Microstructural Development during Hot Working of Mg-3Al-1Zn
A.G.BEER and M.R.BARNETT
The microstructural evolution is examined during the hot compression of magnesium alloy
AZ31for both wrought and as-cast initial microstructures.The in?uences of strain,tempera-
ture,and strain rate on the dynamically recrystallized microstructures are assessed.Both the
percentage dynamic recrysallization(DRX)and the dynamically recrystallized grain size were
found to be sensitive to the initial microstructure and the applied deformation conditions.
Lower Z conditions(lower strain rates and higher temperatures)yield larger dynamically re-
crystallized grain sizes and increased percentages of DRX,as expected.The rate with which the
percentage DRX increases for the as-cast material is considerably lower than for the wrought
material.Also,in the as-cast samples,the percentage DRX does not continue to increase toward
complete DRX with decreasing Z.These observations may be attributed to the deformation
becoming localized in the DRX fraction of the material.Also,the dynamically recrystallized
grain size is generally larger in as-cast material than in wrought material,which may be
attributed to DRX related to twins and the inhomogeneity of deformation.Orientation maps of
the as-cast material(from electron backscattering di?raction(EBSD)data)reveal evidence of
discontinuous DRX(DDRX)and DRX related to twins as predominant mechanisms,with
some manifestation of continuous DRX(CDRX)and particle-stimulated nucleation(PSN).
DOI:10.1007/s11661-007-9207-5
óThe Minerals,Metals&Materials Society and ASM International2007
I.INTRODUCTION
R EDUCING the weight of vehicles,for increased fuel e?ciency,is a high priority for the automotive industry. Due to its low density,magnesium is a potential material for a range of automotive components.Most of the consumption of magnesium alloys by the automotive industry has been in the form of die castings.[1]However, wrought magnesium products have the advantage over castings of higher strength and ductility,and so are more suited to structural applications.
Wrought magnesium alloys can be deformed at elevated temperatures using primary fabrication meth-ods such as rolling,extrusion,and forging,but as the workability of the material is limited,production rates are slow and hence the?nal product is comparatively expensive.Deforming magnesium and its alloys at elevated temperatures is of great metallurgical impor-tance because not only is the workability improved but also the?nal grain size and,to a great extent,the?nal properties of the material are altered.The operation of dynamic recrystallization(DRX)during hot deforma-tion of magnesium is of particular importance,because it reduces the?ow stress during deformation and controls the?nal grain size.[2]Therefore,to be able to successfully control the microstructural evolution dur-ing bulk working operations,an understanding of hot deformation behavior of magnesium,particularly the operation of DRX,is essential.
Magnesium has a high stacking-fault energy and it might thus be expected to dynamically soften by dynamic recovery(DRV)instead of DRX.However, the early work by Humphreys and co-workers[2,3] showed that DRX was in fact an important mechanism during the high-temperature deformation of magne-sium.The presence of DRX was attributed to the constraints imposed by the lack of easily activated slip systems of magnesium,rather than its stacking-fault energy.The operation of DRX in magnesium may also be linked to its high grain boundary di?usion rate.[4] The literature presents a complex picture of the mechanisms by which DRX operates in magnesium.It appears that di?erent types of DRX take place under di?erent deformation conditions,e.g.,References5,6, and7.Twinning also seems to play a signi?cant role in the nucleation of dynamically recrystallized grains,e.g., References5,8,and9.
The mechanism of conventional discontinuous DRX (DDRX)has been identi?ed in magnesium alloys by a number of workers.[3,5–7,10–12]This mechanism involves the development of high-angle grain boundaries via the nucleation and growth of new grains.This typically initiates at high-angle boundaries:original grain bound-aries,the boundaries of dynamically recrystallized grains,or boundaries created during straining.[13]The bulging of grain boundaries is frequently observed prior to DRX,and it is assumed that a mechanism closely related to strain-induced boundary migration,which is observed during annealing of cold-worked metals, operates.[13,14]
The continuous mechanism of DRX(CDRX)has been identi?ed in aluminum alloys(e.g.,References15 and16)and also in some magnesium alloys.[2–7,11,12,17–19]
A.G.BEER,Research Academic,and M.R.BARNETT,QEII Research Fellow,are with the CRC for Cast Metals Manufacturing (CAST),School of Engineering and Technology,Deakin University, Geelong VIC3217,Australia.Contact e-mail:aiden.beer@https://www.doczj.com/doc/043663174.html,.au Manuscript submitted:June30,2006.
Article published online July13,2007.
This mechanism di?ers from that of conventional DRX in that the development of high-angle grain boundaries during hot deformation does not involve the nucleation and growth of grains at pre-existing boundaries.Due to the high e?ciency of DRV,new grains are formed progressively within the deformed original grains from the continuous increase of subgrain boundary misorien-tations.[20]
The in?uence of twinning on the dynamically recrystallized behavior of magnesium is important in coarse-grained alloys.Many studies have reported the nucleation of dynamically recrystallized grains at twins,e.g.,References2,5,9,11,17,and21through 23.The in?uence of twinning on DRX has been shown to be of importance for other hcp metals as well,particularly titanium.[24,25]Sitdikov et al.[8,12] observed DRX associated with twinning in pure magnesium and suggested that the nucleation of ‘‘twin’’DRX occurred by the intersection of primary twins(whereby a crystallite is formed,bordered by twin boundaries),by double twinning,and by the subdivision of primary twins by transverse low-angle boundaries(which are transformed to random high-angle boundaries upon further straining).Myshlyaev et al.[26]also observed the nucleation of dynamically recrystallized grains where twins intersected with grain boundaries.
For enhanced mechanical properties,it is often desirable to obtain the?nest grain size possible.Know-ing the in?uence that the deformation conditions have on the dynamically recrystallized grain size is important for achieving?ne grain sizes.When DRX(continuous or discontinuous)is operating,the size of the new grains is characteristic of the deformation conditions and is generally independent of the initial grain size.In magnesium,however,it has been observed that a smaller initial grain size yields a smaller dynamically recrystallized grain size,[27]although the reason for this phenomenon remains unclear.Several studies have examined the dynamically recrystallized grain size developed during the hot deformation of magnesium and developed equations that relate it to the deforma-tion conditions(either stress or Z).[5–7,12,27–31]While some researchers have shown that the relationship between the dynamically recrystallized grain size and the deformation conditions can be described using a power law(e.g.,References27and29through31), others have found that this relationship was not linear and attributed this to a change in the rate controlling dynamically recrystallized mechanism,i.e.,DDRX operative at high temperatures and CDRX operative at lower temperatures.[5–7,12]
Understanding the degree to which DRX goes to completion is also important,because a homogenous microstructure is desired for optimal?nal properties. The percentage DRX,at constant and increasing strains, has received a small degree of attention(e.g.,References 4through6,12,and30through32),but comparisons between di?erent initial starting conditions is lacking. Many studies have used magnesium alloys in the‘‘as-cast’’condition,but the deformation behavior of wrought magnesium is also of importance.The defor-mation of wrought,or preworked,material occurs in primary fabrication processes such as multistep forging and rolling operations and in the extrusion of billets that have been scalped and pre-extruded.The initial grain size will have a marked e?ect on recrystallization kinetics,with a?ne-grained material recrystallizing more rapidly than a coarse-grained material.[13]The higher density of grain boundaries in a?ner-grained structure means that there is an increase in the nucle-ation sites for DDRX.On the other hand,the reduction in grain size reduces the number of inhomogeneities such as shear and deformation bands,which also act as sites for nucleation.Twinning is also suppressed at smaller grain sizes,and thus,one would expect a smaller contribution of‘‘twin’’dynamically recrystallized mech-anisms to the overall microstructural development of ?ne-grained magnesium alloys.
To examine these issues further,the present article studies the e?ects of strain,deformation conditions,and initial state(wrought or as-cast)on the microstructures developed after the hot deformation of magnesium alloy AZ31.
II.EXPERIMENTAL PROCEDURE
The material used in the current study was commer-cial grade magnesium alloy AZ31(Mg-3pct Al-1pct Zn-0.2pct Mn)in both the‘‘wrought’’and‘‘as-cast’’form.The wrought material was in the form of18-mm-diameter extruded rod,and the machined compression samples were orientated such that the compression direction coincided with the extrusion direction.The initial wrought microstructure has a strong texture,in which the basal planes were aligned parallel to the extrusion direction,[33]and an average grain size of22.5 l m.The as-cast material was received in the form of a direct chill(DC)cast billet that was130mm in diameter. Machined compression samples were subsequently homogenized for24hours at425°C and had an average grain size of316l m.Samples were covered with Polytetra?ouroethylene(PTFE)tape and held at the test temperature for5minutes prior to testing.
To examine the evolution of microstructure during hot deformation,uniaxial compression testing was conducted at a temperature of350°C and at a constant strain rate of0.01s–1.The samples were quenched in water following equivalent strains of0.2,0.4,0.6,0.8, and 1.0.Also,to examine the?nal microstructures developed,uniaxial compression testing was conducted at temperatures ranging between300°C and450°C and at constant strain rates ranging between0.01s–1and 1s–1.These samples were quenched in water following an equivalent strain of1.0.Specimens for metallograph-ic examination,taken from the cross section of deformed samples,were cold mounted in epoxy resin,?ne ground with1200grit SiC paper,and diamond polished through6and3l m.Specimens were chemi-cally polished for45seconds in10pct nital and then etched for5seconds with acetic picral.Samples for examination using electron backscattering di?raction (EBSD)were polished as for optical metallography,
after which they were polished with a colloidal silica suspension and then etched with a solution of10mL HNO3,30mL acetic acid,40mL H2O,and120mL ethanol for5seconds.[34]
III.RESULTS
A.Evolution of Microstructure with Strain
The in?uence of strain on the microstructures that are developed when the wrought and as-cast materials are deformed at a temperature of350°C and a strain rate of 0.01s–1can be seen in Figures1and2,respectively.The initial microstructure of the extruded material displays fairly uniform grain boundaries.As the strain is increased to0.2,the original grain boundaries become serrated and dynamically recrystallized grains are observed at prior grain boundaries.As the strain is further increased to0.4and0.6,dynamically recrystal-lized grains become more prevalent.At strains of0.8, DRX has proceeded further and consumed most of the original grains.The microstructure is almost completely DRX after a strain of1.0.
In line with the observations during the compression of the wrought material,the original grain boundaries of the as-cast material also become serrated and a small number of dynamically recrystallized grains are ob-served at prior grain boundaries after deformation to a strain of0.2.In the deformation of the as-cast material, deformation twinning is evident.Twinning does not occur in all grains and slight serrations can be seen on some of the twin boundaries.At a strain of0.4,further dynamically recrystallized grains nucleate at the original grain boundaries forming a‘‘necklace’’type structure. In some areas,these necklaces have thickened to be several dynamically recrystallized grains wide,an exam-ple of which is the diagonal band of?ne grains running from the lower left corner to the upper right corner of the micrograph(Figure2(c)).The boundaries of
twins, Fig1—Microstructural evolution of wrought AZ31with increasing strain,deformed in compression at temperature of350°C and a strain rate of0.01s–1:(a)initial microstructure,(b)strain of0.2,(c)strain of0.4,(d)strain of0.6,(e)strain of0.8,and(f)strain of
1.0.
Fig2—Microstructural evolution of as-cast AZ31with increasing strain,deformed in compression at a temperature of350°C and a strain rate of0.01s–1:(a)initial microstructure,(b)strain of0.2,(c)strain of0.4,(d)strain of0.6,(e)strain of0.8,and(f)strain of1.0.
which had formed at earlier strains,have since become further serrated and DRX related to twinning is evident.The dynamically recrystallized grains are generally observed to have nucleated on the twin boundaries of thick twins and at twin intersections.It is also clear that dynamically recrystallized grains develop within,and ?ll,narrow twins.This can be more clearly seen in the micrograph at a strain of 0.6(Figure 2(d)),where a large grain has been subdivided by a thin lenticular twin,which has than acted as a nucleation site for DRX.At this strain,the necklaces of dynamically recrystallized grains decorating the pre-existing grains,and the dynamically recrystallized grains associated with twins,have continued to broaden.At a strain of 0.8,the development of DRX is more extensive,but even after strains of 1.0,the structure remains partially DRX.Di?erences between the development of DRX for the wrought and as-cast material are highlighted in Fig-ure 3,where the percentage DRX and the dynamically recrystallized grain size are plotted as a function of strain.For the as-cast material,the rate with which the percentage DRX increases is considerably lower than that observed for the wrought material.The microstruc-ture is close to 65pct DRX at the highest strain examined,and it is not obvious what level of strain will be required to develop a completely dynamically recrys-tallized microstructure.For both initial microstructures,the size of dynamically recrystallized grains (distin-guished from pre-existing grains by their size and morphology)remains virtually unchanged as deforma-tion proceeds (Figure 3(b))This ?nding is in line with the observation of Sah et al.,[35]who examined the conventional dynamically recrystallized behavior during the hot deformation of nickel.The average dynamically recrystallized grain size is approximately 7l m in the wrought material and 9l m in the as-cast material.B.Microstructure at a Strain of 1.0
The in?uence of strain rate and temperature on the microstructure formed when the wrought and as-cast materials are deformed at a strain of 1.0is shown in
Figures 4and 5,respectively.For the wrought samples,the material is not completely DRX for the tests carried out at 300°C.Twins and dynamically recrystallized grains decorating twins can be seen in some of the remnants of pre-existing grains.When the temperature is increased,the percentage of DRX and the size of the dynamically recrystallized grains are increased.A sim-ilar observation,albeit to a lesser degree,is observed for a reduction in strain rate.For temperatures of 350°C and above,many of the microstructures developed when the wrought material is deformed to a strain of 1.0are virtually completely DRX.
A signi?cantly di?erent microstructure from that of the wrought material is observed in the as-cast samples (Figure 5).At 300°C,the material is far from being completely DRX.The dynamically recrystallized grains have developed on pre-existing grain boundaries and also within twins,the latter of which is particularly noticeable in the microstructure developed at the highest strain rate (Figure 5(c)).As with the deformation of the wrought material,as the temperature of deformation is increased,the dynamically recrystallized grain size developed in the as-cast material increases.Also,the percentage of DRX can be seen to increase at higher deformation temperatures.At the highest temperatures examined,the material is still far from being completely DRX,although this is not obvious in the micrographs due to the high magni?cation chosen to su?ciently display the dynamically recrystallized grain size at the low temperatures.
Di?erences between the development of DRX,when the wrought and as-cast material is deformed to a strain of 1.0,are highlighted in Figure 6,where the percentage DRX and the dynamically recrystallized grain size are plotted as a function of the Zener–Hollomon parameter,
Z (?_e
exp Q =R T eT),where Q was taken as the activa-tion energy for self-di?usion,135kJ/mol.[36,37,38]For the wrought material,it can be seen that the percentage dynamic recrystallization is lowest at the highest Z values (high strain rate and low temperature)and increases toward being completely DRX at lower values of Z .It is also evident that the material never
reaches
Fig 3—In?uence of initial microstructure on the development of (a )the percentage DRX and (b )the dynamically recrystallized grain size (plot-ted as a function of strain for AZ31deformed at a temperature of 350°C and a strain rate of 0.01s –1).Lines are drawn to guide the eye.
100pct DRX at a strain of 1.0.For a given value of Z ,the percentage dynamic recrystallization is considerably lower in the as-cast material as compared to the wrought material.The percentage DRX is lowest at the highest Z values in the as-cast material;however,at lower values of Z ,the percentage DRX does not keep increasing toward being completely DRX.Instead,the percentage DRX saturates at a value of approximately 65pct DRX.
Figure 6(b)reveals that the dynamically recrystallized grain sizes developed,after deformation to a strain of 1.0,are larger in the as-cast material,particularly at low values of Z .For both the wrought and as-cast material,the dynamically recrystallized grain sizes decrease with increasing Z according to an inverse power law of the form
d DRX ?AZ àn
?1
where d DRX is the dynamically recrystallized grain size,A is a constant,Z is the Zener–Hollomon parameter,and n is the power-law exponent.Equations of this form have been used to predict the dynamically recrys-tallized grain size developed during hot working of magnesium alloys.[27,29–31]Although a more accurate ?t of the current data could be obtained by applying a calculated apparent activation energy,it is convenient to adopt a standard value of the activation energy,Q ,
for the hot working of magnesium alloys (it has also been shown that Q varies with temperature [21,39–42]).We have therefore opted for the activation energy for self-di?usion (135kJ/mol).[36,37,38]The values of A and n determined in the present work are 57.5and 0.095,respectively,for the wrought material,and 165.5and 0.126,respectively,for the as-cast material.
IV.DISCUSSION
Both continuous and discontinuous DRX have been observed to operate in magnesium alloys.[2–7,10–12,17,18]However,they are expected to yield di?ering responses with respect to the deformation behavior and micro-structural development.Our data will therefore be examined in light of these possible mechanisms.A.Flow Stress and Dynamically Recrystallized Grain Sizes
One characteristic that can help distinguish between the operation of DDRX and CDRX is the stress-strain curve.In DDRX,the dislocation density is lowered by the migration of high-angle grain boundaries that ‘‘sweep’’out the stored dislocations.This results in a reduction of the work hardening rate and
appreciable
Fig 4—Developed microstructures of wrought AZ31hot deformed to a strain of 1.0in compression:(a )300°C,0.01s –1;(b )300°C,0.1s –1;(c )300°C,1s –1;(d )350°C,0.01s –1;(e )350°C,0.1s –1;(f )350°C,1s –1;(g )400°C,0.01s –1;(h )400°C,0.1s –1;(i )400°C,1s –1;(j )450°C,0.01s –1;(k )450°C,0.1s –1;and (l )450°C,1s –1.
?ow softening,leading to a distinct peak in the strain-strain curve.By contrast,CDRX involves very little boundary migration as subgrains rotate and develop high-angle grain boundaries.The dislocation density is not signi?cantly reduced as it is in DDRX and thus the stress-strain curve generally attains a maximum stress
and is expected to show very little softening upon further straining,all else constant.
The ?ow curves obtained from the compression of the wrought and as-cast material,at a temperature of 350°C and a strain rate of 0.01s –1,are displayed in Figure 7.The ?ow curves exhibit a distinct peak in
the
Fig 5—Developed microstructures of as-cast AZ31hot deformed to a strain of 1.0in compression:(a )300°C,0.01s –1;(b )300°C,0.1s –1;(c )300°C,1s –1;(d )350°C,0.01s –1;(e )350°C,0.1s –1;(f )350°C,1s –1;(g )400°C,0.01s –1;(h )400°C,0.1s –1;(i )400°C,1s –1;(j )450°C,0.01s –1;(k )450°C,0.1s –1;and (l )450°C,1s –1
.
Fig 6—In?uence of initial microstructure on (a )the percentage DRX and (b )the dynamically recrystallized grain size,for wrought and as-cast AZ31hot deformed to a strain of 1.0in compression.
stress-strain behavior,as do many magnesium hot working ?ow curves presented in the literature (e.g.,References 2,3,7,22,32,41,and 43through 45),which tends to support the operation of a signi?cant level of DDRX.A higher strain to the peak ?ow stress with a coarser grain size in the as-cast material is also consis-tent with conventional DRX.
The size of the new grains developed when DRX is operating (either continuous or discontinuous)is char-acteristic of the deformation conditions.For DDRX,this is due to the simultaneous deformation occurring where,as the nucleated grains grow,work hardening occurs in the dynamically recrystallized grains and the driving force for further growth is reduced.For CDRX,subgrains developed during DRV rotate and develop high-angle grain boundaries.Because very little bound-ary migration occurs,the recrystallized grains are generally found to be a little larger than the prior DRV subgrains,the size of which is found to be strongly dependent on the deformation stress.[28]
For both DDRX and CDRX,increasing Z will result in a reduced grain size and this relationship is upheld in the present results (as seen in Figure 6(b)).However,the sensitivity of the grain size to Z is expected to di?er between the two mechanisms.Derby [28]compared the DDRX grain size for a wide selection of metals and minerals,including magnesium,and found that it is related to the deformation stress in a uniform manner.A similar relationship,albeit with a di?erent sensitivity to changes in ?ow stress,was found for the DRV subgrain size (which can be expected to be slightly smaller than the CDRX grain size).The dynamically recrystallized and DRV regions presented by Derby [28]are plotted in
Figure 8with the current grain size data.In this plot,the mean steady-state grain size during DRX,normalized by a Burger’s vector of 3.21·10–10(m),[28]is plotted against the steady-state deformation stress (taken at a strain of 1.0),normalized by the shear modulus (equal to 16;6001tà0:49T à300eT=924eTeT? (MPa)[36]).It can be seen that the dynamically recrystallized grain size is more sensitive to changes in stress than the DRV subgrain size.However,a large degree of overlap between the DRX and DRV regions exists,and the present data ?t within both regions.That is,no clear distinction between the operation of DDRX and CDRX can be made from the present grain size data.
Figure 6(b)also reveals that the log-log plot of the dynamically recrystallized grain size against Z is approximately linear for both initial microstructures.Similar relationships developed by Kaibyshev and co-workers for pure magnesium and magnesium alloy ZK60found that the dynamically recrystallized grain size correlates with Z (or the normalized stress,r /G ),but the power-law relationship is only obeyed for speci?ed temperature ranges.[5–7,12]This was attributed to changes in the rate-controlling dynamically recrystal-lized mechanism,i.e.,DDRX operative at high temper-atures and CDRX operative at lower temperatures.The current results,however,do not show such a sharp transition,and so it is unlikely that a dramatic change in the dominant dynamically recrystallized mechanism has occurred in the present tests.A gradual change in the dynamically recrystallized mechanism may,however,still result in a linear log-log plot of the dynamically recrystallized grain size against Z
.
Fig 7—Flow curves for the deformation of AZ31at a temperature of 350°C and a strain rate of 0.01s –1
.
Fig 8—Mean steady-state grain size,normalized by the Burger’s vec-tor,plotted against the deformation stress (at a strain of 1.0),nor-malized by the shear modulus.The shaded regions correspond to the grain size (or subgrain size in the case of DRV)models developed by Derby [26].
B.EBSD and Nucleation Mechanisms
In an attempt to gain more insight into the mecha-nisms by which DRX operates in the present material,orientation maps were generated from EBSD data (Figures 9through 11).This was conducted on as-cast samples that were compressed to various strains at a temperature of 350°C and a strain rate of 0.01s –1.As-cast samples were chosen in preference to wrought samples as a clearer distinction between pre-existing and dynamically recrystallized grains can be made.In all orientation maps presented (apart from Figure 11(b)),thick black lines correspond to high-angle boundaries (misorientations greater than 15deg),while low-angle boundaries are represented by thin white lines (misori-entations greater than 2deg)and thin black lines (misorientations greater than 5deg).
Orientation maps of as-cast AZ31,deformed to a strain of 0.2,reveal evidence of the operation of DDRX (Figures 9(a)and (b)).The bulging of original grain boundaries is frequently observed,as can be seen at the high-angle boundary between parent grains A and B in both orientation maps.An example of where a bridging low-angle boundary has developed behind a bulged section of grain A is indicated by C in Figure 9(a).In the case of Figure 9(b),a bulged section of grain A has almost been completely ‘‘pinched o?’’(indicated by D).
Interestingly,Figure 9(b)reveals that new grains have developed in regions associated with particles.The E indicates a typical example (the dark regions in the bottom right-hand corner of Figure 9(b),where no indexing of the EBSD patterns occurred,are interme-tallic particles).Particle-stimulated nucleation (PSN)has been previously reported for several magnesium alloys.[46,47,48]However,in the case of the present alloy,this mechanism is rarely observed and is not believed to signi?cantly contribute to the development of DRX.The CDRX is typically identi?ed by an increase in misorientation from the center to the edge of pre-existing grains and subgrain development near the boundary.[13]This was not observed in orientation maps of the sample that had been deformed to a strain of 0.2.However,when the material was deformed to a strain of 0.6,microstructural features consistent with the opera-tion of CDRX could be observed.Figure 10(a)shows an orientation map of a large pre-existing grain bordered by a necklace of dynamically recrystallized grains.A plot of the cumulative misorientation along the dotted line from the interior of the grain (A)to the grain boundary reveals a large increase in misorientation (Figure 10(b)).The development of low-angle bound-aries immediately adjacent to the necklace can also be seen,as indicated by B and C,and it could be envisaged that,upon further deformation,these may
progressively
Fig 9—(a )and (b )Orientation maps revealing evidence of DDRX (grain boundary bulging and the development of bridging subgrain bound-aries)in as-cast AZ31,deformed in compression to a strain of 0.2at a temperature of 350°C and a strain rate of 0.01s –1(thin white line >2deg misorientation,thin black line >5deg misorientation,and thick black line >15deg misorientation).Evidence of PSN is also present in
(b).
Fig 10—EBSD measurements revealing evidence of CDRX (an increase in misorientation from the center to the edge of a pre-existing grain and low-angle boundaries developing in the grain boundary region).(a )Orientation map of as-cast AZ31,deformed in compression to a strain of 0.6at a temperature of 350°C and a strain rate of 0.01s –1(thin white line >2deg misorientation,thin black line >5deg misorientation,and thick black line >15deg misorientation)and (b )the cumulative misorientation along the dotted line from A to the grain boundary.
increase in misorientation and develop dynamically
recrystallized grains.
This observation,however,di?ers from what has been observed for CDRX of Al-5pct Mg,[13]which exhibited a well-developed subgrain structure that extended fur-ther toward the grain interior.A well de?ned substruc-ture of subgrains is not visible in grain interiors in the present alloy.However,further possible evidence of CDRX may be taken from the small grain indicated by D in Figure 10(a).In this case,a new grain has formed near the parent grain boundary (similar examples can be seen toward the top right corner of Figure 9(b)).
Intragranular nucleation,via DRX related to twin-ning,is frequently observed in the as-cast material,an example of which is given in Figure 11(a)for a sample deformed to a strain of 0.2.It can be seen that an apparent primary twin spanning diagonally across the image has been subdivided into many new grains,possibly by the development of transverse low-angle boundaries that transformed to high-angle boundaries upon further straining.While the grains may develop via a continuous strain-induced process,the dynamically recrystallized grains do appear to have grown out of the twin and into the surrounding material (this can be clearly seen in Figure 2at a strain of 0.6).This points to an appreciable role of boundary migration in the development of new grains.
The misorientation between the dynamically recrys-tallized grains in the twin (A)and the surrounding material (B)was examined to determine the twin type.{10"12}twin boundaries (86deg around <1"210>)are marked as a thick white line in Figure 11(b).It can be seen that the boundaries between A and B do not share this relationship.However,region C does share a {10"12}twin boundary with both the twin grain and the matrix adjacent to it.Boundaries between A and B exhibit a (10"12)–(0"112)twin boundary relationship (60.4deg around <8"1"70>[49]),which is marked by a thick black line in Figure 11(b).Not all twin dynamically recrystal-lized grains exhibit this relationship,possibly due to small changes in misorientation with continuing defor-mation after the dynamically recrystallized grains were formed.
It appears that A and B are twins that formed on separate {10"12}planes in a parent grain with the orientation of C (i.e.,C is a small segment of the parent grain that remained untwinned).This is supported by the corresponding {0001}pole ?gure (Figure 11(c)),which shows that orientation C is favorably orientated for extension of the c -axis when compression is applied parallel to Z (and hence it expected to readily undergo {1012}twinning).Once formed,twin(s)B grew with further deformation,meeting with twin A and reorien-tating a majority of the parent grain.Twin A did not grow during further deformation;instead,developing dynamically recrystallized grains.
Another example of twin-related DRX is given in Figure 12for a sample deformed to a strain of 0.2.In this case,A is probably a small segment of the parent grain that remained untwinned and B is the twinned material (the boundaries between A and B share a {10"12}twin boundary relationship,marked as a thick white line).This is supported by the corresponding {0001}pole ?gure (Figure 12(b)),which shows that A is favorably orientated to undergo {1012}twinning for compression along Z .Region A is also subdivided by many low-angle boundaries,and a plot of the cumula-tive misorientation in region A,along the dotted line from lower left to upper right,illustrates the consider-able orientation gradients that have developed (Fig-ure 12(c)).The mechanism proposed for the development of dynamically recrystallized grains from twins,by the progressive increase in misorientation of transverse low-angle boundaries,may also apply to narrow untwinned regions in the material.
From the orientation maps presented,it is clear that evidence supporting discontinuous DRX (DDRX)and DRX related to twins is abundant.However,a continuous strain-induced process may be responsible for the increase in misorientation required to develop new grains,such as the rotation of subgrains adjacent to pre-existing grain boundaries or the subdivision of twins,and must also provide the bridging bound-aries behind bulges during the formation of DDRX
nuclei.
Fig 11—EBSD measurements revealing evidence of DRX related to twins.(a )Orientation map of as-cast AZ31,deformed in compression to a strain of 0.2at a temperature of 350°C and a strain rate of 0.01s –1(thin white line >2deg misorientation,thin black line >5deg misorienta-tion,and thick black line >15deg misorientation);(b )the identi?cation of twin and twin-twin boundaries,with thin black line >15deg misori-entation,thick white line marking {10"12}twin boundaries (86deg around <1"210>),and thick black line marking (10"12)–(0"112)twin
boundaries (60.4deg around <8"1"70>[49]);and (c )the corresponding {0001}pole ?gure,with the compression direction parallel to Z .
C.In?uence of Initial Structure on the Percentage DRX Both Figures 3(a)and 6(a)reveal that,at higher strains,the percentage DRX is signi?cantly lower in the as-cast material.This may be attributed to the enhanced kinetics of discontinuous DRX in the wrought material,in consequence of the higher speci?c grain boundary area and hence higher density of nucleation sites.The accelerated nucleation of discontinuous DRX in the re?ned initial microstructure of the wrought material may account for the lower ?ow stress compared to that of the as-cast material deformed under the same conditions (Figure 7).
An important ?nding is that,in the as-cast samples,the development of DRX saturates at a value of approximately 70pct with increasing strain and decreas-ing Z .These trends may be related to a lack of further nucleation sites once the original grain boundaries have been completely decorated with dynamically recrystal-lized grains.They may also be related to deformation becoming localized in the dynamically recrystallized fraction of the material,brought about by the large di?erence between the initial and dynamically recrystal-lized grain sizes.This process has been observed during the hot deformation of magnesium by other research-ers [2,3,19,50]and also in superplasticity studies (low strain rates).[51]
In a review on DDRX,Sellars [14]notes that if the deformation conditions change within the dynamically recrystallized fraction,then further nucleation may cease in the unrecrystallized fraction.This results in stabilization of the band of recrystallized grains,and as DRX does not go to completion,large unrecrystallized grains will remain.The study by Ion et al.[2]proposed
that,upon further straining,the bands of dynamically recrystallized grains continue to broaden and merge to form a shear zone.Shear zones were also observed in the current work (Figure 13).Once shear zones form,the accommodation of strain within the band of dynami-cally recrystallized grains occurs,possibly by slip and repeated DRX.This would only be expected to accom-modate a limited amount of deformation,and so at higher strains,voids followed by cracks are likely to form in order to relieve local stress concentrations.[2]The formation of a crack within the shear zone is also evident in Figure 13.This reinforces the link between dynamically recrystallized regions and enhanced rates of
?ow.
Fig 12—EBSD measurements revealing evidence of DRX related to untwinned regions.(a )Orientation map of as-cast AZ31,deformed in com-pression to a strain of 0.2at a temperature of 350°C and a strain rate of 0.01s –1(thin white line >2deg misorientation,thin black line >5deg misorientation,thick black line >15deg misorientation,and thick white line marking {10"12}twin boundaries (86deg around <1"210>);(b )the corresponding {0001}pole ?gure,with the compression direction parallel to Z ;and (c )the cumulative misorientation in region A,along the dot-ted line from lower left to upper
right.
Fig 13—The formation of a shear band in an as-cast sample de-formed in compression to a strain of 1.0,at a temperature of 300°C and a strain rate of 0.01s –1.The arrow indicates a crack.
D.In?uence of Initial Structure on the Dynamically Recrystallized Grain Size
The larger average dynamically recrystallized grain sizes developed in the as-cast material,as compared to the wrought material(Figures3(b)and6(b)),may be due to inhomogeneous deformation in the coarse-grained samples.If deformation becomes substantially accommodated in the recrystallized fraction of the material,the strain rate in this region would be higher and?ner dynamically recrystallized grains would be expected.In regions where deformation is not being substantially accommodated,however,the strain rate would be slower,and dynamically recrystallized grains, if they form,would be able to grow to larger sizes.If the rate of growth in the nondeforming regions was su?-ciently high,the resultant average dynamically recrys-tallized grain size may be coarser than for homogeneous deformation.This is not expected to be signi?cant until high deformation strains are applied and deformation has become localized.
The DRX related to twinning may also be partially responsible for the larger average dynamically recrys-tallized grain sizes observed in the as-cast material, particularly at low strains.Twinning is more pro-nounced in the as-cast material,and it is generally observed that‘‘twin’’dynamically recrystallized grains are larger than those developed at grain boundaries.As deformation becomes accommodated in the dynami-cally recrystallized necklace regions,dynamically re-crystallized grains that develop from twins that form across the pre-existing grains are able to grow to a larger size.
V.CONCLUSIONS
1.Dynamic recrystallization is operative during the
hot deformation of AZ31.Orientation maps of the as-cast material reveal abundant evidence of DDRX and DRX related to twins as predominant mecha-nisms,with some manifestation of CDRX and PSN.
2.The rate the percentage DRX increases for the as-
cast material is considerably lower than for the wrought material.Also,in the as-cast samples,the percentage DRX does not continue to increase to-ward complete DRX with decreasing Z.These observations may be attributed to the deformation becoming localized in the dynamically recrystallized fraction of the material.
3.The dynamically recrystallized grain size is generally
larger in as-cast material than in the wrought mate-rial.This di?erence may be attributed to inhomoge-neous deformation and DRX related to twins.
4.Bands of dynamically recrystallized grains(shear
zones)were developed in the as-cast material.The formation of cracks within shear zones was also observed,which reinforces the link between dynam-ically recrystallized regions and enhanced rates of ?ow.
ACKNOWLEDGMENTS
The CAST CRC was established under,and is sup-ported by,the Australian Government’s Co-operative Research Centres Programme.
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