me als
A icle
Tex u e E olu ion in Biocompa ible Mg-Y-Re Alloy
A e F ic ion S i P ocessing
Lenka Kunˇcická1,*, Pe K ál1, Jiˇ íD oˇ ák1and Radim Kocich 2
1Ins i u e o Physics o Ma e ials, ASCR, 61662 B no, Czech Republic; [email p o ec ed] (P.K.);
[email p o ec ed] (J.D.)
2VŠB—Technical Uni e si y o Os a a, 17. lis opadu 15, 708 00 Os a a 8, Czech Republic;
[email p o ec ed]
*Co espondence: [email p o ec ed]; Tel.: +420-532-290-371
Recei ed: 30 Sep embe 2019; Accep ed: 29 Oc obe 2019; Published: 1 No embe 2019


Abs ac :
The p esen ed s udy deals wi h he in es iga ion o biocompa ible WE 43 Mg-based
alloy p ocessed ia he combina ion o o a y swaging (RS) and ic ion s i p ocessing (FSP) a
h ee di e en o a ional speeds o 400 RPM, 800 RPM, and 1200 RPM. The s uc u e obse a ions
p ima ily ocused on ex u e de elopmen and cha ac e iza ions o g ain sizes and g ain bounda ies.
The esul s showed ha swaging plus p ocessing a 400 RPM and 1200 RPM lead o subs an ial
ec ys alliza ion and g ain e inemen . The ac ions o low angle g ain bounda ies wi hin he 400
RPM and 1200 RPM samples we e app oxima ely 11%, while o he 800 RPM sample exhibi ing
seconda y ec ys alliza ion i was abou 22%. The g ains we e also he ines in he 1200 RPM sample
(a e age g ain diame e o 1.8
µ
m). The p ocessed s uc u es exhibi ed a sligh endency o o m he
{10-10} <0001>p e e en ial ibe ex u e (especially he 800 RPM sample). Tensile es ing showed he
FSP o ha e posi i e in luence on he ul ima e ensile s ess, as well as duc ili y o all he samples; he
mechanical p ope ies imp o ed wi h inc easing FSP a e.
Keywo ds: magnesium alloys; g ains and in e aces; mic os uc u e; ex u e
1. In oduc ion
Thei ad an ageous p ope ies, such as low densi y, good cas abili y and machinabili y, a ou able
speci ic s eng h, and supe io he mal and elec ical conduc i i y, make magnesium-based alloys
pe spec i e solu ions o demanding applica ions, such as ligh weigh s uc u es, anspo a ion
equipmen , and ai c a o je engines [
1
,
2
]. Gi en by hei a ou able biocompa ibili y, ligh weigh ,
and low elas ic modulus simila o he moduli o human bones, selec ed Mg-based alloys a e also
applicable o biodeg adable implan componen s [
3
–
5
]. The mechanical and physical p ope ies o
pu e Mg a e no excep ional, howe e , hey can be enhanced—p ima ily in he wo ollowing ways.
The i s way is addi ion o alloying elemen s. The Mg-Al and Mg–Zn sys ems a e popula o
cons uc ional pu poses [
6
], addi ions o zinc a e ad an ageous also o biocompa ible Mg alloys [
3
].
Op imized addi ions o a e ea h (RE) elemen s ha e supe io e ec s on enhancemen o p ope ies
o Mg; Mg-Zn-Y based alloys we e epo ed o ea u e good cy ocompa ibili y and o ha e posi i e
an ibac e ial e ec s [
7
,
8
]. RE, such as Y, Nd, Dy, o Gd gene ally imp o e he co osion esis ance
o Mg-based alloys and suppo g ain e inemen and imp o emen and homogeniza ion o he
mechanical p ope ies [
9
–
12
]. RE elemen s ha e also been shown o educe he c i ical esol ed shea
s ess (CRSS) o non-basal slip sys ems and e agonal aspec a io o p isma ic and py amidal slip
sys ems, which consequen ly inc eases o mabili y (gene ally low o pu e Mg) [
13
]. The bene icial
e ec s o Y ium a e nume ous, i inc eases o mabili y, suppo s ex u e andomiza ion and ac i a ion
Me als 2019,9, 1181; doi:10.3390/me 9111181 www.mdpi.com/jou nal/me als
Me als 2019,9, 1181 2 o 13
o non-basal slip sys ems by lowe ing he ma ix ene gy and p omo ing solu e d ag (simila ly o
neodymium [14]), which u he dec eases aniso opy o he mechanical p ope ies [15].
The second way o enhancemen o Mg-based alloys’ p ope ies is ia s uc u e changes impa ed
by inno a i e plas ic de o ma ion echnologies, which ha e d awn a en ion no only o hei abili y
o e ec i ely e ine s uc u al uni s and consequen ly enhance he inal p ope ies o he p ocessed
ma e ials, bu also o hei abili y o p ocess ma e ials wi h ela i ely low o mabili ies [
16
–
18
].
The mos e ec i e echnologies a e he me hods o se e e plas ic de o ma ion (SPD), such as equal
channel angula p essing (ECAP) [
19
,
20
], accumula i e oll bonding (ARB) [
21
], high p essu e o sion
(HPT) [
22
], o o a y swaging (RS) [
23
,
24
] and ic ion s i p ocessing and welding—FSP and FSW [
25
].
P e iously published s udies in es iga ed he e ec s o he mos e icien SPD me hods—ECAP
(e.g., Kocich e al. [
26
] epo ed he e ec s o ic ion and die angle pa ame e s on de o ma ion
beha iou o he AZ63 alloy, and Cabibbo e al. [
27
] epo ed he g ain size wi hin he WN43 o
dec ease down o 380 nm a e 8 ECAP passes), and HPT (p o en o be e ec i e o homogeniza ion o
dis ibu ion o RE-based p ecipi a es [
28
]) on a ious Mg-based alloys. Howe e , he main d awback
o hese me hods is ha hey a e limi ed o ela i ely small samples o de ini e dimensions ( he la ges
known HPT-p ocessed samples ha e he diame e s o app oxima ely 20–30 mm). On he o he hand,
echnologies such as RS and FSP can be designed o p ocess la ge bulk samples.
RS is an in ensi e plas ic de o ma ion p ocess enabling educ ion o diame e s o axisymme ic
p oduc s by a epea ed ac ion o o a ing dies a ec ing opposi e su aces o he swaged-piece wi h
comp essi e swaging o ces ha ing wo componen s, axial and angen ial [
29
,
30
]. The axial
componen con ibu es p ima ily o elonga ion o he inal p oduc , whe eas he angen ial
componen in oduces shea s ain con ibu ing o agmen a ion o he g ains and accumula ion o
disloca ions leading o subsequen o ma ion o disloca ion cells and subg ains, and consequen g ain
e inemen . FSP imposes se e e shea s ain o hin laye s o ma e ials, which gene ally in oduces
subs an ial g ains’ ec ys alliza ion. The me hod is ad an ageous especially o inal p ocessing
o demanding ma e ials
[31,32]
. Besides hei e sa ili y, bo h he RS and FSP me hods con ibu e
subs an ially o s uc u e homogeniza ion, p og essi e g ain e inemen , and enhancemen o u ili y
and mechanical p ope ies.
Repo s on he e ec s o swaging on Mg-based alloys a e no nume ous. Gan e al. [
33
,
34
] s udied
he e ec s o swaging on comme cially pu e cas Mg and ound p og essi e g ain e inemen and
he endency o he swaged ma e ial o o m he (0002) basal ex u e. Wang e al. [
35
] in es iga ed
he op imum swaging/annealing condi ions o WE43 alloy and inc eased bo h, he s eng h and
duc ili y o he alloy ia swaging and annealing a 400
◦
C, and Ma ynenko e al. [
36
,
37
] op imized
he swaging ea men and epo ed he g ains wi hin WE43 alloy o e ine down o 0.5
µ
m and
he s eng h o inc ease o mo e han 400 MPa a e se e e swaging a empe a u es be ween 325
and 400
◦
C. Se e al s udies epo ing he e ec s o FSP on Mg-based alloys ha e been published,
bu hey a e sca ce. Wang e al. [
38
] documen ed he posi i e e ec o o a ional speed o FSP on
mechanical p ope ies o he p ocessed Mg–Zn–Y–Z sample, Kuma e al. [
39
] epo ed he posi i e
in luence o FSP on g ain e inemen wi hin a lase -p oduced WE43 sample and cha ac e ized he
a ying s eng hening mechanisms, Va gas e al. [
40
] impa ed signi ican g ain e inemen (g ain size
o 1.2
µ
m a e single FSP pass and submic on g ain size a e he second pass) and duc ili y inc ease
wi hin Mg-Zn-Ca-Z samples, Liu e al. [
41
] homogenized he g ain size and e ined he g ains down o
app ox. 2
µ
m and imp o ed he co osion esis ance o Mg–9Li–1Zn alloy ia ex u e andomisa ion,
and Khan e al. [
42
] inc eased he duc ili y o QE22 alloy o almos 25% ia impa ing nuclea ion o
new g ains and ex u e al e a ion.
In o de o s udy mu ual e ec s o he wo abo e cha ac e ized ad an ageous SPD echnologies,
he p esen ed s udy deals wi h he in es iga ion o a cas and solu ion ea ed Mg–based Mg-4Y-3Nd
(WE43) alloy p ocessed ia o a y swaging (RS) and subsequen ic ion s i p ocessing (FSP) unde
h ee di e en o a ional speeds. The s uc u es we e s udied ia scanning elec on mic oscopy;
Me als 2019,9, 1181 3 o 13
he analyses ocused p ima ily on cha ac e iza ion o g ains and hei bounda ies and on ex u e
cha ac e iza ion. The mechanical p ope ies we e examined ia ensile es s.
2. Ma e ials and Me hods
The Mg-4Y-3Nd (WE43) alloy was p oduced by acuum induc ion mel ing (VIM), cas , and
u he p ocessed ia o a y swaging (RS). Wi hin his s udy, RS was implemen ed p ima ily o inc ease
o mabili y and homogenize he cas WE43 alloy s uc u e. The inal educ ion a io calcula ed using
Equa ion (1), whe e S
0
, S
n
a e he c oss-sec ion a eas a he inpu and ou pu o he swaging dies,
espec i ely, was 1.20.
ϕ=lnS0
Sn, (1)
Swaging was ollowed by T4 hea ea men —8 hou s’ dwell a 520
◦
C, and inal quenching in
wa e (annealing p ocedu e simila o ou p e ious s udies [
12
,
28
]). A e he solu ion ea men (ST),
samples o he dimensions o 40
×
20
×
20 mm we e cu using elec ic discha ging machine (Sodick
SL400Q, Sodick Eu ope, L d, Wa wick, UK) and mechanically polished. The p epa ed samples we e
u he subjec ed o FSP p ocessing a he a e se a e o 60 mm
·
min
−1
and h ee di e en o a ional
speeds o 400 RPM, 800 RPM, and 1200 RPM, and subsequen ly quenched in wa e . The conical ic ion
ool wi h a 3 mm long pin wi h he diame e a ying om 6 mm o 4 mm was made o a ool s eel and
had a la shoulde wi h 15 mm in diame e (simila se up as e.g., in e s. [
31
,
39
]). The schema ics o
he en i e p oduc ion p ocess in depic ed in Figu e 1.
Figu e 1. Schema ic depic ion o p oduc ion p ocess. FSP: ic ion s i p ocessing.
A e p ocessing, samples o s uc u e analyses aken in he di ec ion o ic ion ool mo emen
we e mechanically g ound and polished, and inally polished elec oly ically. The s uc u es o he
FSP-p ocessed, as well as o he o a y swaged and solu ion ea ed (RS-ST) ma e ial s a es we e scanned
ia Tescan Ly a 3 scanning elec on mic oscope (TESCAN B no s. .o, B no, Czech Republic) equipped
wi h an elec on backsca e di ac ion (EBSD) de ec o (Ox o d Ins umen s, Abingdon-on-Thames,
UK). The scans aken wi h he s eps o 0.25
µ
m we e e alua ed using OXFORD Ins umen s [
43
]
so wa e (Ox o d Ins umen s, Abingdon-on-Thames, UK). The g ain size was de e mined as he
a i hme ic a e age o he esul s o measu emen s aken in wo pe pendicula (ho izon al, e ical)
di ec ions. The miso ien a ion angle
θ
=15
◦
was se as he minimum angle o iden i ica ion o g ains
wi h high angle g ain bounda ies (HAGBs), he bounda ies wi h
θ
lowe han 15
◦
a e deno ed as low
angle g ain bounda ies (LAGBs). The g ain is cha ac e ized as a ec ys allized g ain when he in e nal
a e age miso ien a ion angle in he g ain is lowe han 1
◦
. In he case he g ain con ains subg ain
bounda ies wi h miso ien a ion angles highe han 1
◦
, he g ain is cha ac e ized as subs uc u ed.
The alues o he in e nal a e age miso ien a ion angles highe han 1
◦
a e cha ac e is ic o de o med
g ains. The ex u es we e e alua ed in he di ec ion pe pendicula o he di ec ion o ic ion pin
mo emen and we e e alua ed by using 20
◦
maximal de ia ion om he speci ic ex u e componen s.
The specimens o ensile es ing we e cu ia elec o-discha ging machine in he di ec ion pa allel
o he di ec ion o ic ion ool mo emen . The ensile es s specimens we e 10 mm long and had
Me als 2019,9, 1181 4 o 13
2×4 mm
c oss-sec ions, he oom empe a u e es s we e ca ied ou wi h he s ain a e o 10
−3
s
−1
using a Zwick es ing machine (Zwick/Roell—Messphysik KAPPA LA, Zwick /Roell CZ s. .o., B no,
Czech Republic). The inal ul ima e ensile s eng h and elonga ion we e calcula ed as he a e age
alue om h ee indi idual es ed samples.
3. Resul s
3.1. G ains and Bounda ies Cha ac e iza ion
The esul s o g ain size analyses o he o a y swaged and solu ion- ea ed (RS-ST) ma e ial and
he samples a e RS and FSP a e shown in Figu e 2a–d. The mean g ain size o he RS-ST sample
was 58.5
µ
m. The g ains e ined o he a e age diame e s o 4.2
µ
m, 7.1
µ
m, and 1.8
µ
m, a e RS
plus 400, 800, and 1200 RPM FSP, espec i ely. These esul s indica e ha all he FSP echnologies
led o signi ican g ain e inemen wi hin he WE43 alloy. The FSP a e o 1200 RPM e en led o he
o ma ion o s uc u e he a e age g ain size wi hin which app oached he ul a- ine (UF) le el, and
app oxima ely 18% o he g ains wi hin his sample we e smalle han 1
µ
m. The g ains we e he
coa ses in he 800 RMP sample.
Figu e 2.
G ain size dis ibu ions o samples: Ro a y swaged and solu ion ea ed (RS-ST) (
a
); RS+400
RPM FSP (b); RS+800 RPM FSP (c); RS+1200 RPM FSP (d).
The analyses o g ain bounda ies showed ha he RS-ST sample ea u ed a as majo i y o high
angle g ain bounda ies (HAGBs); Figu e 3a shows he cas s uc u e o he p epa ed WE43 alloy,
whe eas Figu e 3b depic s he RS-ST band con as EBSD image wi h HAGBs highligh ed in g een, and
Figu e 3c shows he co esponding miso ien a ions cha o he RS-ST sample.. The image also clea ly
documen s he p esence o ela i ely coa se p ecipi a es. A e RS+400 RPM FSP, he ac ion o low
angle g ain bounda ies (LAGBs) inc eased o app oxima ely 11%; Figu e 3d depic s he EBSD image
wi h HAGBs in g een, while Figu e 3e shows co esponding miso ien a ions cha . This phenomenon
indica es he de elopmen o subs uc u e due o he se e e imposed s ain impa ing accumula ion
Me als 2019,9, 1181 5 o 13
o s uc u e de ec s (disloca ions), which u he clus e in o angles, cells, and disloca ions walls
and e en ually o m LAGBs [
44
]. Figu e 3 ,g depic he EBSD image wi h highligh ed HAGBs and
miso ien a ions cha , espec i ely, o he RS+800 RPM FSP sample. The ac ion o LAGBs o his
sample inc eased o 16%. RS+1200 RMP FSP p ocessing in oduced LAGBs ac ion simila o RS+400
RPM p ocessing, i.e., app oxima ely 11% (Figu e 3h,i).
Figu e 3. Con .

Me als 2019,9, 1181 6 o 13
Figu e 3.
SEM-BSE scan o cas s uc u e (
a
); band con as elec on backsca e di ac ion (EBSD) image
wi h high angle g ain bounda ies (HAGBs) highligh ed in g een o samples: RS-ST (
b)
; RS+400 RPM
FSP (
d
); RS+800 RPM FSP (
); RS+1200 RPM FSP (
h
). G ain bounda ies miso ien a ions dis ibu ions
o samples: RS-ST (c); RS+400 RPM FSP (e); RS+800 RPM FSP (g); RS+1200 RPM FSP (i).
3.2. Rec ys allized F ac ion
The abo e-men ioned analyses can be con i med ia examina ions o ec ys allized ac ion.
Figu e 4a demons a es an example o e alua ion o he analyses o he ec ys allized (g een colou ),
subs uc u ed (yellow colou ), and de o med ( ed colou ) ac ions o he RS+400 RPM FSP sample.
Figu e 4b hen summa izes he esul s o he analyses o all he p ocessed samples. Ob iously, he
ac ion o g owing g ains wi h de eloped subs uc u e was he highes o he RS+800 RPM FSP
sample. The RS+1200 RPM FSP sample, he imposed ene gy o which was inc eased o he le el
su icien o u he g ains es o a ion (also suppo ed by he highes ac ual p ocessing empe a u e),
ea u ed he highes ac ion o ec ys allized g ains. This esul is in acco dance wi h he g ains size
analyses, as well as ex u e analyses epo ed in Sec ion 3.3.
Figu e 4.
Rec ys allized, subs uc u ed, and de o med ac ions: example o analyses e alua ion o
RS+400 RMP FSP sample (a); summa y o RS+FSP-p ocessed samples (b).
3.3. Tex u e
The ex u es o all he RS-ST, and RS plus 400 RPM, 800 RPM, and 1200 RPM FSP samples we e
e alua ed wi h he emphasis on he possible p esence o p e e en ial ibe s/o ien a ions and hei
in ensi ies, and on he co ela ion o p e e en ial o ien a ions and ec ys allized ac ions. Figu e 5a–d
depic in e se pole igu es (IPFs) o he RS-ST, and RS plus 400 RPM, 800 RPM, and 1200 RPM
FSP samples, espec i ely, wi h highligh ed p e ailing ideal o ien a ions -{0001} <11-20>in g een
colou , and {0001} <10-10>in ed colou . Figu e 6a–d hen depic IPFs wi h ex u e in ensi ies o he
men ioned samples. Las bu no leas , Figu e 7a–d show pole igu es (PF) depic ing ec ys allized
g ains o he RS-ST, and RS plus 400 RPM, 800 RPM, and 1200 RPM FSP samples, espec i ely. In he
Me als 2019,9, 1181 7 o 13
PFs, he colou s cha ac e ize he indi idual g ain sizes om blue o he new smalles ec ys allized
g ains, h ough g een o he g owing g ains, o ed o he la ges ones.
Figu e 5.
In e se Pole Figu es o samples: RS-ST (
a
); RS+400 RPM FSP (
b
); RS+800 RPM FSP (
c
);
RS+1200 RPM FSP (
d
). O ien a ion {0001} <10-10>depic ed in ed, o ien a ion {0001} <11-20>depic ed
in g een.
Figu e 6.
In e se pole igu es o samples: RS-ST (
a
); RS+400 RPM FSP (
b
); RS+800 RPM FSP (
c
);
RS+1200 RPM FSP (d).
The RS-ST sample ob iously ea u ed andomly sca e ed g ains wi h no p e e en ial o ien a ions
(Figu es 5a and 7a), he IPF depic ed in Figu e 6a shows ha he maximum ex u e in ensi y
o his sample was less han 2 imes andom and he g ains did no clea ly incline o a single
dis inguishable o ien a ion. A e RS+400 RPM FSP p ocessing, he g ains e ined signi ican ly and
he esh newly nuclea ed g ains (blue do s in Figu e 7b) did no exhibi any p e e en ial ex u e
o ien a ion. Ne e heless, he g owing g ains exhibi ed he endency o o m he {0001} <11-20>and
{0001} <10-10>p e e en ial ex u es (Figu es 5b and 6b). The la e was mo e in ense, as depic ed in
Figu e 6b, al hough i s o al in ensi y was only sligh ly highe han 2 imes andom. Mu ual co ela ion
o Figu es 6b and 7b impa s he supposi ion ha he g owing g ains had he endency o align along
he {0001} <11-20>p e e en ial o ien a ions a i s , and la e on, as hey con inued o g ow, ended
o acqui e he {0001} <10-10>o ien a ion. This conclusion abou ex u e e olu ion is suppo ed by
Figu e 5c, Figu e 6c, and Figu e 7c depic ing he IPFs and PFs o he RS+800 RPM FSP sample, which
exhibi ed simila endencies; he maximum in ensi ies o he p e e en ial ex u e o ien a ions we e
again app oxima ely wo imes andom o his sample. Ne e heless, he <10-10>ideal ex u e
o ien a ion was app oxima ely by 25
◦
shi ed, which is e iden in Figu es 6c and 7c. The IPFs and PFs
depic ing ex u e cha ac e is ics o he RS+1200 RPM FSP p ocessed sample can be seen in Figu es
Me als 2019,9, 1181 8 o 13
5d, 6d and 7d. The ma e ial exhibi ed ine g ains ha ing bo h he {0001} <11-20>and {0001} <10-10>
p e e en ial o ien a ions in simila in ensi ies.
Figu e 7.
Pole igu es emphasizing g ains o ien a ions in ela ion wi h g ain g ow h o samples: RS-ST
(a); RS+400 RPM FSP (b); RS+800 RPM FSP (c); RS+1200 RPM FSP (d).
3.4. Mechanical Tes ing
The samples aken om he cas ma e ial, ma e ial a e RS and ST, and he ma e ials a e RS
plus 400 RPM, 800 RPM, and 1200 RPM FSP we e subjec ed o ensile es s, he esul s o which a e
summa ized in Table 1. E iden ly, plas ici y o all he p ocessed samples inc eased signi ican ly when
compa ed o he cas ma e ial. Ro a y swaging in combina ion wi h solu ion ea men inc eased he
ul ima e ensile s eng h (UTS) o mo e han 200 MPa, and also inc eased he maximum elonga ion
o ailu e. The RS+400 RPM and RS+800 RPM samples also ea u ed bo h enhanced s eng h and
elonga ion, howe e , he elonga ion o he RS+800 RPM sample was he lowes o all he de o ma ion
p ocessed ones. RS+FSP a 1200 RPM led o he UTS inc ease up o almos 280 MPa.
Table 1. Mechanical p ope ies o in es iga ed ma e ial s a es.
Sample YS (MPa) UTS (MPa) Elonga ion o Failu e (%)
As cas 115 142 ±15 3.8 ±0.5
RS-ST 135 201 ±10 5.8 ±0.8
400 RPM 166 232 ±22 10.9 ±0.9
800 RPM 172 246 ±19 9.2 ±1.5
1200 RPM 184 279 ±28 11.7 ±1
4. Discussion
RS is known o i s posi i e in luence on g ain e inemen and agmen a ion o la ge pa icles
and p ecipi a es consequen ly esul ing in enhanced p ope ies o he p ocessed ma e ials [
45
,
46
].
I s a ou able e ec s on imp o emen o mechanical p ope ies we e documen ed by o he s.
Fo example, Wang e al. [
35
] p e iously epo ed he UTS o he WE43 alloy o inc ease o
mo e han 350 MPa a e swaging a 300
◦
C and subsequen annealing, howe e , hei sample was
subjec ed o he signi ican educ ion a io o 73% and ea u ed he g ain size o 4
µ
m a e swaging.
As p o en by he he ein p esen ed esul s, he UTS o he RS-ST sample exceeded 200 MPa, bu he
Me als 2019,9, 1181 9 o 13
elonga ion o his sample was subs an ially lowe han o he samples a e RS plus FSP p ocessing.
As ob ious om Figu e 3b, he RS-ST sample ea u ed no only ela i ely la ge g ains bu also
p ecipi a ed pa icles, which mos p obably nuclea ed in he ma e ial du ing cooling. The p ecipi a es
ac ed as obs acles o disloca ions mo emen du ing he ensile de o ma ion unde cold condi ions
and esul ed in highe UTS (compa ed o he cas ma e ial), bu lowe plas ici y (compa ed o he FSP
samples) [47].
Subsequen FSP impa ed dissolu ion o mos o he p ecipi a es in o solid solu ion, which sligh ly
inc eased he UTS ia solid solu ion s eng hening, bu p ima ily enhanced plas ici y [
48
]. The analyses
indica e ha RS+FSP a 400 RPM in oduced su icien ene gy o impa o ma ion o new g ains
wi hin he s uc u e. RS and p ocessing a he FSP a e o 800 RPM impa ed ec ys alliza ion, oo.
Howe e , he inc eased imposed ene gy oge he wi h he highe ac ual p ocessing empe a u e also
caused g ain coa sening and u he subs uc u e de elopmen — he ac ion o g owing g ains wi h
de eloped subs uc u e was he highes o his sample—which sligh ly dec eased he elonga ion o
ailu e when compa ed o he o he samples a e FSP ( he accumula ed ene gy no only caused he
pos -p ocess g ain g ow h, bu mos p obably also impa ed e-nuclea ion o ine p ecipi a es, howe e ,
hese we e no obse ed a he ac ual SEM magni ica ion).
Chang e al. [
49
] p e iously documen ed he endency o g ains wi hin ex uded Mg-based AZ31
alloy subjec ed o FSP o coa sen wi h inc easing FSP a e. This epo is simila o he he ein obse ed
esul s o 400 RPM and 800 RPM samples. P ima y ec ys alliza ion equi es a ce ain amoun o ene gy
o be imposed in o he p ocessed ma e ial, bu he ene gy necessa y o he pos - ec ys alliza ion g ain
g ow h is a smalle (by up o wo o de s o magni ude) [
44
]. By his eason, he accumula ed ene gy
in he 800 RPM sample, which was no consumed by he p ima y ec ys alliza ion, caused he g ain
bounda ies o mig a e o he poin o ene gy exhaus ion, which esul ed in he inc ease in LAGBs
ac ion and in oduced g ain g ow h. Howe e , he g ain g ow h was inhomogeneous and occu ed
p ima ily in loca ions in which he new g ains nuclea ed as i s (shea bands, e c.) [
44
], which was
documen ed also by Figu e 2c, he p esence o g ains wi h he size o a ew mic ons in which can be
obse ed oge he wi h he p esence o g ains la ge han 10
µ
m. G ains ea angemen occu ing
a e he p ima y ec ys alliza ion was also documen ed by he p omo ed endency o he g ains o
align along he {0001} <10-10>p e e en ial ex u e o ien a ion. Ne e heless, he ideal o ien a ion
was app oxima ely by 25
◦
shi ed, which is e iden in Figu es 6c and 7c. Simila phenomenon was
obse ed by Yuan and Mish a [
50
], who p ocessed AZ31 alloy ia 600–900 RPM FSP and ound he
basal ex u es o be il ed by app oxima ely 35
◦
. The he ein p esen ed lowe de ia ion o app ox. 25%
can be a ibu ed o he addi ions o Y and Nd, which agg a a ed g ain bounda y mig a ion ia solu e
d ag [14].
Inc easing he FSP a e o 1200 RPM u he inc eased he imposed ene gy o he le el su icien
o u he ec ys alliza ion, which was also suppo ed by he highes ac ual p ocessing empe a u e.
Fo his sample, he UTS as well as plas ici y inc eased ema kably. The g ea es con ibu ion o his
beha iou can be a ibu ed o he signi ican g ain e inemen and o ma ion o UFG s uc u e wi h
g ains ha ing mo e o less andom o ien a ions [
50
]. The RS+1200 RPM FSP sample ea u ed he highes
ac ion o new g ains wi hou seconda y g ain g ow h. The inc eased imposed ene gy in oduced ia
he inc ease in he FSP o a ional a e o 1200 RPM impa ed gene a ion o new disloca ions and he
endency o he s uc u e o o m new disloca ion angles, cells, and walls, and subsequen o ma ion
o LAGBs [
48
]. A e eaching he c i ical le el o accumula ed ene gy, g ains cha ac e ized wi h
HAGBs s a o o m wi hin he s uc u e and he LAGBs ac ion dec eases [
45
]. The phenomenon o
signi ican g ain e inemen a e p ocessing a 1200 RPM can be explained by he mu ual e ec s o
occu ing ec ys alliza ion and he p esence o RE elemen s suppo ing g ain bounda y pinning and
o ma ion o ul a- ine g ains [
51
,
52
]. Simila conclusions we e also d awn by Va gas e al. [
40
] o
FPS-p ocessed ZKX50 alloy con aining Z , he a e age g ain size o which a e p ocessing by 1000
RPM was ~ 1 µm.