of DRV which has been con_rmed by electron microscopic analysis ð14\ 15Ł although there have been reports of DRX in MMCs[ In the work of Tuler et al[ ð05\ 07Ł\ DRX was observed in SiCp:5950 Al under hot compression[ They found at 499>C\ 4[9 s−0\ a completely recrystallized structure for a 19 v:o composite but for 09 v:o\ recrystallization was just beginning in the vicinity of the SiC particle clusters[ They concluded that DRX through particle enhanced nucleation occurs at higher temperatures and strain rates[ This agrees reasonably well with Humphreys et al[ ð6\ 06\ 39Ł that particle enhanced nucleation is dependent on the

volume percentage of particles ^ however\ the results pointed primarily to static recrystallization[ Although some DRX nuclei were observed presently\ there was no evidence that they spread throughout the volume as was con_rmed in many other studies ð16Ð25Ł ^ this leaves the doubt that the reports above ð05\ 07Ł should have been of static recrystallization[ It is apparent from Fig[ 0 that no superplastic behavior is observed for either 04 v:o SiCp:A245 Al or A245 Al alloy ^ nor was it observed in 04 v:o SiCp:5950 Al[ For a material to behave superplastically\ its strain rate sensitivity m must be greater than 9[2 ð4\ 5\ 7\ 06\ 30Ð32Ł[ The value of m for A245 is in the range "³9[03# generally observed for other aluminum alloys and for the present composite\ m is similar to its matrix alloy[ Pickens et al[ ð03Ł determined similar m values for composites and their matrix alloys ] at 314>C both 19 v:o SiCp:6989 Al and 6989 had equal m values of 9[05[ Superplastic behavior is possible if recrystallization has su.ciently re_ned the grain size of the matrix ð5\ 7Ł[ Mahoney et al[ ð31Ł demonstrated that normal micrograin superplasticity can occur in aluminum MMCs containing 09 v:o SiC particles of 4 mm diameter\ when these are processed to a _ne grain size[ It was found that ideal

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H[ J[ MCQUEEN et al[ ] BEHAVIOR OF A245:04 VOL) SiCp AND A245 ALLOY

Fig[ 7[ Optical micrographs of unetched longitudinal sections of A245:04 vol) SiCp showing SiC particles and the cracks "all deformed at 0[9 s−0# ^ "a# 299>C\ main crack very irregular linking!up decohesions at SiC particles ^ cracks in matrix ^ "b# 399>C\ main fracture fairly smooth ^ bordering on some of SiC particles ^ "c\ d# 499>C\ of  1[7\ fracture surface strongly dimpled on scale of SiC particles with auxiliary _ssures showing decohesion[

Fig[ 8[ Short crack entering from edge of A245:04 vol) SiCp specimen deformed at 439>C\ 0 s−0 to of  1[7\ showing linking up of decohesions at particles by crack progressing through regions of _ne Si particles at edge of dendrites "bar 099 mm#[

H[ J[ MCQUEEN et al[ ] BEHAVIOR OF A245:04 VOL) SiCp AND A245 ALLOY

Fig[ 09[ TEM micrographs of hot torsioned A245 alloy exhibiting DRV substructures interacting with small Mg1Si and Si particles ] "a# 299>C\ 4 s−0\ small subgrains in two grains ^ "b# 299>C\ 4 s−0\ neat dislocation arrays in SGB ^ "c# 299>C\ 9[0 s−0\ subgrains with Mg1Si particles ^ "d# 399>C\ 4 s−0\ subgrains with heavy interior network ^ "e# 399>C\ 9[0 s−0\ polygonized subgrains with Si precipitates ^ "f# 499>C\ 4 s−0\ large subgrains with a complex wall ^ and "g# 499>C\ 9[0 s−0\ subgrain boundary with interior network Mg1Si particles just to left ^ "h# 439>C\ 4 s−0\ large subgrains with interior network and Mg1Si particles[

022

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H[ J[ MCQUEEN et al[ ] BEHAVIOR OF A245:04 VOL) SiCp AND A245 ALLOY

Fig[ 00[ Transmission electron microscopy illustrating recovered sub!structures in A245:04) SiCp after hot torsion ] "a# 299>C\ 4 s−0\ heavily dislocated subgrains near two Si particles ^ "b# 299>C\ 4 s−0\ some neat polygonized SGB ^ "c# 399>C\ 4 s−0\ small subgrains around a Si particle ^ "d# 399>C\ 9[0 s−0\ subgrain structure with high interior density near a large SiC particle and a small Si particle ^ "e# 499>C\ 4 s−0\ large subgrains ^ and "f# 439>C\ 9[0 s−0\ large subgrains around a Si particle[

H[ J[ MCQUEEN et al[ ] BEHAVIOR OF A245:04 VOL) SiCp AND A245 ALLOY

024

Fig[ 01[ TEM evidence of high!subgrain misorientations in A245:04) SiCp arising from particle constraints in hot torsion at 399>C\ 0 s−0^ "a# bright _eld revealing di}use subgrains subtending di}ractions of 4Ð09>^ "b# dark _eld from large head of arc revealing related subgrains in a block ^ "c# enlargement of complex dislocation wall "in b# and "d# dark _eld from narrow end of the arc[

strain rates were considerably higher than for conventional superplastic aluminum alloys[ Even with _ne grains\ AlÐSiC composites undergo rapid formation of voids which nucleate near or at the SiC particles and which can lead to considerable reduction in ductility ð06Ł[ The value of m was measured after deformation at 499 or 439>C for 5950:09) Al1O2 which also has ~ow curves with an extended steady state ^ although m was near 9[2\ small equiaxed dislocation!free grains never formed ð32Ł[ A high QHW for the A245 composite can be attributed to the ~ow stress being similar at 499>C but higher at 299 and 399>C than A245 Al[ As a result\ the Arrhenius slope s increases from 2[5 to 3[2\ thus making the activation energy higher[ At high T\ in contrast to the alloy\ partial DRX from PSN ð6\ 06\ 22Ð25Ł arises from the SiC particles generating surrounding regions of _ne cells[ In previous reports on similar composite materials\ both Tuler et al[ ð05Ł and Pickens et al[ ð03Ł chose to use the power law in their analyses for which plots of ln s versus 0:T were not linear[ They concluded that no single activation energy could be computed because the deformation process was much more complex than in creep[ However\ the sinh relationship\

gave linear behavior in the Arrhenius plot "Fig[ 3# such that a single QHW can be calculated ð12Ð14\ 22Ð24Ł[ A245 Al alloy exhibited a slightly larger ~ow stress than 5950 Al ^ similarly for the composite systems\ 04 v:o SiCp:A245 Al is generally 4Ð 19) stronger than 04 v:o SiCp:5950 Al[ QHW of 152 kJ:mol for 04 v:o SiCp:A245 Al is higher than 122 kJ:mol for 04 v:o SiCp:5950 Al ð12\ 22Ð24Ł[ However\ 04 vol) SiCp:5950 Al revealed a lower n value from 4[8 to 1[6 but a higher s from 0[6 to 3[4 resulting in greater QHW compared to its matrix alloy[ The value of QHW for A245 Al is lower than 5950 Al "050Ð077 kJ:mol#[ The constitutive constants indicate that the alloy "050 kJ:mol# behaves similarly to Al "Q\ "w  049 kJ:mol# and that the composite with QHW  152 kJ:mol is subject to constraints on DRV as T declined[ Activation energies for alloys commonly increase with solute or precipitate content as the operation of DRV becomes more complex ð28Ł[ Increases in activation energy from 049 to 154 kJ:mol with rise in Mg and impurities have been reported ð37Ł[ The strengthening of the A245 material is related primarily to the silicon particles and to a lesser degree

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H[ J[ MCQUEEN et al[ ] BEHAVIOR OF A245:04 VOL) SiCp AND A245 ALLOY

to the Si dissolved in the Al matrix[ It is known that Mg in solution causes signi_cant strengthening to the Al\ much more than does the Si in solution ð28Ł[ The Si particles are su.ciently large and hard that they might cause PSN which has been found when 0) Mg solute is present but not in commercial purity Al ð6\ 06\ 39\ 31\ 36Ł[ The high level of DRV can be inferred from the strength at 399>C\ 0 s−0 ] the 59 MPa falls between 39 MPa for commercial Al and 69 MPa for AlÐ9[54 Fe with Al2Fe eutectic rods of about 9[1 mm diameter ð28\ 38\ 49Ł[ In the AlÐ Fe alloy the particles strongly stabilize the substructure up to 299>C but above 399>C the dislocations easily bypass the particles[ The matrix in the composite has _ner Si particles\ so it is likely to be even more similar to AlÐFe than the bulk alloy[ The low level of ductility exhibited by the alloy at 299 and 399>C compared to Al\ AlÐMg and AlÐMgÐSi alloys ð03\ 12\ 13\ 28Ł is undoubtedly the e}ect of the Si particles[ Ductility of the A245 matrix alloy is much lower than 5950 reaching only a maximum fracture strain of 4 at 499>C compared to about 19 at 399>C ð03\ 12Ł ^ however\ of of 5950 declined substantially to the point where it was lower than A245 as the temperature approached that for melting[ Decohesion takes place at the interface of the eutectic particles due to the high stresses and tearing takes place in the Al!rich phase constrained by _ne Si particles and solute ð13\ 24\ 28Ł[ As sp declines markedly due to DRV with rising T and falling o¾\ the ductility rises rapidly ^ the marked rise in AlÐ9[54 Fe takes place between 299 and 399>C as a result of the earlier enhancement of DRV associated with the lower density of particles\ which\ moreover\ are so small that decohesion is much less frequent ð37Ð49Ł[ In agreement with the present results\ an AlÐ4[9 Si alloy exhibits rises in tension ductility from 299>C to a maximum at 264>C at 7 s−0 and from 149 to 214>C at 9[1 s−0 ð00Ł[ The formation of voids ceased in these ranges somewhat below the present one due to _ner Si particles "0Ð2 mm#[ The rise in of could result from DRX ^ in Cu\ gÐFe or Ni\ the migrating GB isolate intergranular cracks so that they become blunted ð35\ 40Ł[ This hypothesis su}ers from two problems ] "i# GB migration is restricted by the particles "DRX does not progress# ^ and "ii# the cracks originate at particles and not at triple junctions so cannot be isolated[ The role of the Si particles in the A245 is clear since deco! hesion is seen at many of them away from the main fracture[ In longer _ssures\ the crack path clearly lies along particle! metal interfaces[ The failure occurs by linking up the mic! rocracks through tearing with limited plastic ~ow at 299 and 399>C\ but more extensive ~ow with dimple formation due to necking of the interparticle regions at higher T ð13Ł[ There is also evidence of some di}usional GB pore formation or _ssuration at 439>C which likely accounts for the decrease in ductility ð24\ 28Ł[ The fracture strain of A245 composite was 19 to 29) less than that of its matrix alloy but is almost double that of the 5950 composite[ In the composite\ the SiC particles undergo decohesion and failure progresses by the linking up of such microcracks ð24Ł[ However\ the matrix responds in a relatively more ductile manner by necking particularly in the regions of the primary Al dendrites[ While the _ne Si particles give rise to little _ssure initiation\ the fracture appears to propagate in a less ductile manner through regions of high concentration of _ne Si particles that lie between adjacent SiC particles in the

interdendritic regions[ The SiCp:A245 composite exhibits remarkably high ductility relative to A245 since one would think that the SiC particles would make the e}ects of the Si particles worse[ In comparison to 04 vol) SiCp:5950\ the MMC ductility is higher and is also much more similar to its matrix than in the 5950 case where alloy ductilities are of the order of 19 ð03\ 12\ 13Ł[ The addition of SiC to the 5950 Al alloy considerably reduces the ductility because the particles initiate cracking which the matrix strengthened by the Mg and Si solute cannot adequately inhibit ð03Ł[ The present e}ect may be related to the _ve fold reduction in Si particle sizes of the matrix compared to those of the alloy in bulk ð13Ł[ The matrix behaves in a fairly ductile manner as can be inferred from the more dimpled appearance of the failures[ The substructures observed by TEM in the A245 alloy reveal a uniformity and a level of DRV which is somewhat reduced compared to Al because of the presence of the eutectic and precipitate particles "Fig[ 09#[ The sizes listed agree closely with the sizes measured in AlÐ9[54 Fe alloy where eutectic rods of Al2Fe had a marked e}ect on subgrain stability below 399>C ð38\ 49Ł[ The reduced level of DRV at 299>C contributes to limited ductility but cracking is enhanced by stress con! centrations near eutectic particles[ The heterogeneity of substructure in A245:04) SiCp makes it necessary to consider three categories ð22Ð25Ł[ Subgrain regions are similar in appearance to those in the bulk alloy and vary similarly with changes in T and o¾ "Fig[ 00#[ However\ the subgrains are always smaller and less polygonized than those in A245 itself[ This behavior is similar to that of 5950 with 09 to 19) of either Al1O2 or SiCp ð16Ð21Ł[ The range of mis! orientations is demonstrated by Fig[ 01[ The second type are regions of high dislocation density with little evidence of cellular development\ such substructure is not observed in the alloy itself and becomes more prevalent with increasing particle frac! tion and with declining T ð18Ð23Ł[ The third type seems to develop from the second in so far as cellular features develop with an appearance of high misorientation boundaries as con! _rmed by selected area di}raction patterns "Fig[ 02#[ These regions could be considered as nuclei of DRX grains which are not observed in the alloy itself ð22Ð25Ł[ These became more numerous as T rises ^ however\ they do not appear to grow to engulf signi_cant regions in the composite as usually occurs quite quickly in classical DRX[ The partial DRX contributes to restoration and declining ~ow stress but is not su.cient to cause rapid work softening as in classical DRX ð35\ 40Ł[ The above types of substructures have also been observed in 5950\ 6964 and 1507 alloy matrices ð16Ð27Ł[ The reason for their development is associated with the constraints exerted by the large angular rigid ceramic particles which induce additional plastic ~ow and dislocations in their surroundings[ Certain dis! tributions of particles relative to the external stresses applied can also lead to shear zones in some particle!free regions[ Finally\ due to the DCTE of particles and matrix\ new dis! locations are introduced whenever the composites are altered in temperature such as being preheated for deformation with the e}ect being greater for higher T although static recovery causes some reduction during T equilibration ð8\ 22\ 23Ł[ The long steady state strain at 499 and 439>C has some charac! teristics of superplastic deformation[ However at the strain rates employed\ there is considerable generation of dislocations and

H[ J[ MCQUEEN et al[ ] BEHAVIOR OF A245:04 VOL) SiCp AND A245 ALLOY

026

Fig[ 02[ In A245:04) SiCp deformed by hot torsion\ illustrating high dislocation densities associated with SiC particles "a# 399>C\ 9[0 s−0\ heavily dislocated interface with SiC particles ^ "b# 499>C\ 0 s−0\ high dislocation density with some cellular patches ^ "c# 439>C\ 0 s−0\ highly dislocated region with two DRX nuclei of high misorientation "patterns for A and B above# ^ and "d# 439>C\ 0 s−0\ planar defects in SiC particle

no general development of new _ne grains[ Similar behavior is found in the torsion of other MMCs ð12\ 16Ð20Ł and the ques! tion gave rise to strain rate change tests in 5950:09) Al1O2 in the interval 09−3Ð09−0 s−0 after a prestrain of 1 ð32Ł[ The strain rate sensitivity measured was slightly above 9[2 indicative of superplastic tendencies but there was no evidence of devel! opment of strain free equiaxed grains as observed in AlÐLi alloys in similar circumstances ð41Ł[ The development of high density substructures leads one to a comparison with cold working[ The observations of Hansen and collaborators ð42Ð45Ł on high strain rolling shows sub! structures of much greater regularity because of the simple plane strain deformation path\ the transverse plane "normal to rolling plane and direction# being equivalent to the tangential

plane in torsion where the grains appear as thin helicoids becoming progressively normal to the torsion axis ð40Ł[ In the composites the initial DCTE dislocation distributions\ the indi! vidual particle constraints and the additional e}ects of clusters such as shear zones superimpose a disorganized arrangement on the simple torsion strain pattern[ In consequence there is no formation of cell blocks with microbands which at high strain rotate into the rolling plane as boundaries of layers of cells similar in size and orientation[ The original dendrites do become elongated but internally are subjected to more diverse in~uences than the normal Taylor constraints in a single phase polycrystal ð42Ł[ Furthermore the elevated temperature and the temporal inhomogeneity in local stain allow reorganization of the dense dislocation tangles into high misorientation cells which do not

027

H[ J[ MCQUEEN et al[ ] BEHAVIOR OF A245:04 VOL) SiCp AND A245 ALLOY

have a chance to grow substantially due to the high local strain rates and particle inhibitions[

CONCLUSIONS The ~ow stress decreases uniformly with increasing defor! mation temperature for both composite and matrix alloy[ The ~ow stress for 04 v:o SiCp:A245 Al is generally higher than A245 with a quite small di}erence at higher temperatures ^ yet at 499>C\ they remain about 49) stronger than Al[ The constants in the sinh!Arrhenius constitutive equation indicate that the 04 vol) SiCp:A245 has higher temperature depen! dence of the stress than that of the alloy which is only slightly higher than Al and is probably related to dynamic recovery[ The ductility of the composite is 19Ð29) lower than the alloy for the high temperature range[ Although the ductility of A245\ due to decohesion at the large Si particles\ is considerably lower than that of wrought Al alloys ^ that of A245 MMC is higher than 5950 matrix composites due to increased dynamic recov! ery[ In the composite\ decohesion occurs at SiC particles ^ the matrix exhibits considerable plastic ~ow because of the very _ne Si particles that lead to a dimpled fracture surface at high temperatures[ The cracking is mainly initiated at particle!matrix interfaces with the fraction of broken particles remaining small[ The microstructure of the matrix is very heterogeneous[ In about a third of the volume\ there are subgrains which are not as well!recovered as in the alloy itself[ In the remainder\ there are dense dislocation arrays in about half of which DRX nuclei are observed[ As temperature rises\ the subgrains become more recovered and widespread and DRX nuclei proliferate but do not grow over substantial regions of the matrix[ Acknowled`ements*The authors wish to acknowledge strategic fund! ing from the NSERC of Canada which made this project possible[ Additional assistance came from FCAR de Quebec and the Russian Foundation for Fundamental Research[ A NATO Linkage Grant facili! tated collaboration[

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